Tracing Earth's Volatile Delivery With Tin

Abstract

Earth's accretion history for volatile elements, and the origin of their depletions with respect to the Sun and primitive meteorites, continue to be debated. Two end-member scenarios propose either that volatile elements were delivered during the main phases of accretion and differentiation, or that the Earth accreted from materials largely devoid of volatiles with late addition of volatile-rich materials. Experiments evaluating the effect of metal–silicate equilibrium on elemental and isotopic distribution of volatile and siderophile elements such as Sn can help to distinguish between these scenarios. In this study, we have systematically investigated the relative influence of temperature, pressure, oxygen fugacity, and metal and silicate composition on the metal–silicate partioning behavior of Sn, from 2 to 20 GPa and 1,700 to 2,573 K, indicating that Sn siderophility noticeably decreases with temperature and S content of the metal but increases dramatically with pressure. A resolvable isotopic fractionation factor between metal and silicate suggests that core–mantle equilibrium temperatures (∼3,000 K) could potentially generate a Sn isotopic composition of the mantle lighter than the core by 150–200 ppm/amu. Core formation modeling shows that the volatiles were added during the last 10% of the accretion history. A final core containing 2.5 to 3.5 wt.% S is required. Furthermore, modeling of the BSE isotopic composition argues for a late Sn delivery on Earth with carbonaceous chondrite-like material as the most likely source of volatiles. Therefore, both elemental and isotopic approaches converge toward an identical volatile accretion scenario, involving a late volatile delivery.