The ¹⁷⁶Lu decay constant determined by Lu–Hf and U–Pb isotope systematics of Precambrian mafic intrusions

Abstract

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.

Introduction

After 25 years of Hf isotopic work on both terrestrial and extraterrestrial samples, it has recently become clear that the decay constant (λ) of ¹⁷⁶Lu is not defined to better than 5% (see review by Begemann et al. [2]). Physical determinations of λ¹⁷⁶Lu display considerable internal disagreement, with a range of more than 13% for determinations over the past 20 years [3], [4], [5], [6], [7], [8]. Partly for this reason the attention of the isotope earth science community has focused on determination of λ¹⁷⁶Lu by geochronologic analysis of samples of known age. So far, age comparison experiments based on meteorites (e.g. [9], [10]) suggest higher values for λ¹⁷⁶Lu than those based on terrestrial igneous intrusions dated by U–Pb systematics [1]. There is an urgent need to determine the true λ¹⁷⁶Lu, and to discover what causes the scattered values between samples of different origins.

In this study we have investigated two Precambrian dolerites for determination of λ¹⁷⁶Lu. The value of the decay constant is calculated from the relationship λ¹⁷⁶Lu=ln(m+1)/t, where m equals the slope of the Lu–Hf mineral isochron and t is the age of intrusion constrained by U–Pb baddeleyite dates. In addition to the abundance of baddeleyite, these samples were selected on basis of their extremely fresh mineralogy and textures typical for crystallization from a homogeneous magma. To confirm that the Lu–Hf results fulfill the isochron criteria we determined the Lu–Hf systematics of all main mineral phases in the dolerites. This may be important because, until now, age comparison experiments on terrestrial samples have been limited to Lu–Hf analysis of a few accessory phases, which precludes evaluating whether the result satisfies the isochron requirements. If those samples suffered from slow cooling, magma heterogeneity or later isotopic disturbance might explain the inconsistent results determined from terrestrial and extraterrestrial data sets.