Geochemistry of basalts from the southeast Indian Ridge, 115°E–138°E

Abstract:

The ocean basin south of Australia contains the Australian-Antarctic Discordance, an anomalously deep portion of the Southeast Indian Ridge that marks a boundary between isotopic provinces characteristic of the Indian and Pacific oceans. Samples recovered from the ridge within the discordance display unusual chemical compositions compared to normal mid-ocean ridge basalt (N-MORB) of the same MgO contents, including low iron, high silica, and high sodium abundances and elevated abundances of highly incompatible trace elements. In contrast, samples from the ridge east of the discordance, where the ridge is of average axial depth, display major and trace element systematics more typical of N-MORB. Major and moderately incompatible trace elements show no evidence of a discontinuity in source composition corresponding to the location of the known isotopic discontinuity within the discordance. Ratios of highly incompatible trace elements, however, reveal a gradational change in the range of values across the location of the isotopic discontinuity. Modelling of along-strike variations in major element chemistry suggest they may result from systematic variations in the extent and pressure of melting. The lowest solidus pressures and least extents of melting occur in the mantle beneath the discordance, supporting geophysical inferences based on bathymetric, gravity, and seismic evidence that the discordance overlies a region of cooler mantle temperatures.

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