Quantitative textural analysis of ilmenite in Apollo 17 high-titanium mare basalts

Quantitative textural analysis is a powerful tool in the investigation of basalt crystallization. We present the first comprehensive crystal size distribution analysis of Apollo 17 high-titanium lunar basalts, with a focus on ilmenite. Crystal size distributions of ilmenite, pyroxene, plagioclase, olivine and armalcolite were determined for 18 high-Ti mare basalt samples from the Apollo 17 mission. A subset of the ilmenite size distribution (size bins of <0.6 mm in length) reflects growth at post-eruption or post-emplacement cooling rates. Growth of these small ilmenite crystals is controlled by cooling rate and not bulk composition or ilmenite abundance. CSD characteristics tied to cooling rate determined by experiments yield estimates of cooling rate in natural samples. Matrix ilmenite grew at rates up to 250 °C/h, while most samples contained phenocrysts that originated in environments cooling at <3 °C/h. Textural characteristics of ilmenite phenocrysts are used to develop a relative stratigraphy for the samples within a lava flow based upon comparisons with terrestrial analogues.