Partial melting of a depleted peridotite metasomatized by a MORB-derived hydrous silicate melt - Implications for subduction zone magmatism
Partial melting of a depleted peridotite metasomatized by a MORB-derived hydrous silicate melt - Implications for subduction zone magmatism
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DOI：
10.1016/j.gca.2020.09.001
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发表时间：
2020-12-01
影响因子：
5
通讯作者：
Dasgupta, Rajdeep
中科院分区：
文献类型：
--
作者：
Lara, Michael;Dasgupta, Rajdeep
最近的地球动力学模型和地质温度计表明，板在中到热俯冲带的水饱和的玄武岩固相线交叉，表明含水硅酸盐熔体是重要的代理商的质量转移从板到地幔楔弧下。然而，玄武质地壳来源的含水熔体通量地幔楔熔融的影响知之甚少。在这里，我们提出了熔融相关系的亏损橄榄岩+MORB衍生的含水硅酸盐熔体在熔体：岩石质量比为0.1和0.05（3.5和1.7重量%）H2O，分别）来模拟流体存在的部分熔融的亏损橄榄岩，这已被交代的含水硅酸盐熔体来自俯冲玄武岩地壳。实验在2-3 GPa和900-1250 ℃下在活塞缸中进行，使用Au和Au 75 Pd 25胶囊。角闪石（7-10重量%）是稳定的，直到1000摄氏度，在2和3 GPa共存的组合为主的橄榄石和opx和小部分的cpx和石榴石在3 GPa。表观流体饱和的固相线，我们的散装组成位于1000-1050摄氏度，符合枯竭角闪石在2和3 GPa。角闪石在0和5重量%之间耗尽熔融在2和3 GPa和主导的熔融反应，在这个熔融间隔沿着与opx，产生SiO2和Al 2 O3丰富，FeO* 和MgO贫乏的原始安山岩流体饱和条件下。熔融反应在低度，流体饱和熔融是不一致的，消耗opx和生产橄榄石+ SiO2丰富的熔体，并观察到在很宽的范围内的起始组合物和压力从这项研究和其他。随着熔融程度的增加和自由流体相的消耗，产生了玄武安山岩到玄武质安山岩的光谱。从这个和其他含水橄榄岩熔融研究的实验部分熔体与天然原始弧岩浆的比较表明，具有不同的整体组成，但具有2.5 - 4.2重量%的橄榄岩熔融H2O可以再现全球原始弧岩浆的主要氧化物分布和趋势。从这个比较中，很明显，仅在含水地幔熔融的压力，其中部分熔体是流体饱和下的差异，可以解释在实验和自然数据中观察到的一阶趋势，温度和成分的差异有助于这些趋势内的成分分布。普遍存在的安山岩成因在广泛的压力和散装组合物在含水流体饱和熔融表明，原始安山岩熔体通量通过地壳的相对罕见的可能与这样的事实，即这种熔体只产生在地幔楔的底部，温度相对较低。由于流体饱和的安山质熔体上升到地幔楔的较热的核心，他们可能会消耗更高的程度，流体不饱和熔融产生更常见的含水玄武质熔体。(C)2020爱思唯尔有限公司保留所有权利。
Recent geodynamic models and geothermometers suggest that slabs in intermediate to hot subduction zone cross the water-saturated basalt solidus, indicating that hydrous silicate melts are important agents of mass transfer from slab to mantle wedge beneath arcs. Yet the effects of basaltic crust-derived hydrous melt fluxing on mantle wedge melting are poorly known. Here we present the melting phase relations of a depleted peridotite + a MORB-derived hydrous silicate melt at a melt:rock mass ratio of 0.1 and 0.05 (3.5 and 1.7 wt.% H2O, respectively) to simulate fluid-present partial melting of a depleted peridotite, which has been metasomatized by a hydrous silicate melt derived from subducting basaltic crust. Experiments were performed at 2-3 GPa and 900-1250 degrees C in a piston cylinder, using Au and Au75Pd25 capsules. Amphibole (7-10 wt.%) is stable up to 1000 degrees C at 2 and 3 GPa coexisting with an assemblage dominated by olivine and opx and with minor fractions of cpx and garnet at 3 GPa. The apparent fluid-saturated solidus of our bulk composition is located at 1000-1050 degrees C, coinciding with the exhaustion of amphibole at 2 and 3 GPa. Amphibole is exhausted between 0 and 5 wt.% melting at 2 and 3 GPa and dominates the melting reactions in this melting interval along with opx, generating SiO2 and Al2O3-rich, and FeO*and MgO-poor primitive andesites under fluid-saturated conditions. The melting reactions during low-degree, fluid-saturated melting are incongruent, consuming opx and producing olivine + SiO2-rich melts and is observed over a wide range of starting compositions and pressures from this study and others. As extent of melting increases and the free fluid phase is consumed, a spectrum of basaltic andesites to basanites are produced. Comparison of experimental partial melts from this and other hydrous peridotite melting studies with natural primitive arc magmas suggests that melting of peridotites with varying bulk compositions but with 2.5 - 4.2 wt.% H2O can reproduce the major oxide spread and trends of primitive arc magmas globally. From this comparison, it is clear that differences solely in the pressure of hydrous mantle melting, where the partial melts are fluid-under saturated, can account for the first order trends observed in experimental and natural data, with differences in temperature and composition contributing to the compositional spread within these trends. The ubiquity of andesite genesis over a wide range of pressures and bulk compositions during aqueous fluid-saturated melting suggests that the relative rarity of primitive andesitic melt flux through the crust could be related to the fact that such melts are only produced at the base of the mantle wedge where temperatures are relatively low. As fluid-saturated andesitic melts ascend into the hotter core of the mantle wedge, they are likely consumed by higher-degree, fluid-undersaturated melting generating more common hydrous basaltic melts. (C) 2020 Elsevier Ltd. All rights reserved.