Strong degassing at ridge crests: The behaviour of dissolved carbon and water in basalt glasses at 14°N, Mid-Atlantic Ridge

Abstract

The popping rock 2πD43 dredged at 14°N on the Mid-Atlantic Ridge contains the largest gas content so far reported in MORB [11], with most of the gas preserved in vesicles (17 vol%). This gas has chemical ( CO 2 H 2O , C He , C N , He Ar ) and isotopic ( δ 13C) characteristics that are remarkably constant throughout the dredge. We have taken this opportunity to study the behaviour and variability of dissolved species (carbon and water) against such a well-defined fluid. Sixteen new crushing experiments, carried out in steps and on much smaller (38–170 mg) fragments than in our previous study [11], have been conducted to investigate small-scale variations in vesicle carbon composition. The results confirm the overall constancy of δ 13C ( −3.62 ± 0.05‰ vs. −3.68 ± 0.07‰ for the large fragments). However, variations of 0.5–3.5‰ occur in about one-third of the aliquots analyzed, reflecting outgassing effects on the smaller vesicles and a slight chemical evolution. δ 18O variations from 9.29 to 20.89‰ are associated with the δ 13C variations, and correspond to variations in H 2O concentrations in vesicles ranging from ∼ 5 ± 2 vol% for most of the aliquots to high values of 40–85 vol% for small vesicles. The amount of carbon extracted below 900°C decreases with decreasing grain size, and its isotopic composition can be modelled as a variable mixture of CO 2 from small vesicles, precipitated carbonate and amorphous carbon deposits. On samples crushed down to complete exhaustion of vesicle CO 2, carbon deposits on vesicles walls and cracks have been shown to be about 40 ppm C ( ∼ 1% of vesicle CO 2) at δ 13 C = −16 ± 4‰ . The carbon recovered on fusion, which corresponds to carbon dissolved in the glass, is constant within a given batch of samples but varies from 57 to 94 ppm C from batch to batch, with the corresponding δ 13C values varying from −5.6 to −11.8‰. Water contents vary from 4160 to 5300 ppm, with a δD of −64 ± 2‰, except for an atypical, vesicle-poor sample (2620 ppm). The 13C fractionations between gas and dissolved carbon vary widely, between 2 and 8‰, with most of these values occurring in samples of constant vesicle δ 13C (ca. −3.7‰). Dissolved carbon and water concentrations vary from approximate equilibrium solubility at the seafloor up to 100% and 30% supersaturation respectively. Given their respective solubility laws, this corresponds to similar saturation depths of 0.6–1.3 km below the seafloor. 13C fractionations between vesicle CO 2 and dissolved carbon record disequilibrium outgassing features of deeper origin during the eruptive process of the magma, from ∼ 2 to 4 km below the seafloor, which is most likely the depth of the magma chamber. These outgassing distillation effects lead to variable levels of supersaturation and isotopic disequilibrium that depend on the style (i.e., ascent rate) of eruption, until, at the very end of the process, kinetic isotope effects eventually partially reverse the isotopic trend. These results suggest that, in most occurrences, submarine basalt glasses display the characteristics of magma chambers outgassed to more than 90% (i.e., 100–200 ppm total (vesicle + dissolved) carbon), with δ 13C lowered from an initial value of ca. −4‰ to between −6 and −8‰.