1. Department of Geological Sciences, San Diego State University, 5500 Campanile Drive, San Diego, California 92182-1020, USA
2. Ecole Normale Superieure de Lyon, Laboratoire des Sciences de la Terre, CNRS UMR 5570, 46 Allee d'Italie, 69364 Lyon Cedex 7, France
3. School of Ocean and Earth Science Technology, University of Hawaii, 1680 East-West Road, POST 612A, Honolulu, Hawaii 96822, USA
4. College of Oceanic and Atmospheric Sciences, Oregon State University, 104 COAS Admin Bldg, Corvallis, Oregon 97331-5503, USA

Correspondence to: Barry B. Hanan¹ Email: Barry.Hanan@sdsu.edu

Abstract

The origin of the isotopic signature of Indian mid-ocean ridge basalts has remained enigmatic, because the geochemical composition of these basalts is consistent either with pollution from recycled, ancient altered oceanic crust and sediments, or with ancient continental crust or lithosphere. The radiogenic isotopic signature may therefore be the result of contamination of the upper mantle by plumes containing recycled altered ancient oceanic crust and sediments¹, detachment and dispersal of continental material into the shallow mantle during rifting and breakup of Gondwana², or contamination of the upper mantle by ancient subduction processes^{3, 4}. The identification of a process operating on a scale large enough to affect major portions of the Indian mid-ocean ridge basalt source region has been a long-standing problem. Here we present hafnium and lead isotope data from across the Indian–Pacific mantle boundary at the Australian–Antarctic discordance region of the Southeast Indian Ridge, which demonstrate that the Pacific and Indian upper mantle basalt source domains were each affected by different mechanisms. We infer that the Indian upper-mantle isotope signature in this region is affected mainly by lower continental crust entrained during Gondwana rifting, whereas the isotope signature of the Pacific upper mantle is influenced predominantly by ocean floor subduction-related processes.

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