Abstract
AbstractLunar mare basalts provide insights into the compositional diversity of the Moon's interior. Basalt fragments from the lunar regolith can potentially sample lava flows from regions of the Moon not previously visited, thus, increasing our understanding of lunar geological evolution. As part of a study of basaltic diversity at the Apollo 12 landing site, detailed petrological and geochemical data are provided here for 13 basaltic chips. In addition to bulk chemistry, we have analyzed the major, minor, and trace element chemistry of mineral phases which highlight differences between basalt groups. Where samples contain olivine, the equilibrium parent melt magnesium number (Mg#; atomic Mg/[Mg + Fe]) can be calculated to estimate parent melt composition. Ilmenite and plagioclase chemistry can also determine differences between basalt groups. We conclude that samples of approximately 1–2 mm in size can be categorized provided that appropriate mineral phases (olivine, plagioclase, and ilmenite) are present. Where samples are fine‐grained (grain size <0.3 mm), a “paired samples t‐test” can provide a statistical comparison between a particular sample and known lunar basalts. Of the fragments analyzed here, three are found to belong to each of the previously identified olivine and ilmenite basalt suites, four to the pigeonite basalt suite, one is an olivine cumulate, and two could not be categorized because of their coarse grain sizes and lack of appropriate mineral phases. Our approach introduces methods that can be used to investigate small sample sizes (i.e., fines) from future sample return missions to investigate lava flow diversity and petrological significance.
Highlights
Mare basalt samples provide us with information on the composition of the Moon’s upper mantle and partial melting history (e.g. Neal et al, 1994a; 1994b; Snyder et al, 1997; Shearer et al, 2006; Hallis et al, 2014)
In order to place our subsequent discussion in context, we briefly summarise what is known about the existing Apollo 12 basalt suites recognized in the literature
The equilibrium parent melt Mg# has been modelled from olivine compositions in the samples and the liquidus olivine Mg# has been predicted from the bulk compositions using the methods and equations described in many previous publications (e.g. Roeder and Emslie, 1970; Papike et al 1976; Dungan and Brown, 1977; Joy et al, 2008) and applying a distribution coefficient (Kd) of 0.33, applicable for lunar melts (Grove and Vaniman, 1978; Longhi et al, 1978)
Summary
Mare basalt samples provide us with information on the composition of the Moon’s upper mantle and partial melting history (e.g. Neal et al, 1994a; 1994b; Snyder et al, 1997; Shearer et al, 2006; Hallis et al, 2014). By examining the petrology and geochemistry of lunar basalts, and dating the samples studied (e.g. Nyquist and Shih, 1992), we can learn about the composition and heterogeneity of the lunar mantle, and the evolution of lunar volcanism over time. Sample 12030,187 consists of a single basaltic fragment (2.4 × 2.3 mm) sourced from an immature soil sample (maturity index Is/FeO = 14: Morris, 1978), which is mainly composed of pale breccia fragments, possibly from a large breccia outcrop in the vicinity (McKay et al, 1971). This soil sample was collected near Head crater (Supplementary Information S2), but the exact collection location is not known (Meyer, 2011). Repeatability of the NIST 610 standard measurements over all measurement sessions has a total relative standard deviation range of between 0.01 and 0.05% for all elements analysed and was typically
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