H_2O and halogen (F, Cl, Br) behaviour during shallow magma degassing processes

摘要

Degassing paths of silicic H_2O-rich magmas during explosive Plinian and dome-forming eruptions may be represented in simple evolution diagrams relating the residual volatile content of the melt to the volume ratio of gas over melt (V_g/V_1). These parameters are measurable in erupted magma fragments. They are used for interpreting the H_2O and halogen contents determined in a series of variously degassed volcanic clasts from the 650 y. B.P. eruption at Mt. Pelee (Martinique, FWI) which is characterized by the succession of a dome extrusion and a Plinian activity. H_2O degassing of Plinian pumice clasts are modeled by an equilibrium closed system evolution which allows to calculate the partition coefficients of halogen between the aqueous fluid and the rhyolitic melt (d_i~(v-1)): 1 for F, 10 +- 1.5 for Cl and 3.7 +- 1 for Br, at around 900 deg C. The strong increase of the d_i~(v-1) values of Cl and Br during dome extrusion is explained by an open system degassing model with a simultaneous crystallization of the melt. Some significant Br enrichments relative to Cl in dome fragments are attributed to interactions with an hydrothermal fluid close to seawater in composition. The models allow calculation of Cl and Br contents of aqueous fluids generated by the different degassing paths: Cl contents are approx= 0.5 mol/l for closed system evolution and vary between 0.2 and 1 mol/l during open system evolutions. These fluids are able to extract significant amounts of chloride complexed metals and silica from the magma during dome forming eruptions. They also may generate mineralization of the wall rocks or the rising dome itself (e.g. silica precipitation) which may seal the volcanic system and be responsible for the dome explosivity and the transition from dome-forming to Plinian eruptive style. The Cl/Br ratio of initial melts of Pl eruption (approx= 250) is much lower than the basaltic and exospheric ratios (approx= 400), suggesting that strong Cl-Br fractionation may occur during the production or the differentiation of subduction related magmas.