New high-precision trace-element analyses of magmatic olivines point to a pyroxenite-dominated source for recent alkali basalts erupted above slab windows formed along the Antarctic Peninsula. Melting occurred at ambient mantle temperature, and basalts have geochemical compositions that are indistinguishable from ocean-island basalts (OIBs). We propose that the pyroxenite component originally resided in the upper mantle beneath the subducted slab; formation of a slab window allowed limited decompression and the generation of melts of garnet-pyroxenite, but little or no melting of mantle peridotite. The pyroxenite component in the mantle formed ca. 550 Ma, an age that does not require long-term recycling of subducting slabs to the core-mantle boundary. Enriched mid-ocean ridge basalt (E-MORB) from the adjacent extinct Phoenix Ridge owes its enriched trace-element compositions to mixing between small melt fractions of pyroxenite and peridotite during a period of decreased spreading rate prior to the death of the ridge ca. 3.3 Ma. It is likely that the variable trace-element enrichment seen in East Pacific Rise E-MORB distal from hotspots results from the same process of interactions between small-melt-fraction (<~5%) melts of pyroxenite and peridotite.

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