STRONG DEGASSING AT RIDGE CRESTS - THE BEHAVIOR OF DISSOLVED CARBON AND WATER IN BASALT GLASSES AT 14-DEGREES-N, MID-ATLANTIC RIDGE

The popping rock 2piD43 dredged at 14-degrees-N on the Mid-Atlantic Ri dge contains the largest gas content so far reported in MORB [11], wit h most of the gas preserved in vesicles (17 vol%). This gas has chemic al (CO2/H2O, C/He, C/N, He/Ar) and isotopic (deltaC-13) characteristic s that are remarkably constant throughout the dredge. We have taken th is opportunity to study the behaviour and variability of dissolved spe cies (carbon and water) against such a well-defined fluid. Sixteen new crushing experiments, carried out in steps and on much smaller (38-17 0 mg) fragments than in our previous study [11], have been conducted t o investigate small-scale variations in vesicle carbon composition. Th e results confirm the overall constancy of deltaC-13(-3.62 +/- 0.05 pa rts per thousand vs. -3.68 +/- 0.07 parts per thousand for the large f ragments). However, variations of 0.5-3.5 parts per thousand occur in about one-third of the aliquots analyzed, reflecting outgassing effect s on the smaller vesicles and a slight chemical evolution. DeltaO-18 v ariations from 9.29 to 20.89 parts per thousand are associated with th e deltaC-13 variations, and correspond to variations in H2O concentrat ions in vesicles ranging from approximately 5 +/- 2 vol% for most of t he aliquots to high values of 40-85 vol% for small vesicles. The amoun t of carbon extracted below 900-degrees-C decreases with decreasing gr ain size, and its isotopic composition can be modelled as a variable m ixture of CO2 from small vesicles, precipitated carbonate and amorphou s carbon deposits. On samples crushed down to complete exhaustion of v esicle CO2, carbon deposits on vesicles walls and cracks have been sho wn to be about 40 ppm C (approximately 1% of vesicle CO2) at deltaC-13 = - 16 +/- 4 parts per thousand. The carbon recovered on fusion, whic h corresponds to carbon dissolved in the glass, is constant within a g iven batch of samples but varies from 57 to 94 ppm C from batch to bat ch, with the corresponding deltaC-13 values varying from -5.6 to -11.8 parts per thousand. Water contents vary from 4160 to 5300 ppm, with a deltaD of -64 +/- 2 parts per thousand, except for an atypical, vesic le-poor sample (2620 ppm). The C-13 fractionations between gas and dis solved carbon vary widely, between 2 and 8 parts per thousand, with mo st of these values occurring in samples of constant vesicle deltaC-13 (ca. -3.7 parts per thousand). Dissolved carbon and water concentratio ns vary from approximate equilibrium solubility at the seafloor up to 100% and 30% supersaturation respectively. Given their respective solu bility laws, this corresponds to similar saturation depths of 0.6-1.3 km below the seafloor. C-13 fractionations between vesicle CO2 and dis solved carbon record disequilibrium outgassing features of deeper orig in during the eruptive process of the magma, from approximately 2 to 4 km below the seafloor, which is most likely the depth of the magma ch amber. These outgassing distillation effects lead to variable levels o f supersaturation and isotopic disequilibrium that depend on the style (i.e., ascent rate) of eruption, until, at the very end of the proces s, kinetic isotope effects eventually partially reverse the isotopic t rend. These results suggest that, in most occurrences, submarine basal t glasses display the characteristics of magma chambers outgassed to m ore than 90% (i.e., 100-200 ppm total (vesicle + dissolved) carbon), w ith deltaC-13 lowered from an initial value of ca. - 4 parts per thous and to between - 6 and - 8 parts per thousand.