11B-rich fluids in subduction zones: The role of antigorite dehydration in subducting slabs and boron isotope heterogeneity in the mantle
11B-rich fluids in subduction zones: The role of antigorite dehydration in subducting slabs and boron isotope heterogeneity in the mantle
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DOI：
10.1016/j.chemgeo.2014.03.015
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发表时间：
2014-05-29
期刊：
影响因子：
3.9
通讯作者：
Teresa Gomez-Pugnaire, Maria
中科院分区：
文献类型：
--
作者：
Harvey, Jason;Garrido, Carlos J.;Teresa Gomez-Pugnaire, Maria
蛇纹岩由地幔橄榄岩的水化作用形成，是进入俯冲带的流体活动元素的最大潜在储集层。其中一种元素硼的同位素比率可以区分来自地壳和蛇纹岩来源的流体贡献。尽管深海橄榄岩中85%的硼在从蜥蜴岩到反花岗岩的俯冲开始时丢失了，但仍保留了足够的硼来解释岛弧岩浆的组成(约7微克(-1)，增量B-11为+22%)，直到下沉的板片到达反花岗岩出来的等值线为止。此时释放出富B-11的流体，能够提供岛弧玄武岩独特的d11B特征。除了西班牙Btic Cordillera Cerro del Almirez蛇纹岩中保存独特的反蛇纹岩外，前进岩(反蛇纹岩-绿泥石-斜方辉石-橄榄石蛇纹岩、花岗岩-结构绿泥石-方辉橄榄岩和尖晶石-结构绿泥石-方辉橄榄岩)具有非常不同的硼同位素特征(增量B-11-3至+6%)，但与isograd低P-T侧的反蛇纹岩-蛇纹岩相比，硼含量没有显著差异。富含B-11的流体至少与远洋沉积物部分平衡，与这些在开放体系条件下脱水的前进岩的组成有关。在脱水早期失去的蛇纹岩中的硼很容易被合并到弧前橄榄岩中。反过来，由于俯冲侵蚀的结果，这些岩石可能被拖到弧下深度，并并入由弧前蛇纹岩、蚀变洋壳和远洋沉积物组成的混杂岩系中。在反戈尔特岩出来的等值线上，它脱水，因此可能提供了另一个富含B-11的流体来源。(C)2014爱思唯尔B.V.保留所有权利。
Serpentinites form by hydration of mantle peridotite and constitute the largest potential reservoir of fluid-mobile elements entering subduction zones. Isotope ratios of one such element, boron, distinguish fluid contributions from crustal versus serpentinite sources. Despite 85% of boron hosted within abyssal peridotite being lost at the onset of subduction at the lizardite-to-antigorite transition, a sufficient cargo of boron to account for the composition of island arc magma is retained (c. 7 mu gg(-1), with a delta B-11 of + 22%) until the down-going slab reaches the antigorite-out isograd. At this point a B-11-rich fluid, capable of providing the distinctive d11B signature of island arc basalts, is released. Beyond the uniquely preserved antigorite-out isograd in serpentinites from Cerro del Almirez, Betic Cordillera, Spain, the prograde lithologies (antigorite-chlorite-orthopyroxene-olivine serpentinite, granofels-texture chlorite-harzburgite and spinifex-texture chlorite-harzburgite) have very different boron isotope signatures (delta B-11 - -3 to + 6%), but with no significant difference in boron concentration compared to the antigorite-serpentinite on the low P-T side of the isograd. B-11-rich fluid, which at least partly equilibrated with pelagic sediments, is implicated in the composition of these prograde lithologies, which dehydrated under open-systemconditions. Serpentinite-hosted boron lost during the early stages of dehydration is readily incorporated into forearc peridotite. This, in turn, may be dragged to sub-arc depths as a result of subduction erosion and incorporated in a melange comprising forearc serpentinite, altered oceanic crust and pelagic sediment. At the antigorite-out isograd it dehydrates, thus potentially providing an additional source of B-11-rich fluids. (C) 2014 Elsevier B.V. All rights reserved.