Thallium elemental behavior and stable isotope fractionation during magmatic processes
Thallium elemental behavior and stable isotope fractionation during magmatic processes
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DOI：
10.1016/j.chemgeo.2016.11.007
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发表时间：
2017-01
期刊：
影响因子：
3.9
通讯作者：
J. Prytulak;A. Brett;M. Webb;T. Plank;M. Rehkämper;P. Savage;J. Woodhead
中科院分区：
文献类型：
--
作者：
J. Prytulak;A. Brett;M. Webb;T. Plank;M. Rehkämper;P. Savage;J. Woodhead
稳定铊（Tl）同位素是一种极其敏感的示踪剂，可用于指示少量沉积物或低温蚀变物质添加到幔源熔体中。Tl追踪这些物质的能力是由于地幔（Tl < 2 ng/g）和可能的外来输入（Tl ~ 100 ng/g至> μg/g）之间的浓度差异很大，这些外来输入也经常显示出分馏的Tl同位素组成。然而，单独由火成岩过程引起的铊同位素分馏的幅度还没有得到系统的评估。在这里，两套同生岩浆，跨越大范围的分化，从Hekla，冰岛，和Anatahan，在马里亚纳弧，被用来评估的行为，铊及其稳定同位素的变化在岩浆过程中。在整个岩浆演化过程中，铊表现为一种近乎完全不相容的亲石元素，与Rb、Cs和K等元素形成镜像。来自Hekla的熔岩具有有限的Cs/Tl比值和稳定的Tl同位素组成，这与地幔估计重叠。俯冲相关的Anatahan火山的熔岩也有一个有限的范围内的Tl同位素组成，与Hekla和MORB重叠，表明分馏结晶和部分熔融不分馏稳定的Tl同位素。俯冲环境显示变量Cs/Tl，表明俯冲过程中通常分馏这两个元素。铊稳定同位素对岩浆分馏的免疫力，加上其对远洋沉积物、铁锰结壳和低温蚀变洋壳的极高灵敏度，突出了其在阐明大洋玄武岩和弧熔岩地幔来源性质方面的价值。重要的是，对铊同位素组成的有意义的解释不必局限于原始熔岩。
Stable thallium (Tl) isotopes are an extremely sensitive tracer for the addition of small amounts of sediments or materials altered at low temperatures to the source(s) of mantle-derived melts. The ability of Tl to trace such materials is due to the large concentration contrast between the mantle (Tl < 2 ng/g) and possible exotic inputs (Tl ~ 100 ng/g to > μg/g), which also often display fractionated Tl isotope compositions. However, the magnitude of Tl isotope fractionation induced by igneous processes alone has not been systematically assessed. Here, two suites of co-genetic magmas, spanning a large range of differentiation, from Hekla, Iceland, and Anatahan, in the Mariana arc, are used to assess the behavior of thallium and its stable isotope variations during magmatic processes. Thallium behaves as a near-perfectly incompatible lithophile element throughout magmatic evolution, mirroring elements such as Rb, Cs, and K. Lavas from Hekla have restricted Cs/Tl ratios and stable Tl isotope compositions, which overlap with mantle estimates. Lavas from subduction-related Anatahan volcano also have a restricted range in Tl isotope composition, which overlaps with Hekla and MORB, demonstrating that fractional crystallisation and partial melting does not fractionate stable Tl isotopes. Subduction environments display variable Cs/Tl, indicating that the subduction process commonly fractionates these two elements. The immunity of thallium stable isotopes to fractionation by magmatic processes coupled with its extreme sensitivity for tracing pelagic sediments, FeMn crusts and low temperature altered oceanic crust highlight its value in elucidating the nature of mantle sources of both oceanic basalts and arc lavas. Critically, meaningful interpretation of thallium isotope compositions need not be restricted to primitive lavas.