Thallium isotope compositions of subduction-zone fluids: Insights from ultra-high pressure eclogites and veins in the Dabie terrane, eastern China
Thallium isotope compositions of subduction-zone fluids: Insights from ultra-high pressure eclogites and veins in the Dabie terrane, eastern China
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俯冲带流体铊同位素组成：来自中国东部大别地体超高压榴辉岩和矿脉的见解
DOI：
10.1016/j.chemgeo.2022.120843
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发表时间：
2022-03
期刊：
影响因子：
3.9
通讯作者：
Fang Huang
中科院分区：
文献类型：
--
作者：
Yunchao Shu;Sune G. Nielsen;Veronique Le Roux;Jerzy Blusztajn;Shun Guo;Fang Huang
俯冲板块的脱水作用是弧状岩浆形成的关键过程。然而，板坯脱水的矿物学控制仍然不确定。铊(Tl)同位素已作为俯冲板片成分的示踪剂出现，但板岩流体的Tl同位素特征尚不清楚。大别地体(中国)中的高压变质脉和寄主榴辉岩为俯冲带流体的组成和演化提供了重要信息。在这项研究中，我们首次提供了港河和花梁亭地区两个高压变质富绿帘石脉体的Tl浓度和Tl同位素数据，以及寄主榴辉岩和多硅白云母分离体。多硅白云母分离中的高Tl含量以及多硅白云母与寄主榴辉岩具有相同的Tl同位素组成，说明多硅白云母控制了寄主榴辉岩中Tl的总储量。总体而言，两个地区的脉状样品的Tl含量都远低于寄主榴辉岩，这表明矿脉的沉积流体来自残留了Tl的含多硅白云母榴辉岩。岗河榴辉岩与岗河绿辉石-绿帘脉具有相似的热释光同位素组成。这种相似性表明，成脉流体可能来源于寄主榴辉岩，Tl同位素在流体释放过程中没有分馏。花粮亭地区三代脉体记录了不同的热释光同位素值，可能来源于非均质寄主榴辉岩的多阶段脱水作用，其中一些可能含有沉积物成分。我们认为，榴辉岩脱水流体的Cs/Tl比值可能高于其寄主榴辉岩。具有高Cs/Tl(即~gt；15)的弧岩浆可能直接记录了弧下深部残留多硅白云母的存在，这种高Cs/Tl不是板岩源区成分变化所致，再加上相对较低的Ba/Th比值。
Dehydration of the subducted slab is a crucial process in the generation of arc magmas. However, the mineralogical controls of slab dehydration remain uncertain. Thallium (Tl) isotopes have emerged as tracers of subducted slab components, but the Tl isotope characteristics of slab fluids are not known. High-pressure metamorphic veins and host eclogites in the Dabie terrane (China) provide important information on the composition and evolution of subduction zone fluids. In this study, we present the first Tl concentration and Tl isotope data for two high-pressure metamorphic epidote-rich veins formed through breakdown of lawsonite, as well as host eclogites and phengite separates, from the Ganghe and Hualiangting areas. The high Tl concentrations in phengite separates and identical Tl isotope compositions between phengites and their host bulk eclogites demonstrate that phengite controls the overall inventory of Tl in the host eclogites. Generally, the vein samples from both areas have much lower Tl concentrations than their host eclogites, suggesting that the fluids from which the veins precipitated were derived from phengite-bearing eclogites that retained Tl in the residue. The Ganghe eclogites display similar Tl isotope compositions compared with Ganghe omphacite-epidote vein. This similarity indicates that the vein-forming fluid was likely derived from the host eclogites, and that Tl isotopes did not fractionate during fluid-release. Three generations of veins in the Hualiangting area record variable Tl isotope values and were likely derived from multi-stage dehydration of heterogeneous host eclogites, some of which may have contained a sediment component. We argue that fluids derived from eclogite dehydration are likely characterized by higher Cs/Tl ratios compared to their host eclogites. Arc magmas that display high Cs/Tl (i.e. >15) that cannot be caused by variations in slab source composition, coupled with relatively low Ba/Th ratios, are likely to directly record the presence of residual phengite at subarc depths.
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