Low-Ca boninite formation by second-stage melting of spinel harzburgite residues at mature subduction zones: new evidence from veined mantle xenoliths from the West Bismarck Arc

Abstract

We report the abundances of major and lithophile trace elements and volatiles (H2O, Cl and S) in orthopyroxenite veins cutting mantle-derived, spinel harzburgite xenoliths from the active Ritter volcano in the West Bismarck Arc (Papua New Guinea). The veins preserve sulfide-bearing glass coexisting with crystals. The glass formed by the quench of residual liquids left after crystallisation of abundant orthopyroxene, variable amounts of clinopyroxene, and minor olivine and spinel from silicate melts intruding spinel harzburgite. The glass compositions range from low-Ti (≤ 0.2 wt% TiO2), intermediate to magnesian (2–5 wt% MgO) and high-Mg# (0.45–0.6) andesite to dacite, with Mg# = Mg/(Mg + Fet) where Fet indicates all Fe treated as Fe2+. The glass is depleted in moderately incompatible lithophile trace elements and contains 1.2 to 2.5 wt% H2O, 400–1800 ppm Cl and 100–400 ppm S. Thermo-barometric calculations indicate that the original vein-forming melts intruded the shallow sub-arc mantle lithosphere between 0.5 and 1 GPa, where they partially crystallised (50–60%) to form residual liquids during cooling from ~ 1200 °C down to ~ 650 °C. Our data and calculations show that West Bimarck orthopyroxenite veins are of high-temperature magmatic origin and bear no relation to the carrier magmas of the xenoliths. Instead, petrological modelling demonstrates that the veins formed from the intrusion of primitive low-Ca boninite (LCB) melts in the sub-arc mantle lithosphere. These melts were originally produced by low degrees of melting (≤ 5%) of spinel harzburgite at ≥ 1360 °C and ≤ 1.5 GPa. Some key features of the parental LCB vein-forming melts from West Bismarck (e.g. lithophile trace element signatures and moderate volatile abundances and oxygen fugacity) suggest that, contrary to the widely held belief, their formation does not require persistent mantle wedge fluxing by slab-derived components. Instead, we propose that adiabatic decompression melting of spinel harzburgite residues may also form LCB in the sub-arc mantle. West Bismarck veins are very similar to those previously described in mantle xenoliths from the Kamchatka Arc; active boninite magmatism in these mature arcs shows that the generation of these melts is not as restricted as previously thought. In fact, the growing evidence for the prevalence of boninite in sub-arc mantle peridotites is consistent with the presence of boninite-like geochemical components in many island arcs.