{"title":"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","authors":"Yifan Lu, Yicheng Sun, Guoxin Xia, Xiandong Liu, Guo-Guang Wang, Xiancai Lu","doi":"10.1016/j.gca.2025.05.015","DOIUrl":null,"url":null,"abstract":"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 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 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 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. 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 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 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 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 fluid, we observed that Q<ce:sup loc=\"post\">0</ce:sup> and Q<ce:sup loc=\"post\">1</ce:sup> species are the predominant types. In the water content range of 20 wt% to 30 wt%, the proportional distribution of Q<ce:sup loc=\"post\">n</ce:sup> species in the supercritical 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 fluid is: Q<ce:sup loc=\"post\">1</ce:sup> > Q<ce:sup loc=\"post\">0</ce:sup>. However, when the water content exceeds 30 wt%, the order of Q<ce:sup loc=\"post\">n</ce:sup> abundance shifts to: Q<ce:sup loc=\"post\">0</ce:sup> > Q<ce:sup loc=\"post\">1</ce:sup>. The content of Q<ce:sup loc=\"post\">2</ce:sup> species is relatively low, while the concentrations of Q<ce:sup loc=\"post\">3</ce:sup> and Q<ce:sup loc=\"post\">4</ce:sup> species are negligible. This study reveals that the supercritical 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 fluid not only contains a significant amount of low-Q<ce:sup loc=\"post\">n</ce:sup> (n ≤ 1) species, but also a large proportion of OH<ce:sup loc=\"post\">–</ce:sup> 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 [SiO<ce:inf loc=\"post\">4</ce:inf>]. 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<ce:sup loc=\"post\">–</ce:sup> during the formation of supercritical fluid from ultramafic melts serves as an important source of OH<ce:sup loc=\"post\">–</ce:sup> in deep subduction zones.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"47 1","pages":""},"PeriodicalIF":4.5000,"publicationDate":"2025-05-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":"0","resultStr":"{\"title\":\"Structures and transport properties of Mg2SiO4-H2O system under high temperature and pressure: insights for supercritical fluids in deep subduction zones\",\"authors\":\"Yifan Lu, Yicheng Sun, Guoxin Xia, Xiandong Liu, Guo-Guang Wang, Xiancai Lu\",\"doi\":\"10.1016/j.gca.2025.05.015\",\"DOIUrl\":null,\"url\":null,\"abstract\":\"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 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 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 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. 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 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 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 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 fluid, we observed that Q<ce:sup loc=\\\"post\\\">0</ce:sup> and Q<ce:sup loc=\\\"post\\\">1</ce:sup> species are the predominant types. In the water content range of 20 wt% to 30 wt%, the proportional distribution of Q<ce:sup loc=\\\"post\\\">n</ce:sup> species in the supercritical 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 fluid is: Q<ce:sup loc=\\\"post\\\">1</ce:sup> > Q<ce:sup loc=\\\"post\\\">0</ce:sup>. However, when the water content exceeds 30 wt%, the order of Q<ce:sup loc=\\\"post\\\">n</ce:sup> abundance shifts to: Q<ce:sup loc=\\\"post\\\">0</ce:sup> > Q<ce:sup loc=\\\"post\\\">1</ce:sup>. The content of Q<ce:sup loc=\\\"post\\\">2</ce:sup> species is relatively low, while the concentrations of Q<ce:sup loc=\\\"post\\\">3</ce:sup> and Q<ce:sup loc=\\\"post\\\">4</ce:sup> species are negligible. This study reveals that the supercritical 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 fluid not only contains a significant amount of low-Q<ce:sup loc=\\\"post\\\">n</ce:sup> (n ≤ 1) species, but also a large proportion of OH<ce:sup loc=\\\"post\\\">–</ce:sup> 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 [SiO<ce:inf loc=\\\"post\\\">4</ce:inf>]. 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<ce:sup loc=\\\"post\\\">–</ce:sup> during the formation of supercritical fluid from ultramafic melts serves as an important source of OH<ce:sup loc=\\\"post\\\">–</ce:sup> in deep subduction zones.\",\"PeriodicalId\":327,\"journal\":{\"name\":\"Geochimica et Cosmochimica Acta\",\"volume\":\"47 1\",\"pages\":\"\"},\"PeriodicalIF\":4.5000,\"publicationDate\":\"2025-05-11\",\"publicationTypes\":\"Journal Article\",\"fieldsOfStudy\":null,\"isOpenAccess\":false,\"openAccessPdf\":\"\",\"citationCount\":\"0\",\"resultStr\":null,\"platform\":\"Semanticscholar\",\"paperid\":null,\"PeriodicalName\":\"Geochimica et Cosmochimica Acta\",\"FirstCategoryId\":\"89\",\"ListUrlMain\":\"https://doi.org/10.1016/j.gca.2025.05.015\",\"RegionNum\":1,\"RegionCategory\":\"地球科学\",\"ArticlePicture\":[],\"TitleCN\":null,\"AbstractTextCN\":null,\"PMCID\":null,\"EPubDate\":\"\",\"PubModel\":\"\",\"JCR\":\"Q1\",\"JCRName\":\"GEOCHEMISTRY & GEOPHYSICS\",\"Score\":null,\"Total\":0}","platform":"Semanticscholar","paperid":null,"PeriodicalName":"Geochimica et Cosmochimica Acta","FirstCategoryId":"89","ListUrlMain":"https://doi.org/10.1016/j.gca.2025.05.015","RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"","PubModel":"","JCR":"Q1","JCRName":"GEOCHEMISTRY & GEOPHYSICS","Score":null,"Total":0}
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.
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.