Phase Q - A carrier for subsolar noble gases

Introduction: In our series of experiments to determine the noble gases of phase Q in various classes of meteorites [1,2] we analysed the HF/HCl-resistant residue of an enstatite chondrite [3]. We have chosen the EH5 chondrite St. Mark’s because of its short cosmic-ray exposure age (~1Ma [4]) which is essential to reliably determine the He-Q and Ne-Q isotopic composition. E-chondrites might have accreted in regions closer to the Sun (e.g. [5]) than the ordinary and carbonaceous chondrites previously analysed [1,2]. Echondrites contain, in addition to common Q-gases [6], abundant “subsolar” noble gases [4,7]. This component has elemental and Ar-Xe isotopic compositions intermediate between solar and meteoritic and is supposed to reside in enstatite [8]. Here, we present new data, obtained by pyrolysis, for an aliquot of the residue and its oxidised remnants that remained after on-line stepped etching [3] to analyse the presolar noble gas contents of St. Mark’s and to assess the gas release efficiency. The on-line etch data yield He/He = (1.80±0.06)x10, Ne/Ne = 11.72±0.25 (by extrapolation to Ne/Ne = 0.0294), and Ar/Ar = 5.44±0.13 for the subsolar component. The latter is in agreement with 5.46±0.04 obtained from South Oman [7]. In addition, we re-determine the elemental composition of the subsolar component including its previously unknown He abundance.