Reactions between basalt and CO2-rich seawater at 250 and 350°C, 500bars: Implications for the CO2 sequestration into the modern oceanic crust and the composition of hydrothermal vent fluid in the CO2-rich early ocean
Reactions between basalt and CO2-rich seawater at 250 and 350°C, 500bars: Implications for the CO2 sequestration into the modern oceanic crust and the composition of hydrothermal vent fluid in the CO2-rich early ocean
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DOI：
10.1016/j.chemgeo.2013.08.044
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发表时间：
2013-11
期刊：
影响因子：
3.9
通讯作者：
T. Shibuya;Motoko Yoshizaki;Y. Masaki;Katsuhiko Suzuki;K. Takai;M. Russell
中科院分区：
文献类型：
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作者：
T. Shibuya;Motoko Yoshizaki;Y. Masaki;Katsuhiko Suzuki;K. Takai;M. Russell
本研究旨在了解玄武岩在海底高温蚀变带中吸收co2的过程，重建太古宙绿岩碳酸化等热液蚀变过程。为此，我们进行了两次室内实验，模拟玄武岩（石英-费长石-磁铁矿氧逸度条件下合成）与富co2 NaCl流体（25℃pH = 6.5）在高温高压下的热液反应。在250°C和350°C 500 bar条件下，随着水/岩反应的进行，总碳酸浓度（ΣCO2）分别从最初的400 mmol/kg降低到接近0和100 mmol/kg，同时方解石作为蚀变矿物在玄武岩中形成。这表明，在h2o - co2 -玄武岩体系中，方解石随着温度的升高而变得不稳定，地壳玄武岩可以以方解石的形式吸收流体中几乎所有的co2，至少在温度和初始co2浓度分别低于250℃和400 mmol/kg时是如此。虽然在实验中实现了第二个目标，但在蚀变产物中没有发现太古宙绿岩中存在的绢云母、白云石、铁云母和菱铁矿等矿物，这可能是由于初始溶液中缺乏K、Mg和Fe。水/岩反应过程中流体中SiO2、Mg和K的稳态浓度与现代玄武岩热液体系中高温流体（bb0 ~ 250℃）的稳态浓度相似。然而，在250°C和350°C时，最终实验pH值分别为6.6和7.2，高于现代热液流体（约5），高于250 - 350°C， 500 bar时的中性pH值（5.5-5.6）。结果表明，初始流体中存在丰富的co2，导致玄武岩碳化；因此，pH值被方解石的沉淀和溶解所缓冲。由于phin -si的升高，流体中溶解的Fe和Mn浓度比现代热液流体低2 ~ 3个数量级。在现代海洋中，高温热液喷口流体是第二大铁来源（仅次于河流输入）。然而，由于碱性、缺乏金属的热液流体是在富含二氧化碳的系统中产生的，因此富含二氧化碳的海底热液系统可能在早期海洋中扮演了铁水槽的角色。
This study aims to understand how basaltic rocks absorb CO2in high-temperature alteration zones in the subseafloor, and to reconstruct hydrothermal alteration processes such as carbonatization of Archean greenstones. To this end, we conducted two laboratory experiments, simulating hydrothermal reactions between basalt (synthesized under quartz–fayalite–magnetite oxygen fugacity) and CO2-rich NaCl fluid (pH = 6.5 at 25 °C) at high temperature and pressure. As the water/rock reactions progressed at 250 °C and 350 °C, 500 bars, total carbonic acid concentration (ΣCO2) reduced from its initial 400 mmol/kg to near 0 and 100 mmol/kg, respectively, meanwhile calcite was formed in the basalt as an alteration mineral. This indicates that calcite destabilizes as temperature increases in the H2O–CO2–basalt system and that crustal basalts can absorb almost all CO2in the fluid as calcite, at least at temperatures and initial CO2concentrations below 250 °C and 400 mmol/kg, respectively. Although the second aim was realized in the experiments, minerals such as sericite, dolomite, ankerite, and siderite present in Archean greenstones were not identified in the alteration products, possibly because K, Mg, and Fe were lacking in the initial solutions. Steady-state concentrations of SiO2, Mg, and K in the fluids during water/rock reactions were similar to those of high-temperature fluids (> 250 °C) in modern basalt-hosted hydrothermal systems. However, the final experimental pHin-situwas 6.6 and 7.2 at 250 °C and 350 °C, respectively, higher than that in modern hydrothermal fluids (approximately 5) and higher than the neutral pH (5.5–5.6) at 250–350 °C, 500 bars. The results suggest that the presence of abundant CO2in the initial fluid induced carbonatization of basalt; consequently, pH was buffered by precipitation and dissolution of calcite. Because pHin-situwas elevated, the dissolved Fe and Mn concentrations in the fluid were two to three orders of magnitude lower than those of modern hydrothermal fluids. In modern oceans, high-temperature hydrothermal vent fluids are the second-largest iron source (after riverine input). However, because alkaline, metal-poor hydrothermal fluids are generated in CO2-rich systems, CO2-rich seafloor hydrothermal systems may have behaved as iron sinks in early oceans.