GCA: Core-mantle fractionation of carbon in Earth and Mars: The effects of sulfur

Tsuno, K., Grewal, D.S., Dasgupta, R. (2018). Core-mantle fractionation of carbon in Earth and Mars: The effects of sulfur. Geochimica et Cosmochimica Acta 238: 477-495.

 

Abstract: Constraining carbon (C) fractionation between silicate magma ocean (MO) and core-forming alloy liquid during early differentiation is essential to understand the origin and early distribution of C between reservoirs such as the crust-atmosphere,
mantle, and core of Earth and other terrestrial planets. Yet experimental data at high pressure (P)-temperature (T) on the
effect of other light elements such as sulfur (S) in alloy liquid on alloy-silicate partitioning of C and C solubility in
Fe-alloy compositions relevant for core formation is lacking. Here we have performed multi-anvil experiments at
6–13 GPa and 1800–2000 °C to examine the effects of S and Ni on the solubility limit of C in Fe-rich alloy liquid as well
as partitioning behavior of C between alloy liquid and silicate melt (Dc). The results show that C solubility in the alloy
liquid as well as Dc decreases with increasing in S content in the alloy liquid. Empirical regression on C solubility in
alloy liquid using our new experimental data and previous experiments demonstrates that C solubility significantly increases
with increasing temperature, whereas unlike in S-poor or S-free alloy compositions, there is no discernible effect of Ni on C
solubility in S-rich alloy liquid.
Our modelling results confirm previous findings that in order to satisfy the C budget of BSE, the bulk Earth C undergoing
alloy-silicate fractionation needs to be as high as those of CI-type carbonaceous chondrite, i.e., not leaving any room for
volatility-induced loss of carbon during accretion. For Mars, on the other hand, an average single-stage core formation at
relatively oxidized conditions (1.0 log unit below IW buffer) with 10–16 wt% S in the core could yield a Martian mantle with
a C budget similar to that of Earth’s BSE for a bulk C content of 0.25–0.9 wt%. For the scenario where C was delivered to
the proto-Earth by a S-rich differentiated impactor at a later stage, our model calculations predict that bulk C content in the
impactor can be as low as ~0.5 wt% for an impactor mass that lies between 9 and 20% of present day Earth’s mass. This value
is much higher than 0.05–0.1 wt% bulk C in the impactor predicted by Li et al. (Li Y., Dasgupta R., Tsuno K., Monteleone B.,
and Shimizu N. (2016) Carbon and sulfur budget of the silicate Earth explained by accretion of differentiated planetary
embryos. Nat. Geosci. 9, 781–785) because C-solubility limit of 0.3 wt% in a S-rich alloy predicted by their models is significantly
lower than the experimentally derived C-solubility of 1.6 wt% for the relevant S-content in the core of the impactor.

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