Ridge-hotspot interaction along the Mid-Atlantic Ridge between 37 degrees 30' and 40 degrees 30'N: The U-Th disequilibrium evidence

This study reports U-Th disequilibrium data obtained by mass spectrometry for basaltic glasses collected along the Azores platform portion of the Mid-Atlantic Ridge (37 degrees 30'-40 degrees 30'N), a region characterized by both a geochemical and bathymetric gradient. High Th and U concentrations, as well as Th/U ratios, document an enriched geochemical signature. (Th-230/U-238) activity ratios range from 1.20 to 1.35 and are thus systematically larger than most EPR MORBs reported in the literature. (Th-230/Th-232) activity ratios show remarkable homogeneity for multiple samples taken from single dredge hauls. Additionally, samples with the highest Th concentrations (2.4 ppm) have among the highest Th isotope ratios. Taken together, these observations rule out assimilation of Th-230-rich sediment as an explanation for the Th-230-U-238 systematics. The relatively large Th-230 excesses in the erupted lavas may be related to the influence of the enriched Azores mantle plume source. The lack of observed correlations between Th-230 excess and trace element and isotopic indices of source enrichment, however, rules out source composition as an explanation for the variations in (Th-230/U-238). Excess Th-230 is correlated with the axial depth of the ridge in the study area, with the shallowest portions showing the largest extents of disequilibrium. This may reflect more melting in the presence of garnet for the shallow segments, and suggests that melting begins well within the garnet peridotite stability field (similar to 35 kbar) in the mantle beneath the Azores segment of the MAR. At the ridge segment scale, Th-230 excesses tend to be smaller near segment boundaries. This could reflect differences in the melting process or less frequent magmatism in these zones. These results demonstrate the potential for U-Th systematics to constrain the depth and degree of melting as well as the rate of mantle upwelling, even in the presence of source chemical heterogeneity.