Experimentally based water budgets for dehydrating slabs and consequences for arc magma generation
Experimentally based water budgets for dehydrating slabs and consequences for arc magma generation
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DOI：
10.1016/s0012-821x(98)00142-3
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发表时间：
1998-11-01
影响因子：
5.3
通讯作者：
Poli, S
中科院分区：
文献类型：
--
作者：
Schmidt, MW;Poli, S
根据俯冲带特有的条件，编制了含水大洋中脊玄武岩(MOR)和含水橄榄岩(250 Lan)的相图。我们根据实验测定的玄武岩中绿泥石、绿柱石、绿帘石、角闪石、滑石、拟绿泥石、滑石和多硅白云母的相关系，以及文献中蛇纹石、滑石、绿泥石、角闪石和超镁铁石中的A相的相关系，合成了玄武岩中绿泥石、绿帘石、绿帘石、角闪石、拟绿泥石和多硅白云母的相关系，并根据文献中的蛇纹石、滑石、绿泥石、角闪石和超镁铁石中的A相的相关系，计算了天然成分中的水合相组合中的水含量。这就产生了计算下降板的水量平衡所需的信息。对于水化洋壳，从H2O含量在5-6wt%之间的低品位蓝片岩条件(10-20 lan深度)开始，作为单个板岩地温的函数，在70-300 lan深度之间实现了完全脱水。在火山弧以下深层分解的水合相有镁铁质成分中的劳钠铝石、橄榄石、绿泥石和滑石(+/-多硅白云母)，橄榄岩中有绿泥石和蛇纹石。大约15%-35%的初始俯冲的H2O被释放到火山弧下方。角闪石脱水对水收支的贡献很小(5-20%)，而且发生在相对较浅的深度(65-90公里)。在任何预测的热结构中，脱水是一个逐步和连续的过程的组合，通过许多不同的反应，这些反应同时发生在下降板的不同部分。这种脱水特征与“单相脱水模型”不相容，“单相脱水模型”将流体流动集中在一个独特的重大脱水事件中，以解释火山前缘。作为不断进行脱水的结果，从板材上升的水通常可达到约150-200兰的深度。从俯冲岩石圈上升的流体将在地幔楔形的热部分引起部分熔融。我们认为，火山前锋只是形成在地幔楔形等温线之上，那里的熔融程度足以允许对母质岩浆的机械提取。热模型表明，这样的等温线(约1300摄氏度)位于火山锋面以下，低于这样的等温线的板片表面深度与火山前锋下板片表面的观测深度是一致的。(C)1998 Elsevier Science B.V.保留所有权利。
Phase diagrams of hydrous mid-ocean ridge (MOR) basalts to 330 lan depth and of hydrous peridotites to 250 lan depth are compiled for conditions characteristic for subduction zones. A synthesis of our experimentally determined phase relations of chlorite, lawsonite, epidote-zoisite, amphibole, paragonite, chloritoid, talc, and phengite in basalts and of phase relations from the literature of serpentine, talc, chlorite, amphibole, and phase A in ultramafics permits calculation of H2O contents in hydrous phase assemblages that occur in natural compositions. This yields the information necessary to calculate water budgets for descending slabs. Starting from low-grade blueschist conditions (10-20 lan depth) with H2O contents between 5 and 6 wt% for hydrated oceanic crust, complete dehydration is achieved between 70 and >300 lan depth as a function of individual slab geotherms. Hydrous phases which decompose at depth below volcanic arcs are lawsonite, zoisite, chloritoid, and talc (+/- phengite) in mafic compositions and chlorite and serpentine in peridotite. Approximately 15-35% of the initially subducted H2O are released below volcanic arcs. The contribution of amphibole dehydration to the water budget is small (5-20%) and occurs at relatively shallow depth (65-90 km). In any predicted thermal structure, dehydration is a combination of a stepwise and a continuous process through many different reactions which occur simultaneously in the different portions of the descending slab. Such a dehydration characteristic is incompatible with 'single phase dehydration models' which focus fluid flow through a unique major dehydration event in order to explain Volcanic fronts. As a consequence of continuously progressing dehydration, water ascending from the slab will be generally available to depth of ca. 150-200 lan. The fluid rising from the subducting lithosphere will cause partial melting in the hot portion of the mantle wedge. We propose that the volcanic front simply forms above the mantle wedge isotherm where the extent of melting is sufficient to allow for the mechanical extraction of parental are magmas. Thermal models show that such an isotherm (ca. 1300 degrees C) locates below volcanic fronts, slab surface depths below such an isotherm are compatible with the observed depths of the slab surface below volcanic fronts. (C) 1998 Elsevier Science B.V. All rights reserved.