TERRESTRIAL AND CHONDRITIC SM-ND AND LU-HF ISOTOPIC SYSTEMS: ARE THEY IDENTICAL?

Abstract

This paper presents a critical evaluation of popular concepts on the 143Nd/144Nd, 176Hf/177Hf, 87Sr/86Sr, Sm/Nd, Lu/Hf, and Rb/Sr ratios in the Earth's primitive mantle. We examined the major controversies ensuing from the model of the chondrite uniform reservoir (CHUR) as a proxy for the composition of the primitive mantle (DePaolo and Wasserburg, 1976a). Among them are (1) the high magmatic productivity of the depleted mantle (DM) with no regular isotopic signal from the primitive mantle (PM); (2) the abundance of geochemically enriched rocks, in particular, alkali basalts, with the isotopic characteristics of the DM; (3) the depletion of the source of HIMU basalts in the Nd-Sr-Hf system in contrast to enrichment in the U-Pb system; (4) mass balance calculations for the Sm-Nd isotopic system of the crust and mantle constrain the mass of the DM, which served as a source of the crust, as 1/4-1/5 of the bulk mantle, but these estimates are in conflict with mass balances for the Rb-Sr and U-Pb systems and some other elements; and (5) the neodymium isotopic compositions and Sm/Nd values of high-degree melts from the PM must be similar to those of the source; however, rocks resembling the CHUR in both these parameters have never been reported. These controversies can be resolved only by assuming that the Sm/Nd ratio of the primitive mantle differs from that of the CHUR by 8%. The present-day compositional parameters of the primitive mantle are estimated as εNd = +9, 143Nd/ 144Nd = 0.51310, Sm/Nd = 0.350; εHf = +13, 176Hf/177Hf = 0.28323, Lu/Hf = 0.264; and ε Sr = -24, 87Sr/86Sr = 0.7028, Rb/Sr = 0.020. The uncertainty of the estimated neodymium isotopic parameters can be up to ±1 for εNd. The isotopic effects in the Sm-Nd, Lu-Hf, and Rb-Sr systems from the extraction of the Earth's crust from the mantle material is much smaller than the observed isotopic variations in mantle-derived rocks.