Noble gases in pyroxenites and metasomatised peridotites from the Newer Volcanics, southeastern Australia: implications for mantle metasomatism

The elemental and isotopic compositions of five noble gases (He, Ne, Ar, Kr and Xe) have been determined in selected, well-documented, ultramafic xeno liths from southeastern Australia. These xenoliths include both spinel-bear ing peridotites with an apparent metasomatic overprint and garnet-bearing p yroxenites. In general, helium, neon, argon and xenon isotopic ratios from gases trapped in fluid inclusions of the samples are mid-ocean ridge basalt (MORB)-like, as has previously been found in anhydrous Iherzolite xenolith s from the same area. In addition to the MORB-like components, radiogenic ( He-4*) and nucleogenic (Ne-21* and Ne-22*) components were found in the pre sent samples, reflecting relatively high U and Th contents in the metasomat ic minerals such as amphibole, apatite and clinopyroxene. These components are only released upon melting, thus are likely to be trapped in the crysta l lattices of the minerals. The MORB-like noble gas component found in fluid inclusions (invariably CO2 -rich) of these samples probably was introduced into the lithospheric mantl e by metasomatising melts derived from the upper mantle. The noble gases di ssolved in the metasomatising melt are likely to be effectively decoupled f rom incompatible elements during segregation of a CO2-rich fluid in the asc ending melt. In the metasomatising melt, the noble gases would partition in to the CO2-rich fluid whereas the incompatible elements would remain in the melt phase. As a result, the noble gas composition in fluid inclusions of the minerals appears to be independent of their degree of metasomatism indi cated by their host rock mineralogy and trace elemental geochemistry. Once noble gases are trapped in CO2-rich fluid inclusions within the minerals, t hey would preserve their source signatures without being affected significa ntly by the ingrowth of radiogenic and nucleogenic products, because the U, Tn and K would remain in the melt. The predominance of MORB-like noble gas signatures in fluid inclusions of the xenoliths suggests that the metasoma tising components were derived from the asthenospheric mantle underlying so utheastern Australia. (C) 2000 Elsevier Science B.V. All rights reserved.