Noble gas and carbon isotope ratios in Argyle diamonds, Western Australia: Evidence for a deeply subducted volatile component

Abstract

The Argyle lamproite pipe of Western Australia contains diamonds formed at depths exceeding 150 km. We undertook noble gas and carbon isotope ratio (δ13C) analyses of three diamonds (likely of eclogitic paragenesis) from the Argyle lamproite to test for the possible presence of deeply subducted volatile components, and to further constrain the noble gas evolution of the Earth's mantle. The Argyle diamonds are characterised by mantle 3He (with 3He/4He ratios of 0.79 R A to 0.25 R A, where R A is the atmospheric 3He/4He ratio of 1.4 × 10–6), small excess Ar and Xe isotope anomalies relative to atmospheric components, and δ13C values of –11.6 to –10.2‰ VPDB. These observations indicate that noble gas and carbon isotopic compositions of the mantle where the Argyle diamonds formed, represent mixtures of an intrinsic mantle component with sedimentary and atmospheric components that may have been introduced through subduction processes.