High-pressure Hydrous Phase Relations of Radiolarian Clay and Implications for the Involvement of Subducted Sediment in Arc Magmatism
High-pressure Hydrous Phase Relations of Radiolarian Clay and Implications for the Involvement of Subducted Sediment in Arc Magmatism
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DOI：
10.1093/petrology/egq054
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发表时间：
2010-11
影响因子：
3.9
通讯作者：
S. Skora;J. Blundy
中科院分区：
文献类型：
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作者：
S. Skora;J. Blundy
俯冲大洋沉积物的熔融被认为在弧岩浆特征的产生中起着关键作用。我们对微量元素掺杂的放射虫粘土在3 GPa、700~12508C的温度下进行了含水熔融的实验研究。将 7^15 wt% H2O 添加到沉积物中，以模拟源自底层脱水岩性（例如蛇纹岩）的流体的冲刷效果。由于多硅白云母+单斜辉石+柯石英的分解，熔化在 7508C 开始，含水熔体与大部分石榴石+蓝晶石石英共存，温度高达 12508C 左右。金红石和 Fe^Ti 氧化物的温度高达 10008C。在相对较低的超固相线温度下会发生非常高的熔化程度（例如，添加 15 wt% 的 H2O，粘土在 8008C 时熔化 54%），这与类似岩石成分的无流体熔化形成鲜明对比，后者在类似温度下产生可忽略不计的熔化分数 (5±10%)。这项研究的一个特别重点是残余独居石，它优先掺入轻稀土元素 (LREE) 和 Th，从而对沉积物衍生流体和熔体的 Th/La 比率发挥强大的控制作用。与之前的研究相比，我们发现 DTh/La 变化很大，并且可能显着高于或低于统一。我们的数据集表明，出现这种模式是因为独居石固溶体系列的各个成员独立地受到不同参数的影响。我们还证明，由于独居石^钙辉石固溶体，基于掺杂实验的独居石^熔体分配系数不能不加批判地用于预测自然界中的分馏过程。然而，将我们的结果外推到自然浓度水平表明，在大多数情况下，在独居石存在的情况下，Th 会从 La 中分离出来。我们提出，需要一种几乎没有或没有残留独居石的固体残渣来解释弧岩浆的各种地球化学特征，包括 Th/La 比率。如果提供足够的水（例如通过叶蛇纹石分解）以促进充分熔化以溶解沉积物的全部 LREE+Th 预算，则可以在相对较低的亚弧温度下获得不含独居石的残留物。
Melting of subducted oceanic sediment is considered to play a key role in the generation of the arc magmatic signature.We have carried out an experimental study on hydrous melting of trace element-doped radiolarian clay at 3 GPa and temperatures from 700 to 12508C; 7^15 wt % H2O was added to the sediment to simulate the effects of flushing by fluids derived from underlying dehydrating lithologies, such as serpentinites. Melting begins at 7508C owing to the breakdown of phengiteþ clinopyroxeneþ coesite and a hydrous melt coexists with mostly garnetþ kyanite quartz up to around 12508C. Rutile and Fe^Ti oxides are present to 10008C.Very high degrees of melting occur at relatively low, supra-solidus temperatures (e.g. with 15 wt % added H2O, the clay is 54% molten at 8008C), in marked contrast to fluid-absent melting of similar rock compositions, which yields negligible melt fractions (5 10%) for similar temperatures. A particular focus of this study is residual monazite, which preferentially incorporates light rare earth elements (LREE) and Th, thereby exerting a powerful control on the Th/La ratio of sediment-derived fluids and melts. In contrast to previous studies, we find that DTh/La varies widely and can be significantly above or below unity. Our dataset suggests that this pattern arises because the various members of the monazite solid solution series are influenced independently by different parameters. We also demonstrate that monazite^melt partition coefficients based on doped experiments cannot be used uncritically to predict fractionation processes in nature because of monazite^huttonite solid solution. However, extrapolation of our results to natural concentration levels suggests fractionation of Th from La in the presence of monazite in most cases. We propose that a solid residue with little or no residual monazite is needed to explain a wide range of geochemical features of arc magmas, including Th/La ratios. A monazite-free residue can be achieved at relatively low sub-arc temperatures provided that enough water is made available (e.g. through antigorite breakdown) to promote sufficient melting to dissolve the entire LREEþTh budget of the sediment.