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 Lu-176 needed for Lu-Hf isotopic studies. We have determined lambda(176)Lu 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 lambda(176)Lu of 1.867 +/- 0.008 x 10(-11) year(-1). The pristine character of the rocks and the agreement of the lambda(176)Lu values with those from other terrestrial data sets [E. Scherer et al., Science 293 (2001) 683-687] suggest that the true value of lambda(176)Lu lies between 1.86 and 1.87 x 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 Lu-176/Hf-177 ratio in apatite suggests that intrusive ages can be determined at a precision of a few million years or better. (C) 2004 Elsevier B.V. All rights reserved.