Molybdenum isotope fractionation between Mo4+ and Mo6+ in silicate liquid and metallic Mo
Molybdenum isotope fractionation between Mo4+ and Mo6+ in silicate liquid and metallic Mo
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DOI：
10.1016/j.chemgeo.2018.11.014
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发表时间：
2019-01
期刊：
影响因子：
3.9
通讯作者：
R. Hin;Antony D. Burnham;D. Gianolio;M. Walter;T. Elliott
中科院分区：
文献类型：
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作者：
R. Hin;Antony D. Burnham;D. Gianolio;M. Walter;T. Elliott
以前的工作表明，即使在适合于核心形成的温度下，Mo同位素也可以在金属和硅酸盐液体之间测量到断裂。然而，硅酸盐液体中Mo结构环境的变化（特别是作为价态的函数）对Mo同位素分馏的影响仍然研究得很少。我们研究了价态在金属硅酸盐实验中的作用，实验是在1400° C的气体控制炉中进行的，氧逸度在10− 12.7和10-9.9之间，即在铁-方铁矿缓冲层以下3到0.2个对数单位之间。进行了两组实验，都是在CaO-Al 2 O3-SiO2系统中使用硅酸盐液体。一组使用钼金属线环作为金属源，另一组使用在石英玻璃管中含有2.5wt% Mo的液态金合金。X射线吸收近边光谱分析表明，在氧逸度为10-12.0和10-9.9的线环实验中，硅酸盐玻璃中的Mo 6+/Al 2 O3 Mo在0.24和0.77之间变化;在Au-Mo合金实验中，在10-11.4和10- 9.9的线环实验中，硅酸盐玻璃中的Mo 6+/Al 2 O3 Mo在0.15和0.48之间变化。Mo同位素组成的双加标分析进一步表明，金属-硅酸盐体系中Mo同位素分馏与硅酸盐玻璃中的Mo 6+/Al 2 O3 Mo成线性关系，纯Mo 4+与纯Mo 6+硅酸盐体系中的98 Mo/95 Mo相差0.51‰。前者为八面体配位，后者为四面体配位。我们的研究表明，以前的实验工作中包含的硅酸盐液体中的Mo 4+和Mo 6+物种的混合物。我们对金属和硅酸盐之间Mo同位素分馏的精确参数化可以描述为Δ 98/95 Mo金属-硅酸盐=− 1.43±0.14× 10 6 Mo 6+/Mo+ 8±6× 10 4 T2因此，如果Mo 6+/Mo +8的参数化可以作为限制行星体核形成过程中氧逸度的替代指标，钼的变化与氧逸度扩大，例如，包括含铁系统。在地球上，文献数据表明，上地幔相对于大部分地球而言，重钼同位素是贫化的，如钙钛矿所代表的。如前所述，这种差异很可能不是由地核形成引起的，地核形成要么使地幔中的重Mo同位素富集，要么不引起明显的分馏，这取决于温度和我们在这里确定的Mo 6+含量。我们重申，核心的形成不占现代上地幔的钼同位素组成，这可能会反映在俯冲过程中的分馏的影响，作为全球板块循环的一部分。
Previous work has shown that Mo isotopes measurably fractionate between metal and silicate liquids, even at temperatures appropriate for core formation. However, the effect of variations in the structural environment of Mo in the silicate liquid, especially as a function of valence state, on Mo isotope fractionation remained poorly explored. We have investigated the role of valence state in metal-silicate experiments in a gas-controlled furnace at 1400° C and at oxygen fugacities between 10− 12.7 and 10–9.9, ie between three and 0.2 log units below the iron-wüstite buffer. Two sets of experiments were performed, both with a silicate liquid in the CaO-Al 2 O 3-SiO 2 system. One set used molybdenum metal wire loops as the metal source, the other liquid gold alloyed with 2.5 wt% Mo contained in silica glass tubes. X-ray absorption near-edge spectroscopy analysis indicates that Mo 6+/ΣMo in the silicate glasses varies between 0.24 and 0.77 at oxygen fugacities of 10–12.0 and 10–9.9 in the wire loop experiments and between 0.15 and 0.48 at 10–11.4 and 10–9.9 in the experiments with Au-Mo alloys. Double-spiked analysis of Mo isotope compositions furthermore shows that Mo isotope fractionation between metal and silicate is a linear function of Mo 6+/ΣMo in the silicate glasses, with a difference of 0.51‰ in 98 Mo/95 Mo between purely Mo 4+-bearing and purely Mo 6+-bearing silicate liquid. The former is octahedrally and the latter tetrahedrally coordinated. Our study implies that previous experimental work contained a mixture of Mo 4+ and Mo 6+ species in the silicate liquid. Our refined parameterisation for Mo isotope fractionation between metal and silicate can be described as Δ 98/95 Mo metal–silicate=− 1.43±0.14× 10 6 Mo 6+/Σ Mo+ 8±6× 10 4 T 2 Molybdenum isotope ratios therefore have potential as a proxy to constrain the oxygen fugacity during core formation on planetary bodies if the parameterisation of Mo 6+/ΣMo variation with oxygen fugacity is expanded, for instance to include iron-bearing systems. On Earth literature data indicate that the upper mantle is depleted in heavy Mo isotopes relative to the bulk Earth, as represented by chondrites. As previously highlighted, this difference is most likely not caused by core formation, which either enriches the mantle in heavy Mo isotopes or causes no significant fractionation, depending on temperature and, as we determined here, Mo 6+ content. We reaffirm that core formation does not account for the Mo isotope composition of the modern upper mantle, which may instead reflect the effect of fractionation during subduction as part of global plate recycling.