Noble gases in anhydrous lherzolites from the newer volcanics, southeastern Australia: a MORB-like reservoir in the subcontinental mantle

Abstract

In order to understand the nature of noble gas reservoirs beneath continents, elemental and isotopic compositions of noble gases were analyzed from spinel-lherzolites with anhydrous mineral assemblages (i.e., olivine > clinopyroxene ∼ orthopyroxene > spinel); these are considered to be typical of the upper lithospheric mantle. The observed noble gas elemental and isotopic compositions are very similar to those found in MORBs. Characteristic features include (1) 3He/ 4He ratio of about 1 × 10 −5, (2) linearly correlated 20Ne/ 22Ne and 21Ne/ 22Ne ratios plotting on the well-established MORB-line, (3) relatively high 40Ar/ 36Ar ratios, and (4) correlated excesses in 129Xe and 136Xe with respect to atmospheric xenon. In addition, 3He to 22Ne ratios are close to those of a solar component, reinforcing the view that the Earth contains primordial helium and neon of solar composition. The MORB-like noble gases are trapped predominantly in CO 2-rich fluid inclusions, which are inferred to be of secondary origin. These gases probably are of metasomatic origin, having been introduced into the lithospheric mantle from the convective upper mantle by CO 2-bearing melts. The results indicate that a MORB-like noble gas reservoir may characterize the upper part of the mantle on a global scale. This reservoir may be underlain by a relatively less degassed mantle reservoir, as plume-like mantle noble gases also have been identified beneath southeastern Australia (Matsumoto et al., 1997). Thus, some form of chemical layering appears to be required to explain the noble gas signatures both in the subcontinental and suboceanic mantle.