Degassing of CO 2 and H 2O in submarine lavas from the Society hotspot

Abstract

We analyzed CO 2– δ 13C–H 2O– δD in vesicles of a suite of 23 vesicular submarine volcanic glasses (4 alkalic basalts and 19 trachytes) from Teahitia, Volcano 4 and Rocard seamounts and Mehetia Island in the active area of the Society hotspot (French Polynesia, South Pacific). Vesicle C and H 2O concentrations ([C vesicle] and [H 2O vesicle]) vary in the range 1–1487 ppm C and ∼0–5431 ppm H 2O, respectively. The δ 13C of CO 2 in vesicles ( δ 13C vesicle) vary from − 3.8‰ to − 9.7‰ and the δD of H 2O in vesicles ( δD vesicle) vary from − 32‰ to − 71‰. The δ 13C vesicles values are correlated with the vesicularity, Log([C vesicle]) and the δD vesicle values. The Log([H 2O vesicle]) is also correlated to the δD vesicle values and Log([C vesicle]). These observations show that late-stage open-system degassing, following a first stage of closed-system degassing, is the chief process controlling the concentration and isotopic ratio variations observed in the vesicles. The highest measured δ 13C vesicle values fall in a typical mantle range (− 4.0 ± 0.5‰). Taking into account the effect of vesicle bursting and crystal fractionation, the reconstructed pre-eruptive carbon and water concentrations in the alkalic basalt parental magma are ∼3810 ppm C and 14,360 ppm H 2O (1.43 wt.%). These concentrations correspond to carbon and water fluxes for the Society hotspot during the last 900 kyr of 2.57 × 10 4 tons C yr − 1 and 97 × 10 3 tons H 2O yr − 1 and volatile concentrations of the source of 190 ppm C and 720 ppm H 2O. These values are similar in magnitude to previous estimates for other hotspots. Our study emphasizes that H 2O degassing must be taken into account before discussing H 2O content and δD value variations in the lavas in terms of source variation in the case of the Society hotspot.