Journal of Petrology Advance Access originally published online on July 20, 2005
Journal of Petrology 2005 46(12):2427-2464; doi:10.1093/petrology/egi060
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Hafnium Isotope and Trace Element Constraints on the Nature of Mantle Heterogeneity beneath the Central Southwest Indian Ridge (13°E to 47°E)
1 DEPARTMENT OF TERRESTRIAL MAGNETISM, CARNEGIE INSTITUTION OF WASHINGTON, 5241 BROAD BRANCH ROAD, NW, WASHINGTON, DC 20015, USA
2 DEPARTMENT OF GEOLOGICAL SCIENCES, UNIVERSITY OF CAPE TOWN, PRIVATE BAG, RONDEBOSCH 7701, SOUTH AFRICA
RECEIVED JULY 6, 2004; ACCEPTED JUNE 10, 2005
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 
Hf values for a given Nd 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 
Hf) and low Lu/Hf ratios, from an Indian Ocean-type MORB signature, present east of 32°E, characterized by relatively high Hf 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 Hf values, respectively. The low Hf 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 Hf and Sr and Pb isotopic and trace element ratios clearly delineate a high-Hf ‘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.
KEY WORDS: mid-ocean ridge basalt; isotopes; incompatible elements; Indian Ocean
* Corresponding author. Present address: Department of Geology, The Field Museum of Natural History, 1400 S. Lake Shore Drive, Chicago, IL 60605, USA. Telephone: (312) 665-7099. Fax: (312) 665-7641. E-mail: pjanney{at}fieldmuseum.org
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