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MIL 07006 |
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Sayh al Uhaymir 449
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Shişr
160 |
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Shişr 162 |
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NWA 2998
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Dhofar 1442
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Three chemical parameters are useful
for first-order chemical classification of lunar meteorites: (1) The concentration of aluminum, usually reported as mass % Al2O3, (2) The ratio of magnesium to iron, usually reported as mass % MgO/FeO or magnesium number, which is the mole % MgO/[MgO+FeO], and (3) The concentration of any of the numerous "incompatible elements." These include P, K, Y, Zr, Nb, rare earth elements, Hf, Ta, Th, and U. We use Th (thorium) here because Th concentrations were measured from orbit on the lunar surface by the Lunar Prospector mission. A fourth parameter, the titanium TiO2 concentration is used to classify Apollo mare basalts. Thus far, however, all the basaltic lunar meteorites are have low concentrations of TiO2 compared to Apollo mare basalts. For the brecciated lunar meteorites, another useful parameter is the concentration of any of the highly siderophile (iron-loving) elements such as nickel, gold, and iridium. In lunar rocks, these elements derive almost entirely from asteroidal meteorites (e.g., chondrites) that have impacted the Moon. Regolith breccias that have high concentrations of siderophile elements also usually have high concentrations of solar-wind implanted gases. Such breccias consist of "mature" regolith, that is, much of the fine-grained material of the breccia was exposed at the lunar surface for a long time where it absorbed solar wind and was contaminated by impacting micrometeorites. Brecciated lunar meteorites with low concentrations of siderophile elements tend to be fragmental or impact-melt breccias that consist mainly of material that was deeper in the Moon and not so contaminated by micrometeorites. |
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Aluminum and calcium concentrations anticorrelate with iron and magnesium concentrations in lunar rocks. This correlation is a necessary mathematical consequence of the fact that only four minerals account for 98+% of the crystalline material on the lunar crust. Plagioclase carries essentially all of aluminum and most of the calcium whereas pyroxene, olivine, and ilmenite carry nearly all of the iron, magnesium, and titanium. Highlands rocks consist mainly of plagioclase, thus they are rich in aluminum. For most lunar rocks, dividing the %Al2O3 by 35.5% yields the normative plagioclase abundance. Mare basalts consist mainly of pyroxene, with some plagioclase, olivine, and ilmenite, thus they are rich in iron. All of the "mingled" lunar meteorites are breccias and most are mixtures of mare basalt and feldspathic breccias from the highlands. In this and other figures, lines connect points representing different lithologies in the multilithologic meteorites Dhofar 287, SaU 169, and NWA 773/2700/2727/2977/3160/3333. |
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The ratio of magnesium to iron is greater in highlands rocks than in mare basalts. Within either group, there is considerable variation in the ratio among the meteorites. The MgO/FeO ratio correlates with the olivine/pyroxene ratio. For example, among the feldspathic lunar meteorites, olivine is rare in the low-MgO/FeO meteorites but increases in abundance with increasing MgO/FeO. The olivine cumulate lithology of Northwest Africa 773 consists in large part of olivine. |
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Incompatible elements like thorium vary greatly among lunar meteorites (note logarithmic scale). A characteristic of lunar meteorites that distinguished some of them from any other type of meteorite is high concentrations of incompatible elements. We are unaware of any nonlunar meteorite that has more than 0.5 ppm Th. SaU 169 is neither from the maria nor the feldspathic highlands; it must derive from the Procellarum KREEP Terrane. |
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Distribution of thorium on the lunar surface. The Procellarum KREEP Terrane is the Th-rich area (>3.5 ppm) that is mainly left (west) and above (north) of the center of the map. |
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The ratio of samarium to europium is also also useful for distinguishing
among lunar meteorites. |
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