The influence of crustal recycling on the molybdenum isotope composition of the Earth's mantle
The influence of crustal recycling on the molybdenum isotope composition of the Earth's mantle
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DOI：
10.1016/j.epsl.2022.117760
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发表时间：
2022-10
影响因子：
5.3
通讯作者：
R. Hin;K. Hibbert;Shuo Chen;M. Willbold;M. B. Andersen;E. Kiseeva;B. Wood;Y. Niu;K. Sims
中科院分区：
文献类型：
--
作者：
R. Hin;K. Hibbert;Shuo Chen;M. Willbold;M. B. Andersen;E. Kiseeva;B. Wood;Y. Niu;K. Sims
一些研究表明，地球的上地幔相对于大块地球（由陨石定义）略微富含轻钼同位素，但对于这种微妙但可能值得注意的签名的存在没有达成共识。为了更好地确定地球上地幔的98 Mo/95 Mo是否确实是亚南极的，我们分析了来自太平洋，大西洋和印度洋盆地的贫（即球粒陨石标准化La/Sm< 1）洋中脊玄武岩（MORB）的手工挑选的玻璃。相对于参考NIST SRM 3134（δ 98/95 Mo NIST SRM 3134），我们的贫化MORB的平均Mo同位素组成为− 0.22±0.03‰（95%置信区间，ci），而大块地球的值为− 0.15±0.01‰（95% ci）。我们对这些样品的234 U/238 U活度比的高精度分析在统一的不确定度范围内，这排除了可能的次级海底过程的影响，这是其低δ 98/95 Mo NIST SRM 3134的主要原因。实验结果表明，在1400° C下，硫化物液体的δ 98/95 Mo NIST SRM 3134比玄武质硅酸盐液体低0.25±0.01‰。这种分馏是太小，显着改变玄武岩的钼同位素组成相对于他们的来源在熔融或分化。我们的MORB数据表明，可分辨的亚南极钼同位素组成是常见的上地幔。此外，从本研究和文献中提取的亏损和富集MORB的适当加权平均δ 98/95 Mo NIST SRM 3134得出的地幔值估计为− 0.20±0.01‰，表明上地幔作为一个整体是亚南极洲的。由于先前的工作表明，岩心形成不会产生具有亚南极δ 98/95 Mo NIST SRM 3134的残留硅酸盐储层，因此我们认为，这一特征是由于俯冲脱水期间Mo同位素分馏而使具有低δ 98/95 Mo NIST SRM 3134的大洋地壳再循环的结果。这样的起源是符合亚南极洲的Th/U和低Ce/Pb的亏损地幔，功能不能用简单的熔体提取解释。我们提出的板块构造旋回的质量平衡模型，定量地说明，地球地幔的δ 98/95 Mo NIST SRM 3134可以适当地降低这种海洋地壳再循环。我们的钼同位素研究增加了这样一个概念，即亏损地幔已被俯冲处理的洋壳的地球动力学循环大大修改。
Several studies have suggested that the Earth's upper mantle is slightly enriched in light molybdenum isotopes relative to bulk Earth, defined by chondrites, but there is no consensus on the presence of this subtle but potentially notable signature. To establish better whether or not the 98 Mo/95 Mo of Earth's upper mantle is indeed sub-chondritic, we have analysed hand-picked glasses of depleted (ie chondrite normalised La/Sm< 1) mid-ocean ridge basalts (MORB) from the Pacific, Atlantic and Indian ocean basins. The mean Mo isotope composition of our depleted MORB relative to reference NIST SRM 3134 (δ 98/95 Mo NIST SRM 3134) is− 0.22±0.03‰(95% confidence interval, ci) compared to a value of− 0.15±0.01‰(95% ci) for bulk Earth. Our high precision analyses of the 234 U/238 U activity ratios of these samples are within uncertainty of unity, which rules out the effect of possible secondary, sea-floor processes as the dominant cause of their low δ 98/95 Mo NIST SRM 3134. We further report experimental data showing that sulphide liquid has δ 98/95 Mo NIST SRM 3134 0.25±0.01‰ lower than basaltic silicate liquid at 1400° C. This fractionation is too small to significantly alter the Mo isotope composition of basalts relative to their sources during melting or differentiation. Our MORB data show that resolvably sub-chondritic Mo isotope compositions are common in the upper mantle. Moreover, an appropriately weighted average δ 98/95 Mo NIST SRM 3134 of depleted and enriched MORB, taken from this study and the literature, yields an estimated mantle value of− 0.20±0.01‰, indicating that the upper mantle as a whole is sub-chondritic. Since prior work demonstrates that core formation will not create a residual silicate reservoir with a sub-chondritic δ 98/95 Mo NIST SRM 3134, we propose that this feature is a result of recycling oceanic crust with low δ 98/95 Mo NIST SRM 3134 because of Mo isotope fractionation during subduction dehydration. Such an origin is in keeping with the sub-chondritic Th/U and low Ce/Pb of the depleted mantle, features which cannot be explained by simple melt extraction. We present mass balance models of the plate tectonic cycle that quantitatively illustrate that the δ 98/95 Mo NIST SRM 3134 of the Earth's mantle can be suitably lowered by such oceanic crustal recycling. Our Mo isotope study adds to the notion that the depleted mantle has been substantially modified by geodynamic cycling of subduction-processed oceanic crust.