RE-THINKING OOS (ORTHOPYROXENE, OLIVINE, SPINEL) ASSEMBLAGE ON THE MOON: IMPLICATIONS FOR MINERALOGICAL DIVERSITY OF THE LUNAR CRUST

Remote sensing observations at Moscoviense basin led to the proposal of OOS (orthopyroxene, olivine, spinel) assemblage based on the discrete but proximal occurrence of lithologies dominated by these minerals [1]. OOS has been suggested to have originated via excavation of deep-seated differentiated plutons. We are building upon this existing knowledge and making a case for re-evaluation of the OOS assemblage based on our recent detection of OOS lithologies in the Ingenii-Thomson region, making it only the third such reported occurrence so far. Ingenii-Thomson OOS components are likely associated with the Thomson ejecta on Ingenii basin floor. We note that the spatial distribution of individual OOS components is significantly different and potentially provides clues to their source. Orthopyroxene is a common mineral of lunar highlands, and its pervasive distribution in and around impact basins potentially suggests Opx-dominated megaregolith exposures as the preferred source instead of plutonic outcrops, which would otherwise be required in large numbers. Olivine-dominated lithology is a minor component in and around the impact structures, especially compared to Opx and Spinel. With these limited exposures, the origin of olivine-dominated lithology seems uncertain, but few occurrences might result from localized crustal heterogeneity (basin excavations and recycling of small-scale plutons). The third OOS component, Mg-Spinel bearing lithology occurs as small outcrops and has a regional distribution at the OOS locations. Wall-rock alteration has been proposed to be a viable formation mechanism of this lithology which is quite distinct from formation through differentiated plutons [2].

The widely different spatial distribution of the OOS components does not support a common formation mechanism which may also be separated in space and time but juxtaposed to each other in the current geological setting. Hence, OOS lithologies may not have a genetic relationship. This has direct relevance for interpreting the mineralogical diversity of the lunar crust. Detection of additional OOS occurrences (or the lack of it) might provide further insights in this direction.

References: [1] Pieters et al. (2011) JGR, vol 116, E00G08. [2] Prissel et al. (2014) EPSL, 403, p. 144-156.