Fluid-Peridotite Interactions in Mantle Wedge Xenoliths

Brent I.A. McInnes, GEMOC Key Centre and CSIRO Exp & Mining, Sydney, Australia (B.McInnes@syd.dem.csiro.au)

Sub-arc mantle xenoliths of spinel lherzolite, harzburgite, websterite, and orthopyroxenite were collected from a shoshonitic submarine cinder cone (Tubaf volcano; Herzig et al., EOS 75(44), 513-515) near Lihir Island, Papua New Guinea. The mineralogy of the anhydrous lherzolites (olivine: Mg# 0.87-0.91; orthopyroxene: Mg# 0.92 and average Al2O3=1.7 wt.%; Cr-diopside: En48Wo44Fs3, with average Cr2O3=0.75, Na2O=0.13 and Al2O3=2.0 wt.%; and spinel: Cr# 0.35-0.56) is similar to that of the Cr-spinel lherzolite group. These characteristics and the predominance of harzburgite (87%) over lherzolite (13%) indicates that the sub-arc mantle is a depleted peridotite residue from an earlier episode of melting in a mid-ocean ridge environment. The spinel peridotite xenoliths record 2-pyroxene closure temperatures of 775-1100_C and oxidation states ranging from +2 to +4 log fO2 units greater than oceanic lithosphere represented by abyssal spinel peridotites (FMQ-1).

Oxidation occurred during late stage metasomatism by hydrous supercritical fluids, as evidenced by the occurrence of zoned spinels in orthopyroxenite veins in the peridotites. The veins are planar, interconnected networks ranging from1 to 6 cm in width. The orthopyroxenite veins contain fibrous, radiating orthopyroxene with fine-grained Fe-Ni sulfides and minor olivine. The presence of H2O-rich fluid inclusions in the orthopyroxene and the lack of shear structures in these veins indicates that metasomatism of peridotite occurred via hydraulic fracturing caused by the influx of slab-derived hydrous fluids into the mantle.

The mineral assemblage orthopyroxene+ olivine+H2O is stable at temperatures above 700°C and below 1000°C (wet solidus) in hydrated peridotite at 5-20 kbars pressure. Under these conditions, hydrous fluids dissolve substantial amounts of silicate components (1 to 10 wt.%). The formation of orthopyroxenite veins in peridotite occurs via silicification of olivine by infiltrating hydrous fluids with a high Si/Mg ratio. Incongruent dissolution of primary peridotite minerals leaches Al but leaves the bulk mantle Mg/Mg+Fe unchanged. The orthopyroxenite "residua" has a geochemical signature similar to a strongly depleted peridotite with Al-poor diopside and orthopyroxene (Al2O3= 0.3 wt.%) and Cr-rich spinel (Cr# 0.80).

Small patches of Na- and K-bearing aluminosilicate melt present in opened fluid inclusions represent quenched fluid products, and have an atomic Na:K:Al ratio of 2:1:10. Apatite, phlogopite and amphibole are absent in the orthopyroxenites, but are present in the websterites indicating that crystallization of these components occurred as the supercritical fluids became solute-saturated at a higher level in the mantle wedge.