Evidence for channelized external fluid flow and element transfer in subducting slabs (Raspas Complex, Ecuador)
Evidence for channelized external fluid flow and element transfer in subducting slabs (Raspas Complex, Ecuador)
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DOI：
10.1016/j.chemgeo.2012.03.023
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发表时间：
2012-06
期刊：
影响因子：
3.9
通讯作者：
P. Herms;T. John;R. Bakker;V. Schenk
中科院分区：
文献类型：
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作者：
P. Herms;T. John;R. Bakker;V. Schenk
由俯冲板块脱水而在极深处释放的流体在物质运输中起主要作用，并对上覆地幔楔的交代作用和部分熔融负有责任。为了研究大洋俯冲岩石圈内的流体流动机制和元素流动性，选择了厄瓜多尔的拉斯帕斯杂岩，在那里暴露了蓝片岩、榴辉岩、变质长岩和超镁岩组成的蛇绿岩组合。该蛇绿岩组合以高压黝帘石脉、伴生的交代黝帘石榴辉岩和沙化、假刺状织构绿泥石黑兹伯尔岩而突出，表明在深度约60km处流体流动和元素可迁移性。交代榴辉岩中脉状黝帘石和石榴石边缘的振荡分带可以解释为在反长岩击穿反应后，高压沙化过程中的循环流体排出。流体包裹体研究表明，水相组成均匀，低矿化度，ch4和co2含量较少，符合开放体系流体外源渗透的特征。砂化绿泥石辉石是低矿化度水溶液的潜在来源。与morb型榴辉岩相比，黝帘石脉与富含LREE、MREE、Pb、Sr、HFSE、Th、U等元素的黝帘石榴辉岩的结合，体现了大量微量元素的交代作用和流体流动性。流体流动微量元素B、Rb、Pb、Sr富集于辉石中流体包裹体中。大多数微量元素不能从蛇纹岩中获得，但可以通过在强烈的流体-岩石相互作用带中从变质岩和变质岩中浸出来解释。缺乏轻稀土、重稀土和锶的石榴石角闪孔岩可能是这种浸出的变质岩。B、Rb、Th和U可由元长岩衍生。通过通道化流体流动和高流体通量，流体可移动的微量元素可能被输送到地幔楔体中。
Fluids released at great depth by dehydrating subducting plates play a major role in mass transport and are responsible for metasomatism and partial melting in the overlying mantle wedge. To investigate the fluid-flow regime and element mobility within a subducting oceanic lithosphere, the Raspas Complex in Ecuador has been selected where an ophiolite association of blueschists, eclogites, metapelites and ultramafitites is exposed. This ophiolite association stands out by high-pressure zoisite veins, associated metasomatized zoisite eclogites and deserpentinized, pseudo-spinifex textured chlorite harzburgites indicating fluid flow and element mobility at a depth of about 60km. Oscillatory zoning in vein zoisite and garnet rims in the metasomatized eclogites is explained by cyclic fluid expulsion during high-pressure deserpentinization after overstepping of the antigorite breakdown reaction. Fluid inclusion studies reveal a homogeneous low-salinity aqueous fluid composition with minor CH4and CO2which is in accord with open-system fluid infiltration derived from an external source. The deserpentinized chlorite harzburgite is a potential source for the low-salinity aqueous fluid. The association of zoisite veins with zoisite eclogites which are enriched in LREE, MREE, Pb, Sr, HFSE, Th, and U, compared to MORB-type eclogites, documents the metasomatic effects and the fluid mobility of a large range of trace elements. In addition, the fluid-mobile trace elements B, Rb, Pb, and Sr are enriched in fluid inclusions in omphacite. Most trace elements cannot be derived from the serpentinites but might be explained by leaching from metabasites and metapelites in zones of intense fluid–rock interaction. A garnet–amphibole rock, deficient in LREE, MREE and Sr, could represent such a leached metabasite. B, Rb, Th and U could be derived from metapelites. By channelized fluid flow and high fluid flux, the fluid-mobile trace elements might be transported into the mantle wedge.