Hafnium Isotope and Trace Element Constraints on the Nature of Mantle Heterogeneity beneath the Central Southwest Indian Ridge (13{degrees}E to 47{degrees}E) |
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Authors: | JANNEY P E; LE ROEX A P; CARLSON R W |
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Institution: | 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 |
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Abstract: | Hafnium isotope and incompatible trace element data are presentedfor a suite of mid-ocean ridge basalts (MORB) from 13 to 47°Eon the Southwest Indian Ridge (SWIR), one of the slowest spreadingand most isotopically heterogeneous mid-ocean ridges. Variationsin NdHf isotope compositions and Lu/Hf ratios clearlydistinguish an AtlanticPacific-type MORB source, presentwest of 26°E, characterized by relatively low Hf valuesfor a given Nd relative to the regression line through all NdHfisotope data for oceanic basalts (termed the NdHfmantle array line; the deviation from this line is termed![{Delta}](http://petrology.oxfordjournals.org/math/Delta.gif) Hf) and low Lu/Hf ratios, from an Indian Ocean-type MORB signature,present east of 32°E, characterized by relatively high ![{Delta}](http://petrology.oxfordjournals.org/math/Delta.gif) Hfvalues and Lu/Hf ratios. Additionally, two localized, isotopicallyanomalous areas, at 1315°E and 3941°E,are characterized by distinctly low negative and high positive![{Delta}](http://petrology.oxfordjournals.org/math/Delta.gif) Hf values, respectively. The low ![{Delta}](http://petrology.oxfordjournals.org/math/Delta.gif) Hf MORB from 13 to 15°Eappear to reflect contamination by HIMU-type mantle from thenearby Bouvet mantle plume, whereas the trace element and isotopiccompositions of MORB from 39 to 41°E are most consistentwith contamination by metasomatized Archean continental lithosphericmantle. Relatively small source-melt fractionation of Lu/Hfrelative to Sm/Nd, compared with MORB from faster-spreadingridges, argues against a significant role for garnet pyroxenitein the generation of most central SWIR MORB. Correlations between![{Delta}](http://petrology.oxfordjournals.org/math/Delta.gif) Hf and Sr and Pb isotopic and trace element ratios clearly delineatea high-![{Delta}](http://petrology.oxfordjournals.org/math/Delta.gif) Hf Indian Ocean mantle component that canexplain the isotope composition of most Indian Ocean MORB asmixtures between this component and a heterogeneous AtlanticPacific-typeMORB source. The Hf, Nd and Sr isotope compositions of IndianOcean MORB appear to be most consistent with the hypothesisthat this component represents fragments of subduction-modifiedlithospheric mantle beneath Proterozoic orogenic belts thatfoundered into the nascent Indian Ocean upper mantle duringthe Mesozoic breakup of Gondwana. KEY WORDS: mid-ocean ridge basalt; isotopes; incompatible elements; Indian Ocean |
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Keywords: | : mid-ocean ridge basalt isotopes incompatible elements Indian Ocean |
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