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

Abstract

We report on 91 analyses of CO 2 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 CO 2 in the glasses range from 30 to 1014 ppm and always exceed the experimentally determined pressure-dependent equilibrium CO 2 solubility curve for tholeiitic basalts at 1200°C determined by Stolper and Holloway [1]. The measured CO 2 content mostly represents oversaturation of CO 2 in the magma and, to a lesser extent, CO 2 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. The C/ 3 He ratio ranges from 1.4 × 10 9 to 4.3 × 10 9, as compared to a mean of 2 ± 1 × 10 9 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 H 2O 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 CO 2 outgassing and the percentage fusion along the ridge are proportional to crustal thickness or excess ridge elevation.