Abstract
Spinel-group minerals and ilmenite in 28 mare basalts (pyroxene-phyric basalts, olivine-phyric basalts, olivine microgabbros, feldspathic microgabbros, and feldspathic peridotites), 3 rocks of high-alumina basalt and 2 of ANT composition from the Apollo 15 rake samples were analyzed with an electron microprobe. In mare basalts chromite is usually early, euhedral, and often included within olivine. Later chromite often has irregular outlines (resorbtion), and is often mantled by ulvispinel, which also occurs as a groundmass mineral. Ilmenite crystallizesrelatively late. The compositional gap between chromite and ulvöspinel and the change in compositional trend is most pronounced in pyroxene-phyric basalts, smaller in olivine-phyricbasalts and olivine microgabbros, and absent in feldspathic peridotites; feldspathic micro-gabbro has only ulvöspinel. Ilmenite is least magnesian in pyroxene-phyric basalts, intermediate in olivine-phyric basalts and olivine microgabbros, and most magnesian in feldspathic peridotites, with the feldspathic microgabbro being variable. Three rocks of high-alumina basalt composition have either spinel, chromite, or ulvöspinel. Ilmenite is high in MgO in spinel- and chromite-bearing rocks, but low in the ulvöspinel-bearing rock. Rocks of ANT Composition have chromite, some with high Fe/(Fe + Mg) C2O3. in these chromites in-creases with increasing Fe/(Fe + Mg), in contrast to mare basalts, probably as the resultof simultaneous crystallization with early plagioclase, rather than with early olivine alone, as in mare basalts. The spinel-group mineral relationships in mare basalts are more satis-factorily explained by the pyroxene peritectic hypothesis than by the miscibility gap hypo-thesis; comparison of pyroxene zoning trends with spinel compositions in pyroxene-phyric rocks indicates the existence of a time gap between crystallization of chromite and ulvöspinel.Compositionally, spinel-group minerals and ilmenite can be correlated to the bulk composition of the melts from which they crystallized; high MgO content of ilmenite is not the resultof crystallization at high pressures.
