Temperature dependent loss of noble gases from insoluble organic matter and presolar grains during hydrothermal alteration

Abstract

The effects of hydrothermal alteration on primordial noble gases were investigated by analysing noble gases in aliquots of insoluble organic matter (IOM) from the Murchison meteorite (CM2) that were hydrothermally altered for different durations (27–163 days) and at a range of temperatures (250–450 °C). The samples contained Q gases, HL from presolar nanodiamonds, and Ne-E from presolar SiC and graphite. We observed changes in the noble gases that correlated with changes in the experimental reaction temperature. Losses of Ne-E occurred in samples reacted at 250–300 °C (23 % of the Ne-E present in unaltered samples) and in samples reacted at 350–450 °C (72 %). This indicates that temperature drives degassing of Ne-E from presolar SiC and graphite and that Ne-E can be degassed at temperatures below 400 °C under hydrothermal conditions. Elemental ratios of ²⁰Ne/³⁶Ar correlate with reaction temperature. This relationship can be used to estimate the peak hydrothermal alteration temperature experienced, as Ne is predominantly carried in presolar diamonds and Ar in Phase Q, which respond differently to hydrothermal alteration. The calculated temperatures using the correlation between ²⁰Ne/³⁶Ar and temperature with ²⁰Ne/³⁶Ar ratios from previously published data agree well with temperatures in the literature determined by other techniques when ratios are between 0.15 and 0.30. Therefore ²⁰Ne/³⁶Ar ratios have the potential to be used as a parent body hydrothermal alteration thermometer.