B4 - The atmo- and hydrophile element (H and halogens) inventory of the Earth and Moon

Research areas: Geochemistry, petrology, mineralogy

Summary

In the Earth–Moon system, the origins of atmophile and hydrophile elements, such as H, C, and the halogens, and the processes that govern their abundance remain poorly understood. These elements either accreted and partially degassed during the main stages of Earth’s construction, or they accreted at least partially late after the core was formed. Furthermore, to understand the overall volatile element budget of the Earth and Moon, we must know about the abundances of the atmophile and hydrophile elements in the crust and mantle, as well as the constraints on their behaviour during the giant impact, magma ocean crystallisation, and magmatic differentiation. Compared to CI chondrites, all terrestrial planets are depleted in water and other volatiles, reflecting the variable loss of volatile elements during solar system condensation and possibly during planet formation and differentiation. Lunar volcanic rocks are even more depleted in volatiles, which supports the prevailing giant impact model. Even though atmophile elements occur only in trace amounts in the terrestrial planets, they are instrumental for establishing and maintaining plate tectonics, and they greatly influence Earth’s internal dynamics and global biogeochemical cycles. For example, high abundances of certain hydrophile elements (e.g., H, Cl, and F) lowers the liquidus of mantle rocks, which results in lower viscosity and enhanced transport of heat and matter within the planetary interior. At shallower depths, these elements facilitate tectonic responses to plate motion, magma generation, and element mobilisation during a wide range of high and low temperature processes. Therefore, the atmophile and hydrophile element budget of the Earth–Moon system is key for understanding its formation and subsequent geodynamic evolution (subproject C4 and C5).

Earth’s atmophile and hydrophile element budget also provides clues about the time of planetary volatile accretion and offers constraints on the subsequent redistribution of volatile elements during planetary differentiation. The Moon likely formed at the end of Earth’s main accretion phase, and it remains debated whether both bodies were initially dry or wet, or if it accreted dry and received its volatiles exclusively from late-impacting asteroids or comets (see also other B subprojects). Therefore, volatiles on the Earth and Moon may have, at least in part, a different origin. Moreover, a resolvable signal of late volatile element delivery as proposed for the Earth may also be discernible from the lunar record. Hence, the proposed project will investigate the atmophile and hydrophile element inventory of Earth and Moon from three different angles, employing geochemical–petrological studies on terrestrial (Part 1) and lunar (Part 2) samples as well as experimental partitioning studies (Part 3) that will provide key constraints for quantitatively interpreting the volatile inventory. We specifically aim to (1) better constrain the terrestrial volatile element content by determining the H, halogen and the H, Cl, and S isotope composition of olivine-hosted melt inclusions in a greater variety of mantle-derived rocks than investigated to date. Furthermore, we aim to (2) determine volatile element contents (H and halogens) and the isotope ratios of H, Cl, and S in phosphates from a range of lunar rocks. To complement interpretation of the analytical results, partitioning experiments will (3) quantify the behaviour of atmo- and hydrophile elements during the early evolution of Earth and Moon. In combination with the experimental data, the new atmophile and hydrophile element data on lunar and terrestrial samples will enable us to better constrain the origin, present budget, and behaviour of the volatile elements during formation and differentiation of the Earth and Moon. In combination with information provided from projects B1 and B2 on the relative contribution of late-accreted material (and its volatile content), the new data will ultimately allow us to determine whether Earth and Moon accreted wet or dry and how much of the Earth’s and Moon’s volatile element budget was acquired during the late accretion stage.