Statistics – Computation
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30..603x&link_type=abstract
Meteoritics, vol. 30, no. 5, page 603
Statistics
Computation
3
Scientific paper
Mass fractionation is well established for the isotopes of several elements in lunar soils, notably O, Si, K, Ca (to a lesser extent), and S (see [1,2]). Zinc is nearly as volatile as S and is associated with it in the coatings of pyroclastic deposits (see [3]). It therefore seemed plausible that zinc might also be fractionated and that the degree of fractionation might be correlated with that of sulfur. To test this inference, we have begun to measure Zn isotopic abundances in lunar soils. Here we report results for four Apollo 16 samples from which the magnetic fraction had been previously removed. The samples were ground, weighed, and dissolved in HF, HNO3 and HCl in a Teflon bomb. Zn was separated by anion exchange and analyzed isotopically by using ICP/MS. Only the data for 64Zn, 66Zn, and 68Zn are reported: The molecular ion ^35Cl^16O(sub)2+ interfered with the measurement of 67Zn and counting statistics for 70Zn gave only %-level precision. We used Zn atomic absorption solutions as standards with a known concentration of Ga added to control for instrumental drift. Measured elemental Zn contents are consistent with literature values [4-6]. The isotopic ratios of the samples were expressed as delta-values calculated relative to the corresponding isotopic ratios of the standards. No fractionation (i.e., < 0.2%/AMU) was observed for any of several ordinary chondrites, among them Suizhou and Dhajala. In all four lunar soils, however, we found delta 68Zn~2xdelta 66Zn and delta 66Zn>0 at the one-sigma level. These results indicate mass dependent fractionation. If we make the oversimplified assumption that the Zn isotopic fractionation reflects Rayleigh distillation from a single well-mixed reservoir of material from which Zn evaporated as the atom, then the fraction of Zn retained is given approximately by (1+delta ^YZn/1000)^(-2M(sub)Y/Delta M where Y=66 or 68, M is mass, and Delta M is the mass difference M(sub)Y - M(sub)64. The computations suggest that the lunar soils analyzed lost 20 to 40% of their Zn. The calculated losses of Zn are comparable to those estimated for sulfur by [1]. The degree of S mass fractionation in lunar soils appears to correlate with S content and exposure age [7,8]; Kerridge [8,9] has therefore proposed that meteorite impact vaporizes some S but delivers even more. So far, we do not see a similar correlation with exposure age in the Zn data, but the number of samples analyzed is small. The nominal losses of potassium calculated by [1] are also in the neighborhood of 30%, but may actually be much higher for certain fractions of the soil [2]. To explain the K isotopic enrichments, and in particular the contrast to Mg and Ca in which any fractionations are much smaller, Humayun and Clayton [2] point to K's volatility and tendency to concentrate in the fine fractions. Zn has higher volatility than K and is also enriched in small grains [10]. References: [1] Clayton et al. (1974) Proc. LSC 5th, 1801-1809. [2] Humayun M. and Clayton R. N. (1995) GCA, 59, 2115-2130. [3] Haskin L. and Warren P. (1991) Lunar Sourcebook: A User's Guide to the Moon (G. Heiken et al., eds.), pp. 357-474, Cambridge Univ. [4] Kr henb hl et al. (1973) Proc. LSC 4th, 1325-1348. [5] Rose et al. (1973) Proc. LSC 4th, 1149-1158. [6] Finkelman et al. (1975) Proc. LSC 6th, 1385-1398. [7] Cripe J. D. and Moore C. B. (1976) Proc. LSC 7th, 469-479. [8] Kerridge et al. (1975) GCA, 39, 137-162. [9] Kerridge et al. (1975) Proc. LSC 6th, 2151-2162. [10] Kr henb hl et al. (1977) Proc. LSC 8th, 3901-3916.
Hall Graham S.
Herzog Gregory F.
Xue She-Sheng
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