Cautionary Notes on Cosmogenic W and Other Nuclei in Lunar Samples

Introduction: Leya et al. (2000) [1] showed that neutron capture on Ta results in a production rate of Ta (decays with a half-life of 114 days to W) sufficiently high to cause significant shifts in W abundances considering the neutron fluences due to the cosmic ray cascade that were known to occur near the lunar surface (Lingenfelter et al., 1972; Russ et al. 1972; Curtis and Wasserburg, 1975; Woolum et al., 1975; Nishiizumi et al., 1997) [2, 3, 4, 5, 6]. Leya et al. [1] concluded that this cosmogenic production of W may explain the large positive ε182W values that Lee et al. (1997) [7] had reported in some lunar samples rather than being produced from decay of now extinct Hf ( 6 13 10 τ = × yr). If the large range in ε182W of lunar samples (0 to +11 in whole rock samples) was due to decay of now extinct Hf, it would require a very early time of formation and differentiation of the lunar crust-mantle system (with high Hf/W ratios) during the earliest stages of Earth’s accretion. This result was both surprising and difficult to understand. The ability to explain these results by a more plausible mechanism is therefore very attractive. In a recent report Lee et al. (2002) [8] showed that there were excesses of W and that ε182W was correlated with the Ta/W ratios in the mineral phases of individual lunar rock samples. This is in accord with W variations in lunar samples being produced by cosmic-ray induced neutron capture on Ta. Results and Discussions: While our considerations of W production due to neutron capture on Ta are in general agreement with the results presented by Leya et al. [1] and Lee et al. [8], there are some additional points that merit attention. The first of these is analytical, the second is isotope systematics, and the third is the relationship between irradiation ages and neutron fluence for different systems. From an analytical consideration there is always the issue of eliminating isobaric interferences. One possible source of apparent W excess is the presence of TaH. To test this, several solutions of normal Ta and W were prepared with different proportions and analyzed with the MC-ICP-MS (Micromass IsoProbe) in the EPS Department at Harvard. The results are shown in Figure 1. It was found that there was a very precise linear correlation between the apparent ε182W (“ε182”) and the Ta/W in the solution, where ε182 = [(I/W) / (W/W) 1] x 10, and I = W + TaH. -1.0 0.0 1.0 2.0 3.0 4.0