Isotope fractionation of cadmium in lunar material

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Abstract

The double spike technique has been used to measure the isotope fractionation and elemental abundance of Cd in nine lunar samples, the Brownfield meteorite and the Columbia River Basalt BCR-1, by thermal ionisation mass spectrometry. Lunar soil samples give a tightly grouped set of positive isotope fractionation values of between + 0.42% and + 0.50% per mass unit. Positive isotope fractionation implies that the heavy isotopes are enhanced with respect to those of the Laboratory Standard. A vesicular mare basalt gave zero isotope fractionation, indicating that the Cd isotopic composition of the Moon is identical to that of the Earth. A sample of orange glass from the Taurus-Littrow region gave a negative isotope fractionation of − 0.23 ± 0.06% per mass unit, presumably as a result of redeposition of Cd from the Cd-rich vapour cloud associated with volcanism. Cadmium is by far the heaviest element to show isotope fractionation effects in lunar samples. The volatile nature of Cd is of importance in explaining these isotope fractionation results. Although a number of mechanisms have been postulated to be the cause of isotope fractionation of certain elements in lunar soils, we believe that the most likely mechanisms are ion and particle bombardment of the lunar surface.

Introduction

The primary objective of this experiment is to confirm the preliminary results of Sands et al. [1] for the isotope fractionation of Cd in lunar soils and orange glass, and to extend the original work by analysing additional lunar samples, including a mare basalt, using the double spike technique [2]. In particular, this paper will (i) Verify the magnitude of the reported isotope fractionation in lunar soils so that the tentative inverse relationship between elemental abundance and isotope fractionation [1] can be tested; (ii) Verify the magnitude of negative isotope fractionation of the orange glass (sample 74220,125) and explain the nature of this fractionation; (iii) Measure the isotope fractionation of a lunar basalt (sample 10017,341) and a terrestrial basalt (BCR-1), to investigate the extent of the isotope variation between the Earth and Moon; and (iv) Measure the elemental abundance of Cd in the lunar samples by the double spike technique and compare the results with earlier published data.

Isotope fractionation in meteoritic, lunar and planetary materials is a phenomenon which is essential to our understanding of the physico-chemical evolution of the Solar System. Isotopes of some elements may show significant fractionation effects as a result of physical, chemical or biological processes, the rates or equilibrium states of which are mass dependent. In terms of this study, an isotope effect is defined as some physical or chemical process whose rate, magnitude or position of equilibrium results in isotope fractionation.

Isotope fractionation studies of meteoritic and lunar samples have been extensively investigated because of their applicability to understanding early Solar System processes. Isotope fractionation in lunar soils has been reported for O and Si [3], [4], [5]; S [6], [7], [8]; K [9], [10], [11], [12]; Cd [1]; Cu and Zn [13]; Fe [14] and Mg [15]. The isotope fractionation of Ca in lunar samples has been investigated, but revealed no definitive fractionation effects [16].

A number of theoretical explanations for these fractionation effects on the lunar surface have been given:

  • 1.

    Volatilization by micrometeorite impact [5], [17],

  • 2.

    Ion sputtering [18], [19],

  • 3.

    Redeposition of volatilized or sputtered material after gravitational separation [18], [17].

Ion sputtering calculations have shown that elements of similar mass and volatility should be fractionated by similar amounts, but that volatility plays an important role in determining the magnitude of the isotope fractionation [19]. The marked difference in the magnitude of the isotope fractionation between K and Ca, two elements of similar mass, may reflect the importance of volatility [12]. In the case of K, the magnitude of isotope fractionation in the 41K / 39K ratio is approximately + 1.3% per mass unit, whereas for Ca the data show no intrinsic isotope fractionation within the limits imposed by experimental uncertainties [16].

Cadmium, with a condensation temperature of 430 K at 7.5 × 10 7 Pa, is a more volatile element than either S, with a condensation temperature of 684 K, or K with a condensation temperature of 1000 K [20]. Cadmium possesses eight stable isotopes, ranging in mass number from 106 to 116, and has an atomic number Z = 48. Cadmium is therefore the heaviest element in the Periodic Table in which isotope fractionation in lunar samples has been reported. The large mass range of Cd represents a 9% relative variation in mass, which compares favourably with the corresponding 5% relative variation for the K isotopes.

The first evidence of isotope fractionation in Cd was reported in 1976 in certain unequilibrated ordinary chondrites using Thermal Ionization Mass Spectrometry (TIMS) [21]. The Brownfield H3 chondrite was shown to be enriched in the heavier isotopes, with an average value of + 0.27 ± 0.01% per mass unit [22]. Negative isotope fractionation (in which the lighter isotopes are enhanced with respect to the Laboratory Standard), was observed in the LL3 chondrite Semakona to an extent of − 0.21% per mass unit, whilst the LL3 chondrite Bishunpur showed the more customary positive fractionation to a magnitude of + 0.27% per mass unit [23]. In each of these studies, the magnitude of isotope fractionation was determined by the double spike technique using the methodology of Russell [2].

The isotopic composition of Cd has recently been measured by Multiple Collector-Inductively Coupled Plasma-Mass Spectrometry (MC-ICP-MS) in a range of stony meteorites and terrestrial samples [24]. These authors found that terrestrial rock and mineral samples display little variation in isotopic composition, (except for a tektite), as compared to a Laboratory Standard, as was the case for carbonaceous chondrites, whereas some ordinary chondrites and the Rumurita chondrite, display both positive and negative isotope fractionation [24].

The thermal neutron energy spectrum on the lunar surface was determined by measuring the isotopic composition of elements possessing isotopes with very high thermal neutron capture cross-sections—113Cd,155,157Gd and 149Sm [25]. The measurements showed that the isotopic composition of Cd indicated evidence of isotope fractionation, in contrast to Gd and Sm which showed no evidence of isotope fractionation on the same samples [25]. A detailed examination of the isotopic composition of Cd enabled the magnitude of isotope fractionation for the analysed lunar samples to be determined [1]. The magnitude of the isotope fractionation varied from + 0.01 ± 0.02% per mass unit to + 0.63 ± 0.05% per mass unit for five lunar soil samples, and − 0.13 ± 0.02% per mass unit for a sample of orange glass from the vicinity of Shorty Crater [1]. In contrast, samples of the terrestrial Columbia River Basalt BCR-1, which were subjected to the same ion exchange chemistry and mass spectrometry as the lunar samples, showed no evidence of isotope fractionation with respect to the Laboratory Standard.

Unfortunately the Cd isotope fractionation data reported by Sands et al. [1] were confined to five lunar samples and the results were only carried out on unspiked samples, so the report was of a preliminary nature. Only a relatively small number of elements have been studied which exhibit isotope fractionation effects on lunar samples, so that our understanding of the processes responsible for this phenomenon is somewhat limited. A detailed study of isotope fractionation in lunar samples for other elements would extend our present understanding. As has already been pointed out, Cd is a good candidate for such a study, provided the double spike technique is used to accurately establish the magnitude of isotope fractionation. This is the primary objective of the present study.

Section snippets

Samples

Lunar samples were stored in the original aluminium capsules received from the NASA lunar sample curators. After weighing each sample inside a Class 1000 clean-air laboratory, the material was immediately sealed. Nine lunar samples were analysed in this study: 10017,341 (vesicular basalt, high-Ti mare basalt), 14163,848 (surface soil sample), 14310,615 (polymict rock melted on impact), 15041,188 (near surface soil sample), 15059,240 (interior chips from a regolith breccia containing mixed

Results

The Laboratory Standard comprised a sample of spectroscopically pure Cd metal (Johnson Matthey Chemicals Ltd.), which was taken into solution in HCl. The mean value of the isotope ratio data set for the Laboratory Standard is given in Table 1. The isotopic composition of Cd in eight Cd-bearing minerals was measured by TIMS [27]. No natural variations in isotopic composition as compared to the Laboratory Standard were found. Subsequently, a number of other terrestrial samples have been analysed

Discussion

An objective of this experiment was to show unequivocally that Cd is isotopically fractionated in some lunar samples in order to confirm the preliminary results of Sands et al. [1]. The double spiking technique, was successfully exploited for this purpose [2]. There is now definitive evidence that lunar soils exhibit isotope fractionation for Cd of approximately + 0.30% to + 0.50% per mass unit. Lunar sample 15059,240, representing interior chips from a regolith breccia, also yielded a positive

Conclusions

Positive, mass-dependent isotope fractionation has been determined experimentally in five lunar soil samples. The isotope fractionation for these samples varies from + 0.42% to + 0.50% per mass unit given that the revised value for sample 60501,105 is accepted. The magnitude of the fractionation for these samples, together with the elemental abundance of Cd for each sample, were determined by the Double Spike technique using TIMS. Micrometeorite impact and ion sputtering of the lunar surface by

Acknowledgements

The authors acknowledge the provision of nine lunar samples by NASA and the assistance of the NASA lunar sample curators. In particular, we thank Professor S. R. Taylor for his advice and assistance. The Mass Spectrometry laboratory at Curtin University is supported by the Australian Research Council and the Western Australian Government.

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    Present address: School of Physics, The University of Western Australia, Nedlands, Western Australia 6009.

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