Anomalous Plagioclase Compositions from Apollo 16 Impact Melts: Modification of Impact Melts on the Lunar Surface

Details Anomalous Plagioclase Compositions from Apollo 16 Impact Melts: Modification of Impact Melts on the Lunar Surface Anomalous Plagioclase Compositions from Apollo 16 Impact Melts: Modification of Impact Melts on the Lunar Surface

Dec 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p11i..06f&link_type=abstract

American Geophysical Union, Fall Meeting 2011, abstract #P11I-06

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[3672] Mineralogy And Petrology / Planetary Mineralogy And Petrology, [6250] Planetary Sciences: Solar System Objects / Moon

Scientific paper

A study of Apollo 16 impact melts has been initiated as part of the NASA Lunar Science Institute's Center for Lunar Science and Exploration. The investigation uses a novel approach to investigate the crystallization conditions for the igneous textured samples returned by Apollo 16. This approach uses a combination of crystal size distributions and mineral chemistry to develop a crystal stratigraphy approach to examine crystallization conditions. The Apollo 16 impact melts are enriched in Al2O3 (~25-28%) and are highly feldspathic with only minor amounts of pyroxene, olivine, and opaque minerals. Initial studies focused on sample 60635 where previous whole rock analyses show that, surprisingly this sample possesses a small negative Eu anomaly [1,2]. This is intriguing given the feldspathic nature of the sample (~70% modal plagioclase), which should impart a strong positive Eu anomaly on the whole rock signature. The major element compositions of the plagioclases are relatively homogeneous with a range of An94-98 and an average of An97.2. Plagioclase trace element compositions exhibit a much wider compositional range with some crystals having the predicted positive Eu anomaly but other showing an unexpected negative Eu anomaly. There are about the same number of crystals with a negative Eu anomaly as there are with a positive anomaly and both are equally distributed through the thin section (60635,2). Intriguingly, 5 plagioclase crystals from this sample contain both positive and negative Eu anomalies in different sections of the crystals. Subsequent analyses of other impact melt samples show that a further 4 samples from the LM/Station 10 site also contain plagioclase with both positive and negative Eu anomalies (60235,5; 60335,13; 60615,8; 60618,3 and ,4), along with one sample from Station 4 (64817,3). The hypothesis we are considering is that the impact melt(s) represented by these anomalous samples were generated by impact into typical lunar highlands materials and flowed across the lunar surface. Crystal stratigraphic studies show that crystallization began with the positive Eu anomaly plagioclases, but during this stage a KREEPy component with a marked negative Eu anomaly was assimilated and the new magma composition produced plagioclases with a negative Eu anomaly. [1] Deutsch, A. and D. Stöffler (1987) GCA, 51, 1951-1964. [2] Korotev, R.L. (1994) GCA, 58, 3931-3969.

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