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.\"