Abstract-Crystalline lunar spherules (CLS) from three thin sections of Apollo 14 regolith breccias (143 18,6; 14318,48 and 14315,20) have been examined. The objects have been classified and their abundances, size distributions, bulk compositions, and (where possible) plagioclase compositions determined. By number, 64% consist predominantly of very tine-grained equant plagioclase grains but can also contain larger (-50 pm) feldspar crystals (type X), while 22% contain plagioclase lathes in a fine-grained mafic mesostasis (type Y ) . Plagioclase in both spherule types displays bright yellow cathodoluminescence that is conspicuous among the blue CL of the normal feldspar of the breccias. Type Z spherules (5%) contain feldspar with blue CL and minor amounts of olivine and pyroxene. Type Q spherules (4%) contain feldspar with yellow CL but in a luminescent mesostasis (of quartz or feldspar?). A few spherules are mixtures of type Y and type X textures. Most type X spherules, and a few type Y spherules, have fine-grained opaque rims. Compound objects were also found and consist of two or more CLS that appear to have collided while still plastic or molten. The CLS are thought to be impact spherules that crystallized in free flight, their coarse textures suggesting fairly slow cooling rates (-< 1 "Us). The abundance of the CLS resembles that of chondrules in the CM chondrite Murchison, and their cumulative size-frequency distributions are very similar to those of the chondrules in several meteorite classes. The bulk compositions of the CLS do not resemble regoliths at any of the Apollo sites, including Apollo 14, or any of the common impact glasses, but they do resemble the bulk compositions of several lunar meteorites and the impact glasses they contain. The Apollo 14 site is located on a region containing Imbrium ejecta, and we suggest that the CLS derive from the Imbrium impact. Ballistic calculations indicate that only impact events of this size on the Moon are capable of producing melt spherules with the required free flight times and slow cooling rates. Smaller impacts produce glassy spherules and agglutinates. As has been pointed out many times, the CLS have many properties in common with meteoritic chondrules. While much remains unclear, difficulties with a nebular origin and new developments in chondrule chronology, studies of asteroid surfaces and impact ejecta behavior, and the present observations indicate that meteoritic chondrules could have formed by impact.
[1] Cathodoluminescence (CL) petrography provides a means of observing petrographic and compositional properties of geological samples not readily observable by other techniques. We report the low-magnification CL images of 60 sections of extraterrestrial materials. The images we report include ordinary chondrites (including type 3 ordinary chondrites and gas-rich regolith breccias), enstatite chondrites, CO chondrites and a CM chondrite, eucrites and a howardite, lunar highland regolith breccias, and lunar soils. The CL images show how primitive materials respond to parent body metamorphism, how the metamorphic history of EL chondrites differs from that of EH chondrites, how dark matrix and light clasts of regolith breccias relate to each other, how metamorphism affects eucrites, the texture of lunar regolith breccias and the distribution of crystallized lunar spherules (''lunar chondrules''), and how regolith working affects the mineral properties of lunar soils. More particularly, we argue that such images are a rich source of new information on the nature and history of these materials and that our efforts to date are a small fraction of what can be done.
Abstract. Chondritic meteorites appear to have undergone metal-silicate fractionations and chondrule size sorting that resulted in unique distributions for each chondrite class. We present evidence from laboratory experiments for size sorting of meteoritic chondrules and metal by fluidization during parent body degassing caused by evaporation and sublimation of water in the parent bodies. Experiments using chondritic analogs in an upward flowing stream of air indicate that metal can travel upward in a fluidized bed, resulting in a metal-rich surface and metal-poor regions at depth. When the outgassing velocity drops below the minimum fluidization velocity for chondrules, metal grains still suspended in the gas travel upward through the pore spaces between chondrules, provided the size ratio bernTeen chondmles and metal is >3. These experiments also suggest that chondmle size sorting can occur but with less efficiency than metal-silicate sorting. Fluidization can account for the sorting observed in enstatite chondrites, suggesting that only one parent body is required for EH and EL chondrites. Ordinary chondrite components are also sorte& with H chondrites deriving from locations near the surface of the parent body, while L and LL chondrites originate from greater regolith depths. The upward migration of metal to asteroid surfaces may help explain the reddened spectral features of asteroids likely to be the parent bodies of ordinary chondrites.
Abstract-We report induced thermoluminescence (TL) data for separates from three howardite, eucrite and diogenite (HED) meteorites and the Vaca Muerta mesosiderite. The results of thermal modeling of the surface of their parent body are also described. The TL sensitivities for matrix samples from the LEW 85300, 302 and 303 paired eucrites and the Bholghati howardite are lower than the TL sensitivities for the clasts, which is consistent with regolith working of the matrix in fairly mature regoliths. Within an isochemical series of HED meteorites, TL sensitivity reflects metamorphic intensity, but clast-to-clast variations in the TL sensitivities of the Vaca Muerta mesosiderite and clasts in the EET 87509, 5 13 and 53 1 paired howardite primarily reflect differences in mineralogy and petrology. Thermoluminescence peak temperatures indicate that all the components from the LEW 85300,302 and 303 paired eucrites experienced a reheating event involving temperatures >SO0 "C, which is thought to have been due to impact heating, and therefore that the event was concurrent with or postdated brecciation. The Vaca Muerta clasts are essentially unmetamorphosed, but the induced TL data indicate that the remaining howardite, eucrite, dioenite and mesosiderite (HEDM) meteorites experienced metamorphism to a variety of intensities but involving temperatures -400 "C. Laboratory heating experiments show that temperatures 3 0 0 "C cause a change in TL peak temperature. Feldspars from a variety of terrestrial and extraterrestrial sources show this behavior, and x-ray diffraction and kinetic studies suggest that it is indirectly related to AI,Si disordering.Cooling rates are not consistent with autometamorphism following the initial igneous event or with heating by subsequent eruptions of lava onto the surface of the HED parent body. Instead, our thermal models suggest that the metamorphism occurred within a regolith ejecta blanket of up to a few kilometers thick, with different levels of metamorphism corresponding to different thicknesses of blanket, between essentially 0 and -2 km, rather than different burial depths in a regolith of uniform thickness. We argue that metamorphism occurred 3.9 Ga ago and was associated with the resetting of the Ar-Ar system for the HED meteorites.
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