No abstract

In the Central Volcanic Zone of the Andean cordillera (CVZ), thickening of the continental crust 30 Ma ago led to the production of increasingly differentiated magmas. Better constrain on the interactions between ascending magmas and the thick continental crust are needed to understand the contamination processes and their implications on the geochemical diversity of arc magmas. Lascar volcano figures among the most active Quaternary volcano of the CVZ and yet geochemical studies on its volcanic products remain scarce. We lead major-, trace elements and Pb-Sr isotopes measurements on 10 Lascar samples representative of the different petrologic types (andesitic to dacitic) in order to better characterize the Pb-Sr isotopic signature and diversity of Lascar magmas yet defined based on only 8 samples.

Formation of secondary sulphate 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 (X VI O4)2 with M I = NH4 + , Na + , K + ; M III = Al 3+ , Fe 3+ and X VI = S 6+) in the 330°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 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 g/g) and to a lesser extent Mo (hundreds of g/g). The uptake of gaseous Tl, V and Mo is not due to saturation of the gas with Tl, V and Mo compounds but to the possibility for the elements to enter the crystal structure of alum: Tl + , V 3+ and Mo 6+ might substitute for the M I , M III and X VI ions, respectively. The process of Te uptake remains uncertain, but must be related to the incorporation of Tl and V with which Te tightly correlates. Thallium, V and Te concentrations increase by a factor > 20 from core to rim, where they reach 8760, 8508, and 1687 g/g respectively. This trend correlates with the decrease of crust growth rate. This indicates that the low rate of rock cations supply to the outer edge of the crust favours the substitution of Tl, V and Te in the crystal network. Such surface enrichment does not occur for Mo, because Mo competes with S, another element from the gas. This suggests that the surface of mature alum crust has a high adsorption capacity for those gaseous metals able to compensate for the lack of rockderived 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10 7), V (1.110 7) and Te (0.6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.