Silicon isotopes in angrites and volatile loss in planetesimals

Abstract

Inner solar system bodies, including the Earth, Moon, and asteroids, are depleted in volatile elements relative to chondrites. Hypotheses for this volatile element depletion include incomplete condensation from the solar nebula and volatile loss during energetic impacts. These processes are expected to each produce characteristic stable isotope signatures. However, processes of planetary differentiation may also modify the isotopic composition of geochemical reservoirs. Angrites are rare meteorites that crystallized only a few million years after calcium-aluminum-rich inclusions and exhibit extreme depletions in volatile elements relative to chondrites, making them ideal samples with which to study volatile element depletion in the early solar system. Here we present high-precision Si isotope data that show angrites are enriched in the heavy isotopes of Si relative to chondritic meteorites by 50-100 ppm/amu. Silicon is sufficiently volatile such that it may be isotopically fractionated during incomplete condensation or evaporative mass loss, but theoretical calculations and experimental results also predict isotope fractionation under specific conditions of metal-silicate differentiation. We show that the Si isotope composition of angrites cannot be explained by any plausible core formation scenario, but rather reflects isotope fractionation during impact-induced evaporation. Our results indicate planetesimals initially formed from volatile-rich material and were subsequently depleted in volatile elements during accretion.volatiles | accretion | isotopes | angrites | silicon

Early stages of core segregation recorded by Fe isotopes in an asteroidal mantle

“…platinum group elements) demonstrate that ureilites experienced separation of S-rich metallic melts, with possibly entrainment of metal (Warren et al, 2006;Rankenburg et al, 2008;Goodrich et al, 2013b), but the process (batch or fractional) and temperatures of segregation are not well constrained by these data. The heavy Fe isotopic compositions and the low S contents of the ureilites are consistent with the separation of S-rich metallic melts (Fig.…”

“…This explanation was initially proposed for the Earth and the Moon by Poitrasson et al (2004) but is controversial for these bodies (e.g., Halliday, 2013). It is probably valid for the angrite parent body (APB), where a strong depletion in volatile elements such as the alkalis, is accompanied by heavy Fe and Si isotopic compositions (Wang et al, 2012;Pringle et al, 2014). The UPB did not accrete from volatile depleted materials, because the magmas generated by the melting of its mantle were alkali and volatile-rich as shown by the compositions of the plagioclases in the feldspathic clasts found in polymict ureilites and the geochemistry of ALM-A, a trachyandesitic lava from the UPB (e.g., Cohen et al, 2004;Bischoff et al, 2014).…”

“…Nevertheless, it was heated enough to generate magmas (e.g. Cohen et al, 2004;Bischoff et al, 2014) and a small S-rich core (Warren et al, 2006;Rankenburg et al, 2008;Goodrich et al, 2013b).…”

Silicon isotope fractionation in rice and cucumber plants over a life cycle: Laboratory studies at different external silicon concentrations

“…The study of silicon isotopes in deep‐sea water could improve our understanding of the impact that the silicon biogeochemical cycle has on the silicic acid concentration gradient and the corresponding silicon isotopic compositions [ Grasse et al ., ]. On the current status of research, the study on silicon isotope has been even extended to the field of space [ Pringle et al ., ].…”

“…compositions [Grasse et al, 2013]. On the current status of research, the study on silicon isotope has been even extended to the field of space [Pringle et al, 2014].…”