Abstract We have performed experiments at 0.5-2GPa and 1200-1500°C to investigate the dissolution behaviour of chlorine in silicate melts. The experiments were performed with chlorine fugacities controlled by mixtures of Ag, AgCl and AgI and oxygen fugacity buffered at C-CO-CO2 (CCO) and Re-ReO2. The results demonstrate that the initial chlorine dissolution mechanism involves the replacement of O2- in the silicate melt by two dissociated Cl- ions according to the reaction: Cl2 + [O2-]melt = 2[Cl-]melt+ 0.5O2 The same dissolution mechanism applies to hydrous, fluid-saturated basalt at 100-200 MPa/1050°C. Experiments using both an Fe-free haplobasaltic composition (An50Di28Fo22) and an Icelandic basalt followed the predicted dependence of Cl concentration on f(Cl2)0.5 and f(O2)0.25. This Henrian behaviour extends from 0 to at least 2.6 wt% Cl dissolved in the haplobasaltic composition, 1.6 wt% Cl in anhydrous basalt and ∼1.5 wt% Cl in fluid-saturated basalt. Deviations from Henry’s Law behaviour at higher concentrations are consistent with progressive association of Cl- ions. In the Henry’s Law region Cl concentration in the An50Di28Fo22 composition is given by (wt%): log Cl melt = 1.206 32 - 940 40 P T - 0.25 l o g f O 2 + 0.5 l o g f Cl 2 P is in GPa, T in kelvin, values in brackets are 1 standard error, and f(Cl2) and f(O2) refer to standard states of pure gas at 0.1MPa and the temperature of interest. For the natural anhydrous basalt we obtain: log Cl melt = 0.984 64 - 930 70 P T - 0.25 l o g f O 2 + 0.5 l o g f Cl 2 By considering the P-T dependences of the Cl contents of melts we find that the concentrations observed in nature are extremely stable in basalt to very low pressures. Basalts containing the typical concentration range of 0.05-0.5 wt% Cl should, for example, only begin to degas their chlorine significantly, as HCl, at pressures in the range 0-5MPa. Data on hydrous, fluid-saturated basalt at 100-200 MPa are when corrected for dissolution of Ca, Na and K in the fluid, broadly consistent with our results for anhydrous basalt. Finally, we use recently evaluated thermodynamic data for sodalite (Na4Al3Si3O12Cl) to calculate the conditions under which this phase would stabilise in trachytes and phonolites. We find that the appearance of sodalite as a liquidus phase reflects a combination of low liquidus temperature and high Na2O activity rather than unusually high chlorine fugacity.

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硅酸盐熔体中氯的化学行为

摘要 我们在 0.5-2GPa 和 1200-1500°C 下进行了实验，以研究氯在硅酸盐熔体中的溶解行为。这些实验是用由 Ag、AgCl 和 AgI 的混合物控制的氯逸度以及缓冲在 C-CO-CO2 (CCO) 和 Re-ReO2 的氧逸度进行的。结果表明，最初的氯溶解机制涉及根据反应由两个离解的 Cl- 离子置换硅酸盐熔体中的 O2-： Cl2 + [O2-]melt = 2[Cl-]melt+ 0.5O2 相同的溶解机制适用于 100-200 MPa/1050°C 的含水、流体饱和玄武岩。使用无铁单质玄武岩成分 (An50Di28Fo22) 和冰岛玄武岩的实验遵循预测的 Cl 浓度对 f(Cl2)0.5 和 f(O2)0.25 的依赖性。这种亨利行为从 0 扩展到至少 2。6 wt% Cl 溶解在单基玄武岩组合物中，1.6 wt% Cl 在无水玄武岩中，约 1.5 wt% Cl 在流体饱和玄武岩中。较高浓度下亨利定律行为的偏差与 Cl- 离子的渐进结合一致。在亨利定律区域中，An50Di28Fo22 组合物中的 Cl 浓度由 (wt%) 给出： log Cl 熔体 = 1.206 32 - 940 40 PT - 0.25 logf O 2 + 0.5 logf Cl 2 P 单位为 GPa，T 单位为开尔文括号内为 1 个标准误差，f(Cl2) 和 f(O2) 是指纯气体在 0.1MPa 和目标温度下的标准状态。对于天然无水玄武岩，我们得到：log Clmelt = 0.984 64 - 930 70 PT - 0.25 logf O 2 + 0。5 logf Cl 2 通过考虑熔体中 Cl 含量的 PT 依赖性，我们发现在自然界中观察到的浓度在玄武岩中对非常低的压力非常稳定。例如，含有 0.05-0.5 wt% Cl 的典型浓度范围的玄武岩应仅在 0-5MPa 的压力范围内开始显着地将其氯气脱气，如 HCl。100-200 MPa 下含水、流体饱和玄武岩的数据是针对流体中 Ca、Na 和 K 的溶解进行校正的，与我们对无水玄武岩的结果大体一致。最后，我们使用最近评估的方钠石 (Na4Al3Si3O12Cl) 热力学数据来计算该相在粗面岩和响岩中稳定的条件。