Subsolar Noble Gases in an Acid-resistant Residue of the Eh5 Chondrite St

Introduction: Enstatite chondrites have remarkable properties: they were formed in a reducing environment, they are putatively similar to precursor material of Earth, Venus, or Mercury, have a terrestrial oxygen isotopic composition and an uncommon noble gas signature [1-5]. Trapped noble gases in Echondrites are intermediate between solar and chondritic composition and consist of two components: “Subsolar”, enriched in Ar and Kr relative to Xe and Q-gas, possibly residing in enstatite, micro-vesicles or graphite within enstatite [6,7] and Q-gases [5,8]. He and Ne isotopic and elemental abundances of Q and subsolar gases in E-chondrites are not well defined, but essential to determine the origin of these gases. Here, we present preliminary data of a stepwise online etching experiment [9] on a carbonaceous HF/HCl-resistant residue of EH5 St. Mark’s, especially suitable due to a cosmic-ray exposure age of only 1Ma [10]. Results and discussion: The first steps show relatively constant elemental abundances, indicating the release of a single component. Surprisingly, these steps yield a subsolar element pattern with Ar/Xe ~400 and Kr/Xe ~1.7, similar to the bulk composition [10]. We cannot decide yet whether the oxidisable acid-resistant residue, by definition phase Q, represents the main carrier, because ~4% of the bulk concentrations were released so far. Enstatite [6] is clearly not the only subsolar gas carrier.