Deep-sea explosive activity on the Mid-Atlantic Ridge near 34°50′N: a stable isotope (C, H, O) study

The carbon and water contents and the corresponding isotopic compositions have been measured on a set of glassy samples collected by dives on the Mid-Atlantic Ridge (MAR) near 34°50′N where volcanoclastic deposits are present. The volatile phases have been extracted by crushing under vacuum and stepwise heating. 8O of the glasses shows that N-MORB are depleted in 18O (down to 5.14‰) whereas all the other lavas fall in the mantle range, 5.4–5.8‰. These data preclude strong interaction between seawater and magmas before eruption. steruptive contents of dissolved water and carbon measured on N-, T-, E-MORB and alkali basalts range from 1125 to 5253 ppm and from 20 to 119 ppm, respectively. The vesicle gas is dominated by CO2 in N- and T-MORB. Water is present as small amounts in E-MORB vesicles and represents up to 17 vol.% of the total gas in alkali-basalt vesicles. The preeruptive water and carbon concentrations of these magmas have been reconstructed. They range from 1130 to 8497 ppm and from 343 to 15677 ppm, respectively. The isotopic data demonstrate that seawater contamination is detectable only when most of the vesicles have been broken during eruption and quenching. Otherwise, all the δ13C and δD values obtained in vesicles or dissolved in glasses, fall in a typical mantle range, −4‰ to −7‰ and −50‰ to −88‰, respectively. f the lavas have degassed under conditions close to chemical and isotopic equilibrium, except enriched magmas for which kinetic processes were predominant during the last eruptive event and precluded the attainment of chemical and isotopic equilibrium. into account the effects of crystal fractionation, the primitive magmas had initial concentrations of water ranging from 1100 to 8000 ppm. Because of the important CO2 degassing (80–99.9%), a reconstruction, based on the two-steps degassing model of Pineau and Javoy [Earth. Planet. Sci. Lett. (1994) 123] was made. As the enriched character increases, carbon saturation is reached at increasing depths and pressures: between 2.6 and 6.6 kbars for N- and T-MORB, between 6 and 12.8 kbars for E-MORB and between 15 and 27.5 kbars for alkali basalts. gh water and carbon contents found in the alkali-rich lavas are likely linked to an enriched fertile mantle source beneath this ridge segment. That source provides the additional supply of incompatible elements needed to produce (1) The E-MORB by magma mixing and (2) The explosive eruption characteristics.