An assessment of local and regional isotopic equilibrium in the mantle

The assumption of local equilibrium during partial melting is fundamental to the interpretation of isotope and trace element data for mantle-derived rocks. If disequilibrium melting is significant, the scale of the chemical and isotopic heterogeneity in the mantle indicated by the data could be as small as the grain size of the mantle rock, and the isotope data themselves are then of doubtful value to the understanding of mantle processes. To assess the scale of isotopic heterogeneity in a partially molten asthenosphere we review the Sr isotopic data of volcanic rocks from oceanic regions and the available experimental data on diffusion kinetics in minerals and melts similar to those existing in the mantle. Although diffusion data are scarce and afflicted with uncertainties, most of the diffusion coefficients for cations in mantle minerals at temperatures of 1000–1200°C appear to be greater than 10^?13 cm² s^?1. Sr diffusion in liquid basalt is more rapid, with diffusion coefficients of D = 10^?7 to 10^?6cm²s^?1 near 1300°C. Simple model calculations show that, with these D values, a fluid-free mantle can maintain a state of disequilibrium on a centimeter scale for periods of 10⁸ to 10⁹ years. The state of disequilibrium found in many mantle-derived xenoliths is thus easily explained. A partially molten mantle, on the other hand, will tend to equilibrate locally in less than 10⁵ to 10⁶ years. The analytical data on natural rocks likewise indicate that the inhomogeneities are both old (>FX1.5 b.y.) and regional in character and that the consistent isotopic difference between ocean island and ocean floor volcanics cannot be explained by small-scale heterogeneity of the source rock.