Cryptic Messages From Recycled Terrigeneous Sediments in the Slab Graveyards of the Lower Mantle: Some Experimental Insights

Isotope geochemists have been telling us for almost 50 years now that crustal material is present in the lower mantle, recycled there by subduction, where it has created geochemically "enriched" reservoirs sampled by mantle plumes that feed "hot-spot" ocean-island basalt (OIB) volcanism in places like Samoa, where "EM-type" OIB lavas are attributed to the presence of continental-derived sediments in slab graveyards sitting at the top of the lower mantle. Phase equilibria studies of dry "average continental crust" show that a phase assemblage of K-hollandite, stishovite, and Ti-rich aluminous perovskite is stable deep into the lower mantle. However, K-hollandite is destabilized in the presence of modest amounts of water, triggering a peritectic melting reaction in which both K-hollandite and Al-perovskite break down to form a hydrous supercritical fluid, enriched in Ti and other high-field strength elements (e.g., Zr, Nb, U), in equilibrium with a two-phase residue of stishovite and potassium-rich NAL ("new aluminous") phase; this reaction begins in terrigenous sediments containing ~3-6 wt% H2O at 23-24 GPa and 1400-1500°C. Subsequent experiments in which a layer of peridotite has been placed above a layer of sediments provide insights into how these fluids infiltrate and react with a "lower mantle" assemblage of Mg-bridgmanite (Mg-Pv) + ferropericlase (FP). At 23 GPa and 1500-1600°C, a metasomatic zone forms in the overlying peridotite, with the original Mg-PV transformed to aluminous perovskite enriched in Ti, Fe3+, and HFSE (possibly H2O), with these changes most pronounced at the sediment-peridotite interface. A "modal" metasomatic front in which ferropericlase is absent can be distinguished from a more penetrative "cryptic" front, manifest in a lower mantle assemblage with metasomatic FP with variable Mg-numbers (range: 0.58-0.82) and elevated Ni and Al contents and Mg-Pv with minor enrichment in Al, Ti, and Fe3+. Certain mineral inclusions in ultra-deep diamonds are likely representatives of residues from melting of recycled "continental" sediments (e.g., K-rich NAL phase) in the slab graveyards of the lower mantle, and others appear to represent lower mantle phase assemblages metasomatized by sediment-derived, water-rich fluids (e.g., Mg-Pv variably enriched in Ti and Fe3+, FP with low Mg-number).