The effect of F and Cl on the liquidus temperature of a hydrous (~3.5–4 wt% H2O) trachytic melt (~66 wt% SiO2) at 925 to 990 °C and at 100 MPa has been experimentally investigated. We employed a novel disequilibrium approach involving diffusion couple experiments with the two-diffusion couple end-members differing solely in halogen concentrations. A shift of the liquidus temperature by ~50 °C was observed between a halogen-poor and halogen-enriched melt. Each experiment spanned the entire range of F and Cl concentrations between the two end-member compositions. We determined the halogen concentrations at the transition from crystal-bearing to crystal-free melt. These concentrations correspond to the liquidus halogen concentrations of the melt at each experimental temperature. We demonstrate that there is a limiting halogen concentration (~0.19–0.52 wt% F; ~0.07–0.24 wt% Cl), below which the melt crystallizes spherulitic clinopyroxene during heating to the run temperature. At high temperatures, upon diffusion of F and Cl into the halogen-poor melt, those crystals dissolve, leaving behind a dissolution front parallel to the diffusion interface. We propose that the dissolution is a consequence of F and Cl complexing with some of the main cationic components of clinopyroxene (Mg, Fe, Ca), thereby destabilizing this phase. Thus, the experimental dissolution of clinopyroxene is a manifestation of a liquidus depression caused by increased halogen content. Our results show that the liquidus shifts at a rate of ~1575(379) K/mol% of F and Cl in the melt, which is a minimum estimate, assuming both halogens equally drive dissolution. This liquidus depression is valid for a range of halogen concentrations (~0.06–0.87 wt% F; ~0.06–0.36 wt% Cl) and the experimental temperatures. Our findings illustrate that the degassing of halogens during or prior to an eruption can enhance crystallization in the melt and therefore influence magma physical properties that may ultimately affect eruption style.

中文翻译：

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卤素 (F, Cl) 对含水粗面石熔体近液相线结晶度的影响

已经通过实验研究了 F 和 Cl 在 925 至 990 °C 和 100 MPa 下对含水（~3.5~4 wt% H2O）粗面晶熔体（~66 wt% SiO2）液相线温度的影响。我们采用了一种新颖的不平衡方法，涉及扩散偶实验，其中两个扩散偶末端成员仅在卤素浓度上有所不同。在贫卤素熔体和富卤素熔体之间观察到液相线温度移动了约 50°C。每个实验都跨越了两个末端成员组成之间的 F 和 Cl 浓度的整个范围。我们确定了从含晶体熔体到无晶体熔体过渡时的卤素浓度。这些浓度对应于每个实验温度下熔体的液相线卤素浓度。我们证明有一个限制卤素浓度（~0.19-0。52 重量% F；~0.07–0.24 wt% Cl)，低于此值的熔体在加热至运行温度期间会结晶出球晶型单斜辉石。在高温下，当 F 和 Cl 扩散到贫卤素熔体中时，这些晶体溶解，留下平行于扩散界面的溶解前沿。我们认为溶解是 F 和 Cl 与单斜辉石的一些主要阳离子成分（Mg、Fe、Ca）络合的结果，从而使该相不稳定。因此，单斜辉石的实验溶解是由卤素含量增加引起的液相线下降的表现。我们的结果表明，假设两种卤素同样驱动溶解，熔体中 F 和 Cl 的液相线以 ~1575(379) K/mol% 的速率移动，这是一个最低估计值。这种液相线下降适用于一系列卤素浓度（~0.06-0.87 wt% F；~0.06-0.36 wt% Cl）和实验温度。我们的研究结果表明，在喷发期间或之前对卤素进行脱气可以增强熔体中的结晶，从而影响可能最终影响喷发方式的岩浆物理性质。