The non‐trivial problem of meteorite pairing

Abstract— Pairing is the procedure of identifying fragments of a single meteorite fall (that were separated during atmospheric passage or during terrestrial history) by establishing the similarity of two or more meteorite fragments. We argue that pairing is governed by two principles, that only a single mismatch of properties is required to refute a proposed pairing, and that virtually all pairings bear some degree of uncertainty. Using data distributions for modern falls, we take a probability approach to estimate degrees of certainty associated with proposed pairings, emphasizing the importance of unusual features. For new pairing criteria or new analytical additions to old criteria, the degree of variation within individual meteorites must be delineated and the degree of variation within meteorite classes must be quantified. Criteria for pairing can be divided into (1) parent body history indicators, (2) meteoroid space history indicators, and (3) terrestrial history indicators. Included in these categories are 11 specific criteria, including petrographic textures, mineralogy and mineral composition, terrestrial age estimates, cosmic‐ray exposure ages, and natural thermoluminescence (TL) levels. Not all criteria are applicable to all meteorite types. About 2275 pairings suggested in the literature have been subjected to this analysis. Many literature pairings, especially those involving common meteorite types, bear large uncertainties due to lack of data.     

Meteorite fusion crust variability

Abstract— Two assumptions commonly employed in meteorite interpretation are that fusion crust compositions represent the bulk‐rock chemistry of the interior meteorite and that the vesicles within the fusion crust result from the release of implanted solar wind volatiles. Electron microprobe analyses of thin sections from lunar meteorite Miller Range (MIL) 05035 and eucrite Bates Nunataks (BTN) 00300 were performed to determine if the chemical compositions of the fusion crust varied and/or represented the published bulk rock composition. It was determined that fusion crust compositions are significantly influenced by the incorporation of fragments from the substrate, and by the composition and grain size of those minerals. Because of compositional heterogeneities throughout the meteorite, one cannot assume that fusion crust composition represents the bulk rock composition. If the compositional variability within the fusion crust and mineralogical differences among thin sections goes unnoticed, then the perceived composition and petrogenetic models of formation will be incorrect. The formation of vesicles within these fusion crusts were also compared to current theories attributing vesicles to a solar wind origin. Previous work from the STONE‐5 experiment, where terrestrial rocks were exposed on the exterior of a spacecraft heatshield, produced a vesicular fusion crust without prolonged exposure to solar wind suggesting that the high temperatures experienced by a meteorite during passage through the Earth's atmosphere are sufficient to cause boiling of the melt. Therefore, the assumption that all vesicles found within a fusion crust are due to the release of implanted volatiles of solar wind may not be justified.     

Modal abundances of pyroxene,olivine, and mesostasis in nakhlites: Heterogeneity,variation, and implications for nakhlite emplacement

Nakhlites, clinopyroxenite meteorites from Mars, share common crystallization and ejection ages, suggesting that they might have been ejected from the same place on Mars by the same ejection event (impact) and are different samples of the same thick volcanic flow unit or shallow sill. Mean modal abundances and abundance ranges of pyroxene, olivine, and mesostasis vary widely among different thin‐sections of an individual nakhlite. Lithologic heterogeneity is the main factor contributing to the observed modal‐abundance variations measured in thin‐sections prepared from different fragments of the same stone. Two groups of nakhlites are distinguished from one another by which major constituent varies the least and the abundance of that constituent. The group consisting of Nakhla, Lafayette, Governador Valadares, and the Yamato nakhlite pairing group is characterized by low modal mesostasis and pyroxene‐olivine covariance, whereas the group consisting of the Miller Range nakhlite pairing group and Northwest Africa 5790 is characterized by low modal olivine and pyroxene‐mesostasis covariance. These two groups sample the slowest‐cooled interior portion and the chilled margin, respectively, of the nakhlite emplacement body as presently understood, and appear to be also related to recently proposed nakhlite groups independently established using compositional rather than petrographic observations. Phenocryst modal abundances vary with inferred depth in the nakhlite igneous body in a manner consistent with solidification of the nakhlite stack from dynamically sorted phenocryst‐rich magmatic crystal‐liquid mush.     

The Bur-Gheluai Meteorite (Mineralogy,Structure and Metamorphism)

Abstract The principal data about the fall and the distribution of the fragments of the Bur-Gheluai, Bur Acaba, Somalia (CN = EO436,024) meteorite are collected. A complete individual, weighing 744.2 g, preserved in the Geological Institute, Agricultural Dept., University of Perugia, Italy, is described in some detail. Crust morphology, mineralogical composition and structure are studied. Optical data are established by microscopical analysis of eight thin sections and two polished surfaces. Compared with electron-probe analysis, they are found in good agreement. Bur-Gheluai is an holocrystalline, olivine-bronzite chondrite, with evident features of recrystal-lization and metamorphism.     

THE BORGO SAN DONINO METEORITE: MINERALOGY AND CHEMISTRY

The principal data are collected about the fall, April 19, 1808, and the distribution of the fragments of the Borgo San Donino, Italy, meteorite (long. 44° 52′ N.; lat. 11° 03′ E.). An Almost complete individual, weighing 447 g is described in some detail. Crust morphology, mineralogical composition, chemical composition and petrology were studied. Optical data were established by microscopic analysis of both thin and polished sections. The Borgo San Donino is an ordinary LL-group chondrite, with a fundamentally hypochondritic texture and evident features of recrystallization and metamorphism. The presence of some anomalous chondrules is discussed     

The Puerto Lápice eucrite

Abstract— Puerto Lápice is a new eucrite fall (Castilla‐La Mancha, Spain, 10 May 2007). In this paper, we report its detailed petrography, magnetic characterization, mineral and bulk chemistry, oxygen and noble gas isotope systematics, and radionuclide data. Study of four thin sections from two different specimens reveal that the meteorite is brecciated in nature, and it contains basaltic and granulitic clasts, as well as recrystallized impact melt and breccia fragments. Shock veins are ubiquitous and show evidence of at least three different shock events. Bulk chemical analyses suggest that Puerto Lápice belongs to the main group of basaltic eucrites, although it has a significantly higher Cr content. Oxygen isotopes also confirm that the meteorite is a normal member of the HED suite. Noble gas abundances show typical patterns, with dominant cosmogenic and radiogenic contributions, and indicate an average exposure age of 19 ± 2 Ma, and a Pu‐fission Xe age well within typical eucrite values. Cosmogenic radionuclides suggest a preatmospheric size of about 20–30 cm in diameter.     

New Chilean iron meteorites: Medium octahedrites from Northern Chile are unique

Abstract— We report data on three new iron meteorites from Northern Chile and propose names. All are unnamed iron meteorites from the meteorite collection at the Universidad de La Serena. For two, the provenance is unknown; for the third, the presumed discovery site is in the countryside east of Iquique. The three meteorites have been analyzed by instrumental neutron activation analysis (INAA) and their structures examined with a binocular microscope. La Serena is a complete 663 g iron, a new member of group IIICD; it is not paired with any other iron. Elqui has a mass of 260 g; two faces are fractures, possibly produced by human actions, but fusion crust appears to be present on some of the remaining surface. It is a hexahedrite and a member of group IIAB, but its composition differs from that of all other Chilean hexahedrites. The third iron, Pozo Almonte, is a medium octahedrite member of group IIIAB, one of the most common meteorite groups. To find out whether it is paired, we assembled a full set of IIIAB iron meteorites from Northern Chile. Our compositional data show that Pozo Almonte is not paired with any other IIIAB iron, and that there are no pairings within the full set with the possible exception of Joel's Iron and Sierra Sandon, which differ only in their contents of Ir, 0.39 and 0.34 μg/g, respectively. However, Buchwald's (1975) structural observations rule out this possible pairing. We find appreciable differences in Cu, As and Au between the previously paired IIIAB irons Chañaral and Ilimaës and conclude that these should not be paired.     

Determination of bulk density for small meteorite fragments via visible light 3‐D laser imaging

Abstract– Bulk density is an important intrinsic property of meteorites, but the necessary bulk volume measurement is difficult to do in a truly nondestructive way. Archimedean methods involving the displacement of a 40–100 μm beads “fluid” are commonly applied, but can encounter systematic errors. Herein, we report a visible light laser imaging technique for the nondestructive measurement of meteorite surface features, allowing for the subsequent assembly of 3‐D volumetric models; the method is particularly applicable to small meteorite fragments and to fragile specimens. We have acquired laser image data for 24 fragments from 18 ordinary chondrites, carbonaceous chondrites, and achondrites, with masses ranging from 265.0 to 1.2 g. Laser imaging bulk density is consistent between sister fragments of meteorites down to sizes of about 0.5 cm³, an order of magnitude smaller than can be reliably measured with Archimedean beads techniques. Uncertainty is less than 2% for fragments >4 cm³, and typically between 2 and 4% for small fragments <4 cm³. For 10 fragments, 3‐D laser imaging volumes are on average 1.3% smaller than those obtained with Archimedean beads. In a wider comparison using 21 meteorite fragments, 3‐D laser imaging bulk densities are on average 2.14 ± 2.36% greater than the corresponding Archimedean method literature values for these meteorites. Difficulties in the procedure of 3‐D image alignment may lead to a slight overestimation of meteorite bulk density, and so laser imaging‐based bulk densities are maximum estimates that can be viewed as being complementary to the minimum bulk density estimates obtained using Archimedean beads methods.     

Reclassification of Villalbeto de la Peña—Occurrence of a winonaite‐related fragment in a hydrothermally metamorphosed polymict L‐chondritic breccia

The Villalbeto de la Peña meteorite that fell in 2004 in Spain was originally classified as a moderately shocked L6 ordinary chondrite. The recognition of fragments within the Villalbeto de la Peña meteorite clearly bears consequences for the previous classification of the rock. The oxygen isotope data clearly show that an exotic eye‐catching, black, and plagioclase‐(maskelynite)‐rich clast is not of L chondrite heritage. Villalbeto de la Peña is, consequently, reclassified as a polymict chondritic breccia. The oxygen isotope data of the clast are more closely related to data for the winonaite Tierra Blanca and the anomalous silicate‐bearing iron meteorite LEW 86211 than to the ordinary chondrite groups. The REE‐pattern of the bulk inclusion indicates genetic similarities to those of differentiated rocks and their minerals (e.g., lunar anorthosites, eucritic, and winonaitic plagioclases) and points to an igneous origin. The An‐content of the plagioclase within the inclusion is increasing from the fragment/host meteorite boundary (approximately An₁₀) toward the interior of the clast (approximately An₅₂). This is accompanied by a successive compositionally controlled transformation of plagioclase into maskelynite by shock. As found for plagioclase, compositions of individual spinels enclosed in plagioclase (maskelynite) also vary from the border toward the interior of the inclusion. In addition, huge variations in oxygen isotope composition were found correlating with distance into the object. The chemical and isotopical profiles observed in the fragment indicate postaccretionary metamorphism under the presence of a volatile phase.     

Description of a very dense meteorite collection area in western Atacama: Insight into the long‐term composition of the meteorite flux to Earth

We describe the geological, morphological, and climatic settings of two new meteorite collections from Atacama (Chile). The “El Médano collection” was recovered by systematic on‐foot search in El Médano and Caleta el Cobre dense collection areas and is composed of 213 meteorites before pairing, 142 after pairing. The “private collection” has been recovered by car by three private hunters and consists of 213 meteorites. Similar to other hot desert finds, and contrary to the falls and Antarctica finds, both collections show an overabundance of H chondrites. A recovery density can be calculated only for the El Médano collection and gives 251 and 168 meteorites larger than 10 g km^?2, before and after pairing, respectively. It is by far the densest collection area described in hot deserts. The Atacama Desert is known to have been hyperarid for a long period of time and, based on cosmic‐ray exposure ages on the order of 1–10 Ma, to have been stable over a period of time of several million years. Such a high meteorite concentration might be explained invoking either a yet unclear concentration mechanism (possibly related to downslope creeping) or a previously underestimated meteorite flux in previous studies or an average terrestrial age over 2 Myr. This last hypothesis is supported by the high weathering grade of meteorites and by the common terrestrial fragmentation (with fragments scattered over a few meters) of recovered meteorites.     

The classification of micrometeorites

Abstract— Due to their small size, the mineralogical and chemical properties of micrometeorites (MMs) are not representative of their parent bodies on the centimeter to meter scales that are used to define parent body groups through the petrological study of meteorites. Identifying which groups of MM are derived from the same type of parent body is problematic and requires particles to be rigorously grouped on the basis of mineralogical, textural, and chemical properties that reflect the fundamental genetic differences between meteorite parent bodies, albeit with minimal bias towards preconceived genetic models. Specifically, the interpretation of MMs requires a rigorous and meaningful classification scheme. At present the classification of MMs is, however, at best ambiguous. A unified petrological‐chemical classification scheme is proposed in the current study and is based on observations of several thousand MMs collected from Antarctic ice.     

The densest meteorite collection area in hot deserts: The San Juan meteorite field (Atacama Desert,Chile)

Abstract– We describe the geological, morphological, and climatic setting of the San Juan meteorite collection area in the Central Depression of the Atacama Desert (Chile). Our recovery activities yielded 48 meteorites corresponding to a minimum of 36 different falls within a 3.88 km² area. The recovery density is in the range 9–12 falls km^?2 depending on pairing, making it the densest among meteorite collection areas in hot deserts. This high meteorite concentration is linked to the long‐standing hyperaridity of the area, the stability of the surface pebbles (> Ma), and very low erosion rates of surface pebbles (approximately 30 cm Ma^?1 maximum). The San Juan meteorite population is characterized by old terrestrial ages that range from zero to beyond 40 ka, and limited weathering compared with other dense collection areas in hot desert. Chemical weathering in San Juan is slow and mainly controlled by the initial porosity of meteorites. As in the Antarctic and other hot deserts, there is an overabundance of H chondrites and a shortage of LL chondrites compared with the modern falls population, suggesting a recent (< few ka) change in the composition of the meteorite flux to Earth.     

Multi-technique investigation reveals new mineral, chemical, and textural heterogeneity in the Tagish Lake C2 chondrite

The Tagish Lake meteorite, an ungrouped C2 chondrite that is related to CI and CM chondrites, is a heterogeneous accretionary breccia with several distinct lithologies that, in bulk, are thought to represent the first known sample of a primitive carbonaceous D-type asteroid. Textural and chemical zoning of clasts and matrix have been little studied and promise additional insight into early solar system processes in both the solar nebula and on the Tagish Lake parent asteroid. We have examined an intact 2.9 g fragment and two polished thin sections from the spring 2000 (non-pristine) Tagish Lake collection to ascertain the major mineralogy and textures of notable features such as chondrules, amoeboid olivine aggregates (AOAs), inclusions, clasts, matrix, and fusion crust. We designed three stages of analysis for this friable meteorite: an initial, non-destructive in situ reconnaissance by μXRD to document meteorite mineralogy and textures and to identify features of interest, followed by spatially correlated μXRD, SEM-EDX and colour SEM-CL analysis of polished thin sections to fully understand mineralogy and the record of texture development, and finally higher resolution SEM-BSE mapping to document smaller scale relationships.Our analyses reveal several previously unreported or poorly characterized features: (1) distinctive colour cathodoluminescence (CL) zoning in relict CAI spinel, in chondrule and AOA forsterite, and in calcite nodules occurring throughout the Tagish Lake matrix. Forsterite frequently shows CL colour and intensity zonation that does not correspond with major or minor element differences resolvable with EPMA, indicating a trace element and/or structural CL-activation mechanism for the zonation that is likely of secondary origin; (2) an irregular inclusion dominated by magnesioaluminate spinel, dolomite, and phyllosilicates with traces of a Ca, Ti oxide phase (likely perovskite) interpreted to be a relict CAI; (3) variable preservation of mesostasis glass in porphyritic olivine chondrules. We anticipate that our multi-technique methodology, particularly non-destructive μXRD, can be successfully applied to other rare and friable materials such as the pristine Tagish Lake fragments.     

A NEW SPECIMEN OF THE MOUNT DOOLING IRON METEORITE FROM MOUNT MANNING,WESTERN AUSTRALIA

A 701 kg iron meteorite has recently been discovered near the Mount Manning Range in Western Australia. The meteorite has a fan-shaped or delta wing configuration, one side being smooth and slightly concave with a well-defined fusion crust, whilst the other side is rough, convex and possesses numerous regmaglypts. It is probable that the meteorite pentrated much of the earth's atmosphere in an aerodynamically stable orientation, typical of the stalled attitude of delta wing aircraft. The meteorite is a member of Chemical Group 1C. A comparison of the chemical composition, surface features, microstructure and location of this meteorite with the Mount Dooling meteorite confirms that the find is a larger specimen of Mount Dooling. It is possible that other fragments of the Mount Dooling meteorite may be found in the Mount Manning Range region.     

The accumulation rate of meteorite falls at the Earth's surface: The view from Roosevelt County,New Mexico

Abstract— The discovery of 154 meteorite fragments within an 11 km² area of wind-excavated basins in Roosevelt County, New Mexico, permits a new calculation of the accumulation rate of meteorite falls at the Earth's surface. Thermoluminescence dating of the coversand unit comprising the prime recovery surface suggests the maximum terrestrial age of the meteorites to be about 16.0 ka. The 68 meteorite fragments subjected to petrological analyses represent a minimum of 49 individual falls. Collection bias has largely excluded carbonaceous chondrites and achondrites, requiring the accumulation rate derived from the recovered samples to be increased by a factor of 1.25. Terrestrial weathering destroying ordinary chondrites can be modelled as a first-order decay process with an estimated half-life of 3.5 ± 1.9 ka on the semiarid American High Plains. Having accounted for the age of the recovery surface, area of field searches, pairing of finds, collection bias and weathering half-life, we calculate an accumulation rate of 9.4 × 10² falls/a per 10⁶ km² for falls > 10 g total mass. This figure exceeds the best-constrained previous estimate by more than an order of magnitude. One possible reason for this disparity may be the extraordinary length of the fall record preserved in the surficial geology of Roosevelt County. The high accumulation rate determined for the past 16 ka may point to the existence of periods when the meteorite fall rate was significantly greater than at present.     

The catalog of the meteorite collection of the Italian Museum of Planetary Sciences in Prato (Italy)

For the first time, this paper presents to the planetary scientists' community the catalog of the meteorite collection preserved at the Italian Museum of Planetary Sciences (Museo Italiano di Scienze Planetarie, henceforth MISP) in Prato (Italy). Founded in 2005, MISP is a type specimen official repository approved by the Nomenclature Committee of the Meteoritical Society. It represents one of the few museums worldwide entirely devoted to planetary sciences. The catalog of its meteorite collection encompasses 430 meteorites for a total of 1536 specimens, including 291 thin sections, 184 thick sections, and 278 specimens that MISP has classified. Furthermore, MISP is currently classifying 57 other meteorites. Some samples were found during meteorite recovery expeditions in hot deserts, promoted by MISP in collaboration with diverse Italian universities and national research institutions. MISP also keeps an impact rocks collection comprising 257 samples. In a country like Italy, where most of the collected meteorites are housed in museums whose catalogs are not available online, the publication of the MISP meteorite collection catalog, together with the catalog of the impact rocks collection, represents not only a significant scientific primary source but also a remarkable tool for disseminating meteoritics to nonresearch audiences in educational activities and citizen science projects.     

Meteorite falls in China and some related human casualty events

Abstract— Statistics of witnessed and recovered meteorite falls found in Chinese historical texts for the period from 700 B.C. to A.D. 1920 are presented. Several notable features can be seen in the binned distribution as a function of time. An apparent decrease in the number of meteorite reports in the 18th century is observed. An excess of observed meteorite falls in the period from 1840 to 1880 seems to correspond to a similar excess in European data. A x² probability test suggests that the association between the two data sets are real. Records of human casualties and structural damage resulting from meteorite falls are also given. A calculation based on the number of casualty events in the Chinese meteorite records suggests that the probability of a meteorite striking a human is far greater than previous estimates. However, it is difficult to verify the accuracy of the reported casualty events.     

Cover

Cover: The 153 kg Kumtag 016 L5 meteorite fragments are being recovered from the Kumtag Desert, Xinjiang Province, China. XiaoJia Zeng et al. discuss the strewn fi eld in their article on p. 1113. Image credit: Peng Wang.     

Is the Kaidun Meteorite a Sample from Phobos?

The Kaidun meteorite exhibits an incredible diversity of extraterrestrial material. The parent body of the meteorite is mainly composed of carbonaceous chondrite material of the second petrological type. The meteorite is specific in its composition: it contains numerous fragments and inclusions formed at an early stage of the Solar System evolution by nebular condensation, gaseous metasomatosis, agglomeration, and other processes, and two different fragments of alkaline-enriched differentiated material, which entered the parent body as a result of different events. The data on the lithologic composition of the Kaidun meteorite give strong arguments for considering the meteorite's parent body to be a carbonaceous chondrite satellite of a large differentiated planet. Phobos, the moon of Mars, is the most probable candidate. Many features of the Kaidun meteorite can be well explained within the framework of the popular hypothesis of Phobos' origin based on the nebular capture model.     

Helium,neon, and argon in meteorites: A data collection

Abstract‐ Noble gases have been measured in meteorites for more than 100 years. The last 50 years have been especially fruitful, with concentration and isotopic compositional analysis of He, Ne, Ar, Kr, and Xe making important contributions to meteorite research. Differently trapped noble gas components are the basis for understanding planetary atmospheres and even different stages of stellar evolution. Noble gases are a valuable tool to detect pairing of meteorite specimens or even to prove whether a rock is a meteorite or not. Noble gas data, however, are distributed over a large number of publications. Sometimes, only concentrations are given for selected isotopes or just a simple derivative quantity is published. We have tried to collect all available measurements of He, Ne, and Ar in meteorites. Here, we present the data in a form that will help easily calculate isotopic or elemental ratios for selected measurements. The present compilation contains all data available as of March 2004.