Cooling Regimes of Apollo 11 High-Ti Basalts - Insights from Quantitative Textural Analysis of Ilmenite and Plagioclase

Introduction: Determining a crystal size distribution (CSD) is a quantitative textural analysis that can provide critical information about crystallization histories of magmas. Previous studies have suggested that ilmenite was an early and prolonged crystallizing phase in lunar high-Ti basalts [1-3], which can be a good choice for studying and comparing the evolution history of different high-Ti basalts [4,5]. We present an ilmenite and plagioclase CSD analysis of Apollo 11 (A-11) highTi lunar basalts in order to investigate the cooling regimes of different chemical groups. 12 thin sections of A-11 high-Ti basalts (Fig. 1) representing 5 different geochemical groups [6] were analyzed: 5 from Group A, 3 from Group B1, 1 from Group B2, 2 from Group B3, and 1 from Group U (Unclassified). Preliminary CSD analyses of ilmenite and plagiocalse from these samples highlighted the variability of textures and thus cooling rates among different A-11 high-Ti basalts [7,8]. Method: The method and principle of CSDs are described in [9]. To summarize, each thin section was photographed using a 4x objective in plain-polarized (ppl), cross-polarized (xpl), and reflected light (rl), and these images were stitched together using Microsoft Image Composite Editor to create a digital photomosaic (Fig.1). Using Corel Paintshop Pro, ilmenite, plagioclase and the whole sample were traced and filled in and exported as individual files. For ilmenite tracing, rl images were preferred, while for plagioclase tracing, both ppl and xpl images were used. Crystals touching the edge of the section were not included as they may have not been whole crystals. In cases where crystals are overlapping, multiple tracing layers were necessary, as a single layer would erroneously record these crystals as one. The filled-in layers (for crystals & the sample area) were imported into ImageJ [10]. ImageJ does a pixel analysis and applies a best-fit ellipse to each crystal determining the major and minor axes, roundness, and precise area of each crystal (and the whole sample), from which modal percentages of ilmenite and plagioclase can be determind. Crystals with minor axes <0.03 mm were not included in the final analysis; they are more likely to be a projection of the crystal due to the thickness of the section [9,11]. Major and minor axes of the remaining crystals are exported to CSDSlice [12], and all data to CSDCorrections [13]. The former determines the approximate shape of the 2D traced crystals in 3D. The latter sorts the crystals based on major axis length and plots these in size bins (5 bins per decade) based on the number of crystals present vs. the natural log (ln) of population density. All sections were traced for ilmenite, however, plagioclase in two fine-grained Group A samples 10049 and 10069 were too tiny to be identified and traced (Figs. 1-3). Results: In most CSDs, there were downturns at the smallest size bins (Fig. 2) because of the low resolution limit of the tracing process. CSD slopes and intercepts were calculated using size bins for which population density increased with decreasing size (Figs. 2,3), and for plagioclase, only size bins >0.3 mm were used for slope and intercept calculation [9]. The largest size bins of these CSDs are excluded for calculation because of their relatively large errors.