Metasomatic Mantle Xenoliths from the Bismarck Microplate (Papua New Guinea)--Thermal Evolution, Geochemistry and Extent of Slab-induced Metasomatism

A suite of ultramafic mantle xenoliths from the TUBAF and EDISONseamounts in the Bismarck Archipelago NE of Papua New Guineawas sampled by video-guided grab. The xenoliths, which weretransported to the sea floor by rift-related, Quaternary trachybasalts,mainly represent part of the oceanic mantle. Mineral zoningin peridotite xenoliths testifies to slow cooling after mantleformation at a mid-ocean ridge. Cooling rates in the range of1°C/Ma were calculated from zoning of Ca in olivine usingthe Lasaga algorithm. Subsequent to this cooling, a strong metasomatismaffected the mantle peridotites when metasomatic agents emergedfrom the underlying slab of a subduction zone, which was stalledabout 15 my ago. This resulted in the formation of orthopyroxene-,clinopyroxene-, phlogopite- and hornblende-bearing veins crosscuttingspinel peridotites and olivine clinopyroxenites, as well aspervasively metasomatized plagioclase lherzolites. The metasomaticxenoliths reveal strong chemical disequilibria between the metasomaticminerals and the adjacent, unaltered host rock minerals, whichare especially prominent in the veined samples. Temperaturesduring the metasomatic overprint, estimated using spinel–olivinethermometry, range between 660 and 950°C. Oxygen barometryreveals an elevated oxygen fugacity, with