俯冲带条件下菱镁矿在水和NaCl溶液中的溶解度和碳形态的原位测定

IF 2 4区地球科学 Q3 GEOSCIENCES, MULTIDISCIPLINARY

Solid Earth Sciences Pub Date : 2022-09-15 DOI:10.1016/j.sesci.2022.06.002

Wan-Cai Li , Qinxia Wang

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引用次数: 2

摘要

碳酸盐岩在俯冲带流体中的溶蚀行为尚未得到很好的约束。在这项研究中，我们研究了菱镁矿在纯水和NaCl溶液(高达19 wt% NaCl)中的溶解度，并通过拉曼光谱测定了流体中的碳形态。在0.7 ~ 2.4 GPa和635 ~ 940℃条件下，菱镁矿在纯水中的溶解度在0.01 ~ 0.05 mol/kg范围内，温度对其溶解度有明显的增强作用。对实验数据进行最小二乘拟合得到菱镁矿在纯水中的溶解度经验表达式为:CMgCO3=(964510±41362)exp(−6953±690T)exp[0.0200±0.0098T(P−1)]，其中CMgCO3的单位为μg/g, T为温度，单位为K, P为压力，单位为bar。在0.7 ~ 2.4 GPa、640 ~ 940℃下，MgCO3的溶解度比CaCO3低一个数量级。NaCl对菱镁矿溶解度的增强因子(m/m°，m和m°分别为菱镁矿在NaCl溶液和水中的溶解度)在NaCl摩尔分数0 ~ 0.07范围内呈抛物线趋势，在XNaCl = 0.035时最大放大5.2，这与以往研究中在高盐度条件下溶解度随盐度的增加而不断增加不同，说明菱镁矿在稀释NaCl溶液中的溶解反应不同。在200 ~ 800℃和1.9 ~ 3.8 GPa范围内，HCO3-的含量高于CO32 -;在400 ~ 600℃范围内，HCO3-的含量受温度的影响，但不受压力的影响。我们的实验数据表明，在没有熔融的情况下，只有少量的菱镁矿可以被亚弧深度的板状释放流体所动员。

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In situ determination of magnesite solubility and carbon speciation in water and NaCl solutions under subduction zone conditions

The dissolution behavior of carbonates in subduction zone fluids has not been well constrained. In this study we investigated the solubility of magnesite in pure water and NaCl solutions (with up to 19 wt% NaCl) in situ with a hydrothermal diamond anvil cell, and determined carbon speciation in the fluid by Raman spectroscopy. The solubility of magnesite in pure water falls in the range of 0.01–0.05 mol/kg at 0.7–2.4 GPa and 635–940 °C, enhanced strongly by temperature. Least squares fitting of experimental data leads to the following empirical expression for magnesite solubility in pure water: $C_{{MgCO}_{3}} = (964510 \pm 41362) \exp (\frac{- 6953 \pm 690}{T}) \exp [\frac{0.0200 \pm 0.0098}{T} (P - 1)]$ where $C_{{MgCO}_{3}}$ is given in μg/g, T is the temperature in K, and P is the pressure in bar, respectively. The solubility of MgCO₃ is about an order of magnitude lower than that of CaCO₃ at 0.7–2.4 GPa, 640–940 °C. The solubility enhancement factor by NaCl (m/m° with m and m° being magnesite solubility in NaCl solution and water, respectively) presents as a parabolic trend with the mole fraction of NaCl in the range of 0–0.07, with a maximum amplification of 5.2 at X_NaCl = 0.035, which is different from the continuously increase of solubility with salinity increasing at high salinity conditions in previous studies and suggests the dissolution reaction of magnesite in dilute NaCl solution is different. Despite slight contamination of CH₄ formed by the reaction of the diamond anvils, we were able to identify CO₃²⁻ and HCO₃^- to be the aqueous carbon species, HCO₃^- was predominant over CO₃²⁻ in the range of 200–800 °C and 1.9–3.8 GPa and its proportion was affected by temperature, but not affected by pressure at 400–600 °C. Our experimental data suggest that in the absence of melting, only a small amount of magnesite can be mobilized by the slab-released fluid at subarc depths.

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来源期刊

Solid Earth Sciences GEOSCIENCES, MULTIDISCIPLINARY-

CiteScore

3.60

自引率

5.00%

发文量

审稿时长

103 days