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GEOPHYSICAL RESEARCH LETTERS, VOL. 22, NO. 17, PAGES 2325–2328, 1995

⁴He/³He Dispersion and Mantle Convection

Claude J. Allègre

Laboratoire de Géochimie et Cosmochimie, Institut de Physique du Globe de Paris et UFR de Sciences de la Terre Universités de Paris VI et VII, 4 place Jussieu, 75252 Paris Cedex 05, France

Manuel Moreira

Laboratoire de Géochimie et Cosmochimie, Institut de Physique du Globe de Paris et UFR de Sciences de la Terre Universités de Paris VI et VII, 4 place Jussieu, 75252 Paris Cedex 05, France

Thomas Staudacher

Laboratoire de Géochimie et Cosmochimie, Institut de Physique du Globe de Paris et UFR de Sciences de la Terre Universités de Paris VI et VII, 4 place Jussieu, 75252 Paris Cedex 05, France

Abstract

Histograms of the ⁴He/³He ratios analysed in MORB glasses from individual ridge segments show a linar correlation between the ridge spreading rate and the standard deviation of the corresponding averaged ⁴He/³He ratio. We interpret the form of these histograms in terms of stirring time (τ_stir) of the upper mantle. This stirring time corresponds to the time that perturbations brought in from outside into the upper mantle, in the form of oceanic island source material or slabs, were attenuated by a factor of 1/e through the effect of convection.

Using ⁴He/³He ratios of oceanic basalts, we calculate a stirring time close to 250 Ma, distinct from the residence time of ∼1 Ga. This may indicate the existence of two scale upper mantle convection, rapid convection being responsible for the homogeneisation of helium in the upper mantle, and the consequent uniformity of the ⁴He/³He ratio, and slower convection being responsible for mantle outgassing and plate tectonic motion.

Received 28 February 1995; accepted 21 July 1995.

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Citation: Allègre, C. J., M. Moreira, and T. Staudacher (1995), ⁴He/³He Dispersion and Mantle Convection, Geophys. Res. Lett., 22(17), 2325–2328.

Copyright 1995 by the American Geophysical Union.