Behavior of fluid-mobile elements in serpentines from abyssal to subduction environments: Examples from Cuba and Dominican Republic
Behavior of fluid-mobile elements in serpentines from abyssal to subduction environments: Examples from Cuba and Dominican Republic
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DOI：
10.1016/j.chemgeo.2012.04.009
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发表时间：
2012-06-18
期刊：
影响因子：
3.9
通讯作者：
France, Lyderic
中科院分区：
文献类型：
--
作者：
Deschamps, Fabien;Godard, Marguerite;France, Lyderic
俯冲环境的蛇纹岩是流体活动元素的重要汇。为了限制俯冲过程中流体流动元素的地球化学行为，我们进行了一系列的蛇纹岩和堆积物的地球化学研究（微量元素和铅同位素）从大加勒比海（古巴和多米尼加共和国）的增生楔。采用LA-HR-ICP-MS技术原位分析了原生相和蚀变相关相的微量元素组成。所研究的样品代表俯冲的原大西洋海洋岩石圈的一部分，经历了低到高级变质作用（绿片岩榴辉岩相），被挖出之前，这些样品的一个子集来自地幔楔。这一取样提供了在大洋俯冲背景下沿沿着冷地热进行俯冲变质作用期间追踪流体活动元素化学活动性的机会。蛇纹岩显示出流体活动元素的强烈富集，表明广泛的流体-岩石相互作用。原位分析允许区分三种类型的蛇纹岩有关的原生矿物（橄榄石，正或单斜辉石）的性质。俯冲样品的成分，特别是流体活动元素的成分，与深海橄榄岩的成分比较接近，但在俯冲相关的俯冲变质作用期间，微量元素的活动性没有明显的证据，除了B。这证实了所观察到的富集是在海洋中停留期间海水/橄榄岩相互作用的结果。这也表明，在俯冲过程中，大多数储存在蛇纹石矿物中的移动的元素是不移动的。这一观察的一个主要结果是，蛇纹石矿物是俯冲带中移动的元素的良好汇，直到它们脱水。此外，铅同位素和过度富集在高级俯冲蛇纹岩（叶蛇纹石）的沉积成分的贡献，在二次水化发生在利蛇纹石/叶蛇纹石过渡。我们认为，这一新的蛇纹岩化事件，发生在更深的深度，从俯冲通道中的沉积物和蛇纹岩之间的混合的结果。地幔楔蛇纹岩的热液流体的印记：他们是富硼，但没有强烈的富集在As和Sb，并显示证据的放射性铅成分的中等贡献。这表明，地幔楔蛇纹岩的流体来源于洋壳的脱水，与中等或没有贡献的沉积物。地幔楔状蛇纹岩化作用发生在20-25 km深度，在此深度和温度条件下（T>200 ℃），俯冲沉积物仍释放出富硼孔隙流体，而与含水矿物（多硅白云母、硬硅钙石）结合的结构水保持稳定。在俯冲带环境（俯冲蛇纹岩，地幔楔蛇纹岩，以及俯冲沉积物和蚀变洋壳），可能会释放他们的流体在不同的深度有很强的影响弧熔岩形成的流体流动元素的各种潜在的水库的存在。(C)2012爱思唯尔有限公司版权所有。
Serpentinites from subduction environments represent an important sink for fluid-mobile elements. In order to constrain geochemical behavior of fluid-mobile elements hosted by serpentine phases during subduction processes, we carried out a geochemical study (trace elements and Pb isotopes) of a series of serpentinites and cumulates from the accretionary wedge of Greater Caribbean (Cuba and Dominican Republic). The trace element compositions of the primary and alteration-related phases were analyzed in situ using LA-HR-ICP-MS techniques. The studied samples represent parts of the subducted proto-Atlantic oceanic lithosphere, which has experienced low to high grade metamorphism (greenschist to eclogite facies), before being exhumed; a subset of these samples were derived from the mantle wedge. This sampling provides the opportunity to trace the chemical mobility of fluid-mobile elements during prograde metamorphism along a cold geotherm in an oceanic subduction setting.Serpentinites display strong enrichment in fluid-mobile elements indicating extensive fluid-rock interaction. In situ analyses allow distinction of three types of serpentines related to the nature of primary minerals (olivine, ortho- or clinopyroxene). Compositions of subducted samples, especially in fluid-mobile elements, are relatively close to those of abyssal peridotites without noticeable evidence of mobility for trace elements during subduction-related prograde metamorphism, with the exception of B. This confirms that the observed enrichment results from seawater/peridotite interactions during residence time in the ocean. It also suggests that most mobile elements stored in serpentine minerals are immobile during subduction processes. A major consequence of this observation is that serpentine minerals are a good sink for mobile elements in subduction zones, until their dehydration. Additionally, Pb isotopes and over-enrichment in As-Sb in high-grade subducted serpentines (antigorite) suggest the contribution of a sedimentary component during a secondary hydration taking place at the lizardite/antigorite transition. We propose that this new serpentinization event, taking place at greater depth, results from mixing between sediments and serpentinites in the subduction channel. Mantle wedge serpentinites present imprints of hydrothermal fluids: they are B-rich but without strong enrichment in As and Sb, and show evidence for moderate contributions of a radiogenic Pb-component. This suggests that the fluids that produced the mantle wedge serpentinites derived from the dehydration of the oceanic crust, with moderate to no contribution of sediments. We posit that mantle wedge serpentinization took place around 20-25 km depth: at such depth and temperature conditions (T>200 degrees C), the subducted sediments still released their B-rich pore fluids while their structural water incorporated in hydrous minerals (phengite, lawsonite) remained stable. The existence of various potential reservoirs for fluid-mobile elements in subduction zone environments (subducted serpentinites, mantle wedge serpentinites, as well as subducted sediments and altered oceanic crust) that potentially release their fluids at different depths has strong implications for arc lava formation. (C) 2012 Elsevier B.V. All rights reserved.