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Geochemical characteristics and Sr–Nd–Hf isotope compositions of mantle xenoliths and host basalts from Assab, Eritrea: implications for the composition and thermal structure of the lithosphere beneath the Afar Depression

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Teklay, M and Scherer, EE and Mezger, K and Danyushevsky, LV (2010) Geochemical characteristics and Sr–Nd–Hf isotope compositions of mantle xenoliths and host basalts from Assab, Eritrea: implications for the composition and thermal structure of the lithosphere beneath the Afar Depression. Contributions to Mineralogy and Petrology, 159. pp. 731-751.

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Abstract

The Afar Depression offers a rare opportunity to study the geodynamic evolution of a rift system from continental rifting to sea floor spreading. This study presents geochemical data for crustal and mantle xenoliths and their alkaline host basalts from the region. The basalts have enriched REE patterns, OIB-like trace element characteristics, and a limited range in isotopic composition (87Sr/86Sr = 0.70336–0.70356, eNd = ?6.6 to ?7.0, and eHf = ?10.0 to ?10.7). In terms of trace elements and Sr–Nd isotopes, they are similar to basalts from the Hanish and Zubair islands in the southern Red Sea and are thus interpreted to be melts from the Afar mantle. The gabbroic crustal xenoliths vary widely in isotope composition (87Sr/86Sr = 0.70437–0.70791, eNd = -8.1 to ?2.5, and eHf = -10.5 to ?4.9), and their trace element characteristics match those of Neoproterozoic rocks from the Arabian–Nubian Shield and modern arc rocks, suggesting that the lower crust beneath the Afar Depression contains Neoproterozoic mafic igneous rocks. Ultramafic mantle xenoliths from Assab contain primary assemblages of fresh ol ? opx ? cpx ? sp ± pl, with no alteration or hydrous minerals. They equilibrated at 870–1,040C and follow a steep geothermal gradient consistent with the tectonic environment of the Afar Depression. The systematic variations in major and trace elements among the Assab mantle xenoliths together with their isotopic compositions suggest that these rocks are not mantle residues but rather series of layered cumulate sills that crystallized from a relatively enriched picritic melt related to the Afar plume that was emplaced before the eruption of the host basalts.

Item Type: Article
Keywords: Afar Depression Xenoliths Cumulates Rifting Sr–Nd–Hf isotopes
Journal or Publication Title: Contributions to Mineralogy and Petrology
Page Range: pp. 731-751
Identification Number - DOI: 10.1007/s00410-009-0451-0
Additional Information: The final publication is available at http://www.springerlink.com
Date Deposited: 08 Dec 2010 04:51
Last Modified: 18 Nov 2014 04:15
URI: http://eprints.utas.edu.au/id/eprint/10457
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