Meteoritic Component Research Articles

Oral histories in meteoritics and planetary science: VII. Alastair G. W. Cameron

Abstract— In this interview, Alastair Cameron recounts how he started his career as a nuclear physicist but taught himself astrophysics after he read a paper that required an astrophysical explanation for the presence of technetium in red giant stars. Subsequently, as new analytical data became available, he periodically updated the Suess‐Urey tables of elemental abundances to enhance the value of the systematic approach they provided to understanding individual processes of nucleosysthesis. Since many of these new data were based on analyses of carbonacous chondrites, he taught himself meteoritics. In recent decades, Cameron has focused his research interests on problems such as the provenance of certain components of meteorites (calcium‐aluminum‐rich inclusions, FUN (fractionated and unknown nuclear) anomalous inclusions, amoeboid olivine aggregates, and presolar grains) that he believes to have formed in the supernova envelope prior to formation of the solar nebula, the origin of chondrules in the primitive solar nebula, and the origins of the solar system and of the Earth‐Moon system. To investigate these subjects he has pioneered the use of advanced computer technology to make lengthy calculations of nucleosysthesis in complicated networks. After teaching courses and advising graduate students at several research institutes and colleges, Cameron served as a Professor of Astronomy at Harvard University from 1973 to 1997 when he was appointed to the Donald H. Menzel Research Professorship of Astrophysics. In 1994, The Meteoritical Society honored him with the Leonard Medal at its meeting in Prague, the Czech Republic.

Kgagodi Basin: The first impact structure recognized in Botswana

Abstract— The 3.4 km wide, so‐called Kgagodi Basin structure, which is centered at longitude 27°34.4′ E and latitude 22°28.6′ S in eastern Botswana, has been confirmed as a meteorite impact structure. This crater structure was first recognized through geophysical analysis; now, we confirm its impact origin by the recognition of shock metamorphosed material in samples from a drill core obtained close to the crater rim. The structure formed in Archean granitoid basement overlain and intruded by Karoo dolerite. The crater yielded a gravity model consistent with a simple bowl‐shape crater form. The drill core extends to a depth of 274 m and comprises crater fill sediments to a depth of 158 m. Impact breccia was recovered only between 158 and 165 m depth, below which locally brecciated basement granitoids grade into fractured and eventually undeformed crystalline basement, from ∼250 m depth. Shock metamorphic effects were only found in granitoid clasts in the narrow breccia zone. This breccia is classified as suevitic impact breccia due to the presence of melt and glass fragments, at a very small abundance. The shocked grains are exclusively derived from granitoid target material. Shock effects include multiple sets of planar deformation features in quartz and feldspar; diaplectic quartz, and partially and completely isotropized felsic minerals, and rare melt fragments were encountered. Abundances of some siderophile elements and especially, Ir, in suevitic breccia samples are significantly elevated compared to the contents in the target rocks, which provides evidence for the presence of a small meteoritic component.Kgagodi is the first impact structure recognized in the region of the Kalahari Desert in southern Africa. Based on lithological and first palynological evidence, the age of the Kgagodi structure is tentatively assigned to the upper Cretaceous to early Tertiary interval. Thus, the crater fill has the potential to provide a long record of paleoclimatic conditions.

The Dar al Gani meteorite field (Libyan Sahara): Geological setting, pairing of meteorites, and recovery density

Abstract— As of July 2001, 1238 Libyan meteorites have been reported. Most were found in two areas called Dar al Gani and Hamadah al Hamra. Dar al Gani is located on a plateau of marine carbonate rocks with marly components. Eight‐hundred and sixty‐nine meteorites between 6 g and 95 kg totalling 687 kg have been found here but the calculated mean recovery density is comparatively low with one meteorite on 6.5 km2.Dar al Gani is a perfect site for the recognition and preservation of meteorites. The existence of meteorites is the result of a combination of specific geological and geomorphological conditions: there is a bright‐colored, old limestone plateau (<2 Ma), under arid weather conditions over long periods of time, with rapid elimination of surface water if present and low erosion rates. The preservation of meteorites is guaranteed through the absence of quartz sand on the plateau, strongly reducing wind erosion and a basic environment emerging from the carbonate ground retards rusting of metallic meteorite components. A supposed soil cover during pluvial times has probably protected older meteorites and led to a concentration of meteorites of different periods.An evaluation of Dar al Gani meteorites suggests the existence of at least 26 strewnfields and 26 meteorite pairs reducing the number of falls to, at most, 534. Shock and weathering grades as a tool for the recognition of pairings turned out to be problematic, as several strewnfields showed paired meteorites which had been classified to different shock and weathering grades.

Diverse supernova sources of pre-solar material inferred from molybdenum isotopes in meteorites.

Variations in the isotopic composition of some components in primitive meteorites demonstrate that the pre-solar material was not completely homogenized, nor was it processed at sufficiently high temperatures to erase the signatures of the diverse stellar sources. This is in accord with the observation that accretion disks of young stellar objects are at relatively low temperatures. Carbonaceous chondrites are considered to represent the 'average' Solar System composition; the rare pre-solar grains in the matrixes of carbonaceous chondrites have been used to identify some sources of the pre-solar material. Here we report that the molybdenum isotopic composition of bulk carbonaceous chondrites is distinctly different from the accepted average solar value. We show that the Mo data require the presence of material produced in at least two different r-processes, and that the contribution from the p-process material is decoupled from the r-process, all occurring in supernova explosions. This is consistent with the emerging picture of diverse sources inferred from short-lived isotopes in the early Solar System and elemental analyses of metal-poor stars.

The composition of the solar wind

Accurate knowledge of the composition of the solar wind and of solar system abundances allows us to improve our understanding about various processes affecting the elemental, isotopic, and charge-state abundances of the solar wind. During the evolution of the Sun, isotopic and elemental abundances have been affected by migration processes at the interface between the well mixed outer convective zone and the radiative zone. In the solar atmosphere, the isotopic and elemental composition can be affected by Coulomb drag and wave-particle interactions. In addition, elemental abundances are clearly fractionated by some process that is controlled by the first ionization potential (FIP) of the elements. The abundances of the charge states are determined by processes in the corona, and, in interplanetary space, serve as valuable tracers for the coronal origin of the solar wind. The abundances of certain key elements and their isotopes can safely and accurately be determined from their meteoritic values. Their abundances in the solar wind can be used to obtain estimates for the degree of fractionation undergone by other elements and their isotopes, for which the meteoritic abundances do not necessarily reflect photospheric values, or for which it is not clear which meteoritic component corresponds to solar values. In short, the composition of the solar wind can yield valuable information about processes in the early solar system and its evolution, the evolution of the Sun, processes in the solar interior, atmosphere, and corona, as well as in interplanetary space.

日本における隕鉄成分(Fe-Ni含有)粒子の新たな存在試料について

日本における隕鉄成分(Fe-Ni含有)粒子の新たな存在試料について

Heterogeneous chemical processes as a source of persistent meteor trains

Abstract— Observations of long‐lasting persistent trains following the entry of some meteoroids into the Earth's atmosphere are suggested to arise in part from the interaction between meteoroid components and the atmosphere and in the heterogeneous recombination reaction of atmospheric O atoms with NO. The latter occurs on the surfaces of dust left by the explosive fragmentation of larger meteoroids. A strong role is attributed to reactions of troilite (FeS), a meteorite component, with the atmosphere at elevated temperatures. The suggestions made in this paper complement previous work that suggested that long‐lived emissions results from a variety of species made in the shock of larger meteoroids.

Sulfate oxygen-17 anomalies in desert varnishes

Rock varnishes are ubiquitous in arid regions on Earth, and are believed to be commonly present on Mars. Here we report high water-soluble sulfate and nitrate contents in desert varnishes from the Death Valley region of southwestern U.S.A., and that sulfate in varnishes possess δ 17O/δ 18O ratios that do not fall on terrestrial mass-dependent fractionation line. Sulfate from wet and dry atmospheric deposition is probably the source of the δ 17O anomalies. The anomalies are only moderately lower than that of aerosol sulfates collected from the greater Los Angeles area, indicating probably more than half of the sulfate in desert varnish is supplied by atmospheric deposition. This finding suggests that Earth surface environments are constantly accumulating δ 17O-anomalous sulfate from the atmosphere; arid and stable conditions facilitate the preservation of these atmospheric signatures. This finding also indicates that different δ 17O/δ 18O ratios found in different components in the Martian meteorites may result from atmospheric chemical processes.

Petrography, geochemistry, and argon‐40/argon‐39 ages of impact‐melt rocks and breccias from the Ames impact structure, Oklahoma: The Nicor Chestnut 18‐4 drill core

Abstract— The 15 km diameter Ames structure in northwestern Oklahoma is located 2.75 km below surface in Cambro‐Ordovician Arbuckle dolomite, which is overlain by Middle Ordovician Oil Creek Formation shale. The feature is marked by two concentric ring structures, with the inner ring of about 5 km diameter probably representing the collapsed remnant of a structural uplift composed of brecciated Precambrian granite and Arbuckle dolomite. Wells from both the crater rim and the central uplift are oil‐ and gas‐producing, making Ames one of the economically important impact structures.Petrographic, geochemical, and age data were obtained on samples from the Nicor Chestnut 18‐4 drill core, off the northwest flank of the central uplift. These samples represent the largest and best examples of impact‐melt breccia obtained so far from the Ames structure. They contain carbonate rocks, which are derived from the target sequence. The chemical composition of the impact‐melt breccias is similar to that of target granite, with variable carbonate admixture. Some impact‐melt rocks are enriched in siderophile elements indicating the possible presence of a meteoritic component. Based on stratigraphic arguments, the age of the crater was estimated at 470 Ma. Previous 40Ar‐39Ar dating attempts of impact‐melt breccias from the Dorothy 1–19 core yielded plateau ages of about 285 Ma, which is in conflict with the stratigraphic age. The new 40Ar‐39Ar age data obtained on the melt breccias from the Nicor Chestnut core by ultraviolet (UV) laser spot analysis resulted in a range of ages with maxima around 300 Ma. These data could reflect processes related either the regional Nemaha Uplift or resetting due to hot brines active on a midcontinent‐wide scale, perhaps related to the Alleghenian and Ouachita orogenies. The age data indicate an extended burial phase associated with thermal overprint during Late Pennsylvanian‐Permian.

Noble gases in mantle plumes.

Trieloff et al . ([1][1]) provided neon isotope data to argue that mantle neon originates as a trapped component in meteorites rather than having a pristine solar composition. That finding, if true, has important consequences for understanding the mechanism of volatile input into the mantle, because

Sulfur isotopic compositions of individual sulfides in Martian meteorites ALH84001 and Nakhla: implications for crust–regolith exchange on Mars

Atmospheric chemical reactions on Mars have been invoked to explain non-mass-dependent Δ33S anomalies (Δ33S=δ33S−0.516δ34S) reported from bulk analyses of Martian meteorites. To explore this signature in detail, a new ion microprobe multi-collector technique was developed to obtain precise in situ 32S, 33S and 34S measurements of individual sulfide grains from Martian meteorites ALH84001 (>4.0 Ga) and Nakhla (1.3 Ga). This technique permits high-precision simultaneous measurement of multiple isotopes to uniquely evaluate Δ33S at the grain scale (<30 μm). Our data reveal resolvable non-mass-dependent Δ33S anomalies in two separate ALH84001 pyrite grains (Δ33S=−0.74±0.39‰ and −0.51±0.38‰, 2σ); none were detectable in Nakhla pyrrhotite (total range in Δ33S=−0.4±0.5‰ to −0.07±0.5‰, 2σ). Our results might reflect a difference in how these meteorites exchanged sulfur with the Martian regolith and/or differences in their sources (atmospheric versus meteoritic) of anomalous sulfur. Nebular heterogeneities in sulfur isotope composition are indicated by Δ33S anomalies preserved in, for example, the ureilites. The Δ33S anomalies in ALH84001 pyrite could suggest that early (pre-4 Ga) additions of a meteoritic component carried isotopically anomalous sulfur to the Martian regolith, and was stored there as seen in the detection of Δ33S anomalies from bulk measurements of Nakhla. Therefore, meteoritic contributions should also be considered in addition to atmospheric effects when explaining the large non-mass-dependent anomalies seen in Martian meteorites. These studies provide insight into how hydrothermal systems have facilitated exchange between volatile reservoirs on Mars, a planet that lacks efficient crustal recycling mechanisms and preserves ancient (and anomalous) Δ33S signatures.

Non-chondritic distribution of the highly siderophile elements in mantle sulphides.

The abundances of highly siderophile (iron-loving) elements (HSEs) in the Earth's mantle provide important constraints on models of the Earth's early evolution. It has long been assumed that the relative abundances of HSEs should reflect the composition of chondritic meteorites--which are thought to represent the primordial material from which the Earth was formed. But the non-chondritic abundance ratios recently found in several types of rock derived from the Earth's mantle have been difficult to reconcile with standard models of the Earth's accretion, and have been interpreted as having arisen from the addition to the primitive mantle of either non-chondritic extraterrestrial material or differentiated material from the Earth's core. Here we report in situ laser-ablation analyses of sulphides in mantle-derived rocks which show that these sulphides do not have chondritic HSE patterns, but that different generations of sulphide within single samples show extreme variability in the relative abundances of HSEs. Sulphides enclosed in silicate phases have high osmium and iridium abundances but low Pd/Ir ratios, whereas pentlandite-dominated interstitial sulphides show low osmium and iridium abundances and high Pd/Ir ratios. We interpret the silicate-enclosed sulphides as the residues of melting processes and interstitial sulphides as the crystallization products of sulphide-bearing (metasomatic) fluids. We suggest that non-chondritic HSE patterns directly reflect processes occurring in the upper mantle--that is, melting and sulphide addition via metasomatism--and are not evidence for the addition of core material or of 'exotic' meteoritic components.

Determining the ages of comets from the fraction of crystalline dust.

The timescale for the accretion of bodies in the disk surrounding a young star depends upon a number of assumptions, but there are few observational constraints. In our own Solar System, measurements of meteoritic components can provide information about the inner regions of the nebula, but not the outer parts. Observations of the evolution of more massive protostellar systems (Herbig Ae/Be stars) imply that significant changes occur in the physical properties of their dust with time. The simplest explanation is that thermal annealing of the original, amorphous grains in the hot inner nebula slowly increases the fractional abundance of crystalline material over time. Crystalline dust is then transported outward, where it is incorporated into comets that serve as a long-term reservoir for dust disks, such as that surrounding Beta Pictoris. Here we show that when applied to our own Solar System, this process can explain observed variations in both the volatile and dusty components of comets, while also providing a natural indicator of a comet's mean formation age. Studies of comets with different dust contents can therefore be used to investigate the timescales of the early Solar System.

Woodleigh, Carnarvon Basin, Western Australia: a new 120 km diameter impact structure

The Woodleigh multi-ring structure, buried by Cretaceous and, at its centre, Lower Jurassic lacustrine sediments, east of Hamelin Pool, Carnarvon Basin, Western Australia, is identified as an impact structure, the largest discovered to date on the Australian continent. An impact origin is indicated by: a central core of uplifted granitoid basement probably less than 25 km in diameter, which displays shock-induced planar deformation features in quartz, pervasive diaplectic vitrification of feldspar and penetrative pseudotachylite veining; and an inner ring syncline containing a ∼70 m thick thermally modified diamictite overlain by ∼380 m of Lower Jurassic lacustrine deposits. An outermost diameter of 120 km, defined by gravity, magnetic and surface drainage, indicates a ring fault that sharply intersects the NS-striking regional structure. At the centre of the basement uplift shock metamorphosed granitoid was intersected at a depth of 171 m, at least 1800 m higher than the gravity-modelled level of regional basement. Pseudotachylite vein systems within the shocked granitoid are strongly enriched in Al, Ca, Mg, Ni, Co, Cr, V and S, and depleted in K and Si, suggesting chemical fractionation attendant on shock volatilisation, enrichment by an injected and volatilised meteoritic component, and potentially of sulfide mineralisation. The impact age is constrained by overlying Lower Jurassic strata, reworked Early Permian palynomorphs in the Jurassic lacustrine section, and deformed Lower Devonian and older units. A regional thermal event identified by apatite fission track at 280–250 Ma hints at a possible Permian–Triassic boundary age for the impact, although the lack of Triassic fossils in the crater fill favours a late Triassic age.

A petrographical and geochemical study of quartzose nodules, country rocks, and dike rocks from the Upheaval Dome structure, Utah

Abstract— Upheaval Dome, in Canyonlands National Park, Utah, USA, is a unique structure on the Colorado Plateau. It has earlier been interpreted as an impact structure or as a pinched‐off salt diapir. Some subrounded quartzose fragments were found in a ring depression near the eastern margin of the structure and, based on vesicularity and apparent flow structure, the fragments were interpreted by early researchers as “impactites.” Our petrographic studies show no indication of a high‐temperature history and are in agreement with a slow, low‐temperature formation of the quartz nodules. Composi***ionally, the lag deposit samples are almost pure SiO2. They show no chemical similarity to any of the possible target rocks (e.g., Navajo Sandstone), from which they should have formed by melting if they were impactites. Instead, the samples have relatively high contents of elements that indicate fluid interaction (e.g., hydrothermal growth), such as As, Sb, Ba, and U, and show positive Ce anomalies. Thus, we interpret the “lag deposit samples” as normal low‐temperature (hydrothermally‐grown?) quartz that show no indication of being impact‐derived. In addition, a petrographic and geochemical analysis of a series of dike samples yielded no evidence for shock metamorphism or a meteoritic component.

Astronomical constraints on nebular temperatures: Implications for planetesimal formation

Abstract—Motivated by recent observations of T‐Tauri stars and the interpretation of these observations in terms of the properties of circumstellar disks, we derive internal (midplane) temperatures for disks around mature (age ∼1 Ma) T‐Tauri stars. The estimates are obtained by combining published results for disk masses, sizes, accretion rates, and surface temperatures. For 26 stars (for which adequate data are available), we derive midplane temperatures at 1 AU primarily in the range 200–800 K, and 100–400 K at 2.5 AU. It is likely that the solar nebula, at the same stage of evolution, contained planetesimals and objects destined to become meteorite parent bodies. Observations of young stellar objects at earlier stages of evolution (age ∼0.1 Ma) imply that accretion rates were, on the average, at least two orders of magnitude greater than the 10−8M⊙/year rates typical for mature T‐Tauri stars. Such high values would result in midplane temperatures at or near the silicate vaporization temperature in the terrestrial planet region.If cooling of the solar nebula from such a hot epoch was responsible for establishing the pervasive elemental fractionation patterns found in chondritic meteorites, then objects in the asteroid belt must have grown rapidly (within 0.1 Ma) to sizes of ∼1 km, a conclusion consistent with current theories of planetesimal formation. However, the fact that primitive meteorite parent bodies escaped being melted by the decay of26Al then implies that further growth of at least some objects was essentially delayed for 2 Ma or more. Such a diminished growth rate appears to be consistent with simulations of the dynamics of solid bodies in the asteroid belt.Other hypotheses seem less attractive. One might assume that the final cooling occurred only after the decay of26Al (i.e., more than a million years after calcium‐aluminum rich inclusion formation), or that26Al was not ubiquitous in the early solar system. But the first of these conjectures is incompatible with astronomical observations of T‐Tauri systems, and the second appears to be contradicted by the evidence for26Al in diverse meteoritic components. The remaining alternative would then appear to be that, despite a lack of supporting evidence, chondritic fractionation patterns reflect the net effect of many local heating and cooling events and have nothing to do with global nebular cooling. We conclude that the most plausible hypothesis is that both nebular cooling and coagulation of solids to kilometer‐sized objects occurred rapidly and that a substantial number of planetesimals in the asteroid belt remained smaller than a few kilometers in radius for at least 2 Ma.

Yallalie: a buried structure of possible impact origin in the Perth Basin, Western Australia

An enigmatic buried structure, located in Mesozoic sedimentary rocks in the Perth Basin, Western Australia, was discovered in 1990 by Ampol Exploration. The basin-like Yallalie structure (centred on 30° 26′ 40.3″ S, 115° 46′ 16.4″ E) is circular in plan view and about 12 km in diameter. High-resolution, seismic-reflection profiles across the structure show a basin-shaped area of chaotic reflections that extend to a depth of approximately 2 km below the surface. The structure has sharp boundaries with surrounding faulted, but otherwise relatively undisturbed, rocks. In the centre of the structure there is an uplifted area approximately 3–4 km across, similar to those described from complex meteorite impact structures. The seismically defined structure coincides with a circular topographic depression, and image processing of digital elevation data has allowed recognition of concentric and radial structures extending as far as 40 km from the centre of the depression. Gravity surveys show the structure to be associated with a positive gravity anomaly of about 30 gu. Aeromagnetic surveys have defined annular anomalies associated with the central uplifted section, and possibly margins, of the structure. A search for siderophile element enrichments (by neutron activation analysis) in the rocks of the structure, which would indicate the presence of a meteorite component, proved negative. Quartz grains in cores that penetrate the structure show the development of prismatic cleavage fractures and irregular, slightly curved planes formed by brittle fracture. An allochthonous breccia of Late Cretaceous rocks occurs a few kilometres west of the western margin of the structure. Quartz grains from a thin veneer of Tertiary sediments that drape the structure are essentially undeformed. However, multiple sets of closely spaced planar deformation features in quartz, characteristic of highly shocked rocks, have yet to be observed in the rocks of the Yallalie structure and the allochthonous breccia. The morphology of the Yallalie structure determined from topographic and geophysical data suggests strongly that it is of impact origin. Geological and geochemical evidence is equivocal, but is not inconsistent with this interpretation.

Small-scale hydrous pyrolysis of macromolecular material in meteorites

The hydrous pyrolysis method, usually performed on several hundred grams of terrestrial rock sample, has been scaled down to accommodate less than two grams of meteorite sample. This technique makes full use of the high yields associated with hydrous pyrolysis experiments and permits the investigation of the meteorite macromolecular material, the major organic component in carbonaceous meteorites. The hydrous pyrolysis procedure transforms the high molecular weight macromolecular material into low molecular weight fragments. The released entities can then be extracted with supercritical fluid extraction. In contrast to the parent structure, the pyrolysis products are amenable for analysis by gas chromatography-based techniques. When subjected to hydrous pyrolysis, two carbonaceous chondrites (Orgueil and Cold Bokkeveld) released generally similar products, which consisted of abundant volatile aromatic and alkyl-substituted aromatic compounds. These results revealed the ability of small-scale hydrous pyrolysis to dissect extraterrestrial macromolecular material and thereby reveal its organic constitution.

Petrology, chemistry, and isotopic compositions of the lunar highland regolith breccia Dar al Gani 262

Abstract— Lunar meteorite Dar al Gani 262 (DG 262)—found in the Libyan part of the Sahara—is a mature, anorthositic regolith breccia with highland affinities. The origin from the Moon is undoubtedly indicated by its bulk chemical composition; radionuclide concentrations; noble gas, N, and O isotopic compositions; and petrographic features. Dar al Gani 262 is a typical anorthositic highland breccia similar in mineralogy and chemical composition to Queen Alexandra Range (QUE) 93069.About 52 vol% of the studied thin sections of Dar al Gani 262 consist of fine‐grained(100 μm) constituents, and 48 vol% is mineral and lithic clasts and impact‐melt veins. The most abundant clast types are feldspathic fine‐grained to microporphyritic crystalline melt breccias (50.2 vol%; includes recrystallized melt breccias), whereas mafic crystalline melt breccias are extremely rare (1.4 vol%). Granulitic lithologies are 12.8 vol%, intragranularly recrystallized anorthosites and cataclastic anorthosites are 8.8 and 8.2 vol%, respectively, and (devitrified) glasses are 2.7 vol%. Impact‐melt veins (5.5 vol% of the whole thin sections) cutting across the entire thin section were probably formed subsequent to the lithification process of the bulk rock at pressures below 20 GPa, because the bulk rock never experienced a higher peak shock pressure.Mafic crystalline melt breccias are very rare in Dar al Gani 262 and are similar in abundance to those in QUE 93069. The extremely low abundance of mafic components and the bulk composition may constrain possible areas of the Moon from which the breccia was derived. The source area of Dar al Gani 262 must be a highland terrain lacking significant mafic impact melts or mare components.On the basis of radionuclide activities, an irradiation position of DG 262 on the Moon at a depth of 55–85 g/cm3and a maximum transit time to Earth &lt;0.15 Ma is suggested. Dar al Gani 262 contains high concentrations of solar‐wind‐implanted noble gases. The isotopic abundance ratio 40Ar/36Ar &lt; 3 is characteristic of lunar soils.The terrestrial weathering of DG 262 is reflected by the occurrence of fractures filled with calcite and by high concentrations of Ca, Ba, Cs, Br, and As. There is also a large amount of terrestrial C and some N in the sample, which was released at low temperatures during stepped heating. High concentrations of Ni, Co, and Ir indicate a significant meteoritic component in the lunar surface regolith from which DG 262 was derived.

Discovery of microscopic evidence for shock metamorphism at the Serpent Mound structure, south-central Ohio: confirmation of an origin by impact

The origin of the Serpent Mound structure in south-central Ohio has been disputed for many years. Clearly, more evidence was needed to resolve the confusion concerning the origin of the Serpent Mound feature either by endogenic processes or by hypervelocity impact. A petrographic study of 21 samples taken from a core 903 m long drilled in the central uplift of the structure provides evidence of shock metamorphism in the form of multiple sets of planar deformation features in quartz grains, as well as the presence of clasts of altered impact-melt rock. Crystallographic orientations of the planar deformation features show maxima at the shock-characteristic planes of {10 1 ̄ 3} and {10 1 ̄ 2} and additional maxima at {10 1 ̄ 1} , {21 3 ̄ 1} , and {51 6 ̄ 1} . Geochemical analyses of impact breccias show minor enrichments in the abundances of the siderophile elements Cr, Co, Ni, and Ir, indicating the presence of a minor meteoritic component.