Hafnium Isotope and Trace Element Constraints on the Nature of Mantle Heterogeneity beneath the Central Southwest Indian Ridge (13°E to 47°E)

Abstract

Hafnium isotope and incompatible trace element data are presented for a suite of mid-ocean ridge basalts (MORB) from 13 to 47 E on the Southwest Indian Ridge (SWIR), one of the slowest spreading and most isotopically heterogeneous mid-ocean ridges. Variations in Nd–Hf isotope compositions and Lu/Hf ratios clearly distinguish an Atlantic–Pacific-type MORB source, present west of 26 E, characterized by relatively low eHf values for a given eNd relative to the regression line through all Nd–Hf isotope data for oceanic basalts (termed the ‘Nd–Hf mantle array line’; the deviation from this line is termed DeHf) and low Lu/Hf ratios, from an Indian Ocean-type MORB signature, present east of 32 E, characterized by relatively high DeHf values and Lu/Hf ratios. Additionally, two localized, isotopically anomalous areas, at 13–15 E and 39–41 E, are characterized by distinctly low negative and high positive DeHf values, respectively. The low DeHf MORB from 13 to 15 E appear to reflect contamination by HIMU-type mantle from the nearby Bouvet mantle plume, whereas the trace element and isotopic compositions of MORB from 39 to 41 E are most consistent with contamination by metasomatized Archean continental lithospheric mantle. Relatively small source-melt fractionation of Lu/Hf relative to Sm/Nd, compared with MORB from faster-spreading ridges, argues against a significant role for garnet pyroxenite in the generation of most central SWIR MORB. Correlations between DeHf and Sr and Pb isotopic and trace element ratios clearly delineate a highDeHf ‘ Indian Ocean mantle component’ that can explain the isotope composition of most Indian Ocean MORB as mixtures between this component and a heterogeneous Atlantic–Pacific-type MORB source. The Hf, Nd and Sr isotope compositions of Indian Ocean MORB appear to be most consistent with the hypothesis that this component represents fragments of subduction-modified lithospheric mantle beneath Proterozoic orogenic belts that foundered into the nascent Indian Ocean upper mantle during the Mesozoic breakup of Gondwana.