Smith Evan M, Ni Peng, Shirey Steven B, Richardson Stephen H, Wang Wuyi, Shahar Anat
Gemological Institute of America, New York, NY 10036, USA.
Earth and Planets Laboratory, Carnegie Institution for Science, Washington, DC 20015, USA.
Sci Adv. 2021 Mar 31;7(14). doi: 10.1126/sciadv.abe9773. Print 2021 Mar.
Subducting tectonic plates carry water and other surficial components into Earth's interior. Previous studies suggest that serpentinized peridotite is a key part of deep recycling, but this geochemical pathway has not been directly traced. Here, we report Fe-Ni-rich metallic inclusions in sublithospheric diamonds from a depth of 360 to 750 km with isotopically heavy iron (δFe = 0.79 to 0.90‰) and unradiogenic osmium (Os/Os = 0.111). These iron values lie outside the range of known mantle compositions or expected reaction products at depth. This signature represents subducted iron from magnetite and/or Fe-Ni alloys precipitated during serpentinization of oceanic peridotite, a lithology known to carry unradiogenic osmium inherited from prior convection and melt depletion. These diamond-hosted inclusions trace serpentinite subduction into the mantle transition zone. We propose that iron-rich phases from serpentinite contribute a labile heavy iron component to the heterogeneous convecting mantle eventually sampled by oceanic basalts.
俯冲的构造板块将水和其他地表成分带入地球内部。先前的研究表明，蛇纹石化橄榄岩是深部再循环的关键部分，但这条地球化学路径尚未得到直接追踪。在此，我们报告了来自360至750公里深度的岩石圈下地幔钻石中富含铁镍的金属包裹体，其铁同位素偏重（δFe = 0.79至0.90‰）且锇不具放射性（Os/Os = 0.111）。这些铁值超出了已知地幔成分或深部预期反应产物的范围。这种特征代表了来自磁铁矿和/或在大洋橄榄岩蛇纹石化过程中沉淀的铁镍合金的俯冲铁，大洋橄榄岩这种岩石类型已知携带了从先前对流和熔体亏损继承而来的不具放射性的锇。这些含于钻石中的包裹体追踪了蛇纹岩俯冲到地幔过渡带的过程。我们提出，蛇纹岩中的富铁相为最终被大洋玄武岩采样的非均匀对流地幔贡献了一种不稳定的重铁成分。
