Institut für Mineralogie, Corrensstrassee 24,
D-48149 Münster, Tel.: +49 251-8333047,
In current formation models of the Earth, the budgets of the siderophile volatile elements (SVE) S, Se, and Te and the highly siderophile elements (HSE) in the silicate Earth are assumed to predominantly derive from the late accretion of material that had a broadly chondritic bulk composition. But, some HSEs, most notably Pd, appear to display less siderophile behaviour at the very high temperatures and pressures that prevailed during Earth’s core formation. Thus, a late veneer is not needed to explain the elevated Pd abundances in the Earth’s mantle. However, the abundances of other HSEs (Ir, Os) cannot be accounted for by equilibrium partitioning during core formation, even under very high temperatures and pressures. These contrasting results can be reconciled if the HSE abundances in the Earth’s mantle reflect not only the late veneer but also a residual signature of core formation. This would also be consistent with the elevated Pd/Ir and Pd/Os of the Earth’s mantle, although these non-chondritic ratios may likewise be a distinctive feature of the late-accreted material.
Similar to the HSEs, the budget of the SVEs S, Se, and Te in the silicate Earth are commonly thought to derive almost entirely from the late veneer. This view has been challenged, however, by recent data on the S isotopic composition of ocean ridge basalts derived from the Earth’s mantle. These data suggest that the S isotopic composition in the Earth’s mantle and in chondrites is different, which was interpreted to reflect mass-dependent fractionation of S isotopes during metal–silicate segregation. This difference would imply that about half of the S in the Earth’s mantle might be left over from metal–silicate segregation during core formation, meaning that only the remaining S was delivered by the late veneer. This conclusion, however, contradicts available data on the metal–silicate partitioning of S, Se, and Te and on the chondritic abundance ratios of S, Se, and Te in mantle rocks, which suggest that >90% of the budget of S and essentially the complete budget of Se and Te were delivered by a late veneer.
Therefore, in light of these discrepancies, the objectives of this project are twofold: first, we aim to study the stable isotope composition of S, Te, and Pd in samples derived from the terrestrial mantle, as well as the stable isotope composition of Te and Pd in chondritic bulk rocks. Second, we aim to experimentally calibrate the stable isotope fractionation imparted by metal–silicate equilibration during core formation by performing isotope fractionation experiments at a range of relevant conditions.
Overall, this study will contribute critical data to help resolve the discrepancy between the isotopic and concentration data of S, and it will also help to assess whether the elevated Pd abundances in the Earth’s mantle can or cannot be a residual signature of core formation. As such, by determining how core formation and the late veneer affected the budget of HSEs and SVEs in the Earth’s mantle, this study will help us quantitatively constrain the relative roles of these crucial events in Earth’s history.
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.
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Steenstra, E. S., Agmon, N., Berndt, J., Klemme, S., Matveev, S., van Westrenen, W., 2018: Depletion of potassium and sodium in mantles of Mars, Moon and Vesta by core formation, Scientific Reports 8, Article number: 7053. 10.1038/s41598-018-25505-6
Grützner, T., Klemme, S., Rohrbach, A., Gervasoni, F., Berndt, J., 2018: The effect of fluorine on the stability of wadsleyite: Implications for the nature and depths of the transition zone in the Earth’s mantle. Earth and Planetary Science Letters, Vol. 482, pp. 236-244. 10.1016/j.epsl.2017.11.011
Grützner, T., Kohn, S. C., Bromiley, D. W., Rohrbach, A., Berndt, J., Klemme, S., 2017: The storage capacity of fluorine in olivine and pyroxene under Upper Mantle conditions. Geochimica et Cosmochimica Acta, Vol. 208, pp. 160-170. 10.1002/2016JE005221.
Grützner, T., Klemme, S., Rohrbach, A., Gervasoni, F., Berndt, J., 2017: The role of F-clinohumite in volatile recycling processes in subduction zones. Geology, Vol. 45, pp. 443-446. 10.1130/G38788.1