Investigating solid mantle upwelling beneath mid-ocean ridges using U-series disequilibria. II. A local study at 33°S Mid-Atlantic Ridge

Abstract

Uranium series disequilibria have been used to study the physical melting regime along a section of the southern Mid-Atlantic Ridge where the largest mantle Bouguer anomaly yet observed on ridges exists. Zero age excesses for ( 230Th)/( 238U) and ( 231Pa)/( 235U) are age constrained by the presence of measured ( 226Ra)/( 230Th) disequilibria. By this criterion, 7 out of 15 samples are younger than 8000 years. Zero age ( 230Th)/( 238U) range between 1.06 and 1.25 while zero age ( 231Pa)/( 235U) range between 1.77 and 3.57. Concentrations of Th range from 100 to 866 ppb and strongly correlate with K/Ti. The U-series data differentiate the segment with the large gravity anomaly from the surrounding segments. Specifically, this segment shows combined 231Pa and 230Th systematics unlike any observed in MORB thus far, having exceptionally low zero-age 231Pa excesses. The U-series data can be modeled using upwelling rates similar to the half-spreading rate of this area of ridge. The disequilibria systematics for the anomalous segment 4 possibly suggest a solid upwelling rate that is a factor of two higher than the adjacent segments. The similarity between melting at segment 4 and at ocean islands is noted.