EVIDENCE OF SUBDUCTED ALTERED OCEANIC CRUST INTO DEEP MANTLE FROM INCLUSIONS OF IAB DIAMONDS

Nitrogen is one of the most common impurities in diamonds, and its aggregation styles in diamonds have been used as criteria for diamond classification. Pure IaB diamonds (with 100% nitrogen in B aggregation) are rather rare among natural diamonds. The occurrence of B center is generally associated with high temperature and long resident time of the host diamond, which would potentially provide information of Earth’s deep interior. Seawater circulation is the unique process that shapes the surface of our planet and potentially has profound effect on Earth’s interior due to slab subduction. Here, from 50 IaB diamonds with detectable micro inclusions submitted to the Gemological Institute of America (GIA) for screening, we found more than 70% of them content typical mineral assemblage from sublithosphere. Jeffbenite (TAPP), majorite garnet, enstatite pyroxene, ferropericlase have been observed which could be retrograde products of former bridgemanite. CaSiO₃ Walstromite with larnite, and titanite is the dominate phase present in ~40% of all diamond samples. Direct evidence from oxygen isotope ratios measured by SIMS (δ¹⁸O_VSMOW in the range +11.1 to +12.5 ‰) of CaSiO₃ walstromite with coexisting larnite and titanite that retrograde from CaSiO₃ perovskite suggest that hydrothermally altered oceanic basalt can subduct to depths of > 410 km in the transition zone. Incorporation of materials from subducted altered oceanic crust into deep mantle produced diamond inclusions that have both lower mantle and subduction signatures. Ca(Si,Al)O₃ perovskite was observed with high concentration of REE elements (> 1.7 wt.%) that could be enriched under lower mantle PT conditions. Evidence from ringwoodite with hydroxide bond, precipitates of NH₃ phase, and cohenite with trace amounts of Cl implies that the subducted brines can potentially introduce hydrous fluid to the bottom of transition zone. Increasing trends of carbon isotope ratio from core to rim region detected by SIMS (δ ¹³C from -5.5 ‰ to -4 ‰ ) of the diamonds with subducted materials suggests that an oxidized carbonate-dominated fluid was associated with recycling of the subducted hydrous material and played an important role in balancing redox exchange with the reduced lower mantle indicated by precipitated iron nano particles and coexisting hydrocarbons and carbonate phases.