Noble gases in anhydrous lherzolites from the Newer Volcanics, southeastern Australia: A MORB-like reservoir in the subcontinental mantle

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 X 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 CO₂-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 CO₂-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.