Bulk Rock and Mineral Chemistry of Peridotites From Mariana Forearc Seamounts (ODP Leg 195, Site 1200A, and ODP Leg 125, Sites 779A and 784A) and the Petrogenesis of Mantle Wedge Peridotites

Ocean Drilling Program Leg 195 recovered serpentinised peridotites from the South Chamorro Seamount, a serpentinite mud volcano located in the Mariana forearc (Shipboard Scientific Party, 2002). At Site 1200A the mud encloses clasts of mostly harzburgite, with minor dunite and lherzolite, strongly serpentinised and tectonised. The primary mineralogy includes olivine, orthopyroxene, clinopyroxene and chromium-spinel. Bulk-rock chemistry shows Mg# = 91-93, and high Ni and Cr contents. Low CaO and Al₍₂₎O₍₃₎ contents, and very low REE contents with U-shaped, strongly LREE-enriched and HREE-depleted, patterns confirm that most peridotites are highly depleted and suffered extensive partial melting, and contain an enriched component. The only lherzolite sample shows a significantly lower degree of depletion. A new set of peridotite clasts from Sites 779A and 784A, located in two Mariana forearc mud volcanoes drilled during ODP Leg 125, have been investigated. Some of the peridotite clasts have experienced more extensive melt extraction than those from South Chamorro Seamount. Modeling of the REE composition of the clinopyroxenes provide evidence that the onset of partial melting of most of Leg 125 and Leg 195 peridotites occurred under garnet-facies condition and followed during the upwelling of the asthenosphere at shallow, spinel-facies, level. The effect of melt migration or entrapping have also been recognised. The late stages of the evolution comprise sub-solidus re-equilibration, with clinopyroxene exsolution from primary high-Ca orthopyroxene. Reference: Shipboard Scientific Party, In Salisbury, M.H., Shinohara, M., Richter, C., et al., Proc. ODP, Init. Repts., 195, College Station, TX (Ocean Drilling Program), 1-233, [CD-ROM] (2002).