Degassing of CO2 and H2O in submarine lavas from the Society hotspot

We analyzed CO2-delta C-13-(HO)-O-2-delta D 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 H2O concentrations ([C-vesicle] and [H2Ovesicle) vary in the range 1-1487 ppm C and similar to 0-5431 ppm H2O, respectively. The delta C-13 of CO2 in vesicles (delta C-13(vesicle)) vary from -3.8 parts per thousand to -9.7 parts per thousand and the delta D of H2O in vesicles (delta D-vesicle) vary from -32 parts per thousand to -71 parts per thousand. The delta C-13(vesicles) values are correlated with the vesicularity, Log([C-vesicle]) and the delta D-vesicle values. The Log([H2Ovesicle) is also correlated to the delta D-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 delta C-13(vesicle) values fall in a typical mantle range (-4.0 +/- 0.5 parts per thousand). 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 similar to 3810 ppm C and 14,360 ppm H2O (1.43 wt.%). These concentrations correspond to carbon and water fluxes for the Society hotspot during the last 900 kyr of 2.57 x 10(4) tons C yr(-1) and 97 x 10(3) tons H2O yr(-1) and volatile concentrations of the source of 190 ppm C and 720 ppm H2O. These values are similar in magnitude to previous estimates for other hotspots. Our study emphasizes that H2O degassing must be taken into account before discussing H2O content and delta D value variations in the lavas in terms of source variation in the case of the Society hotspot. (c) 2005 Elsevier B.V. All rights reserved.