The 176Lu decay constant determined by Lu-Hf and U-Pb isotope systematics of Precambrian mafic intrusions
At present, there is large uncertainty in the decay constant of 176Lu needed for Lu-Hf isotopic studies. We have determined λ176Lu by cross-calibration of U-Pb and Lu-Hf isotopic systems on mineral fractions from the Proterozoic Karlshamn and Sorkka dolerites in Sweden and Finland. The dolerites crystallized at shallow depths from homogeneous, high-temperature magmas, carry olivine that is nearly 100% unaltered, and show no signs of post-magmatic isotopic disturbance. The Lu and Hf isotopic compositions of plagioclase, olivine, pyroxene, apatite, ilmenite and baddeleyite were determined by multicollector-inductively coupled plasma mass spectrometry (MC-ICPMS). Calibrating the Lu-Hf results against baddeleyite U-Pb dates of 954.1±1.2 and 1256.2±1.4 Ma for the dolerites yields a mean λ176Lu of 1.867±0.008×10-11 year-1. The pristine character of the rocks and the agreement of the λ176Lu values with those from other terrestrial data sets [E. Scherer et al., Science 293 (2001) 683-687] suggest that the true value of λ176Lu lies between 1.86 and 1.87×10-11 year-1. Calibration experiments on extraterrestrial samples give significantly higher (4-6%) values, a discrepancy that may be due to plotting of non-cogenetic samples on the same Lu-Hf isochron diagram, or may have other, as yet undetermined, causes. The result of this study also indicates that the Lu-Hf method is capable for dating the crystallization of mafic rocks. The high 176Lu/177Hf ratio in apatite suggests that intrusive ages can be determined at a precision of a few million years or better.