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

Abstract

The elemental and isotopic compositions of five noble gases (He, Ne, Ar, Kr and Xe) have been determined in selected, well-documented, ultramafic xenoliths from southeastern Australia. These xenoliths include both spinel-bearing peridotites with an apparent metasomatic overprint and garnet-bearing pyroxenites. 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 lherzolite xenoliths from the same area. In addition to the MORB-like components, radiogenic ( 4He*) and nucleogenic ( 21Ne* and 22Ne*) components were found in the present samples, reflecting relatively high U and Th contents in the metasomatic minerals such as amphibole, apatite and clinopyroxene. These components are only released upon melting, thus are likely to be trapped in the crystal lattices of the minerals. The MORB-like noble gas component found in fluid inclusions (invariably CO 2-rich) of these samples probably was introduced into the lithospheric mantle by metasomatising melts derived from the upper mantle. The noble gases dissolved in the metasomatising melt are likely to be effectively decoupled from incompatible elements during segregation of a CO 2-rich fluid in the ascending melt. In the metasomatising melt, the noble gases would partition into the CO 2-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 indicated by their host rock mineralogy and trace elemental geochemistry. Once noble gases are trapped in CO 2-rich fluid inclusions within the minerals, they would preserve their source signatures without being affected significantly by the ingrowth of radiogenic and nucleogenic products, because the U, Th and K would remain in the melt. The predominance of MORB-like noble gas signatures in fluid inclusions of the xenoliths suggests that the metasomatising components were derived from the asthenospheric mantle underlying southeastern Australia.