Hf-Nd Isotopic Covariance in the Crust and Mantle and Constraints on the Evolution of the Depleted Mantle

Abstract

It has long been recognized that Lu-Hf and Sm-Nd isotope systems behave analogously during most magmatic processes but the details of Hf-Nd isotopic behaviour in the major crust and mantle reservoirs have not been fully demonstrated. How Hf and Nd isotopes behave in the depleted mantle through time, for example, has been uncertain because of the apparent different Hf-Nd variations in MORBs and OIBs and the potential for decoupling these systems in a number of important mantle (e.g. Salters and Hart, 1991) and crustal (e.g. Patchett et al., 1984; Vervoort and Patchett, 1996) environments. New HfNd results in combination with existing data, allow us to make some first-order observations of Hf-Nd behaviour in some of the Earth's major reservoirs. Oceanic Basalts. Hf-Nd data for plot along an array defined by the line eHr = 1.3 eNd + 3.1. The OIB array is indistinguishable from the array for all oceanic basalts (including MORBs and IABs) of eHf = 1.3 eNd + 3.6. Crustal Samples. Muds and shales, recent to Archaean, plot along an array enf = 1.4 end + 2.9, similar to that for OIBs and all ocean basalts. Sands and sandstones are more scattered and plot along a steeper array (eHf = 1.8 eNd + 2.5) due to the less radiogenic Hf character in some sands. This 'zircon effect' occurs in some sands derived from older crustal sources where the Hf in the sands is dominated by zircons less radiogenic than the SmNd in the bulk rock. All whole-rock data. All terrestrial whole-rock Hf and Nd data define a remarkably coherent array of eHf = 1.4 aN~ + 2.9 (Fig. 1). Mn crusts and pelagic red clays, some peridotite xenoliths, and a few sandstones with extreme zircon effects plot outside of the main array and are not included in this regression. This array contains diverse samples of wide-ranging compositions and ages. The close similarity of the Hf-Nd arrays for continental crust (as defined by muds) and the mantle (as defined by all oceanic basalt data) indicates no large-scale decoupling of Hf and Nd between crust and mantle. This tight Hf-Nd covariation may also argue for the efficacy of crust to mantle recycling in modulating Hf-Nd behaviour in the silicate Earth. With this background, we report new whole-rock Hf-Nd isotope data for 1) mantle-derived rocks, midArchaean to Mesozoic in age and 2) early Archaean gneisses from West Greenland. Hf and Nd isotopic compositions are well correlated in the juvenile rocks, corresponding to eHf = 1.4 end +2. l, and plot well within the collective Hf-Nd array of terrestrial samples (eHf = 1.4 r +3.1). The early Archaean Greenland gneisses, in contrast, have an extreme range in eNd values (--4.4 to +4.2; Bennett et at., 1993) while corresponding eHf values are more restricted and entirely positive (0 to +3.4) resulting in a Hf-Nd array for these rocks (eHf = --0.1 eNd +2.3) quite different from that defined by other terrestrial samples. This Nd isotopic heterogeneity is not a result of extreme depletions and enrichments in the early Earth, but from disturbances in the Sm-Nd isotope system of these rocks.