AN APOLLO-15 MARE BASALT FRAGMENT AND LUNAR MARE PROVINCES

Lunar sample 15474,4 is a tiny fragment of olivine-augite vitrophyre t hat is a mare basalt. Although petrographically distinct from all othe r Apollo 15 samples, it has been ignored since its first brief descrip tion. Our new petrographic and mineral chemical data show that the oli vines and pyroxenes are distinct from those in other basalts. The basa lt cooled and solidified extremely rapidly; some of the olivine might be cumulate or crystallized prior to extrusion. Bulk-chemical data sho w that the sample is probably similar to an evolved Apollo 15 olivine- normative basalt in major elements but is distinct in its rare earth e lement pattern. Its chemical composition and petrography both show tha t 15474,4 cannot be derived from other Apollo 15 mare basalts by shall ow-level crystal fractionation. It represents a distinct extrusion of magma. Nonetheless, the chemical features that 15474,4 has in common w ith other Apollo 15 mare basalts, including the high FeO/Sc, the gener al similarity of the rare earth element pattern, and the common (and c hondritic) TiO2/Sm ratio, emphasize the concept of a geochemical provi nce at the Apollo 15 site that is distinct from basalts and provinces elsewhere. In making a consistent picture for the derivation of all of the Apollo 15 basalts, both the commonalities and the differences amo ng the basalts must be explained. The Apollo 15 commonalities and diff erences suggest that the sources must have consisted of major silicate phases with the same composition but with varied amounts of a magma t rapped from a contemporary magma ocean. They probably had a high olivi ne/pyroxene ratio and underwent small and reasonably consistent degree s of partial melting to produce the basalts. These inferences may be i nconsistent with models that suggest greatly different depths of melti ng among basalts, primitive sources for the green glasses, or extensiv e olivine fractionation during ascent. An integrated approach to lunar mare provinces, of which the Apollo 15 mare basalts constitute only o ne, offers advances in our understanding of the physical and chemical processes of source formation and mare production but has so far not b een utilized.