TRACE ELEMENT ENRICHMENT SIGNATURES BY SLAB-DERIVED CARBONATE FLUIDS IN THE CONTINENTAL MANTLE WEDGE: AN EXAMPLE FROM THE SIERRA NEVADA, CALIFORNIA

Miocene volcanic rocks from the central Sierra Nevada contain upper mantle xenoliths. These rocks represent rare direct fragments of the mantle wedge beneath a continental arc. Several spinel+garnet and garnet peridotites contain calcite veins and pockets that represent products of metasomatic enrichment in the sub-arc mantle. We report preliminary trace element and isotopic studies of carbonated peridotite samples from the Big Creek pipe, central Sierra Nevada. Petrographic observations show that the fluids that ultimately led to the formation of calcite veins have reacted with the matrix peridotite, leading to the generation of secondary diopside and accessory apatite. The delta (18) O values of calcite separated from two garnet peridotite samples are ~19-20 permil relative to SMOW, clearly indicating that the metasomatizing carbonate fluids are recycled, presumably with the subducting slab. Trace element analyses of the two carbonate separates were measured by quadrupole ICP-MS. The patterns are typical for “arc” or subduction-related magmas, e.g. pronounced negative anomalies of the high field strength elements such as Nb and Ta. We also determined the trace element abundances of clinopyroxenes that are associated with the carbonate veins and clinopyroxenes in peridotites from the same xenolith suite that do not show any sign of metasomatic enrichment. Pyroxenes from carbonated peridotites show the “arc” trace element signatures, whereas pyroxenes from carbonate-free peridotites do not. We interpret these results to indicate that CO2-rich fluids released from the subducting slab have imprinted the well-known “arc” signature on the mantle wedge – at least in this case. Forward models of partial fusion of a peridotite with the trace element compositions of carbonated peridotites from Big Creek shows that melts generated in such an environment would closely resemble the trace element signatures determined in arc basalts and andesites worldwide.