Thallium Isotope Fractionation During Magma Degassing: Evidence From Experiments and Kamchatka Arc Lavas
Thallium Isotope Fractionation During Magma Degassing: Evidence From Experiments and Kamchatka Arc Lavas
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DOI：
10.1029/2020gc009608
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发表时间：
2021-04
期刊：
影响因子：
3.7
通讯作者：
S. Nielsen;Yunchao Shu;Bernard J. Wood;J. Blusztajn;M. Auro;C. Ashley Norris;G. Wörner
中科院分区：
文献类型：
--
作者：
S. Nielsen;Yunchao Shu;Bernard J. Wood;J. Blusztajn;M. Auro;C. Ashley Norris;G. Wörner
铊（Tl）同位素比值是一种新兴的工具，可用于跟踪弧熔岩和洋岛玄武岩（OIB）中的地壳再循环过程。铊是一种易挥发的金属，在火山熔岩中富集，但尚不清楚陆上熔岩中铊的脱气是否对其残留铊同位素组成有显著影响。在这里，我们提出的Tl同位素和浓度数据脱气实验，最好的解释瑞利动力学同位素分馏过程中Tl损失。我们的数据密切遵循预测的同位素分馏模型，其中TlCl是主要的脱气物种，其中Tl损失由扩散和自然对流控制，与我们的实验中使用的缓慢气体平流速度一致。我们计算出，脱气到空气中应与净铊同位素分馏因子α净= 0.99969的扩散和天然气对流（低气体速度）和α净= 0.99955的扩散和强制气体对流（高气体速度）。我们还表明，从堪察加半岛弧的三个火山的熔岩表现出Tl同位素和浓度模式，图之间的两个不同的气体对流制度，这意味着脱气控制所观察到的Tl同位素组成在这三个火山中发挥了重要作用。陆上熔岩的铊同位素数据的文献检查显示，这些大多数似乎只有轻微的影响，脱气，虽然一些样品从OIB和弧火山可以确定，可能经历了一些铊脱气。
Thallium (Tl) isotope ratios are an emerging tool that can be used to trace crustal recycling processes in arc lavas and ocean island basalts (OIBs). Thallium is a highly volatile metal that is enriched in volcanic fumaroles, but it is unknown whether degassing of Tl from subaerial lavas has a significant effect on their residual Tl isotope compositions. Here, we present Tl isotope and concentration data from degassing experiments that are best explained by Rayleigh kinetic isotope fractionation during Tl loss. Our data closely follow predicted isotope fractionation models in which TlCl is the primary degassed species and where Tl loss is controlled by diffusion and natural convection, consistent with the slow gas advection velocity utilized during our experiments. We calculate that degassing into air should be associated with a net Tl isotope fractionation factor of αnet = 0.99969 for diffusion and natural gas convection (low gas velocities) and αnet = 0.99955 for diffusion and forced gas convection (high gas velocities). We also show that lavas from three volcanoes in the Kamchatka arc exhibit Tl isotope and concentration patterns that plot in between the two different gas convection regimes, implying that degassing played an important role in controlling the observed Tl isotope compositions in these three volcanoes. Literature inspection of Tl isotope data for subaerial lavas reveals that the majority of these appear only minorly affected by degassing, although a few samples from both OIBs and arc volcanoes can be identified that likely experienced some Tl degassing.