Petrology and Mineralogy of Mantle Peridotites from the Southern Marianas

Abstract

Plagioclase-bearing mantle peridotites were dredged from two sites along the scarp of a fault (144°E) on the inner wall of the southern Mariana Trench, during cruise KH-98-1 of the R/V Hakuho-maru. The peridotites are weakly to moderately serpentinized, with most of the primary minerals retained, at least in part. The spinels have been modified by an impregnating melt, and their compositions indicate that the peridotites represent the residue left over from as much as 15% partial melting. The impregnating melt yielded large amounts of plagioclase and clinopyroxene (up to 7%), and also enriched spinel and pyroxene in TiO2. The texture of the peridotites, and the degree of melting, suggest that these rocks originated in a back-arc basin. However, the degree of melting and melt impregnation in these peridotites is higher than that in peridotites from the Mariana Trough.