Implications of Differentiated Late Accretion for the Volatile Inventory of the Bulk Silicate Earth

Abstract

Earth is believed to have acquired its highly siderophile element (HSE) inventory through the late accretion of ∼0.3%–0.5% of its mass in chondrite-like materials, following the main stage of its growth. Late accretion, particularly if it originated from the outer solar system, could have significantly contributed to the bulk silicate Earth’s (BSE = mantle + crust + hydrosphere + atmosphere) carbon–nitrogen–hydrogen (C–N–H) inventory. However, recent studies, noting differences between the HSE inventory of the Earth and Moon’s mantle, suggest that relatively large lunar-sized differentiated impactors, rather than small chondritic projectiles, delivered HSEs to Earth’s mantle during late accretion. The implications of a differentiated late accretion event for the BSE’s C–N–H inventory remain unclear. In this study, we modeled the equilibrium partitioning of highly volatile C–N–H and moderately volatile sulfur–selenium–tellurium (S–Se–Te) between the atmosphere, magma ocean (MO), and core of lunar-sized or slightly larger impactors. The impactor’s MO-degassed atmosphere contained most of its C–N–H inventory, whereas almost all of the S–Se–Te was present in its core or mantle. Given the low escape velocity of lunar-sized impactors, the MO-degassed atmosphere was likely dissipated quickly after core formation. As a result, in contrast to S–Se–Te, the contribution of differentiated late accretion to BSE’s C–N–H inventory was limited, irrespective of its inner or outer solar system origin. The C–N–H-depleted nature of differentiated objects suggests that most of BSE’s highly volatile inventory was delivered by primitive chondritic materials toward the final stages of Earth’s accretion, before the Moon-forming event.

Keywords

• Earth (planet)
• Solar system formation
• Planetary system formation
• Planetary science