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Published May 2005 | public
Journal Article

Subducted carbonates, metasomatism of mantle wedges, and possible connections to diamond formation: An example from California

Abstract

We investigated calcite globules and veins in two spinel-garnet peridotite xenoliths from the sub-Sierra Nevada mantle. The studied xenoliths were entrained in a Miocene (11 Ma) volcanic plug. These carbonates are associated spatially with silicate glass inclusions, suggesting that they are primary inclusions—inclusions that formed at high temperature in the mantle and not at or close to the Earth's surface. The host peridotites represent samples of the lithospheric mantle wedge beneath the Mesozoic California magmatic arc, as indicated by radiogenic isotopic ratios measured on clinopyroxene separates [^(87)Sr/^(86)Sr(11 Ma) = 0.7058–0.7061, ε_Nd (11 Ma) = −1.9 to −0.7]. Mineral chemistry of the peridotite major phases is typical of a mantle section that was depleted of melt. The δ^(18)O values of olivine and orthopyroxene from the two samples are also typical of mantle rocks (δ^(18)O = 6–6.5‰). In contrast, calcite veins have δ^(18)O of 18–20‰ and δ^(13)C 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 CO_2- or CO_2-H_(2)O-rich fluids or melts. The trace-element patterns of two analyzed calcite veins are typical of the arc signatures (e.g., depletions in high-field-strength elements) seen in calc-alkaline magmatic rocks worldwide. However, the cores of peridotite clinopyroxenes do not show that pattern, suggesting that the arc-like 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 based on these observations.

Additional Information

© 2005 Mineralogical Society of America. Manuscript received April 11, 2004; manuscript accepted February 5, 2005; manuscript handled by William Carlson. This research was supported by NSF-EAR grants 0081725 (Ducea) and NSF-EAR 0087347 (Saleeby). Ducea also acknowledges interactions with Patrick Hourican (Silverstone Prospecting) and John Ryder (Diadem Resources), who provided insights into the state of exploration for Sierra Nevada diamonds. Constructive journal reviews by Associate Editor Bill Carlson, reviewers Robert Stem. Brent Mcinnes, and an anonymous reviewer have significantly improved the quality of this manuscript.

Additional details

Created:
August 22, 2023
Modified:
October 23, 2023