Between a Hotspot and a Cold Spot: Isotopic Variation in the Southeast Indian Ridge Asthenosphere, 86degreesE-118degreesE

Abstract

Glasses from a 2600 km section of the Southeast Indian Ridge west of the Australian–Antarctic Discordance all possess Nd–Pb–Sr isotopic signatures typical of Indian Ocean ridge basalt. The boundary between Pacific- and Indian-Ocean-type ridge basalt within the Discordance thus marks the westernmost extent of shallow Pacific-type asthenosphere beneath the ridge. Along-axis He, Nd, Pb, and Sr isotopic patterns are largely independent of ridge segmentation, but a weak tendency is evident for the most strongly Indian-Ocean-type mantle to be relatively fusible and for shallower asthenosphere to have lower 3He/4He. On average, εNd appears slightly lower than for ridges in the western Indian Ocean far from hotspots. Importantly, the regional isotopic patterns cannot be explained by a previously proposed eastward flow of Kerguelen–Heard or Amsterdam–St. Paul hotspot mantle. Nd, Pb and (to a much lesser extent) Sr isotopes correlate roughly with many incompatible element ratios, including parent–daughter ratios. If interpreted as mantle errorchrons, the latter correlations imply ‘ages’ of 200–300 Ma, significantly greater than the oldest known age of the Kerguelen–Heard hotspot (119–135 Ma) commonly postulated to have played an important role in creating the isotopic signature of Indian Ocean mantle. Rather than reflecting relatively recent mixing involving mantle from hotspots in the region, much of the observed isotopic heterogeneity may be the result of other mixing or of past intra-mantle chemical fractionation, probably associated with melting.