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Petrological Implications of Seafloor Hydrothermal Alteration of Subducted Mid-Ocean Ridge Basalt
Journal of Petrology ( IF 3.9 ) Pub Date : 2020-08-26 , DOI: 10.1093/petrology/egaa086
David Hernández-Uribe 1 , Richard M Palin 1, 2 , Kim A Cone 1 , Wentao Cao 3
Affiliation  

Determining the mineralogical changes occurring in subducted oceanic crust is key to understanding short- and long-term geochemical cycles. Although numerous studies have explored the mineral assemblages that form in mid-ocean ridge basalt (MORB) at different depths below the Earth’s surface, it is widely recognized that seafloor hydrothermal alteration of the uppermost portion of the oceanic crust can change its composition between a ridge and a trench prior to subduction. In this study, we use petrological modelling to explore the effects of different types of pre-subduction hydrothermal alteration on the phase changes that occur during seafloor alteration of MORB-like compositions during subduction along an average Phanerozoic geotherm. We consider a representative composition of altered oceanic crust, as well as extreme end-member scenarios (pervasive spilitization, chloritization, and epidotization). Our models show that epidotization and chloritization of MORB strongly affects phase equilibria at different depths, whereas spilitization and an average style of alteration produce relatively fewer changes on the mineral assemblage to those expected in a pristine MORB. Devolatilization of MORB during subduction occurs mostly in the forearc region, although the type and extent of alteration strongly control the depth and magnitude of fluid released. Altered compositions carry significantly more H2O to sub- and postarc depths than unaltered compositions; the H2O carrying capacity of unaltered and altered compositions is further enhanced during subduction along colder geotherms. Extremely localized areas affected by epidotization can transport up to 22 times more H2O than unaltered MORB and up to two times more than average altered oceanic crust compositions to depths beyond the arc. Regardless of the extent and style of alteration, the stability of hydrous phases, such as epidote and phengite (important trace element carriers), is expanded to greater pressure and temperature conditions. Thus, hydrothermal alteration of the subducted oceanic slab-top represents a viable, and probably common, mechanism that enhances geochemical recycling between the Earth’s hydrosphere and shallow interior.

中文翻译:

俯冲洋中脊玄武岩海底热液蚀变的岩石学意义

确定俯冲洋壳中发生的矿物学变化是了解短期和长期地球化学循环的关键。尽管大量研究已经探索了在地球表面以下不同深度的中海洋脊玄武岩(MORB)中形成的矿物组合,但人们普遍认识到,洋壳最上部的海底热液蚀变会改变其在海脊之间的组成在俯冲之前先挖一条沟 在这项研究中,我们使用岩石学模型来探索不同类型的俯冲前热液蚀变对沿沿平均生代地热的俯冲期间MORB样成分海底变化期间发生的相变的影响。我们考虑了地壳变化的典型组成,以及极端的终端成员场景(普遍的碎片化,氯化和结石化)。我们的模型表明,MORB的上位化和氯化作用在不同深度强烈影响相平衡,而碎片化和平均蚀变方式对矿物组合的变化相对于原始MORB所预期的要少。尽管改变的类型和程度强烈地控制着所释放流体的深度和大小,但在俯冲过程中,MORB的脱挥发分主要发生在前臂区域。改变的成分携带更多的H 相对于原始的MORB而言,成矿和平均改变方式对矿物组合产生的变化相对较少。尽管改变的类型和程度强烈地控制着所释放流体的深度和大小,但在俯冲过程中,MORB的脱挥发分主要发生在前臂区域。改变的成分携带更多的H 相对于原始的MORB而言,成矿和平均改变方式对矿物组合产生的变化相对较少。尽管改变的类型和程度强烈地控制着所释放流体的深度和大小,但在俯冲过程中,MORB的脱挥发分主要发生在前臂区域。改变的成分携带更多的H比未改变的组成要大2 O到弧下和弧后的深度;在沿较冷的地热俯冲过程中,未改变和改变的组分的H 2 O携带能力进一步增强。受极点影响的极端局部区域可以传输多达22倍的H 2比未改变的MORB的O值高出弧线以外的深度,是平均改变后的海洋硬壳成分的两倍。不管改变的程度和方式如何,水相(如附子和硫铁矿(重要的微量元素载体))的稳定性都会扩展到更大的压力和温度条件下。因此,俯冲的海洋平板顶部的热液蚀变是一种可行的并且可能是常见的机制,可以增强地球水圈与浅层内部之间的地球化学循环。
更新日期:2020-08-26
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