The regolith evolution of the lunar meteorites Dhofar (Dho) 081, Northwest Africa (NWA) 032, NWA 482, NWA 773, Sayh al Uhaymir (SaU) 169, and Yamato (Y-) 981031 was investigated by measuring the light noble gases He, Ne, and Ar. The presence of trapped solar neon in Dho 081, NWA 773, and Y-981031 indicates an exposure at the lunar surface. A neon three-isotope diagram for lunar meteorites yields an average solar 20Ne/22Ne ratio of 12.48 +/- 0.07 representing a mixture of solar energetic particles neon at a ratio of 11.2 and solar wind neon at a ratio of 13.8. Based on the production rate ratio of 21Ne and 38Ar, the shielding depth in the lunar regolith of NWA 032, NWA 482, SaU 169, and Y-981031 was obtained. The shielding depth of these samples was between 10.5 g/cm2 and >500 g/cm^2. Based on spallogenic Kr and Xe, the shielding depth of Dho 081 was estimated to be most likely between 120 and 180 g/cm^2. Assuming a mean density of the lunar regolith of 1.8 g/cm^3, 10.5 g/cm^2 corresponds to a depth of 5.8 cm and 500 g/cm^2 to 280 cm below the lunar surface. The range of regolith residence time observed in this study is 100 Ma up to 2070 Ma.

Previous studies of unmelted micrometeorites (>50 micrometers) recovered from Antarctic ice have concluded that chondrules, which are a major component of chondritic meteorites, are extremely rare among micrometeorites. We report the discovery of eight micrometeorites containing chondritic igneous objects, which strongly suggests that at least a portion of coarse-grained crystalline micrometeorites represent chondrule fragments. Six of the particles are identified as composite micrometeorites that contain chondritic igneous objects and fine-grained matrix. These particles suggest that at least some coarse-grained micrometeorites (cgMMs) may be derived from the same parent bodies as fine-grained micrometeorites. The new evidence indicates that, contrary to previous suggestions, the parent bodies of micrometeorites broadly resemble the parent asteroids of chondrulebearing carbonaceous chondrites.

We have evaluated various mechanisms proposed for the formation of the Widmanstätten pattern in iron meteorites and propose a new mechanism for low P meteoritic metal. These mechanisms can also be used to explain how the metallic microstructures developed in chondrites and stony-iron meteorites. The Widmanstätten pattern in high P iron meteorites forms when meteorites enter the three-phase field alpha + gamma + Ph via cooling from the gamma + Ph field. The Widmanstätten pattern in low P iron meteorites forms either at a temperature below the (alpha + gamma)/(alpha + gamma + Ph) boundary or by the decomposition of martensite below the martensite start temperature. The reaction gamma --> alpha + gamma, which is normally assumed to control the formation of the Widmanstätten pattern, is not applicable to the metal in meteorites. The formation of the Widmanstätten pattern in the vast majority of low P iron meteorites (which belong to chemical groups IAB-IIICD, IIIAB, and IVA) is controlled by mechanisms involving the formation of martensite alpha2. We propose that the Widmanstätten structure in these meteorites forms by the reaction gamma --> alpha2 + gamma --> alpha + gamma, in which alpha2 decomposes to the equilibrium alpha and gamma phases during the cooling process. To determine the cooling rate of an individual iron meteorite, the appropriate formation mechanism for the Widmanstätten pattern must first be established. Depending on the Ni and P content of the meteorite, the kamacite nucleation temperature can be determined from either the (gamma + Ph)(alpha + gamma + Ph) boundary, the (alpha + gamma)/(alpha + gamma + Ph) boundary, or the Ms temperature. With the introduction of these three mechanisms and the specific phase boundaries and the temperatures where transformations occur, it is no longer necessary to invoke arbitrary amounts of under-cooling in the calculation of the cooling rate. We conclude that martensite decomposition via the reactions gamma --> alpha 2 --> alpha + gamma and gamma --> alpha2 + gamma --> alpha + gamma are responsible for the formation of plessite in irons and the metal phases of mesosiderites, chondrites, and pallasites. The hexahedrites (low P members of chemical group IIAB) formed by the massive transformation through the reaction gamma --> alpha-m --> alpha at relatively high temperature in the two-phase alpha + gamma region of the Fe-Ni-P phase diagram near the alpha/(alpha + gamma) phase boundary.

Studies of several samples of the large Caddo County IAB iron meteorite reveal andesitic material enriched in Si, Na, Al, and Ca, which is essentially unique among meteorites. This material is believed to have formed from a chondritic source by partial melting and to have further segregated by grain coarsening. Such an origin implies extended metamorphism of the IAB parent body. New 39Ar-40Ar ages for silicate from three different Caddo samples are consistent with a common age of 4.50-4.51 Gyr. Less well-defined Ar-Ar degassing ages for inclusions from two other IABs, EET (Elephant Moraine) 83333 and Udei Station, are ~4.32 Gyr, whereas the age for Campo del Cielo varies considerably over about 3.23-4.56 Gyr. New 129I-129Xe ages for Caddo County and EET 83333 are 4557.9 +/- 0.1 Myr and 4557-4560 Myr, respectively, relative to an age of 4562.3 Myr for Shallowater. Considering all reported Ar-Ar degassing ages for IABs and related winonaites, the range is ~4.32-4.53 Gyr, but several IABs give similar Ar ages of 4.50-4.52 Gyr. We interpret these older Ar ages to represent cooling after the time of last significant metamorphism on the parent body and the younger ages to represent later 40Ar diffusion loss. The older Ar-Ar ages for IABs are similar to Sm-Nd and Rb-Sr isochron ages reported in the literature for Caddo County. Considering the possibility that IAB parent body formation was followed by impact disruption, reassembly, and metamorphism (e.g., Benedix et al. 2000), the Ar-Ar ages and IAB cooling rates deduced from Ni concentration profiles in IAB metal (Herpfer et al. 1994) are consistent if the time of the postassembly metamorphism was as late as about 4.53 Gyr ago. However, I-Xe ages reported for some IABs define much older ages of about 4558-4566 Myr, which cannot easily be reconciled with the much younger Ar-Ar and Sm-Nd ages. An explanation for the difference in radiometric ages of IABs may reside in combinations of the following: a) I-Xe ages have very high closure temperatures and were not reset during metamorphism about 4.53 Gyr ago; b) a bias exists in the 40K decay constants which makes these Ar-Ar ages approximately 30 Myr too young; degrees C) the reported Sm-Nd and Rb-Sr ages for Caddo are in error by amounts equal to or exceeding their reported 2-sigma uncertainties; and d) about 30 Myr after the initial heating that produced differentiation of Caddo silicate and mixing of silicate and metal, a mild metamorphism of the IAB parent body reset the Ar-Ar ages.

Numerous circular depressions north of Burghausen in eastern Bavaria, with diameters ranging from meters to tens of meters in size and dispersed over an area of at least 11 7 km, are suspected to have an extraterrestrial origin since they resemble other small meteorite impact craters. The depressions are bowl-shaped, have high circularity and a characteristic rim. Most of them were formed in unconsolidated glacial gravels and pebbles intermixed with fine-grained sand and clay. Magnetic investigations reveal weak anomalies with amplitudes of less than +/- 10 nanoTesla (nT). In some cases, the origins of the anomalies are suspected to be due to human activity within the structures. So far, no traces of meteoritic material have been detected. An evident archaeological or local geological explanation for the origin of the craters does not exist. A World War I and II explosive origin can be excluded since trees with ages exceeding 100 years can be found in some craters. One crater was described in 1909. Carbon-14 dating of charcoal found in one crater yielded an age of 1790 +/- 60 years. Hence, a formation by meteorite impacts that occurred in Celtic or early medieval times should be considered. A systematic archaeological excavation of some structures and an intensified search for traces of meteoritic material are planned.

We report the spectral reflectance of Martian meteorites from 0.3-2.6 microns for the purpose of cataloguing spectra and the association of their spectral properties with mineralogy and petrology. We fit the spectra to a series of overlapping, modified Gaussian absorptions using least squares fitting. The results are validated against established relationships between photon interactions with mineral chemistry and the band parameters. These resultant band parameters can be used to constrain interpretations of Martian reflectance spectra in the search for the source region of meteorites from Mars. The limitations of the fitting method are discussed.