Evidence for Mo isotope fractionation in the solar nebula and during planetary differentiation
Evidence for Mo isotope fractionation in the solar nebula and during planetary differentiation
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DOI：
10.1016/j.epsl.2014.01.037
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发表时间：
2014-04-01
影响因子：
5.3
通讯作者：
Bourdon, Bernard
中科院分区：
文献类型：
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作者：
Burkhardt, Christoph;Hin, Remco C.;Bourdon, Bernard
质量相关的钼同位素分馏已针对多种陨石进行了研究，包括球粒陨石（顽火辉石、普通球粒陨石和碳质球粒陨石）、铁陨石和无球粒陨石（锂辉石、红陨石和火星陨石），以及月球和陆地样品。岩浆铁陨石与顽火辉石、普通和大多数碳质球粒陨石一起定义了一个常见的 delta Mo-98/95 值为 -0.16 +/- 0.02%（相对于 NIST SRM 3134 Mo 标准），该值被解释为反映了太阳系内部大块行星体的 Mo 同位素组成。 IAB 铁陨石的重钼同位素组成很可能反映了这些陨石中撞击引起的钼蒸发损失。碳质球粒陨石定义了 delta Mo-98/95 与金属含量之间的负相关性，以及 delta Mo-98/95 与基质丰度之间的正相关性。这些相关性主要由 CM 和 CK 球粒陨石定义，并且可能反映碳质球粒陨石形成区域中同位素轻金属和/或同位素重基体成分的不均匀分布。或者，CM 和 Cl（球粒陨石）的 δ Mo-98/95 升高可能是由于挥发性轻同位素 Mo 氧化物的损失造成的，这些氧化物是在形成这些球粒陨石的典型氧化条件下形成的。与球粒陨石和铁陨石的平均成分相比，源自分化行星体硅酸盐部分的样品的 Mo 同位素成分很重。这种差异是 与金属和硅酸盐之间钼同位素分馏的实验证据定性一致。来自不同地球化学储层的月球样品的常见 Delta Mo-98/95 值为 0.05 +/- 0.03%0，表明月球上不存在因硅酸盐分异或撞击变质作用/挥发而导致的显着 Mo 同位素分馏。对钼同位素组成最直接的解释 月幔对应于金属硅酸盐平衡温度为 1800 200 摄氏度的月球核心的形成。所研究的火星陨石、红陨石和赤霞石表现出更多变化的 Mo 同位素组成，对于几个样本来说，其值延伸到高于与核心形成相关的最大 delta Mo-98/95 = +0.14%0 的值。对于这些样品的后核形成过程，例如 部分熔化、变质作用以及陨石发现的陆地风化一定导致了钼同位素分馏。因此，对火星金属硅酸盐平衡温度（2490 + 770 摄氏度）和红榴石母体（1790 +/- 230 摄氏度）的估计比对月球的估计更加不确定。尽管大部分钼同位素组成 作为本研究的一部分，硅酸盐地球尚未确定，值为 -0.16% < delta Mo-98/95
Mass-dependent Mo isotope fractionation has been investigated for a wide range of meteorites including chondrites (enstatite, ordinary and carbonaceous chondrites), iron meteorites, and achondrites (eucrites, angrites and martian meteorites), as well as for lunar and terrestrial samples. Magmatic iron meteorites together with enstatite, ordinary and most carbonaceous chondrites define a common delta Mo-98/95 value of -0.16 +/- 0.02% (relative to the NIST SRM 3134 Mo standard), which is interpreted to reflect the Mo isotope composition of bulk planetary bodies in the inner solar system. Heavy Mo isotope compositions for IAB iron meteorites most likely reflect impact-induced evaporative losses of Mo from these meteorites. Carbonaceous chondrites define an inverse correlation between delta Mo-98/95 and metal content, and a positive correlation between delta Mo-98/95 and matrix abundance. These correlations are mainly defined by CM and CK chondrites, and may reflect the heterogeneous distribution of an isotopically light metal and/or an isotopically heavy matrix component in the formation region of carbonaceous chondrites. Alternatively, the elevated delta Mo-98/95 of the CM and Cl (chondrites could result from the loss of volatile, isotopically light Mo oxides, that formed under oxidized conditions typical for the formation of these chondrites.The Mo isotope compositions of samples derived from the silicate portion of differentiated planetary bodies are heavy compared to the mean composition of chondrites and iron meteorites. This difference is qualitatively consistent with experimental evidence for Mo isotope fractionation between metal and silicate. The common delta Mo-98/95 values of 0.05 +/- 0.03%0 of lunar samples derived from different geochemical reservoirs indicate the absence of significant Mo isotope fractionation by silicate differentiation or impact metamorphism/volatilization on the Moon. The most straightforward interpretation of the Mo isotope composition of the lunar mantle corresponds to the formation of a lunar core at a metal-silicate equilibration temperature of 1800 200 degrees C. The investigated martian meteorites, angrites and eucrites exhibit more variable Mo isotope compositions, which for several samples extend to values above the maximum delta Mo-98/95 = +0.14%0 that can be associated with core formation. For these samples post-core formation processes such as partial melting, metamorphism and in the case of meteorite finds terrestrial weathering must have resulted in Mo isotope fractionation. Estimates of the metal-silicate equilibration temperatures for Mars (2490 + 770 degrees C) and the angrite parent body (1790 +/- 230 degrees C) are thus more uncertain than that derived for the Moon. Although the Mo isotope composition of the bulk silicate Earth has not been determined as part of this study, a value of -0.16% < delta Mo-98/95