Abstract 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 provide unidirectional dissolution rates at 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 the 29Si 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), but 29Si/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

中文翻译：

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使用 Si 同位素示踪剂在接近平衡时解耦长石溶解和沉淀速率：对模拟硅酸盐风化的影响

摘要 在这里，我们展示了用于动力学研究的同位素示踪实验方法，借助多接收器电感耦合等离子体质谱 (MC-ICP-MS) 分析非传统稳定同位素的最新进展和可访问性，可以提供单向溶解速率在接近平衡的条件下。长期以来，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的，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 29Si 同位素示踪剂在接近平衡条件下在 50 °C 的近中性 pH 溶液中进行钠长石和钾长石溶解实验。结果表明，钠长石和钾长石溶液的饱和指数 (SI) 在约 240-360 小时 (h) 后接近零，但实验溶液的 29Si/28Si 比率表明另外 720-1440 小时的持续溶解。总硅沉淀速率远小于硅溶解速率。实验溶液对于无定形 Al(OH)3、三水铝石、石英、水铝石、伊莫缨石和高岭石是过饱和的。对于这些相，溶液的 SI 保持不变，而 Al 浓度略有下降，Si 浓度略有增加，表明耦合长石溶解和第二相沉淀，例如钠长石→无定形Al(OH)3+石英或钠长石→溶液+Al-Si相，而不是明显的钠长石和钾长石沉淀（逆反应）在 50°C。Si、Al、Na 和 pH 的时间演变的反应路径模型显示钠长石溶解（没有显着的逆反应）加上 Si:Al 比为 ~2:1 的第二相的沉淀可以成功地匹配实验数据。鉴于低温系统中的长石沉淀反应可以忽略不计（例如，T Na 和 pH 值表明钠长石溶解（没有显着的逆反应）加上 Si:Al 比为 2:1 的第二相的沉淀可以成功地匹配实验数据。鉴于低温系统中的长石沉淀反应可以忽略不计（例如，T Na 和 pH 值表明钠长石溶解（没有明显的逆反应）加上 Si:Al 比为 2:1 的第二相的沉淀可以成功地匹配实验数据。鉴于低温系统中的长石沉淀反应可以忽略不计（例如，T