Serpentinite Subduction: Implications for Fluid Processes and Trace-Element Recycling
Serpentinite Subduction: Implications for Fluid Processes and Trace-Element Recycling
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DOI：
10.2747/0020-6814.46.7.595
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发表时间：
2004-07
影响因子：
2.6
通讯作者：
M. Scambelluri;J. Fiebig;N. Malaspina;O. Müntener;T. Pettke
中科院分区：
文献类型：
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作者：
M. Scambelluri;J. Fiebig;N. Malaspina;O. Müntener;T. Pettke
蛇纹岩是大洋岩石圈的重要组成部分，被认为是俯冲带的主要水源。对一组在俯冲深度增加时达到平衡的超镁铁岩（大洋和高压叶蛇纹石蛇纹岩、橄榄石斜方辉石岩）的研究表明，这些岩石可以充当Cl、B、Sr、Rb、Cs和碱金属的载体。大洋地幔的蛇纹石化作用产生Sr、Cl、B和碱金属的富集。叶蛇纹石稳定性区内的俯冲路径伴随着海洋Cl、B、Sr和碱金属再循环进入微咸水流体（基于质量平衡计算，NaClequiv含量为4-8重量%;基于流体包裹体分析，NaClequiv含量为10 - 50重量%）。在释放最大量的2wt%H2O的第一（橄榄油内）脱水反应期间产生流体。高压叶蛇纹石蛇纹岩的氧同位素组成基本上反映了俯冲前原岩的氧同位素组成。这被解释为由于高压变质作用期间有限的流体生产和流动性而缺乏再平衡。叶蛇纹石蛇纹岩分解为橄榄石-斜方辉石岩石释放6.5重量%的H2O，形成低盐度流体（基于质量平衡计算为0.4-2重量%的NaClequiv）。俯冲流体的含盐量似乎随着深度的增加而减少：因此可以预期流体中Cl含量的“分化”，较浅的含盐流体之后是较深的、含盐量较少的溶液。橄榄石-斜方辉石岩中的原生流体包裹体代表了闪长岩破裂流体的残余。它们的微量元素组合物富含几种大离子亲石元素（Rb、Sr、Cs、Pb）、Li、B和碱金属，并贫HFSE;它们的微量元素模式与许多现今弧熔岩的模式相似。橄榄石-斜方辉石岩石的氧同位素组成相当均匀，表明脱水过程中流体具有较大的活动性。叶蛇纹石的分解反应似乎产生了一种移动的流体，这种流体在弧下地幔的交代作用中起作用。因此，蛇纹石化的海洋地幔代表了一个有价值的候选水库流体和不相容的元素：因此，它的作用在整个元素循环中的俯冲设置应重新评估。
Serpentinites are important components of the oceanic lithosphere and are viewed as major water sources in subduction zones. Study of a set of ultramafic rocks equilibrated at increasing subduction depths (oceanic and high-pressure antigorite serpentinites, olivine-orthopyroxene rocks), shows that these rocks can act as carriers of Cl, B, Sr, Rb, Cs, and alkalis. Serpentinization of the oceanic mantle produces enrichment in Sr, Cl, B, and alkalis. The subduction path within the stability field of antigorite serpentine is accompanied recycling of oceanic Cl, B, Sr, and alkalis into variably saline fluids (4-8 wt% NaClequiv based on mass balance calculations; 10 to 50 wt% NaClequiv based on fluid inclusion analysis). Fluids are produced during a first (olivine-in) dehydration reaction releasing a maximum amount of 2 wt% H2O. The oxygen isotope compositions of the high-pressure antigorite serpentinites largely mirror those of the pre-subduction protoliths. This is interpreted as a lack of re-equilibration due to limited fluid production and mobility during high-pressure metamorphism Breakdown of the antigorite serpentinites to olivine-orthopyroxene rocks releases 6.5 wt% H2O with formation of a low salinity fluid (0.4-2 wt% NaClequiv based on mass balance calculations). The salt contents of subduction fluids appear to decrease with increasing depth: a "differentiation" in the Cl content of fluids can thus be expected, with shallower saline fluids being followed by deeper, less saline, solutions. Primary fluid inclusions in olivine-orthopyroxene rocks represent remnants of the antigorite-breakdown fluid. Their trace-element compositions are enriched in several LILE (Rb, Sr, Cs, Pb), Li, B, and alkalis and are depleted in HFSE; their trace-element patterns are similar to those of many present-day arc lavas. Oxygen isotope compositions of olivine-orthopyroxene rocks are rather homogeneous, and are suggestive of larger fluid mobility during dehydration. The antigorite breakdown reaction appears to produce a mobile fluid that can play a role in the metasomatism of sub-arc mantle. Serpentinized oceanic mantle thus represents a valuable candidate reservoir for fluids and incompatible elements: consequently, its role in the overall element cycle in subduction settings should be re-evaluated.