The Earth's mantle is isotopically heterogeneous on length scales ranging from centimetres to more than 10⁴ kilometres^{1, 2}. This heterogeneity originates from partial melt extraction and plate tectonic recycling, whereas stirring during mantle convection tends to reduce it. Here we show that mid-ocean ridge basalts from 2,000 km along the southeast Indian ridge (SEIR) display a bimodal hafnium isotopic distribution. This bimodality reveals the presence of ancient compositional striations (streaks) in the Indian Ocean upper mantle. The number density of the streaks is described by a Poisson distribution, with an average thickness of 40 km. Such a distribution is anticipated for a well-stirred upper mantle, in which heterogeneity is continually introduced by plate tectonic recycling, and redistributed by viscous stretching and convective refolding.

1. College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon 97331, USA
2. Laboratoire des Sciences de la Terre, Ecole Normale Superieure, 69364 Lyon, France
3. SOEST, University of Hawaii, Honolulu, Hawaii 96822, USA

Correspondence to: D. W. Graham¹ Correspondence and requests for materials should be addressed to D.W.G. (Email: dgraham@coas.oregonstate.edu).

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