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The Luobusa chromitite and ophiolite present a captivating geological feature marked by peculiar mineralogical and geochemical characteristics. Abundant platinum-group minerals (PGM), base-metal sulfides (BMS), and PGE-sulfides and alloys in the chromitite reveal a multistage genesis, encompassing partial mantle melting, melt-rock interactions, and dynamic shifts in oxygen and sulfur fugacities (fO2, fS2). The geochemical signatures and PGE patterns of these mineral inclusions elucidate the evolutionary process of the Luobusa ophiolite, tracing its transition from a sub-ridge environment to a sub-arc setting. The variable ΣPGE values (40–334 ppb) in chromitite, coupled with notably lower ΣPGE values (10–63 ppb) in dunite imply extensive melt fractionation and melt-rock interactions. Coexisting well-crystallized Os-Ir alloys alongside interstitial BMS likely reflect low fS2 and high temperatures during the early formational stages, whereas abundant anhedral or irregular sulfarsenide and pyrite inclusions in chromite point to lower temperatures and higher fS2 during the late stages. The trace element composition of pyrite inclusions displays some of the characteristics of mid-ocean ridge (MOR) and oceanic island rocks, manifesting the interplay of diverse magmatic sources during the evolution of the Luobusa ophiolite. [ABSTRACT FROM AUTHOR]

Boron geochemistry can track fluid–rock interaction during subduction zone metamorphism. Rare tourmaline-bearing blueschists, which are associated with ultrahigh-pressure (UHP) serpentinites are first recognized in SW Tianshan, China. Detailed petrology, whole-rock and mineral chemistry, B isotope analysis, and modeling characterized two consecutive stages of tourmaline crystallization (Tur-I , Tur-II). Tourmaline included in, or intergrown with, garnet and the cores of tourmaline in rock matrixes and veins are Tur-I , which grew during prograde metamorphism at 430°C to 460°C/470°C, ~1.9–2.1 GPa. The rims of tourmaline in rock matrixes and veins are Tur-II , which formed during initial exhumation at 460°C to 490°C, ~2.1–1.7 GPa. Variable δ11B values of tourmaline (+8‰, Tur-I to −2‰, Tur-II) point to a 11B-rich signature of the fluid infiltrating at Stage I. With progressing metamorphism, δ11B decreased in the fluid. The high-δ11B Tur-I (up to +8‰) could not have crystallized from fluid released from the high-pressure metapelites (−12‰ to −7‰) and metabasites (−15‰ to −5‰) surrounding the tourmaline host rocks given the lower δ11B values. Modeling of B isotope fractionation yields the δ11B values of −9‰ to −5‰, −11‰ to −1‰, and +8‰ to +17‰ for the fluids equilibrium with the restitic metapelites, metabasites, and serpentinites, respectively. The tourmaline and whole-rock B isotope data, along with the tourmaline compositions, point to the associated serpentinites as source of the fluid that infiltrated the metamorphic rocks. This fluid was released by the partial dehydration of serpentinites through the reaction antigorite + brucite = olivine + water at forearc depth. We propose that metabasites in subduction zones can acquire 11B-rich signatures through interaction with serpentinite-derived fluids, leading to the formation of robust tourmaline minerals at shallow levels. As a new reservoir of heavy boron, these metabasites can then transport this signature to greater depths. [ABSTRACT FROM AUTHOR]

The ultra-deep genesis of ophiolitic peridotite has reshaped our perception of the genesis of the oceanic mantle. Although ultra-high pressure (UHP) mineral assemblages have been unearthed in dozens of ophiolites in different orogenic belts around the world, the vast majority of them have been limited to podiform chromitites formed in suprasubduction zone (SSZ) settings, leaving uncertainty about whether such UHP minerals are intrinsic to the oceanic mantle or influenced by a specific mantle rock type. Here, we report on the occurrence of diamonds recovered from the harzburgites within the Skenderbeu massif, Mirdita ophiolite. The whole-rock, mineralogical major and trace element compositions, and redox states of the harzburgites align with modern abyssal harzburgites. Trace element modeling of clinopyroxene indicates that harzburgites have endured varying degrees of garnet-facies melting (~2%–5%) before progressing to spinel-facies melting (~10%–12%). Mineralogical characteristics further support that the Skenderbeu harzburgites underwent late-period MORB-like melt metasomatism in a forearc spreading center. An unusual mineral assemblage of diamonds has been separated from the studied harzburgites. The first occurrence of ophiolite-hosted diamonds discovered in the forearc harzburgites, together with previous similar discoveries in the SSZ ophiolitic chromitites, suggest that the ophiolite-hosted diamonds are not specific to certain mantle rocks. [ABSTRACT FROM AUTHOR]