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Sample 60639 is a polymict fragmental breccia rake sample collected near the Apollo 16 lunar module and containing a variety of lithic clasts including mare basalt, anorthosite, and a variety of impact-melt breccias (poikilitic, aphanitic, and glassy [1]. A basalt clast from 60639 was examined by [2], who described the fragment as being chemically similar to subophitic Luna 16 basalts. A subsequent study [3] suggested that the major- and trace-element concentrations in the 60639 basalt clast indicated that it is similar to low alkali Apollo 11 and 17 high-Ti basalts. The basalt fragment 60639,2 is a coarse-grained mare basalt [4] with plagioclase laths ~300-500 microns in length [1] and is composed of approximately 5% olivine, 35% plagioclase, 5-10% ilmenite, and 50% pyroxene and accessory spinel [4]. The main objective of this study is to characterize newly discovered basalt clasts from 60639, as well as re-examine the previously studied 60639 basalt clast [3,4], with modern analytical methods. The chemical composition of ilmenite, pyroxene, olivine, and plagioclase phases were determined for three thin-sections from 60639, both from the breccia and basalt portions. Whole-rock major and trace elements were also quantified using solution mode ICP-OES and ICP-MS.
Individual crystal chemistry analyses show that plagioclase grains from different thin-sections have similar major element compositions. Plagioclase grains from the breccia portions have higher An% than the basalt portions with an average (97.9%) similar to the that of plagioclase from Apollo 16 impact melt (97.2%). Plagioclase cores in the breccia portions are more enriched in La/Sm than the rims, while the opposite is true for plagioclase found in the basalt portions. Pyroxene grains are predominately augite with a few being enstatitic in composition. Average composition of olivine grains from the basalt is 66.8% Fo and 70.4% Fo from the breccia. The presence of olivine within these samples is particularly intriguing as it is rarely found in high-Ti Lunar basalts.
Whole-rock results indicate that basalts from 60639 are similar in major-element composition (e.g. TiO2 and FeO) to Apollo 11 high-Ti basalts, which is consistent with the findings of [3]; the combination of chemical composition and textural data is largely distinct from other previously analyzed Apollo 16 basalts [5] indicating that basalts from the Apollo 16 site likely represent ejecta from several localities. Although major element data is similar to Apollo 11 high-Ti basalts, 60639 does not fall easily within any of the defined high-Ti basalt groups described by [6], but appears to bridge the gap between group A and the group B basalts. To further complicate the characterization, the general shape of the whole-rock REE profile of each clast, defined by the ratios Dy/Dy* and Dy/Yb normalized to chondrite, is similar to Apollo 12 basalts and not Apollo 11. All of the 60639 basalt clast whole-rock aliquots have similar chemical compositions to each other indicating that all three were likely ejected from the same locality. Whole rock data indicate that although there are similarities with Apollo 11 high-Ti basalts, the 60639 basalt clasts may represent a new type of high-Ti basalt not completely representative of Apollo 11 nor Apollo 12.
This work was funded through NASA Lunar Science Institute subcontract 02713-05.
References: [1] Ryder G and Norman M (1980), NASA Cur. Branch Pub. 52, JSC 16904. [2] Dowty E et al., (1974), PLPSC., 5th, 431-445. [3] Murali AV et al. (1976), LPSC, 7th, 583-584. [4] Delano JW (1975), PLPSC 6th, 15-47. [5] Zeigler R et al. (2006), MAPS, 41, 263-284. [6] Jerde EA et al. (1994), GCA, 58, 515-527.
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