Silicon isotope fractionation during magmatic differentiation
Silicon isotope fractionation during magmatic differentiation
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DOI：
10.1016/j.gca.2011.07.043
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发表时间：
2011-10
影响因子：
5
通讯作者：
P. Savage;R. B. Georg;H. Williams;Kevin W. Burton;A. Halliday
中科院分区：
文献类型：
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作者：
P. Savage;R. B. Georg;H. Williams;Kevin W. Burton;A. Halliday
地幔的Si同位素组成被认为是均匀的（δ 30 Si =-0.29 ± 0.08 ‰，2s. d.）并且不受部分熔化和再循环的很大影响。以前对演化的火成岩物质的分析表明，这些岩石相对于地幔来说是同位素重的。为了理解这种变化，有必要研究岩浆分异过程中硅同位素分馏的程度。在这里，我们报告的硅同位素组成的熔岩从Hekla火山，冰岛，形成在一个地区没有老的，地球化学多样的地壳。我们发现，硅同位素组成线性变化的功能，二氧化硅含量，具有较重的同位素组成更分化的岩石。来自阿法尔裂谷带的样品数据以及各种火成岩USGS标准与Hekla趋势共线，提供了岩浆分异与Si同位素之间基本关系的证据。分馏的效果已被测试，通过研究累积从Skaergaard复杂的，这表明，橄榄石和辉石是同位素轻，斜长石重，相对于地球的地幔的Si同位素组成。因此，硅同位素可以用来模拟镁铁质和长英质矿物分馏的竞争效应，在不断发展的硅酸盐液体和累积。在平均SiO2含量为约60wt. %时，仅由岩浆分异作用产生的陆壳δ 30 Si值预测值为-0.23 ± 0.05 ‰（2s.e.），几乎不比地幔重这至多是一个最大估计值，因为这没有考虑风化物质，风化物质的形成使产物的δ 30 Si值更轻。质量平衡计算表明，从上地幔中移除这种成分的大陆地壳不会影响地幔的Si同位素组成。
The Si isotopic composition of Earth’s mantle is thought to be homogeneous (δ30Si=−0.29±0.08‰, 2s.d.) and not greatly affected by partial melting and recycling. Previous analyses of evolved igneous material indicate that such rocks are isotopically heavy relative to the mantle. To understand this variation, it is necessary to investigate the degree of Si isotopic fractionation that takes place during magmatic differentiation. Here we report Si isotopic compositions of lavas from Hekla volcano, Iceland, which has formed in a region devoid of old, geochemically diverse crust. We show that Si isotopic composition varies linearly as a function of silica content, with more differentiated rocks possessing heavier isotopic compositions. Data for samples from the Afar Rift Zone, as well as various igneous USGS standards are collinear with the Hekla trend, providing evidence of a fundamental relationship between magmatic differentiation and Si isotopes. The effect of fractionation has been tested by studying cumulates from the Skaergaard Complex, which show that olivine and pyroxene are isotopically light, and plagioclase heavy, relative to the Si isotopic composition of the Earth’s mantle. Therefore, Si isotopes can be utilised to model the competing effects of mafic and felsic mineral fractionation in evolving silicate liquids and cumulates. At an average SiO2content of ∼60wt.%, the predicted δ30Si value of the continental crust that should result from magmatic fractionation alone is −0.23±0.05‰ (2s.e.), barely heavier than the mantle. This is, at most, a maximum estimate, as this does not take into account weathered material whose formation drives the products toward lighter δ30Si values. Mass balance calculations suggest that removal of continental crust of this composition from the upper mantle will not affect the Si isotopic composition of the mantle.