高温高压下Mg2SiO4-H2O体系的结构与输运性质:深俯冲带超临界流体研究

IF 4.5 1区 地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS
Yifan Lu, Yicheng Sun, Guoxin Xia, Xiandong Liu, Guo-Guang Wang, Xiancai Lu
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Our study confirmed the previous experimental observation that ultramafic melts can undergo polymerization due to dissolved water. The cause of the polymerization is the increase in 5-fold Si-O coordination, and it only occurs within specific ranges of water content at a given temperature. In the Mg<ce:inf loc=\"post\">2</ce:inf>SiO<ce:inf loc=\"post\">4</ce:inf>-H<ce:inf loc=\"post\">2</ce:inf>O system, water and OH are primarily bonded to Mg, forming Mg-H<ce:inf loc=\"post\">2</ce:inf>O<ce:italic><ce:inf loc=\"post\">m</ce:inf></ce:italic> and Mg-OH species. The results demonstrate that as the water content increases, the viscosity of the Mg<ce:inf loc=\"post\">2</ce:inf>SiO<ce:inf loc=\"post\">4</ce:inf>-H<ce:inf loc=\"post\">2</ce:inf>O system exhibits a rapid initial decrease followed by a gradual reduction. 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引用次数: 0

摘要

了解超临界流体的结构和输运性质对于深入了解它们在俯冲带中的行为至关重要。虽然以前的研究已经提供了来自长硅熔体的超临界流体性质的理解,但我们对深俯冲带超镁铁质熔体形成的超临界流体的认识仍然有限。在这项研究中,我们采用第一性原理分子动力学系统地研究了Mg2SiO4-H2O体系在2000 K和3000 K温度下,在大约10 GPa的压力下的结构、形态、自扩散和粘度,覆盖了0到70 wt%的含水量。我们的研究证实了先前的实验观察,即超镁铁熔体由于溶解的水而发生聚合。聚合的原因是5倍Si-O配位的增加,它只发生在给定温度下的特定含水量范围内。在Mg2SiO4-H2O体系中,水和OH主要与Mg结合,形成Mg- h2om和Mg-OH。结果表明:随着水含量的增加,Mg2SiO4-H2O体系的粘度呈现先快速下降后逐渐降低的趋势;在2000 K时观察到的粘度的快速下降不是由于结构的解聚;相反,在这个过程中,体系的结构变得更加聚合。驱动粘度快速下降的关键因素是质子化硅酸盐单元比例的增加。超临界Mg2SiO4-H2O流体的低粘度使其流动性比玄武岩熔体高2 ~ 3个数量级,比碳酸盐熔体高1.7 ~ 20倍。通过将这些发现与来自长硅熔体的超临界流体进行比较,我们提出由俯冲带不同硅酸盐组分形成的超临界流体具有相似的低粘度,但差异较小。超临界流体促进元素迁移的能力主要取决于这些元素在超临界流体中的溶解度。在超临界Mg2SiO4-H2O流体中,我们观察到Q0和Q1种是主要类型。在含水量为20 wt% ~ 30 wt%范围内,超临界Mg2SiO4-H2O流体中Qn组分的比例分布为:Q1 >;Q0处。然而,当含水量超过30% wt%时,Qn丰度的顺序为:Q0 >;Q1。Q2种的含量相对较低,而Q3和Q4种的浓度可以忽略不计。本研究发现,超临界Mg2SiO4-H2O流体中不仅含有大量的低qn (n≤1)物质,还含有大量OH -物质。在深俯冲带,由俯冲洋壳释放的流体所产生的超临界流体将更富含单体[SiO4]。这种结构有利于与特定元素(如高场强元素)形成配合物,并促进其被超临界流体动员。此外,超镁质熔体在形成超临界流体过程中大量释放的OH -是深俯冲带OH -的重要来源。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
Structures and transport properties of Mg2SiO4-H2O system under high temperature and pressure: insights for supercritical fluids in deep subduction zones
Understanding the structure and transport properties of supercritical fluids is crucial for gaining insights into their behavior in subduction zones. While previous research has provided an understanding of the properties of supercritical fluids derived from felsic melts, our knowledge regarding supercritical fluids formed from ultramafic melts in deep subduction zones remains limited. In this study, we employed first-principles molecular dynamics to systematically investigate the structure, speciation, self-diffusion, and viscosity of the Mg2SiO4-H2O system at temperatures of 2000 K and 3000 K under a pressure of approximately 10 GPa, covering water contents ranging from 0 to 70 wt%. Our study confirmed the previous experimental observation that ultramafic melts can undergo polymerization due to dissolved water. The cause of the polymerization is the increase in 5-fold Si-O coordination, and it only occurs within specific ranges of water content at a given temperature. In the Mg2SiO4-H2O system, water and OH are primarily bonded to Mg, forming Mg-H2Om and Mg-OH species. The results demonstrate that as the water content increases, the viscosity of the Mg2SiO4-H2O system exhibits a rapid initial decrease followed by a gradual reduction. The rapid decrease in viscosity observed at 2000 K is not due to the depolymerization of the structure; on the contrary, the structure of the system becomes more polymerized during this process. The crucial factor driving the rapid viscosity decrease is the increasing proportion of protonated silicate units. The low viscosity of supercritical Mg2SiO4-H2O fluid allows its mobility to reach 2 to 3 orders of magnitude greater than that of basalt melt and 1.7 to 20 times greater than that of carbonate melt. By comparing these findings with supercritical fluids derived from felsic melts, we propose that supercritical fluids formed from different silicate components in subduction zones exhibit similarly low viscosities with minor differences. The ability of supercritical fluids to facilitate element migration primarily depends on the solubility of these elements within the supercritical fluids. In the supercritical Mg2SiO4-H2O fluid, we observed that Q0 and Q1 species are the predominant types. In the water content range of 20 wt% to 30 wt%, the proportional distribution of Qn species in the supercritical Mg2SiO4-H2O fluid is: Q1 > Q0. However, when the water content exceeds 30 wt%, the order of Qn abundance shifts to: Q0 > Q1. The content of Q2 species is relatively low, while the concentrations of Q3 and Q4 species are negligible. This study reveals that the supercritical Mg2SiO4-H2O fluid not only contains a significant amount of low-Qn (n ≤ 1) species, but also a large proportion of OH species. In deep subduction zone, the supercritical fluids generated by the release of fluids from the subducting oceanic crust will be more enriched in monomers [SiO4]. This structure is favorable for the formation of complexes with specific elements (such as high field strength elements) and promotes their mobilization by supercritical fluids. Furthermore, the substantial release of OH during the formation of supercritical fluid from ultramafic melts serves as an important source of OH in deep subduction zones.
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来源期刊
Geochimica et Cosmochimica Acta
Geochimica et Cosmochimica Acta 地学-地球化学与地球物理
CiteScore
9.60
自引率
14.00%
发文量
437
审稿时长
6 months
期刊介绍: Geochimica et Cosmochimica Acta publishes research papers in a wide range of subjects in terrestrial geochemistry, meteoritics, and planetary geochemistry. The scope of the journal includes: 1). Physical chemistry of gases, aqueous solutions, glasses, and crystalline solids 2). Igneous and metamorphic petrology 3). Chemical processes in the atmosphere, hydrosphere, biosphere, and lithosphere of the Earth 4). Organic geochemistry 5). Isotope geochemistry 6). Meteoritics and meteorite impacts 7). Lunar science; and 8). Planetary geochemistry.
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