Exposure history of glass and breccia phases of lunar meteorite EET87521

Abstract— Glass-rich separates were prepared from a sample of the basaltic lunar meteorite EET87521 rich in dark glass. Noble gas isotopic abundances and ²⁶Al and ¹⁰Be activities were measured to find out whether shock effects associated with lunar launch helped to assemble these phases. Similar ¹⁰Be and ²⁶Al activities indicate that all materials in EET87521 had a common exposure history in the last few million years before launch. However, the glass contains much higher concentrations of trapped gases and records a much longer cosmic-ray exposure, 100 Ma–150 Ma, in the lunar regolith than does the bulk sample. The different histories show that the glass existed long before the ejection of EET87521. The trapped ⁴⁰Ar/³⁶Ar ratio of 1.6 ± 0.1 implies that the lunar exposure that produced most of the stable cosmogenic noble gases began 500 Ma ago. Cosmogenic and trapped noble gas components correlate strongly in various temperature-release fractions and phases of EET87521, which is probably because the glass contains most of the gas. The trapped solar ratios, ²⁰Ne/²²Ne = 12.68 ± 0.20 and ³⁶Ar/³⁸Ar = 5.24 ± 0.05 can be understood as resulting from a mixture consisting of ～60% solar wind and 40% solar energetic particles (SEP). All EET87521 phases show a ⁴⁰K-⁴⁰Ar gas retention age of ～3300 Ma, which is in the range of typical lunar mare basalts.     

Exposure history of the Torino meteorite

Abstract— We determined He, Ne, Ar, ¹⁰Be, ²⁶Al, ³⁶Cl, and ¹⁴C concentrations, as well as cosmic-ray track densities and halogen concentrations in different specimens of the H6 chondrite Torino, in order to constrain its exposure history to cosmic radiation. The Torino meteoroid had a radius of ～20 cm and travelled in interplanetary space for 2.5–10 Ma. Earlier, Torino was part of a larger body. The smallest possible precursor had a radius of 55 cm and a journey through space longer than ～65 Ma. If the first-stage exposure took place in a body with a radius of >3 m or in the parent asteroid, then it lasted nearly 300 Ma. The example of Torino shows that it is easy to underestimate first-stage exposure ages when constructing two-stage histories.     

Exposure history of the Peekskill (H6) meteorite

Abstract— The Peekskill H6 meteorite fell on 1992 October 9. We report extensive measurements of cosmic-ray produced stable nuclides of He, Ne, and Ar, of the radionuclides ²²Na, ⁶⁰Co, ¹⁴C, ³⁶Cl, ²⁶Al, and ¹⁰Be, and of cosmic-ray track densities. After correction for shielding via the ²²Ne/²¹Ne ratio, the concentrations of cosmic-ray produced ³He, ²¹Ne and ³⁸Ar give an average exposure age of 25 Ma, which is considered to be a lower limit on the true value. The ¹⁰Be/²¹Ne age is 32 Ma and falls onto a peak in the H-chondrite exposure age distribution. The activities of ²⁶Al, ¹⁴C, ³⁶Cl, and ¹⁰Be are all close to the maximum values expected for H-chondrites. Together with cosmic-ray track densities and the ²²Ne/²¹Ne ratio, these radionuclide data place the samples at a depth >20 cm in a meteoroid with a radius >40 cm. In contrast, the ⁶⁰Co activity requires a near-surface location and/or a much smaller body. Calculations show that a flattened geometry for the Peekskill meteoroid does not explain the observations in the context of a one-stage irradiation. A two-stage model can account for the data. We estimate an upper bound of 70 cm on the radius of the earlier stage of irradiation and conclude that Peekskill's radius was <70 cm when it entered the Earth's atmosphere. This size limit is somewhat smaller than the dynamic determinations (Brown et al., 1994).     

The early geological history of the Moon inferred from ancient lunar meteorite Miller Range 13317

Miller Range (MIL) 13317 is a heterogeneous basalt‐bearing lunar regolith breccia that provides insights into the early magmatic history of the Moon. MIL 13317 is formed from a mixture of material with clasts having an affinity to Apollo ferroan anorthosites and basaltic volcanic rocks. Noble gas data indicate that MIL 13317 was consolidated into a breccia between 2610 ± 780 Ma and 1570 ± 470 Ma where it experienced a complex near‐surface irradiation history for ~835 ± 84 Myr, at an average depth of ~30 cm. The fusion crust has an intermediate composition (Al₂O₃ 15.9 wt%; FeO 12.3 wt%) with an added incompatible trace element (Th 5.4 ppm) chemical component. Taking the fusion crust to be indicative of the bulk sample composition, this implies that MIL 13317 originated from a regolith that is associated with a mare‐highland boundary that is KREEP‐rich (i.e., K, rare earth elements, and P). A comparison of bulk chemical data from MIL 13317 with remote sensing data from the Lunar Prospector orbiter suggests that MIL 13317 likely originated from the northwest region of Oceanus Procellarum, east of Mare Nubium, or at the eastern edge of Mare Frigoris. All these potential source areas are on the near side of the Moon, indicating a close association with the Procellarum KREEP Terrane. Basalt clasts in MIL 13317 are from a very low‐Ti to low‐Ti (between 0.14 and 0.32 wt%) source region. The similar mineral fractionation trends of the different basalt clasts in the sample suggest they are comagmatic in origin. Zircon‐bearing phases and Ca‐phosphate grains in basalt clasts and matrix grains yield ²⁰⁷Pb/²⁰⁶Pb ages between 4344 ± 4 and 4333 ± 5 Ma. These ancient ²⁰⁷Pb/²⁰⁶Pb ages indicate that the meteorite has sampled a range of Pre‐Nectarian volcanic rocks that are poorly represented in the Apollo, Luna, and lunar meteorite collections. As such, MIL 13317 adds to the growing evidence that basaltic volcanic activity on the Moon started as early as ~4340 Ma, before the main period of lunar mare basalt volcanism at ~3850 Ma.     

Thermal and irradiation history of lunar meteorite Dhofar 280

Dhofar 280 recorded a complex history on the Moon revealed by high‐resolution ⁴⁰Ar‐³⁹Ar dating. Thermal resetting occurred less than 1 Ga ago, and the rock was exposed to several impact events before and afterwards. The cosmic ray exposure (CRE) age spectrum indicates a 400 ± 40 Ma CRE on the lunar surface. A unique feature of this lunar sample is a partial loss of cosmogenic ³⁸Ar, resulting in a (low‐temperature) CRE age plateau of about 1 Ma. This was likely caused by the same recent impact event that reset the (low‐temperature) ⁴⁰Ar‐³⁹Ar age spectrum and preceded the short transit phase to Earth of ≤1 Ma. Dhofar 280 may be derived from KREEP‐rich lunar frontside terrains, possibly associated with the Copernicus crater or with a recent impact event on the deposits of the South Pole–Aitken basin. Although Dhofar 280 is paired with Dhofar 081, their irradiation and thermal histories on the Moon were different. An important trapped Ar component in Dhofar 280 is “orphan” Ar with a low ⁴⁰Ar/³⁶Ar ratio. It is apparently a mixture of two components, one endmember with ⁴⁰Ar/³⁶Ar = 17.5 ± 0.2 and a second less well‐constrained endmember with ⁴⁰Ar/³⁶Ar ≤10. The presence of two endmembers of trapped Ar, their compositions, and the breccia ages seem to be incompatible with a previously suggested correlation between age or antiquity and the (⁴⁰Ar/³⁶Ar)_trapped ratio (Eugster et al. 2001; Joy et al. 2011a). Alternatively, “orphan” Ar of this impact melt breccia may have an impact origin.     

Shock‐thermal history of the Agoudal (IIAB) iron meteorite from microstructural studies

The Agoudal IIAB iron meteorite exhibits only kamacite grains (~6 mm across) without any taenite. The kamacite is homogeneously enriched with numerous rhabdite inclusions of different size, shape, and composition. In some kamacite domains, this appears frosty due to micron‐scale rhabdite inclusions (~5 to 100 μm) of moderate to high Ni content (~26 to 40 wt%). In addition, all the kamacite grains in matrix are marked with a prominent linear crack formed during an atmospheric break‐up event and subsequently oxidized. This feature, also defined by trails of lowest Ni‐bearing (mean Ni: 23 wt%) mm‐scale rhabdite plates (fractured and oxidized) could be a trace of a pre‐existing γ–α interface. Agoudal experienced a very slow rate of primary cooling ~4 °C Ma^?1 estimated from the binary plots of true rhabdite width against corresponding Ni wt% and the computed cooling rate curves after Randich and Goldstein (1978). Chemically, Agoudal iron (Ga: 54 ppm; Ge: 140 ppm; Ir: 0.03 ppm) resembles the Ainsworth iron, the coarsest octahedrite of the IIAB group. Agoudal contains multiple sets of Neumann bands that are formed in space and time at different scales and densities due to multiple impacts with shock magnitude up to 130 kb. Signatures of recrystallization due to postshock low temperature mild reheating at about 400 °C are also locally present.     

Shock and thermal history of Martian meteorite Allan Hills 84001 from transmission electron microscopy

Abstract— Microstructures in the Allan Hills 84001 meteorite were studied using optical and electron microscopy, putting emphasis on shock effects, which are widespread. Some orthopyroxene exhibits only (100) slip, but more typical grains suffered extensive slip, microfracturing, and frequently contain (100) clino‐inversion lamellae. In fracture zones, shock deformation of orthopyroxene has produced all three effects in profusion, together with intergranular pockets of orthopyroxene glass and intragranular glass lamellae, which were apparently created by shearing on low index planes, usually (100) or {110}. Both types of plane are loci that pseudo‐planar fractures tend to follow. Thus, the glass lamellae, which have not been observed in other meteorites, probably formed by frictional heating during the sliding of microscale corrugated surfaces, one over another, leading to local melting. We infer that the orthopyroxene glass and the fracture zones both formed from shear stresses created by strong shock. Ubiquitous undeformed micrometer and submicrometer euhedral chromites in orthopyroxene and plagioclase glasses and carbonate probably crystallized after shock heating and fracture zone formation. Nanocrystals of eskolaite (Cr₂O₃) coating silica glass grains are probably also a result of shock‐induced thermal decomposition of chromite. Iron sulfides (pyrite and pyrrhotite were identified) tended to be associated with plagioclase glass. A carbonate disk showing no evidence for shock deformation had a substructure of elongated, slightly misoriented subcells in the exterior; interior regions had more eqiaxed subcells. Both microstructures probably formed during growth, but the conditions are undetermined. Chemical composition varied on a micron scale, but the rim of the disk was more ferroan; oxide precipitates and voids were widely distributed as in fracture‐filling carbonates. If the fracture zones and opx glass are the result of strong shock, as we deduce, it is very unlikely that pores could have filled by carbonate long after the fracture zones formed. We infer that the carbonate, like the phosphate, olivine, pyrrhotite, eskolaite, and many euhedral, submicrometer chromites, crystallized during the final stages of the impact that created the fracture zones and glasses with compositions of plagioclase, silica, and orthopyroxene.     

The igneous crystallization history of an ancient Martian meteorite from rare earth element microdistributions

Abstract— Rare earth element (REE) and other selected trace and minor element concentrations were measured in individual grains of orthopyroxene, feldspathic glass (of plagioclase composition) and merrillite of the ALH 84001 Martian meteorite. Unlike in other Martian meteorites, phosphate is not the main REE carrier in ALH 84001. The REE pattern of ALH 84001 bulk rock is dependent on the modal abundances of three REE-bearing phases, namely, orthopyroxene, which contains most of the heavy rare earth elements (HREEs); feldspathic glass, which dominates the Eu abundances; and merrillite, which contains the majority of the light rare earth elements (LREEs). Variations in the REE abundances previously observed in different splits of ALH 84001 can easily be explained in terms of small variations in the modal abundances of these three minerals without the need to invoke extensive redistribution of LREEs. At least some orthopyroxenes (i.e., those away from contacts with feldspathic glass) in ALH 84001 appear to have preserved their original REE zonation from igneous fractionation. An estimate of the ALH 84001 parent magma composition from that of the unaltered orthopyroxene “core” (i.e., zoned orthopyroxene with the lowest REE abundances) indicates that it is LREE depleted. This implies that the Martian mantle was already partly depleted within ～100 Ma of solar system formation, which is consistent with rapid accretion and differentiation of Mars. Although equilibration and exchange of REEs between phases (in particular, transport of LREEs into the interstitial phases, feldspathic glass and merrillite) cannot be ruled out, our data suggest that the LREE enrichment in melts “in equilibrium” with these interstitial phases is most likely the result of late-stage infiltration of the cumulate pile by a LREE-enriched melt.     

The Orgueil meteorite: 150 years of history

The goal of this paper is to summarize 150 yr of history of a very special meteorite. The Orgueil meteorite fell near Montauban in southwestern France on May 14, 1864. The bolide, which was the size of the full Moon, was seen across Western France, and almost immediately made the news in local and Parisian newspapers. Within a few weeks of the fall, a great diversity of analyses were performed under the authority of Gabriel Auguste Daubrée, geology professor at the Paris Museum, and published in the Comptes Rendus de l'Académie des Sciences. The skilled scientists reported the presence of iron sulfides, hydrated silicates, and carbonates in Orgueil. They also characterized ammonium salts which are now gone, and observed sulfates being remobilized at the surface of the stone. They identified the high water and carbon contents, and noted similarities with the Alais meteorite, which had fallen in 1806, 300 km away. While Daubrée and his colleagues noted the similarity of the Orgueil organic matter with some terrestrial humus, they were cautious not to make a direct link with living organisms. One century later, Nagy and Claus were less prudent and announced the discovery of “organized” elements in some samples of Orgueil. Their observations were quickly discredited by Edward Anders and others who also discovered that some pollen grains were intentionally placed into the rock back in the 1860s. Orgueil is now one of the most studied meteorites, indeed one of the most studied rocks of any kind. Not only does it contain a large diversity of carbon‐rich compounds, which help address the question of organo‐synthesis in the early solar system but its chemical composition is also close to that of the Sun's photosphere and serves as a cosmic reference. Secondary minerals, which make up 99% of the volume of Orgueil, were probably formed during hydrothermal alteration on the parent‐body within the first few million years of the solar system; their study is essential to our understanding of fluid–rock interaction in asteroids and comets. Finally, the Orgueil meteorite probably originated from a volatile‐rich “cometary” outer solar system body as indicated by its orbit. Because it bears strong similarities to other carbonaceous chondrites that originated on dark asteroids, this cometary connection supports the idea of a continuum between dark asteroids and comets.     

Exposure history of the Sutter's Mill carbonaceous chondrite

The Sutter's Mill (SM) carbonaceous chondrite fell in California on April 22, 2012. The cosmogenic radionuclide data indicate that Sutter's Mill was exposed to cosmic rays for 0.082 ± 0.008 Myr, which is one of the shortest ages for C chondrites, but overlaps with a small cluster at approximately 0.1 Myr. The age is significantly longer than proposed ages that were obtained from cosmogenic noble gas concentrations, which have large uncertainties due to trapped noble gas corrections. The presence of neutron‐capture ⁶⁰Co and ³⁶Cl in SM indicates a minimum preatmospheric radius of approximately 50 cm, and is consistent with a radius of 1–2 m, as derived from the fireball observations. Although a large preatmospheric size was proposed, one fragment (SM18) contains solar cosmic ray–produced short‐lived radionuclides, such as ⁵⁶Co and ⁵¹Cr. This implies that this specimen was less than 2 cm from the preatmospheric surface of Sutter's Mill. Although this conclusion seems surprising, it is consistent with the observation that the meteoroid fragmented high in the atmosphere. The presence of SCR‐produced nuclides is consistent with the high SCR fluxes observed during the last few months before the meteorite's fall, when its orbit was less than 1 AU from the Sun.     

Orthopyroxene as a geospeedometer: Thermal history of Kapoeta,Old Homestead 001, and Hughes 002 howardites

Abstract— Single crystals of orthopyroxene from small fragments of the Kapoeta, Old Homestead 001, and Hughes 002 howardites were studied by x‐ray diffraction and microprobe analyses. The Fe‐Mg equilibrium distribution coefficients k_D of the crystals were used to calculate the closure temperatures (T_c) using the calibration by Stimpfl et al. (1999). The compositions, the presence of exsolved augite lamellae, and the T_c values (from 365 to 385 °C) obtained for Kapoeta orthopyroxene s suggest that our fragment comes from a diogenitic cumulate clast. The more Fe‐rich composition, the absence of exsolved lamellae, and the higher T_c values (from 583 to 605 °C) measured in the Old Homestead 001 orthopyroxenes suggest that this fragment comes from a cumulitic clast affected by fast cooling at high temperature. For the Hughes 002 orthopyroxenes, close in composition to Old Homestead 001, the different T_c values (339, 358, and 607 °C) recorded by the various crystals and the presence of augite lamellae in the crystals with the lowest T_c support the hypothesis that this howardite sample is an unheated breccia containing a mixture of cumulitic orthopyroxenes with different thermal histories.     

Two distinct assemblages of high‐pressure liquidus phases in shock veins of the Sixiangkou meteorite

Abstract— Shock‐produced complex veins, including earlier and later veins, are identified in the Sixiangkou L6 chondrite. The early vein is intersected by the late vein and consists of coarse‐grained aggregates of ringwoodite, majorite, and lingunite, and fragments of olivine, pyroxene, plagioclase, metal, and troilite, as well as a fine‐grained matrix of garnet, ringwoodite, metal, and troilite. The late vein mainly consists of a fine‐grained matrix of garnet, magnesiowüstite, metal, and troilite, as well as a small amount of coarse‐grained aggregates. The amount of fine‐grained matrix suggests that the late vein was nearly completely melted, whereas the early vein underwent partial melting. Both fine‐grained assemblages of garnet plus ringwoodite in the early vein and garnet plus magnesiowüstite in the late vein are liquidus phases crystallized from shock‐induced melt. Based on our understanding of the liquidus assemblages, the late vein experienced a higher pressure and temperature than the early vein.     

A unique ultrarefractory inclusion from the Murchison meteorite

Abstract— Through freeze-thaw disaggregation of the Murchison meteorite, we have recovered a refractory inclusion, HIB-11, that is unique in terms of its texture, mineral compositions, and bulk composition. It consists of anhedral, Y-rich (1.6 wt% Y₂O₃) perovskite and lathlike spinel grains enclosed in a matrix of fine-grained, Sc-rich (10.5 wt% SC₂O₃ avg.), Ti-rich (12.6 wt% TiO₂ avg., reporting all Ti as TiO₂) clinopyroxene. The chondrite-normalized rare earth element (REE) pattern is complex, with light REE (LREE) at ～10× C1, abundances increasing from Gd through Ho (the latter at ～10⁴× C1), decreasing through Yb at 200× C1, and Lu at ～400× C1. The pattern reflects several stages of high-temperature volatility fractionation. Removal of Lu and Er from the source gas in the first condensation event was followed by partial to complete removal of the somewhat less refractory heavy REE, Gd through Ho, in the HIB-11 precursors by condensation from the fractionated residual gas in a second event. Both of these events probably reflect condensation of REE into ZrO₂ or a mixed Zr-, Sc-, Ti-, Y-oxide at temperatures too high for hibonite stability. A second, lower-temperature component, which was subsequently added, had fractionated (Nd-poor, Ce-rich) LREE abundances that resulted from condensation from a gas that had undergone prior removal of the more refractory LREE, resulting in enrichment in Ce and the most volatile REE, Eu and Yb. The aggregate was then melted and quickly cooled, forming a fine-grained spherule. This is the first reported inclusion in which the two most refractory REE, Lu and Er, are strongly fractionated from the other REE. An absence of mass fractionation among the Ti isotopes indicates that HIB-11 is not an evaporative residue, implying that volatility fractionation of trace elements took place during condensation. The fact that the two most refractory heavy REE could be separated from the other, only slightly less refractory heavy REE suggests that a wide variety of REE patterns is possible, and that ultrarefractory inclusions with other unusual REE patterns, important recorders of nebular condensation, may yet be discovered.     

The cosmic‐ray exposure history of the Twannberg iron meteorite (IIG)

The Twannberg iron meteorite is one out of only six members of the group IIG. The combined noble gas and radionuclide data obtained in this new systematic study indicate that Twannberg with its ~570 recently recovered specimens was a large object with a preatmospheric radius in the range of ~2 m, which corresponds to ~250 × 10³ kg. The cosmic‐ray exposure age for Twannberg is 182 ± 45 Ma. The most surprising result is the long terrestrial age of T_terr =  ka, which is unexpected considering the humid conditions in Switzerland. However, this age is in accord with glaciation events, indicating that the less shielded samples from Mt. Sujet were found close to the position of the original strewn field, whereas the samples from Gruebmatt and Twannbach, which are from more shielded positions, were glacially transported to the east–northeast during the second last ice age (185–130 ka ago) from an original position west of Mt. Sujet.     

The Sutter's Mill meteorite: Thermoluminescence data on thermal and metamorphic history

A piece of the Sutter's Mill meteorite, fragment SM2‐1d, has been examined using thermoluminescence techniques to better understand its thermal and metamorphic history. The sample had very weak but easily measureable natural and induced thermoluminescence (TL) signals; the signal‐to‐noise ratio was better than 10. The natural TL was restricted to the high‐temperature regions of the glow curve suggesting that the meteorite had been heated to approximately 300 °C within the time it takes for the TL signal to recover from a heating event, probably within the last 10⁵ years. It is possible that this reflects heating during release from the parent body, close passage by the Sun, or heating during atmospheric passage. Of these three options, the least likely is the first, but the other possibilities are equally likely. It seems that temperatures of approximately 300 °C reached 5 or 6 mm into the meteorite, so that all but one of the small Sutter's Mill stones have been heated. The Dhajala normalized induced TL signal for SM2‐1d is comparable to that of type 3.0 chondrites and is unlike normal CM chondrites, the class it most closely resembles, which do not have detectable TL sensitivity. The shape of the induced TL curve is comparable to other low‐type ordinary, CV, and CO chondrites, in that it has a broad hummocky structure, but does not resemble any of them in detail. This suggests that Sutter's Mill is a unique, low‐petrographic–type (3.0) chondrite.     

Exposure history and terrestrial ages of ordinary chondrites from the Dar al Gani region,Libya

Abstract— We measured the concentrations of noble gases in 32 ordinary chondrites from the Dar al Gani (DaG) region, Libya, as well as concentrations of the cosmogenic radionuclides ¹⁴C, ¹⁰Be, ²⁶Al, ³⁶Cl, and ⁴¹Ca in 18 of these samples. Although the trapped noble gases in five DaG samples show ratios typical of solar or planetary gases, in all other DaG samples, they are dominated by atmospheric contamination, which increases with the degree of weathering. Cosmic ray exposure (CRE) ages of DaG chondrites range from ?1 Myr to 53 Myr. The CRE age distribution of 10 DaG L chondrites shows a cluster around 40 Myr due to four members of a large L6 chondrite shower. The CRE age distribution of 19 DaG H chondrites shows only three ages coinciding with the main H chondrite peak at ?7 Myr, while seven ages are <5 Myr. Two of these H chondrites with short CRE ages (DaG 904 and 908) show evidence of a complex exposure history. Five of the H chondrites show evidence of high shielding conditions, including low ²²Ne/²¹Ne ratios and large contributions of neutron‐capture ³⁶Cl and ⁴¹Ca. These samples represent fragments of two or more large pre‐atmospheric objects, which supports the hypothesis that the high H/L chondrite ratio at DaG is due to one or more large unrecognized showers. The ¹⁴C concentrations correspond to terrestrial ages <35 kyr, similar to terrestrial ages of chondrites from other regions in the Sahara but younger than two DaG achondrites. Despite the loss of cosmogenic ³⁶Cl and ⁴¹Ca during oxidation of metal and troilite, concentrations of ³⁶Cl and ⁴¹Ca in the silicates are also consistent with ¹⁴C ages <35 kyr. The only exception is DaG 343 (H4), which has a ⁴¹Ca terrestrial age of 150 ± 40 kyr. This old age shows that not only iron meteorites and achondrites but also chondrites can survive the hot desert environment for more than 50 kyr. A possible explanation is that older meteorites were covered by soils during wetter periods and were recently exhumed by removal of these soils due to deflation during more arid periods, such as the current one, which started ?3000 years ago. Finally, based on the ²⁶Al/²¹Ne and ¹⁰Be/²¹Ne systematics in 16 DaG meteorites, we derived more reliable estimates of the ¹⁰Be/²¹Ne production rate ratio, which seems more sensitive to shielding than was predicted by the semi‐empirical model of Graf et al. (1990) but less sensitive than was predicted by the purely physical model of Leya et al. (2000).     

Infrared spectroscopy of interstellar nanodiamonds from the Orgueil meteorite

Abstract— We present results from an ongoing study of the infrared (IR) and optical properties of nanodiamonds, an objective of which is to identify spectral features in the laboratory that could also be used telescopically to trace the presence of these particles in the interstellar medium (ISM). Fourier transform mid-and far-infrared spectra of nanodiamond residue extracted from the Orgueil (CI) chondrite were acquired. All of the mid-IR bands initially present were found to diminish, with the exception of a band at ～1100 cm^?1, following additional oxidation of the diamonds. The ～1100 cm^?1 band can be predominantly attributed to adsorbed species, especially an ether-type linkage, while the “oxidisable” features seem to be associated with less stable, surface-bonded species and residual carbonaceous material. We obtained three far-IR features but are uncertain about the origin of those at 475 and 188 cm^?1. We did not obtain a feature at ～120 cm^?1 reported by another group but do not discount the possibility that the band at 188 cm^?1 could be related to it. The weak absorption band at 475 cm^?1 (21 μm) is especially interesting because it may be strong in emission from hot nanodiamonds and, therefore, related to the unidentified infrared feature (UIF) observed at this wavelength in the spectra of some C-rich protoplanetary nebulae.     

Mineralogy,petrology, chronology,and exposure history of the Chelyabinsk meteorite and parent body

Three masses of the Chelyabinsk meteorite have been studied with a wide range of analytical techniques to understand the mineralogical variation and thermal history of the Chelyabinsk parent body. The samples exhibit little to no postentry oxidation via Mössbauer and Raman spectroscopy indicating their fresh character, but despite the rapid collection and care of handling some low levels of terrestrial contamination did nonetheless result. Detailed studies show three distinct lithologies, indicative of a genomict breccia. A light‐colored lithology is LL5 material that has experienced thermal metamorphism and subsequent shock at levels near S4. The second lithology is a shock‐darkened LL5 material in which the darkening is caused by melt and metal‐troilite veins along grain boundaries. The third lithology is an impact melt breccia that formed at high temperatures (~1600 °C), and it experienced rapid cooling and degassing of S₂ gas. Portions of light and dark lithologies from Chel‐101, and the impact melt breccias (Chel‐102 and Chel‐103) were prepared and analyzed for Rb‐Sr, Sm‐Nd, and Ar‐Ar dating. When combined with results from other studies and chronometers, at least eight impact events (e.g., ~4.53 Ga, ~4.45 Ga, ~3.73 Ga, ~2.81 Ga, ~1.46 Ga, ~852 Ma, ~312 Ma, and ~27 Ma) are clearly identified for Chelyabinsk, indicating a complex history of impacts and heating events. Finally, noble gases yield young cosmic ray exposure ages, near 1 Ma. These young ages, together with the absence of measurable cosmogenic derived Sm and Cr, indicate that Chelyabinsk may have been derived from a recent breakup event on an NEO of LL chondrite composition.