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首页> 外文期刊>Geochemical Journal >Dissolution processes of elements from subducting sediments into fluids: Evidence from the chemical composition of the Sanbagawa pelitic schists
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Dissolution processes of elements from subducting sediments into fluids: Evidence from the chemical composition of the Sanbagawa pelitic schists

机译：从沉积物俯冲到流体中的元素溶解过程：Sanbagawa胶生片岩的化学成分的证据

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摘要

In order to evaluate the dissolution processes of elements from subducting sediments into fluids during early stages of metamorphism (up to oligoclase–biotite zone at about 30 km depth), the chemical composition of Sanbagawa pelitic schists, Sanbagawa Metamorphic Belt, Japan was studied. Samples from different metamorphic grades show similarities in their major element and Rb compositions, suggesting that the Sanbagawa schists experienced insignificant dissolution of these elements. Arsenic, N. (as ammonium, NH_4~+) and Cs contents decrease with increasing metamorphic grade, dem-onstrating that their dissolution is enhanced under respective metamorphic stages (As: upper–garnet zone, NH_4~+: chlorite oligoclase–biotite zone, Cs: garnet – albite–biotite zone). Dissolution mechanisms proposed are as follows. The NH_4~+ dissolution accompanies dehydration of the pelitic schists, which may produce fluids with a high potential to dissolve or oxidize NH_4~+. The thermal structure of the subducted slab is likely to influence the depth of the NH_4~+ dissolution. The Cs dissolution is caused mainly by fluid flow from underlying meta-mafic rocks and the amount of fluids possibly controls the degree of the Cs dissolution. Heterogeneous dissolution of Cs suggests that the fluid flow from the underlying metamafic rocks has a channelized structure. The mechanism of As dissolution is not clearly understood; however, it may be strongly related to graphitization of kerogens in the pelitic schists. Differences in Cs dissolution were observed between the Sanbagawa pelitic schists and other well-studied meta-sedimentary sequences, particularly the Catalina Schist, California and New Caledonia Schistes Lustres. In the case of the Catalina Schists, Cs was dissolved into fluids more effectively than the Sanbagawa pelitic schists. In contrast, meta-sedimentary rocks of the Schistes Lustres nappe did not experience any Cs dissolution, although rocks subducted deeper than their dehydration depth. One of the reasons for these differences may be variations in the amount of channelized structures among subduction zones.
机译：为了评估在变质的早期阶段（直到大约30 km深度的寡聚橄榄岩-黑云母带）从沉积物俯冲到流体中的元素的溶解过程，研究了日本Sanbagawa变质带的Sanbagawa片岩片岩的化学成分。来自不同变质等级的样品在其主要元素和Rb组成上显示出相似性，表明Sanbagawa片岩经历了这些元素的微不足道的溶解。 N，砷（以铵态NH_4〜+形式）和Cs含量随变质等级的增加而降低，表明它们在各个变质阶段的溶解度均得到增强（如：上石榴石带，NH_4〜+：绿泥石寡糖-黑云母带，CS：石榴石-钠长石-黑云母带）。提出的溶解机制如下。 NH_4〜+的溶解伴随着胶质片岩的脱水，这可能产生具有高溶解或氧化NH_4〜+的潜力的流体。俯冲板的热结构可能会影响NH_4〜+溶解的深度。 Cs的溶解主要是由下伏的基性岩体中的流体流引起的，流体的数量可能控制Cs的溶解程度。 Cs的非均质溶解表明，来自下伏的基质岩的流体流具有通道化的结构。砷溶解的机理尚不清楚。然而，它可能与胶粉岩中干酪根的石墨化作用密切相关。在Sanbagawa泥质片岩和其他经过充分研究的元沉积层序之间，尤其是在Catalina Schist，加利福尼亚和New Caledonia Schistes Lustres之间观察到Cs溶出度的差异。就Catalina片岩而言，Cs比Sanbagawa胶体片岩更有效地溶解在流体中。相反，尽管Schistes Lustres推覆的准沉积岩的俯冲比其脱水深度更深，但它们并未经历Cs溶解。这些差异的原因之一可能是俯冲带之间通道化结构数量的变化。

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