Lunar Pyroxene-Phyric Basalts: Crystallization Under Supercooled Conditions

Pyroxene-phyric basalts constitute a distinct petrologic group in samples from lunar maria at both Apollo 12 and Apollo 15 sites. Textures of pyroxene-phyric basalts from both sites are similar, but bulk compositions and mineralogy are somewhat different. Pyroxene-phyric basalts are characterized by large pyroxene phenocrysts with cores of magnesian pigeonite and mantles of augite grading to ferroaugite, usually set in a distinctly finer groundmass of iron-rich pyroxene, plagioclase, ilmenite, and other minor minerals. Olivine is scarce or absent modally and normatively. Controversy has arisen over whether the porphyritic texture is a result of (1) a two-stage cooling history, e.g. phenocrysts formed at depth, and groundmass formed on extrusion; or (2) single stage, rapid cooling under supercooled conditions (cooling history here refers to cooling conditions imposed by external factors, and is not to be equated with ‘crystallization history’). A study of six rocks belonging to this group from Apollo 15 rake samples is reported here. A considerable range of textures is present in these rocks, and they may be ranked in order of decreasing late-stage cooling rate (15125, 15666, 15682, 15118, 15684, 15116) on the basis of groundmass crystal size. The same ranking is obtained from delta-beta (measured in X-ray precession photos) of pigeonite and augite exsolved from once homogeneous crystals, or of epitaxially overgrown augite and pigeonite. The size of the phenocrysts in these rocks tends to be positively correlated with the coarseness of the groundmass. Furthermore the same correlation is evidently present in almost all other Apollo 12 and Apollo 15 pyroxene-phyric rocks. This constitutes a strong argument in favor of a single-stage cooling history for all pyroxene-phyric rocks, because the correlation would be fortuitous for a two-stage cooling history. On the basis of this and many other arguments advanced previously and in this paper, it is concluded that all the pyroxene-phyric rocks originated in a single-stage cooling process. A crystallization model for pyroxene-phyric rocks accounts for the bimodal distribution of crystal size by two episodes of supercooling and nucleation during the continuous cooling process, the first of pyroxene, which nucleates homogeneously, the second of plagioclase, which nucleates heterogeneously on the pyroxene phenocrysts. The more rapidly cooled rocks attained greater degrees of supercooling in both stages, hence greater nucleation rates and smaller crystals.