Core-mantle fractionation of carbon in Earth and Mars: The effects of sulfur
Core-mantle fractionation of carbon in Earth and Mars: The effects of sulfur
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DOI：
10.1016/j.gca.2018.07.010
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发表时间：
2018-10
影响因子：
5
通讯作者：
K. Tsuno;D. Grewal;R. Dasgupta
中科院分区：
文献类型：
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作者：
K. Tsuno;D. Grewal;R. Dasgupta
在地球和其他类地行星的地壳-大气、地幔和地核等储层中，控制硅酸盐岩浆海洋（MO）和成核合金液体在早期分异过程中的碳(C)分异对理解碳的起源和早期分布至关重要。然而，在高压(P)-高温(T)条件下，合金液体中其他轻元素（如硫(S)）对与岩心形成相关的铁合金成分中C -硅酸盐分配和C溶解度的影响的实验数据缺乏。本文在6 - 13gpa和1800-2000℃条件下进行了多砧实验，研究了S和Ni对C在富铁合金液中的溶解度极限的影响，以及C在合金液和硅酸盐熔体之间的分配行为（D - C / l / S, l - C / S, l - C / t）。结果表明：随着合金液中s含量的增加，合金液中C的溶解度和D - C的溶解度均随合金液中s含量的增加而降低；利用我们的新实验数据和之前的实验对合金液中的C溶解度进行了经验回归，结果表明，C溶解度随着温度的升高而显著增加，而与贫s或无s合金成分不同，Ni对富s合金液中的C溶解度没有明显的影响。我们的模拟结果证实了之前的发现，即为了满足BSE的C预算，进行合金硅酸盐分馏的地球C的体积需要与ci型碳质球粒陨石的体积一样高，即在吸积过程中不留下任何挥发性碳损失的空间。另一方面，对于火星来说，在相对氧化的条件下（低于IW缓冲层1.0 log单位），平均单阶段岩心形成，在岩心中含有10-16 wt%的S，可以产生一个碳收支类似于地球BSE的火星地幔，总体碳含量为0.25-0.9 wt%。对于C在后期由富含s的差异化撞击器传递到原地球的情况，我们的模型计算预测，撞击器中的总体C含量可低至0.5 wt%，而撞击器的质量介于当今地球质量的9%至20%之间。这个值远高于Li等人预测的撞击器中体积C的0.05-0.1 wt% (Li Y., Dasgupta R., Tsuno K., Monteleone B., and Shimizu N.（2016）硅酸盐地球的碳和硫收支由分化的行星胚胎增生解释。Nat. geosi . 9, 781-785)，因为他们的模型预测的富s合金的c溶解度极限为0.3 wt%，明显低于撞击器核心中相关s含量的实验得出的c溶解度~ 1.6 wt%。
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 (D C a l l o y/s i l i c a t e). The results show that C solubility in the alloy liquid as well as D C a l l o y/s i l i c a t e 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, ie, 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.