ALBERT

Description: A suite of ultramafic mantle xenoliths from the TUBAF and EDISON seamounts in the Bismarck Archipelago NE of Papua New Guinea was sampled by video-guided grab. The xenoliths, which were transported to the sea floor by rift-related, Quaternary trachybasalts, mainly represent part of the oceanic mantle. Mineral zoning in peridotite xenoliths testifies to slow cooling after mantle formation at a mid-ocean ridge. Cooling rates in the range of 1°C/Ma were calculated from zoning of Ca in olivine using the Lasaga algorithm. Subsequent to this cooling, a strong metasomatism affected the mantle peridotites when metasomatic agents emerged from the underlying slab of a subduction zone, which was stalled about 15 my ago. This resulted in the formation of orthopyroxene-, clinopyroxene-, phlogopite- and hornblende-bearing veins crosscutting spinel peridotites and olivine clinopyroxenites, as well as pervasively metasomatized plagioclase lherzolites. The metasomatic xenoliths reveal strong chemical disequilibria between the metasomatic minerals and the adjacent, unaltered host rock minerals, which are especially prominent in the veined samples. Temperatures during the metasomatic overprint, estimated using spinel–olivine thermometry, range between 660 and 950°C. Oxygen barometry reveals an elevated oxygen fugacity, with Δlog(fo2)FMQ values of 0·4 to 〉4. A geochemical study of the ultramafic rocks shows that all types of xenoliths have been metasomatized. Pervasively metasomatized plagioclase lherzolites and cumulate olivine clinopyroxenites have high contents of middle and heavy rare earth elements compared with veined peridotites. Cryptic metasomatism, indicated by increased light rare earth elements and Nd concentrations, results from LREE-rich hydrous fluid circulation. The investigated peridotites underwent a three-stage evolution from depleted oceanic ridge residues via repeated depletion to metasomatic imprint within a supra-subduction-zone setting.