Metal–silicate partitioning of Mo and W at high pressures and temperatures: Evidence for late accretion of sulphur to the Earth
Metal–silicate partitioning of Mo and W at high pressures and temperatures: Evidence for late accretion of sulphur to the Earth
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DOI：
10.1016/j.gca.2012.01.010
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发表时间：
2012-05
影响因子：
5
通讯作者：
J. Wade;Bernard J. Wood;J. Tuff
中科院分区：
文献类型：
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作者：
J. Wade;Bernard J. Wood;J. Tuff
为了更好地限制地球和其他行星的核形成条件，我们在1.5- 24 GPa和1803- 2723 K的压力下进行了Mo和W在液态富铁金属和液态硅酸盐之间分配的实验。在1.5GPa/1923 K下在MgO胶囊中进行的实验表明，在低于IW（Fe-FeO）缓冲层的氧逸度>2log单位时，Mo在硅酸盐中处于+4氧化态。在1.5GPa/1923 K和低于IW缓冲区1.8-3.3log单位的温度下，硅酸盐中W ~（6+）是钨的主要氧化态。当我们的15个数据在6到24 GPa之间的压力与Cottrell等人（2009）的数据相结合时，我们发现在6 GPa以上的W氧化态变化和在实验fO 2范围内的分配压力依赖性变化都没有证据。钼和W的金属-硅酸盐分配显示出强烈的依赖于硅酸盐熔体组合物，这两种元素变得更加亲铁的熔体变得更加SiO2丰富。虽然在分区数据的趋势可以与硅酸盐熔体组合物中的非桥氧四面体阳离子NBOT的比例，我们发现，使用一个定期的解决方案模型的硅酸盐熔体的结果在一个显着更好地拟合的数据。我们结合我们的研究结果与文献中的那些得到适用于地球的分割方程。在重量分配方面，我们定义Diwtand（KDi）wtas如下：实验数据，当校正成分效应时，产生以下的地幔岩表达式：括号中的值对应于1个标准误差的拟合。这些表达式与Wade和Wood（2005）的连续吸积模型相结合，以研究它们对吸积过程的约束。然而，我们发现，增生路径与硅酸盐地球的Ni，Co，V，Cr和Nb的含量相一致，W应分区的两倍强烈的核心钼。这与估计的核幔分配系数形成鲜明对比，W的核幔分配系数为40 - 40，Mo的核幔分配系数为90-140。无论是对增生路径的改变，还是对局部不平衡的假设，都不能轻易改变这一结果。答案似乎在于核心中的一个轻元素的身份。我们研究了我们的吸积模型的影响，通过添加2%的这种元素（符合宇宙化学估计）的核心。如果在整个吸积过程中以恒定的S/Fe比加入S，则净效应可以忽略。然而，如果S只在吸积的最后10-20%期间加入，DMo和DW将与这些元素的硅酸盐地球含量相一致。仅需要对模型进行小的额外调整以适应Ni、Co、V、Cr和Nb的分配变化。我们的结论是，钼和钨含量的硅酸盐地球表明，S（和其他中等挥发性元素）被添加到地球的核心形成过程中，但只有在最后的20%的吸积。这一结论与Schönbächler等人（2010年）从硅酸盐地球的Ag同位素组成得出的结论相同。
In order to place better constraints on the conditions of core formation on Earth and other planetary bodies we have performed experiments to determine the partitioning of Mo and W between liquid Fe-rich metal and liquid silicate at pressures of 1.5–24GPa and temperatures of 1803–2723K. Experiments performed in MgO capsules at 1.5GPa/1923K indicate that Mo is in the +4 oxidation state in the silicate at oxygen fugacities >2log units below the IW (Fe–FeO) buffer. In contrast W6+is the dominant tungsten oxidation state in the silicate at 1.5GPa/1923K and 1.8–3.3log units below the IW buffer. When our 15 data for pressures between 6 and 24GPa are combined with those of Cottrell et al. (2009) we find evidence neither for a change in oxidation state of W above 6GPa nor for a change in pressure dependence of partitioning in the experimental fO2range. Metal–silicate partitioning of both Mo and W shows strong dependence on silicate melt composition with both elements becoming more siderophile as the melt becomes more SiO2-rich. Although the trends in the partitioning data can be related to silicate melt composition in terms of the ratio of nonbridging oxygens to tetrahedral cations NBOT we find that use of a regular solution model for the silicate melt results in a significantly better fit to the data. We combined our results with those in the literature to obtain partitioning equations applicable to the Earth. In terms of weight partitioning we define Diwtand (KDi)wtas follows: The experimental data, when corrected for compositional effects, yield the following expressions for a pyrolite mantle:The value in brackets corresponds to 1 standard error of the fit. These expressions were combined with the continuous accretion model of Wade and Wood (2005) to investigate the constraints which they place on the accretionary process. We find, however, that, for accretionary paths consistent with the silicate Earth contents of Ni, Co, V, Cr and Nb, W should partition twice as strongly into the core as Mo. This is in stark contrast to the estimated core–mantle partition coefficients of ∼40 for W and 90–140 for Mo. Neither changes to the accretionary path nor the assumption of partial disequilibrium can readily alter this result. The answer appears to reside with the identity of one of the light elements in the core. We investigated the effect of S on our accretionary model by adding 2% of this element (consistent with cosmochemical estimates) to the core. If S is added at constant S/Fe ratio throughout accretion the net effect is negligible. If, however, S is added exclusively during the last 10–20% of accretion DMoand DWbecome consistent with the silicate Earth contents of these elements. Only small additional adjustments to the model are required to accommodate changes in partitioning of Ni, Co, V, Cr and Nb. We conclude that the Mo and W contents of the silicate Earth indicate that S (and other moderately volatile elements) was added to the Earth during core formation but only during the last ∼20% of accretion. This conclusion is the same as that reached by Schönbächler et al. (2010) from the Ag isotopic composition of silicate Earth.