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This document contains suggestions for best practices by authors who refer to meteorites in publications. It can also be taken as a guide for publishers in establishing guidelines for authors. The following best practices are recommended in addition to acknowledging the loaning institution or loaning individual (unless required otherwise). The main motivations are to: help ensure that research on meteorites is reproducible, prevent confusion in the literature, and enhance tracking of specimens and related data., SCOPUS: le.j, info:eu-repo/semantics/published

X-ray microcomputed tomography is a useful means of characterizing cosmochemical samples such as meteorites or robotically returned samples. However, there are occasional concerns that the use of μCT may be detrimental to the organic components of a chondrite. Small organic compounds such as amino acids comprise up to ~10% of the total solvent extractable carbon in CM carbonaceous chondrites. We irradiated three samples of the Murchison CM carbonaceous chondrite under conditions akin to and harsher than those typically used during typical benchtop x-ray μCT imaging experiments to determine if detectable changes in the amino acid abundance and distribution relative to a non-exposed Murchison control sample occurred. After subjecting three meteorite samples to ionizing radiation dosages between ~300 Gray (Gy) and 3 kGy with bremstrahlung X-rays, we analyzed the amino acid content of each sample. Within sampling and analytical errors, we found no differences in the amino acid abundances and amino acid enantiomeric ratios when comparing the control samples (non-exposed Murchison) and the irradiated samples. We conclude that a polychromatic X-ray μCT experiment has no detectable effect on the amino acid content of a CM type carbonaceous chondrite.

Miller Range 07273 is a chondritic melt breccia that contains clasts of equilibrated ordinary chondrite set in a fine-grained ( 3 GPa, possibly to ~15–19 GPa) and temperature (>1350 °C, possibly to ≥2000 °C). The higher pressures and temperatures would have involved back-reaction of high-pressure polymorphs to pyroxene and olivine upon cooling. Silicates outside of melt matrix have compositions that were relatively unchanged owing to brief heating duration.

X-ray computed tomography has become a popular means for examining the interiors of meteorites and has been advocated for routine curation and for the examination of samples returned by missions. Here, we report the results of a blind test that indicate that CT imaging deposits a considerable radiation dose in a meteorite and seriously compromises its natural radiation record. Ten vials of the Bruderheim L6 chondrite were placed in CT imager and exposed to radiation levels typical for meteorite studies. Half were retained as controls. Their thermoluminescence (TL) properties were then measured in a blind test. Five of the samples had TL data unaltered from their original (~10 cps) while five had very strong signals (~20,000 cps). It was therefore very clear which samples had been in the CT scanner. For comparison, the natural TL signal from Antarctic meteorites is ~5000–50,000 cps. Using the methods developed for Antarctic meteorites, the apparent dose absorbed by the five test samples was calculated to be 83 ± 5 krad, comparable with the highest doses observed in Antarctic meteorites and freshly fallen meteorites. While these results do not preclude the use of CT scanners when scientifically justified, it should be remembered that the record of radiation exposure to ionizing radiations for the sample will be destroyed and that TL, or the related optically stimulated luminescence, are the primary modern techniques for radiation dosimetry. This is particularly important with irreplaceable samples, such as meteorite main masses, returned samples, and samples destined for archive.

We report on a suite of microchondrules from three unequilibrated ordinary chondrites (UOCs). Microchondrules, a subset of chondrules that are ubiquitous components of UOCs, commonly occur in fine-grained chondrule rims, although may also occur within matrix. Microchondrules have a variety of textures: cryptocrystalline, microporphyritic, radial, glassy. In some cases, their textures, and in many cases, their compositions, are similar to their larger host chondrules. Bulk compositions for both chondrule populations frequently overlap. The primary material that composes many of the microchondrules has compositions that are pyroxene-normative and is similar to low-Ca-pyroxene phenocrysts from host chondrules; primary material rarely resembles olivine or plagioclase. Some microchondrules are composed of FeO-rich material that has compositions similar to the bulk submicron fine-grained rim material. These microchondrules, however, are not a common compositional type and probably represent secondary FeO-enrichment. Microchondrules may also be porous, suggestive of degasing to form vesicles. Our work shows that the occurrence of microchondrules in chondrule rims is an important constraint that needs to be considered when evaluating chondrule-forming mechanisms. We propose that microchondrules represent melted portions of the chondrule surfaces and/or the melt products of coagulated dust in the immediate vicinity of the larger chondrules. We suggest that, through recycling events, the outer surfaces of chondrules were heated enough to allow microchondrules to bud off as protuberances and become entrained in the surrounding dusty environment as chondrules were accreting fine-grained rims. Microchondrules are thus byproducts of cyclic processing of chondrules in localized environments. Their occurrence in fine-grained rims represents a snapshot of the chondrule-forming environment. We evaluate mechanisms for microchondrule formation and hypothesize a potential link between the emergence of type II chondrules in the early solar system and the microchondrule-bearing fine-grained rims surrounding type I chondrules.

Low-iron, manganese-enriched (LIME) olivine grains are found in cometary samples returned by the Stardust mission from comet 81P/Wild 2. Similar grains are found in primitive meteoritic clasts and unequilibrated meteorite matrix. LIME olivine is thermodynamically stable in a vapor of solar composition at high temperature at total pressures of a millibar to a microbar, but enrichment of solar composition vapor in a dust of chondritic composition causes the FeO/MnO ratio of olivine to increase. The compositions of LIME olivines in primitive materials indicate oxygen fugacities close to those of a very reducing vapor of solar composition. The compositional zoning of LIME olivines in amoeboid olivine aggregates is consistent with equilibration with nebular vapor in the stability field of olivine, without re-equilibration at lower temperatures. A similar history is likely for LIME olivines found in comet samples and in interplanetary dust particles. LIME olivine is not likely to persist in nebular conditions in which silicate liquids are stable.

The Stardust mission captured comet Wild 2 particles in aerogel at 6.1 km s{sup -1}. We performed high-resolution three-dimensional imaging and X-ray fluorescence mapping of whole cometary tracks in aerogel. We present the results of a survey of track structures using laser scanning confocal microscopy, including measurements of track volumes, entry hole size, and cross-sectional profiles. We compare various methods for measuring track parameters. We demonstrate a methodology for discerning hypervelocity particle ablation rates using synchrotron-based X-ray fluorescence, combined with mass and volume estimates of original impactors derived from measured track properties. Finally, we present a rough framework for reconstruction of original impactor size, and volume of volatilized material, using our measured parameters. The bulk of this work is in direct support of nondestructive analysis and identification of cometary grains in whole tracks, and its eventual application to the reconstruction of the size, shape, porosity, and chemical composition of whole Stardust impactors.

– Northwest Africa 5492 is a new metal-rich chondrite breccia that may represent a new oxygen reservoir and new chondrite parent body. It has some textural similarities to CB and CH chondrites, but silicates are more reduced, sulfides are more common and not associated with metal, and metal compositions differ from CB and CH chondrites. Oxygen isotope ratios indicate that Northwest Africa (NWA) 5492 components (chondrules and lithic fragments) formed in at least two different oxygen reservoirs. The more common, and presumably host, component plots in a region above the terrestrial fractionation line, below ordinary chondrite compositions, and just above enstatite chondrites in 3-oxygen space. The only other chondritic materials that plot in this region are chondrules from the Grosvenor Mountains (GRO) 95551 ungrouped metal-rich chondrite. The other rare component plots near the CR, CB, and CH chondrites. Based on petrologic characteristics and oxygen isotopic compositions, NWA 5492 appears to be related to the ungrouped metal-rich GRO 95551 chondrite.

– Here, we show that several geochemical indicators point to number densities during chondrule formation that were far higher than can be accounted for by known nebula processes. The number densities implied by compound chondrules and nonspherical chondrules are shown to be significantly higher than estimated in previous studies. At the implied chondrule number densities, if a chondrule formation region survived a formation event it would have been gravitationally bound and would have collapsed quite rapidly to form an asteroidal-sized body. The diversity of chondrule compositions and textures in a chondrite group could have formed in a single event in subvolumes of a formation region that were chemically isolated from one another because of slow diffusion in the gas. Within these subvolumes, equilibration between chondrules with different compositions would have been fairly rapid, although small isotopic mass fractionations in elements like Fe, Si, Mg, and O may persist. This could explain the existence of the small isotopic mass fractionations in these elements that have been observed in chondrules. However, the evidence for recycling of chondrules requires that there was more than one chondrule formation event prior to formation of a parent asteroid. Finally, we argue that OC and CO chondrule Mg-Al systematics are both consistent with single ages or narrow ranges of ages, and that the CO, and possibly the OC, ages date parent body alteration. This would resolve the conundrum of needing to preserve in a turbulent nebula physically and chemically distinct CO and OC chondrule populations for 1–2 Myr.

Evaporation rates of K2O, Na2O, and FeO from chondrule-like liquids and the associated potassium isotopic fractionation of the evaporation residues were measured to help understand the processes and conditions that affected the chemical and isotopic compositions of olivine-rich Type IA and Type IIA chondrules from Semarkona. Both types of chondrules show evidence of having been significantly or totally molten. However, these chondrules do not have large or systematic potassium isotopic fractionation of the sort found in the laboratory evaporation experiments. The experimental results reported here provide new data regarding the evaporation kinetics of sodium and potassium from a chondrule-like melt and the potassium isotopic fractionation of evaporation residues run under various conditions ranging from high vacuum to pressures of one bar of H2+CO2, or H2, or helium. The lack of systematic isotopic fractionation of potassium in the Type IIA and Type IA chondrules compared with what is found in the vacuum and one-bar evaporation residues is interpreted as indicating that they evolved in a partially closed system where the residence time of the surrounding gas was sufficiently long for it to have become saturated in the evaporating species and for isotopic equilibration between the gas and the melt. A diffusion couple experiment juxtaposing chondrule-like melts with different potassium concentrations showed that the diffusivity of potassium is sufficiently fast at liquidus temperatures (DK>2-10-4cm2/s at 1650-C) that diffusion-limited evaporation cannot explain why, despite their having been molten, the Type IIA and Type IA chondrules show no systematic potassium isotopic fractionation.

We report analyses of aerogel tracks using (1 synchrotron X-ray computed microtomography (XRCMT), (2) laser confocal scanning microscopy (LCSM), and (3) synchrotron radiation X-ray fluorescence (SRXRF) of particles and their paths resulting from simulated hypervelocity impacts (1-2), and a single ~1 mm aerogel track from the Stardust cometary sample collector (1-3). Large aerogel pieces can be imaged sequentially, resulting in high spatial resolution images spanning many tomographic fields of view (lambda-tomography). We report calculations of energy deposited, and tests on aromatic hydrocarbons showing no alteration in tomography experiments. Imaging at resolutions from ~17 to ~1 micron/pixel edge (XRCMT) and to

We document the size distributions and locations of voids present within five highly porous equilibrated ordinary chondrites using high-resolution synchrotron X-ray microtomography (μCT) and helium pycnometry. We found total porosities ranging from ~10 to 20% within these chondrites, and with μCT we show that up to 64% of the void space is located within intergranular voids within the rock. Given the low (S1-S2) shock stages of the samples and the large voids between mineral grains, we conclude that these samples experienced unusually low amounts of compaction and shock loading throughout their entire post accretionary history. With Fe metal and FeS metal abundances and grain size distributions, we show that these chondrites formed naturally with greater than average porosities prior to parent body metamorphism. These materials were not fluffed on their parent body by impact-related regolith gardening or events caused by seismic vibrations. Samples of all three chemical types of ordinary chondrites (LL, L, H) are represented in this study and we conclude that incomplete compaction is common within the asteroid belt.

Fountain Hills is a metal-rich chondrite with mineral and whole chondrite oxygen isotope compositions that suggest it is a CB chondrite. However, its petrologic characteristics suggest that it has been modified by shock and recrystallization to a greater degree than other CB chondrites. It differs texturally from the CB chondrites in that its metal is interstitial to the silicate and does not occur as discrete clasts as in the other CB chondrites. Portions of Fountain Hills appear to be recrystallized and it contains large (mm-size) olivine rimmed by pyroxene. A characteristic of the CB chondrites is the presence of small sulfide blebs in large metal clasts and anomalously heavy (15N-enriched) nitrogen often associated with metal surrounding the sulfide blebs, but Fountain Hills lacks sulfide and its nitrogen is relatively light. The differences between Fountain Hills and the other CB chondrites can be attributed to a secondary process, most likely impact-melting and recrystallization, that overprinted its primary features and it is inferred that Fountain Hills is an impact-modified CB chondrite.

We used synchrotron X-ray microtomography to image in 3-dimensions (3D) eight whole chondrules in a ~1 cm^3 piece of the Renazzo (CR) chondrite at ~17 μm per volume element (voxel) edge. We report the first volumetric (3D) measurement of metal/silicate ratios in chondrules and quantify indices of chondrule sphericity. Volumetric metal abundances in whole chondrules range from 1 to 37 volume % in 8 measured chondrules and by inspection in tomography data. We show that metal abundances and metal grain locations in individual chondrules cannot be reliably obtained from single random 2D sections. Samples were physically cut to intersect representative chondrules multiple times and to verify 3D data. Detailed 2D chemical analysis combined with 3D data yield highly variable whole-chondrule Mg/Si ratios with a supra-chondritic mean value, yet the chemically diverse, independently formed chondrules are mutually complementary in preserving chondritic (solar) Fe/Si ratios in the aggregate CR chondrite. These results are consistent with localized chondrule formation and rapid accretion resulting in chondrule + matrix aggregates (meteorite parent bodies) that preserve the bulk chondritic composition of source regions.

Methods of synchrotron X-ray computed microtomography (XRCMT) are described, which allow nondestructive, high spatial and contrast resolution imaging of the density structures of meteorites and their components in three dimensions. Images of bulk chondrites (to one cubic centimeter in size) reveal compound chondrules, chondrule/matrix volumetric ratios, metal and sulfide distribution, petrofabrics, and 3-D chondrule and calcium-aluminum inclusion (CAI) sizes and shapes. Images of separated chondrules and CAIs reveal void spaces, mineral intergrowth textures, and the true locations of crystal rims and cores, at resolutions to

Amoeboid olivine aggregates (AOAs) are irregularly shaped, fine-grained aggregates of olivine and Ca, Al-rich minerals and are important primitive components of CR chondrites. The AOAs in CR chondrites contain FeNi metal, and some AOAs contain Mn-rich forsterite with up to 0.7 MnO and Mn:Fe ratios greater than one. Additionally, AOAs in the CR chondrites do not contain secondary phases (nepheline and fayalitic olivine) that are found in AOAs in some CV chondrites. The AOAs in CR chondrites record a complex petrogenetic history that included nebular gas-solid condensation, reaction of minerals with the nebular gas, small degrees of melting, and sintering of the assemblage. A condensation origin for the Mn-rich forsterite is proposed. The Mn-rich forsterite found in IDPs, unequilibrated ordinary chondrite matrix, and AOAs in CR chondrites may have had a similar origin. A type A calcium, aluminum-rich inclusion (CAI) with an AOA attached to its Wark-Lovering rim is also described. This discovery reveals a temporal relationship between AOAs and type A inclusions. Additionally, a thin layer of forsterite is present as part of the Wark-Lovering rim, revealing the crystallization of olivine at the end stages of the Wark-Lovering rim formation. The Ca, Al-rich nodules in the AOAs may be petrogenetically related to the Ca, Al-rich minerals in Wark- Lovering rims on type A CAIs. AOAs are chondrite components that condensed during the final stage of Wark-Lovering rim formation but, in general, were temporally, spatially, or kinetically isolated from reacting with the nebula vapor during condensation of the lower temperature minerals that were commonly present as chondrule precursors.

— In addition to the Mg-, Al-, 16O-rich spinels that are known to occur in refractory inclusions, the Murchison meteorite contains Cr-rich, 16O-poor spinels, most of whose sources are unknown because they are rarely found in situ. Here we report the in situ occurrence in Allende of Cr-rich spinels, found in 13 chondrules and 4 “olivine-rich objects”. The Allende spinels exhibit major and minor element contents, isotopic compositions, and zoning of Cr2O3 contents like those of the Cr-spinels from Murchison. Some chondrules contain patchy-zoned spinel (Simon et al., 1994), which suggests that such grains did not form by sintering but perhaps by formation of overgrowths on relic grains. Unlike the olivine-rich objects, phases in all three chondrules that were analyzed by ion microprobe have uniform, near-normal O-isotopic compositions. One olivine-rich object, ALSP1, has a huge (1 mm) fragment of chevron-zoned spinel. This spinel has near-normal O-isotopic compositions that are quite distinct from those of adjacent forsteritic olivine, which are relatively 16O-rich and plot on the calcium-aluminum-inclusion (CAI) line, like some isolated forsterite grains found in Allende. The spinel and olivine in this object are therefore not genetically related to each other. Another olivine-rich object, ALSP11A, contains a rectangular, 150 ×s 100 μm, homogeneous spinel grain with 50 wt% Cr2O3 and 23 wt% FeO in a vuggy aggregate of finer-grained (5–90 μm), FeO-rich (Fo47–55) olivine. The magnesian core of one olivine grain has a somewhat 16O-rich isotopic composition like that of the large spinel, whereas the FeO-rich olivine is relatively 16O-poor. The composition of the spinel in ALSP11A plots on the CAI line, the first Cr-rich spinel found to do so. Chevron-zoned spinel has not been observed in chondrules, and it is unlikely that either ALSP1 or ALSP11A were ever molten. Calculations show that a spinel with the composition of that in ALSP1 can condense at 1780 K at a Ptot of 10−3 atm and a dust/gas ratio of 100 relative to solar. The Cr-rich spinel in ALSP11A could condense at ∼1420 K, but this would require a dust/gas enrichment of 1000 relative to solar. The data presented here confirm that, as in Murchison, the coarse Cr-rich spinels in Allende are relatively 16O-depleted and are isotopically distinct from the 16O-enriched MgAl2O4 from CAIs. Sample ALSP11A may represent a third population, one that is Cr-rich and plots on the CAI line. That the O-isotopic composition of ALSP1 is like those of Cr-rich spinels from chondrules indicates that O-isotopic compositions cannot be used to distinguish whether grains from such unequilibrated objects are condensates or are fragments from a previous generation of chondrules.

An algorithm is described for the calculation of equilibrium compositions of multiple highly nonideal liquid and solid solutions, as well as pure stoichiometric phases, coexisting with a mixture of ideal gas species at fixed temperature and pressure. The total Gibbs free energy of the system is approximated as a quadratic function of the compositions of the gas phase and stable condensed phases, in an orthogonal basis set of pure elements. Only changes in thermal energy and energy related to pressure-volume work are considered. The total Gibbs energy is minimized directly by use of both the slope and the curvature of the Gibbs energy surface with respect to the gas and condensed phase compositions in the basis elements. The algorithm described has been implemented in a computer code for the calculation of condensation sequences for cosmic nebular gases enriched in dust. Machine, compiler and library requirements for performing these calculations in the C programming language are compared. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 247–256, 2000

In the sulfide mineral assemblages commonly found in terrestrial rocks, the major elements are S with the metals Fe, Zn, Cu, Pb, and Ag and semimetals As and Sb. Minor elements include Te, Se; the metals Ni, Cd, Co, Mn and the semimetals Hg and Bi. Particular ore deposits can be characterized using specific subsystems containing these elements (e.g., McKinstry 1963). Previous reviews have summarized in detail the phase relations in binary, ternary, and higher-order systems. These include Barton (1970), the first volume in this series (Craig and Scott 1976), excellent chapters by Barton and Skinner (1967, 1979), and Vaughan and Craig (1997) in Barnes’ valuable compendium, and particularly the comprehensive volume by Vaughan and Craig (1978, chapter 8). Characteristics of a few important subsystems are briefly presented here, but our emphasis is on new results since 1980, with an emphasis on equations of state suitable for computer-assisted calculation of phase relations applicable to terrestrial ore deposits, and generally to assemblages quenchable from natural systems. Most of these new results focus on the properties of sulfide mineral solutions and phase relations in portions of the supersystem Ag2S-Cu2S-ZnS-FeS-PbS-Sb2S3-As2S3-Bi2S3-Au-S2. Solid solutions of a few minerals in this multisystem constitute a significant fraction of many hydrothermal ores, particular when they are initially deposited. Some of these solid solutions are not quenchable in phase (e.g., (Ag,Cu)2S solid solutions) or composition (e.g., PbS-AgSbS2-AgBiS2 galena solid solution), their instability with cooling giving rise to a multiplicity of more nearly stoichiometric sulfides and/or an exceeding diverse suite of modular sulfosalts (e.g., Skinner 1966; Hall and Czamanske 1972; Anthony et al. 1990; Makovicky 1997; see also in …

Planet formation: Theory, observations, and experiments, by Hubert Klahr and Wolfgang Brandner