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This paper describes secondary crystallized melt inclusions trapped in magnesiochromite of lherzolite xenolith from the V. Grib kimberlite pipe (Arkhangelsk diamondiferous province). It is shown that the inclusions are microportions of melt related to magmatism, which was further formed this pipe. Daughter minerals assemblage of inclusions in magnesiochromite contain Na‒K‒Ca-, Na‒Mg-, Ca‒Mg-, Mg-, and Ca-bearing carbonates; Na–Mg carbonates with additional PO , Cl–, and SO anions; chlorides; sulfate; phosphate; and silicate. The mineral assemblage of daughter phases, the amount of carbonates (77 vol %) and silicates (tetraferriphlogopite) (15 vol %) and Ca : Na : K ratios within the inclusions indicate that this melt was an alkali-enriched carbonate liquid with a low content of SiO2 (≤6 wt %) and H2O (≤0.6 wt %). As is known, serpentine in kimberlites is a major H2O-bearing mineral, but the problem of water sources during serpentinization of kimberlites and an actual H2O content in kimberlite melts is a controversial issue. The absence of serpentine and the low H2O content (≤0.6 wt %) of the studied melt inclusions in comparison with those of kimberlites of the V. Grib kimberlite pipe (10‒14 wt %) indicate the key role of external fluids during serpentinization of these kimberlites. [ABSTRACT FROM AUTHOR]

ABSTRACT This study provides new mineral chemistry data together with micro‐thermometric measurements on fluid inclusions hosted in ultramafic xenoliths (lherzolite, wehrlite, and dunite) brought to the surface by the last Mt. Vulture volcano activity (140 ka; southern Italy), and fed by melilitite‐carbonatite magmas. Petrographic evidence and mineralogical compositions of Mt. Vulture xenoliths are consistent with an origin in the upper mantle. Fluid inclusions in rock‐forming minerals of lherzolite and wehrlite xenoliths are CO2‐dominated. The equilibrium temperature calculated by geothermometric estimates ranges from 1039 C (±36°C) to 1142°C (±15°C), and entrapment pressures of fluid inclusions with post‐trapping re‐equilibration correspond to the local crust–mantle boundary (32 km depth), and to a shallow reservoir located at 12–14 km depth. These results contribute to constrain the origin of these xenoliths and the depth of storage of magmas erupted from Mt. Vulture, where carbonatite‐like metasomatism and mantle‐derived CO2 degassing occur. [ABSTRACT FROM AUTHOR]

The Nagaland-Manipur ophiolites (NMO), part of the Phanerozoic (538.8–0 Ma) Tethyan ophiolites, occur in the NNE-SSW trending Indo-Myanmar Orogenic Belt (IMOB), northeast India. The NMO hosts both high-Al (0.46 < Cr# < 0.53) and high-Cr chromitites (0.71 < Cr# < 0.79). These chromitite bodies are hosted in lherzolite, harzburgite, and dunite and show various textures, including massive, disseminated, nodular, and granular. The high-Al chromitite compositions in conjunction with the calculated Al2O3[melt] (15.66–16.39 wt.%), TiO2[melt] (0.65–0.94 wt.%), and FeO/MgO[melt] (0.65–0.83 wt.%) values indicate that they were derived from the tholeiitic melt that formed at the mid-ocean ridge centre through low-degree partial melting. In contrast, the high-Cr chromitites, coupled with the Al2O3[melt] (11.24–12.99 wt.%), TiO2[melt] (0.21–0.33 wt.%), and FeO/MgO[melt] (0.58–1.54) values show similar geochemical affinities to those derived from boninitic melts produced by partial melting of already depleted mantle due to the subduction of oceanic plate in a supra-subduction zone environment. The total platinum group element (PGE) contents (60–190 ppb) of high-Al chromitites are lower than the total PGE contents (118–2341 ppb) in high-Cr chromitites. Chondrite-normalized PGE patterns in high-Al chromitites are flat from Os to Rh and negatively sloping from Rh to Pd, whereas high-Cr chromitites show strongly fractionated chondrite-normalized PGE patterns. Total PGE contents and low Pd/Ir ratios (0.02–0.64) of chromitites are consistent with typical ophiolitic chromitites. Mineral chemistry and PGE systematics suggest that NMO chromitites were generated in two separate tectonic settings. Thus, we argue that the upper mantle of the NMO of the IMOB has been modified by a substantial amount of supra-subduction zone components after initially being formed in a mid-ocean ridge tectonic environment. [ABSTRACT FROM AUTHOR]

Basalts from the Late Carboniferous to Early Permian are extensively developed in the central Lhasa subterrane, southern Tibet. Studying the petrogenesis of these rocks may have implications for the late Paleozoic arc magmatism along the central Lhasa subterrane uncovering more of the evolution of the Sumdo Paleo‐Tethys Ocean and its dynamic mechanism. Basalt samples from the Luobadui Formation in the Leqingla area, NW of Linzhou City in the central Lhasa subterrane, southern Tibet exhibit arc‐like geochemical signatures in a subduction‐zone tectonic setting characterized by high Al2O3 and low TiO2 contents, fractionated REE patterns with low Nb/La ratios and high LREE concentrations, and negative HFSE anomalies. Based on their higher Th/Ce, Nb/Zr, and lower Ba/Th, Pb/Nd ratios, slightly negative to positive εNd(t) values, and the relatively high Sr‐Pb isotopic compositions, these samples were probably derived from partial melting of a depleted mantle source of garnet + spinel lherzolite, metasomatized by subducted sediments around 297 Ma. Modeling of the trace elements indicates that these basalts experienced fractional crystallization of olivine, clinopyroxene and minor plagioclase during magma ascent and eruption. It is proposed that these Late Carboniferous–Early Permian basalts are associated with the northward subduction of the Sumdo Paleo‐Tethys Ocean seafloor along the southern margin of the central Lhasa subterrane. [ABSTRACT FROM AUTHOR]

Peridot has a long history and is deeply loved by people for its unique olive-green color. The Yiqisong peridot deposit in Jilin Province is a newly discovered peridot deposit that still deserves systematic research. In this study, gemological and chemical analyses of thirty-three Yiqisong peridot samples were carried out to investigate the gemological characteristics, as well as the mantle properties and formation conditions of the Yiqisong. In addition, we identified gemological differences in peridot between Yiqisong, Tanzania, and Arizona. The Yiqisong peridot samples have typical peridot gemological characteristics. The UV–visible spectrum indicated that Fe is the chromogenic element. The infrared spectra and Raman spectra of different samples are consistent, which indicates that the Yiqisong peridot belongs to forsterite. The contents of Ni and V in Yiqisong peridot are generally low, distinguishing it from peridot found in Tanzania and Arizona. The major and trace elements of samples show that the Yiqisong peridot is derived from the spinel lherzolite xenoliths with the P–T formation conditions of 813–1087 °C and 21–22 kbar. The Yisqisong peridot samples have relatively high Fo values (up to 91.6), supporting their origin from a moderate refractory lithosphere mantle. Therefore, this study provides gemological, mineralogical, and chemical evidence that fills the research gap in peridot deposit studies and lays the foundation for follow-up investigations of gem-grade peridot deposits. [ABSTRACT FROM AUTHOR]

Garnet peridotite xenoliths have been rarely reported from suboceanic mantle. Petrographic and geochemical characteristics of garnet-bearing oceanic peridotite xenoliths provide precious information on dynamics of the suboceanic lithosphere and asthenosphere interaction. We examined a lherzolite xenolith included in olivine nephelinite lava from Aitutaki Island, a member of the Cook-Austral volcanic chain. The lherzolite xenolith contains reddish fine-grained (< 5 µm in size) mineral aggregates (FMAs) with size range of 0.5–6 mm, consisting of olivine, calcic and sodic plagioclases, aluminous spinel, native iron, and nepheline. Microstructural observations and chemical data corroborate that the FMA is a decomposed pyrope-rich garnet including chromian spinel grains with an irregular highly indented morphology in the center. The FMA is surrounded by pyroxene-poor and olivine-rich aureole. The spatial and morphological relationships of FMA and chromian spinel with pyroxene-depleted margin suggest a reaction of aluminous spinel + pyroxenes → pyrope-rich garnet + olivine, which requires a compression before decomposition of the garnet to FMA. An orthopyroxene grain shows slight but clear chemical zoning characterized by increase in Al, Ca, and Cr from the grain center to the rim. The zoning patterns of Al and Ca in the orthopyroxene grain can be modeled by diffusion-controlled solid-state reactions induced by pressure and temperature changes, keeping surface concentrations in equilibrium with the other coexisting mineral phases. The results indicate that the mantle, from which the lherzolite xenolith was derived, underwent isothermal decompression followed by a weak heating on a time scale of a few tenths of million years before the xenolith extraction. From the deduced compression and decompression histories, we hypothesize that the mantle beneath Aitutaki Island was once dragged down to a garnet-stable deep mantle region and brought up later by small-scale sublithospheric convection. [ABSTRACT FROM AUTHOR]

The mechanisms governing a commonly observed seismic velocity drop in the cratonic lithosphere, referred to as the mid‐lithospheric discontinuity (MLD), have been widely debated. To identify the composition and seismic structure of MLDs, we have analyzed Sp receiver functions (SRF) and mantle xenocrysts for six regions across Australia. We utilize locations where seismic stations and kimberlite‐hosted mantle xenocrysts are both available, allowing for comparison between seismological and petrological constraints. Our results show negative SRF phases indicative of the MLD coincide with clinopyroxene‐depleted zones at 60–140 km depth. Clinopyroxenes with different chemical compositions across the MLD define a litho‐chemical discontinuity. Modeling and experimental data show that MLDs may be explained by modified lherzolite with 10%–20% modal pargasite. Pargasite MLDs may form when rising H2O‐bearing melts cross the amphibole dehydration curve and react with clinopyroxene in lherzolite. Because the amphibole dehydration curve is isobaric at 80–120 km, pargasite will be precipitated as horizontal channels. Plain Language Summary: Seismic imaging of Earth's lithosphere has revealed a seismic velocity drop at 60–120 km depth, termed the mid‐lithosphere discontinuity (MLD). The mechanisms governing the MLD have been extensively debated. To advance the understanding of the MLD we carried out an interdisciplinary study to investigate the seismic structure and composition of MLDs identified in the cratonic lithosphere of Australia. We integrated analysis of seismic converted signals from discontinuities recorded by permanent seismometers with geochemical data for kimberlite‐hosted mantle xenocrysts collected in the vicinity. The method allows direct comparison between seismological and petrological constraints. Our results suggest that the MLD comprises anomalously low abundances of clinopyroxene and separates geochemically distinct layers within the lithospheric mantle. Experimental results and modeling suggests that the observed decrease in seismic velocity and absence of clinopyroxene may relate to the formation of pargasite channels in modified lherzolite. Key Points: Sp receiver functions and mantle xenocrysts are used to study mid‐lithosphere discontinuities beneath Australian cratonsMid‐lithosphere discontinuity at 60–120 km depth corresponds with mantle xenocryst populations that are depleted in clinopyroxeneModeling and experimental data suggests the mid‐lithosphere discontinuity is caused by pargasite‐bearing lherzolite [ABSTRACT FROM AUTHOR]

Reaction experiments have confirmed that phlogopite websterite can be formed by the interaction of peridotite with hydrous alkaline- or silica-rich melts. Phlogopite websterites commonly occur as xenoliths in orogenic and intraplate volcanism but do not receive much attention. We have experimentally investigated the melting behaviour of a phlogopite websterite at 1.5 GPa (1050–1300 °C), 3.0 GPa (1100–1500 °C), and 4.5 GPa (1200–1500 °C) to contribute to understanding the sources of ultrapotassic rocks that occur in different settings. The solidus temperature of the investigated phlogopite websterite rises with increasing pressure, bracketed between 1050 and 1100 °C at 1.5 GPa, 1100 and 1150 °C at 3.0 GPa, and between 1200 and 1250 °C at 4.5 GPa. At 1.5 GPa, phlogopite websterite melts incongruently to form olivine and melt, whereas orthopyroxene, garnet, and melt are formed at 3.0 and 4.5 GPa. The transition of orthopyroxene from reactant to product with increasing pressure results in changes in the SiO2 content of melts. The experimental melts reach a maximum K2O content when phlogopite is consumed completely at temperatures ~150 °C above the solidus. The melting reactions are similar to those of phlogopite lherzolite, but the low Al2O3 starting materials result in lower Al2O3 in the melt than in melts of phlogopite lherzolite. Comparison with natural ultrapotassic rock compositions reveals that the sources of ultrapotassic rocks in convergent settings may be dominated by phlogopite websterite, phlogopite lherzolite, and phlogopite harzburgite. Sources of ultrapotassic rocks in intraplate settings are more likely to include phlogopite clinopyroxenite ± CO2 and K-richterite. In all melting experiments on phlogopite-bearing rocks, K2O from phlogopite passes into the melt, and hence the highest K2O contents in ultrapotassic rocks must be an indication of the minimum stoichiometric coefficient of phlogopite in the melting reaction. In cases where phlogopite websterite or phlogopite lherzolite is identified as the source, the minimum modal percentage of phlogopite in the source can be inferred from the highest K2O content. When applied to the Milk River minettes and New South Wales leucitites, the estimated modal proportion of phlogopite in the sources is greater than 20 wt %. Phlogopite can survive the subduction process and melt later in the post-collisional environment, whereas thermal perturbations are necessary to trigger the melting of phlogopite-bearing assemblages at the base of the lithosphere in intraplate settings. [ABSTRACT FROM AUTHOR]

Orogenic peridotites in the crystalline basement of the northwestern Bohemian Massif contain multiphase solid inclusions (MSI), which are interpreted to be crystallisation products of trapped former carbonate–silicate melts metasomatizing their host rocks. We applied conventional thermobarometry and forward thermodynamic modelling to constrain the P–T evolution ranging from the peak metamorphic conditions of the investigated harzburgite and lherzolite, through entrapment of the melts in the outer parts of garnets, to the (re)-equilibration of the MSI assemblages. The peak conditions of c. 1100 °C/4.5–5.5 GPa are recorded by garnet cores and large pyroxene porphyroclasts. The melt entrapment, during which garnet outer parts grew, was associated with influx of the metasomatizing liquids and probably took place during the early stage of the exhumation. Thermodynamic model of amphibole-free MSI assemblage comprising kinoshitalite/Ba-rich phlogopite (approximated by phlogopite in the model), dolomite, magnesite, clinopyroxene, orthopyroxene, garnet and chromite provided robust estimate of P and T of its (re)-equilibration, c. 900–1000 °C, 1.8–2.2 GPa. Furthermore, the lack of olivine reflects co-existence of COH fluid with high X(CO2) = CO2/(CO2 + H2O) ≥ 0.6. Models employing identical P–T–X(CO2) parameters successfully reproduced the other two amphibole-bearing assemblages observed. The modelled stability fields show perfect alignment with a characteristic isobaric segment of the solidus curve of carbonated peridotite. This co-incidence implies that the (re)-equilibration corresponds to the termination of the melt crystallisation once the near-isothermal exhumation path intersected the solidus. Decreased solubility of silicates at the carbonated peridotite "solidus ledge", inferred from the published experimental data, as well as concentric textures of some MSI indicates sequential crystallisation from the early silicates to late dolomite. The carbonated "solidus ledge" is a relatively narrow boundary in the lithospheric mantle capable of an abrupt immobilisation of fluxing or transported carbonated melts. The investigated rocks are estimated to store approximately 0.02 kg C/m3 (or 6 ppm C) occurring as carbonates in the MSI. [ABSTRACT FROM AUTHOR]

Replicating lithospheric diamonds experimentally helps reveal their histories at depth, which may be complicated by successive growth and dissolution episodes. Here, we present diamond growth and dissolution experiments to constrain the conditions of diamond formation and residence in the Earth's lithosphere before their transport to the surface. Experiments were performed on mixtures of carbonates, natural lherzolite or MORB, water, diamond seeds, and with or without graphite using multi-anvil presses over a few hours to more than a day at conditions relevant to the lithosphere (7 GPa, 1300–1670 °C). We observed growth within a volatile-rich melt resulting from the miscibility of silicate-carbonate melt and aqueous fluid via the reaction: Carbonate minerals + Silicate minerals/glasses + H 2 O = [volatile-rich melt] ± minerals + C diamond + O 2. In the absence of graphite, we observed diamond dissolution within the same hydrous-carbonate–silicate-bearing melts and under similar pressure and temperature conditions used to grow diamond, indicating that diamond formation requires an oxygen sink (graphite, metallic and/or sulphide melts). In dissolution experiments, we also observed resorption features similar to those described in lithospheric monocrystalline diamonds, which we thus attribute to mantle fluids and not to kimberlite-induced resorption during magma ascent. We show that dissolution and growth may alternate in the mantle, and that protracted periods are not necessary to explain natural diamond histories. During experimental growth, inclusions were trapped in diamonds as they are in nature. We observed both syngenetic (formed during growth, representing the parental fluid) and protogenetic inclusions (here particles from the capsule), indicating that both kinds of inclusions can be trapped within a single growth event. Our experiments confirm that inclusions trapped in natural diamonds do not necessarily represent remnants of their parental fluids and are not necessarily contemporaneous: two inclusions near each other within a single monocrystalline diamond may have different histories, and inclusions must be shown to have achieved chemical and isotopic equilibrium before being considered synchronous. [ABSTRACT FROM AUTHOR]

In Northwestern Cameroon, the northwest flank of Oku Volcanic Complex forms part of the continental sector of the Cameroon Volcanic Line (CVL). Detailed study of petrology and mineral chemistry of nine mantle xenoliths from the NW flank of Oku Volcanic Complex with derived teleseismic data have been conducted to determine the nature and depth of the lithospheric mantle beneath the flank and CVL as a whole. Petrographically, mantle xenoliths are olivine websterite and plagioclase-spinel lherzolite consisting of a five-phase mineral assemblage (olivine, orthopyroxene, clinopyroxene, spinel and plagioclase) with varied textural changes (protogranular, porphyroclastic and porphyroblastic) characterizing the equilibrium conditions of the upper mantle. Mineralogically, magnesian olivine (forsterite and chrysolite), clinopyroxene (diopside), and orthopyroxene (clinoenstatite) with plagioclases (labradorite with rare bytownite) dominate. We propose hereditary representations to explain the occurrence of plagioclase in the mantle peridotites. (1) Subsolidus transformation of spinel to plagioclase, (2) crystallisation of melt pockets from partial melting of the upper mantle and (3) accumulations from melts that impregnated the lithospheric mantle. Velocity and synthetic models obtained from the inversion of the receiver functions indicate that the Moho depth or crustal discontinuity is 33.2 km for CM20 and 40.8 km for CM23 with a mean thickness of 37 km. The lithospheric mantle thickness is 28.1 km and 20.5 km for CM20 and CM23 respectively with a mean thickness of 24.3 km. The low velocity zone is located at intervals of 61.3 to 113 km, exhibiting a thickness of 51.7 km and 42.5 km for CM20 and CM23 respectively, resulting in a mean thickness of 47.05 km. The discontinuities are located at 37 km for the Mohorovicic discontinuity and between 61.3 km and 61.5 km for the Lithospheric-Asthenospheric Boundary. The occurrence of plagioclase in the mantle peridotite, coupled with results from the receiver function techniques, provides evidence of mantle upwelling beneath this region, particularly within the continental sector of the CVL. [ABSTRACT FROM AUTHOR]

Late Neoproterozoic mafic-ultramafic rocks exposed in the Gabal Um Selim–Zeiatit area, northernmost Arabian-Nubian Shield include an old ophiolite sequence and a young mafic-ultramafic intrusion. The ophiolite sequence is represented by serpentinized peridotite and metagabbro, which are highly deformed, metamorphosed, and tectonically thrust over island arc metavolcanics. The mafic-ultramafic intrusion, known as the Um Selim–Zeiatit mafic-ultramafic intrusion (SZMUI), shows no evidence of deformation or metamorphism. The SZMUI consists of cumulate layered rocks and is composed of an upper suite of mafic rocks and a basal suite of ultramafic rocks. The upper suite consists of olivine gabbro, gabbronorite, and normal gabbro, while the basal suite includes lherzolite and olivine websterite. The SZMUI forms a lopolith-like intrusion and displays modal layering at outcrop scale as well as cryptic layering due to the variations in mineral compositions. The lherzolite contains green spinel (pleonaste) characteristic of layered intrusions. The spinel and olivine compositions of the ophiolitic serpentinites are consistent with residual mantle rocks that experienced high degrees of partial melt extraction typical for suprasubduction-zone forearc ophiolites, whereas those in the SZMUI ultramafic layers are typical of stratiform complexes and clearly different from those found in ophiolitic and Alaskan-type complexes. The differences in whole-rock Mg# between the ultramafic samples are consistent with a distinction based on mineral chemistry, with higher values in the ophiolitic peridotite than in the SZMUI peridotite. The mafic and ultramafic units of the SZMUI are clearly related and derived from a common parent magma, but they were not emplaced coevally and represent different pulses of magma. The ultramafic units of the SZMUI are cumulates and are derived from a high-Mg tholeiitic parent magma. Geochemical characteristics of the SZMUI confirm that it is not an Alaskan-type body but is more akin to a layered mafic-ultramafic intrusion. It was emplaced at a late stage of orogeny, during a transition from a subduction to an intraplate setting. [ABSTRACT FROM AUTHOR]

Petit-spot volcanoes, occurring due to plate flexure, have been reported globally. As the petit-spot melts ascend from the asthenosphere, they provide crucial information of the lithosphere–asthenosphere boundary. Herein, we examined the lava outcrops of six monogenetic volcanoes formed by petit-spot volcanism in the western Pacific. We then analyzed the 40Ar/39Ar ages, major and trace element compositions, and Sr , Nd , and Pb isotopic ratios of the petit-spot basalts. The 40Ar/39Ar ages of two monogenetic volcanoes were ca. 2.6 Ma (million years ago) and ca. 0 Ma. The isotopic compositions of the western Pacific petit-spot basalts suggest geochemically similar melting sources. They were likely derived from a mixture of high- μ (HIMU) mantle-like and enriched mantle (EM)-1-like components related to carbonatitic/carbonated materials and recycled crustal components. The characteristic trace element composition (i.e., Zr , Hf , and Ti depletions) of the western Pacific petit-spot magmas could be explained by the partial melting of ∼ 5 % crust bearing garnet lherzolite, with 10 % carbonatite flux to a given mass of the source, as implied by a mass-balance-based melting model. This result confirms the involvement of carbonatite melt and recycled crust in the source of petit-spot melts. It provides insights into the genesis of tectonic-induced volcanoes, including the Hawaiian North Arch and Samoan petit-spot-like rejuvenated volcanoes that have a similar trace element composition to petit-spot basalts. [ABSTRACT FROM AUTHOR]

The results of study of the REE ID‒ICP‒MS, 86Sr/87Sr, and 147Sm‒143Nd AL ID-TIMS isotope systematics of spinel lherzolite from the Northern Kraka Massif, which is part of the largest (>900 km2) lherzolitic allochthon thrust over the bathyal and shelf deposits of the passive continental margin of the East European Platform, are reported. As a result, an isochron dependence (MSWD = 0.85) was revealed for the first time, which determines the age of 545 ± 26 Ma and the high value of the initial ratio (143Nd/144Nd)0 = 0.512390 ± 0.000054, corresponding to εNd = +8.9 within the model. The resulting REE, 87Sr/86Sr, and 147Sm–143Nd isotopic signatures indicate the melting of an already depleted protolith, which can be identified as a mantle source, with MORB-like parameters. The calculated isochron age of homogenization of the 147Sm–143Nd isotope system in combination with the available complex of geological and geochemical data allows us to place the Late Vendian phase (epoch) of folding and orogeny in the Urals in the interval of 545 ± 26 Ma. Comparison of these data with materials on the geology of Central and Western Europe allows us to correlate the Timanide structures formed as a result of this phase of folding with the Cadomian, which, based on global reconstructions of continents for the end of the Proterozoic, will ultimately authorize the hypothesis of the existence of the Cadomian orogeny on the periphery of Gondwana. [ABSTRACT FROM AUTHOR]

The mantle section of the central Palawan Ophiolite (CPO) in western Philippines is dominantly composed of dunites and harzburgites. Basal lherzolites and harzburgites juxtaposed with the metamorphic sole of the CPO are also exposed within the vicinity of Ulugan Bay, Puerto Princesa. These basal peridotites provide novel information regarding the processes transpiring in the upper mantle during the evolution of this preserved oceanic lithosphere. The Nasuduan lherzolite is characterized by low Cr# of spinel and high concentrations of REEs in clinopyroxenes. The Oyster Bay lherzolite displays similar fertile characteristics and occurs with harzburgites. Both contain fine-grained and interstitial clinopyroxenes derived from interstitial melts. Tremolite and talc in the Oyster Bay lherzolite and harzburgite and in other CPO harzburgites are attributed to slab-fluid metasomatism. Harzburgites and dunites from Bentoan Beach and other CPO harzburgites have high spinel Cr# and low clinopyroxene REE concentrations. These indicate that these peridotites have undergone extensive degrees of melting facilitated by fluids. The petrological and geochemical signatures of the CPO peridotites record the transition of a supra-subduction zone from back-arc spreading, incipient subduction, to the obduction of an ancient oceanic lithosphere. [ABSTRACT FROM AUTHOR]

We present the geochemical and Sr–Nd–Pb isotopic data and the petrological evolution of primitive basaltic lavas that erupted from the Miocene to Quaternary in the East Anatolia Collision Zone to understand the geodynamic conditions and the change in the lithospheric mantle over time. Major trace element abundances, Sr–Nd–Pb isotopic compositions and petrological models show that the primitive basaltic samples were not affected by crustal contamination and fractional crystallization. They are derived from a depleted MORB mantle modified by melts derived from subducted sediments. The primitive melts of the Miocene and Quaternary series were derived from an amphibole-bearing garnet lherzolitic mantle and an amphibole-bearing garnet–spinel lherzolite mantle source, respectively. In contrast, the Pliocene basaltic melts were formed by mixing melts originating from both an amphibole-bearing spinel and garnet lherzolite. Our thermodynamic calculations indicate that the lithosphere–asthenosphere boundary (LAB) is about 30 km shallower in the Pliocene than in the Miocene.This may be explained by lithospheric delamination in the Early Pliocene. In contrast, the LAB in the Quaternary is approximately 9 km deeper than in the Pliocene, which can be explained by relamination of the mantle lithosphere. Thermal calculations have shown that about 5–11 km of the relamination can occur within 5–6 Ma and that asthenospheric melts can relaminate the base of the thinned lithospheric mantle by cooling, and the presence of the relaminated mantle lithosphere is documented throughout the whole EACZ. [ABSTRACT FROM AUTHOR]

We experimentally investigated the phase relations of a primary basaltic glass erupted on the 10–15°E ultraslow spreading (∼0.4 cm/yr) oblique amagmatic segment of the Southwest Indian Ridge (SWIR). Piston‐cylinder experiments were conducted at 9–13 kbar and 1270–1310°C, in accordance with the predicted pressure and temperature (9.5 ± 0.8 kbar and 1290 ± 12°C) of saturation with a mantle lherzolite assemblage using the recently developed multiphase reverse fractional crystallization model ReversePetrogen (RevPet, Krein et al., 2021, https://doi.org/10.1029/2020JB021292). The experiments were multiply saturated with a plagioclase lherzolite assemblage (olivine + augite + orthopyroxene + plagioclase) at 10 kbar and 1290°C, within the uncertainty of the RevPet prediction. We use the forward model Petrogen (Krein et al., 2020, https://doi.org/10.1029/2020JB019612) to find the best fitting mantle composition and melting conditions that could produce glass KN162‐9 48‐21. This model independently finds a melting pressure and temperature of 9.5 kbar and 1285°C, which agree remarkably well with the RevPet predictions and our experiments. This sample is also significantly more depleted in incompatible trace elements than other glasses from the same ridge segment, suggesting its mantle source experienced prior episodes of depletion. This is consistent with previous work suggesting that ultraslow spreading at the SWIR preserves heterogeneities on a local scale. The variability of erupted compositions largely reflects aggregation and mixing of heterogeneous near‐fractional melts produced in the spinel and plagioclase stability fields along similar melting paths and their fractional crystallization products. More generally, plagioclase‐field mantle melting does occur at mid‐ocean ridges and is preserved in some melting environments. Plain Language Summary: The mantle melting conditions that produce seafloor basalts are important for understanding the Earth's temperature structure. The two most important parameters to constrain are the pressure and temperature at which primary magmas are produced, yet different lines of evidence point to different conclusions, in part because the composition of these magmas are modified en route to the surface. In this work, we investigate a very primitive basalt erupted at the Southwest Indian Ridge, one of the slowest spreading ridges which also displays extreme geochemical variability. We perform experiments that allow us to constrain the pressure and temperature of melting, which match almost perfectly with predictions from the recently developed model ReversePetrogen. These point to shallow melting of a depleted mantle, which is likely a common end‐member at the Southwest Indian ridge as well as other mid‐ocean ridges globally. Our analysis suggests that while a geochemically heterogeneous mantle is required, slow spreading conditions favor the preservation and eruption of these depleted primitive melts. Key Points: A near‐primary basalt from the Southwest Indian Ridge originates from shallow mantle melting in the plagioclase stability fieldShallow melting at slow and ultraslow spreading ridges may explain the preservation of primitive melts with low apparent mantle potential temperaturesMixing of deep, enriched melts, and shallow depleted melts can explain most of the geochemical variability at the Southwest Indian Ridge [ABSTRACT FROM AUTHOR]

The Late Paleozoic tectonic evolution of the Xing'an block in the eastern Central Asian orogenic belt has long been the subject of debate. In this paper, a comprehensive study of U-Pb zircon ages, Lu-Hf isotopes and whole-rock elemental analyses was carried out on Hadayang schists. Representative samples of the epidote-biotite-albite schist and biotite-albite schist yielded the weighted mean 206Pb/238U ages of 360 ± 2 Ma and 355 ± 3 Ma, respectively. This indicated the presence of Late Devonian–Early Carboniferous intermediate-basic rocks in the eastern Xing'an block. The Hadayang schists exhibited a Na-rich, tholeiitic and calc-alkaline affinity in composition with low Mg# (35.2–53.0), Cr (23.7–86.5 ppm), Ni (21.1–40.0 ppm) and Co (12.1–30.6 ppm). They were characterized by enrichment of LILEs, depletion of HFSEs and highly positive zircon εHf(t) values (the average values were +8.93 and +9.29, respectively). The magma source of the Hadayang schists was a mantle that consisted of both spinel and garnet lherzolite, with a partial melting degree of 1%–5%, and it had undergone fractional crystallization of olivine, orthopyroxene and plagioclase. The Hadayang schists, together with other Late Devonian–Early Carboniferous intermediate-basic magmatic rocks in the eastern Xing'an block, were formed in an intracontinental extension tectonic setting similar to that of the North American Basin and Range basalt. Moreover, Late Devonian–Early Carboniferous ophiolite under a similar tectonic background in the western Xing'an block has been reported. We believe that the Xing'an block would have been in the stage of intracontinental extension during the Late Devonian–Early Carboniferous. [ABSTRACT FROM AUTHOR]

Numerous mantle xenoliths 6–12 cm in size and sub-angular to rounded in shape occur within Mio-Pliocene basanite lavas of the monogenic volcano of Hosséré Sédé in the Adamawa plateau. Xenoliths of spinel lherzolite exhibit protogranular, equigranular or porphyroclastic texture. Microprobe chemical analyses show that olivine is highly magnesian (Fo88-90), clinopyroxene crystals are diopside and augite (Wo41.6–49.6 En45.3–53.7 Fs4.2–6.2), orthopyroxene crystals are enstatite (Wo1.4–1.5 En88.6–89.0 Fs9.6–9.9) and spinel crystals are mainly Al-spinel associated to minor Cr-spinel. Estimated temperatures and pressures through empirical formulas show that Hosséré Sédé xenoliths have equilibrated between 1085 and 1204 °C and 1.08 to 1.57 GPa, corresponding to sampling depths of 36 and 52 km. Detailed petrographical and mineral chemistry of Hosséré Sédé xenoliths evidences the complex nature and composition of the subcontinental lithosphere under the Adamawa plateau. This may involve a probable uplift of the whole area after a limited extensional event, possible metasomatism through melt infiltration during shearing of the lithospheric mantle along the Pan African strike-slip fault system. [ABSTRACT FROM AUTHOR]

The world-class Jinchuan magmatic Ni-Cu-PGE (platinum-group element) sulfide deposit comprises four large orebodies (No. 1, 2, 3 and 24). Underground fan drilling discovered significant lateral extension of No. 2 orebody, referred as No. 2a in this study. From base to top, the No. 2a orebody comprises disseminated sulfide in olivine pyroxenites, net-textured ore in lherzolite, and massive ore. Despite different host rocks, the contents of Ni, Cu and PGE of the same types of sulfide ores from No. 2 and No. 2a orebodies are comparable. Mass balance calculation indicates that the parental magmas contained 0.01–0.05 ppb Ir and 0.2-0.8 ppb Pd, which are about one order of magnitude less than PGE abundances of undepleted continental flood basalts. PGE tenors (recalculated to 100% sulfides) of the net-textured ores from both orebodies increase upward. We attribute this to variations in magma:sulfide ratios of successive batches of recharged magmas and/or reaction of sulfides with less evolved magma. We propose that the No. 2a and No. 2 orebodies were located in the upstream and downstream parts of an originally sub-horizontal magma conduit, respectively. Sulfide liquids accumulated in the wider parts of the magma conduit to form No. 2a and No. 2 orebodies progressively. Variable locations within the intrusion and sharp contacts with other types of sulfide ore indicate that the massive ores formed by injection of pooled sulfide melt. Significant Pt-depletion of the massive ores and Pt enrichment in the adjacent net-textured ores suggest migration of residual fractionated sulfide liquids. [ABSTRACT FROM AUTHOR]

We present a new experimental dataset for reaction experiments between natural amphibole-clinopyroxene metasomes (hornblendite) and synthetic lherzolite that produced Na-rich alkaline melts. Experiments were conducted at 1, 3 and 4 GPa and 1000–1300 °C. The generated melts range from foidite over basanite to phonotephrite. At 1 GPa between 1000 and 1100 °C amphibole decompression-breakdown products generate a phonotephritic melt. Among the breakdown components rhönite was found to be stable up to 1100 °C and 1 GPa. At 3 and 4 GPa the melt compositions are affected by phlogopite melting and shift to more foiditic compositions. We find that the melting of hornblendites and the reaction of the melt with the lherzolite produce wehrlitic residues with different olivine/clinopyroxene ratios. Wehrlite formation does not always require separate metasomatic processes but can be a direct by-product of alkaline volcanism. We applied a metasome melting model to the magmas of the Kula volcanic province, Turkey, and show that at 1 GPa basanite melts and phonotephrite melts cover the whole range of known Kula lava compositions. The Kula lava compositional trend can be therefore generated by basanite-phonotephrite melt mixing. A comparison of high-pressure (3–4 GPa) melts with natural nephelinite data shows overlap with many major, minor, and trace elements but differences in SiO2, FeO, and TiO2 argue that the natural nephelinite data do not represent primary metasome melts. [ABSTRACT FROM AUTHOR]

The findings of platinum group metal mineralization (PGM) and the distribution of platinum group elements (PGE) in lherzolites of the Northern Kraka massif are described. The total contents of PGE are approximately two orders of magnitude lower than those in chondrite and are close to pyrolite, relative to which the studied lherzolites are enriched in Pd and depleted in Ru. In segregations of PGMs, the presence of all PGEs (except rhodium) in various proportions was established. All found grains are divided into three contrasting types: the refractory triad Os–Ir–Ru, essentially platinum with the participation of Pd, and Cu–Pd. Almost all found PGM grains are localized either in the peripheral parts of grains of altered sulfides (heazlewoodite, pentlandite) or in the silicate matrix in the immediate vicinity of sulfide segregations. Based on the mineral associations and PGE distribution, a probable genesis of segregations has been suggested. Associations of Cu–Pd and Pd–Pt(+Cu) composition most likely formed during the crystallization of sulfides from the extracted partial melts. This is indicated by their close association with clinopyroxenes and the presence of relatively fusible platinoids and copper. The associations of Pt–Ir and Os–Ir–Ru(+Pt) composition are most likely restitic, formed in place of primary mantle sulfides as a result of extraction of more fusible elements and further desulphurization. The isolation of platinoids as their own mineral phases is associated with the influence of superimposed low-temperature processes – subsolidus redistribution during cooling and subsequent serpentinization.

The mafic–ultramafic cumulate rocks and mantle peridotites of an ophiolite are excellent recorders of the geodynamic evolution of a fossil subduction zone. The spectro-radiometric absorption features of olivine, orthopyroxene, clinopyroxene and serpentine, which are the dominant constituents of these ultramafic rocks, have emerged as essential tools for their identification. This study uses six different kinds of ultramafic rock samples – spinel lherzolite, spinel harzburgite, wehrlite, orthopyroxenite, olivine websterite, and dunite compositions from the Nagaland Ophiolite Complex, NE India. These are investigated with integrated petrographic, mineral and bulk rock compositional and reflectance spectroscopic studies to identify the constituent minerals. The study indicates that the most dominant olivine absorption features vary from 1038 to 1049 nm in Ni-poor ultramafic cumulates of wehrlite (Mg# = 89, Ni = 355 ppm) and olivine websterite (Mg# = 90.5, Ni = 361 ppm) to 1066–1097 nm in nickeliferous mantle peridotites of dunite (Mg# = 87, Ni = 3658 ppm) and spinel lherzolite (Mg# = 87, Ni = 2708 ppm) compositions. Reversal of the 625 nm reflectance maximum in olivine occurs for Ni concentrations in the mineral between 2200 and 2955 ppm. The low-Ca pyroxene (cf. orthopyroxene) species show characteristic absorption features at 902–926 nm in lherzolite and harzburgite samples and 918 nm in monomineralic orthopyroxenite. High-Ca pyroxene (cf. clinopyroxene) in most rocks records absorption features at 643–666 and 709–811 nm. The study also recognises major absorption bands at 1382–1400, 1913–1948, and 2304–2326 nm as indicative of secondary hydrous minerals as a product of alteration in the investigated rocks. The significance of major (Mg–Fe) and minor (Mn, Ni and Cr) element compositions of these ultramafic rocks to explain specific spectral signatures in olivine, orthopyroxene and clinopyroxene is discussed. The findings are expected to aid the interpretation of remotely sensed data from terrestrial and extra-terrestrial bodies, mapping ultramafic cumulate and mantle peridotite rocks in remote, covered, and unmapped terranes of ophiolitic origin, and in the process, identification of fossil oceanic subduction systems. [ABSTRACT FROM AUTHOR]

The lithology, geochemistry, and architecture of the continental lithospheric mantle (CLM) underlying the Kimberley Craton of north‐western Australia has been constrained using pressure‐temperature estimates and mineral compositions for >5,000 newly analyzed and published garnet and chrome (Cr) diopside mantle xenocrysts from 25 kimberlites and lamproites of Mesoproterozoic to Miocene age. Single‐grain Cr diopside paleogeotherms define lithospheric thicknesses of 200–250 km and fall along conductive geotherms corresponding to a surface heat flow of 37–40 mW/m2. Similar geotherms derived from Miocene and Mesoproterozoic intrusions indicate that the lithospheric architecture and thermal state of the CLM has remained stable since at least 1,000 Ma. The chemistry of xenocrysts defines a layered lithosphere with lithological and geochemical domains in the shallow (<100 km) and deep (>150 km) CLM, separated by a diopside‐depleted and seismically slow mid‐lithosphere discontinuity (100–150 km). The shallow CLM is comprised of Cr diopsides derived from depleted garnet‐poor and spinel‐bearing lherzolite that has been weakly metasomatized. This layer may represent an early (Meso to Neoarchean?) nucleus of the craton. The deep CLM is comprised of high Cr2O3 garnet lherzolite with lesser harzburgite, and eclogite. The peridotite components are inferred to have formed as residues of polybaric partial mantle melting in the Archean, whereas eclogite likely represents former oceanic crust accreted during Paleoproterozoic subduction. This deep CLM was metasomatized by H2O‐rich melts derived from subducted sediments and high‐temperature FeO‐TiO2 melts from the asthenosphere. Key Points: The Kimberley Craton has retained a thick (>220 km) thermally stable lithospheric root since the MesoproterozoicThe lithospheric mantle comprises a depleted shallow layer (<100 km) and metasomatized deep layer (>150 km) separated by a diopside‐depleted mid‐lithosphere discontinuityDiamondiferous lamproites at the craton margin are associated with eclogite and high levels of K2O metasomatism of the lithospheric mantle [ABSTRACT FROM AUTHOR]

The lithosphere--asthenosphere boundary (LAB), which can be seismically detected, stabilizes plate tectonics. Several conflicting hypotheses have been proposed as the causes of LAB discontinuity, such as the contribution of hydrated minerals, mineral anisotropy, and partial melts. The petit-spot melts ascending from the asthenosphere, owing to subducting plate flexures, support the partial melting at the LAB. Here, we observed the lava outcrops of six monogenetic volcanoes formed by petit-spot volcanism in the western Pacific. Thereafter, we determined the 40Ar/39Ar ages, major and trace element compositions, and Sr, Nd, and Pb isotopic ratios of the petit-spot basalts. The 40Ar/39Ar ages of two monogenetic volcanoes were ca. 2.6 Ma (million years ago) and ca. 0 Ma, respectively. The isotopic compositions of the western Pacific petit-spot basalts suggest their geochemically similar melting sources. They were likely derived from a mixture of high-μ (HIMU) mantle-like and enriched mantle (EM) -1-like components related to carbonatitic/carbonated materials and recycled crustal components. A mass balance-based melting model implied that the characteristic trace element composition (i.e., Zr, Hf, and Ti depletions) of the western Pacific petit-spot magmas could be explained by the partial melting of garnet lherzolite with a small degree of carbonatite melt flux with crustal components. This result confirms the involvement of carbonatite melt and recycled crust in the source of petit-spot melts and provides an implication for the genesis of tectonic-induced volcanism with similar geochemical signatures to those of petit-spots. [ABSTRACT FROM AUTHOR]

The lithosphere–asthenosphere boundary (LAB), which can be seismically detected, stabilizes plate tectonics. Several conflicting hypotheses have been proposed as the causes of LAB discontinuity, such as the contribution of hydrated minerals, mineral anisotropy, and partial melts. The petit-spot melts ascending from the asthenosphere, owing to subducting plate flexures, support the partial melting at the LAB. Here, we observed the lava outcrops of six monogenetic volcanoes formed by petit-spot volcanism in the western Pacific. Thereafter, we determined the 40Ar/39Ar ages, major and trace element compositions, and Sr, Nd, and Pb isotopic ratios of the petit-spot basalts. The 40Ar/39Ar ages of two monogenetic volcanoes were ca. 2.6 Ma (million years ago) and ca. 0 Ma, respectively. The isotopic compositions of the western Pacific petit-spot basalts suggest their geochemically similar melting sources. They were likely derived from a mixture of high-μ (HIMU) mantle-like and enriched mantle (EM) -1-like components related to carbonatitic/carbonated materials and recycled crustal components. A mass balance-based melting model implied that the characteristic trace element composition (i.e., Zr, Hf, and Ti depletions) of the western Pacific petit-spot magmas could be explained by the partial melting of garnet lherzolite with a small degree of carbonatite melt flux with crustal components. This result confirms the involvement of carbonatite melt and recycled crust in the source of petit-spot melts and provides an implication for the genesis of tectonic-induced volcanism with similar geochemical signatures to those of petit-spots. [ABSTRACT FROM AUTHOR]

The Zhaheba ophiolite is an ocean relic of the Zhaheba-Aermantai oceanic slab, a branch of the early Paleozoic Paleo-Asian Ocean. The peridotites consist mainly of harzburgite, lherzolite and minor dunite, chromitite. This study describes the whole-rock geochemistry and mineral chemistry of the Zhaheba peridotite and chromitite for the purpose of constraining their tectonic environment and genesis. The major oxides and the trace element concentrations of the peridotites are comparable with abyssal peridotite, but fall outside the field of SSZ (suprasubduction zone) peridotite and the fore-arc peridotite. The massive chromites belong to the high-Cr group, with an average Cr# (Cr/(Cr + Al)) atomic ratio) value of chromian spinel of 0.77, whereas the average Mg# value is 0.60. The disseminated chromites give a lower concentration of Cr2O3 (38.96–42.15 wt.%, average 40.35 wt.%) and lower Cr# values (0.50–0.56, average 0.53), but slightly higher contents of MgO (13.23 wt.%) and Mg# (0.61) than the massive chromites. In the diagrams of Cr#-Mg#, NiO-Cr# and TiO2-Cr#, the massive chromites fall in the field of boninite, and the disseminated chromite in the peridotite plot fall in the field of abyssal peridotite and mid-oceanic ridge basalt (MORB). The massive chromitites, with high-Cr, display a boninite affinity, whereas the disseminated chromite plot in the high-Al and abyssal peridotite type field may be generated by the extension of the Zhaheba ocean in the MOR environment then experienced deep subduction and exhumation. The calculated degrees of partial melting for the massive chromites are 21%−22%, and for the disseminated chromites in peridotites the degrees are 17%−18%. The calculated values of fO2 for the massive chromites range from −1.44 to +0.20, and the values for the disseminated chromites range from −0.32 to +0.18. The inferred parental melt composition for massive chromitite falls in the field of boninite in an arc setting, whereas the disseminated chromite in peridotites are in the field of a MORB setting. This indicates that the parental magmas of the former were more refractory than the latter. A two-stage evolution model for the chromites was proposed, in which disseminated chromites were first formed in an MOR environment and then modified by later-stage melts and fluids, and formed massive chromites were formed in an SSZ setting during intra-oceanic subduction. [ABSTRACT FROM AUTHOR]

The lithospheric mantle as sampled by peridotite xenoliths in some continental settings resembles the source of mid-ocean ridge basalts (MORB). Whether this resemblance is a primary feature or the result of post-formation secondary processes remains controversial. Here, the age, origin and thermochemical evolution of fertile continental mantle are constrained based on the chemical composition of minerals in spinel-facies lherzolite and websterite xenoliths from the Wum maar and Befang cinder cone of the Oku Volcanic Group (Cameroon Volcanic Line, West Africa), combined with in-situ Sr isotope compositions of clinopyroxene and fabric investigation by Electron Backscatter Diffraction (EBSD). The majority of lherzolites (here assigned to Group I) consist of minerals with fertile composition (olivine Fo89, Al-rich pyroxenes, spinel Cr# 0.08–0.10). Clinopyroxene is LREE-depleted and has depleted 87Sr/86Sr (0.7017–0.7020). Crystal-preferred orientation determined by EBSD reveals that clinopyroxene, and sporadically both clino- and orthopyroxene, post-date the olivine framework. Subordinate Group II lherzolites also contain secondary clinopyroxene which is LREE-enriched and has higher 87Sr/86Sr (0.7033). In contrast, the scarce lherzolites of Group III are more refractory: they contain 72–78 vol.% olivine, Al-poor pyroxenes, and spinel with Cr# 0.18. Clinopyroxene (87Sr/86Sr 0.7021) is texturally coeval with olivine and orthopyroxene. Few lherzolites contain amphibole (87Sr/86Sr 0.7031) which post-dates the nominally anhydrous minerals. Most of the websterites (Group A) are aluminous (spinel Cr# 0.04–0.06) with LREE-depleted clinopyroxene having depleted 87Sr/86Sr ratios (0.7017–0.7020) similar to Group I lherzolites. Chemical characteristics of minerals coupled with the crystal-preferred orientation data suggests that Group I lherzolites originated in the spinel stability field by reactive intergranular percolation of an incompatible element-depleted MORB-like melt. Group A websterites likely formed as cumulates from that melt. The Group II lherzolites supposedly occur close to lithosphere-asthenosphere boundary and record interaction with lavas of the Cameroon Volcanic Line, whereas Group III lherzolites occur in the shallow part of the mantle profile and represent the protolith from which the Group I lherzolites were formed. Local crystallization of amphibole and concomitant recrystallization of the host lherzolite were driven by supply of water in an event post-dating the formation of LREE-depleted rejuvenated rocks. Migration of alkaline melts of the CVL apparently did not significantly affect the mineral and chemical composition of the lithospheric mantle, which allowed Group I lherzolites and Group A websterites to retain very low 87Rb/86Sr (average 0.002) and depleted 87Sr/86Sr ratios in clinopyroxene. This not only indicates their formation in the Paleoproterozoic (~2.0–2.25 Ga), possibly during the Eburnean orogeny at the margin of the Congo craton, but also indicates surprisingly little influence of the regionally recognized Pan-African event. [ABSTRACT FROM AUTHOR]

The Almogholagh Intrusive Complex (AIC) in the central part of the Sanandaj-Sirjan zone (C-SaSZ) includes gabbros to granites. Zircon U-Pb dating of mafic, intermediate, and felsic intrusions yield age of 146 to 141 Ma (Late Jurassic-Early Cretaceous). Bulk chemistry shows a wide range for SiO2 (46.8 to 73.3 wt.%), MgO (0.08 to 8.95 wt.%), and Fe2O3(T) (0.47 to 9.25 wt.%) and also alkali elements ranging from 3.59 to 10.12 wt.%. These rocks are characterized by a high content of large-ion lithophile elements (LILEs; e.g. Rb, U, Th, and K) and LREEs with weak Nb and Ta negative anomalies, showing many similarities to an extensional setting. The 176Hf/177Hf ratios vary from 0.282802 to 0.282985 with positive εHf(t) values from + 1.67 to +10.74 (avg: 7.85). Trace-element and isotope ratios modelling indicate that the AIC mafic-intermediate rocks were generated by partial melting of spinel lherzolite and then underwent considerable crustal contamination. The present work in combination with previous data confirm an extensional tectonic regime for the northern part of the C-SaSZ during the Late Jurassic to Early Cretaceous. Magmatic differentiation with crustal contamination played a fundamental role in the wide compositional range from mafic to felsic types that formed the AIC. [ABSTRACT FROM AUTHOR]

Reverse fractionation modeling considering energy-constrained assimilation-fractional crystallization is performed to estimate primary magma compositions, degree of crustal contamination, pressure–temperature of equilibrium with mantle, and potential temperatures for the origin of the Paleoproterozoic (~ 2.37–1.88 Ga) basaltic dikes in central and eastern Dharwar Craton and sills and volcanics in the adjoining Cuddapah Basin, southern India. Mineral thermobarometry indicates that the dikes crystallized at upper crustal conditions (~ 1–6 kbar/ ~ 1120–1210 °C). Hence, the reverse fractionation calculations are performed at low pressures by adding olivine + plagioclase + clinopyroxene, olivine + plagioclase and only olivine in equilibrium with melt, and simultaneously subtracting an upper crustal partial melt in small steps until the melt is multiply saturated with lherzolite at a high pressure. The results indicate that the basalts are 5–30% contaminated, and their enriched light rare earth element (REE) patterns can be attributed to upper crustal assimilation. The upper crust was pre-heated to 665–808 °C during dike emplacement. The primary magmas of all basalts were last equilibrated with spinel lherzolite at 10–16.5 kbar/1291–1366 °C, and they resemble pooled polybaric incremental melts generated along a ~ 1450 °C adiabat. The estimated mantle potential temperatures (1293–1515 °C) are similar to Paleoproterozoic ambient mantle temperatures. All basalts and their primary magmas show lower chondrite-normalized DyN/YbN ratios than the plume-derived mid-Proterozoic Mackenzie dikes of Canadian Shield, and the primary magmas show flat REE patterns indicating spinel lherzolite melting. The low estimated potential temperatures, low DyN/YbN ratios, and a spinel-bearing mantle source are at odds with an origin of the basalts from mantle plumes. [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. [ABSTRACT FROM AUTHOR]

The voluminous Choke Shield basalts and flood basalts are distributed in the central Ethiopian Plateau. They are tholeiitic in composition and have OIB-like geochemical features. The ca. 23 Ma Choke Shield basalts have SiO2 (47.1 wt.%–59.6 wt.%,), MgO (1.01 wt.%–7.8 wt.%,), Na2O + K2O (2.7 wt.%–8.4 wt.%), and display right inclined REE patterns ((La/Yb)N = 21.4–24.2) with enrichment of Nb, Ta, Zr, Hf and Pb in the primitive mantle-normalized trace element diagrams. They show low initial 87Sr/86Sr ratios (0.703 47–0.703 77) and high εNd(t) values (+4.4 to +5.0). In comparison, the 24 Ma high-Ti (HT1) flood basalts have SiO2 (38.9 wt.%–50.8 wt.%), MgO (3.9 wt.%–11.4 wt.%), Na2O + K2O (1.6 wt.%–5.8 wt.%), and display right inclined REE patterns ((La/Yb)N = 24–130.3) with enrichment of Nb, Ta, Zr, Hf, and Pb. They also show low initial 87Sr/86Sr ratios (0.703 30–0.704 44) and high εNd(t) values (+2.2 to +5.3). Both types of basalts were contaminated by minor crustal materials and underwent fractional crystallization of clinopyroxene, plagioclase, olivine, and minor Fe-Ti oxide. The Choke Shield basalts were generated by 1%–5% melting of garnet-spinel to phlogopite-bearing spinel lherzolite in a shallow zone of the mantle plume, while the flood basalts were formed by <20% melting of amphibole-bearing garnet to garnet-spinel lherzolite in a deeper zone of the same mantle plume. The mantle source beneath the central Ethiopian Plateau was significantly heterogeneous during the Tertiary. It was characterized by EMI and EMII end-members that were formed by the metasomatism of the different components. [ABSTRACT FROM AUTHOR]

New geological and geochemical data on the age and composition of Neogene rocks in the northern part of the Central Kamchatka volcanic belt are presented. K–Ar dating of the all studied volcanic rocks yielded a Late Miocene age. The geochemical characteristics of the rocks show that the northern part of the Central Kamchatka volcanic belt is characterized by a complex geological history, in which magma generation processes are governed by the type of mantle matter. The "earliest" Late Miocene rocks that make up the Umuvayam and Tolyatovayam complexes (6.7–9.0 Ma) are represented by typical island-arc rocks, while "later" (5.4–7.7 Ma) volcanic rocks of the Alney and Veemgetver complexes are characterized by an association of rocks with a high proportion of enrichment from the OIB-type source. The magma source corresponded in composition to spinel lherzolite. The enrichment of volcanic rocks in large-ion lithophile elements is explained by the role of fluids introduced into the melts during the melting of the supra-subduction mantle wedge, which probably also was modified by supra-subduction metasomatism in the geological past. [ABSTRACT FROM AUTHOR]

This paper reports the first discovery of zoned aragonite, a high-pressure CaCO3 polymorph, in a mantle xenolith from kimberlite. Aragonite is the most common epigenetic mineral in the studied xenolith and is located in the interstitial space, where it can occupy up to 80 vol %. According to the P‒T parameters of the last equilibrium (6.9 GPa and 1350°C), the studied sheared lherzolite belongs to the deepest mantle xenoliths from kimberlites. Based on the stability of aragonite, it could have formed in sheared lherzolite both in situ in the lithospheric mantle and during ascent of the kimberlite magma to a depth of ~80 km. The studied aragonite has high contents of SrO (from ~0.7 to 8.3 wt %) and Na2O (up to ~1.1 wt %). Diffusion modeling at mantle temperatures of 1100–1350°C shows that the aragonite grains lose zonation in SrO over ~1–7 years. Thus, the zonation pattern indicates a link between the formation of aragonite in the sheared lherzolite and magmatism, which formed the eastern body of the Udachnaya pipe. In general, such high contents of aragonite in the interstitial space of the studied xenolith are direct evidence of the existence of carbonatite melts in the lithospheric mantle of ancient cratons at depths below ~230 km shortly before the formation of kimberlites on the surface. The results in the present study again raise the question regarding the compositions of primitive kimberlite melts and true concentrations of alkalis (in particular, Na2O) in them. [ABSTRACT FROM AUTHOR]

The Neo‐Tethyan ophiolites of the Tuting‐Tidding Suture Zone (TTSZ), Eastern Himalaya (viz., Tidding Ophiolite Complex and Mayodia Ophiolite Complex) and Indo‐Myanmar Orogenic Belt (IMOB, i.e., Nagaland Ophiolite Complex and Manipur Ophiolite Complex) which lie along the southern extension of the Indus‐Tsangpo Suture Zone have been collectively investigated through the mantle‐derived peridotite sequence. The peridotites (harzburgite and dunites) in the Eastern Himalaya ophiolites are refractory, constrained by the high Fo olivine content (~95), high Cr# [Cr/(Cr + Al)] (0.90–0.99), as well as by the parental melt composition of Cr‐spinels (Al2O3 melt = 3.05–7.55 wt%, FeO/MgO = 0.69–6.46). They have slightly "U‐shaped" chondrite‐normalized Rare Earth Elements (REE) patterns with slight enrichment in Light Rare Earth Elements (LREE) relative to the patterns expected for residues of partial melting, thereby indicating a reaction with the LREE‐enriched melt. These peridotites represent residual portions of a depleted/enriched mantle that underwent partial melting up to 23% in the nascent forearc of an intra‐oceanic subduction zone, and later metasomatized by high‐temperature silicate melts and low‐temperature hydrous fluids. They are composed of a very refractory olivine‐spinel assemblage (Fo: 91.68–96.44; Cr#: 0.90–0.99), corroborating a boninitic parentage, with influence from melt‐rock interactions. In the case of the IMOB ophiolites, a wide range of chemical compositions is observed in the mantle sequence. Lherzolites display low Cr# (0.12–0.26) and TiO2 (<0.11) associated with high Mg# [Mg/(Mg + Fet)] (0.69–0.76) in the Cr‐spinels present in them. They represent the residual product of a fertile mantle that underwent low‐degree partial melting (2%–10%) in a divergent mid‐ocean ridge (MOR) tectonic setting. On the other hand, the harzburgites and dunites of the IMOB have high Cr# (0.84–0.90) and low TiO2 (<0.06 wt%) Cr‐spinels and exhibit slightly U‐shaped REE distributions indicating their derivation from a highly depleted mantle source, which experienced higher degree partial melting (17%–24%) similar to those of supra‐subduction zone (SSZ) harzburgites and dunites of the TTSZ peridotites. The occurrence of both MOR and SSZ types of melting regimes indicates that the peridotites in the IMOB ophiolites formed at two major different stages of the pre‐subduction and subduction events, respectively. However, the peridotites of Eastern Himalaya ophiolites were formed only during subduction tectonics. Thus, we argue that the mantle peridotites in the ophiolites of northeast India evolved from lherzolite through clinopyroxene‐harzburgite and harzburgite then to highly refractory dunite, supporting multistage melting and melt‐rock reaction processes during their generation. [ABSTRACT FROM AUTHOR]

An attempt for the calculation of primordial mineralogy of the source rock of the ultramafics of Manipur ophiolite, north‐east India is made with the use of an established schematic ternary phase diagram of the anhydrous system Forsterite‐Diopside‐Enstatite at 20 kb pressure for a hypothetical upper mantle rock (X). For the case of batch partial melting, if the proportions of the phases entering the melt (Pα) are different from their initial proportion in the rock (X0α), then the weight fraction of the melt (F) at which each phase in question is consumed is given by F = X0α/Pα. From the phase diagram, the bulk composition of the source rock X can be expressed as 0.4 Di, 0.2 En, and 0.4 Fo (these are the X0α values). The proportion (Pα) in which the various phases entered the melt is initially given by the proportion in the eutectic composition E, that is, 0.7 Di, 0.2 En, 0.1 Fo. With the value of Fcpx = 0.4/0.7 = 0.57, it is found that the diopside is the only phase to be completely consumed into the partial melt, which occurs at 57% partial melting. The least serpentinized group of ultramafics of the Ophiolite Belt in Manipur is considered to be the residue of those which have suffered the least percent of partial melting (about 5%), and the average proportions of the three main phases of such rocks are taken as yardsticks for necessary calculation of the primordial mineralogy of the upper source rock. From the computed value of Fcpx (Mo) = 0.14, it is inferred that the diopside in the original source rock of Manipur Ophiolite must be completely melted at about 15% partial melting as against 57% of the hypothetical mantle rock discussed in the phase diagram. Thus, at about 15% partial melting of the source rock of Manipur Ophiolite, the levels of melting of the three main phases, as derived from the phase diagram, is calculated to be, Diopside = 100%, Enstatite = 57%, and Olivine = 14%. From these values of levels of melting, the different degrees of melting of the three main phases have been computed separately for 5%, 10%, 15%, and 20% bulk partial melting. From the modal proportions of the main phases of the least serpentinized ultramafics of Manipur Ophiolite (considered to be a residue of 5% partial melting), the mineralogical proportions of the original source rock are computed and these are found to correspond to Spinel Lherzolite composition. [ABSTRACT FROM AUTHOR]

Since the Early Cenozoic, the Philippine Sea Plate (PSP) has undergone a complex tectonic evolution. During this period the Parece Vela Basin (PVB) was formed by seafloor spreading in the back‐arc region of the proto‐Izu‐Bonin‐Mariana (IBM) arc. However, until now, studies of the geological, geophysical, and tectonic evolution of the PVB have been rare. In this study, we obtained in situ trace element and major element compositions of minerals in basalts collected from two sites in the southern part of the PVB. The results reveal that the basalts from site CJ09‐63 were likely formed via ∼10% partial melting of spinel‐garnet lherzolite, while the basalts from site CJ09‐64 were likely formed via 15%–25% partial melting of garnet lherzolite. The order of mineral crystallization for the basalts from site CJ09‐64 was olivine, spinel, clinopyroxene, and plagioclase, while the plagioclase in the basalts from site CJ09‐63 crystallized earlier than the clinopyroxene. Using a plagioclase‐liquid hygrometer and an olivine‐liquid oxybarometer, we determined that the basalts in this study have high H2O contents and oxygen fugacities, suggesting that the magma source of the Parece Vela basalts was affected by subduction components, which is consistent with the trace element composition of whole rock. [ABSTRACT FROM AUTHOR]

Complex multiphase reaction rims that form during garnet breakdown are known as kelyphite coronae and are common amongst exhumed mantle xenoliths. It has long been established that a reaction of garnet and olivine produces kelyphite corona consisting of spinel and pyroxenes, and that preservation of high-pressure garnet cores requires sufficiently rapid uplift of material through the spinel lherzolite stability field from depths of at least 60 km. We present new high-pressure, high-temperature experiments of garnet breakdown in the spinel–lherzolite stability field demonstrating that a series of cascading reactions can reproduce the multilayer, multiphase kelyphites seen in nature. In all experiments where breakdown occurred, a melt appears to have moderated the reactions towards equilibrium; we believe this to be the first experimental confirmation of the importance of such melts in garnet breakdown reactions. In our experiments at least three distinct zones of concentric kelyphite growth can occur at a single pressure, temperature condition; we suggest, therefore, that such kelyphites seen in natural samples do not have to be caused by a multistage uplift path as is often assumed. Kelyphitic coronae surrounding garnet have previously been used to estimate uplift rates; however, the lack of kinetic data for relevant exhumation reactions has limited their use for PTt pathway estimations and the understanding of emplacement mechanisms. In order to constrain accurate PTt pathways we use reaction rim thickness as a proxy for reaction progress and present preliminary results for the kinetics of garnet breakdown. [ABSTRACT FROM AUTHOR]

Mantle intrusive rocks (lamproites and basalts) were collected from different units of the Uzbek Tian Shan, Central Asia to investigate xenoliths and xenocrysts aiming at tracking the shallow or deep nature of the mantle source. Whole-rock geochemistry and petrography revealed that these rocks are mostly of basaltic composition and may contain various exotic materials. These rocks originate from composite mantle magmas; they often result from the mixing between an evolved primary melt and more primitive melts, as indicated by the composition and textures of clinopyroxenes. Olivine, orthopyroxene and spinel mineral geochemistry from xenocrysts and lherzolitic xenoliths is consistent with a shallow mantle source (spinel lherzolite). Trace element content of clinopyroxenes shows LREE depletion supporting the removal of the magma from an undepleted sub-continental lithospheric mantle. The latter one is heterogenous, though and some parts recorded evidence of intense metasomatism. The upwelling asthenospheric magmas, emplacing in trans-lithospheric structures, were probably directly transferred into the crust or just below the crust, without significantly changing the geochemical composition of the sub-continental lithospheric mantle. [ABSTRACT FROM AUTHOR]

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]

• High-MgO basaltic andesites from Klyuchevskoy volcano are products of mantle melting. • Pressures of melting correspond to near-Moho depths (23–36 km). • Melting temperatures are 1220 to 1240 °C; melt fraction is 8 to 11 wt%. • Melting is driven by rifting-induced decompression of amphibole-bearing lherzolite. • Primary arc magma chemistry is controlled by wedge thermal structure and H 2 O activity. Primary magmas in volcanic arcs exhibit wide compositional diversity on both local and global scales. Processes responsible for this diversity are generally ascribed to some combination of mantle melting or crustal differentiation processes. One widespread view is that arc magmagenesis is driven by combination of H 2 O-fluxed and decompression melting of peridotitic mantle wedge, and that primary, mantle-derived melts are high-MgO basalt. However, a variety of other mantle-derived primitive arc magmas, ranging in composition from high-Mg andesite to picrite, has been recognised and it remains unclear to what extent this diversity can be generated by mantle melting processes modulated, for example, by changes in the thermal state of the mantle wedge or the supply of fluid from the slab. Here we use high pressure and temperature experiments to constrain magma generation conditions of a primitive magnesian (8.8 wt% MgO) basaltic andesite from Klyuchevskoy volcano, Kamchatka arc, Russia. We use an inverse experimental approach to define a multiple saturation point on the liquidus surface of the basaltic andesite. The experimental multiple saturation point defines the pressure and temperature at which an erupted melt could have last been in equilibrium with a polymineralic source rock, such as mantle peridotite, and hence provides a robust estimate of magma source conditions. Equilibrium piston-cylinder experiments were carried out between 0.5 and 1.0 GPa under hydrous conditions (3 to 6 wt% added H 2 O) at f O 2 = ΔNNO+1. We show that Klyuchevskoy basaltic andesite is multiply saturated with the lherzolite assemblage olivine (Fo 90) + clinopyroxene + orthopyroxene + Cr-spinel close to its liquidus (≥ 95% melt) in the pressure range of 0.6 to 1 GPa (23 to 36 km depth) and 1220–1240 °C. Amphibole is present at temperatures just below the multiple saturation point (≤ 1200 °C). Our results show that basaltic andesite was produced by 8 to 11 wt% partial melting of amphibole-lherzolite source and therefore represents a primary, undifferentiated magma extracted from its source at near-Moho depths. These findings are in a good agreement with geophysical studies of Klyuchevskoy volcano that show magmas are supplied directly from a reservoir at near-Moho depths (25–30 km) through a sub-vertical, pipe-like feeder system. Coeval high-MgO basalts from the volcano may correspond to higher degree mantle melts extracted at slightly greater depths. Our results provide a tight constraint on the thermal structure of the mantle wedge beneath Klyuchevskoy. Experimental temperatures are higher than those calculated from steady-state thermal models suggesting that upwelling asthenosphere might directly impinge the Moho in a similar fashion to mid-ocean ridges. Intra-arc rifting promotes asthenospheric decompression beneath the Central Kamchatka Depression. The presence of amphibole in our experiments at temperatures up to 1200 °C indicates that dehydration melting of amphibole peridotite formed by metasomatism of mantle wedge by slab-derived fluids is the primary magma generating process. Amphibole stability exercises an important control on melting conditions. Integrating our results with published multiple-saturation experiments we show that Klyuchevskoy basaltic andesite is one of a family of primary arc magmas whose compositions depend on mantle wedge thermal structure and H 2 O activity. [ABSTRACT FROM AUTHOR]

Carbon isotope data is desired to be increased to promote the understanding of carbon cycle throughout in the Earth. Diamond is a key carbonaceous tool to study deep carbon cycle, but most diamond occurrences are limited from kimberlite pipes in the continental region. Recently, micron-sized diamonds have been discovered from the oceanic region and investigated to understand deep carbon cycle in the oceanic mantle. However, some fundamental cautions have been issued on the oceanic diamonds because some of them could be of artificial origin. Hence, alternative oceanic mantle-derived carbonaceous material is needed to increase oceanic carbon isotope data. We report micron-sized calcite vein in a lherzolite xenolith hosted by enriched mantle I (EM1)-type olivine nephelinite from Aitutaki Island, Cook Islands in the southern Pacific. With employing various techniques to determine carbon and oxygen isotope compositions from sub-micrograms of calcite, we demonstrate that carbonaceous fluid originated from EM1-type mantle source exhibited organic carbon signature based on its light carbon isotope composition along with petrographic characteristics of the calcite vein. The oceanic mantle hosts organic carbon in places due to the recycling of surface materials. • Calcite in oceanic lherzolite xenolith exhibited light carbon isotope composition. • Carbonaceous fluid was originated form enriched mantle I-type mantle source. • Oceanic mantle hosts organic carbon due to the recycling of surface materials. [ABSTRACT FROM AUTHOR]