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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]

Mantle-melt evolution processes in the earliest stage of subduction initiation (SI) are still ambiguous. Here, we report the systematic petrogeochemical and physical records of the SI processes demonstrated by the Gomsiqe massif, Mirdita ophiolite. The Gomsiqe ultramafic body consists of basal lherzolites, upper harzburgites containing lentoid dunites and disseminated chromitites, displaying transition of petrological (Cr# of spinels range from 12.7 to 47.1) and geochemical signatures. Clinopyroxene trace element modeling indicates that lherzolites and harzburgites have undergone variable garnet-facies melting prior to spinel-facies melting. Harzburgites encountered higher degrees of spinel-facies melting compared with those of lherzolites (∼ 8–10% vs. ∼ 3–5%), and late-stage MORB melt metasomatism. Lherzolites escaped metasomatism. The dunites and chromitites formed by interactions between the harzburgitic lithosphere and MORB-like transitional magmas continued, but with limited hydrous fluid involvement during subduction. The transitional characteristics of metasomatic melt composition and physical condition (i.e., oxygen fugacity [ f o 2 ]) of different lithologies are consistent with the corresponding archive in the Izu-Bonin-Mariana forearc. Lherzolites and harzburgites have comparable f o 2 to that of the abyssal peridotites. The dunites are apparently more oxidized compared to the associated chromitites, further supporting a hypothesis that sulfur-saturated MORB melts were responsible for the formation of chromitites and reduced Cr# of chromitite spinels. Thermometry showed that lherzolites preserved higher equilibration temperatures than harzburgites. Both experienced a later lower-temperature re-equilibration process. Thus, the Mirdita ophiolite preserves multi-stage mantle-melt evolution processes, which are the comprehensive physicochemical responses of the earliest stage of SI. • Gomsiqe mantle rocks reveal time-progressive mantle-melt evolution process. • Gomsiqe massif records the earliest subduction initiation-related signatures. • Ophiolitic mantle provide complementary insights of subduction initiation process. [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]