Trace-element-rich brines in eclogitic veins: implications for fluid composition and transport during subduction
Trace-element-rich brines in eclogitic veins: implications for fluid composition and transport during subduction
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DOI：
10.1007/bf00321985
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发表时间：
1991-02
影响因子：
3.5
通讯作者：
P. Philippot;J. Selverstone
中科院分区：
文献类型：
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作者：
P. Philippot;J. Selverstone
原生和假次生流体包裹体产于西阿尔卑斯山Monviso蛇绿混杂岩榴辉岩脉中的扇形和扇形带绿辉石中。包裹体主要由卤水和石盐、钾盐、方解石、白云石、钠长石、硬石膏、重晶石、斜锆石、金红石、辉石、铁氧化物、黄铁矿、独居石等矿物的子晶体组成。这一子矿物系列表明Na、K、Ca、Mg、Fe、Zr、Ti、P、Ba、Ce、La、Th和S物种的高可溶物，并为俯冲过程中高场强、大离子亲石和轻稀土元素作为溶解物种的迁移提供了直接证据。流体包裹体的不均匀性保存在静脉内和相邻的颗粒之间，但是，表明流体平衡的规模很小。一些矿脉中的裂缝封闭几何形状意味着流体以脉冲形式释放，而不是稳定的流动控制矿脉中的矿物沉积和生长。从这些观察，我们开发了一个模型的流体释放和截留在其中的流体脉冲与剪切和拉伸破坏的增量的时间;流体释放的速率和孔隙度的减少都取决于塑性流动的速率。脉流体最初可能来自于寄主榴辉岩中早期流体包裹体的爆裂，脉中流体包裹体所暗示的小尺度流体非均质性最好解释为有限的流体流动，因此有限的交代作用。我们的结论是，元素回收到地幔楔在俯冲过程中将至少强烈地依赖于流体的传输机制，在流体相的元素溶解度。在Monviso，尽管有证据表明微量元素在盐水中的溶解度很高，但在教学深度为1040 km之前，这些元素没有从下行板中去除。
Primary and pseudosecondary fluid inclusions occur in oscillatory-and sector-zoned omphacite in eclogitic veins from the Monviso ophiolitic complex in the Western Alps. The inclusions contain aqueous brines and daughter crystals of halite, sylvite, calcite, dolomite, albite, anhydrite and/or gypsum, barite, baddeleyite, rutile, sphene, Fe oxides, pyrite and monazite. This daughter mineral suite indicates high solubilites of Na, K, Ca, Mg, Fe, Zr, Ti, P, Ba, Ce, La, Th, and S species and provides direct evidence for transport of high-fieldstrenght, large-ion-lithophile, and light-rare-earth elements as dissolved species during subduction. Fluid-inclusion heterogeneities preserved within and between adjacent grains in the veins, however, suggest that the scale of fluid equilibration was small. A crack-seal geometry in some of the veins implies that fluid release in pulses rather than steady flow controlled mineral deposition and growth in the veins. From these observations, we develop a model of fluid release and entrapment in which pulses of fluid are associated in time with increments of shear and tensile failure; the rate of fluid release and the reduction in porosity both depend on the rate of plastic flow. Vein fluids may initially be derived from decreptitation of early fluid inclusions in the host eclogites, Small-scale fluid heterogeneities implied by the fluid inclusions in the veins are best interpreted in terms of limited fluid flow, and hence limited metasomatism. We conclude that element recycling into the mantle wedge during subduction will depend at least as strongly on fluid transport mechanisms as on element solubilities in the fluid phase. At Monviso, despite evidence for high trace element solubilities in saline brines, the elements were not removed from the downgoing slab prior to teaching depths of ∼40 km.