Mathematics – Logic
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.284r&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 284
Mathematics
Logic
1
Scientific paper
A poikilitic impact melt rock fragment in a sample from Boulder 1, Station 2, is distinct and important: 1) no such poikilitic- textured melt rock has been observed in Boulder 1 before; 2) although superficially similar to typical coarse samples of the Serenitatis melt sheet, it is chemically and petrographically distinct from them. It extends the known range of significant low-K Fra Mauro impact melt rock compositions that potentially can provide us with information about lunar crustal components and structure. Lunar sample 72255 is a fine-grained aphanitic melt representing Boulder 1 on the South Massif. It contains many clasts, including granitic fragments, but more typically fine basaltic-composition impact melts, feldspathic breccias, and granulitic impactites. The aphanitic melt groundmass of 72255 is distinct in composition from typical Serenitatis impact melt samples, but even if it formed in a distinct event, the relative ages are radiometrically indistinguishable (at present). The discovery of a poikilitic impact melt rock fragment during new investigations of 72255 at first led me to conclude that Boulder 1 contained fragments of the Serenitatis melt sheet, hence must be slightly younger. However, a closer inspection and analysis reveals that this fragment is distinct from the Serenitatis melt sheet samples, thus stratigraphy cannot be inferred from it. The distinction of the fragment instead gives it a different potential significance, that of furthering our understanding of crustal components. The fragment was observed in saw-cut faces as a sharply defined pale brown clast embedded in the groundmass. It was about 7 mm in diameter, although not all was exposed. The fragment was crystalline, with dark brown pyroxenes, white plagioclases, and a pale yellow minor phase. Its texture and the presence of small vugs suggested an igneous rock, and it was chipped for chemical and thin section studies. The thin section revealed a clast-bearing impact melt rock with pyroxene oikocrysts, plagioclase chadacrysts, and ilmenite chains, very similar to Apollo 15, 16, and 17 coarser poikilitic impact melt samples. The chemistry of its mineral phases (both clasts and groundmass) is identical with Apollo 17 poikilitic boulder samples, e.g., pigeonite oikocrysts with a range of En82- 70. It does contain rather more high-Ca pyroxene oikocrysts than do the poikilitic boulders: pig/cpx of 2 rather than about 4. The oikocrysts, about a millimeter across, contain many plagioclase chadacrysts. They are normally zoned outward from their cores, with superimposed normal zoning toward each plagioclase chadacryst. The clasts are dominantly plagioclases, with some olivine and rare lithic fragments such as a feldspathic granulite. A small amount of olivine may have crystallized from the melt, but none forms oikocrysts. The INAA shows the melt rock fragment to be a little higher in Ca and a little lower in Fe than the poikilitic boulders. The main difference is the much lower abundance of all large ion lithophile elements in the new clast, e.g., Sm 8 ppm cf. 15 ppm in the poikilitic boulders. This result was confirmed by analysis of both a second chip and a second analysis of the first chip. The difference is far too great to result merely from a larger clast content in the new fragment; the chemistry suggests an entirely separate impact event, not a phase of Serenitatis. The higher modal clinopyroxene suggests the same. The melt rock fragment is also distinct from the few other unique Apollo 17 melts, such as 76055, or any of the Apollo 15 and 16 poikilitic melts. It extends the range of known low-K Fra Mauro samples, and gives us more potential to understand both the lunar crust, and Boulder 1. Its investigation will continue with more complete mineralogical and chemical data, an age determination, and a detailed comparison with all other known poikilitic melt samples.
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