Subducted Carbonates And Their Role In The Metasomatism Of Mantle Wedges: An Example From California

We investigated calcite veins in two spinel-garnet peridotite xenoliths from the sub-Sierra Nevada mantle. These peridotites represent samples of the lithospheric mantle wedge beneath the Mesozoic California magmatic arc, as indicated by radiogenic isotopic ratios measured on clinopyroxenes separates (Sr/Sr8 Ma= 0.7058-0.7061, Nd/Nd8 Ma= 0.512538-0.512601). Mineral chemistry of the peridotite major phases is typical of a mantle section that was depleted of melt. The δO values of olivine and orthopyroxene from the two samples are also typical of mantle rocks (δO = 6-6.5 ‰). Calcite veins, in contrast have δO of 18-20 ‰ and δC of –14 ‰, arguing for a subducted sedimentary origin for these carbonates. Presumably, the carbonates were expelled from the downgoing slab and fluxed into the overlying mantle wedge as CO2or CO2-H2O rich fluids or melts. The trace element patterns of two analyzed calcite veins are typical of the arc signatures (e.g. depletions in Nb, Ta) seen in calc-alkaline magmatic rocks worldwide. However, the cores of peridotite clinopyroxenes do not show that pattern, suggesting that at least in this case, the arc trace element signature was introduced via the recycled carbonate agent. A connection between mantle wedge carbonation and diamond formation in a subduction environment is proposed.