Carbon in Mid-Atlantic Ridge basalt glasses from 28°N to 63°N: Evidence for a carbon-enriched Azores mantle plume

We report on 91 analyses of CO2 in fresh, ‘macrovesicle-free’ Mid-Atlantic Ridge (MAR) basalt glasses from 48 dredge stations located between 28°N and Iceland. A stepped heating gas chromatographic technique was used for the analyses. Concentrations of carbon as CO2 in the glasses range from 30 to 1014 ppm and always exceed the experimentally determined pressure-dependent equilibrium CO2 solubility curve for tholeiitic basalts at 1200°C determined by Stolper and Holloway [1]. The measured CO2 content mostly represents oversaturation of CO2 in the magma and, to a lesser extent, CO2 trapped in microvesicles during the latest stage of magma emplacement and quenching on the seafloor. The degree of ‘oversaturation’ varies from 1 to 9 times the equilibrium solubility values at any given depth. It is highest over the deeper part of the MAR profile, along the so-called normal ridge segments, and lowest close to Iceland and over the center of the Azores platform, where the oceanic crust is thicker and the mantle hotter than usual. 3 He ratio ranges from 1.4 × 109 to 4.3 × 109, as compared to a mean of 2 ± 1 × 109 previously reported for normal MOR segments. The C/3 ratio in MAR glasses over the Azores province correlates positively with 87Sr/86Sr ratio and reaches a maximum over the Azores platform. Based on this correlation, binary mixing modeling indicates minimum enrichments of carbon in the Azores plume ranging from 3.3 to 4.6, using 87Sr/86Sr ratios, and 4.9-;8.8, using 143Nd/144Nd ratios, depending on the enrichment of Sr and Nd estimated in the plume relative to the depleted asthenosphere. For comparison, the H2O content in the Azores plume is enriched by a factor of 2–4. The carbon enrichment over the Azores platform (plume) is in agreement with an independent predictive model which assumes that the likelihood of CO2 outgassing and the percentage fusion along the ridge are proportional to crustal thickness or excess ridge elevation.