Experimental Simulation of Closed-System Degassing in the System Basalt-H2O-CO2-S-Cl
Experimental Simulation of Closed-System Degassing in the System Basalt-H2O-CO2-S-Cl
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DOI：
10.1093/petrology/egr027
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发表时间：
2011-09-01
影响因子：
3.9
通讯作者：
Behrens, Harald
中科院分区：
文献类型：
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作者：
Lesne, Priscille;Kohn, Simon C.;Behrens, Harald
岩浆脱气过程通常通过研究喷发斑晶中的熔融包裹体和测量火山口的气体排放来阐明，并与挥发性溶解度的实验约束模型相关联。在这里，我们开发了一种替代的实验方法，旨在直接模拟减压驱动的，封闭系统的玄武岩浆在氧化条件下（f（O2）的1中心点0-2中心点4日志单位以上的Ni-NiO缓冲）与H-C-O-S-Cl流体平衡脱气。合成实验起始材料基于Stromboli（意大利）和马萨亚（尼加拉瓜）持续脱气火山喷发的玄武岩浆，初始挥发性库存与每个火山的最未脱气熔体包裹体相匹配。实验在25-400 MPa下在超液相线条件（1150 ℃）下进行。用电子探针、二次离子质谱、傅里叶变换红外光谱、卡尔-费休滴定、Fe ~（2+）/Fe ~（3+）比色法和CS分析仪对成品玻璃和原料进行了分析。通过质量平衡测定每次运行中的出溶蒸气的组成。结果表明，H2O/CO2比值随压力的降低而增大，而CO2/S比值在100-300 MPa时达到最大值。在低压下，S优先于Cl释放，导致蒸汽S/Cl比急剧增加，玻璃的S/Cl比急剧下降。这与已发表的斯特龙博利（和埃特纳）熔体包裹体和基质玻璃中挥发物浓度的测量结果一致。不同S丰度的实验表明，在流体饱和熔体的H2O和CO2的含量不受影响。使用两组起始材料的实验中的CO2溶解度与使用公布的模型计算的溶解度很好地匹配。模型一致高估了水的溶解度的Stromboli样的组合物，导致计算的蒸汽组合物，更丰富的CO2和计算的脱气轨迹，更强烈的弯曲比在实验中观察到的。对于Masaya类成分来说，这种差异不那么严重，强调了溶解度和熔体-蒸汽分配的重要成分依赖性。我们新的实验方法可以很容易地扩展到其他散装组合物。
Magma degassing processes are commonly elucidated by studies of melt inclusions in erupted phenocrysts and measurements of gas discharge at volcanic vents, allied to experimentally constrained models of volatile solubility. Here we develop an alternative experimental approach aimed at directly simulating decompression-driven, closed-system degassing of basaltic magma in equilibrium with an H-C-O-S-Cl fluid under oxidized conditions (f(O2) of 1 center dot 0-2 center dot 4 log units above the Ni-NiO buffer). Synthetic experimental starting materials were based on basaltic magmas erupted at the persistently degassing volcanoes of Stromboli (Italy) and Masaya (Nicaragua) with an initial volatile inventory matched to the most undegassed melt inclusions from each volcano. Experiments were run at 25-400 MPa under super-liquidus conditions (1150 degrees C). Run product glasses and starting materials were analysed by electron microprobe, secondary ion mass spectrometry, Fourier transform infrared spectroscopy, Karl-Fischer titration, Fe2+/Fe3+ colorimetry and CS analyser. The composition of the exsolved vapour in each run was determined by mass balance. Our results show that H2O/CO2 ratios increase systematically with decreasing pressure, whereas CO2/S ratios attain a maximum at pressures of 100-300 MPa. S is preferentially released over Cl at low pressures, leading to a sharp increase in vapour S/Cl ratios and a sharp drop in the S/Cl ratios of glasses. This accords with published measurements of volatile concentrations in melt inclusion and groundmass glasses at Stromboli (and Etna). Experiments with different S abundances show that the H2O and CO2 contents of the melt at fluid saturation are not affected. The CO2 solubility in experiments using both sets of starting materials is well matched to calculated solubilities using published models. Models consistently overestimate H2O solubilities for the Stromboli-like composition, leading to calculated vapour compositions that are more CO2-rich and calculated degassing trajectories that are more strongly curved than observed in experiments. The difference is less acute for the Masaya-like composition, emphasizing the important compositional dependence of solubility and melt-vapour partitioning. Our novel experimental method can be readily extended to other bulk compositions.