Mid-ocean ridge basalt generation along the slow-spreading, South Mid-Atlantic Ridge (5-11°S) : inferences from ²³⁸U-²³⁰Th-²²⁶Ra disequilibria

摘要

U-series disequilibria have provided important constraints on the physical processes of partial melting that produce basaltic magma beneath mid-ocean ridges. Here we present the first ²³⁸U-²³⁰Th-²²⁶Ra isotope data for a suite of 83 basalts sampled between 5°S and 11°S along the South Mid-Atlantic Ridge. This section of the ridge can be divided into 5 segments (A0-A4) and the depths to the ridge axis span much of the global range, varying from 1429 to 4514m. Previous work has also demonstrated that strong trace element and radiogenic isotope heterogeneity existed in the source regions of these basalts. Accordingly, this area provides an ideal location in which to investigate the effects of both inferred melt column length and recycled materials. ²²⁶Ra-²³⁰Th disequilibria indicate that the majority of the basalts are less than a few millennia old such that their ²³⁰Th values do not require any age correction. The U-Th isotope data span a significant range from secular equilibrium up to 32% ²³⁰Th excess, also similar to the global range, and vary from segment to segment. However, the (²³⁰Th/²³⁸U) ratios are not negatively correlated with axial depth and the samples with the largest ²³⁰Th excesses come from the deepest ridge segment (A1). Two sub-parallel and positively sloped arrays (for segments A0-2 and A3 and A4) between (²³⁰Th/²³⁸U) and Th/U ratios can be modelled in various ways as mixing between melts from peridotite and recycled mafic lithologies. Despite abundant evidence for source heterogeneity, there is no simple correlation between (²³⁰Th/²³⁸U) and radiogenic isotope ratios suggesting that at least some of the trace element and radiogenic isotope variability may have been imparted to the source regions > 350kyr prior to partial melting to produce the basalts. In our preferred model, the two (²³⁰Th/²³⁸U) versus Th/U arrays can be explained by mixing of melts from one or more recycled mafic lithologies with melts derived from chemically heterogeneous peridotite source regions.