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Ridge-hotspot interaction along the Mid-Atlantic Ridge between 37°30′ and 40°30′N: the UTh disequilibrium evidence

Citation:

Bourdon B, Langmuir CH, Zindler A. Ridge-hotspot interaction along the Mid-Atlantic Ridge between 37°30′ and 40°30′N: the UTh disequilibrium evidence. Earth and Planetary Science Letters. 1996;142 (1) :175 - 189.

Abstract:

This study reports UTh disequilibrium data obtained by mass spectrometry for basaltic glasses collected along the Azores platform portion of the Mid-Atlantic Ridge (37°30′–40°30′N), a region characterized by both a geochemical and bathymetric gradient. High Th and U concentrations, as well asTh/U ratios, document an enriched geochemical signature. (230Th238U) activity ratios range from 1.20 to 1.35 and are thus systematically larger than most EPR MORBs reported in the literature. (230Th232Th) 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 of230Th-rich sediment as an explanation for the230Th238U systematics. The relatively large230Th excesses in the erupted lavas may be related to the influence of the enriched Azores mantle plume source. The lack of observed correlations between230Th excess and trace element and isotopic indices of source enrichment, however, rules out source composition as an explanation for the variations in (230Th238U). Excess230Th 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 (∼ 35 kbar) in the mantle beneath the Azores segment of the MAR. At the ridge segment scale,230Th 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 UTh 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.

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