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

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.