Halogen Bearing Amphiboles, Aqueous Fluids, and Melts in Subduction Zones: Insights on Halogen Cycle From Electrical Conductivity
Halogen Bearing Amphiboles, Aqueous Fluids, and Melts in Subduction Zones: Insights on Halogen Cycle From Electrical Conductivity
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DOI：
10.1029/2020jb021339
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发表时间：
2021-03-01
影响因子：
3.9
通讯作者：
Mookherjee, M.
中科院分区：
文献类型：
--
作者：
Manthilake, G.;Koga, K. T.;Mookherjee, M.
角闪石是大洋地壳中通过热液蚀变作用形成的含水矿物。卤素部分取代OH-使角闪石成为俯冲板片中Cl和F的主要寄主之一。在这项研究中，我们研究了一组含卤素角闪石矿物在1.5 GPa至1,400 K的电导率。不连续的电行为表明角闪石在915 K附近脱水。随后在高于1,070 K的温度下发生脱水诱导的含水熔融。我们发现，释放的水性流体具有类似于0.1 S/m的电导率。这种高电导率可能解释了在某些俯冲带环境中观察到的异常高的电导率。这种比在类似条件下纯水性流体的电导率大一个数量级的高电导率可能是由于F和Cl分配到水性流体中。我们还注意到，在脱水之后，由于原生含卤素角闪石的分解而形成次生相。这些次生相的化学分析表明，它们是F和Cl的储存库。因此，我们推断，在脱水的初级卤素轴承角闪石，首先的F和Cl被分配到含水流体，然后卤素被分配回次生矿物相。这些次生矿物可能将卤素输送到地球深处，并可能部分解释在洋岛玄武岩中观察到的卤素浓度。
Amphiboles are hydrous minerals that are formed in the oceanic crust via hydrothermal alteration. The partial substitution of halogens for OH- makes amphibole one of the principal hosts of Cl and F in the subducting slab. In this study, we investigated the electrical conductivity of a suite of halogen bearing amphibole minerals at 1.5 GPa up to 1,400 K. The discontinuous electrical behavior indicates dehydration of amphibole at similar to 915 K. This is followed by dehydration induced hydrous melting at temperatures above 1,070 K. We find that the released aqueous fluids have an electrical conductivity of similar to 0.1 S/m. This high electrical conductivity is likely to explain anomalously high electrical conductivity observed in certain subduction zone settings. This high electrical conductivity of an order of magnitude greater than the electrical conductivity of pure aqueous fluids at similar conditions is likely due to the partitioning of the F and Cl into the aqueous fluids. We also noted that subsequent to the dehydration, secondary phases form due to the breakdown of the primary halogen bearing amphibole. Chemical analyses of these secondary phases indicate that they are repositories of F and Cl. Hence, we infer that upon dehydration of the primary halogen bearing amphibole, first the F and Cl are partitioned into the aqueous fluids and then the halogens are partitioned back to the secondary mineral phases. These secondary minerals are likely to transport the halogen to the deep Earth and may in part explain the halogen concentration observed in ocean island basalt.