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Petrology and Mineralogy of Mantle Peridotites
from the Southern Marianas
Hiroshi Sato and Teruaki Ishii
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 composi-
tions 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 TiO 2 . The
texture of the peridotites, and the degree of melting, suggest that these rocks origi-
nated 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.
Keywords Mantle peridotite • Impregnation • Back-arc basin • Southern Mariana
The Izu-Bonin-Mariana (IBM) arc-trench system, along the eastern boundary of
the Philippine Sea Plate, is characterized by exposures of serpentinite-dominant
seamounts in the fore-arc area (Fryer and Fryer 1987 ; Fryer et al. 1995 ). These
seamounts are composed of rocks with island-arc affinities and lithologies similar
to those of onland ophiolites; therefore, they are classified as “island-arc ophiolites”
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