Abstract

Formation of secondary sulfate minerals during the reaction between volcanic gases and rocks modulates the composition and flux of gaseous emanations. We report on the sub-surface formation of anhydrous alum (M-I M-III ((XO4)-O-VI)(2) with M-I = -Na-4(+), Na+, K+; M-III = Al3+, Fe3+ and X-VI = S6+, Mo6+) in the 330 degrees C fumaroles of the Lascar volcano (Chile). The alum occurs as a few millimetres thick crust that grew internally by two-way diffusion of reaction gases and diffusive influx of rock cations within the crust. The average growth rate is estimated at ca. 0.3 mu m/day, based on the 19-year-long activity of the degassing fracture hosting the crust. The growth rate is controlled by the slow migration of the rock cations and decreases towards crust rim. The crust selectively concentrates Tl, V and Te (thousands of mu g/g) and to a lesser extent Mo (hundreds of mu g/g). The uptake of gaseous Tl, V and Mo is due to the possibility for these elements to enter the M-I, M-III and X-VI sites of alum, respectively. The process of Te uptake must be related to the incorporation of Tl and V with which Te tightly correlates. Extensive substitution of Tl, V and Te occurs at the surface of the crust where the supply of rock cations is the lowest. Such surface enrichment does not occur for Mo, because Mo substitutes for S, another element from the gas. These findings suggest that the surface of mature alum crust has a high adsorption capacity for those gaseous metals able to compensate for the lack of rock-derived cations. Based on the composition of gases escaping from the fracture hosting the crust, it is estimated that the partition coefficients of Tl (3.3 x 10(7)), V (1.1 x 10(7)) and Te (0.6 x 10(7)) between crust surface and gases are two to four orders of magnitude higher than for other volatile metals and metalloids. It follows that gases equilibrating with anhydrous alums lose between 77 and 95% of their initial Tl content, but less than 1% of Pb. Given the Tl emission rate of Lascar volcano (5 g/day), between 17 and 104 g of toxic Tl would deposit every day if all Lascar gases were to equilibrate with anhydrous alums. This study suggests that anhydrous alums significantly immobilize Tl, V and Te in the ground of quiescent volcanoes, reducing the atmospheric emissions of these three elements. (C) 2020 Elsevier Ltd. All rights reserved.