Lunar Meteorite: Sayh al Uhaymir 169
from The Meteoritical Bulletin, No. 87 Sayh al Uhaymir 169 (SaU 169)Oman Lunar meteorite (KREEP-rich mafic impact melt breccia and adhering regolith) History: A complete, light grey-greenish rounded stone (70 x 43 x 40 mm) weighing 206.45 g was found in the Sayh al Uhaymir region of Oman. Finders, mineralogy and classification: E. Gnos, B. A. Hofmann and A. Al-Kathiri (UniBern): Dark brown fusion crust is only locally preserved. The impact melt breccia (87 vol% of the stone, based on 8 X-ray tomographic sections) contains 25-40 vol% of strongly shocked magmatic rocks and crystal clasts (up to 17 mm) derived from norites, evolved magmatites, and granulites set in a fine-grained (<0.1 mm) crystalline matrix. Most crystal fragments are shocked plagioclases, locally associated with enstatite. In addition to plagioclase metallic iron, spinel, olivine, and orthopyroxene clasts are present. The fine-grained impact melt matrix consists mainly of short prismatic low-Ca pyroxene (En61-64,Wo2-4), interstitial plagioclase (An75-81) intergrown with potassium feldspar. The remaining minerals are poikilitic ilmenite, whitlockite, olivine (Fo58- 59), zircon, and traces of troilite, kamacite and tridymite. The regolith (13 vol.%) present one one side of the meteorite comprises crystalline and glassy volcanic rocks, magmatic lithic fragments, breccia fragments, fragments of mafic granulites, and crystal fragments. The impact melt breccia contains 32 ppm Th and 8.5 ppm U, 0.47% K (K/U= 553), indicating a lunar origin. This is further confirmed by fusion-crust Fe/Mn of 75.1 (microprobe, n=14) and impact melt bulk Fe/Mn of 79. Oxygen isotope composition (I. A. Franchi, OU) are also consistent with a lunar origin (∆17O = 0.001 ± 0.032‰). This impact melt breccia is the most strongly KREEP enriched lithology among all known lunar rocks. Weathering: W1 (Fe metal shows only little oxidation). Specimens: All in NMB. |
Randy Says… SaU 169 unique. The impact-melt lithology has the greatest concentrations of geochemically incompatible elements (K, P, Zr, REE, Th, U, etc.) of any lunar meteorite or any other kind of meteorite. Concentrations of incompatible elements are very high even compared to most Apollo samples. It is, however, compositionally indistinguishable from one type of impact-melt breccia from Apollo 12. The meteorite is the first for which the concentrations of naturally occurring thorium and uranium are so high that the radioactivity of the rock can be detected on a laboratory radiation survey meter (“Geiger counter”). [So as not to be too sensational, the radioactivity of most terrestrial granites and some other kinds of Earth rocks can also be detected with a radiation survey meter. Meteorites in general have less radioactivity than most Earth rocks.] It is an unusual dilithologic meteorite (two main rock types in one rock). It is the first lunar meteorite that must originate from the nearside feature known as the Procellarum KREEP Terrane. |
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