44 Earth and Planetary Science Letters, 38 (1978) 44-62 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands AN ASSESSMENT OF LOCAL AND REGIONAL ISOTOPIC EQUILIBRIUM IN THE MANTLE A.W. HOFMANN and S.R. HART * Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, D.C. 20015 [USA) 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 ofD = 10 -7 to 10 -6 cm2 s-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 (>1.5 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. 1. Introduction The isotopic composition of Sr and Pb has been used to set constraints on the origin and composition of the source material of volcanic rocks. Gast [ 1 ] pioneered this approach and showed by comparing meteoritic and terrestrial 87Sr/86Sr ratios and trace element abundances that the earth (or at least the upper mantle and crust) is depleted in alkalis relative to chondritic meteorites. Starting with the work of Hurley et al. [2], Hedge and Walthall [3], and Faure and Hurley [4], the initial 87Sr/86Sr ratio was used to determine, for example, whether granites were derived from the crust or the mantle. Gast et al. [5] for the first time showed that there are consistent differences between individual oceanic islands in the Pb and Sr isotopic composition, and they concluded that there are regional variations in the composition of the upper mantle. Hedge and Peterman [6] and Hart [7] compared 87Sr/86Sr ratios from ocean floor * Present address: Department of Earth and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massa- chusetts 02139, U.S.A. basalts (i,e. rocks derived from mid-ocean ridges) with ratios from oceanic island basalts and showed that the isotopic ratios from the ocean floor are systematically lower. More recent work has shown that, although significant regional differences do exist, the ocean floor basalts are the most nearly constant in their Sr isotopic composition of all the volcanic rocks investi- gated. The 87Sr/86Sr ratios of these rocks usually vary within the narrow range 0.7025-0.7035, with the large majority having values less than 0.7030. In con- trast, the 87Sr/86Sr ratios of oceanic island basalts are higher and much more variable. The pattern that emerges from these results is one of distinct groupings of basalts from the different environments in terms of isotopic compositions. This is illustrated in Fig. 1 where 87Sr/86Sr ratios are shown for basalts derived from the ocean floor, and from ocean islands. These results imply that the mantle is compositionally inhomogeneous both laterally and vertically, because island volcanoes situated near a mid-ocean ridge usually erupt lavas that differ in isotopic composition from the mid-ocean ridge material itself (see, for example, the data for Iceland, the Azores and Tristan