Molybdenum isotope variations in calc-alkaline lavas from the Banda arc, Indonesia: Assessing the effect of crystal fractionation in creating isotopically heavy continental crust
Molybdenum isotope variations in calc-alkaline lavas from the Banda arc, Indonesia: Assessing the effect of crystal fractionation in creating isotopically heavy continental crust
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DOI：
10.1016/j.chemgeo.2018.02.037
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发表时间：
2018-05
期刊：
影响因子：
3.9
通讯作者：
M. Wille;O. Nebel;T. Pettke;P. Vroon;S. König;R. Schoenberg
中科院分区：
文献类型：
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作者：
M. Wille;O. Nebel;T. Pettke;P. Vroon;S. König;R. Schoenberg
最近的研究报告说，在收敛的边缘熔岩中，钼同位素的变异性高达1‰(以δ98/95MoNIST3134表示)。这些同位素变化与俯冲带过程有关，并最终可能解释演化大陆地壳中重和可变的同位素特征。与球粒陨石的平均值(δ98/95MoNIST3134−0.16 ± 0.02‰)相比，弧状熔岩的Mo同位素有轻有重，其浓度加权平均同位素组成(δ98/95MoNIST3134+0.03‰)明显高于地幔(δ98/95MoNIST3134~−0.21至−0.16‰)。拉斑玄武岩的火成岩分异过程中没有同位素分馏，表明收敛的边缘岩石中重的Mo同位素特征是俯冲带过程的内在特征，要么是俯冲成分的循环，要么是岩浆分异过程中的Mo同位素分馏，要么是两者的结合。为了更详细地了解弧熔岩中Mo同位素的变化，我们研究了印度尼西亚班达弧沿线钙碱性熔岩中的Mo同位素及其浓度，班达弧是一个洋内俯冲带，以俯冲大陆物质的不同贡献而闻名。班达弧熔岩在−98/95MoNIST3134中显示出从0.48‰到+0.24‰的大范围的Mo同位素组成。我们发现，结合来自其他俯冲带的已公布数据，大多数弧形数据比其推测的地幔楔形源或大洋中脊玄武岩的同位素重。此外，弧状熔岩的钼同位素特征表明，角闪石-单斜辉石从熔体中分馏的指数之间存在明显的协变。通常，钼同位素特征越重，稀土分馏程度越高，表现为λ1、λ2和Dy*/Dy，高Ca/Al和高Sc含量。这些观察结果可以用开放系统岩浆库得到最好的解释，在那里，分离结晶、补给和喷发紧密联系在一起。在班达地区值得注意的例外是四个具有同位素轻钼的样品。其中3个轻同位素熔岩指示了重δ~(18)O和放射性成因~(87)Sr/~(86)Sr-~(206)Pb/~(204)Pb的同化作用，初步确定为下弧壳。总之，我们的数据表明，与以前的发现一致，岩浆作用改变了弧状熔岩中的钼同位素组成，从而增加了俯冲带固有的钼同位素体系的复杂性。
Recent studies report a large Mo isotope variability of up to 1‰ (expressed in δ98/95MoNIST3134) in convergent margin lavas. These isotopic variations have been associated with subduction zone processes and ultimately may account for heavy and variable isotope signatures in evolved continental crust. Arc lavas show both lighter and heavier Mo isotopic values when compared to the chondritic average (δ98/95MoNIST3134−0.16 ± 0.02‰), with a concentration weighted mean isotopic composition (δ98/95MoNIST3134+0.03‰) distinctly heavier than that of the mantle (δ98/95MoNIST3134~−0.21 to −0.16‰). The absence of isotopic fractionation during tholeiitic igneous differentiation indicates that heavy Mo isotope signatures in convergent margin rocks are intrinsic to subduction zone processes, caused either by recycling of subducted components, by Mo isotope fractionation during magmatic differentiation, or by a combination thereof. In order to gain a more detailed understanding of the Mo isotope variability in arc lavas, we have studied Mo isotopes and concentrations in calc-alkaline lavas sampled along the Banda Arc in Indonesia, an intra-oceanic subduction zone well known for variable contributions of subducting continental meterial.Banda Arc lavas exhibit Mo isotopic compositions covering a large range from −0.48 to +0.24‰ in δ98/95MoNIST3134. We find that, combined with published data from other subduction zones, the majority of arc data are isotopically heavier compared to its presumed mantle wedge source or mid-ocean-ridge basalts. Furthermore, arc lava Mo isotope signatures show an apparent covariation between indices of amphibole-clinopyroxene fractionation from the melt. Generally, heavier Mo isotopic signatures are associated with higher degrees of REE fractionation expressed as λ1, λ2, and Dy*/Dy, high Ca/Al, and higher Sc contents. These observations can be best explained with an open system magma reservoir where fractional crystallisation, recharge and eruption are tightly linked. Notable exceptions in the Banda region are four samples with isotopically light Mo. Three of these isotopically light lavas indicate the assimilation of an isotopically light Mo reservoir with heavy δ18O and radiogenic87Sr/86Sr-206Pb/204Pb, tentatively identified as lower arc crust. Together, our data demonstrate, in line with previous findings, that magmatic processes modify the Mo isotopic composition in arc lavas, thus adding complexity to the Mo isotope systematics inherent to subduction zones.