The Osmium Isotopic Composition of Tagish Lake and Other Chondrites, Implications for Late Terrestrial Planetary Accretion

Introduction: The prevailing models for terrestrial planet origin and differentiation call upon magma oceans generally concurrent with core formation very early during differentiation. Tungsten isotope measurements of Earth and Mars materials are consistent with core formation occurring in terrestrial planets within 30 m.y. or less after the onset of condensation of the solar system [1-4]. In such models, extraction of the core likely leaves the silicate portions of the Earth and Mars strongly depleted in the highly siderophile elements (HSE), In addition, magma ocean development and other processes occurring during early accretion and differentiation would result in strong degassing, and would leave early interiors of Earth and Mars relatively volatile depleted [9]. A result of such processes is that late accretion of materials subsequent to core formation and magma ocean development is may be necessary to replenish both volatiles and HSE to their estimated and measured bulk silicate Earth and Mars concentrations [5-10]. A question that is crucial for constraining early Earth and Mars differentiation is thus, was an acquisition of volatiles and highly siderophile elements during late accretionary processes coupled? If the abundances of these two elemental groups are coupled, then materials must have existed that are volatile-rich, and have the requisite HSE abundances and Os isotopic compositions that would signify that one type of material could be the supplier. Alternatively, materials that are strongly HSE depleted but volatile-rich, and vice versa, could have both contributed, or the mechanisms of volatile and HSE replenishment are decoupled. Each of these scenarios has important consequences to the earliest history of terrestrial planetary differentiation and implications for the types of materials present in the inner solar system during those events. There are drawbacks to having late accretion material with elevated HSE that is volatile-rich. The relatively volatile-rich carbonaceous (C) chondrites have present day Os isotopic compositions that are too low to explain the proposed Earth's primitive upper mantle (EPUM), reflecting a time-integrated lower Re/Os [11]. However, the Tagish Lake C-chondrite may be material that could fulfill these requirements. Brown et al. [12] measured concentrations for Re, Os, and Pt for one aliquot of Tagish Lake. The calculated Re/Os