Lu–Hf and Sm–Nd isotopic systematics in chondrites and their constraints on the Lu–Hf properties of the Earth

Abstract

Sm–Nd and Lu–Hf isotopic data are presented for 19 chondritic meteorites: six carbonaceous chondrites, five L-chondrites, seven H-chondrites, and a single enstatite chondrite. The primary goal of the study is to better define the Bulk Silicate Earth (BSE) reference values for Hf isotopes. Except for one sample with lower Sm/Nd, the Sm–Nd data define a cluster around the accepted reference values for chondrites and terrestrial planets, giving a mean 147Sm/ 144Nd of 0.1960±0.0005, and a mean 143Nd/ 144Nd of 0.512631±0.000010 (uncertainties are two standard errors). It seems appropriate to retain the presently accepted Sm–Nd reference parameters, 147Sm/ 144Nd=0.1966 and 143Nd/ 144Nd=0.512638 (when fractionation-corrected to 146Nd/ 144Nd=0.7219).Lu–Hf isotopic data are not clustered, but spread along an approximate 4.5-Ga isochron trend, with a range of 176Lu/ 177Hf from 0.0301 to 0.0354. The data are similar to many of the samples of chondrites presented by Bizzarro et al. [Nature 421 (2003) 931], but lack the range to lower Lu/Hf shown by those authors. Our chondrite data define a regression line of 4.44±0.34 Ga when 1.867×10 −11 year −1 is used for the decay constant of 176Lu [Science 293 (2001) 683; Earth Planet. Sci. Lett. 219 (2004) 311–324]. Combining our data with the main population of analyses from Bizzarro et al. [Nature 421 (2003) 931] yields 4.51±0.24 Ga. Unless samples of eucrite meteorites and deviating replicates of chondrites with 176Lu/ 177Hf less than 0.030 are employed, no combination of the main population of chondrite Lu–Hf data yields a regression with sufficiently low error to constrain the decay constant of 176Lu. Sample heterogeneity seems to hinder the acquisition of reproducible Lu–Hf analyses from small, manually ground pieces of chondrites, and we suggest that analysis of powders prepared from large volumes of meteorite will be needed to adequately characterize the Lu–Hf isotope systematics of chondritic reservoirs and of BSE. Our results for carbonaceous chondrites show higher average 176Lu/ 177Hf and 176Hf/ 177Hf than ordinary chondrites, and the mean of carbonaceous chondrites also coincides with replicate analyses of a powder representing a large volume of meteorite, the Allende powder from the Smithsonian Institution. Use of the carbonaceous chondrite mean for BSE Lu–Hf characteristics results in a BSE Hf–Nd point that lies well within the array of terrestrial compositions, and leads to plausible initial ε Hf values for Precambrian rocks. An improved objective resolution of meteorite data and of meteoritic models for the Earth needs to occur before BSE can be established for Lu–Hf.