Decoupling feldspar dissolution and precipitation rates at near-equilibrium with Si isotope tracers: Implications for modeling silicate weathering
Decoupling feldspar dissolution and precipitation rates at near-equilibrium with Si isotope tracers: Implications for modeling silicate weathering
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用硅同位素示踪剂解耦近平衡状态下的长石溶解和沉淀速率：对模拟硅酸盐风化的影响
DOI：
10.1016/j.gca.2019.12.024
复制
发表时间：
2020
影响因子：
5
通讯作者：
Yuan, Honglin
中科院分区：
文献类型：
--
作者：
Zhu, Chen;Donald Rimstidt, J.;Zhang, Yilun;Kang, Jinting;Schott, Jacques;Yuan, Honglin
在这里，我们表明，同位素示踪动力学研究的实验方法，辅助的最新进展和多接收器电感耦合等离子体质谱（MC-ICP-MS）分析非传统的稳定同位素的可访问性，可以提供单向溶解率在近平衡条件。在很长一段时间内，在接近平衡时唯一可用的速率是净反应速率-溶解速率减去沉淀速率。这是因为动力学研究的传统实验方法是基于元素浓度，只能提供净速率。单向速率的可用性使我们能够重新审视地球化学中模拟风化的一些基本概念和实践。在这项研究中，我们使用29 Si同位素示踪剂在50 °C的近中性pH溶液中进行了近平衡条件下的钠长石和钾长石溶解实验。结果表明，钠长石和钾长石在240-360 h（h）后，溶液的饱和指数（SI）接近于零，而实验溶液的29 Si/28 Si比值表明，钠长石和钾长石在720-1440 h（h）后仍在继续溶解。总Si析出速率远小于Si溶解速率。实验溶液相对于无定形Al（OH）3，三水铝石，石英，水铝英石，伊毛沸石，高岭石过饱和。相对于这些相，溶液的SI保持恒定，而Al浓度略微降低，Si浓度略微增加，表明在50 °C下，长石溶解和第二相沉淀耦合，例如钠长石→无定形Al（OH）3+石英或钠长石→溶液+ Al-Si相，而不是显著的钠长石和钾长石沉淀（逆反应）。Si、Al、Na和pH的时间演变的反应路径建模揭示，钠长石溶解（没有显著的反向反应）与Si：Al比为1.02：1的第二相的沉淀耦合可以成功地匹配实验数据。T< 100 °C），我们建议在近平衡、长石-欠饱和系统中使用单向正演速率和次级相沉淀速率来模拟长石风化。这可以通过对地球化学建模软件或输入文件进行微小修改来实现。长石溶解与第二相沉淀的耦合作用使体系处于近平衡稳定状态。使用基于亲和力的速率方程，如经典的线性过渡态理论速率定律或Burch经验关系与远离平衡的速率数据一起，将预测显着的长石沉淀在不饱和的解决方案，但接近平衡的长石，这是不可能在近环境温度。
Here we show that the isotope tracer experimental method for kinetic studies, aided by the recent advance and accessibility of multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) analysis for non-traditional stable isotopes, can provideunidirectional dissolution ratesat near-equilibrium conditions. For a long time, the only rates available at near-equilibrium were net reaction rates—dissolution rates minus precipitation rates. This is because the conventional experimental method of kinetic studies is based on element concentrations and can only provide net rates. The availability of unidirectional rates allows us to re-examine some fundamental concepts and practices of modeling weathering in geochemistry.In this study, we used the29Si isotope tracer to conduct albite and K-feldspar dissolution experiments at near-equilibrium conditions in near-neutral pH solutions at 50 °C. Results show that the saturation indices (SI) of solutions approached zero with respect to albite and K-feldspar after ∼240–360 h (h), but29Si/28Si ratios of the experimental solutions indicated continual dissolution for another 720–1440 h. The rates of total Si precipitation were much smaller than the rates of Si dissolution. The experimental solutions were supersaturated with respect to amorphous Al(OH)3, gibbsite, quartz, allophane, imogolite, and kaolinite. The SI of the solutions remained constant with respect to these phases while Al concentrations slightly decreased and Si concentrations slightly increased, indicating the coupled feldspar dissolution and precipitation of secondary phases, such as albite → amorphous Al(OH)3+ quartz or albite → solution + Al-Si phase(s), instead of significant albite and K-feldspar precipitation (the reverse reaction) at 50 °C. Reaction path modeling of the temporal evolution of Si, Al, Na, and pH revealed that albite dissolution (without significant backward reaction) coupled with the precipitation of a secondary phase with a Si:Al ratio of ∼2:1 can successfully match the experimental data.Given the negligible feldspar precipitation reactions in low-temperature systems (e.g.,T< 100 °C), we recommend modeling feldspar weathering using unidirectional forward rates together with secondary phase precipitation rates in near-equilibrium, feldspar-undersaturated systems. This can be accomplished with minor modifications in geochemical modeling software or input files. The coupled feldspar dissolution with secondary phase precipitation arrests the system in a near-equilibrium steady state. Using affinity-based rate equations such the classical linear Transition State Theory rate law or the Burch empirical relation together with far-from-equilibrium rate data will predict significant feldspar precipitation in solutions undersaturated but close to equilibrium with respect to feldspars, which is unlikely at near ambient temperatures.
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地球化学速率模型：地球化学动力学简介
DOI：
--
发表时间：
2013
期刊：
影响因子：
--
作者：
J. Rimstidt
通讯作者：
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影响因子：
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作者：
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通讯作者：
Rimstidt, J. Donald
影响因子：
5
作者：
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通讯作者：
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影响因子：
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Tournassat, Christophe
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DOI：
--
发表时间：
2007
期刊：
影响因子：
--
作者：
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通讯作者：
Chen Zhu