LUNAR MANTLE HETEROGENEITY AND THE APOLLO MARE BASALTS: EXAMPLES FROM APOLLOBASALTDB

Mare basalts contain information on the equilibrium and kinetic environments of the lunar interior through their geochemical, petrological, geochronological, and textural characteristics. On geologic timescales representing the majority of lunar volcanism (~3.9 to ~ 3.0 Ga), both environments are likely tied to a heterogeneous mantle. The origin of this heterogeneity is hypothesized to have occurred before complete solidification of the lunar magma ocean (~4.5 to 4.3 Ga) whereby a shallow, high-Ti cumulate layer trapped between the upper anorthositic crust and the underlying cumulate mantle induced a large-scale mixing event due to gravitational instability. As a consequence, mare basalt characteristics should reflect source heterogeneities although the geochemical, spatial, and temporal natures of the heterogeneities are not well understood. To investigate mantle heterogeneity, we introduce the exploration of mare basalt characteristics contained in ApolloBasaltDB, a freely available and periodically updated Apollo mare basalt database that currently contains major element oxides, textures, isotopic ages, and mineral modes from published literature. Employing multidimensional numerical and statistical approaches as well as simple bivariate analyses, we compare mare basalt patterns against remote-sensing observations (gravity and reflectance imaging) to revisit the evolution of lunar mantle heterogeneity and related source magmas. Early results suggest that Apollo 11 and 17 mare basalts (mostly very-high-Ti), located proximal to the Procellarum KREEP Terrane rift system perimeter but on opposing sides of Mare Tranquillitatis, show similar olivine-to-pyroxene ratios as well as similar average Ti-enrichment (~10-12 wt. % TiO₂) and may reflect localized mantle mixing dynamics that operated ~3.7 Ga. This timing is consistent with a minimum crater-size-frequency-distribution age of ~3.5 Ga for the Procellarum KREEP Terrane rift system. Mare basalts from Apollo 12 and 15 sites (mostly low-Ti) are located distal to Procellarum KREEP Terrane rift system perimeter and show different olivine-to-pyroxene ratios, suggesting locally dissimilar fractionation behavior. Isotopic ages constrain Apollo site 12 and 15 basalts to ~3.4 to 3.2 Ga, which places the timing of crystallization events near the minimum rift age. Age and geochemical/mineralogical diversity in mare basalts may be linked to mantle mixing dynamics as a function of distance from the rift perimeter.