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The dynamic properties and melting behavior of the Earth's mantle are strongly influenced by the presence of volatile species, including water, carbon dioxide, and halogens. The role that halogens play in the mantle has not yet been fully quantified: their presence in only small quantities has dramatic effects on the stability of mantle minerals, melting temperatures, and in generating halogen-rich melts such as lamproites. Lamproites are volumetrically small volcanic deposits but are found on every continent on the planet: they are thought to be melts generated from volatile-rich mantle sources rich in fluorine and water. To clarify the mantle sources of lamproites, we present experimentally determined mineral/melt partition coefficients for fluorine and barium between phlogopite and lamproite melts. Both fluorine and barium are compatible in phlogopite [ D F (Phl/Melt) 0.96 ± 0.02 − 3.44 ± 0.33 , D Ba (phl/Melt) 0.52 ± 0.05 − 3.68 ± 0.43 ] at a range of pressures (5–30 kbar), temperatures (1000–1200 °C), and fluid compositions (C-O-H mixtures). Using our partition coefficients, we model the melt compositions produced by potential lamproite sources, including phlogopite garnet lherzolite, phlogopite harzburgite, and hydrous pyroxenite. The results demonstrate that hydrous pyroxenites and phlogopite garnet lherzolite can produce melts with F and Ba contents similar to lamproites, but only hydrous pyroxenites fully reproduce other geochemical characteristics of lamproites including high K2O, low CaO contents, and high F/H2O ratios. [ABSTRACT FROM AUTHOR]

To improve the understanding of the formation and evolution of the sub‐continental lithospheric mantle (SCLM) underlying the South Australian Craton we have conducted a detailed petrological study on >3,000 mantle xenocrysts from 13 kimberlites emplaced across the craton. Pressure (P) and temperature (T) estimates on Cr diopside and garnet have been coupled with their chemical concentrations to constrain lithospheric thickness and chemo‐lithostratigraphy. We show that lithospheric thickness is greatest beneath the Gawler Craton, whereas thinner lithosphere occurs beneath the Adelaide Fold Belt. Mineral compositions highlight two litho‐chemical domains within the shallow and deep SCLM that are separated by a mid‐lithosphere discontinuity (MLD). The shallow SCLM (60–130 km) comprises low Cr2O3 lherzolite and wehrlite. Shallow SCLM xenocrysts record depleted and refertilized compositions enriched in light rare earth elements related to metasomatism by kimberlite or related melts. The mid‐lithosphere (130–160 km) is depleted in garnet and Cr diopside which may relate to a layer of pargasite lherzolite. The deep SCLM (>160 km) comprises high Cr2O3 lherzolite with elevated TiO2 and FeO. We interpret the litho‐chemical stratification of the SCLM to reflect a multi‐stage top‐down growth. The shallow SCLM reflects an amalgamation of Precambrian cratonic nuclei characterized by heterogeneity in geochemical enrichment and depletion. Interaction of the shallow SCLM with mantle plumes accreted melts along the paleo‐lithosphere‐asthenosphere boundary, which now occurs as a MLD. The deep SCLM represents depleted mantle residue formed during mantle plume impingement and thickened during orogenesis. This domain has been metasomatized and refertilized by high‐T melts from the asthenosphere. Key Points: The South Australian Craton comprises three litho‐chemical layers within the shallow, middle and deep lithospheric mantleXenocrysts define a heterogeneously depleted‐kimberlite melt metasomatized shallow sub‐continental lithospheric mantle (SCLM) and fertile‐melt metasomatized deep SCLMThe mid‐lithosphere is a seismic and geochemical discontinuity marked by negative Vp and lower modal proportions of garnet and diopside [ABSTRACT FROM AUTHOR]

The different olivine fabrics in ultramafic rocks have been widely used to discuss past tectonic settings, given that the olivine fabrics vary with pressure, temperature and water content. However, there are no research related to whether and how the olivine fabrics transform at different metamorphic stages in a natural rock during the process of deep subduction and exhumation. Yinggelisayi garnet lherzolites from South Altyn have experienced deep continental subduction and exhumation. The garnet lherzolites contain well‐preserved residual protolith minerals, and near‐peak (M1), granulite‐facies retrograde (M2), and amphibolite‐facies retrograde (M3) metamorphic mineral assemblages. Olivine grains in M1 formed at P‐T conditions of 2.52–3.08 GPa, 1,095–1,136°C and low water contents (183–213 ppm H/Si), and showed [010] axes sub‐normal to the foliation and [001] axes subparallel to the lineation, which is characteristic of B‐type fabric ((010)[001]). Olivine grains in M2 formed at P‐T conditions of 1.31–1.80 GPa, 851–893°C and also low water contents (93–139 ppm H/Si), and exhibited [010] axes sub‐normal to the foliation and [100] axes subparallel to the lineation, which is characteristic of A‐type fabric ((010)[100]). These observations suggest that olivine fabrics in high pressure‐ultrahigh pressure metamorphosed ultramafic rocks are different in the near‐peak and retrograde metamorphic stages, and also that the olivine fabrics can be transformed during deep continental subduction and exhumation. Therefore, the dispersed or no clear olivine fabric is probably caused by multi‐stage deformation and metamorphism, and the distinct olivine fabrics can also be used as a clue to identify geological processes and better understand metamorphism and deformation during subduction and exhumation. Plain Language Summary: According to the previous research that different olivine fabrics developed under various temprerature, pressure, stress and water content, we try to discuss the transformation of olivine fabric at different metamorphic stages with different P‐T conditions in metamorphic ultramafic rocks. The Yinggelisayi garnet lherzolites from South Altyn, NW China contain residual protolith minerals, and three stages of metamorphic minerals which include the near‐peak (M1), granulite‐facies (M2) retrograde, and amphibolite‐facies (M3) retrograde metamorphic minerals base on the petrography and mineral composition. And then, we analyzed olivine fabrics in the near‐peak (M1) and early retrograde metamorphic stages (M2). Results showed that the olivine in M1 during deep subduction of the continental crust developed the B‐type olivine fabric and the olivine in M2 during exhumation developed the A‐type olivine fabric. Key Points: Yinggelisayi garnet lherzolites contain residual protolith inclusions and three stages of metamorphic mineralsOlivine in the garnet lherzolites exhibited different fabrics during deep subduction and exhumation of the continental crustOlivine fabrics will be dispersed at multi‐stage deformation and metamorphism [ABSTRACT FROM AUTHOR]

Continental formation models invoke subduction or plume‐related processes to create the buoyant, refractory character of continental lithospheric mantle (CLM). From similarities in melt depletion, major element composition, modal clinopyroxene, and Os isotope systematics it has been proposed that oceanic mantle lithosphere is the likely protolith to non‐cratonic CLM, however, a direct link between the two has been difficult to ascertain. Using dredged mantle peridotite xenoliths from the Ferrel Seamount, off the west coast of Baja California, Mexico, we show that tectonic isolation of an oceanic plate may lead to formation of non‐cratonic CLM. Ferrel xenoliths are coarse‐grained spinel lherzolite, or rare harzburgite. Bulk‐rock and clinopyroxene trace element compositions reveal two‐stages of melt refertilization following melt depletion, with infiltration by mid‐ocean ridge basalt‐type melts, followed by melt addition from host alkali basalt. Melt depletion correlations with 187Os/188Os and highly siderophile element abundances indicate preserved melt depletion and refertilization processes are ancient. From these observations, the Ferrel xenoliths represent lithosphere from the abandoned Pacific‐Farallon ridge. The history of melt depletion, followed by MORB‐melt refertilization is consistent with the peridotites representing oceanic mantle lithosphere that was subsequently incorporated into the Baja‐Guadalupe microplate during "ridge jump." These peridotites demonstrate that isolation of oceanic lithosphere that is rafted onto a continental margin provides a viable means for producing non‐cratonic CLM. We suggest that continuation of late‐stage, low degree melt refertilization may provide a link between oceanic lithosphere and non‐cratonic CLM and propose a tectonic model to preserve and facilitate this continued evolution. Key Points: Ancient melt depletion can be preserved despite metasomatism and suggest Ferrel Seamount peridotite was formed by mid‐ocean ridge processesFerrel Seamount peridotites likely represent preserved Pacific‐Farralon lithosphereA tectonic process, ridge jump, can isolate oceanic lithosphere and provides a starting point for the formation of continental lithosphere [ABSTRACT FROM AUTHOR]

New petrological and geochemical data for lherzolite, harzburgite, and gabbros in the Darbut ophiolitic mélange of west Junggar are combined to constrain the geological evolution of the Darbut ophiolite. Lherzolite, consisting of olivine, orthopyroxene, clinopyroxene, and chrome–spinel with low Cr# values (34–39), is analogous to fertile abyssal peridotite. Harzburgite, composed of olivine, orthopyroxene, clinopyroxene, and chrome–spinel with relatively high Cr# values (48–55), is similar to the supra–subduction zone (SSZ) peridotite. Isotropic gabbro, characterized by a flat rare earth element (REE) pattern as well as low Nb/Yb and high Ti/V ratios, is comparable to mid–ocean ridge basalt (MORB). Hornblende gabbro, displaying relative enrichments of fluid-soluble elements and elevated Th/Yb ratios, is similar to that of fore–arc basalt. Geochemical modelling of partial melting suggests that lherzolite samples are compatible with their formation after relatively low-degree (11–16%), anhydrous dynamic melting of the primitive mantle, while harzburgite samples have undergone 5–10% secondary-stage partial melting based on the already 16% depleted primitive mantle. These data suggest that the Darbut ophiolite was generated in a forearc setting. The upwelling asthenosphere triggered by the subduction initiation of the Junggar oceanic lithosphere led to low-degree, anhydrous decompression melting, producing lherzolite as well as the MORB–like melts at Late Silurian period. Increasing slab–derived fluids influx, accompanied by the progressively sinking slab, largely enhanced the partial melting degrees of the depleted mantle, and formed refractory harzburgite. [ABSTRACT FROM AUTHOR]

The Yap trench‐island arc system, where three tectonic plates (i.e., the Philippine Sea Plate, Pacific Plate, and Caroline Plate) interact, has undergone a relatively complicated geological evolution. East of the system, the Caroline Plateau are being subducted, and the plateau has been rifted into east and west Caroline ridges by the development of the Sorol Trough. However, until now, knowledge of the nature of the basement of the Sorol Trough is lacking, which limits our understanding of the complete tectonic evolution of the Yap island arc‐trench system and the Caroline Plateau. In this study, we performed petrological and mineral geochemical (in situ major element compositions and trace element compositions) analysis of newly acquired subducting basalt samples from the Sorol Trough segment, which is being subducted along the Yap Trench, and K‐Ar age of the Sorol basalts is 23.8 ± 0.7 Ma. The results show that phenocryst minerals of basalt from Sorol Trough are mainly olivine and plagioclase, with minor clinopyroxene microlites. The forsterite (Fo) values of the olivine phenocrysts in these rocks vary from 84.97 to 86.85, and plagioclase (Pl) phenocrysts are predominantly labradorite, with a few andesine plagioclase microlites. Bulk‐rock chemical compositions confirm that these rocks are intraplate alkaline basalts and show ocean island basalt (OIB)‐like geochemical characteristics. The mantle source of these basement basalts could be peridotite, and these Sorol Trough basalts may have been formed by 5–10% partial melting of mantle spinel lherzolite. The potential temperature of the mantle beneath the Sorol Trough was inferred to be 1,331–1,393°C, shows that there is no thermal anomaly in the mantle source. In addition, the crystallization temperatures of the clinopyroxenes (1,068–1,263°C) and plagioclase (702–1,216°C) and their compositional characteristics indicate that the magmatic processes have a conspicuously fast rate of magma upwelling. In conclusion, the basalts in the Sorol Trough were the products of low‐degree partial melting of a relatively enriched mantle source that passively upwelled as the Sorol Trough separated the Caroline Ridge into its western and eastern parts. [ABSTRACT FROM AUTHOR]

The features of morphology and composition of accessory Cr-spinels from four ophiolitic peridotite massifs of the Southern Urals are considered. Massifs are localized in the Main Uralian Fault zone (Nurali, Mindyak), at its junction with the Sakmar zone (Kempirsai) and in the northern part of the Zilair zone (Kraka). The Kraka, Nurali and Mindyak massifs are composed mainly of lherzolites with subordinate harzburgites and dunites, while harzburgites predominate in the Kempirsai massif and dunites with large chromitite deposits are significantly developed in its southeastern part. The PT–fO2 formation conditions of lherzolites correspond to the upper mantle below a rift structure: temperature of 700–1000 °C, pressure of 5–12 kbar, and oxygen fugacity varying from –2 to + 0.5 ΔFMQ. The compositional variations of Cr-spinels from primitive lherzolite (Cr# 0.15–0.30, Mg#0.6–0.8) to harzburgite (Cr# 0.3–0.6, Mg#0.5–0.7) and dunite (Cr# 0.6–0.8, Mg#0.4–0.7) and the increase in Mg# value of olivine are a result of synchronous processes of partial melting and plastic flow of the material in the upper mantle. Four main morphological Cr-spinel types are distinguished in lherzolites: (1) fine rods and lamellae within silicate grains and along their boundaries, (2) anhedral and holly-leaf grains closely intergrown with restitic olivine and enstatite, (3) anhedral and subhedral grains in an assemblage with plagioclase and diopside and (4) euhedral grains in dunites. The formation of type 1 grains is interpreted as a result of deformation-induced segregation of trace elements on structural defects of silicates with consequent crystallization of newly formed minerals. The advanced stages of solid-phase transformation produce the larger anhedral and holly-leaf grains in peridotites and euhedral grains in dunites (types 2 and 4 grains). The decompression replacement of a precursor high-pressure mineral (garnet?) is suggested for the formation of the Cr-spinel–plagioclase aggregates. The subhedral and euhedral grains in the assemblage with plagioclase and clinopyroxene could have formed as a result of crystallization from percolating melts or their reaction with restite. [ABSTRACT FROM AUTHOR]

Melt‐peridotite interaction is the key process causing lithological and mineralogical modification in the lithospheric mantle beneath the North China Craton (NCC). However, the lithology, kinetic, mineral, and melt compositional variations of carbonatite melt‐peridotite reaction are poorly constrained relative to those of silicate melt‐peridotite reaction. Here we report Ca‐rich carbonatite melt‐harzburgite reaction experiments at 3 GPa and 1100–1300°C. The experimental results show that the Ca‐rich carbonatite melt‐harzburgite reaction proceeds by diffusion‐governed rapid infiltration of melt into the harzburgite along grain boundaries, simultaneous dissolution of orthopyroxene and olivine, and precipitation of clinopyroxene. These processes lead to the formation of clinopyroxenite at the melt‐rock interface and melt‐impregnated lherzolite/harzburgite. The reactive infiltration of the melt inside the harzburgite matrix is much faster than the harzburgite dissolution and the growth of the clinopyroxene‐rich layer. The extrapolation of experimental melt infiltration rate to natural conditions suggests that carbonatite‐melt peridotite interaction results in large‐scale mantle metasomatism with various extents of clinopyroxene enrichment, whereas silicate melt‐peridotite interaction forms restricted pyroxenite/garnetite veins. Clinopyroxenes produced by silicate and carbonatite melt‐peridotite reactions show two distinct compositional variation trends. The comparison of published clinopyroxene compositions from 433 natural peridotite samples with the experimentally determined compositional variation trends indicates that carbonatite metasomatism was prevailing in the ancient lithospheric mantle of the NCC before its destruction in the early Cretaceous, while silicate metasomatism was predominant in the juvenile lithospheric mantle since the late Cretaceous. Prolonged ubiquitous carbonatite mantle metasomatism may have led to the destruction of the eastern NCC. Plain Language Summary: We study here the infiltration of molten carbonates (carbonatite) in a refractory mantle in the laboratory. Our experimental results show that the reaction of carbonatite melt with mantle peridotite differs significantly from that of silicate melt in respect of grain‐scale process, reaction rate, and mineral composition. Based on the comparison of experimental results with a collection of published clinopyroxene compositions in natural peridotite samples from the North China Craton (NCC), we propose that carbonatite melt‐peridotite interaction was prevailing inside the ancient lithospheric mantle of the NCC before its catastrophic destruction in the early Cretaceous. Silicate melt‐peridotite interaction was predominant in the juvenile lithospheric mantle since the late Cretaceous. Key Points: Diffusion‐governed carbonatite melt‐harzburgite reaction leads to the formation of clinopyroxenite and melt‐infused lherzolite/harzburgiteClinopyroxenes produced by silicate and carbonatite melt‐peridotite reactions show two distinct compositional variation trendsCarbonatite metasomatism was prevailing in the ancient lithospheric mantle of the NCC before its destruction in the early Cretaceous [ABSTRACT FROM AUTHOR]

The reliability of eight Fe–Mg exchange geothermobarometers for garnet-bearing peridotites, pyroxenites and eclogites has been examined using a database comprised of more than 300 published peridotite, pyroxenite and eclogite experiments conducted from 10 to 70 kbar and 850 to > 1650 ∘ C . We have tested Fe–Mg exchange geothermometers suitable for a range of mantle lithologies, including websterite, harzburgite, wehrlite and eclogite. All geothermometers maintained an average difference in experimental and calculated temperature (T) Δ T = T exp - T calc of less than ± 50 °C with a standard deviation of Δ T between ± 50 to 150 °C. Most geothermometers performed well across a narrow range in ln Kd Fe - Mg A - B (where Kd Fe - Mg A - B = Fe A × Mg B (Fe B × Mg A) ), however, systematic overestimation and underestimation of T were observed outside of the optimal range of lnKd Fe - Mg A - B . Increases in experimental pressure (P) adversely affected several geothermometers, particularly those calibrated empirically using natural samples. All previously published calibrations of the garnet-clinopyroxene geothermometer were unable to reliably reproduce the experimental T for both peridotite and eclogite experimental compositions, which hinders their confident application to natural datasets. To improve the state of mantle geothermobarometry we have used our experimental database to recalibrate the (1) garnet-clinopyroxene Fe–Mg exchange geothermometer, and (2) garnet-orthopyroxene Fe–Mg exchange geothermometer. Each geothermometer has been recalibrated across an extended P, T, and compositional range. The inclusion of eclogitic experiments in the calibration for the garnet-clinopyroxene geothermometer permits application to both eclogitic and peridotitic/pyroxenitic assemblages equilibrated under a wide range of PT conditions in the upper mantle. Using multiple linear regression to solve for lnKd, we found the following expressions best reproduced the experimental T (℃) of our dataset: T Fe - Mg grt - cpx (∘ C) = 3356.34 - 0.008 × P kbar + 0.259 × X Ca grt + 0.914 × X Mg grt + - 0.159 × Jd cpx + ln Kd Fe - Mg grt - cpx + 1.265 - 273 T Fe - Mg grt - opx (∘ C) = 1851.85 - 0.007 × P kbar + - 1.83 × X Ca grt + ln Kd Fe - Mg grt - cpx + 1.08 - 273 . where, X Ca grt = Ca Ca + Fe + Mg , Kd Fe - Mg grt - opx = Fe grt × Mg opx (Fe opx × Mg grt) , X Mg grt = Mg Ca + Fe + Mg , Jd cpx = Na - Cr - 2 × Ti , Kd Fe - Mg grt - cpx = Fe grt × Mg cpx (Fe cpx × Mg grt) , with all elements calculated on the basis of 12 oxygen anions in garnet and 6 oxygen anions in clino- and orthopyroxene. Fe2+ = total Fe. Our updated calibrations resolve several issues with earlier calibrations, including a poor performance at elevated P and compositional limitations. An improvement in precision and accuracy has been demonstrated through application to the experimental calibration dataset, a second independent set of published experimental data, and to natural peridotites, pyroxenites and eclogites from on and off craton settings. Iterative PT estimates on natural datasets calculated using our updated calibrations compare well with estimates from widely used calibrations such as the Taylor (1998) two-pyroxene solvus geothermometer. We anticipate that this contribution will provide an important reference for the reliability of mantle geothermometers and that our updated calibrations will be used in future studies on peridotite, pyroxenite and eclogite inclusions in diamond and mantle-derived xenoliths. [ABSTRACT FROM AUTHOR]

The La Quinta Formation is a Triassic-Jurassic volcano-sedimentary unit exposed in Colombia and Venezuela, that crops out along both flanks of the Serranía del Perijá and in western Mérida Andes. It is composed of reddish mudstones and sandstones interbedded with basaltic-to-rhyolitic lava flows and ignimbrites. This succession is locally intruded by dacitic-rhyolitic hypabyssal bodies, felsic dikes and small monzodioritic intrusions. The volcanism recorded in La Quinta comprises the whole subalkaline compositional spectrum from basaltic andesites-alkali basalts (plagioclase-augite), though andesites-trachyandesites-dacites (plagioclase-augite-ß quartz-amphibole) and rhyolites-alkali rhyolites (ß quartz-sanidine-plagioclase-biotite). Magnetite, ilmenite, hematite, zircon and apatite are recurrent accessory phases and suggest dominant oxidized conditions (+1.2 Ni-NiO to hematite-magnetite buffers) in the magmas. Clinopyroxene and ilmenite-magnetite geothermobarometry suggest an early crystallization of the former (ca. 1120 °C and 4.6 kbar) in the middle portion of a normal-to-thickened continental crust (26-49 km), and later fractionation of the Fe oxides (ca. 720 °C). In addition, some samples exhibit calc-silicate alteration formed of hydrated grossular-andradite, epidote and zeolites in association with possible sediment-hosted stratiform copper occurrences. According to mineralogical and geochemical evidence, these rocks belong to a single comagmatic suite in which differentiation processes were driven by low-pressure fractional crystallization and middle-crustal assimilation. Moreover, these rocks exhibit geochemical features indicative of low degrees of partial melting of a spinel lherzolite (or an enriched source) in a hybrid setting between subduction-related arc and intracontinental rifting. Thus, we propose a mixed tectonic scenario in which subduction of the Farallon plate beneath western South America combined with transtensional tectonics resulting from the break-up of Pangea, generated volcanic activity with an inherited arc-signature behind (landward) the arc axis. [ABSTRACT FROM AUTHOR]

Continental intraplate basalts usually exhibit diverse compositions varying from silica-deficient alkaline basalts to silica-excess tholeiites. However, the mechanism of compositional diversity remains controversial. Here, we conducted comprehensive petrology, mineral chemistry, whole-rock geochemistry, and Sr–Nd–Hf isotopic compositions, as well as thermodynamic modeling for the coexisting Datong alkaline and tholeiitic basalts, North China, to put constraints on the source characteristics and melting conditions of these basalts. The alkaline basalts and tholeiites have contrasting major elemental and Sr–Nd–Hf isotopic compositions (87Sr/86Sr, 0.703431–0.703603 vs. 0.703912–0.704010; Ɛ Nd , 5.5–7.0 vs. 2.9–4.8; Ɛ Hf , 9.5–10.6 vs. 8.2–9.6) and trace-element patterns. The observed compositional differences could be explained by neither magmatic processes (e.g., assimilation and/or fractional crystallization), varying melting degrees due to lithospheric thickness variations, nor the melt-peridotite interaction. Instead, lithological heterogeneous mantle sources containing different types of pyroxenite should be considered to account for the compositional diversity, as indicated by olivine (high Ni, Zn/Fe, and low Mn/Zn) and whole-rock chemistry (low CaO, high Fe/Mn, FC3MS, and FCKANTMS ratios). The incompatible trace element-based thermodynamic modeling was carried out to put quantitative constraints on the mantle source characteristics and melting conditions. The modeling results suggest that the source of alkaline basalts contains 30% silica-deficient pyroxenite +70% lherzolite with melting pressure and temperatures of 2 GPa, 1370 °C, whereas the tholeiites have 15% silica-excess pyroxenite +85% harzburgite in the source with melting conditions of 1.5 GPa, 1425 °C. The estimations, together with the thermobarometric calculations using major-element compositions, suggest that the tholeiitic and alkaline basalts were generated by melting of lithological heterogeneous mantle sources under lithosphere with different thicknesses. The lithological heterogeneity originated from recycled components from historical peripheral subducted plates, and melting of such lithological heterogeneity is likely related to asthenosphere upwelling and convection induced by the deeply subducted Pacific slab. Our study emphasizes the fundamental role of source lithological heterogeneity in the generation of co-existing Datong alkaline and tholeiitic basalts and may provide insights into the origin of compositional diversity of other continental intraplate basalts. • Melting conditions of the Datong basalts was constrained using thermodynamic methods. • Silica-deficient and silica-excess pyroxenite is in the source of Datong alkaline and tholeiitic basalts, respectively. • Source lithological heterogeneity is important in the compositional diversity of continental intraplate basalts. [ABSTRACT FROM AUTHOR]

• The Daihai basalts are similar to other Cenozoic basalts in the western NCC, with OIB-like geochemical signatures and 40Ar-39Ar plateau ages of 18.22 ± 1.84 Ma and 26.86 ± 0.72 Ma. • The Daihai basalts were derived from an enriched mantle source and were products of partial melting of lherzolite. • There was significant heterogeneous lithospheric thinning in the western NCC. • Mantle flow convection and blocking triggered the mantle melting and thinning of the lithosphere in the western NCC. The Daihai Cenozoic intraplate basalts are distributed in the western North China Craton (NCC), which is a part of the Cenozoic volcanic province in eastern China, and they are all alkaline basalts. The fine-scale determination of their mantle source region properties, partial melting mechanisms, and petrogenesis can provide crucial information for exploring lithospheric destruction and thinning in the western NCC. 40Ar - 39Ar dating of potassium feldspar grains from the Daihai alkaline basalts yielded plateau ages of 18.22 ± 1.84 Ma and 26.86 ± 0.72 Ma, indicating that the Daihai basalts underwent multiple eruptive cycles during the Late Oligocene - Middle Miocene. These basalts exhibit ocean island basalt (OIB)-like geochemical features and were subjected to negligible crustal contamination. Moreover, basaltic magmas underwent intense fractional crystallization of olivine and clinopyroxene. The geochemical differences in the Daihai basalts were controlled by partial melting. The Daihai basaltic magmas were composed of at least two types (type I-enriched mantle and prevalent mantle) of mantle end-members that partially melted and then mixed, and the lithology of the mantle source region was predominantly peridotite. Under enriched mantle conditions, the mixing of garnet lherzolite partial melts (<1%) and spinel lherzolite partial melts (2 - 5%) can reasonably explain the elemental variations characteristic of the Daihai basalts. Most importantly, the melting depth and lithospheric thickness of the Daihai basalts were < 70 km or even close to 50 km, implying that the western NCC underwent lithospheric destruction and thinning during the Cenozoic. However, these effects were spatially heterogeneous. The most plausible genetic mechanism for the generation of the Daihai basalts was the coupled effects of subduction of the Pacific slab and subduction–collision of the Indo–Eurasian Plate since the Oligocene. The mantle flows generated by these two events convected, blocked and triggered upwelling mantle flows at the eastern margin of the Ordos Block. The upwelling mantle flows resulted in frequent magmatism in the region. [ABSTRACT FROM AUTHOR]

Subduction process plays an important role in triggering chemical metasomatism of the lithospheric mantle beneath orogenic belts. The lithospheric mantle beneath the West Qinling orogen is extensively heterogeneous, but the lithospheric modification process and related metasomatic agents remain poorly constrained. In this paper, we report in-situ elemental and isotopic compositions of minerals in mantle xenoliths from the West Qinling orogen. The mantle xenoliths are lherzolites and can be categorized into two types. Lherzolite with refractory compositions represents the residue of lithospheric mantle that have undergone high degrees of melt extraction. In contrast, lherzolites with fertile compositions contain clinopyroxenes variably enriched in incompatible elements and radiogenic Sr isotopes. These fertile lherzolites represent lithospheric mantle modified by metasomatic process in different degrees. High (La/Yb) N , Ca/Al, Nb/Ta, Nd/Yb, 87Sr/86Sr ratios and low Ti/Eu ratios of the clinopyroxenes suggest the metasomatic agent was dominated by carbonate-rich melts. This inference is supported by the identification of carbonate inclusion, reaction structures, and clinopyroxene chemical zoning in the lherzolites. Considering the tectonic background of the West Qinling orogen, the orogenic lithospheric mantle would have been metasomatized by carbonate-rich melts derived from partial melting of subducted carbonate-bearing Paleo-Tethyan oceanic slab. In this regard, petrological observations, along with in-situ elemental and Sr isotopic compositions of clinopyroxenes in mantle xenoliths can be powerful means to identify carbonate metasomatism and constrain the evolution of the lithospheric mantle beneath orogenic belts. • Mantle xenoliths are abundant in the Cenozoic basalt from West Qinling orogen. • In-situ trace element and Sr isotope are operated in these mantle xenoliths. • The results indicate orogenic mantle was metasomatized by carbonate-rich melts. [ABSTRACT FROM AUTHOR]

To investigate the process and chemistry of mineral reaction zone formation, we conducted detailed petrographic observations and chemical analysis of rocks and minerals of spinel lherzolite xenoliths from basanites of Tumusun volcano (Baikal Rift Zone). The reaction zones gradually disappear from contact toward the center of the xenoliths. The influence of basanite melt on major and trace element composition of secondary minerals of reaction zones is notable only at a distance up to 100–200 μm from the contact. At a distance of 0.3–1.0 mm from the contact, the major and trace composition of secondary clinopyroxenes from the orthopyroxene reaction zone indicates their formation from a melt formed by dissolution of orthopyroxene and influenced by the element diffusion from basanite melt. Inside xenoliths, the secondary minerals have Mg# values equal to or higher than Mg# of primary minerals, and secondary clinopyroxenes inherit their depleted or enriched REE pattern from primary pyroxenes. The compositional variations in secondary clinopyroxenes testify melt heterogeneity. Clinopyroxene rims have slightly higher LILE and similar abundances of other trace elements compared to clinopyroxene cores. This is consistent with the model developed from experimental studies: due to the interaction with basanite, incongruent dissolution of orthopyroxene occurs to form a melt which circulates in lherzolite and leads to pyroxenes and spinel dissolution. Diffusion of elements from basanite results in lherzolite enrichment in K, Na, Rb, Ba, La, and Ce, which are incorporated in feldspars and clinopyroxene of reaction zones as well as in feldspar veinlets. Non-dissolved mineral cores are homogenous and similar in major and trace element composition to primary minerals without reaction rims.

Reconstruction of the mechanisms of carbonatitic melt evolution is extremely important for understanding metasomatic processes at the base of the continental lithospheric mantle (CLM). We have studied the interaction between garnet lherzolite and a carbonatitic melt rich in molecular CO2 and H2O in experiments at 6.3 GPa and 1200–1450 °C. The interaction with garnet lherzolite and H2O-bearing carbonatite melt leads to wehrlitization of lherzolite, without its carbonation. Introduction of molecular CO2 and H2O initiates carbonation of olivine and clinopyroxene with the formation of orthopyroxene and magnesite. Partial carbonation leads to the formation of carbonate–silicate melts that are multiphase saturated with garnet harzburgite. Upon complete carbonation of olivine already at 1200 °C, melts with 27–31 wt% SiO2 and MgO/CaO ≈ 1 are formed. At 1350–1450 °C, the interaction leads to an increase in the melt fraction and the MgO/CaO ratio to 2–4 and a decrease in the SiO2 concentration. Thus, at conditions of a thermally undisturbed CLM base, molecular CO2 and H2O dissolved in metasomatic agents, due to local carbonation of peridotite, can provide the evolution of agent composition from carbonatitic to hydrous silicic, i.e., similar to the trends reconstructed for diamond-forming high density fluids (HDFs) and genetically related proto-kimberlite melts. [ABSTRACT FROM AUTHOR]

The composition of spinel lherzolite xenolith KLB-1 from the Kilbourne Hole volcanic crater, United States, which is close to the composition of the Earth's primitive mantle, was used for thermodynamic modeling of phase relations in the Na2O–CaO–FeO–MgO–Al2O3–SiO2 system (NCFMAS) using the Perple_X software package in the temperature range of 900–2000 °C and pressures of 0.0001–30 GPa. The calculated phase diagram is in good agreement with published thermodynamic data on KLB-1 composition and reveals the peculiarities of mantle mineral assemblages at P–T parameters on which experimental data are insufficient or absent. The results showed that the mineral assemblage of garnet wehrlite (garnet + olivine + clinopyroxene), the least common type of mantle peridotite on the Earth's surface, prevails in the upper mantle since the Archean. Mineral assemblage of garnet lherzolite (garnet + olivine + clinopyroxene + orthopyroxene), which is a variety of mantle peridotites most widely found on the Earth's surface, is formed in the lithospheric mantle because its temperatures are lower than those of the convecting mantle. Thermodynamic modeling reveals a ringwoodite-free field in the P–T diagram (located at the bottom of the mantle transition zone), which is crosscut by Archean adiabats and by the geotherms of Archean and the hottest Phanerozoic plumes. This area causes a change, from negative to positive, in the slope of the boundary between the lower mantle and the mantle transition zone. A positive slope of the boundary in the Archean should have stimulated the ascend of lower mantle plumes through the transition zone. Conversely, this boundary has a negative slope for most Phanerozoic plumes, rising from the lower mantle, and as a result, the plumes either slow down or stop. [ABSTRACT FROM AUTHOR]

Two types of lherzolite xenoliths appear in the Cerro Gordo maar from the Cenozoic Calatrava volcanic field. Hydrous lherzolites (group-1) show a major proportion of metasomatic amphibole (>5 modal%), whereas group-2 lherzolites have only accessory amounts or an absence of amphibole. Group-1 lherzolites show a higher LREE-Ba-Sr-Th content in whole-rock composition compared to group-2 lherzolites. The most depleted group-2 lherzolite xenoliths indicate a low degree of partial melting (<5%) of the mantle source, similarly to other Calatrava xenolith suites. Trace element content combined with Sr-Nd isotopic ratios suggests a protolith mantle source with the characteristics of a depleted MORB Mantle (DMM type). This mantle protolith was later refertilized by subduction-related metasomatism, causing the formation of modal amphibole and obliterating the original MORB-like chemical features in the most transformed xenoliths (group-1 lherzolites). The entrainment of these mantle fragments in the volcanic alkaline magma promoted amphibole breakdown reactions during its transport, generating silica-rich glasses with the associated unusual formation of secondary orthopyroxene. The suprasubduction character of the protolithic mantle combined with previously published Re-Os model ages in nearby lherzolite xenoliths suggests that the sampled lherzolites could represent the old Cadomian mantle wedge of the Central Iberian zone. [ABSTRACT FROM AUTHOR]

The Niangzhong diabase dikes, dated at 138.1 ± 0.4 Ma, are located within the outcrop area of the Comei large igneous province (LIP). These diabase samples can be divided into two groups: samples in Group 1 show varying MgO (1.50 wt.%–10.25 wt.%) and TiO2(0.85 wt.%–4.63 wt.%) contents, and enriched initial isotope compositions (87Sr/86Sr(t) = 0.705 6–0.711 2, εKNd(t) = −0.3− +3.8), with OIB-like REEs and trace elements patterns, resulting from low degree melting of garnet-bearing lherzolite mantle sources; in contrast, samples in Group 2 show limited MgO (4.14 wt.%–7.75 wt.%) and TiO2(0.98 wt.%–1.69 wt.%) contents, and depleted initial isotope compositions (87Sr/86Sr(t) = 0.707 5–0.711 2, εNd(0 = +5.5− +6.2), with N-MORB-like REEs and trace elements patterns, resulting from relatively high degree melting of spinel-bearing lherzolite mantle source. Combined with the published representative data about Comei LIP, we summarize that the source components for Comei LIP products include OIB end-member, enriched OIB end-member, and N-MORB end-member, respectively. Melts modeling suggests that magmas in the Comei LIP evolve in a relatively high oxygen fugacity condition, which influenced their fractionation sequences and led to systematic changes of TiO2 contents, Ti/Y and Ti/Ti* ratios. From the spatial and temporal distribution of above three end-member samples, deep process of Kerguelen plume during the Comei LIP formation can be interpreted as the interaction among the Kerguelen plume, the overlying lithospheric mantle, and the upwelling asthenosphere. The magmatism of Comei LIP began at ∼140 Ma and then lasted and peaked at ∼132 Ma with the progressively lithospheric thinning of eastern Gondwana upon the impact of Kerguelen plume. [ABSTRACT FROM AUTHOR]

The Tidding‐Mayodia ophiolites (TMO) exposed along the Lohit and Dibang river valleys in eastern Himalaya that have been considered as the extension of the Indus‐Tsangpo Suture Zone ophiolites are revisited to review their petrogenetic‐tectonic origin. The ophiolites consist of depleted harzburgite and dunite with lesser amounts of mafic rocks (gabbro intrusives, mafic dykes) and carbonates. The serpentinized peridotites consist of antigorite, lizardite, olivine, Cr‐spinel, and bastite with minor sulfide minerals. From SEM‐EDS studies, sulfide minerals were observed to be associated mainly with magnetites. The main sulfide mineral is pentlandite with minor millerite that exists as inclusions inside the pentlandite grains. Elemental mapping of these sulfides shows that they are mainly Ni‐(Co‐)‐bearing sulfides. The olivines are highly forsteritic (Fo = 95–96) while the Cr‐spinels show distinct Cr‐magnetite rims with a chromite core (Cr# = ~93). The serpentinized peridotites have whole‐rock compositions of SiO2 <47 wt% and high MgO (>36.37 wt%) and low Al2O3 (<1.21 wt%), CaO (<0.82 wt%), indicating the depleted nature of the parent rocks. Highly fractionated LREEs as compared to HREEs [(La/Yb)N = 2.62–13.22], and REE and Cr spinel chemistry modelling suggests that the studied peridotites have formed from ~22% partial melting of a depleted spinel lherzolite source which later underwent interactions with a high‐temperature silicate melt that caused enrichment in LREE and Cr of spinels. The parental melt compositions of Cr spinel yield their formation during arc tectonism (Al2O3melt = 6.28–7.65 wt%, FeO/MgO = 1.00–1.33). Furthermore, Mn and Zn concentrations in spinels, the occurrence of Cr magnetite rim in Cr‐spinels, presence of secondary olivine with higher Fo (~98), and occurrence of low‐temperature re‐equilibrated sulfide minerals, indicate that the rocks were subject to low‐temperature metamorphism. Based on this evidence, combined with data from previous studies, a tectonic model has been proposed for the genesis of the studied ophiolites. This model shows that the ophiolites have formed from the entrapment of depleted N‐MORB mantle in the mantle wedge of an intra‐oceanic subduction zone. During the nascent forearc regime, this mantle wedge underwent interactions with high‐temperature melts, which caused changes in their chemistry. Moreover, the rocks underwent interactions with low‐temperature fluids in the mature forearc, which caused the formation of sulfides and metamorphozed these rocks. [ABSTRACT FROM AUTHOR]

Here, we report new geochemical data from mantle peridotite (spinel lherzolite, dunite, and harzburgite) and cumulate peridotite (websterite) rocks from the northernmost portion of the Nagaland Ophiolite Complex. Based on detailed textural, mineral (major element) chemistry, and whole‐rock (major and trace element) chemistry, we reveal the origin of these rocks in different tectonic settings and their modification via the melt–mantle interaction process. The spinel lherzolite rocks with low Cr# (0.16–0.25) chrome spinel are mantle residues formed after low degrees (~10–15%) of partial melting in a mid‐oceanic ridge setting, which has been subsequently modified in a subduction zone setting. However, the dunites and harzburgite samples are interpreted to be depleted mantle residue formed in a supra‐subduction zone (SSZ) setting. Additionally, the dunites are inferred to be of replacive origin representing former harzburgites. Based on Cr# of chrome spinel, the dunites are classified into low Cr# (0.53–0.57), medium Cr# (0.73–0.79), and high Cr# (0.89–0.90) dunites. High Cr# dunite with low‐TiO2 (0.01–0.06 wt%) possibly formed by interaction with a boninitic melt, whereas low‐medium Cr# dunite with high TiO2 (0.14–0.34 wt%) formed by interaction with a high‐Ti melt in an SSZ setting. Harzburgite samples with their highly resorbed orthopyroxene grain boundaries contain chrome spinels (Cr# = 0.84–0.88; TiO2 = 0.00–0.06 wt%) similar to the high Cr# dunite samples and are also interpreted to be formed by interaction with a boninitic melt. The websterite samples with their low Mg# (0.79) olivine and low Ni (345–413 ppm) content are interpreted to be of cumulate origin formed in an SSZ setting. However, the interstitial chrome spinel blebs with low Mg# (0.18–0.19), high Cr# (0.60–0.61), Fe3+ (0.20–0.21), and very high TiO2 (2.03–2.04 wt%) are interpreted to be a result of post‐cumulus interaction of interstitial chrome spinel with high‐Ti intercumulus melt. These mantle peridotites and cumulates of different tectonic origins were obducted and juxtaposed during the final stages of the India‐Asia collision. [ABSTRACT FROM AUTHOR]

Late Mesozoic lamprophyres are widespread in the Chaohu area of the Lower Yangtze region, eastern China. These mantle‐derived rocks are of great significance for understanding the characteristics of mantle source and tectonic environment of eastern China during the Late Mesozoic. The lamprophyres from the Chaohu area can be divided into two types. The type‐I lamprophyres have low SiO2 (43.8–44.5 wt.%) and TiO2 (1.06–1.14 wt.%), high MgO (13.5–17.5 wt.%) and high contents of compatible elements, and are enriched in LREE and LILE but depleted in HFSE. They have high initial 87Sr/86Sr (0.70767–0.70904) and negative εNd (t) (−4.8 to −4.4). The type‐II lamprophyres are characterized by low contents of SiO2 (43.0–45.4 wt.%), high TiO2 (1.74–2.55 wt.%), MgO (9.17–15.4 wt.%), and compatible elements. They are also enriched in LREE and LILE but depleted in HFSE. They show enriched Sr‐Nd isotopic compositions with initial 87Sr/86Sr values (0.70773–0.70886) and εNd (t) values (−8.6 to −7.8). The type‐I lamprophyres show higher Mg# and MgO contents but significant lower TiO2 than the type‐II lamprophyres. In addition, the type‐I lamprophyres have relatively similar initial 87Sr/86Sr ratios but higher εNd(t) values than the type‐II lamprophyres. The difference of geochemical characteristics between the type‐I and type‐II lamprophyres may be caused by different degrees of partial melting of an amphibole‐bearing garnet lherzolite mantle. The lithospheric mantle was metasomatised by the fluids caused by the rollback of the Paleo‐Pacific plate subduction during the Late Cretaceous. [ABSTRACT FROM AUTHOR]

Interaction between peridotite and pyroxenite-derived melts can significantly modify the mineralogy and chemistry of the upper mantle, enhancing its heterogeneity, by creating re-fertilized peridotites and secondary-type pyroxenites. We experimentally investigated the reaction between a fertile lherzolite and MgO-rich basaltic andesite produced by partial melting of an olivine-free pyroxenite at 2 GPa and 1300–1450 ∘ C. The aim was to constrain the rate and style of melt–peridotite reaction mostly as a function of temperature, i.e. assuming variable physical status of the host peridotite. Experiments juxtaposed pyroxenite on a synthesized fertile lherzolite to evaluate the modal and mineral compositional changes in the fertile lherzolite resulting from the reaction with pyroxenite-derived melt. At 1300 and 1350 ∘ C, the reaction produces a thin orthopyroxene-rich reaction zone confined between partially molten pyroxenite and modally unmodified subsolidus lherzolite. Chemical changes in minerals of the pyroxenite crystal mush suggest that element diffusion across the pyroxenite–peridotite interface, coupled with orthopyroxene precipitation, plays a role in the reactive crystallization of mantle pyroxenite veins. At 1380 and 1400 ∘ C, infiltration of pyroxenite-derived melt significantly modifies the mineralogy and chemistry of the host peridotite by creating orthopyroxene-rich websterites and pyroxene-rich lherzolite. At 1450 ∘ C, pyroxenitic melt fluxes into molten peridotite, enhancing peridotite melting and creating a melt-bearing dunite associated with a refractory harzburgite. At a given pressure, bulk compositions of hybrid rocks originating through melt–peridotite interaction are mostly controlled by the chemistry of the reacting melt. Interaction between pyroxenitic melt and peridotite causes XMg[XMg=Mg/(Mg+Fetot)] and XCr[XCr=Cr/(Cr+Al)] decrease and TiO 2 increase in pyroxenes and spinel across the pyroxenite–peridotite boundary. Similar chemical gradients in minerals are observed in pyroxenite–peridotite associations from natural mantle sequences. The comparison with mineral chemistry variations derived by reaction experiments potentially represents a petrologic tool to discriminate between low- versus high-temperature melt–peridotite reactions. [ABSTRACT FROM AUTHOR]

Significant variations in the composition of garnets, both within individual grains and in the rock, are found in the xenolith of deformed garnet lherzolite from the Udachnaya kimberlite pipe. The central parts of the grains, corresponding in composition to the garnets of the lherzolite paragenesis, demonstrate a sinusoidal distribution of rare earth elements (REEs). At the same time, the edge portions have a distribution characteristic of garnet mega-crystals from kimberlites. Despite being depleted in Y and HREE, the cores are enriched in light rare earth elements, Nb, Ta, Th, and U relative to garnet from primitive garnet peridotite. In terms of the REE distribution, the model melts, which are in equilibrium with the edge parts of garnet, are close to kimberlite but are significantly enriched in comparison with kimberlite in Nb, Ta, and Hf and depleted in Sr. Melts in equilibrium with the central parts of garnet are characterized by a steeper negative slope in the region of heavy and medium REEs and approach kimberlite in the region of light REEs. Based on the data obtained, several stages in the evolution of deformed garnet lherzolite are distinguished. The first stage involves the interaction of depleted peridotite with a melt similar in composition to carbonatite melts. This stage is associated with the formation of garnet with a sinusoidal REE distribution. At the next stage, which was preceded by the dissolution of garnet grains, garnet rims with increased Ti, Zr, and Y contents were formed and clinopyroxene appeared. At the final stage, garnet melted, caused by the inflow of a water–carbon dioxide fluid with a high potassium content, leading to polymineral inclusions and kelyphite rims. [ABSTRACT FROM AUTHOR]

To provide new insights into the interaction of ultramafic alkaline melts with the subcontinental lithospheric mantle, we present results of a petrographical-mineralogical study of ilmenite-bearing mantle xenoliths from the Grib kimberlite, Archangelsk, Russia along with results from reaction experiments between harzburgite and Fe–Ti bearing carbonate–silicate melts similar to aillikite. The compositions of orthopyroxene, ilmenite and garnet from our mantle xenoliths are similar to compositions of minerals of the low-Cr megacryst suite from different kimberlite occurrences worldwide including the Grib kimberlite as well as minerals from sheared lherzolite xenoliths captured by the Grib kimberlite. This suggests that ilmenite-bearing xenoliths, megacrysts, and sheared lherzolite xenoliths could have a common origin and/or formed under similar conditions. The reaction experiments were performed at 4 GPa and 1200 °C with varying proportions of aillikite (0, 10, and 50 wt%) that reacted with harzburgite. The experimental runs with 10% and 50% aillikite resulted in two layers within the capsule, with an ilmenite-bearing reaction zone at the contact between aillikite and harzburgite, and an ilmenite-free zone characterized by higher garnet and clinopyroxene abundances. An increase of aillikite melt is directly correlated with increasing TiO2 and decreasing Cr2O3 contents and Mg# values in the mineral phases, most significantly for pyroxenes. Overall, the experiments produce a chemical gradation of minerals from Cr-rich (Fe–Ti-poor) to Cr-poor (Fe–Ti-rich) which is strikingly similar to the chemical gradation observed in minerals from natural mantle-derived xenoliths from kimberlites. In summary, comparison of our experimental data with natural samples indicates possible links between the generation of megacrysts and Ti-rich metasomatism of the lithospheric mantle by ultramafic alkaline (aillikite-related) melts and their possible evolution towards kimberlites. Our results illustrate the importance of melt-rock ratios in generating the mineralogical and chemical diversity in mantle xenolith suites. [ABSTRACT FROM AUTHOR]

The zoning of diamonds was studied using cathodoluminescence (CL) and the chemical composition of mineral inclusions in six typical diamonds from kimberlites of Yakutia. The diamonds were ground on special equipment until inclusions with dimensions of 10–200 μm were brought to the surface. The inclusions are characterized by a morphology reflecting the influence of the host diamonds. Multiple inclusions and intergrowths of magnesiochromite, olivine, pyrope, and phlogopite are located in both the central and peripheral zones of diamonds. In three diamonds, significant differences in the composition of magnesiochromites in different growth zones were observed, while in the other three such differences were not found. The overwhelming majority (five out of the six diamonds studied), according to the compositional features of magnesiochromite, olivine, and phlogopite, belong to the dunite–harzburgite paragenesis prevailing in diamonds from various diamond-bearing provinces of the Earth. In one of the diamonds, a lherzolite paragenesis, identified by the composition of the pyrope inclusion in magnesiochromite, was observed for the first time. The complex history of diamond growth and the variations in the chemical composition of the included minerals indicate the possibility of coexistence of syngenetic and protogenetic inclusions in the same diamond crystal. [ABSTRACT FROM AUTHOR]

Enrichments in light REE without concomitant enrichments in high-field-strength elements in mantle peridotites are usually attributed to inputs from carbonate-rich melts and referred to as 'carbonatite' metasomatism as opposed to interaction with evolved silicate melts. Alternatively, both enrichment types are ascribed to percolating volatile-bearing mafic liquids whose chemical signatures evolve from 'silicate' to 'carbonatite'. Here we compare these models for peridotites in which these enrichment types are combined, as may be common in the mantle. We report new Ca-Sr-Nd isotope and chemical data for lherzolite and harzburgite xenoliths from Spitsbergen that were metasomatized, first, by silicate, then by carbonate-rich melts that formed carbonate-bearing pockets replacing earlier minerals. Seven crushed samples were treated with acetic acid that dissolved carbonates formed in the latest event, but not silicates. The leachates (acid-removed carbonates making up 0.6–1.4% of total sample mass) contain much more Sr than the residues after leaching (277–2923 vs. 16–60 ppm), have a greater overall 87Sr/86Sr range (0.7049–0.7141 vs. 0.7036–0.7055) and higher 87Sr/86Sr in each sample than the residues. The leachates have lower δ44/40Ca range (0.17–0.68‰) than the residues (0.78–1.00‰), as well as lower δ44/40Ca than the residues in all samples but one. By and large, the carbonates are out of Ca-Sr isotope equilibrium with the host peridotites implying that the older silicate and younger carbonatite metasomatism were produced by different parental melts, thus supporting the existence of distinctive carbonate-rich metasomatic media in the lithospheric mantle, possibly including recycled materials. The δ44/40Ca in the leachates (i.e. carbonates, 0.17–0.68‰) are well below bulk silicate Earth (BSE) estimates (0.94 ± 0.05‰) and δ44/40Ca in non-metasomatized melt-depleted mantle. Yet, δ44/40Ca in the non-leached whole rock (WR) carbonate-bearing samples (0.75–0.95‰) fall within, or are only slightly lower than, the BSE range. The 87Sr/86Sr range in these WR samples (0.7030–0.7112) includes very high values for peridotites with large aggregates of dolomite and Mg-calcite. It appears that both carbonatite and silicate metasomatism may produce δ44/40Ca values lower than the BSE such that Ca-isotope data cannot robustly tell apart these two enrichment types, yet carbonatite metasomatism may yield the lowest δ44/40Ca. Carbonates, even at small mass fractions, are significant hosts of Sr in the WR Spitsbergen peridotites (8–51 wt.% of Sr mass) because of very high Sr concentrations, but add little to WR Ca balance (3–12 wt.%). As a result, high Sr content and 87Sr/86Sr ratios may be indices (though not definitive proofs) of carbonatite metasomatism in mantle rocks. [ABSTRACT FROM AUTHOR]

Peridotites in ophiolites exposed along Himalayan suture zone open window to study deep Earth processes. We report c.2 km thick well exposed mantle section of Shergol ophiolite in the Indus Suture Zone (ISZ). The mantle section comprises lherzolite, serpentinised peridotite with cumulate chromitites. The in situ evidences from micron-sized mineral exsolutions are studied using optical microscopy, scanning electron microscopy and micro-Raman spectroscopy. From the ISZ lherzolite, ilmenite mineral exsolution is noted in the olivine grain. The exsolved ilmenites are oriented topotactically following former {111} planes of magnetite, providing evidence of spinel precursor. In another observation, the disoriented 1–3 µm wide and 10–80 µm long ilmenite needles are hosted in the olivine, is characterised by Raman peaks at 685, 370 cm−1 with low peaks at 308 cm−1,523 cm−1, 683 cm−1, 693 cm−1 and 731 cm−1 point to mixed mineral phases of magnetite with Cr–Fe–Al. Based on morphologies, crystal-chemical structure and modal calculation of exsolutions, we infer, exsolved Fe–Ti phases in Shergol peridotite is sourced from deeper part of the upper mantle. With the Fe–Ti phases, tiny silicate inclusions in chrome spinel and accompanying opaque base metal sulphides are also observed in the same peridotite, attributes successive stages of partial melting and subsequent cooling of metal enriched mantle. These observations, challenge shallow intra-oceanic arc setting for Shergol ophiolite and proposes, part of exsolved mineral phases in ophiolite has deep Earth origin. These mineral phases would ascent at the shallow mantle level beneath Neotethyan spreading ridge aided by dunite channel. [ABSTRACT FROM AUTHOR]

The origin of basal lherzolites in the mantle section of harzburgite‐dominated ophiolites is enigmatic. The basal part of the mantle section is well exposed in the Muslim Bagh Ophiolite, Pakistan, which is one of the harzburgite‐dominated ophiolites of the Tethys Ophiolite Belt. In this contribution, we describe the basal lherzolite of the Muslim Bagh Ophiolite, Pakistan, and discuss its origin based on the trace‐element characteristics of its clinopyroxenes. The basal lherzolite exhibits porphyroclastic to mylonitic textures. Primitive mantle‐normalized trace‐element patterns of the porphyroclastic clinopyroxenes were characterized by low ratios of light rare‐earth elements (LREEs) to heavy rare‐earth elements (HREEs), low abundances of HREEs, and positive Sr anomalies. These geochemical characteristics are not consistent with a lherzolite and its clinopyroxenes, which is formed by a residue after a low degree of partial melting and melt extraction from peridotites in a mid‐ocean ridge setting. Instead, the compositions of the clinopyroxenes are consistent with open‐system melting induced by the infiltration of slab‐derived fluids into residual peridotites that had been depleted in REEs. The compositions of chromian spinels in the chromitites of the basal peridotite sequence are also consistent with their formation in an arc setting. We conclude that the basal lherzolites of the Muslim Bagh Ophiolite represent a residue after a relatively high degree of partial melting, and that the clinopyroxenes were added as metasomatic crystallization from slab‐derived arc‐related melts to this residual depleted peridotite in a subduction setting. [ABSTRACT FROM AUTHOR]

The Circum-Pacific subduction zone is a famous gold metallogenic domain in the world, with two important gold metallogenic provinces, the North China Craton and Nevada, which are related to the destruction of the North China Craton and the Wyoming Craton, respectively. Their ore-forming fluids were possibly derived from the stagnant slab in the mantle transition zone. The oceanic lithospheric mantle usually contains serpentine layers up to thousands of meters thick. During plate subduction, serpentine is dehydrated at depths of <200 km and transformed into high-pressure hydrous minerals, known as Phases A to E, which carries water to the depth of >300 km. The overlying big mantle wedge is hydrated during the breakdown of these hydrous facies in the mantle transition zone. The dehydration of the subducted slab in the big mantle wedge releases sulfur-rich fluid, which extracts gold and other chalcophile elements in the surrounding rocks, forming gold-rich fluid. Because the cratonic geotherm is lower than the water-saturated solidus line of lherzolite, the fluid cannot trigger partial melting. Instead, it induces metasomatism and forms pargasite and other water-bearing minerals when it migrates upward to depths of less than 100 km in the cratonic lithospheric mantle, resulting in a water- and gold-rich weak layer. During the destruction of craton, the weak layer is destabilized, releasing gold-bearing fluids that accelerate the destruction. The ore-forming fluids migrate along the shallow weak zone and are accumulated at shallow depths, and subsequently escape along deep faults during major tectonic events, leading to explosive gold mineralization. The ore-forming fluids are rich in ferrous iron, which releases hydrogen at low pressure through iron hydrolysis. Therefore, decratonic gold deposits are often reduced deposits. [ABSTRACT FROM AUTHOR]

We present comprehensive geochemical and isotopic (Sr-Nd-Hf) datasets for two suites of ultramafic rock in Vietnam, namely xenoliths of spinel lherzolite entrained in late Cenozoic alkali basalts, and Paleozoic ultramafic massifs that occur along the Tam Ky-Phuoc Son suture zone in central and southern Vietnam. The ultramafic massifs are the products of high degrees of melt extraction (up to 40%), and have relatively low equilibrium temperatures of 603 to 778 °C. The lherzolites are residues of relatively low degrees of fractional melting (< 1% up to 20%). The compositions of minerals in the xenoliths reveal that the late Cenozoic lithospheric mantle beneath central and southern Vietnam was hotter (825–1058 °C) than during the Paleozoic. The calculated trace element patterns of metasomatic melts that equilibrated with clinopyroxenes in the LREE-enriched xenoliths show enrichments in Th, U, and LREEs, and depletions in Nb. These data, together with the elevated Ti/Eu ratios of the clinopyroxenes, reflect the role of hydrous silicate melts as the main agents of metasomatism. The ultramafic rocks of the Paleozoic massifs contain spinel with TiO2 contents higher than expected for residual spinel, which suggests the influence of boninitic melt(s). In the lherzolite xenoliths, the clinopyroxenes have MORB-like depleted Sr-Nd-Hf isotopic compositions (87Sr/86Sr = 0.70241–0.70416; εNd = +6.6 to 12.3; εHf = +13.1 to 25.1), suggesting metasomatic melts/fluids from upwelling asthenosphere. We suggest that subduction of the (Paleo-)Pacific Plate and continental collision during the Permian–Triassic played key roles in lithospheric replacement and thinning beneath central and southern Vietnam. [ABSTRACT FROM AUTHOR]

Olivine is the most common rock-forming mineral of the majority of the lithospheric mantle rocks beneath ancient cratons. This study provides the information about an epigenetic olivine in a lherzolite xenolith from the Udachnaya kimberlite pipe (Siberian craton), which is characterized by lower Mg# compared to the rock-forming one (Mg# = 87.4). The iron-rich olivine has been observed in the epigenetic mineral assemblage that forms a kelyphite shell around the rock-forming garnet. Olivine from the kelyphite shell occurs as both homogeneous grains (Mg# = 84.3–85.9) and zoned grains (Mg# = 85.1–87.5). The major and minor elements asymmetric zoning patterns have been found in the rock-forming olivine grains at the contact with the kelyphite shell. These olivine grains have an outer low Mg# (up to 85.9) zone at the contact with the kelyphite shell as the epigenetic olivine grains in the kelyphite shell. We suggest that the iron-rich epigenetic olivine was produced as the result of a reaction between the rock-forming garnet and the primitive kimberlite melt. During this reaction, a hybrid melt was formed in the interstitial space. The hybrid melt was iron-enriched relative to the kimberlite melt. The source of iron for the micro-portions of the interstitial hybrid melt was the rock-forming garnet. [ABSTRACT FROM AUTHOR]

The Upper Cretaceous Arangue complex is located in the Makran zone at the SE of Iran. The complex consists of ultra-mafic rocks, microgabbro dykes, pillow lavas and lime stones that pillow lavas are mainly exposed to the northwest and southeast part of study area. There are oval and tubular basalt lavas with cracked bread crust surface. They predominantly have plagioclase, clinopyroxene with minor olivine and opaque minerals in a fine-grained groundmass along with glass. Mineral chemistry data show that plagioclases and clinopyroxene composition varies from An68.27-81.73 Ab18.27-31.57 Or0- 0.41 and Wo38.1- 47.8Fs8.2-19.3 En38.6-48.7 respectively. In the geochemical diagrams, the Arangue complex pillow lavas fall in the basalt and sub-alkaline fields. Geochemical data indicate that the Arangue complex pillow lavas are tholeiitic. The absence of a distinct Eu anomaly (Eu/Eu*= 0.8-1.2), indicates that plagioclase fractionation is not notable, or that the magma is a little oxidized. The Arangue complex pillow lavas show properties similar to transitional basalts between enriched MORB and OIB and some BABB. However, their enrichment in incompatible elements and low Nb and La / Nb ratios (0.8-2.1) display that these have affinity of the BABB. These were produced by approximately 15-25% partial melting of plagioclase lherzolite where fractionation was controlled by removal of clinopyroxene, spinel, and olivine. Petrogenetic study indicates that the source of mantle lherzolite is subjected to enrichment variables in subduction components consisting of fluids for the Arangue Complex pillow lavas. [ABSTRACT FROM AUTHOR]

[Display omitted] • Basalts in Cuona and diabases in Zhegucuo were formed at 137–131 Ma. • Both these basalts and diabases were derived from melts formed via melting of the Kerguelen plume and lithospheric mantle, respectively. • Eastern Gondwana breakup resulted from the reactivated suture zone and the Kerguelen plume. Geochronological, geochemical, and Sr–Nd isotope data for the mafic rocks from Cuona and Zhegucuo areas, Southern Tibet are presented to understand better the connection between the Kerguelen mantle plume and the breakup of eastern Gondwana. Zircon U–Pb dating yielded a concordant, weighted-mean 206Pb/238U age of 131 Ma for the Zhegucuo diabases. Compared to the Cuona basalts, the Zhegucuo diabases have higher Mg# values and MgO, Ni, and Cr contents closer to those of primary mantle melts. The Cuona basalts have ocean island basalt-like compositions, including light rare-earth element enrichments, positive Nb–Ta anomalies, and positive ε Nd (t) values (+1.20 to +2.97). The Zhegucuo diabases are characterized by Nb depletion, Zr and Hf enrichment, and negative ε Nd (t) values (−2.71 to −2.26). Modeling implies that low degrees (4––7%) of partial melting of garnet lherzolites (3–5 % garnet) within the Kerguelen mantle plume can produce Cuona basalts. However, the Zhegucuo diabases were formed by similar degrees (4–7%) of partial melting of spinel-bearing lherzolite (2 vol% spinel) in the shallow enriched lithospheric mantle. The positive anomalies of Zr–Hf in the Zhegucuo diabases are because their sources have melted. The Tethyan Himalaya underwent lithospheric thinning during the Early Cretaceous, resulting from the impact of the Kerguelen mantle plume. The breakup of eastern Gondwana could have been a combination of the reactivation of the former suture zone and the arrival of a new mantle plume beneath the Tethyan Himalaya. [ABSTRACT FROM AUTHOR]

The temperature-dependent exchange of Ni and Mg between garnet and olivine in mantle peridotite is an important geothermometer for determining temperature variations in the upper mantle and the diamond potential of kimberlites. Existing calibrations of the Ni-in-garnet geothermometer show considerable differences in estimated temperature above and below 1100 °C hindering its confident application. In this study, we present the results from new synthesis experiments conducted on a piston cylinder apparatus at 2.25–4.5 GPa and 1100–1325 °C. Our experimental approach was to equilibrate a Ni-free Cr-pyrope-rich garnet starting mixture made from sintered oxides with natural olivine capsules (Niolv ≅ 3000 ppm) to produce an experimental charge comprised entirely of peridotitic pyrope garnet with trace abundances of Ni (10–100 s of ppm). Experimental runs products were analysed by wave-length dispersive electron probe microanalysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). We use the partition coefficient for the distribution of Ni between our garnet experimental charge and the olivine capsule lnD grt / olv Ni ; Ni grt Ni olv , the Ca mole fraction in garnet ( X grt Ca ; Ca/(Ca + Fe + Mg)), and the Cr mole fraction in garnet ( X grt Cr ; Cr/(Cr + Al)) to develop a new formulation of the Ni-in-garnet geothermometer that performs more reliably on experimental and natural datasets than existing calibrations. Our updated Ni-in-garnet geothermometer is defined here as: T ∘ C = - 8254.568 X grt Ca × 3.023 + X grt Cr × 2.307 + lnD grt olv Ni - 2.639 - 273 ± 55 where D grt / olv Ni = Ni grt Ni olv , Ni is in ppm, X grt Ca = Ca/(Ca + Fe + Mg) in garnet, and X grt Cr = Cr/(Cr + Al) in garnet. Our updated Ni-in-garnet geothermometer can be applied to garnet peridotite xenoliths or monomineralic garnet xenocrysts derived from disaggregation of a peridotite source. Our calibration can be used as a single grain geothermometer by assuming an average mantle olivine Ni concentration of 3000 ppm. To maximise the reliability of temperature estimates made from our Ni-in-garnet geothermometer, we provide users with a data quality protocol method which can be applied to all garnet EPMA and LA-ICP-MS analyses prior to Ni-in-garnet geothermometry. The temperature uncertainty of our updated calibration has been rigorously propagated by incorporating all analytical and experimental uncertainties. We have found that our Ni-in-garnet temperature estimates have a maximum associated uncertainty of ± 55 °C. The improved performance of our updated calibration is demonstrated through its application to previously published experimental datasets and on natural, well-characterised garnet peridotite xenoliths from a variety of published datasets, including the diamondiferous Diavik and Ekati kimberlite pipes from the Lac de Gras kimberlite field, Canada. Our new calibration better aligns temperature estimates using the Ni-in-garnet geothermometer with those estimated by the widely used (Nimis and Taylor, Contrib Mineral Petrol 139:541–554, 2000) enstatite-in-clinopyroxene geothermometer, and confirms an improvement in performance of the new calibration relative to existing versions of the Ni-in-garnet geothermometer. [ABSTRACT FROM AUTHOR]

To determine the nonlinear creep characteristics of rocks under cyclic loading and unloading conditions, a nonlinear Kelvin model and damage viscoplastic model are proposed. The models are connected in series with a linear elastic body to establish a nonlinear damage creep model. The differential damage constitutive equations of the proposed creep model under one-dimensional and three-dimensional stress states are derived based on the creep mechanics and elasticity theory. The damage and unloading creep equations are then obtained based on the superposition principle, and a simple and feasible method for determining the model parameters is determined. Finally, the step cyclic loading and unloading creep test data for lherzolite and limestone are used to verify the rationality and feasibility of the nonlinear damage creep model. The results show that the theoretical creep curves of the nonlinear damage creep model are consistent with the experimental curves which indicates that the proposed model can not only determine the creep properties of lherzolite and limestone under cyclic loading and unloading but also determine the nonlinear characteristics of rocks in the transient and steady-state creep stages and particularly within the accelerating creep stage. [ABSTRACT FROM AUTHOR]

Sulfide-bearing polymineralic inclusions in mantle-derived chromium pyrope garnets of lherzolite paragenesis from lamprophyres of the Chompolo field (Aldan shield, southern Siberian craton) have been studied. The inclusions are composed of either only sulfides or sulfides in association with other minerals (carbonates, silicates, oxides, etc.). The sulfide part of the inclusions is represented by up to four minerals. Among the sulfides, minerals rich in Cu and Ni have been found, whereas Fe sulfides (pyrrhotite, troilite) are absent. This distinguishes the inclusions studied from the majority of sulfide inclusions in mantle minerals and diamonds, as well as in mantle xenoliths from kimberlites. The formation of polymineralic inclusions in chromium garnets of the Chompolo field is attributed to the effect of a carbonate–silicate metasomatic melt/fluid on mantle peridotites, as evidenced by the mineral suite associated with the sulfides. The research results indicate significant differences in the nature of metasomatic processes that occurred in the lithospheric mantle of the southern and central parts of the Siberian craton. [ABSTRACT FROM AUTHOR]

Strongly SiO2-undersaturated alkalic rocks (Mg# > 50, SiO2 ≤ 45 wt%, Na2O + K2O ≥ 3 wt%) occur in three early-stage (Sarnu-Dandali, Mundwara, Bhuj) and one late-stage (Murud-Janjira) rift-associated volcanic complexes in the Cretaceous-Paleogene Deccan Traps flood basalt province of India. Thermobarometry based on clinopyroxene-liquid equilibrium suggests that they mostly crystallized beneath the Moho at ~ 15 kbar/1270 °C to ~ 11–12 kbar/1115–1156 °C pressures and temperatures. Primary magma compositions in equilibrium with lherzolite were estimated through reverse fractionation calculations by incrementally adding equilibrium phases to the rocks in olivine:clinopyroxene:spinel:phlogopite = 12:68:20:15 proportions at low temperatures followed by olivine:clinopyroxene:spinel = 12:68:20 proportions at higher temperatures. A comparison of the primary magmas with experimentally generated melts shows that their compositions are consistent with an origin from garnet lherzolite sources with < 1 wt% H2O and CO2. Hornblendite, pyroxenite (except for some Bhuj rocks) and carbonated eclogite are unlikely sources for the Deccan alkalic rocks. The Sarnu-Dandali and Bhuj alkalic rocks and the Murud-Janjira lamprophyres probably originated by < 5% melting of ~ 1.3 times Ti-enriched lherzolitic sources compared to primitive mantle. The primary magmas of the Murud-Janjira basanites calculated through reverse assimilation-fractional crystallization by assimilating lower crustal and mantle xenoliths found in younger lamprophyre dikes of the same area indicate that contamination by the Indian lithosphere was unlikely during their ascent. The basanites evolved by mixing with phonotephritic melts, and they probably originated from a Ti-poor (0.7 times) lherzolite source. The temperature of the melts at the base of the lithosphere was ~ 1325 °C beneath Sarnu-Dandali and ~ 1285 °C beneath Bhuj and Murud-Janjira. [ABSTRACT FROM AUTHOR]

Oxygen isotope composition was studied in 33 monomineralic fractions of olivine from plagioclase lherzolite, dunite, troctolite, and olivine gabbronorite of the Yoko-Dovyren layered massif. The δ18O values of the least altered rocks not contaminated with crustal materials range within 5.8 ± 0.2‰ (n = 27). These values for the dunite contaminated with carbonate material are notably higher and average at 6.2 ± 0.3‰. This is similar to characteristics of the Bushveld and Jinchuan mineralized complexes. Using the COMAGMAT-5 model, the temperatures of equilibrium of 98% crystals with late portions of the residual melts were evaluated at 1131–1266°C (1176 ± 34°C on average). These estimates are viewed as the closing temperatures of the cumulate systems with respect to the efficient exchange of 18O and 16O isotopes between olivine and mobile melt/fluid. For the uncontaminated systems, the δ18O values of the parental melt calculated using the modeled temperatures are 6.6 ± 0.2‰, and are 7.1 ± 0.3 ‰ for melts with signatures of crustal contamination. Such relations are consistent with insignificant (no more than a few percent) contamination of the parental magmas with carbonate materials from the host rocks. [ABSTRACT FROM AUTHOR]

The pressure dependence of the exchange of Cr between clinopyroxene and garnet in peridotite is applicable as a geobarometer for mantle-derived Cr-diopside xenocrysts and xenoliths. The most widely used calibration (Nimis and Taylor Contrib Miner Petrol 139: 541–554, 2000; herein NT00) performs well at pressures below 4.5 GPa, but has been shown to consistently underestimate pressures above 4.5 GPa. We have experimentally re-examined this exchange reaction over an extended pressure, temperature, and compositional range using multi-anvil, belt, and piston cylinder apparatuses. Twenty-nine experiments were completed between 3–7 GPa, and 1100–1400 °C in a variety of compositionally complex lherzolitic systems. These experiments are used in conjunction with several published experimental datasets to present a modified calibration of the widely-used NT00 Cr-in-clinopyroxene (Cr-in-cpx) single crystal geobarometer. Our updated calibration calculates P (GPa) as a function of T (K), CaCr Tschermak activity in clinopyroxene a CaCrTs cpx , and Cr/(Cr + Al) (Cr#) in clinopyroxene. Rearranging experimental results into a 2n polynomial using multiple linear regression found the following expression for pressure: P GPa = 11.03 + - T K ln (a CaCrTs cpx) × 0.001088 + 1.526 × ln Cr # cpx T K where Cr # cpx = Cr Cr + Al , a CaCrTs cpx = Cr - 0.81 · Cr # cpx · Na + K , with all mineral components calculated assuming six oxygen anions per formula unit in clinopyroxene. Temperature (K) may be calculated through a variety of geothermometers, however, we recommend the NT00 single crystal, enstatite-in-clinopyroxene (en-in-cpx) geothermometer. The pressure uncertainty of our updated calibration has been propagated by incorporating all analytical and experimental uncertainties. We have found that pressure estimates below 4 GPa, between 4–6 GPa and above 6 GPa have associated uncertainties of 0.31, 0.35, and 0.41 GPa, respectively. Pressures calculated using our calibration of the Cr-in-cpx geobarometer are in good agreement between 2–7 GPa, and 900–1400 °C with those estimated from widely-used two-phase geobarometers based on the solubility of alumina in orthopyroxene coexisting with garnet. Application of our updated calibration to suites of well-equilibrated garnet lherzolite and garnet pyroxenite xenoliths and xenocrysts from the Diavik-Ekati kimberlite and the Argyle lamproite pipes confirm the accuracy and precision of our modified geobarometer, and show that PT estimates using our revised geobarometer result in systematically steeper paleogeotherms and higher estimates of the lithosphere‒asthenosphere boundary compared with the original NT00 calibration. [ABSTRACT FROM AUTHOR]

Abgarm ultramafic complex in the south of Kerman province is a part of Esfandagheh- Haji Abad ophiolite which is located in the northern edge of Zagros thrust. The complex contains harzburgites, lherzolites, dunites, and chromitites. Harzburgite is the main lithology of Abgarm peridotites and dunites containing lens-shaped chromites are arranged in irregular layers between them. Some evidence such as recrystallization of grain boundaries, curved shapes boundaries, and exsolution lamellae of clinopyroxenes in orthopyroxenes in the harzburgites and lherzolite show that the peridotites of this studied complex, passed the high-temperature deformation in the upper mantle and then they have emplaced in the crust. Petrography and mineral chemistry evidence shows that there are two groups of chrome spinel in Abgarm complex. The first group is the disseminated chrome spinels in the peridotites and the other group is the chrome spinels in dunites and chromitites. Chemical analyses of spinels in studied rocks show that in these crystals the maximum amounts of Cr# belong to those that exist in the dunites and chromitites (60.76 – 63.81) and the minimum is for those in the lherzolites (11 – 13.50). Chemical composition of chrome spinels from the harzburgites and lherzolites are similar to those exist in Alpine peridotites with about 5 to 15% partial melting and probably they were formed in a back-arc basin environment while the dunites and chromitites of the complex can be caused by the reaction of the basaltic melts of the back-arc basin with peridotites. [ABSTRACT FROM AUTHOR]

To investigate channelization during migration of a reactive melt, we performed a series of Darcy‐type experiments in which an alkali basalt infiltrated partially molten harzburgites and lherzolites at a confining pressure of 300 MPa, temperatures of 1200°C and 1250°C, and pore pressure gradients of ~2 to 60 MPa/mm. We compare our results to those from previously published experiments performed on wehrlites. In all experiments, irrespective of the exact mineralogy, a planar reaction layer composed of olivine + melt developed in which all of the pyroxene was consumed. Under specific conditions controlled primarily by the melt flow velocity, finger‐like channels composed of olivine + melt also developed. In wehrlites, these reaction infiltration instabilities formed at 1200°C and 1250°C at pressure gradients >25 and >5 MPa/mm, respectively. In harzburgites, channelization occurred only at 1250°C at a pressure gradient of 35 MPa/mm. In lherzolites, a planar melt‐filled vein developed at 1250°C; no finger‐like channels formed under a pressure gradient of ~25 MPa/mm. Both the finger‐like channels and the planar vein led to very efficient extraction of melt from the reservoir. Channelization established large compositional variations over short distances in the crystalized phases as well as in the local melt and greatly enhanced the abundance of the reaction product, olivine, similar to dunite channels in the Earth. The range of chemical‐mechanical responses displayed by this array of compositions provides a set of targets for reactive transport and mechanical modeling studies. Key Points: Reaction infiltration instability develops in experiments on wehrlites and harzburgitesChannelization establishes large compositional variations over short distances in the crystalized phases as well as in local meltChannelization greatly increases the modal abundance of olivine in the partially molten rock [ABSTRACT FROM AUTHOR]

The first sampling of mantle horizons underlying the ophiolite peridotite complex of the Mariana back-arc basin was carried out based on a spinel lherzolite xenolith in basanites from the submarine Esmeralda Volcano. The paper shows significant differences between the bulk, petrological, geochemical, and mineralogical compositions of this xenolith and similar characteristics of ophiolite peridotites dredged from the Central Trough of the north part of the Mariana Trench, on the west wall of one of the fault zones of the Santa Rosa Bank Fault (southern part of the trough), and ultramafic rocks dredged from the Conical and Torishima seamounts in the fore-arc part of the Mariana and Idzu-Bonin Island Arc Systems. Using different geothermometers and geobarometers, we revealed the PT conditions for the formation of Esmeralda xenolith's mineral paragenesis. The optimal temperature is 950–1050°C and the pressure is 13–15 kbar. These parameters significantly exceed those suggested for the compared ophiolite peridotites. [ABSTRACT FROM AUTHOR]

Interaction between eclogite-derived melt and peridotite has played an important role in modifying the cratonic lithospheric mantle and generation of high-Mg igneous rocks. To better understand the effect of peridotite physical state on melt-peridotite interaction, we conducted reaction experiments of lherzolite with two basaltic andesites and a ferro-basalt at temperatures of 1300 °C, 1375 °C, and 1425 °C and a pressure of 2 GPa using the reaction couple method. At 1300 °C, when lherzolite is subsolidus, its dissolution rate is slow, and it is mineralogically and texturally unchanged. Garnet and clinopyroxene precipitate at the melt-rock interface. The low-temperature reaction enriches the melt with SiO 2 and Na 2 O, and depletes the melt with Al 2 O 3 , FeO, and CaO. At 1375 °C and 1425 °C, when the lherzolite is partially molten, the dissolution and re-equilibration rates are fast due to grain-scale process that involves dissolution, precipitation, and reprecipitation. Dissolution of olivine and precipitation of orthopyroxene produce a melt-bearing orthopyroxene-rich lithology at the melt-rock interface followed by a melt-bearing harzburgite and a melt-bearing lherzolite. The lithology near the interface can be an orthopyroxene-rich harzburgite or an orthopyroxenite depending on the reacting melt composition. The high-temperature reaction produces melt with increased MgO, FeO, CaO, and Mg#, and decreased SiO 2 , Al 2 O 3 , and Na 2 O. The dissolution rate obtained from the experiments is used to assess the survival of mantle xenoliths in the early Cretaceous high-Mg diorites. Combined with field observations from mantle samples, the experimental results provide insight into the style of eclogite-derived melt and peridotite interaction in the lithospheric mantle beneath the North China Craton. At the lithosphere-asthenosphere boundary, the reaction is dominated by the high-temperature regime, which produces orthopyroxene-rich lithologies such as orthopyroxene-rich harzburgite and orthopyroxenite-veined peridotite. Above the lithosphere-asthenosphere boundary where peridotite is subsolidus, the reaction is characterized by the low-temperature regime, which produces garnet-bearing or garnet-rich lithologies such as garnet pyroxenite, garnetite, and high-Mg granulite. Interactions between eclogite-derived melts and peridotite in the two regimes are responsible for the geochemical features of the early Cretaceous high-Mg igneous rocks from the North China Craton. [ABSTRACT FROM AUTHOR]

The timing of the amalgamation of the Xing'an and Songliao–Xilinhot blocks in the eastern Central Asian Orogenic Belt has long been the subject of debate. To explore this question, we studied a suite of late Carboniferous mafic to felsic intrusive rocks in the central Great Xing'an Range, NE China. These rocks yielded late Carboniferous zircon U–Pb isotopic ages of 320–300 Ma. The felsic monzogranites and granodiorites are strongly peraluminous with high A/CNK ratios (≥ 1.1), and they have signatures transitional between I- and S-type granites. Their geochemical characteristics suggest a magmatic source in a heterogeneous juvenile crust that included both clay-rich pelitic material and metabasalt. On the other hand, the intermediate biotite granodiorites have characteristics that suggest a dual magmatic source involving both mantle material and crustal contributions. The mafic gabbros were derived from the partial melting of mantle materials including both spinel and garnet lherzolite. The upwelling of the asthenospheric mantle contributed greatly to the formation of these magmas, with the upwelling as a result of the delamination of dense lithosphere and/or previously trapped mafic slabs. The petrology, geochemical signatures, field characteristics, and rock associations of the late Carboniferous mafic to felsic intrusive rocks imply that they formed in a post-collisional extensional tectonic setting that developed after the amalgamation of the Xing'an and Songliao–Xilinhot blocks and closure of the Nenjiang Ocean between them. Considering that the tectonic setting during the early Carboniferous was one of regional subduction, our data constrain the amalgamation of the Xing'an and Songliao–Xilinhot blocks and the closure of the Nenjiang Ocean to the early late Carboniferous. [ABSTRACT FROM AUTHOR]

A detailed study on petrology and mineral chemistry of 12 mantle xenoliths from Late Cretaceous basaltic lava flows at Daxizhuang has been conducted to constrain the nature and secular evolution of the Mesozoic lithospheric mantle beneath the eastern North China Craton (NCC). The Daxizhuang mantle xenoliths are mainly anhydrous spinel lherzolite, with subordinate olivine websterite and rare spinel wehrlite. Based on petrographic and geochemical characteristics, the Daxizhuang spinel lherzolite can be subdivided into two groups. Group 1 lherzolite has olivine with Fo contents from 89.4 to 90.6 and LREE-depleted through spoon-shaped to LREE-enriched REE (rare earth element) patterns in clinopyroxene grains, reflecting variable degrees of partial melt extraction (up to 18%) overprinted by later incipient metasomatism. In contrast, Group 2 lherzolite is strongly metasomatized and enriched in iron, as evidenced by relatively lower Fo contents of olivine (84.9 to 88.2) than that from the Group 1 lherzolite and convex-upward trace element patterns in clinopyroxene grains. The Daxizhuang olivine websterite and spinel wehrlite are rich in Fe, Al and low Mg# in olivine and pyroxenes, and slight Ti-enrichment in spinel. Numerical modeling of the Mg# shows that the Daxizhuang olivine websterite and wehrlite may have resulted from the reaction between host residual lherzolite and evolved, silicate melts at high melt/rock ratios. The result of this study indicates that the lithospheric mantle beneath Daxizhuang underwent multiple metasomatism through asthenosphere-lithosphere interaction, which plays an important role in the transformation of the lithospheric mantle of the NCC at least during Late Cretaceous time. [ABSTRACT FROM AUTHOR]

We investigated lherzolitic peridotites in the Cretaceous Purang ophiolite along the Yarlung Zhangbo suture zone (YZSZ) in SW Tibet to constrain their mantle–melt evolution history. Coarse-grained Purang lherzolites contain orthopyroxene (Opx) and olivine (Ol) porphyroclasts with embayments filled by small olivine (Ol) neoblasts. Both clinopyroxene (Cpx) and Opx display exsolution textures represented by lamellae structures. Opx exsolution (Opx1) in clinopyroxene (Cpx1) is made of enstatite, whose compositions (Al2O3 = 3.85–4.90 wt%, CaO = <3.77 wt%, Cr2O3 = 0.85–3.82 wt%) are characteristic of abyssal peridotites. Host clinopyroxenes (Cpx1) have higher Mg#s and Na2O, with lower TiO2 and Al2O3 contents than Cpx2 exsolution lamellae in Opx, and show variable LREE patterns. Pyroxene compositions of the lherzolites indicate 10–15% partial melting of a fertile mantle protolith. P–T estimates (1.3–2.3 GPa, 745–1067°C) and the trace element chemistry of pyroxenes with exsolution textures suggest crystallization depths of ~75 km in the upper mantle, where the original pyroxenes became decomposed, forming exsolved structures. Further upwelling of lherzolites into shallow depths in the mantle resulted in crystal–plastic deformation of the exsolved pyroxenes. Combined with the occurrence of microdiamond and ultrahigh-pressure (UHP) mineral inclusions in chromites of the Purang peridotites, the pyroxene exsolution textures reported here confirm a multi-stage partial melting history of the Purang lherzolites and at least three discrete stages of P-T conditions in the course of their upwelling through the mantle during their intra-oceanic evolution. [ABSTRACT FROM AUTHOR]

The lithospheric mantle beneath the South China Block (SCB) underwent a dramatic transformation from depleted to enriched during late Mesozoic. With a view to deeply understand this process, here we investigate the Mesozoic basalts and their melt inclusions from the Daoxian and Ningyuan regions within the central SCB. The geochemical features of the melt inclusions in these basalts suggest that these rocks originated from the lithospheric mantle enriched through interaction with K-rich aqueous fluids released from subducted Palaeo-Pacific oceanic sediments, whereas the Ningyuan basalts were mainly derived from the asthenospheric mantle source. Geochemical modelling indicates that the Daoxian basalts were generated from 15%-25% of partial melting of garnet lherzolite, whereas the Ningyuan basalts originated from 10%-20% of partial melting of garnet pyroxenites. Our data, combined with those from other Jurassic basalts suggest a temporal evolution of the SCB mantle sources during the Late Mesozoic. Diverse crust–mantle interactions through mixing of the asthenospheric melts with variable proportions of subducted Palaeo-Pacific oceanic sediments might account for the spatial heterogeneity of mantle sources observed beneath the SCB. The transition from Tethyan tectonic realm to the Palaeo-Pacific tectonic regime might have played a significant role in the transformation of the lithospheric mantle beneath the SCB. [ABSTRACT FROM AUTHOR]

To investigate the effect of CO2 on water solubility in orthopyroxene coexisting with H2O-CO2 as a buffering fluid, high-pressure experiments were conducted at pressures of 1.5 and 3 GPa and a temperature of 1100 °C. The experiments were performed in a Walker-type multianvil assembly using natural orthopyroxene with various CO2 to CO2-H2O molar ratios as a starting material. The water contents were measured by polarized Fourier transform infrared spectrometry. At 1.5 GPa and 1100 °C, the H2O solubility decreased with increasing CO2 content in the fluid. The water solubility c(H2O) could be quantitatively determined based on water fugacity f(H2O) as c(H2O) = 25.21 × f(H2O)1.24. The addition of 57% CO2 dramatically reduced the water solubility in orthopyroxene from 184 to 90 ppm. In contrast, at 3 GPa and 1100 °C, the water solubility did not change with the CO2 content in the starting material because CO2 is unstable in bulk peridotite due to the reaction between CO2 and olivine at pressures exceeding 2.2 GPa. This study confirms that the additional component in the aqueous fluid can change the water activity and fugacity, thereby directly lowering the water storage capacity in mantle minerals. As a result, previous estimates of the maximum water storage capacity in the shallow mantle may be overestimated by a factor of 3. [ABSTRACT FROM AUTHOR]