Solar Noble Gases in Enstatite Chondrites and Implications for the Formation of the Terrestrial Planets

Introduction: In a companion abstract [1], we report evidence for tiny amounts of most likely primordial noble gases with solar-like elemental and isotopic composition admixed to Q-type primordial noble gases in an E chondrite. Here, we discuss possible implications of this finding for terrestrial planet formation. The genetic relation of the noble gases in planetary bodies to those in the Sun, seemingly their most likely precursors, is largely unknown [2]. This knowledge, however, is essential to assess the processes that have led to the specific noble gas compositions observed now, e.g. the noble gas inventories of the terrestrial interior and atmosphere [3]. The trapping mechanism of the noble gases into Earth and, accordingly, their primordial composition are under debate [see e.g. 2-6]. Major mechanisms that have been proposed include: Gravitational capture by proto-Earth (or sufficiently large precursor planetesimals) could have incorporated noble gases by dissolution into an early magma ocean. This would imply that Earth already had accreted a significant part (~30%) of its present mass [7] and the crust had melted before the solar nebula has completely dissipated (~10 Ma [8]). Implantation of solar wind: The precursors of the Earth (e.g. ~10-100-km-sized planetesimals at present solar wind intensity [9]) could have trapped solar wind in their uppermost layers during an unshielded early solar irradiation. This would require a thin solar nebula at the location where the planetesimals formed. He and Ne in the terrestrial mantle are solar, whereas Ar-Xe are not yet sufficiently constrained [2]. In view of many compositional similarities of Earth to carbonaceous [10] and enstatite [11] chondrites, it seems surprising that primordial solar-like noble gases have hardly ever been reported in meteorites. An exception is olivine in the pallasite Brenham [12]. A quite prominent component with a composition somewhat closer to solar than the typical primordial component in meteorites is the subsolar component, reported in many cases, in particular E chondrites. This component might provide a link between the inferred terrestrial primordial noble gas composition and primitive meteorites, in being enriched in Ar relative to Xe and showing He-Ar isotopic ratios closer to solar [13]. E chondrites: Based on their subsolar noble gases and other observations [14], E chondrites have been suggested to have formed closer to the Sun than other chondrites and thus might be particularly suitable to record processes that occurred to material that accreted to Earth. The oxygen isotopic composition of E chondrites plotting close to the terrestrial fractionation line [15] supports this view.