A brief review of the petrology of lunar mare basalts and a case study

Of all the lunar rock types the basalts which occupy the low-lying mare plains have been studied the most. In general, the mare basalts are chemically classified as: high-Ti group and low Ti-group. Experimental petrologic studies indicate that the chemical variations in the Ti and K contents are due to partial melting of different materials at different depths of the lunar interior corresponding to pressures of approximately 5kb to approximately 20kb. The high-K variety of basalt tends to have an intersertal texture. While the low-K variety tends to be ophitic. Otherwise, the wide range of textures, from pyroxene vitrophyric to plagioclase-pikilitic, displayed by the mare basalts depends essentially on cooling history, not on composition. The cooling history of each rock, as well as the bulk chemical composition of its parent melt have left a definite signature in the chemical zoning of its rock forming minerals, pyroxenes in particular. One particular Apollo 11 high-Ti/Ow-K ophitic medium-grained basalt fragment is described with emphasis on the chemical zoning in pyroxenes. This rock appears to be the product of near-surface crystallization, probably after an earlier stage of olivine and chromite fractionation has increased the Al content of its parent melt. (author)