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In this study we calculate and compare trace element concentrations in parental liquids of basalts from the Apollo 12 landing site. The Apollo 12 collection is known to have sampled several basaltic units based on geochemical analyses; the Olivine, Pigeonite, Ilmenite, and Feldspathic Suite basalts. Based on crater excavation depths and sample placement on the lunar surface, Rhodes et al. (1977, PLSC 8th, 1305-1338) determined that the Pigeonite Suite is older and underlies the Ilmenite Suite and that both flows were about 40 meters thick. They also calculated the parental liquids to those Suites by performing whole rock geochemical modeling of major and trace elements on the most fine-grained vitrophyric sample, taken to represent the 'frozen' parental magma. They found that geochemical differences among samples in the individual suites could be accounted by varying amounts of olivine and pyroxene removal and addition.
Neal et al. (1994) expanded the work of Rhodes et al. (1977) by including many more trace elements and additional samples into the model. Neal et al. (1994) also used the most fine-grained/vitrophyric samples to represent parental liquids of each individual Suite. Both of these studies utilized whole rock analyses, which represent an average of all phases in a sample. We calculate parental liquids of the various suites using a novel approach which utilizes large primitive zoned pyroxene crystals to back-calculate parental liquids. This method provides verification of the parental liquids previously calculated by Neal et al. (1994) and Rhodes et al. (1977). Because of the necessity of pyroxene phenocrysts, we only analyze the Pigeonite, Ilmenite, and Feldspathic basalts. Since our study does not require a vitrophyric sample to represent the parental liquid, we can calculate a parental liquid of each individual basalt. This allows for the comparison of individual parental liquids within the same basalt Suite.
Parental liquids are calculated by first identifying primitive pyroxene crystals with high Mg numbers and low incompatible element concentrations. We use a JEOL JXA 8200 electron microprobe to obtain major element concentrations of individual zones within pyroxene crystals. Next, we analyze trace element concentrations of those zones using a New Wave Research Nd-YAG Laser Ablation system combined with an Element 2 Inductively Coupled Plasma Mass Spectrometer. Analysis of individual spots within pyroxene crystals allows for easy identification of primitive crystals/crystal cores. After trace element concentrations of the most primitive crystals/crystal cores are identified, an appropriate partition coefficient is needed to calculate equilibrium liquids. Trace element partition coefficients are assigned to individual crystals based on similarities in major element geochemistry between the crystal being analyzed and the crystal from which the partition coefficient was calculated. We feel this method is more robust then simply picking a partition coefficient from the list provided on earthref.org or using the outdated values in Snyder et al. (1992, GCA, 56, 3809-3823) that were supposedly appropriate for lunar conditions, although no details are given as to how they were chosen. With an appropriate partition coefficient value, equilibrium liquids are calculated by dividing concentration by the partition coefficient.
We compare our calculated liquids to those calculated by Neal et al. (1994) and Rhodes et al. (1977). Our initial results for the Ilmenite Suite basalts indicate light Rare Earth Element (REE) concentrations are similar to those of Neal et al. (1994) and heavy REE are slightly more enriched than those caluculated by Neal et al. (1994). Ilmenite Suite samples 12056 and 12063 appear to be the most evolved (i.e. highest incompatible trace element concentrations) while samples 12062 and 12064 appear to be the most primitive (i.e. lowest compatible trace element concentrations). Results for the Pigeonite and Feldspathic Suites will be presented at the Lunar Science Forum.
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