Element partitioning during carbonated pelite melting at 8, 13 and 22 GPa and the sediment signature in the EM mantle components

Abstract

Within the subducting oceanic crust, carbonated eclogitic pelites are the lithology with the lowest melting temperature at >5GPa, i.e. at depths beyond major subarc dehydration. 200–400°C below the oceanic mantle geotherm, carbonated pelites generate alkali-rich Ca-carbonate melts that constitute efficient metasomatic agents of the mantle. Partition coefficients between residual minerals and such melts were experimentally determined at 8, 13, and 22GPa at 1100–1500°C. Compared to previous studies, clinopyroxenes have higher jadeite contents (57–82mol%) resulting in a larger compatibility for LILE. In garnet, the compatibility of REE increases from incompatible LREE (DLa~0.005 at 8–22GPa) to slightly compatible Lu (DLu=0.96 to 3.5 at 8–22GPa), DHFSE's increase with pressure from slightly incompatible at 8GPa to highly compatible at 22GPa, always with DHf>DZr. K-hollandite/carbonate melt partition coefficients at 13GPa are all <0.3 except for K itself. At 22GPa, Rb, Sr, Ba, and Pb also become compatible in K-hollandite. Also at 22GPa, FeTi-perovskites appear and have high D-values for HFSEs (DHFSE 28–88), similar to other Ti-rich minerals. In the CAS phase, also occurring at 22GPa, Ti, Sr, La to Gd, and Pb, Th and U are compatible (DPb>DTh>DU>1.7 with a DPb/DU of 12 to 26) leading to a strong fractionation of these elements during melting just above the 660km discontinuity.Calculated bulk residue/carbonate melt partition coefficients increase with pressure for almost all elements. At 22GPa, i.e. for carbonated sediment melting in the transition zone, element fractionation strongly effects the Pb isotopic evolution. Carbonate melt trace element compositions normalized to primitive mantle show strong enrichments in incompatible elements including LILE and LREE and relative negative anomalies for Ti at 8 and 13GPa and for Hf, Zr and Ti at 22GPa at which pressure absolute values are close to mantle concentrations. Primitive mantle normalized patterns for 8GPa carbonate melts are similar to ultrapotassic rocks and many lamproites on one hand confirming the involvement of a sedimentary component in the source region of these rocks, on the other hand defining this component as a carbonated sediment melt. The melting of mantle domains re-enriched by ~0.4wt.% of our 8–13GPa carbonate melts produces the typical trace element signature observed in the group II kimberlites. Finally, the Pb, Nd, and Sr isotopic evolution of mantle domains contaminated by ≤1 wt.% carbonate melt derived from carbonated pelites yields reservoirs which cover most of the compositions identified as EM I and II flavors in OIBs.