Hydrous Phase Relations and Trace Element Partitioning Behaviour in Calcareous Sediments at Subduction-Zone Conditions
Hydrous Phase Relations and Trace Element Partitioning Behaviour in Calcareous Sediments at Subduction-Zone Conditions
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DOI：
10.1093/petrology/egv024
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发表时间：
2015-05
影响因子：
3.9
通讯作者：
S. Skora;J. Blundy;R. Brooker;E. Green;J. D. Hoog;J. Connolly
中科院分区：
文献类型：
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作者：
S. Skora;J. Blundy;R. Brooker;E. Green;J. D. Hoog;J. Connolly
我们报告了在俯冲带条件下（3GPa，750-1200 ℃）对两种具有不同碳酸盐含量的天然海洋沉积物（钙质粘土：CO2 <$4 6 - 1wt%;泥灰岩：CO2 <$4 16 - 2wt%）的实验结果。将水（7-15重量%）加入到起始材料中以模拟从俯冲板内加入外部水的效果。在富含水的实验中，融化的开始是在760摄氏度;融化在800摄氏度时变得丰富。相比之下，在已发表的贫水实验中，熔化的开始发生在可变的温度下，大量熔体部分的产生被限制在900 ℃以上（多硅白云母出）。不同的固相线温度（Tsolidus）可以归因于可变的流体XH 2 O [H2O/（CO2 <$H2O）]，其又取决于本体K2 O、H2O和CO2。与残余石榴石、碳酸盐、石英/柯石英、绿帘石、金红石、蓝晶石、多硅白云母和单斜辉石平衡的部分熔体在成分上是花岗岩，具有大量溶解的挥发物。超固相线运行总是包含硅酸盐熔体和溶质丰富的流体，表明实验条件低于花岗岩-H2O-CO2系统中的第二个关键端点。泥灰岩中1100 ℃以上碳酸盐熔体与硅酸盐熔体及富溶质流体共存。碳酸盐岩在高温下的持久性，与富含CO2的含水熔体相平衡，提供了一种机制，既可以向弧岩浆提供CO2，又可以将碳循环到地球深部。实验玻璃的微量元素组成限制了钙质沉积物对弧岩浆的潜在贡献。残留绿帘石和碳酸盐的存在下，赋予不同的微量元素特征相比，微量元素信号的钙贫海洋沉积物（如远洋粘土）。值得注意的是，绿帘石保留Th和轻稀土元素，使得一些来自钙质沉积物的熔体具有升高的Ba/Th和U/Th，以及低的La/SmPUM，从而类似于通常归因于蚀变洋壳的流体。我们的研究结果强调了残余矿物学的重要性，而不是源岩性，在控制微量元素的板状流体的特性。
We report the results of experiments on two natural marine sediments with different carbonate contents (calcareous clay: CO2 ¼ 6� 1 wt %; marl: CO2 ¼ 16� 2 wt %) at subduction-zone conditions (3 GPa, 750–1200 � C). Water (7–15 wt %) was added to the starting materials to simulate the effects of external water addition from within the subducting slab. The onset of melting is at 760 � Ci n water-rich experiments; melt becomes abundant by 800 � C. In contrast, the onset of melting in published, water-poor experiments occurs at variable temperatures with the production of significant melt fractions being restricted to more than 900 � C (phengite-out). The different solidus temperatures (Tsolidus) can be ascribed to variable fluid XH2O [H2O/(CO2 þ H2O)], which, in turn, depends on bulk K2O, H2O and CO2. Partial melts in equilibrium with residual garnet, carbonate, quartz/coesite, epidote, rutile, kyanite, phengite, and clinopyroxene are granitic in composition, with substantial dissolved volatiles. Supersolidus runs always contain both silicate melt and solute-rich fluid, indicating that experimental conditions lie below the second critical endpoint in the granite–H2O–CO2 system. Carbonatite melt coexists with silicate melt and solute-rich fluid above 1100 � C in the marl. The persistence of carbonate to high temperature, in equilibrium with CO2-rich hydrous melts, provides a mechanism to both supply CO2 to arc magmas and recycle carbon into the deep Earth. The trace element compositions of the experimental glasses constrain the potential contribution of calcareous sediment to arc magmas. The presence of residual epidote and carbonate confers different trace element characteristics when compared with the trace element signal of Ca-poor marine sediments (e.g. pelagic clays). Notably, epidote retains Th and light rare earth elements, such that some melts derived from calcareous sediments have elevated Ba/Th and U/Th, and low La/SmPUM, thereby resembling fluids conventionally ascribed to altered oceanic crust. Our results emphasize the importance of residual mineralogy, rather than source lithology, in controlling the trace element characteristics of slab-derived fluids.