Low solubility of He and Ar in carbonatitic liquids: Implications for decoupling noble gas and lithophile isotope systems

Abstract

In order to asses the importance of carbonatitic liquids in transporting noble gases in the mantle, the solubilities of He and Ar in carbonatitic liquids were estimated from analyses of calcium–potassium carbonate glasses that had been synthesized at 1 bar and temperatures between 850 and 950 °C under He or Ar enriched atmospheres. Despite poor reproducibility related to difficulties synthesizing carbonatite glass, we have been able to estimate He and Ar solubilities in carbonatite liquids to be 1 × 10 −8 and 2 × 10 −9 mol g −1 at 1 bar respectively (with ⩾50% uncertainty). Despite the significant uncertainties on these estimates, it is clear that the noble gases are not massively soluble in carbonatite liquids (within error, these solubilities are identical to their equivalent solubilities in tholeiitic melts). Assuming the results of these low pressure experiments can be applied to mantle conditions, it seems unlikely that carbonatite metasomatism per se transports noble gases within the mantle. It is nevertheless possible that partitioning of lithophile trace elements (including the important radioelements, U, K and Th) and noble gases between a carbonatitic melt and a silicate melt could effectively decouple lithophile and noble gas isotope systematics because the carbonatitic melt expressedly does not transport noble gases, yet is known to efficiently transport incompatible trace elements.