Redox controls on tungsten and uranium crystal/silicate melt partitioning and implications for the U/W and Th/W ratio of the lunar mantle
Redox controls on tungsten and uranium crystal/silicate melt partitioning and implications for the U/W and Th/W ratio of the lunar mantle
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DOI：
10.1016/j.epsl.2014.07.015
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发表时间：
2014-10-15
影响因子：
5.3
通讯作者：
Blanchard, Henrik
中科院分区：
文献类型：
--
作者：
Fonseca, Raul O. C.;Mallmann, Guilherme;Blanchard, Henrik
地核形成的时间对于了解地球和月球的早期分化历史至关重要。由于在金属-硅酸盐分离过程中Hf是亲石的，W是亲铁的，Hf-182向W-182的衰变（半衰期为9 Ma）已被证明是一个有用的核-幔分异事件的计时器。解释Hf-182/W-182数据的一个关键参数是原始（即未枯竭）地幔的Hf/W比值。由于W在地幔熔融过程中是不相容的，其相对于U和玄武岩中其他类似不相容元素（如Th、La）的比值可以作为其地幔来源的替代指标。然而，对于在很大范围内氧逸度（fO(2)）达到平衡的岩石系统来说，W和U同样不相容的假设可能是有缺陷的。虽然W通常被认为是同价的，但有证据表明，在地球和月球的硅酸盐地幔中推断的fO(2)范围内，U是异价的。本文报道了W、U、高场强元素（HFSE）和Th在斜辉石、正辉石、橄榄石、斜长石和硅酸盐熔体之间分配的新数据。与前人的研究一致，我们发现这些元素在地球上地幔的fO(2)特征上表现为同价元素。然而，W和U在低fO(2)时变得更相容，这表明它们的氧化还原状态发生了变化，W在逐渐降低fO(2)时变得更相容。W的这一结果特别出乎意料，因为即使在非常低的fO(2)下，这种元素也被认为是六价的。相对于W6+而言，W4+（在低fO(2)下推断的物种）的相容性要高得多，这意味着即使W4+的一小部分也会对w的整体相容性产生重大影响。我们的结果表明，在月球分化被认为发生的还原条件范围内（即类似于IW-2到IW-0.5）。当我们的分馏数据应用于月球岩浆海的分馏结晶模型时，U/W、Hf/W和Th/W的月球趋势得到了很好的再现。这个模型的结果表明，构成地球和月球的硅酸盐组分的Hf/W实际上是相同的。(C) 2014 Elsevier B.V.版权所有
The timing of core formation is essential for understanding the early differentiation history of the Earth and the Moon. Because Hf is lithophile and W is siderophile during metal-silicate segregation, the decay of Hf-182 to W-182 (half-life of 9 Ma) has proven to be a useful chronometer of core-mantle differentiation events. A key parameter for the interpretation of Hf-182/W-182 data is the Hf/W ratio of the primitive (i.e. undepleted) mantle. Since W is incompatible during mantle melting, its ratio relative to U and other similarly incompatible elements in basalts (e.g. Th, La) may be used as proxies for their mantle sources. However, the assumption that W and U are equally incompatible may be flawed for petrological systems that equilibrated over a large range of oxygen fugacity (fO(2)). Although W is typically perceived as being homovalent, evidence suggests that U is heterovalent over the range of fO(2) inferred for the silicate mantles of the Earth and the Moon.Here we report new partitioning data for W, U, high-field-strength elements (HFSE), and Th between clinopyroxene, orthopyroxene, olivine, plagioclase and silicate melt. In agreement with previous studies, we show that these elements behave as homovalent elements at fO(2) characteristic of Earth's upper mantle. However, both W and U become more compatible at low fO(2), indicating a change in their redox state, with W becoming more compatible at progressively lower fO(2). This result for W is particularly unexpected, because this element was thought to be hexavalent even at very low fO(2). The much higher compatibility of W4+ (the species inferred here at low fO(2)) relative to W6+ means that even a small fraction of W4+ will have a significant effect on the overall compatibility of W. Our results imply that over the range of reducing conditions in which lunar differentiation is thought to have taken place (i.e. similar to IW-2 to IW-0.5), W is likely to become fractionated from U. When our partitioning data are applied to model the fractional crystallization of a lunar magma ocean, lunar trends for U/W, Hf/W and Th/W are well reproduced. The result of this model carries with it the implication that the Hf/W of the bulk silicate fractions that comprise the Earth and the Moon are virtually identical. (C) 2014 Elsevier B.V. All rights reserved.