Mineral Heterogeneity Of Lunar Sub-Milligram Basaltic Clasts And Its Effect On The Production Rates Of Cosmogenic Nuclides

Abstract

: Precise determination of the chemical composition of lunar samples is crucial for obtaining cosmogenic noble gas production rates and reliable cosmic ray exposure (CRE) ages. In this study, we established a new non-destructive method for determining the chemical composition of small mineralogically heterogeneous lunar basaltic clasts (<1 mg) using high-resolution X-ray microcomputed tomography (μCT). The volume of the individual mineral grains in each clast was obtained via μCT and combined with the chemical composition and density of the minerals to estimate the bulk chemical composition of each sample. The calculated chemical compositions were ultimately used to determine cosmogenic nuclide production rates. We used the lunar mare simulant sample (LMS-1) to evaluate the uncertainty of our method on the calculation of production rates of cosmogenic Ne ( 20 Ne, 21 Ne, and 22 Ne) and Ar ( 36 Ar and 38 Ar) (4% and 5% were adopted as suggested values, respectively). By applying this method to five Chang’E-5 basaltic clasts, we demonstrated that the chemical compositions of lunar regolith clastic samples (basalts) were different and the maximum variations of P 21 (the production rate of cosmogenic 21 Ne) and P 38 (the production rate of cosmogenic 38 Ar) among the five basaltic clasts were in the range of 18–20%. Therefore, the average chemical composition cannot be used to represent a single grain. In our study, mineral heterogeneity influenced the theoretical production rate of cosmogenic noble gases. Furthermore, the maximum cosmogenic 21 Ne production rate deviation from the average value reached 18.4% (~2 g/cm 2 ). Our method significantly minimized the uncertainties in the production rate calculations caused by the mineral heterogeneity of the sub-milligram samples and when applied routinely would result in more reliable CRE ages.