Electron microprobe analyses of diffusion profiles in basaltic melt during anhydrite dissolution

Electron microprobe analyses of diffusion profiles in basaltic melt during anhydrite dissolution. The experiments and analyses were performed by Raven Polk and form the basis for her M.Sc. thesis at McGill. Abstract from the thesis: \"We measured the diffusion of sulfur released from anhydrite during dissolution into a basaltic melt to quantify the rates of sulfur contamination. The diffusion experiments were used to measure anhydrite saturation to provide insight into the interaction between mafic melts and evaporitic units during the emplacement of the Siberian Traps. Diffusion experiments were performed using a dike composition from the Central Atlantic Magmatic Province and a mixture of compressed anhydrite powder. Experimental conditions range from 1250-1450 °C at 1 GPa and durations ranging from 600 to 3600 seconds. Diffusion values vary from 5.1 x 10-12 m2/s to 3.0 x 10-11 m2/s and results were used to create an Arrhenius equation below for the diffusion of sulfur within a tholeiitic basalt:D=6.21 x 10^(-6) exp⁡((-173.6±84.0)/RT). The sulfur peak position from wavelength-dispersive spectroscopy (WDS) indicates that the dominant sulfur species in the melt is sulfate; the fraction of sulfur as sulfate in the melt S6+/Stotal, is ~ 0.60. Anhydrite solubility, the sulfur concentration at anhydrite saturation (SCAS), was determined from the measured sulfur concentrations in the melt and extrapolated along the diffusion profile to the anhydrite/melt interface. The SCAS ranges from 0.69 wt.% at 1450 °C to 1.28 wt.% at 1250 °C. There is no strong dependence of the SCAS on temperature.These diffusivity measurements are applied to a model that attempts to predict the rates of volatile uptake within a basaltic magmatic system; this model provides insight into sulfur storage and transport. The modeling results provide estimations of the amount of sulfur that can be transported in the melt and possibly released into the atmosphere and can help to determine the extent of interaction between the evaporites and magma.\"硬石膏溶解过程中玄武质熔体扩散剖面的电子探针分析。本实验与分析由雷文·波尔克（Raven Polk）完成，为其在麦吉尔大学（McGill）的硕士学位论文奠定了核心基础。该论文的摘要如下：“我们测定了硬石膏溶解过程中释放的硫在玄武质熔体中的扩散行为，以量化硫污染的速率。本扩散实验用于测定硬石膏饱和度，以探究西伯利亚暗色岩省（Siberian Traps）就位过程中，镁铁质熔体与蒸发岩地层之间的相互作用机制。本扩散实验采用来自中大西洋岩浆省（Central Atlantic Magmatic Province）的岩脉成分，以及压实硬石膏粉末混合物开展。实验条件为压力1 GPa，温度区间1250~1450 ℃，实验时长600~3600 s。扩散系数取值范围为5.1×10^-12 m²/s至3.0×10^-11 m²/s，研究人员基于实验结果构建了适用于拉斑玄武岩中硫扩散的阿伦尼乌斯（Arrhenius）方程：D=6.21×10^(-6) exp[(-173.6±84.0)/(RT)]。通过波长色散光谱（wavelength-dispersive spectroscopy, WDS）得到的硫峰位信息表明，熔体中主要的硫物种为硫酸盐；熔体中以硫酸盐形式存在的硫占总硫的比例S⁶⁺/S_total约为0.60。硬石膏溶解度，即硬石膏饱和时的硫浓度（sulfur concentration at anhydrite saturation, SCAS），通过实测熔体中的硫浓度，并沿扩散剖面外推至硬石膏/熔体界面处获得。SCAS取值范围为1450 ℃时的0.69 wt.%至1250 ℃时的1.28 wt.%，且SCAS与温度并无显著相关性。本研究将扩散系数测量结果应用于旨在预测玄武质岩浆系统内挥发性组分吸收速率的模型，该模型有助于解析硫的储存与运移机制。模拟结果可估算熔体中可运移并可能释放至大气中的硫总量，同时有助于厘清蒸发岩与岩浆之间的相互作用程度。”