The Lu–Hf and Sm–Nd isotopic composition of CHUR: Constraints from unequilibrated chondrites and implications for the bulk composition of terrestrial planets

The Lutetium–Hafnium radiogenic isotopic system is widely used as a chronometer and tracer of planetary evolution. In order for this isotopic system to fulfill its potential in planetary studies, the Lu–Hf system parameters need to be more tightly constrained, in particular the Lu–Hf isotopic composition of the chondritic uniform reservoir (CHUR) and, by extension, the bulk silicate Earth (BSE). We present new Lu–Hf and Sm–Nd isotopic compositions of unequilibrated carbonaceous, ordinary, and enstatite chondrites of petrologic types 1, 2, and 3 which define a narrow range of Lu/Hf ratios (3%) identical with that of Sm/Nd. This contrasts with previously published data from mostly equilibrated ordinary chondrites of petrologic types 4, 5, and 6 which have a much larger range in Lu/Hf (28%). This heterogeneity has hampered an unambiguous choice for the Lu–Hf isotopic composition of CHUR. Our new determinations of Lu–Hf CHUR parameters are 176Lu/177Hf = 0.0336 ± 1 and 176Hf/177Hf = 0.282785 ± 11 (2σm), which are higher than previous estimates and, together with average Sm–Nd chondrite compositions of unequilibrated chondrites of 147Sm/144Nd = 0.1960 ± 4 and 143Nd/144Nd = 0.512630 ± 11 (2σm), now provide firm constraints on the chondritic parameters for both Lu–Hf and Sm–Nd isotopic systems. A comparison of Lu–Hf and Sm–Nd data show that terrestrial planets, as well as early differentiated planetesimals, converge toward a common initial Hf and Nd isotope composition corresponding to the average of chondrites. Finally, a compilation of Lu–Hf isotopic data of unequilibrated and equilibrated chondrites demonstrates that the 176Lu decay decay-constant value cannot be resolved by age comparison on metamorphosed or shocked planetary materials which have a complex history.