Iron Isotope Composition of the Lunar Mare Regolith: Implications for Isotopic Fractionation During Production of Single Domain Iron Metal

Introduction: The processes of space weathering are dominant in the development of the regolith and soils on all airless bodies in the Solar System. Indeed, it is this processing of the soil that complicates and masks the effective use of remote sensing by spectral reflectance [12]. The lunar regolith provides us with invaluable samples to study these effects of space weathering. In particular, variations in isotopic compositions can constrain some of the processes that act on the lunar soils. It has been observed that formation and maturation of the lunar regolith enrich the heavy isotopes of O, Si, S, K, Cd, Ca, and Mg, in comparison to lunar rocks [3-8], although the effect on Ca and particularly Mg are only minor. The mass-dependent isotopic variations of these elements are variably explained by: 1) ion sputtering [9]; 2) volatilization by micrometeorite impacts [3, 10]; and 3) redeposition of some sputtered or volatilized matter after gravitational fractionation [11]. The relative contributions of these processes to the isotopic fractionations are not well known. However, the abundance of nanophase Fe in lunar soils can provide signatures of some of these competitive processes. Analytical Methodology: We report here the first Fe isotope analyses of lunar mare regolith samples. These samples are from the studies of the Lunar Soil Characterization Consortium, where a selection of mare soils were sieved into sized fractions (<10 μm, 10-20 μm, and 20-45 μm) [1]. In this present study, six of these mare soils were analyzed for their Fe isotope compositions. In addition, the <10 μm size fraction of 79221 was partially leached with 1M HCl, and the leachate (63% of total Fe), and residue were analyzed for their Fe isotope compositions. All analyses were performed using the Univ. of WI IsoProbe [12-13]. Based on replicate analyses of samples processed through the entire analytical procedure two or more times, the average reproducibility for the Fe/Fe is ±0.05 ‰. Analyses are reported using delta notation, where: δFe = ([Fe/Fe] sample /[Fe/ Fe] Terr Ig rocks -1)10 and δFe = ([Fe/Fe] sample /[Fe/ Fe] Terr Ig rocks -1)10 [12]. On this scale the measured Fe isotope composition of the IRMM-014 Fe isotope standard was δFe = -0.09 ± 0.05 ‰ and δFe = -0.11 ± 0.07 ‰. Results: The Fe isotope composition of the various size fractions for the different soils are plotted in Figure 1 versus the I s /FeO values determined for each size fraction of each soil [1]. There is an overall positive correlation between the soil maturity index I s /FeO [14] and δFe. Figure 1. Plot of I s /FeO vs. δFe for lunar soils. All analyses are for bulk samples of the different size fractions. In addition, the <10μm size fraction of 79221 has been leached and the δFe values of the leachate and the residue are given. For comparison, the range for δFe for lunar basalts are shown in the lower left corner. Error bars indicate 1 SD for duplicate or triplicate analyses.