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

Abstract

formation at a mid-ocean ridge. Cooling rates in the range of 1°C/ geochemistry; Bismarck Archipelago‐Papua New Guinea 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-, INTRODUCTION clinopyroxene-, phlogopite- and hornblende-bearing veins crosscutting Evidence for metasomatic processes in the Earth’s mantle spinel peridotites and olivine clinopyroxenites, as well as pervasively is provided by trace element and isotope variations in metasomatized plagioclase lherzolites. The metasomatic xenoliths volcanic rocks (e.g. Hawkesworth et al., 1987), by metareveal strong chemical disequilibria between the metasomatic minerals somatized sections of ultramafic massifs (e.g. Bonatti et and the adjacent, unaltered host rock minerals, which are especially al., 1981) and by metasomatized mantle xenoliths of prominent in the veined samples. Temperatures during the meta- mainly peridotitic composition, which are transported to somatic overprint, estimated using spinel‐olivine thermometry, range the surface by mafic or ultramafic magmas (e.g. Jagoutz between 660 and 950°C. Oxygen barometry reveals an elevated et al., 1979). Two different styles of metasomatism are oxygen fugacity, with log(fo2) FMQ values of 0·4 to >4. A geo- recorded in these rocks. Some ultramafic rocks show chemical study of the ultramafic rocks shows that all types of xenoliths cryptic metasomatism, resulting in trace element enhave been metasomatized. Pervasively metasomatized plagioclase richment without any visible mineral or textural changes lherzolites and cumulate olivine clinopyroxenites have high contents (Dawson, 1984). The second type of metasomatism is of middle and heavy rare earth elements compared with veined characterized by petrographically well-recognizable reperidotites. Cryptic metasomatism, indicated by increased light rare placement textures and/or the presence of secondary, earth elements and Nd concentrations, results from LREE-rich hydrous minerals (e.g. phlogopite, hornblende) as well as secondary clinopyroxene, plagioclase, and accessory hydrous fluid circulation. The investigated peridotites underwent a