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New petrographic and microanalytical studies of mineral inclusions in the Purang ophiolitic chromitites (SW Tibet) are used to scrutinize the evolution of the associated Cretaceous sub-oceanic lithospheric mantle section. Silicate inclusions in the chromite grains include composite and single-phase orthopyroxene, clinopyroxene, amphibole, and uvarovite. Most inclusions are sub-rounded or globular, whereas a few inclusions exhibit cubic/octahedral crystal morphologies. The latter are randomly distributed in the large chromite grains, though discrete aggregates are consistently confined to the grain centers. Abundant micrometer-scale, clinopyroxene inclusions are topotaxially aligned along crystallographic planes. Less-abundant sulfide, wüstite, apatite, and uvarovite inclusions are observed in some samples. The trace element geochemistry of the Purang chromitite evoke parental MORB- and boninite-like melts, consistent with the supra-subduction zone setting. The δ26Mg values of the high-Cr and high-Al chromitites range from –0.25 to –0.29‰ and –0.05 to –0.32‰, respectively. The associated harzburgite has nearly overlapping δ26Mg values of –0.13 to –0.37‰, but pyroxenite sills show distinct δ26Mg values (–0.61 to –0.67‰). The variable Mg isotope signatures, combined with abundant exotic, ultrahigh-pressure and super reduced (UHP-SuR) mineral inclusions in the chromite grains, suggest that recycling and recrystallization under different mantle conditions played an important role in the genesis and evolution of these rocks. Furthermore, discrete silicate, sulfide, and metal alloy inclusions in the Purang chromitites are comparable to those reported in other Tethyan ophiolites, and collectively suggest a common geodynamic evolution.

The North China craton was stabilized in the late Paleoproterozoic but experienced significant removal of ancient lithospheric keel in the late Mesozoic that resulted in the formation of numerous world-class gold deposits with combined reserves of more than 7000 t of gold. However, it remains uncertain whether the North China craton contains older gold deposits formed during generation and final stabilization of the craton. Here, we show that the Shanggong gold deposit (105 t Au at 5.31 g/t) on the southern margin of the North China craton formed in the late Paleoproterozoic during the collision between the Eastern and Western blocks that led to formation of the Trans–North China orogen and final stabilization of the craton. The Shanggong deposit is hosted in amphibolite-facies rocks of the Neoarchean to early Paleoproterozoic Taihua Group and overlying volcanic rocks of the late Paleoproterozoic Xiong’er Group. Gold mineralization is structurally controlled by NE-striking faults and occurs in four segments: the Liuxiugou, Hugou, Shanggong, and Qiliping segments. The ores consist mainly of quartz-ankeritesulfide stockworks and sulfide disseminations in hydrothermally altered wall rocks. Gold is mostly contained in arsenian pyrite that is variably associated with minor sphalerite, galena, and chalcopyrite. Ore-related alteration assemblages comprise mainly quartz, ankerite, K-feldspar, sericite, and tourmaline. Both the stockworks and mineralized alteration assemblages contain hydrothermal accessory minerals, including monazite, apatite, and rutile. Paragenetic relations and textural data show that these accessory phases precipitated synchronously with gold-bearing sulfides. Laser ablation–inductively coupled plasma–mass spectrometry spot analyses of monazite and apatite from the Shanggong segment yielded reproducible U-Pb dates of 1747 ± 20 Ma (2σ, mean square of weighted deviates [MSWD] 0.46) and 1788 ± 200 Ma (2σ, MSWD = 11.3), respectively. These dates are indistinguishable within errors from an apatite U-Pb date of 1743 ± 79 Ma (2σ, MSWD = 1.6) at the Liuxiugou segment and a rutile U-Pb date of 1804 ± 52 Ma (2σ, MSWD = 0.77) at the Hugou segment. These new dates suggest that the Shanggong deposit formed at ca. 1.80–1.74 Ga, coeval with or immediately after formation of the Trans–North China orogen and final stabilization of the North China craton. Sulfides from the Shanggong gold deposit have δ34S values ranging from −18.5‰ to −6.9‰, whereas the coexisting ankerite has δ13CPDB of −6.81‰ to −1.61‰ and δ18OSMOW of 15.70‰–17.62‰. The stable isotope data are distinctively different from values of the Early Cretaceous gold deposits in the southern North China craton, indicating contrasting hydrothermal systems responsible for these two categories of gold deposits. The results presented here, combined with independent geologic evidence, allow Shanggong to be the first confirmed Paleoproterozoic orogenic gold deposit in the North China craton. Recognition of Paleoproterozoic orogenic gold mineralization provides significant new insights into the gold metallogeny of the well-endowed North China craton and has implications for future gold exploration along the three Paleoproterozoic orogenic belts in this craton.

Scandium is important in modern technology and is regarded as a strategic metal in many countries. It is highly dispersed in Earth’s crust and rarely forms independent minerals. Clinopyroxene is the most important Sc-bearing mineral in some world-class deposits hosted in mafic–ultramafic intrusions, which are also the major source of laterite-hosted Sc deposits. However, the factors controlling Sc distribution in minerals have been little explored, impeding the understanding of the geochemical behavior of Sc and why it is common in some clinopyroxene grains. The newly discovered Mouding Sc deposit in SW China is hosted in a zoned intrusion composed, from core to rim, of monzogabbro, syenogabbro, gabbro, magnetite clinopyroxenite, and clinopyroxenite. Clinopyroxene in the intrusion is diopsidic in composition with high Sc contents (80–105 ppm). In-situ trace element mappings of diopside crystals reveal homogeneous, zoned, swallow-tailed, and hourglass internal Sc distribution patterns. These patterns can be produced through kinetically controlled incorporation of Sc on different crystal faces. The preferential substitution of Sc can take place on the {1 0 0}, {1 1 0} and {0 1 0} prism faces because of the high flexibility of the octahedral M1 protosites. The fast growth of diopside, which facilitates kinetically controlled crystallization, is dominated by textural coarsening and promoted by the hydrous parental magmas with low viscosities and active convection. The active flow and efficient interstitial communication of the magma can direct compatible elements from the magma into clinopyroxene, thus favoring formation of Sc-rich grains. Our study provides a feasible way to study intra-grain variations of Sc in minerals and emphasizes that kinetic effects may play a critical role in Sc distribution and enrichment in hydrous magmatic Sc deposits. We also show that disequilibrium crystallization may be more pervasive than previously thought, and the hourglass zoning of clinopyroxene can provide valuable information on this process.

Here we present new occurrences of amphibole in a suite of chromitites, dunites, and harzburgites from the mantle sequence of the Lycian ophiolite in the Tauride Belt, southwest Turkey. The amphibole occurs both as interstitial grains among the major constituent minerals and as inclusions in chromite grains. The interstitial amphibole shows generally decreasing trends in Na2O and Al2O3 contents from the chromitites (0.14–1.54 wt% and 0.04–6.67 wt%, respectively) and the dunites (0.09–2.37 wt%; 0.12–11.9 wt%) to the host harzburgites (

Gold deposits in Precambrian cratons were mostly generated during the formation and stabilization of the cratons, but the North China craton is unusual in that its gold deposits were mainly formed ∼1.7 b.y. after its stabilization. A magmatic-hydrothermal origin or mantle-derived fluid source has been proposed for the giant gold deposits of the Jiaodong District in the eastern North China craton, but direct evidence is sparse, and the mineralization processes remain controversial. Here, we present the results of a comprehensive geological, geochronological, and geochemical study of the gold mineralized Xiejia diorite beneath the Linglong ore field at Jiaodong to link the gold mineralization to underlying magmatism. Magmatic zircon and titanite grains from the Xiejia diorite have laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS) U-Pb ages of 121.3 ± 0.9 Ma to 120.8 ± 1.1 Ma and 121.7 ± 3.9 Ma, respectively, which are indistinguishable from the time of gold deposition throughout the Jiaodong District as constrained by previous studies. The diorite has a shoshonitic composition and is characterized by strong enrichment in large ion lithophile elements (LILEs) and light rare earth elements (LREEs) along with significant depletion in high field strength elements (HFSEs) and heavy rare earth elements (HREEs). Samples of the diorite have high initial 87Sr/86Sr ratios, but low εNd(t) and ɛHf(t) values and low Pb isotope ratios. These geochemical characteristics are akin to those of contemporaneous mafic dikes in most gold mines at Jiaodong, indicating that the Xiejia diorite was most likely derived from an enriched lithospheric mantle source. The upper part of the diorite intrusion is pervasively altered and mineralized, containing an average of 0.32 g/t Au, but locally up to 7.59 g/t. Hydrothermal titanite from the mineralized diorite has a LA-ICP-MS U-Pb age of 122.3 ± 4.3 Ma, which is consistent with the gold-bearing pyrite Re-Os isochron age of 122.5 ± 6.7 Ma. Ore-related sericite aggregates from the Dongfeng gold deposit above the Xiejia diorite have a 40Ar/39Ar plateau age of 122.6 ± 1.3 Ma. Pyrite from the mineralized diorite yielded δ34SCDT values of 2.1‰−9.7‰, which are comparable with those of pyrite (δ34SCDT = 5.8‰−8.1‰, where CDT indicates the Canyon Diablo troilite standard) from gold ores of Dongfeng. Pyrite grains from both groups also have similar Pb isotope compositions. The S and Pb isotope data are consistent with values of mafic dikes that are spatially and temporally associated with gold veins in the Linglong ore field. The results presented here thus indicate a possible genetic link between gold mineralization in the Xiejia diorite and underlying magma presumably represented by the Xiejia diorite. The auriferous fluids exsolved from that magma and migrated upward along the Potouqing fault to form the Dongfeng gold deposit above the Xiejia diorite. The mineralized diorite thus links shallow gold mineralization to deep-seated mantle-derived magmatism generated during the extensive destruction of the North China craton induced by the rollback of the subducted paleo-Pacific plate.

Fractional crystallization of Fe–Ti oxides can induce detectable Mg isotopic changes during late-stage basalt differentiation. Because chromite and olivine are early crystallizing phases during basaltic melt differentiation, the effect of chromite crystallization on the fractionation of Mg isotopes during early-stage basalt differentiation is still poorly understood. Here, we examine the possibility of chromite induced Mg isotope fractionation with a Mg isotopic study of chromite–olivine pairs in dunites and chromitites collected from major types of basaltic intrusive rock suites formed by fractional crystallization in different tectonic settings. The chromite δ26Mg (= [(26Mg/24Mg)sample / (26Mg/24Mg)DSM3–1] × 1000) values range from −0.19‰ to 0.30‰ in the Luobusa ophiolite, −0.09‰ to 0.78‰ in the Kızıldağ ophiolite, −0.04 to 0.42‰ in the Gaositai Alaskan-type complex, similar to those previously reported from the Stillwater layered intrusion (−0.05 to 0.84‰; Bai et al., 2021). They are significantly higher than those of coexisting olivine (δ26Mg = −0.48 to −0.10‰). The Δ26MgChr-Ol (= δ26MgChr − δ26MgOl) values in the rock suites investigated here fall largely between equilibrium fractionation lines of spinel–olivine pairs and magnesioferrite–olivine pairs, indicating equilibrium Mg isotopic fractionation. Furthermore, the Δ26MgChr-Ol values increase with decreasing Cr content of chromite in the dunites and chromitites, showing that high 26Mg has a greater affinity to Al-rich chromite than Cr-rich chromite. Fractional crystallization of such isotopically high chromite is expected to progressively lower the Mg isotope values of the in the remaining magma. Furthermore, continental basaltic magmas typically experience early crystallization of olivine and Al-rich chromite. Their δ26Mg values correlate positively with MgO (FeO, Cr,) and CaO/Al2O3 ratios and negatively with total alkali contents (Na2O + K2O). This indicates that detectable Mg isotopic fractionation occurred in intra-continental basalt magmas, probably by fractional crystallization of olivine and chromite. The observed low-δ26Mg intra-continental basalts can be accurately modeled by olivine + chromite fractionation with fractionation factors (Δ26MgChr-Melt) of 0.20‰, 0.60‰, and 1.18‰ as observed in the chromitites investigated during this study. Therefore, the early-stage basaltic melt differentiation involving separation of olivine and chromite may induce resolvable Mg isotopic fractionation, and the δ26Mg values of continental basalts should be used with caution in petrogenetic studies.

The structure and lithological sequences of ophiolites provide critical information for reconstructing their tectonic setting and thus, the history of the paleo-oceanic basins in which they formed. Our systematic mapping of three of the best-preserved massifs in the Xigaze ophiolite from the middle segment of the Yarlung Zangbo Suture Zone (YZSZ) in South Tibet, revealed a nearly intact structure of the ophiolitic sequence that has largely escaped emplacement and post-emplacement dismemberment. One of the most important results is the identification of a nearly E-W striking, serpentinized shear zone (SPSZ) preserved within the sequence. The SPSZ is rooted in the serpentinized front of the upper mantle and is mainly composed of meta-gabbroic and rodingite lenses floating in a serpentinite matrix. Extensive talc-serpentinite metasomatism, shearing and cataclastic deformation in the SPSZ reveal a complex interplay of fluid metasomatism and progressive ductile-brittle deformation in hundreds of meters of shear zone between the mantle and the crust. The structural and petrological features of the SPSZ are very similar to those of detachment faults within modern in situ oceanic core complexes (OCCs) in the Mid-Atlantic Ridge and the Southwest Indian Ridge. Zircon U Pb ages from the footwall of the fault, and the age of radiolarian chert overlying it suggest that the detachment faulting took place between ca. 125 to 123 Ma. Combined with the stratigraphic and geochronological relationships between the ophiolite and the overlying sediments, we suggest that the Xigaze ophiolite developed in the supra-subduction zone on the southern margin of the Lhasa terrane during rollback of the Neo-Tethyan oceanic slab. Recognition of detachment faulting in the Xigaze ophiolites, along with other features, suggest that the Xigaze ophiolite formed at a spreading ridge, whereas the other large ultramafic massifs in YZSZ, may have formed in an off-axis location on the slow-spreading ridge.

Olivine is a ubiquitous mineral in mafic-ultramafic rocks and has been widely used as a mineral marker in various geological processes. However, its development of trace elements is limited. Here we present newly-obtained trace element data 7Li, 27Al, 29Si, 31P, 43Ca, 45Sc, 49Ti, 51V, 53Cr, 55Mn, 59Co, 60Ni, and 66Zn of olivine in typical mantle xenoliths, mantle peridotites in ophiolites, and plutonic rocks from layered and Alaskan-type intrusions to develop trace element proxies for the petrogenesis, mineralization and discrimination of various mafic-ultramafic rocks. Residual olivine grains in mantle xenoliths and ophiolitic peridotites, which represent residues of mantle melting, have higher Ni/Co (>20) and Ni/Mn (>2) ratios than magmatic olivine (Ni/Co < 20, Ni/Mn < 2), which are consistent with the compatibilities of these elements during partial melting and magma differentiation. Lower Ni content, and lower Ni/Co and Ni/Mn ratios at a given Fo content can distinguish olivine in Alaskan-type intrusions from layered intrusions, reflecting the nature of their mantle sources. The V and Sc contents and V/Sc ratios in olivine can distinguish mantle xenoliths (V > 2 ppm, V/Sc > 0.5) from ophiolitic peridotites (V < 2 ppm, V/Sc < 0.5), indicating a more reduced state of continental lithospheric mantle compared to the oceanic lithospheric mantle. As a consequence, the four occurrences of mafic-ultramafic rocks can be distinguished by olivine with (Sc × 10)-(Ti × 2)-Zn and V/Sc-(Co/Ni × 2)-(Zn/Mn × 5) ternary diagrams. In addition, Li, Ti and P contents in olivine are good tracers of melt/fluid metasomatism, whereas Ni/Co, Ni/Mn and Mn/Zn ratios are indicators of chromite mineralization. Therefore, trace elements in olivine can be used as chemical proxies to distinguish the origin of various mafic-ultramafic rocks, as well as the processes by which they evolved.

Lode gold deposits, which are currently the world’s major gold supply, have been shown to be generated mostly by phase separation of metamorphic fluids and/or interaction between these fluids and wall rocks. Here we use garnet oxygen isotopes by secondary ion mass spectrometry to document the crucial role of magmatic hydrothermal fluids and their mixing with meteoric water in the formation of the world-class Dongping gold deposit in the North China Craton. Garnet grains from quartz veins of various paragenetic stages and the mineralized alteration envelope at Dongping have dynamic δ18O variations of 3.8 to −11.0‰, with large intragrain fluctuations up to 5.3‰. These values correspond to calculated δ18O values of 6.1 to −9.1‰ for the hydrothermal fluids from which the garnet formed. The isotope data, notably the cyclic alternation in δ18O within individual garnet grains, are best interpreted to reflect multiple pulses of magmatically derived fluids and subsequent mixing of each pulse with variable amounts of meteoric water. The results presented here allow us to quantify the significant interplay between magmatic hydrothermal fluids and meteoric water that spanned the entire mineralization history and triggered gold deposition of a lode gold deposit. This study highlights the potential use of in situ oxygen isotope analysis of garnet in tracing the origin and evolution of hydrothermal fluids in the Earth’s crust that may have formed other giant ore deposits.

Although the involvement of hydrous fluids has been widely invoked in formation of podiform chromitites in ophiolites, there is lack of natural evidence to signify the role and mechanism of fluids. In this study, a new model for the genesis of podiform chromitite is proposed on basis of revisits of comprehensive petrological, mineralogical and geochemical results of the well-preserved Kizildag ophiolite and the well-characterized Luobusa chromite deposit. In this model, ascending magmas intruding oceanic lithospheric mantle would presumably form a series of small magma chambers continuously connected by conduits. Tiny chromite nuclei would collect fluids dispersed in such magmas to form nascent droplets. They tend to float upward in the magma chamber and would be easily transported upward by flowing magmas. Chromite-rich droplets would be enlarged via coalescence of dispersed droplets during mingling and circulation in the magma chamber and/or transport in magma conduits. Crystallization of the chromite-rich liquid droplets would proceed from the margin of the droplet inward, leaving liquid entrapped within grains as precursor of mineral inclusions. With preferential upward transportation, immiscible chromite-rich liquids would coalesce to a large pool in a magma chamber. Large volumes of chromite would crystallize in situ , forming podiform chromitite and resulting in fluid enrichment in the chamber. The fluids would penetrate and compositionally modify ambient dunite and harzburgite, leading to significant fractionations of elemental and isotopic compositions between melts and fluids from which dunite and chromitite respectively formed. Therefore, fluid immiscibility during basaltic magma ascent plays a vital role in chromitite formation.

The new mineral jingsuiite (TiB2, IMA-2018-117b), together with osbornite-khamrabaevite solid solution (TiN-TiC), deltalumite, and a potential new mineral, hexagonal Ti10(Si,P,☐)7, constitute four inclusions up to 50 μm across in corundum recovered from the Cr-11 podiform chromitite orebody near Kangjinla, Luobusa ophiolite, Tibet, China. EELS, EDS, and 3D electron diffraction were applied to study the phases. In one inclusion, jingsuiite forms a rounded grain 40 μm across. Associated osbornite-khamrabaevite solid solution forms an irregular mass up to 10 μm across having the composition Ti(N0.5C0.5) and the Ti10(Si,P,☐)7 phase forms an incomplete overgrowth up to 20 μm thick around the grain of jingsuiite. In a second inclusion, jingsuiite, osbornite-khamrabaevite solid solution, Ti10(Si,P,☐)7 and deltalumite form a lamellar intergrowth 100 μm long composed of tablets of the four phases up to 50 μm long × 4 μm in thickness. Jingsuiite has a primitive hexagonal cell with a = 3.04(6), b = 3.04(6), c = 3.22(6) Å, α = 90°, β = 90°, γ = 120°, V = 25.8 (9) Å3, space group P6/mmm, Z = 1. Its structure was determined ab initio and dynamically refined on the basis of three-dimensional electron diffraction data; it is equivalent to that of synthetic TiB2. Results of EELS analyses of jingsuiite in foil no. 5357 (N = 20) gave B 61.87(1.22), C 1.53 (1.26), Ti 36.62 (1.45) at% from which an empirical formula of Ti1.10(B1.86C0.05)Σ1.91 was calculated on the basis of 3 atoms. The ideal formula is TiB2. Our preferred scenario is that corundum with entrapped Ti-Si-P-Fe intermetallic melts was precipitated from basaltic magmas during exhumation following deep subduction. Enrichment of B in the melt pockets is attributed to the highly reducing conditions that led to the segregation of siderophile elements into intermetallic melts and to the siderophile behavior of B, thereby concentrating it in the intermetallic melts in preference to silicate melt. Experimental work on the Ti-Fe-Si system indicates that minerals enclosed in corundum grains such as Ti, FeTiSi2, and TiSi2 could have crystallized from alloy melts at the lowest T accessible on the liquidus, i.e.

In this study, we report systematically field observations, geochronology, whole-rock geochemistry and Sr-Nd-Hf isotopic dataset on the various types of magmatic rocks collected from the Zedang area. Chemically, the diabase and gabbro have a low-K calc-alkaline affinity, whereas the basalt and plagiogranite have medium to high-K calc-alkaline characteristics. In addition, the basalts are highly enriched in light rare earth elements (LREE) and large ion lithophile elements (LILE), but strongly depleted in high strength field elements (HFSE), indicating that their magma source probably was derived from a subduction- or arc-related setting. In contrast, both the gabbro and diabase mainly demonstrate an N-MORB-like affinity consistent with normal mid-oceanic ridge basalt (N-MORB) origin. The zircon U-Pb dating results suggest that the basalts were crystallized earlier at ca. 158–161 Ma (Oxfordian stage), but the gabbro was crystallized at ca. 131 Ma (Hauterivian stage of Early Cretaceous). The zircon U-Pb dating results correspond with the field observations that the veins of gabbro intruded basalt. Furthermore, the plagiogranite has a weighted mean age of ca. 160 Ma (MSWD=2.1) consistent with the basalt within the uncertainty. The basalt and the plagiogranite have significantly positive eHf(t) values (+5.8 to +15.6 and +8.6 to +16.1, respectively), suggesting that they were originated from partial melting of a depleted source. However, basalt and plagiogranite are characterized by the wide variations of eHf(t) values indicating minor amounts of exotic crustal material input during the later magma evolution. Additionally, the basalt shows duplex geochemical features of island-arc and mid-oceanic ridge basalt, corresponding to the supra-subduction zone-(SSZ) type affinity. To sum up, two distinct magmatic events identified in this study probably suggest an intra-oceanic arc system existing in the Zedang area during the Late Jurassic, but the intra-oceanic arc subduction extinguished in the Early Cretaceous as suggested by the N-MORB-like gabbro and diabase. Integrated with regional background and different rock types, as well as geochemical features, we conclude that intra-oceanic arc subduction setting developed during the Late Jurassic in the Zedang area, southern Tibet.

Combined Fe isotopic and olivine fabric analyses have been conducted on the nodular chromitites from the Luobusa ophiolite, Tibet, to study the physico-chemical effects of plastic flow in modifying chromitites and their associated mantle peridotites below spreading centers. The Luobusa nodular chromitites are generally composed of fresh dunitic matrices and well-aligned chromite nodules. Olivine grains in both the dunitic matrices and chromite nodules underwent pressure solution, and those in the dunitic matrices have prevalent Fe-rich stripes parallel to their kink bands and sometimes show lattice dislocation-induced preferred orientations of (1 0 0). Chromite grains in the chromite nodules have Cr# ∼ 75–80 and δ56Fe values from −0.122 to 0.067. The Fo and δ56Fe values of olivine in the dunitic matrices vary from 96 to 99 and 0.165 to 0.294, respectively, distinguishably higher than those of olivine in ordinary ophiolitic dunites (mostly 90–95 and 0.050–0.150). The calculated Mg-Fe exchange temperatures between olivine in the dunitic matrices and chromite in the chromite nodules range from 700 to 900 °C, overall higher than those for ordinary dunites and chromitites (∼600–800 °C). Combined with numerical modeling of diffusion of Fe in olivine, our study reveals that the abnormally high Fo and δ56Fe values of olivine in the dunitic matrices of nodular chromitites were not controlled by their parental magmas but caused by significant loss of Fe from olivine under subsolidus conditions. Intensive plastic deformation in nodular chromitites largely promoted the migration of Fe from olivine and facilitated the Mg-Fe exchange between the chromite nodules and dunitic matrices, maximizing the Fo and δ56Fe values of olivine. However, such an effect is expected to be negligible in chromite-barren dunites and harzburgites because such rocks do not have enough chromite to generate effective chemical-potential gradients for Fe migration. The high Fo values of olivine in nodular and massive chromitites require fast cooling rates (∼0.05–0.1 °C/yr) in the surrounding mantle, consistent with settings below intermediate-fast spreading ridges. Boudinization of small chromitite bodies in less competent dunites under 900–1200 °C is a potential mechanism for generating the nodular texture, also accounting for the en-echelon or bead-like occurrences of chromitite bodies in ophiolites. Effective mantle flow below spreading centers plays an important role in shaping the textures and compositions of mantle rocks, explaining the absence of deformation-related nodular chromitites in layered mafic-ultramafic intrusions.

s of the International Symposium on Deep Earth Exploration and Practices, 1–5 Nov 2021, Nanjing, China

Chromitites in the Kizildag ophiolite mostly occur in the mantle harzburgites and some of them in crustal cumulate dunites. Olivine in both the dunites and chromitites is highly magnesian (Fo = 90.5–92.8 and 90.2–96.1, respectively), while the coexisting chromite varies widely in composition (Cr# = 52.9–68.6 and 57.1–76.2, respectively), placing it between high-Al and high-Cr varieties. Concentrations of Ni, V and Ga in the chromite (Ni: 741–1310 ppm; V: 521–1227 ppm; Ga: 20–51 ppm) are also highly variable, straddling the range of high-Al and high-Cr chromite. Olivine in the dunites and chromitites has a large range of Li concentrations (0.27–2.47 ppm) and δ7Li values (−7.20–29.90‰). Based on the calculated Al2O3 (12.1–17.2 wt.%) contents, FeO/MgO values (0.92–1.57) and trace element concentrations of the parental magmas of the dunites and chromitites, it is suggested that they are transitional between MORB and boninitic melts, and should have been derived by mixing of geochemically and spatially distinct magmas generated by partial melting of different sources. The Li isotopic compositions of olivine from the dunites and chromitites (except KZ14-38 and KZ15-38) are neither controlled by diffusion nor fractional crystallization, and thus may be intrinsic features of their parental melts. The range of δ7Li values of olivine in the dunites and chromitites is analogous to that of arc lavas and marine sediments, indicating that the chromitites record various degrees of melt penetration. The wide range of Li isotopic variations in olivine and chemistry in chromite from the Kizildag chromitites suggest that the parental melts were originated in a proto-forearc mantle during subduction initiation.

The Li-O isotopes of olivine and pyroxene from the mantle sequence of the Luobusa ophiolite, a fragment of Neo-Tethyan forearc lithosphere in Tibet, reveal a series of magmatic processes and geochemical evolution in a nascent mantle wedge during subduction initiation. Olivine grains from the ophiolitic mantle sequence display δ18O values similar to those of normal mantle (+5.18 ± 0.2‰), but have large δ7Li variations, ∼+4‰ to +13‰ in harzburgites and ∼+1‰ to +11‰ in high-Cr chromitites. Clinopyroxene grains in cpx-bearing harzburgites have finger-like protrusions and LREE-depleted patterns comparable to those of clinopyroxene in abyssal peridotites, and also show high Li concentrations (∼2–8 μg/g) and negative δ7Li values (∼−15‰ to −8‰), which are beyond the ranges of normal upper mantle (

The discovery of ultrahigh pressure (UHP) minerals in orogenic belts has revolutionized our understanding of subduction and the return flow of previously deeply subducted material to Earth's surface as part of the cycling and interaction of crustal and mantle systems. One class of UHP minerals is found as inclusions in orogenic peridotite-hosted podiform-chromite systems, within Phanerozoic ophiolites and ophiolitic melanges. Such inclusions have opened a window into processes of recycling of crustal materials to the deep mantle and back through subduction and mantle convection in Phanerozoic orogens. Here, we document the first occurrence of an UHP mineral hosted in an ophiolitic podiform chromitite melange from the Neoarchean paired metamorphic belt of the Central (Taihang) Orogenic Belt, Northern China. Numerous inclusions of rutile, apatite, dolomite, and amphibole are interpreted to be crustal-derived; they occur in podiform chromite grains hosted in a 2.6–2.5 Ga ophiolitic melange now part of the North China Craton and formed by subduction of oceanic and continental material. Microstructures and phase relationships in a multi-phase inclusion of TiO2(II), rutile, apatite, and tremolite yield minimum P-T conditions of 7.5 GPa at 1000 °C, indicating that the crustal host, including carbonates, were subducted to depths > 270 km, transferred to the mantle of the overriding plate, and returned to the surface by 2.5 Ga. We suggest that slab rollback forced upward mantle flow, incorporating entities from the lower plate, perhaps in serpentinite diapirs, resulting in adiabatic melting that allowed crustal material to be trapped in chromite grains crystallizing in high-Mg melts. Contrasting bulk moduli and thermal contraction of the inclusions and host chromite protected the inclusions from P-induced back-reaction during exhumation. Together, these features show that the 1600 km long Central (Taihang) Orogenic Belt is emerging as the world’s first well-documented Phanerozoic style orogen, with classic tectonic zonation, ophiolitic melanges, paired metamorphism, local evidence for UHP conditions, foreland basins, and late to post orogenic magmatism. Applying the null hypothesis, we explain this high degree of similarity by invoking the operation of Phanerozoic style plate tectonics, at least throughout the 1600 km length of the COB, and by geological comparison, in other similar aged geological terrains globally. From this we infer modern-style plate tectonics was operating in the Neoarchean.

The Xigaze ophiolite, which lies in the middle section of the Yarlung Zangbo suture zone (YZSZ) of southern Tibet, is composed from the base upward of tectonized harzburgites with minor dunites, and a magmatic sequence of wehrlites, pyroxenites, troctolites and gabbros. Both abundant high-Cr, metallurgical and sparse, dunites occur within the harzburgites. The peridotites show abundant evidence of mantle melt extraction at various scales as indicated by the Fo values of olivine (92.4–93.5) in the harzburgites and 92.8–94.0 in the dunites. Whole-rock MgO contents are 41–50 wt%, similar in composition to forearc peridotites. Most of the chromitites have high Cr# numbers (61–81), although there are systematic changes in olivine and chromian spinel compositions from harzburgites, to dunites to chromitites, reflecting variable partial melting and melt-rock reaction. The primitive mantle U-shaped rare earth element (REE) patterns of the harzburgites indicate refertilization of depleted peridotites by suprasubduction melts. The PGE abundances in the Xigaze rocks provide additional constraints on their geochemical evolution and show multiple episodes of melt extraction from the mantle source, giving it a subchondritic character. Compositional zoning of minerals likewise attest to modification by migrating melts, formed in a suprasubduction zone mantle wedge.

Chromian-spinel (chromite) is a common mineral in ophiolitic rocks and the study of chromite from the mantle sections of ophiolites can help shed light on their petrogenetic origin and tectonic setting. The Proterozoic Miaowan Ophiolite Complex (MOC) in the Yangtze Craton contains disseminated chromite grains in mantle harzburgites and podiform chromitites associated with serpentinised dunites. Most chromite grains display compositional zoning due to alteration but the fresh cores preserve primary igneous compositions. Podiform chromitites in the MOC dunites and harzburgites are compositionally similar to typical ophiolitic chromitites elsewhere. The chromite grains contain numerous inclusions of clinopyroxene and amphibole with minor amounts of olivine, chlorite, base-metal sulphides (BMS) and platinum-group minerals (PGM). The abundant hydrous mineral inclusions within the chromite grains suggest a hydrous mantle source. Core compositions of chromite grains indicate that the parental melts of the chromitites were similar to typical boninitic melts in a forearc setting. However, chromite grains in the harzburgites show mixed MORB and arc signatures. Thus, the mineralogy and geochemistry of the MOC peridotites suggest that the chromitites in the MOC formed in a forearc tectonic setting during reaction between boninitic melts and MORB-type harzburgite in a supra-subduction zone (SSZ) mantle wedge.

Titanium minerals enclosed in corundum separated from the Cr-11 orebody include native Ti, zamboite (FeTiSi 2 ), osbornite (TiN)-khamrabaevite (TiC) solid solutions, and jingsuiite (TiB 2 ), as well as the new minerals badengzhuite (TiP) and zhiqinite (TiSi 2 ) and two potentially new minerals, Ti 11 (Si,P )10 and Ti 10 (Si,P, □)7 , where □ indicates a vacancy. These minerals together constitute a spheroid 20 µ m across inferred to have crystallized from a droplet of Ti–Si–P intermetallic melt. Energy-dispersive spectroscopy and three-dimensional electron diffraction were applied to characterize the two new minerals. Badengzhuite has a primitive hexagonal cell with a=3.49(7) A, c=11.70(23) A, V=124(4) A 3 , and crystallizes in space group P 6 3∕mmc ( Z=4 ). It is isostructural with synthetic TiP. Two EDX (energy dispersive X-ray spectroscopy) analyses of badengzhuite gave 60.56 wt %Ti and 39.44 wt % P and 62.74 wt % Ti and 37.26 wt % P from which an empirical formula of Ti 1.020 P 0.980 was calculated on the basis of two atoms (ideally TiP). Zhiqinite has a primitive orthorhombic cell with a=8.18(16) A, b=4.85(10) A, c=8.42(17) A, V=334(12) A 3 , and crystallizes in space group Fddd ( Z=8 ). It is isostructural with synthetic TiSi 2 (C54 type). Four EDX analyses of zhiqinite gave 39.58–44.79 wt % Ti and 55.21–60.42 wt % Si, from which an empirical formula of Ti 0.905 Si 2.095 was calculated on the basis of three atoms (ideally TiSi 2 ). We suggest that interaction of mantle-derived CH 4 + H 2 fluids with basaltic magmas in the shallow lithosphere (depths of ∼ 30–100 km) under conditions more reducing than 6 log units below the oxygen fugacities corresponding to the iron–wustite buffer resulted in precipitation of corundum that entrapped intermetallic melts, some of which crystallized to ultra-reduced Ti–P–Si phases. Experimental work on the Ti–Si and Ti–P systems indicates that the minerals enclosed in corundum could have crystallized from the alloy melt at the lowest temperature accessible on the liquidus. It has been alleged that these ultra-reduced phases are anthropogenic contaminants inadvertently introduced with fused alumina abrasive during preparation of mineral separates. Nonetheless, we conclude that the differences between the ultra-reduced minerals in the separates and the ultra-reduced phases in fused alumina are more convincing evidence for these minerals having a natural origin than the similarities between them are evidence for an anthropogenic origin.

We present petrological investigations and mineral chemistry of several Tethyan ophiolites to reveal the occurrence, origin, and fate of water in podiform chromitites. The results show that clinopyroxene and olivine in chromitites have H2O contents of 801–366 and 53–17 ppm, respectively. The highest water contents of olivine occur in massive chromitite and the lowest always in the clinopyroxene-bearing ores because much of the available hydrous fluids was taken up by the clinopyroxene during crystallization. The major and trace elemental and Li isotopic compositions of clinopyroxene associated with chromite and olivine in podiform chromitites indicate formation from a mixture of surface hydrous fluids on chromite grains and evolved melts from which olivine crystallized. The hydrous fluids initially originated from dehydration of a subducting slab as revealed by Li isotopic compositions of clinopyroxene and olivine in the chromitites. High fluid/rock ratios facilitated concentration of chromite to form chromitite, suppressing crystallization of olivine. The hydrous fluids that were collected on the chromite grain surface during crystallization allowed chromite grains to rise via decreasing density in the form of bubbles, thus promoting their gathering and concentration. The fate of these hydrous fluids depends on ambient physical and chemical conditions. Mostly they hydrate adjacent olivine grains in the chromitite or penetrate the surrounding dunite envelope. In some cases, the fluids dissolve into silicate melts to produce water-bearing clinopyroxene and/or hydrous minerals, such as amphibole, or infiltrate silicate and chromite grains to form inclusions, which may exsolve later in the form of mineral lamellae. Our investigations provide direct natural evidence for the presence and importance of water in the formation and evolution of chromite deposits, as inferred by earlier experimental studies.

To investigate the petrogenesis of cyclic units in layered intrusions, we examined chromitite, dunite, poikilitic harzburgite and bronzitite from the Ultramafic Zone of the Stillwater Complex and measured stable isotopes of Li and O in their major minerals. The Li isotopes in olivine range from 4 to 26‰ in δ7Li with uniform Li contents of 1–3 ppm, whereas orthopyroxene and clinopyroxene have Li contents of 0.5–5 ppm and 4–8 ppm, and δ7Li ranges of −13 to 7‰ and −14 to −6‰, respectively. The δ18O values vary from 4.91 to 5.72‰ in olivine, from 5.11 to 5.87‰ in orthopyroxene, and from 4.64 to 5.86‰ in clinopyroxene. For a given sample, olivine displays more variable and higher δ7Li but lower δ18O values than orthopyroxene, indicating that olivine experienced more extensive compositional modification after crystallization relative to orthopyroxene. The general Li and O isotopic compositions are interpreted as the result of re-equilibration between interstitial liquids, from which pyroxenes crystallized, and cumulus minerals. The inter-mineral and inter-sample isotopic variations correlate with mineral assemblages, crystal sizes and major and trace element compositions, revealing that the interstitial liquids varied compositionally mainly due to mixing between fractionated magma and newly injected primitive magma. Abrupt mineralogical and geochemical changes from silicate rocks to chromitites imply that hydrous fluids, which collected on chromite surfaces and were later released from chromite seams, played an additional, critical medium of chemical exchange between minerals in the chromitites.

This paper presents the first detailed geologic map of in situ lower ocean crust; the product of six surveys of Atlantis Bank on the SW Indian Ridge. This combined with major and trace element compositions of primary magmatic phases in 99 seafloor gabbros shows there are both significant vertical and ridge-parallel variations in crustal composition and thickness, but a continuity of the basic stratigraphy parallel to spreading. This stratigraphy is not that of magmatic sedimentation in a large crustal magma chamber. Instead, it is the product of dynamic accretion where the lower crust formed by episodic intrusion, large-scale upward migration of interstitial melt due to crystal mush compaction, and continuous tectonic extension accompanied by hyper- and sub-solidus, crystal-plastic deformation.Five crossings of the gabbro-peridotite contact along the transform wall show that massive mantle peridotite is intruded by cumulate residues of moderately to highly evolved magmas, few of which could be even close to equilibrium with a primary mantle magma. This contact then does not represent the crust-mantle boundary as envisaged in the ophiolite analog for ocean crust. The residues of the magmas parental to the shallow crust must also lie beneath the center of the complex. This, and the nearly complete absence of dunites in peridotites from the transform wall, shows that melt transport through the shallow lithosphere was largely restricted to the central region of the paleo-ridge segment.There is almost no evidence for a melt lens or high-level storage of primitive melt in the upper 1500 m of Atlantis Bank. Thus, the composition of associated mid-ocean ridge basalt appears largely controlled by fractional crystallization of primitive cumulates at depth, near or at the base of the crust, modified somewhat by melt-rock reaction during transport through the overlying cumulate pile to the seafloor.Inliers of the dike-gabbro transition show that the uppermost gabbros crystallized at depth and were then emplaced upward, as they cooled, into the zone of diking. ODP and IODP drilling along the center of the gabbro massif also found few primitive gabbros that could have been in equilibrium with the original overlying lavas. Evidence of large-scale upward, permeable transport of interstitial melt through the gabbros is ubiquitous. Thus, post-cumulus processes, including extensive reaction, dissolution, and re-precipitation within the cumulate pile have obscured nearly all evidence of earlier primitive origins. We suggest that many of the gabbros may have started as primitive cumulates but were hybridized and transformed by later, migrating melts to evolved compositions, even as they ascended to higher levels, while new primitive cumulates were emplaced near the base of the crust. Mass balance for a likely parental melt intruded from the mantle to form the crust, however, requires that such primitive cumulates must exist at depth beneath Atlantis Bank at the center of the magmatic complex.The Atlantis Bank Gabbro Massif accreted by direct magma intrusion into the lower crust, followed by upward diapiric flow, first as a crystal mush, then by solid-state, crystal-plastic deformation, and finally by detachment faulting to the sea floor. The strongly asymmetric spreading to the south, parallel to the transform, was due to fault capture, with the bounding faults on the northern rift valley wall cut off by the detachment fault, which extended across the zone of intrusion causing rapid migration of the plate boundary to the north.

It is becoming commonplace for companies to contribute to open source software (OSS) projects. At the same time, many software organizations are applying Scrum software development practices, for productivity and quality gains. Scrum calls for self-organizing teams, in which the development team has total control over its development process. However, OSS projects typically have their own processes and standards, which might not mesh well with a company's internal processes, such as Scrum. This paper presents an experience report from Sony Interactive Entertainment (SIE), where the "toolchain CPU compiler" team directly participates in the "LLVM" OSS project. The team ran into a number of difficulties when using Scrum to manage their development. In particular, the team often failed to complete Scrum sprints where tasks required interaction with the open source community. We look at how the team redefined task flows to alleviate these difficulties, and eventually evolved a highly modified process, dubbed TWINS (This Workflow Is Not Scrum). We assess the revised process, and compare it to other established agile methods, finding it bears a strong resemblance to Scrumban (the SIE team was not aware of Scrumban previously). The TWINS framework presented here may help other organizations who develop software in-house and engage in OSS projects, to gain the best of both worlds.

On the basis of their mineral chemistry, podiform chromitites are divided into high-Al (Cr# = 20–60) (Cr# = 100 ∗ Cr/(Cr + Al)) and high-Cr (Cr# = 60–80) varieties. Typically, only one type occurs in a given peridotite massif, although some ophiolites contain several massifs that can have different chromitite compositions. We report here the occurrence of both high-Cr and high-Al chromitite in a single massif in China, the Dongbo mafic-ultramafic body in the western Yarlung-Zangbo suture zone of Tibet. This massif consists mainly of mantle peridotites, with lesser pyroxenite and gabbro. The mantle peridotites are mainly composed of harzburgites and minor lherzolites; a few dike-like bodies of dunite are also present. Seven small, lenticular bodies of chromitite ores have been found in the harzburgites, with ore textures ranging from massive through disseminated to sparsely disseminated; no nodular ore has been observed. Individual chromitite pods are 1–3 m long, 0.2–2 m wide and strike NW, parallel to the main trend of the peridotites. Chromitite pods 3, 4, and 5 consist of high-Al chromitite (Cr# = 12–47), whereas pods 1 and 2 are high-Cr varieties (Cr# = 73 to 77). In addition to chromian spinel, all of the pods contain minor olivine, amphibole and serpentine. Mineral structures show that the peridotites experienced plastic deformation and partial melting. The mineralogy and geochemistry of the Dongbo peridotites suggest that they formed originally at a mid-ocean ridge (MOR), and were later modified by suprasubduction zone (SSZ) melts/fluids. We interpret the high-Al chromitites as the products of early mid-ocean ridge basalt (MORB) or arc tholeiite magmas, whereas the high-Cr varieties are thought to have been generated by later SSZ melts.

Cenozoic mafic rocks in Jiangsu and Anhui Provinces, east-central China are chiefly basanites and alkali olivine basalts with subordinate tholeiites, which were erupted in three stages; Paleogene, Neogene and Quaternary. The rocks become increasingly alkaline as they become younger. On a primitive mantle-normalized multi-element plot, these lavas exhibit typical OIB-like trace element patterns, including enrichment in most incompatible elements (LILE and HFSE) and negative K and Pb anomalies. The compositions of the mafic rocks indicate that they were derived from a mantle source mainly containing clinopyroxene and garnet, most probably a mixture of pyroxenite/eclogite and peridotite. A mineral equilibrium projection shows that all the mafic magmas were produced at pressures of 3–4 GPa, implying an asthenospheric origin. Their positive Ba and Sr anomalies and relatively high 87 Sr/ 86 Sr ratios suggest derivation from an EM1-type mantle source. However, poor correlations between 87 Sr/ 86 Sr and 143 Nd/ 144 Nd indicate an isotopically heterogeneous source for the magmas, including DMM, EM1 and EM2, representing mantle peridotite, recycled ancient oceanic crust and seafloor sedimentary rocks, respectively. Variable correlations between 87 Sr/ 86 Sr and 143 Nd/ 144 Nd ratios, CaO–MgO contents and Eu/Eu* and Ce/Ce* anomalies with rock type imply that marine sediments (plus variable amounts of oceanic crust) and peridotites were the dominant source lithologies of the basanites, whereas recycled oceanic crust (pyroxenite/eclogite) was the main source of the weakly alkaline basalts. This hypothesis is supported by seismic tomographic images of the mantle beneath the region, which show the presence of a stagnant subducted slab in the mantle transition zone. Thus, we propose a petrological model in which a hybrid magma column originated from the mantle transition zone and assimilated some of the overlying peridotite during upwelling, to become the parental magmas of these mafic rocks. This process may relate to the documented lithospheric thinning in East China in the Mesozoic and Cenozoic.

The discovery of diamonds and highly reduced minerals in podiform chromitites, which have generally been interpreted as magmatic rocks formed from partial melts of upper mantle peridotites under low-pressure conditions, has raised many questions about the origin of these enigmatic bodies. In order to provide experimental constraints on the formation and emplacement of podiform chromititesin ophiolites, we carried out a number of multi-anvil experiments in the magnesiochromite + SiO 2 system at temperatures of 1000–1600 °C and pressures of 5–15 GPa. The experimental results demonstrate that magnesiochromite is stable up to 14 GPa and decomposed into eskolaite (Cr 2 O 3 ) together with a quenchable modified ludwigite-structured phase [(Fe, Mg) 2 (Al, Cr) 2 O 5 ] at higher pressures, thus placing an approximate maximum depth for chromite crystallization and/or metamorphism. This depth corresponds to the top of the mantle transition zone (MTZ) at 410 km. The ludwigite-structured post-chromite phase has significant implications for understanding phase transformations and Cr incorporation/partitioning of minerals in the MTZ. On the basis of our results, we propose a multi-stage model for the formation of podiform chromitites that incorporates the geochemical, textural and mineralogical features of these bodies.

Models for deep derivation of ophiolitic chromite from the mantle transition zone versus magmatic crystallization at lithospheric level are now highly controversial due to the discovery of ultrahigh-pressure (UHP) and super-reduced (SuR) minerals in ophiolitic chromitites and the presence of silicate mineral exsolution lamellae in chromite grains. According to experimental studies, chromite grains crystallized at low-pressures cannot exsolve silicate phases because of their low contents of CaO and SiO2. However, clinopyroxene needles in chromite grains from the Stillwater Complex, which is a typical layered mafic-ultramafic intrusion crystallized at crustal level, show morphological, compositional and crystallographic features very similar to exsolution lamellae in ophiolitic chromite grains. We propose that the Stillwater clinopyroxene ‘lamellae’ represent minute melt inclusions that were attached parallel to branches of skeletal chromite where they were trapped during continued growth of the grain. This interpretation is supported by the orientations of the lamella-like clinopyroxene and the coexistence of larger clinopyroxene inclusions and apatite crystals along with the clinopyroxene needles. Another possibility is that these lamella-like minerals precipitated from melt/fluid that filled cracks in the chromite grains. Trails of tiny inclusions preserved in some healed fractures are consistent with this mechanism, but are inconsistent with the regular orientations of most of the needle-like ‘inclusions’. The presence of such features in the Stillwater chromitites raises serious questions regarding the origin and significance of silicate lamellae in ophiolitic chromite grains and the use of such features to determine the depth of crystallization.

Ophiolitic chromitites in the eastern sector of the Alpine-Himalayan and Polar Ural orogenic belts preserve evidence of formation under ultra-high-pressure (UHP), deep mantle conditions. The textural and compositional characteristics of microscopic and submicroscopic inclusions in magnesiochromite from the Ray-Iz ophiolites are considered as clues of their complex evolutionary history. Bulk-rock platinum-group-element (PGE) compositions of refractory and metallurgical chromitite ores (102–309 ppb) with generally IPGE>PPGE are consistent with supra-subduction zone (SSZ) metasomatism. The abundant unusual UHP phases (e.g., micro-diamonds, moissanite, coesite…etc.), together with clinopyroxene lamellae, and globular silicate inclusions in the Cr-spinel are seen as indications of deep mantle recycling of the mantle section of the Ray-Iz ophiolites. The calculated fO2 values for dunite and chromitite (+1.17 to +4.16, generally above the FMQ buffer) are in line with interaction of mid-ocean ridge (MOR) or back-arc ophiolites with oxidized, Mg-rich silicic (boninitic) melts in a SSZ environment.

The petrography and compositions of chromitites and peridotites from the Tibetan Luobusa ophiolite, a fragment of Neo-Tethyan forearc lithosphere, are used to study the origin of high-Cr chromite deposits and relevant magmatic evolution in nascent mantle wedges during subduction initiation. The mantle sequence of the ophiolite has a two-layered structure with cpx-poor and cpx-rich harzburgites dominating the upper and lower layers, respectively, and locally contains dunite and chromitite bodies in the upper layer. The cpx-rich and cpx-poor harzburgites contain clinopyroxene having finger-like protrusions and chromite showing both irregular and euhedral shapes, and display spoon-shaped and U-shaped REE patterns, respectively. The dunites and chromitites followed a crystallization sequence Ol-Chr-Cpx and present spoon-shaped to U-shaped REE patterns. Clinopyroxene grains in the cpx-rich harzburgites have major and REE compositions slightly more depleted than those in abyssal peridotites, whereas clinopyroxene grains in the cpx-poor harzburgites and dunites have boninitic affinity. Chromite grains in the chromitites and cpx-rich harzburgites have the highest and lowest Cr#s [100 × Cr/(Cr + Al)] (75–80 vs. 20–40), respectively, whereas those in the cpx-poor harzburgites and dunites have intermediate Cr#s. The dataset above indicates that the Luobusa harzburgitic mantle wedge was once modified by slab-derived fluids and later refertilized by melts with compositions intermediate between MORB and boninitic lavas. Under high geothermal conditions induced by slab rollback and asthenospheric upwelling, subsequent partial melting in the refertilized H2O-bearing mantle formed high-Ca boninitic to arc picritic melts, which reacted with the overlying cpx-rich harzburgites and generated high-Cr chromitites, dunites and cpx-poor harzburgites under conditions of diminishing melt/rock ratios. Chromitites and peridotites in mantle sequences of forearc ophiolites keep important information for reconstructing the geodynamic evolution of subduction initiation.

The Jijal Complex, one of the largest Neo-Tethyan ophiolites (ca. 150 km2) in Pakistan, occupies a deep-level section of the Cretaceous Kohistan Arc that was obducted along the Main Mantle Thrust or Indus Suture Zone. Peridotites of the Jijal Complex consist mainly of harzburgite and dunite with numerous podiform chromitites. In this paper, we present new mineral compositions, whole-rock major elements, trace elements, rare earth elements (REEs) and platinum group elements (PGEs) to evaluate the melt evolution and petrogenesis of these rocks. Chromian spinel in the chromitites is fairly uniform in composition with Cr#s ranging from 75 to 81, Mg#s from 59 to 63 and TiO2

To investigate the factors controlling the mineralization in ophiolites we systematically compared the petrology and mineral compositions of the harzburgites/lherzolites, dunites and chromitites in the Luobusa and Purang ophiolites. Generally, the petrological features and trace element compositions of chromite and olivine in peridotite and chromitite are distinctly different between the two ophiolites. In Luobusa, boninitic melts are inferred to have interacted with the harzburgites and modified the distributions of some trace elements (e.g., Ni, Mn and V) in chromite and olivine. The subsequently formed dunites and chromitites experienced significant elemental exchange. In contrast, the Purang ophiolite contains a wider range of chromitite compositions and records diverse melt activities, such as the growth of relatively abundant secondary clinopyroxene. The metasomatic melts were enriched in Al and depleted in Si, Na and highly incompatible trace elements (e.g., Nb, Zr). Such melts resemble MORB-like melts proposed in the literature but are assumed to be more hydrous than typical MORB because of presence of hydrous minerals. The parental magmas of the Purang dunites and intermediate chromitites are inferred to be compositionally intermediate between boninitic and MORB-like melts. In addition, the more refractory nature of the Luobusa harzburgites facilitated a high Cr concentration gradient with the interacting melts, making it easier to increase Cr in the melts. Crystallization of clinopyroxene and amphibole in the Purang ophiolite accommodated significant amounts of Cr and water, respectively, and negatively affected Cr concentration and chromite crystallization. The concentration of chromite to form chromitites requires the presence of focused melt channels.

We document in this study the geological occurrence of diamonds and other ultrahigh-pressure (UHP) minerals in ophiolitic mantle peridotites and podiform chromitites from different orogenic belts. These minerals exist in both high-Cr and high-Al chromitites. Most ophiolite-hosted diamonds are small (∼ 200-500 μm across), and some contain distinctive inclusions (i.e., coesite, Ni-Mn-Co alloys, spessartite, tephroite). All of the analyzed diamonds have extremely light carbon isotope compositions (δ 13 C = -28.7 to -18.3‰) and variable trace element contents that distinguish them from most kimberlitic and UHP metamorphic varieties. A wide range of highly reduced minerals, such as native elements, Ni-Mn-Co alloys, Fe-Si and Fe-C phases and moissanite (SiC) also occuras accompanying mineral separates confirming the super-reducing conditions of their environment of formation. The presence of exsolution lamellae of diopside and coesite in some chromite grains suggests chromite crystallization depths around >380 km, near the mantle transition zone. Carbon and other recycled crustal materials at these depths are likely to have been derived from previously subducted material. The peridotites encapsulating the podiform chromitites and diamonds were transported to shallow mantle by convection cells beneath oceanic spreading centers. The chromitites may have formed in the deep mantle or in shallow suprasubduction zone environments. Our observations suggest that diamonds, UHP minerals and recycled crustal material are likely to be ubiquitous in the oceanic mantle.

The ultramafic massif of Bulqiza, which belongs to the eastern ophiolitic belt of Albania, is a major source of metallurgical chromitite ore. The massif consists of a thick (> 4 km) sequence, composed from the base upward of tectonized harzburgite with minor dunite, a transitional zone of dunite, and a magmatic sequence of wehrlite, pyroxenite, troctolite and gabbro. Only sparse, refractory chromitites occur within the basal clinopyroxene-bearing harzburgites, whereas the upper and middle parts of the peridotite sequence contain abundant metallurgical chromitites. The transition zone dunites contain a few thin layers of metallurgical chromitite and sparse bodies are also present in the cumulate section. The Bulqiza Ophiolite shows major changes in thickness, like the 41–50 wt.% MgO composition similar with forearc peridotite as a result of its complex evolution in a suprasubduction zone (SSZ) environment. The peridotites show abundant evidence of mantle melt extraction at various scales as the orthopyroxene composition change from core to rim, and mineral compositions suggest formation in a forearc, as Fo values of olivine are in 91.1–93.0 harzburgite and 91.5–91.9 in dunite and 94.6–95.9 in massive chromitite. The composition of the melts passing through the peridotites changed gradually from tholeiite to boninite due to melt–rock reaction, leading to more High Cr# chromitites in the upper part of the body. Most of the massive and disseminated chromitites have high Cr# numbers (70–80), although there are systematic changes in olivine and magnesiochromite compositions from harzburgites, to dunite envelopes to massive chromitites, reflecting melt–rock reaction. Compositional zoning of orthopyroxene porphyroblasts in the harzburgite, incongruent melting of orthopyroxene and the presence of small, interstitial grains of spinel, olivine and pyroxene likewise attest to modification by migrating melts. All of the available evidence suggests that the Bulqiza Ophiolite formed in a suprasubduction zone mantle wedge.

Cenozoic continental basalts from Zhejiang Province, southeast China are tholeiitic to weakly alkalic in composition, with moderate MgO contents (6–11 wt.%) and an average Mg # of 62. They display typical OIB-like trace element features, including enrichment in most incompatible elements, both LILE and LREE, and negative K, Pb, Zr, Hf anomalies. In particular, they are characterized by high Fe/Mn (73 ± 5), La/Yb (19 ± 6) and Nb/Ta (18.8 ± 0.4) ratios, which can be attributed to the presence of residual clinopyroxene, garnet and rutile in the mantle source. Based on these minerals, the following hybrid source rocks are hypothesized: garnet pyroxenite/eclogite and peridotite. Clinopyroxene–liquid thermobarometry indicates clinopyroxene crystallization temperatures of > 1257 °C. This is higher than the assumed temperature at the base of the sub-continental lithospheric mantle (SCLM) (~ 1220 °C) beneath Zhejiang, thus the magmas were presumably derived from the asthenosphere. Some typical geochemical features such as negative K, Pb anomalies, positive Ba, Sr, Nb, Ta anomalies and the extremely high Os isotopic signatures, suggest participation of EM-like mantle sources, indicative of ancient subducted oceanic crust. ( 87 Sr/ 86 Sr) i (0.7037–0.7046) and 143 Nd/ 144 Nd (0.512832–0.512990) isotope ratios point to the presence of mixed components in the source region, i.e., DMM, EM1 and EM2. Recent seismic tomographic images of the mantle beneath Zhejiang suggest the presence of a subducted slab of oceanic lithosphere in the transition zone. Based on the combined geophysical and geochemical evidence, we propose that the major source of the Zhejiang basaltic magmas was the ancient subducted oceanic slab in the transition zone with an EM-like signature. The other magma sources include depleted asthenospheric peridotite possessing a DMM-like signature. The dynamics of this upwelling hybrid magma was apparently related to westward subduction of the Pacific plate underneath the eastern Asian continent. This process may have triggered the widespread Cenozoic volcanism related to the lithospheric thinning in East China in the Mesozoic and Cenozoic.

Diamond, moissanite and a variety of other minerals, similar to those reported from ophiolites in Tibet and northern Russia, have recently been discovered in chromitites of the Hegenshan ophiolite of the Central Asian Orogenic Belt, north China. The chromitites are small, podiform and vein-like bodies hosted in dunite, clinopyroxene-bearing peridotite, troctolite and gabbro. All of the analysed chromite grains are relatively Al-rich, with Cr# [100Cr/(Cr+Al)] of about 47–53. Preliminary studies of mainly disseminated chromitite from ore body No. 3756 have identified more than 30 mineral species in addition to diamond and moissanite. These include oxides (mostly hematite, magnetite, rutile, anatase, cassiterite, and quartz), sulfides (pyrite, marcasite and others), silicates (magnesian olivine, enstatite, augite, diopside, uvarovite, pyrope, orthoclase, zircon, sphene, vesuvianite, chlorite and serpentine) and others (e.g., calcite, monazite, glauberite, iowaite and a range of metallic alloys). This study demonstrates that diamond, moissanite and other exotic minerals can occur in high-Al, as well as high-Cr chromites, and significantly extends the geographic and age range of known diamond-bearing ophiolites.

In recent years diamonds and other exotic minerals have been recovered from mantle peridotites and high-Cr chromitites of a number of ophiolites of different age and different tectonic environments. Here we report a similar collection of minerals from the Sartohay ophiolite of Xinjiang Province, western China, which is characterized by having high-Al chromitites. Several samples of massive podiform chromitite with an aggregate weight of nearly 900 kg yielded diamonds, moissanite and other highly reduced minerals, as well as common crustal minerals. Thus far, more than 20 grains each of diamond and moissanite have been recovered from heavy mineral separates of the chromitites. The diamonds are all 100–200 μm in size and range in color from pale yellow to reddish-orange to colorless. Most of the grains are anhedral to subhedral octahedra, commonly with elongate forms exhibiting well-developed striations. They all display characteristic Raman spectra with shifts between 1325 cm−1 and 1333 cm−1, mostly 1331.51 cm−1 or 1326.96 cm−1. The moissanite grains are light blue to dark blue, broken crystals, 50–150 μm across, commonly occurring as small flakes or fragments. Their typical Raman spectra have shifts at 762 cm−1, 785 cm−1, and 966 cm−1. This investigation extends the occurrence of diamonds and moissanite to a Paleozoic ophiolite in the Central Asian Orogenic Belt and demonstrates that these minerals can also occur in high-Al chromitites. We conclude that diamonds and moissanite are likely to be ubiquitous in ophiolitic mantle peridotites and chromitites.