Trace element enrichment signatures by slab-derived carbonate fluids in the continental
mantle wedge: An example from the Sierra Nevada, California
Mihai Ducea, University of Arizona
Jason Saleeby, Caltech
Miocene volcanic rocks from the central Sierra Nevada contain upper mantle xenoliths.
These rocks represent rare direct fragments of the mantle wedge beneath the Mesozoic
Sierra Nevada 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 on 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.