A theoretical approach to understanding the isotopic heterogeneity of mid-ocean ridge basalt.

Rudge, J. F. and McKenzie, D. P. and Haynes, P. H. (2005) A theoretical approach to understanding the isotopic heterogeneity of mid-ocean ridge basalt. Geochimica et Cosmochimica Acta, 69. pp. 3873-3887. DOI 10.1016/j.gca.2005.03.004

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Abstract

We have developed an idealised mathematical model to understand the isotopic variability of the mantle and its relation to the observed variations in isotopic ratios 143Nd/144Nd, 87Sr/86Sr, 176Hf/177Hf, 208Pb/204Pb, 206Pb/204Pb, and 207Pb/204Pb measured on mid-ocean ridge basalt (MORB). We consider a simple box model of mantle processes. A single melt region produces a melt fraction F of melt, and the average time since a given parcel of mantle material last visited this region is given by the timescale melt. The melt region fractionates the parent/ daughter ratios. Over time this leads to variations in the mantle isotopic ratios as the parent decays to the daughter. Key assumptions are that the half-life of the parent isotope is large compared with melt, that the flow is strongly stirring, and that the mantle has reached a statistical steady state. This enables us to neglect the specifics of the underlying flow. Sampling from our model mantle is dealt with by averaging over a large number N of samples to represent the mixing after melting. The model predicts a probability density for isotopic ratios in MORB which, with exception of the Pb isotopes, are consistent with measurements. Fitting the MORB data to this model gives estimates of the model parameters F, melt, and N. Small melt fractions with F around 0.5% are essential for a good fit, whereas melt and N are less well constrained. melt is estimated at around 1.4 to 2.4 Ga, and N is of the order of hundreds. The model predicts a larger variability for the Pb isotopes than that observed. As has been stated by many previous authors, it appears that fundamental differences exist between the dynamics of Pb isotopes and those of Nd, Sr and Hf isotopes.

Item Type: Article
Uncontrolled Keywords: 2005 AREP 2005 P IA48
Subjects: 05 - Petrology - Igneous, Metamorphic and Volcanic Studies
02 - Geodynamics, Geophysics and Tectonics
Divisions: 02 - Geodynamics, Geophysics and Tectonics
Journal or Publication Title: Geochimica et Cosmochimica Acta
Volume: 69
Page Range: pp. 3873-3887
Identification Number: 10.1016/j.gca.2005.03.004
Depositing User: Sarah Humbert
Date Deposited: 19 Aug 2010 09:22
Last Modified: 23 Jul 2013 09:59
URI: http://eprints.esc.cam.ac.uk/id/eprint/1807

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