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As a case study of spatial and temporal variations in earthquake stress drops between the 2008 Ms 8.0 Wenchuan and 2013 Ms 7.0 Lushan earthquakes, we computed 1828 stress‐drop values for earthquakes with magnitudes 1.7 ≤ ML ≤ 5.0 during an eight‐year time span before and after major earthquakes. We divide the study area into three subregions (the southern segment of the Longmen Shan fault zone; the southwestern junction of the Longmen Shan and Sichuan Basin; and the southwestern margin of the Sichuan Basin) and calculate individual event stress drops in each. The results show that regions of alternating high and low stress drop are found on either side of the southwestern segment of the Longmen Shan fault zone. During the two‐year period after the 2008 Ms 8.0 Wenchuan earthquake, the stress state of the southern Longmen Shan fault shows no significant change. A marked increase in stress level appears about 18 months before the 2013 Ms 7.0 Lushan earthquake near the Lushan hypocenter zone. Two months after the Ms 7.0 event, the stress drops suddenly attenuate, with significantly less seismic energy release per event. We find that changes in the patterns of high and low stress drop values are consistent with the process of stress accumulation or transfer from the pre‐mainshock to post‐mainshock periods. The results indicate that major earthquakes are the dominant cause of temporal and spatial evolution in stress levels. Stress drop variations show obvious temporal and spatial patterns that may suggests subtle changes in the character of stress fields on faults and spatial variations related to local intense compression and tectonic effects. [ABSTRACT FROM AUTHOR]

Farré‐de‐Pablo et al. (2018) report a new occurrence of in situ microdiamonds enclosed in chromite from ophiolitic chromitite pods hosted in the Tehuitzingo serpentinite of southern Mexico. The discovery enlarges the number of occurrence of the ophiolite‐hosted microdiamonds to 7 countries in the world, including India (Das, 2015, 2017), Albania (Xiong et al., 2017; Wu et al., 2017), Turkey (Lian et al., 2017), Myanmar (Chen et al., 2018), Russia (Yang et al., 2015), and China (Bai et al., 1993; Xu et al., 2009). The microdiamonds occur in ophiolitic podiform chromitites and peridotites, and are generally interpreted as UHP phases formed at pressures > 4 GPa (Yang et al., 2014; Griffin et al., 2016; Das et al., 2017). However, Farré‐de‐Pablo et al. (2018) conclude that the Tehuitzingo diamonds were formed under low‐temperature and low‐pressure conditions during serpentinization, which challenges the current knowledge of diamond formation. Here, we discuss several lines of evidence that do not support the authors' conclusion. (1) The authors suggest a two stage of model for the Tehuitzingo microdiamonds crystallization. It begins with crystallization of high‐T, magmatic chromite that produced grains with cores of high‐Al chromite. In the second stage, the magmatic chromite developed cracks, that were then filled with hydrothermal fluids and secondary minerals, such as serpentine, quartz and chlorite, and microdiamonds that are thought to have crystallized in situ. At the same time the rim of the chromite grain was altered to typical porous ferritchromite that is slightly enriched in Fe3+. The authors suggest that these chemical trends reflect reaction of primary high‐Al chromite and matrix olivine with reducing fluids to produce secondary Cr‐ and Fe2+‐rich and Al‐ and Mg‐depleted porous chromite in equilibrium with chlorite (Farré‐de‐Pablo et al., 2018, and references herein). We plotted the Mg number [Mg/(Mg+Fe2+) atomic ratio] and Cr number [Cr/(Cr+Al) atomic ratio] of 191 chromite analysis in Fig. 1, which clearly show that the chromite of the healed fractures has essentially the same composition as that of unaltered magmatic cores, but significantly different from the ferritchromite of the grain rims. Therefore, it is difficult to understand how the diamond‐bearing fracture in chromite was healed by magmatic chromite after "serpentinization"? Thus, we suggest an alternative scenario in which the diamonds, along with a carbon‐rich fluid, were incorporated into the chromite grain at the time it crystallized in the diamond stability field in the mantle (Yang et al., 2015). We note that the Tehuitzingo diamond grains occur as small crystal nuclei associated with amorphous carbon, a feature consistent with in situ diamonds in chromitites of the Luobusa and Ray‐Iz ophiolites that show no signs of serpentinization (Fig. 2) (Yang et al., 2014, 2015; Zhang et al., 2016). Thus, facilitating later crack formation as the chromite cooled, thereby, allowing ingress of late‐stage, serpentinizing fluids. Such fluids could modify the silicate minerals but not the diamonds. It is entirely possible that the silicate minerals in the linear arrays with diamond were retrograded from high pressure phases, i.e., quartz from coesite, and chlorite + serpentine from baiwenjite (BWJ) crystals (Yang et al.., 2007; Griffin et al., 2016). A similar phenomenon has been observed in the Luobusa chromitites where octahedral grains of antigorite define rough linear arrays in healed fractures in chromite grains (Griffin et al. 2016). These antigorite grains are likely hydrated alteration products of BWJ crystals, the latter being predicted to be stable at T∼2000°C and 15 GPa (Kiefer et al., 1999). Similarly, diamonds in healed fractures in chromite may have crystallized in the deep mantle and been preserved during late‐stage serpentinization. Thus, we conclude that it is highly unlikely that the Tehuitzingo diamonds and serpentine were formed by a single process. Otherwise, it is hard to understand why serpentinization is so widespread in mantle rocks, but so few diamonds have been found in serpentinites. Most ophiolite‐hosted diamonds that we have recovered are from completely fresh, unserpentinized chromitites and peridotites (Yang et al., 2015). (2) Can diamond be generated in low P‐T serpentinization environments? The authors quoted a paper by Simakov et al. (2015) to confirm the possibility that "alteration of ultramafic rocks by hydrothermal C‐O‐H fluids at pressures as low as 0.1 GPa and <1000°C produces nano‐ and microdiamonds." We note that Simakov et al., (2015) did report an occurrence of nanodiamonds, primarily of 6 and 10 nm across, in Hyblean asphaltene‐bearing serpentinite xenoliths (Sicily, Italy). However, although the sample is a serpentinized and carbonated block of peridotite that equilibrated in the upper mantle in the spinel‐peridotite stability field, no evidence was offered to confirm that the diamonds were generated by serpentinization. In addition, little is known about the origin and history of the sample because it is an isolated block from a Tortonian tuff‐breccia pipe located in the Hyblean Plateau. The authors also used the experimental work of Lou et al., (2004) to show that diamond crystals can be generated at temperatures ranging from 400 to 600°C. However, this experiment involved reduction of dense carbon dioxide with alkali metals (K, Li), but such a process could not be replicated by serpentinization. The authors also refer to formation of nano‐ and microdiamonds by CVD (chemical vapor deposition), as suggested by Kaminsky et al. (2016) to account for diamonds formed in recent lavas at near‐atmospheric pressures. CVD is an industrial process for growing thermodynamically unstable diamonds in a gas phase at comparatively low pressures (Yan et al., 2002). As its name implies, CVD involves chemical reaction inside a gas‐phase as well as deposition onto a substrate. Various direct and indirect adjustable parameters for CVD have been summarized by many researchers (e.g. Butler and Woodin, 1994; Schwander and Partes, 2011). In CVD the temperature distribution, solubility and internal energy clearly depend on the type of excitation, which can be generated by microwave (MW), radiofrequency (RF), laser inducement (LI), direct current (DC), hot filament (HF) and chemical activation (CA) (Schwander and Partes, 2011). Although Kaminsky et al. (2016) suggested that the crystallization of a carbonado‐like aggregate in the Avacha volcano of Kamchatka was likely due to CVD during or shortly after the course or shortly after one of the volcanic eruption pulses, the energy source for the activation of the chemical processes that liberate and deposit carbon (the source of diamond) was not confirmed. Generally, diamond formation by CVD is a complex process requiring temperatures of ∼600‐1000°C, mostly higher than the temperatures of serpentinization (< 700°C) (Ulmer and Trommsdorff, 1995; Maeda et al., 1998; Battaile et al., 1999; Deschamps et al., 2013), which shows clearly that CVD diamond would rarely, if ever, generated by serpentinization of ultramafic rocks in nature. (3) Another critical evidence is that several thousands of microdiamonds recovered from fresh samples of chromitite and peridotite but serpentinite from many ophiolites in different parts of the world, which does not support that diamonds could be generated by low P‐T serpentinization. Also Farré‐de‐Pablo et al. (2018) have clearly ignored the presence of many other UHP phases occurring both as inclusions in the ophiolite‐hosted diamonds and as associated grains in the host chromitites and peridotites. Yang et al. (2007) reported the discovery of coesite rimming a grain of FeTi alloy in the Luobusa chromitite, which requires minimum pressures of ∼2.8 GPa. They suggested that the coesite is a pseudomorphic replacement of stishovite, and implying a pressure of > 9GPa. Further detailed investigation on the coesite grains revealed the occurrence of submicroscopic inclusions of high‐pressure nitrides, oxides, and native Fe within the coesite (Dobrzhinetskaya et al. 2009), an assemblage interpreted as having formed under high‐pressure and highly reduced conditions at a depth of at least 300 km (Dobrzhinetskaya et al. 2009). Coesite also occurs as inclusions in many ophiolite‐hosted diamonds, which cannot be explained by serpentinization (Yang et al., 2015). The discovery of coesite and clinopyroxene exsolution lamellae in a chromite grain from the Luobusa chromitites provided "real" in‐situ evidence of UHP crystallization of chromite (Yamamoto et al., 2009). Das et al. (2015, 2017) reported UHP minerals including C2/c clinoenstatite, disordered coesite and high‐pressure Mg2SiO4 in peridotite from the Nidar ophiolite, which require derivation from the mantle transition zone (410–660 km) (Das et al., 2015, 2017). The UHP phase TiO2 (II) has been recovered from chromitites of several ophiolites, where it is intergrown with coesite or corundum. These occurrences provide solid data for the deep formation of UHP minerals in chromitites and peridotites, and are much more convincing than speculative models involving low‐pressure serpentinization. In conclusion, serpentinization typically occurs in mantle rocks at mid‐ocean ridges and subduction zone environments, where it can be generated by hydrothermal alteration at temperatures lower than 700°C and pressures less than 1 GPa (Evans, 1977; Ulmer and Trommsdorff, 1995; Deschamps et al., 2013). The fluids responsible for serpentinization are commonly seawater, which may have evolved through interaction with the crust. Oxygen isotopic study indicates that serpentinization typically begins at temperatures of 300–500°C (Mével, 2003). No experimental research has shown that diamond, even microdiamond, can be formed under the low P‐T conditions of serpentinization. If diamonds could be formed in this way they would presumably be widely associated with serpentinites on earth. Obviously, this is not the case. [ABSTRACT FROM AUTHOR]

The Bulqiza ultramafic massif, which is part of the eastern Mirdita ophiolite of northern Albania, is world renowned for its high‐Cr chromitite deposits. High‐Cr chromitites hosted in the mantle section are the crystallized products of boninitic melts in a supra‐subduction zone (SSZ). However, economically important high‐Al chromitites are also present in massive dunite of the mantle‐crust transition zone (MTZ). Chromian‐spinel in the high‐Al chromitites and dunites of the MTZ have much lower Cr# values (100Cr/(Cr+Al)) (47.7–55.1 and 46.5–51.7, respectively) than those in the high‐Cr chromitites (78.2–80.4), harzburgites (72.6–77.9) and mantle dunites (79.4–84.3). The chemical differences in these two types of chromitites are reflected in the behaviors of their platinum‐group elements (PGE). The high‐Cr chromitites are rich in IPGE relative to PPGE with 0.10–0.45 PPGE/IPGE ratios, whereas the high‐Al chromitites have relatively higher PPGE/IPGE ratios between 1.20 and 7.80. The calculated melts in equilibrium with the high‐Cr chromitites are boninitic‐like, and those associated with the high‐Al chromitites are MORB‐like but with hydrous, oxidized and TiO2‐poor features. We propose that the coexistence of both types of chromitites in the Bulqiza ultramafic massif may indicates a change in magma composition from MORB‐like to boninitic‐like in a proto‐forearc setting during subduction initiation. [ABSTRACT FROM AUTHOR]

In recent years diamonds and other unusual minerals (carbides, nitrides, metal alloys and native elements) have been recovered from mantle peridotites and chromitites (both high-Cr chromitites and high-Al chromitites) from a number of ophiolites of different ages and tectonic settings. Here we report a similar assemblage of minerals from the Skenderbeu massif of the Mirdita zone ophiolite, west Albania. So far, more than 20 grains of microdiamonds and 30 grains of moissanites (SiC) have been separated from the podiform chromitite. The diamonds are mostly light yellow, transparent, euhedral crystals, 200-300 μm across, with a range of morphologies; some are octahedral and cuboctahedron and others are elongate and irregular. Secondary electron images show that some grains have well-developed striations. All the diamond grains have been analyzed and yielded typical Raman spectra with a shift at ∼1325 cm−1. The moissanite grains recovered from the Skenderbeu chromitites are mainly light blue to dark blue, but some are yellow to light yellow. All the analyzed grains have typical Raman spectra with shifts at 766 cm−1, 787 cm−1, and 967 cm−1. The energy spectrums of the moissanites confirm that the grains are composed entirely of silicon and carbon. This investigation expands the occurrence of diamonds and moissanites to Mesozoic ophiolites in the Neo-Tethys. Our new findings suggest that diamonds and moissanites are present, and probably ubiquitous in the oceanic mantle and can provide new perspectives and avenues for research on the origin of ophiolites and podiform chromitites. [ABSTRACT FROM AUTHOR]

Geophysical investigations and laboratory experiments show evidence for possible subduction of ancient oceanic crust. Geological and mineralogical observations suggest that subducted oceanic crust is recycled into the upper mantle. The subduction is supported by the recovery of super‐deep diamonds from kimberlites and the presence of crustal materials in ophiolitic chromitites and their host peridotites. What is the mechanism? Here we report the new discovery of ophiolite‐hosted diamonds in the podiform chromitites within the Skenderbeu massif from the Mirdita ophiolite in the western part of Neo‐Tethys (Fig. 1). The diamonds are characterized by exceedingly light C isotopes (δ13CPDB ∼ −25‰), which can be interpreted as evidence for subduction of organic carbon from Earth's surface. The diamonds are also characterized by an exceptionally large range in δ15Nair (−12.9‰ to +25.5‰), accompanied by a low N aggregation state (Fig. 2). On the other hand, materials sparsely included in diamonds include amorphous material, Ni‐Mn‐Co alloy, nanocrystals (20 nm×20 nm) of calcium silicate with an orthorhombic perovskite structure (Ca‐Pv), and fluids (Fig. 3). We consider that the Skenderbeu diamonds nucleated and grew from a C‐saturated, NiMnCo‐rich melt derived from a subducted slab of ocean crust and lithosphere in the deep mantle environment. The environment is in the diamond stability field or near the top of the mantle transition zone. The new discovery of diamonds from the Mirdita ophiolite provides a valuable opportunity to understand deep cycling of subducted oceanic crust and mantle (i.e., composition and process). [ABSTRACT FROM AUTHOR]

A series ophiolitic massifs in the northeastern part of Cuba are generally considered to be relics of proto‐Caribbean or Caribbean lithosphere, emplaced during Paleogene collision of the NW Caribbean and North American plates (Iturralde‐Vinent et al., 2016). However, neither their age nor their geodynamic settings are well constrained. They are generally considered to be Creataceous in age but they have been interpreted as having formed in a back‐arc basin, forearc basin or plume setting (e.g. Giunta et al., 2009; Marchesi et al., 2016; Kerr et al., 1999). The Moa‐Baracoa ophiolitic massif consists of mantle harzburgite, a Moho transition zone, layered and isotropic gabbro, and pillow basalts. The isotropic gabbros, collected west of Moa (20°35′ 17.99″N, 75°02′56.58″W), are composed mainly of subhedral plagioclase and subhedral to euhedral clinopyroxene with minor primary brown amphibole. The plagioclase grains are fresh, whereas the clinopyroxene very weakly altered and partly replaced by secondary amphibole. Cathodoluminescence (CL) images of zircon grains separated from isotropic gabbro display oscillatory zoning and broad zoning, analogous to typical igneous zircon in gabbro. Ten zircon grains with oscillatory zoning in CL images yielded a mean U‐Pb SHRIMP age of 136.7±1.8 Ma. Isotropic gabbro samples have low SiO2 (47.22 wt%–49.25 wt%), TiO2 (0.13wt%–0.26 wt%) and K2O (0.04 wt%–0.08 wt%) and high MgO (8.22 wt%–16.72 wt%), Cr (1337–2065 μg/g) and Ni (192–507 μg/g) contents. Their CaO, Al2O3 and Na2O contents vary from 13.56 wt% to 15.78 wt%, 14.98 wt% to 21.98 wt%, and 0.71 wt% to 1.87 wt%, respectively. In the SiO2 versus Zr/TiO2∗0.0001 diagram, the samples plot in the sub‐alkaline basalt field but show calc‐alkaline affinities in the FeOT/MgO vs. SiO2 diagram. The gabbros contain low REE contents (ΣREE = 4.71–7.71), and their chondrite normalized REE patterns show very weak depletion in light REE (LREE, (La/Yb)N = 0.34–0.50) and flat heavy REE (HREE, (Gd/Yb)N = 1.02–1.15) profiles, with slightly positive Eu anomalies (Eu/Eu∗ = 1.55–1.76) suggesting magmatic accumulation of plagioclase. Furthermore, all samples are depleted in Nb and Th, and enriched in Ba, Pb and Sr in the primitive mantle‐normalized trace element diagram. Geochemical features, such as high values of Ba/La (30–38) and low Th/Yb (0.02–0.03), medium values of (Ta/La)PM (0.30–0.35) and (Hf/Sm)PM (0.58–0.80), indicate enrichment by slab‐derived fluids and subduction‐related metasomatism. These isotropic gabbros are geochemically comparable with those of the Kermanshah and Neyriz ophiolites of Iran (Monsef et al., 2018) that are thought to have originated in a SSZ environment. Therefore, we suggest that the isotropic gabbros in the Moa‐Baracoa massif crystallized from early Cretaceous SSZ magmas that were most likely derived from a mantle source metasomatized by slab fluids in a suprasubduction zone environment. [ABSTRACT FROM AUTHOR]

The article presents a study on the mineralogy, petrology and geochemistry of peridotites from the northern ophiolitic sub-belt of the Western Yarlung Zangbo Suture Zone (YZSZ) in Tibet, China as of December 2015. The focus of the study is the petrological processes in the formation of the peridotites. In the study, new whole-rock major oxide, trace, rare earth element (REE), and platinum group element (PGE) data on the mantle peridotites are examined.

Early Cretaceous magmatism suggested to be related with the Kerguelen mantle plume has been reported in both the eastern and western Tethyan Himalayan terrane. Coeval magmatism (133–138 Ma) recorded by hypabyssal intrusive rocks have been recently discovered in the central Tethyan Himalaya (TH). The hypabyssal intrusions are dominated by OIB‐like basaltic rocks intruded by later porphyritic/ophitic intermediate rocks and are characterized by strongly light rare earth element enrichment and prominent Na‐Ta depletion and Pb enrichment. The basaltic rocks have low 143Nd/144Nd ratios ranging from 0.512365 to 0.512476 but relatively high 87Sr/86Sr ratios ranging from 0.708185 to 0.708966. The ∊Nd(t) ratios of the basaltic rocks are between −4.33 and −2.20 and initial 87Sr/86Sr ratios are 0.707807 to 0.708557. Geochemical data demonstrate that these rocks have experienced combined crustal assimilation and fractional crystallization processes. Magmatic zircons from the hypabyssal rocks exclusively have negative ∊Hf (t) values ranging from −0.7 to −12.7, suggestive of assimilation of crustal material. Zircons from these hypabyssal rocks have U‐Pb ages ranging from 130 to 147 Ma. Inherited zircons have U‐Pb ages from 397 to 2495 Ma. All the zircons are characterized by negative ∊Hf(t) values. The Jiding ocean island basalt (OIB)‐like magmatism is geochemically and geochronologically comparable with that in the western and eastern Tethyan Himalaya, indicating widespread OIB‐like magmatism in the northern margin of Greater India during the Cretaceous. Collectively, these rocks can be correlated with other early Cretaceous magmatism in western Australia and northern Antarctica. Considering the similarities, we suggest that the Jiding hypabyssal rocks are also genetically related to Kerguelen plume. Within the Yarlung Zangbo Suture Zone (YZSZ), there are also numerous occurrences of OIB‐like rocks derived from mantle sources different from those of N‐MORB‐like magmas. The OIB‐like magmatism in the YZSZ is nearly coeval with that in the TH, and the two are geochemically similar. We suggest that the OIB‐like magmatism in the Neo‐Tethyan ocean and the northern margin of Greater India may represent the dispersed fingerprints of the Kerguelen plume preserved in southern Tibet, China. [ABSTRACT FROM AUTHOR]

He Pozanti‐Karsanti ophiolite (PKO) is one of the largest oceanic remnants in the Tauride belt, Turkey. Micro‐diamonds were recovered from the podiform chromitites, and these were investigated based on morphology, color, cathodoluminescence, nitrogen content, carbon and nitrogen isotopes, internal structure and inclusions. The diamonds recovered from the PKO are mainly mixed‐habit diamonds with sectors of different brightness under the cathodoluminescence images. The total δ13C range of the PKO diamonds ranges between −18.8 ‰ and −28.4 ‰, with a principle δ13C mode at −25 ‰. Nitrogen contents of the diamonds range from 7 to 541 μg/g with a mean value of 171 μg/g, and the δ15N values range from −19.1 ‰ to 16.6 ‰, with a δ15N mode of −9 ‰. Stacking faults and partial dislocations are commonly observed in the Transmission Electron Microscopy foils whereas inclusions are rather rare. Combinations of (Ca0.81Mn0.19)SiO3, NiMnCo‐alloy and nano‐size, quenched fluid phases were observed as inclusions in the PKO diamonds, confirming a natural origin of these diamonds. We believe that the δ13C‐depleted carbon signature of the PKO diamonds is a remnant of previously subducted crustal matter. These diamonds may have crystallized in metal‐rich melts in the asthenospheric mantle at depth below 250 km which were subsequently carried rapidly upward by asthenospheric melts/fliuds. We concluded that diamond‐bearing asthenospheric melts were likely involved in the formation of the Pozanti‐Karsanti podiform chromitite. [ABSTRACT FROM AUTHOR]

The Pozanti-Karsanti ophiolite (PKO) in Turkey's eastern Tauride belt comprises mantle peridotites, ultramafic to mafic cumulates, isotropic gabbros, sheeted dikes and pillow lavas. The mantle peridotites are dominated by spinel harzburgites with minor dunites. The harzburgites and dunites have quite depleted mineral and whole-rock chemical composition, suggesting high degrees of partial melting. Their PGEs vary from Pd-depleted to distinct Pd-enriched patterns, implying the crystallization of interstitial sulphides from sulphur-saturated melts (e.g. MORB-like forearc basalt). U-shaped or spoon-shaped REE patterns indicate that the PKO peridotites may have also been metasomatized by the LREE-enriched fluids released from a subducting slab in a suprasubduction zone. Based on the mineral and whole-rock chemical compositions, the PKO peridotites show affinities to forearc peridotites. Chromitites occur both in the mantle peridotites and the mantle-crust transition zone horizon (MTZ). Chromitites from the two different horizons have different textures but similar mineral compositions, consistent with typical high-Cr chromitites. Chromitites hosted by mantle harzburgites generally have higher total platinum-group element (PGE) contents than those of the MTZ chromitites. However, both chromitites show similar chondrite-normalized PGE patterns characterized by clear IPGEs, Rh-enrichments relative to Pt and Pd. Such PGE patterns indicate no or only minor crystallization of Pt- and Pd enriched sulphides during formation of chromitites from a sulphur-undersaturated melt (e.g. boninitic or island arc tholeiitic melt). Dunite enveloping chromitite lenses in the hosting harzburgite resulted from melt-rock reaction. We have performed mineral separation work on samples of podiform chromitite hosted by harzburgites. So far, more than 200 grains of microdiamond and more than 100 grains of moissanite (SiC) have been separated from podiform chromitites. These minerals have been identified by EDX and Laser Raman analyses. The diamonds and moissanite are accompanied by large amounts of rutile. Additionally, zircon, monazite and sulphides are also common phases within the heavy mineral separates. Both diamond and moissanite have been analyzed for carbon and nitrogen isotopic composition using the CARMECA 1280-HR large geometry Secondary Ion Mass Spectrometer at the Helmholtz Zentrum Potsdam. In total, 61 δ13CPDB results for diamond were acquired, exhibiting a range from −28.4 ‰ to −18.8 ‰. 31 δ13CPDB results for Moissanite vary between −30.5 ‰ to −27.2 ‰, with a mean value of −29.0 ‰. Diamond has relatively large variation in nitrogen isotopic composition with 40 δ15NAIR results ranging from −19.1 ‰to 16.6 ‰. The discovery of diamond, moissanite and the other unusual minerals from podiform chromitite of the Pozanti-Karsanti ophiolite provides new support for the genesis of ophiolitic peridotites and chromitites under high-pressure and ultra-high reducing conditions. Considering the unusual minerals, the high Mg# silicate inclusions, and the needle-shaped exsolutions in the PKO chromitites, the parental melts of these chromitites may have been mixed with deep asthenospheric basaltic melts that had assimilated materials of the descending slab when passing through the slab in a subduction zone environment. We suggest melt-rock reactions, magma mixing and assimilation may have triggered the oversaturation of chromites and the formation of PKO chromitites. [ABSTRACT FROM AUTHOR]

The article presents a study to examine characteristics of Yongzhu–Guomang Lake Ophiolitic Melange in Xizang (Tibet), China. Topics discussed include ophiolitic fragments in Yongzhu–Guomang Lake composed of metamorphic harzburgites, gabbros and basalts, characteristics of depleted mantle peridotite, and age of Yongzhu–Guomangcuo ophiolite.