CARBON IN MID-ATLANTIC RIDGE BASALT GLASSES FROM 28-DEGREES-N TO 63-DEGREES-N - EVIDENCE FOR A CARBON-ENRICHED AZORES MANTLE PLUME

We report on 91 analyses of CO2 in fresh, 'macrovesicle-free' Mid-Atla ntic Ridge (MAR) basalt glasses from 48 dredge stations located betwee n 28 degrees N and Iceland. A stepped heating gas chromatographic tech nique was used for the analyses. Concentrations of carbon as CO2 in th e glasses range from 30 to 1014 ppm and always exceed the experimental ly determined pressure-dependent equilibrium CO2 solubility curve for tholeiitic basalts at 1200 degrees C determined by Stolper and Hollowa y [1]. The measured CO2 content mostly represents oversaturation of CO 2 in the magma and, to a lesser extent, CO2 trapped in microvesicles d uring the latest stage of magma emplacement and quenching on the seafl oor. The degree of 'oversaturation' varies from 1 to 9 times the equil ibrium solubility values at any given depth. It is highest over the de eper part of the MAR profile, along the so-called normal ridge segment s, and lowest close to Iceland and over the center of the Azores platf orm, where the oceanic crust is thicker and the mantle hotter than usu al. The C/He-3 ratio ranges from 1.4 x 10(9) to 4.3 x 10(9), as compar ed to a mean of 2 +/- 1 x 10(9) previously reported for normal MOR seg ments. The C/He-3 ratio in MAR glasses over the Azores province correl ates positively with Sr-87/Sr-86 ratio and reaches a maximum over the Azores platform. Based on this correlation, binary mixing modeling ind icates minimum enrichments of carbon in the Azores plume ranging from 3.3 to 4.6, using Sr-87/Sr-86 ratios, and 4.9-8.8, using Nd-143/Nd-144 ratios, depending on the enrichment of Sr and Nd estimated in the plu me relative to the depleted asthenosphere. For comparison, the H2O con tent in the Azores plume is enriched by a factor of 2-4. The carbon en richment over the Azores platform (plume) is in agreement with an inde pendent predictive model which assumes that the likelihood of CO2 outg assing and the percentage fusion along the ridge are proportional to c rustal thickness or excess ridge elevation.