The effect of primary versus secondary processes on the volatile content of MORB glasses: An example from the equatorial Mid-Atlantic Ridge (5 degrees N-3 degrees S)

摘要

We report microanalysis of volatile and trace element compositions, as well as Fe3+/sigma Fe ratios, from 45 basaltic glasses from cruise RC2806 along the equatorial Mid-Atlantic Ridge. The along-strike variations in volatiles result from the complex geodynamical setting of the area, including numerous transform faults, variations in ridge depth, melting degree, and source composition. The strongest gradient is centered on 1.7 degrees N and encompasses an increase of H2O, Cl, and F contents as well as high F/Zr ratio spatially coincident with radiogenic isotope anomalies. We interpret these variations as source enrichment due to the influence of the nearby high--type Sierra Leone plume. South of the St. Paul fracture zone, H2O and F contents, as well as H2O/Ce and F/Zr ratios, decrease progressively. This gradient in volatiles is consistent with progressive dilution of an enriched component in a heterogeneous mantle due to the progressive increase in the degree of melting. These two large-scale gradients are interrupted by small-scale anomalies in volatile contents attributed to (1) low-degree melts preferentially sampling enriched heterogeneities near transform faults and (2) local assimilation of hydrothermal fluids in four samples from dredge 16D. Finally, 20 RC2806 samples described as popping rocks during collection do not show any difference in volatile content dissolved in the glass or in vesicularity when compared to the RC2806 nonpopping samples. Our observations lead us to question the interpretation of the CO2 content in the highly vesicular 2D43 popping rock as being representative of the CO2 content of undegassed mid-ocean ridge basalt.