Garnet-melt noble gas partitioning and its relevance to the deep isolated reservoir hypothesis

Abstract

<p>Noble gas isotopes have been used to argue that hotspot volcanism taps a deep reservoir in the mantle that has remained largely isolated since the accretion of the Earth.  In order to evaluate the viability of this theory, it is important to understand how noble gases are stored at high pressure, and how processes such as melt separation may influence their transport.  Previous work (eg. Heber et al. 2007) has investigated the partitioning of noble gases in upper mantle minerals (olivine and pyroxenes), but as yet no data are available for other important phases, including garnet and higher-pressure minerals.  This study presents data collected from multi-anvil experiments at 6 GPa and 1700 °C – 1900 °C on artificial basalt compositions similar to those found at ocean island hotspots.  This composition has garnet on the liquidus at these conditions, and we have successfully quenched the melt to a glass.  The partitioning of noble gases between liquidus garnets and co-existing melts has been evaluated using a microprobe and laser ablation mass spectrometry to analyse the gas contents of the two phases.  These results shed light on the behaviour of noble gases in the presence of minerals that have, as yet, not been investigated for their ability to store such volatiles, and on the likelihood of the deep-untapped-reservoir theory.</p>