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(2020) Cadmium Stable Isotopes in Lunar Regolith Samples
Abouchami W, Wombacher F, Braukmüller N & Galer SJG
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01c: Plenary Hall, View in program
Listed below are questions that have been submitted by the community that the author will try and cover in their presentation. To submit a question, ensure you are signed in to the website. Authors or session conveners approve questions before they are displayed here.
On the conclusion slide it is written that "Lunar anorthositic crust heavy Cd isotope enrichment is pristine, since no neutron capture effect detected!" Is it possible that the sample could have experienced an impact that fractionated the Cd isotopic composition without spending significant lengths of time at a depth suitable for neutron capture? E.g., too shallow, too deep, or simply not for a long enough time period?
This is a very interesting question regarding lunar anorthosite sample 60025 (i.e. representative of the crust), which is described as “pristine” (~unremelted), with an old age and low 87Sr/86Sr, coarse-grain with low meteoritic siderophiles content (Ni, Ir etc) (Warren & Wasson, 1977). All these features makes an impact-related source for the isotopically heavy Cd signature unlikely, and instead favor our interpretation that this signature is pristine and either primary or formed during the early differentiation. The point is that the 112Cd/110Cd ratio is very fractionated to a positive “heavy” value. By contrast, the 114Cd/110Cd does not indicate any n-capture effects, unlike the data for lunar soils, which suggest it has not been close to the surface for long. You are right that this could be consistent with impact loss of Cd, followed by burial at depth, or insufficient/short exposure time near to the surface when the n-capture takes place. The latter is quite likely given the exposure age of 1.9 Myr (K. Marti, LSC) and would be consistent with the lack of n-capture effects. On the other hand, 60025 has a remarkably high cadmium concentration, that does not suggest Cd loss as being responsible for the heavy cadmium isotope signature. By comparison, the impact melt breccia 65015 has a significantly lower Cd concentration, a factor of 4 lower than that of 60025. Given the arguments above, we favor the interpretation that the fractionated Cd in 60025 is likely a primary feature of this sample. Obviously, we will be able to firm up this idea with data on the additional lunar crustal samples requested, which span a large range in Cd concentrations and for which, Zn isotope data show a large range interpreted by Kato et al. (2015) in terms of mixing of a Zn-poor, isotopically heavy magmatic source and a Zn-rich, isotopically light surface reservoir. It will be interesting to see whether this is substantiated by the Cd isotope data, pending on samples allocation!
Could you explain a bit why TIMS is better than ICPMS for these measurements?
First, TIMS is better than MC-ICPMS due to the higher ionization efficiency (? 0.3%), with down to 1ng of Cd loads measured with a precision of 1 epsilon, not achievable to my knowledge by MC-ICPMS. Considering that the abundance of Cd is less than a ppm in most terrestrial and extra-terrestrial samples, the higher sensitivity of TIMS makes it the method of choice for low-level Cd samples and limited sample availability, such as lunar rocks. Secondly, the accuracy is better as there are no isobaric interferences (Sn, Pd, and In) nor matrix effects which can be an issue for MC-ICPMS measurements. As shown by the double spike Cd stable isotope data on the Apollo samples presented here, the improvement in analytical precision is up to 20-fold compared to previous literature data.
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