Equilibrated ordinary chondrites: Constraints for thermal history from iron-magnesium ordering in orthopyroxene

Abstract— iron-magnesium ordering was determined in orthopyroxenes from two suites of unshocked (shock stage S1, S2), equilibrated L- and LL-chondrites (10 grains from 5 meteorites and 7 grains from 4 meteorites, respectively) by means of single crystal x-ray diffraction (SCXRD). This study, together with a previous investigation of H-chondrites (13 grains from 8 meteorites), produces an internally consistent data set about the thermal record in equilibrated ordinary chondrites (EOCs). The major feature outlined by cation ordering in EOC orthopyroxenes is that H-, L- and LL-chondrites share a common low-temperature record, that is, a common range of similar cooling rates in the 340–480 °C interval for the petrographic types 4 to 6. As a consequence, the thermal evolution of EOCs consists of at least two subsolidus stages; the first stage occurred at temperatures >480 °C where petrographic types were established in distinct environments; the second stage occurred when EOCs, irrespective of chemical class and type, cooled through 340–480 °C in environments characterized by close temperature-time conditions. Quantitative estimates of minimal cooling rates for EOCs range from a few °C/ka to ～10²°C/ka in the 340–480 °C interval. Possibly, final ordering was attained in environments where moderate radiative heat-loss was possible and, thus indicating shallow burial depths in the parent body.     

Microstructures of metal grains in ordinary chondrites: Implications for their thermal histories

Abstract— This paper reports one of the first attempts to investigate by analytical transmission electron microscopy (ATEM) the microstructures and compositions of Fe‐Ni metal grains in ordinary chondrites. Three ordinary chondrites, Saint Séverin (LL6), Agen (H5), and Tsarev (L6) were selected because they display contrasting microstructures, which reflects different thermal histories. In Saint Séverin, the microstructure of the Ni‐rich metal grains is due to slow cooling. It consists of a two‐phase assemblage with a honeycomb structure resulting from spinodal decomposition similar to the cloudy zone of iron meteorites. Microanalyses show that the Ni‐rich phase is tetrataenite (Ni = 47 wt%) and the Ni‐poor phase, with a composition of ～25% Ni, is either martensite or taenite, these two occurring adjacent to each other. The observation that the Ni‐poor phase is partly fcc resolves the disagreement between previous transmission electron microscopy (TEM) and Mössbauer studies on iron meteorites and ordinary chondrite metal. The Ni content of the honeycomb phase is much higher than in mesosiderites, confirming that mesosiderites cooled much more slowly. The high‐Ni tetrataenite rim in contact with the cloudy zone displays high‐Ni compositional variability on a very fine scale, which suggests that the corresponding area was destabilized and partially decomposed at low temperature. Both Agen and Tsarev display evidence of reheating and subsequent fast cooling obviously related to shock events. Their metallic particles mostly consist of martensite, the microstructure of which depends on local Ni content. Microstructures are controlled by both the temperature at which martensite forms and that at which it possibly decomposes. In high‐Ni zones (>15 wt%), martensitic transformation started at low temperature (<300 °C). Because no further recovery occurred, these zones contain a high density of lattice defects. In low‐Ni zones (<15 wt%), martensite grains formed at higher temperature and their lattice defects recovered. These martensite grains present a lath texture with numerous tiny precipitates of Ni‐rich taenite (Ni = 50 wt%) at lath boundaries. Nickel composition profiles across precipitate‐matrix interfaces show that the growth of these precipitates was controlled by preferential diffusion of Ni along lattice defects. The cooling rates deduced from Ni concentration profiles and precipitate sizes are within the range 1–10 °C/year for Tsarev and 10–100 °C/year for Agen.     

Thermoluminescence sensitivity and thermal history of type 3 ordinary chondrites: Eleven new type 3.0–3.1 chondrites and possible explanations for differences among H,L, and LL chondrites

Abstract— We review induced thermoluminescence (TL) data for 102 unequilibrated ordinary chondrites (UOCs), many data just published in abstracts, in order to identify particularly primitive UOCs and further explore TL systematics that may have implications for the history of the chondrites and their parent body. We have identified 11 UOCs of petrologic types 3.0–3.1: Adrar 003, Elephant Moraine (EET) 90066, EET 90161, Grosvenor Mountains (GRO) 95502, Lewis Cliff (LEW) 88477, Meteorite Hills (MET) 96503, Yamato (Y)‐790787, Y‐791324, Y‐791558, Y‐793565, and Y‐793596. These samples represent an important new resource for researchers interested in the nature of primitive solar system materials. Previously reported trends in which TL sensitivity increases with TL peak temperature and TL peak width, which we interpret in terms of crystallization of feldspar in the ordered or disordered forms during metamorphism, are confirmed by the new data. Importantly, the present data strengthen the trend described earlier in which the mean level of metamorphism experienced by UOCs increases along the series LL, L and H. This suggests either different burial depths for the UOCs from each class, or formation at similar depths in regoliths of different thickness.     

Volatiles in unequilibrated ordinary chondrites: Abundances,sources and implications for explosive volcanism on differentiated asteroids

Abstract— Keil and Wilson (1993) proposed that, during partial melting of some asteroidal meteorite parent bodies, explosive pyroclastic volcanism accelerated S-rich Fe, Ni-FeS cotectic partial melts into space. These authors argued that this process was responsible for the S-depletion of many of the magmas from which the magmatic iron meteorites formed. This process only requires the presence of a few hundred to thousand ppm of volatiles in asteroids < ～100 km in radius. If the precursor materials of these magmatic iron meteorite groups were similar in composition to unequilibrated ordinary chondrites, then the volatile contents of the latter may be a measure of the potential effectiveness of the process. Analysis of volatile contents of seven unequilibrated ordinary chondrite falls by dynamic high-temperature mass spectrometry revealed that thousands of ppm of indigeneous volatiles, mostly CO, Cl, Na and S, are released at temperatures near the Fe, Ni-FeS cotectic melting temperature of ～980 °C. If these volatiles are largely retained in the asteroidal parent bodies until onset of partial melting, S depletion of the residual melt might have been achieved by ejection of S-rich partial Fe, Ni-FeS melts by pyroclastic volcanism.     

Microstructure and thermal history of metal particles in CH chondrites

Abstract We have studied metal microstructures in four CH chondrites, Patuxent Range (PAT) 91546, Allan Hills (ALH) 85085, Acfer 214, and Northwest Africa (NWA) 739, to examine details of the thermal histories of individual particles. Four types of metal particles are common in all of these chondrites. Zoned and unzoned particles probably formed as condensates from a gas of chondritic composition in a monotonic cooling regime, as has been shown previously. We have demonstrated that these particles were cooled rapidly to temperatures below 500 K after they formed, and that condensation effectively closed around 700 K. Zoned and unzoned particles with exsolution precipitates, predominantly high‐Ni taenite, have considerably more complex thermal histories. Precipitates grew in reheating episodes, but the details of the heating events vary among individual grains. Reheating temperatures are typically in the range 800–1000 K. Reheating could have been the result of impact events on the CH parent body. Some particles with precipitates may have been incorporated into chondrules, with further brief heating episodes taking place during chondrule formation. In addition to the four dominant types of metal particles, rare Ni‐rich metal particles and Si‐rich metal particles indicate that the metal assemblage in CH chondrites was a mixture of material that formed at different redox conditions. Metal in CH chondrites consists of a mechanical mixture of particles that underwent a variety of thermal histories prior to being assembled into the existing brecciated meteorites.     

Porosity and density of ordinary chondrites: Clues to the formation of friable and porous ordinary chondrites

Abstract— Densities and porosities of meteorites are physical properties that can be used to infer characteristics of asteroid interiors. We report density and porosity measurements of 42 pieces of 30 ordinary chondrites and provide a quantification of the errors of the gas pycnometer method used in this study. Based on our measurements, we find that no significant correlation exists between porosity and petrologic grade, chemical group, sample mass, bulk and grain density, or shock level. To investigate variations in porosity and density between pieces of a meteorite, we examined stones from two showers, Holbrook and Pultusk. Examination of nine samples of Holbrook suggests relative homogeneity in porosity and density between pieces of this shower. Measurements of three samples of Pultusk show homogeneity in bulk density, in contrast to Wilkison and Robinson (2000), a study that reported significant variations in bulk density between 11 samples of Pultusk. Finally, examination of two friable ordinary chondrites, Bjurböle and Allegan, reveal variability in friability and porosity among pieces of the same fall. We suggest that friable ordinary chondrites may have formed in a regolith or fault zone of an asteroid.     

Magnetic classification of stony meteorites: 2. Non‐ordinary chondrites

Abstract— A database of magnetic susceptibility (χ) measurements on different non‐ordinary chondrites (C, E, R, and ungrouped) populations is presented and compared to our previous similar work on ordinary chondrites. It provides an exhaustive study of the amount of iron‐nickel magnetic phases (essentially metal and magnetite) in these meteorites. In contrast with all the other classes, CM and CV show a wide range of magnetic mineral content, with a two orders of magnitude variation of χ. Whether this is due to primary parent body differences, metamorphism or alteration, remains unclear. C3–4 and C2 yield similar χ values to the ones shown by CK and CM, respectively. By order of increasing χ, the classes with well‐grouped χ are: R << CO < CK ≈ CI < Kak < CR < E ≈ CH < CB. Based on magnetism, EH and EL classes have indistinguishable metal content. Outliers that we suggest may need to have their classifications reconsidered are Acfer 202 (CO), Elephant Moraine (EET) 96026 (C4–5), Meteorite Hills (MET) 01149, and Northwest Africa (NWA) 521 (CK), Asuka (A)‐88198, LaPaz Icefield (LAP) 031156, and Sahara 98248 (R). χ values can also be used to define affinities of ungrouped chondrites, and propose pairing, particularly in the case of CM and CV meteorites.     

Sodic plagioclase thermometry of type 6 ordinary chondrites: Implications for the thermal histories of parent bodies

Abstract— The structural states of sodic plagioclase crystals of ～50 μm in size from three H6, two L6, and one LL6 chondritic meteorites have been determined by measuring the Δ131 parameter with a Gandolfi camera after analyzing chemical compositions. The temperature for each sodic plagioclase crystal has been determined by plotting the Δ131 parameter, corrected for the influence of K, on the relation diagram between the Δ131 parameter and the temperature of synthesis of sodic plagioclase by Smith (1972). The temperature obtained is assigned to the crystallization temperature of sodic plagioclase, and the maximum plagioclase temperature for each meteorite can be assumed to correspond to the maximum temperature attained by each meteorite during metamorphism. The maximum metamorphic temperatures estimated are 725–742 °C for the H6 chondrites, 808–820 °C for the L6 chondrites, and 800 °C for the LL6 chondrite. These temperatures are lower than those based on Ca contents of clinopyroxenes (Dodd, 1981; McSween et al., 1988) but are consistent with those based on Ca contents of orthopyroxenes (McSween and Patchen, 1989; Langenhorst et al., 1995; Jones, 1997). The K content of sodic plagioclase correlates with the temperature obtained from the structural state. This positive correlation suggests that sodic plagioclase has formed in the course of equilibration processes of alkali elements in prograde metamorphism (i.e., during heating processes). The results of this study (i.e., the maximum metamorphic temperature of the H6 chondrites is lower than that of the L6 chondrites by ～80 °C, and meteorites of the same chemical group show very similar maximum metamorphic temperatures) are in accordance with the predictions of calculations based on the ²⁶Al heat source and the onion-shell structure model of the parent bodies.     

The porosities of ordinary chondrites: Models and interpretation

Abstract— Densities and porosities for 285 ordinary chondrites have been assembled and analyzed. Measured chondrite porosities are bimodal; finds have an average porosity of <3%, whereas fall porosities average 7% but range from zero to >30%. We conclude that mild degrees of weathering fill pore spaces, lowering grain densities and porosities without significantly changing the bulk size or mass of the sample. By assuming an original pristine grain density (as a function of the meteorite's mineralogy—determined by its class), we can derive model pristine porosities. These model porosities cluster around an average value of 10% for all classes of ordinary chondrites. Ordinary chondrites do not show any correlation of porosity (model or measured) with petrographic grade or sample size (over a range from 0.2 g to 2 kg). However, we do see a correlation between shock state and porosity. Shock-blackened meteorites are less porous than other meteorites. Furthermore, less severely shocked meteorites show a much broader range of porosities, with the maximum porosity seen among meteorites of a given shock class falling linearly as a function of that shock class. This is consistent with the idea that shock compresses and closes pore space. Analysis of meteorite porosity provides a lower bound to the fine-scale porosity of asteroids. Our densities, even with 10% primordial porosity, are significantly higher than inferred densities of possible asteroid parent bodies. These asteroids are probably loose piles of rubble.     

Fe‐Ni metal and sulfide minerals in CM chondrites: An indicator for thermal history

Abstract– CM chondrites were subjected to aqueous alteration and, in some cases, to secondary metamorphic heating. The effects of these processes vary widely, and have mainly been documented in silicate phases. Herein, we report the characteristic features of Fe‐Ni metal and sulfide phases in 13 CM and 2 CM‐related chondrites to explore the thermal history of these chondrites. The texture and compositional distribution of the metal in CM are different from those in unequilibrated ordinary and CO chondrites, but most have similarities to those in highly primitive chondrites, such as CH, CR, and Acfer 094. We classified the CM samples into three categories based on metal composition and sulfide texture. Fe‐Ni metal in category A is kamacite to martensite. Category B is characterized by pyrrhotite grains always containing blebs or lamellae of pentlandite. Opaque mineral assemblages of category C are typically kamacite, Ni‐Co‐rich metal, and pyrrhotite. These categories are closely related to the degree of secondary heating and are not related to degree of the aqueous alteration. The characteristic features of the opaque minerals can be explained by secondary heating processes after aqueous alteration. Category A CM chondrites are unheated, whereas those in category B experienced small degrees of secondary heating. CMs in category C were subjected to the most severe secondary heating process. Thus, opaque minerals can provide constraints on the thermal history for CM chondrites.     

Fe‐Ni metal in primitive chondrites: Indicators of classification and metamorphic conditions for ordinary and CO chondrites

Abstract— We report the results of our petrological and mineralogical study of Fe‐Ni metal in type 3 ordinary and CO chondrites, and the ungrouped carbonaceous chondrite Acfer 094. Fe‐Ni metal in ordinary and CO chondrites occurs in chondrule interiors, on chondrule surfaces, and as isolated grains in the matrix. Isolated Ni‐rich metal in chondrites of petrologic type lower than type 3.10 is enriched in Co relative to the kamacite in chondrules. However, Ni‐rich metal in type 3.15–3.9 chondrites always contains less Co than does kamacite. Fe‐Ni metal grains in chondrules in Semarkona typically show plessitic intergrowths consisting of submicrometer kamacite and Ni‐rich regions. Metal in other type 3 chondrites is composed of fine‐ to coarse‐grained aggregates of kamacite and Ni‐rich metal, resulting from metamorphism in the parent body. We found that the number density of Ni‐rich grains in metal (number of Ni‐rich grains per unit area of metal) in chondrules systematically decreases with increasing petrologic type. Thus, Fe‐Ni metal is a highly sensitive recorder of metamorphism in ordinary and carbonaceous chondrites, and can be used to distinguish petrologic type and identify the least thermally metamorphosed chondrites. Among the known ordinary and CO chondrites, Semarkona is the most primitive. The range of metamorphic temperatures were similar for type 3 ordinary and CO chondrites, despite them having different parent bodies. Most Fe‐Ni metal in Acfer 094 is martensite, and it preserves primary features. The degree of metamorphism is lower in Acfer 094, a true type 3.00 chondrite, than in Semarkona, which should be reclassified as type 3.01.     

Terrestrial weathering of ordinary chondrites in nature and continuing during laboratory storage and processing: Review and implications for Hayabusa sample integrity

Abstract– The Hayabusa mission recently returned the first samples from an ordinary chondrite (OC) parent body. Olivine, low‐Ca pyroxene, and kamacite compositions fall within the known ranges of minerals from LL4 to LL6 chondrites. Hayabusa samples are being processed and stored in a pure N₂ atmosphere. However, during recovery, prior to receiving, and during preliminary examination, some Hayabusa samples were briefly exposed to terrestrial atmosphere. Some of the minerals already identified in the Hayabusa samples (olivine, sulfides) are known to be among the most vulnerable to weathering reactions in moist, oxidizing terrestrial environments. Oxidation of Fe in metal, sulfides, and ferrous silicates is ubiquitous in naturally weathered OC finds, in samples of falls subjected to even a few decades of weathering before recovery, and in OC falls recovered and curated promptly after recovery. All prerecovery oxidation, hydrolysis, hydration, and product‐forming phenomena documented to affect OC finds have been documented to continue in OC samples in curatorial and laboratory settings, producing mineralogical and textural effects at scales easily discernable by electron microscopy, on timescales of decades. Hayabusa samples will be exposed to similar terrestrial conditions at times throughout sample processing, allocation, and examination. Maximizing the science yield from these important samples requires thorough understanding of how LL chondrite minerals like those in the Hayabusa samples react with terrestrial moisture and oxidants in support of proper planning for maintaining Hayabusa sample integrity after allocation, and for proper anticipation of the effects of inevitable exposure to Earth’s atmosphere during storage and examination in terrestrial analytical laboratories.     

Pyroxene structures,cathodoluminescence and the thermal history of the enstatite chondrites

Abstract— In order to explore the thermal history of enstatite chondrites, we examined the cathodoluminescence (CL) and thermoluminescence (TL) properties of 15 EH chondrites and 21 EL chondrites, including all available petrographic types, both textural types 3–6 and mineralogical types α–δ. The CL properties of EL3α and EH3α chondrites are similar. Enstatite grains high in Mn and other transition metals display red CL, while enstatite with low concentrations of these elements show blue CL. A few enstatite grains with >5 wt% FeO display no CL. In contrast, the luminescent properties of the metamorphosed EH chondrites are very different from those of metamorphosed EL chondrites. While the enstatites in metamorphosed EH chondrites display predominantly blue CL, the enstatites in metamorphosed EL chondrites display a distinctive magenta CL with blue and red peaks of approximately equal intensity in their spectra. The TL sensitivities of the enstatite chondrites correlate with the intensity of the blue CL and, unlike other meteorite classes, are not simply related to metamorphism. The different luminescent properties of metamorphosed EH and EL chondrites cannot readily be attributed to compositional differences. But x-ray diffraction data suggests that the enstatite in EH5γ,δ chondrites is predominantly disordered orthopyroxene, while enstatite in EL6β chondrites is predominantly ordered orthopyroxene. The difference in thermal history of metamorphosed EL and EH chondrites is so marked that the use of single “petrographic” types is misleading, and separate textural and mineralogical types are preferable. Our data confirm earlier suggestions that metamorphosed EH chondrites underwent relatively rapid cooling, and the metamorphosed EL chondrites cooled more slowly and experienced prolonged heating in the orthopyroxene field.     

The onset of metamorphism in ordinary and carbonaceous chondrites

Abstract— Ordinary and carbonaceous chondrites of the lowest petrologic types were surveyed by X‐ray mapping techniques. A variety of metamorphic effects were noted and subjected to detailed analysis using electron microprobe, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and cathodoluminescence (CL) methods. The distribution of Cr in FeO‐rich olivine systematically changes as metamorphism increases between type 3.0 and type 3.2. Igneous zoning patterns are replaced by complex ones and Cr‐rich coatings develop on all grains. Cr distributions in olivine are controlled by the exsolution of a Cr‐rich phase, probably chromite. Cr in olivine may have been partly present as tetrahedrally coordinated Cr³⁺. Separation of chromite is nearly complete by petrologic type 3.2. The abundance of chondrules showing an inhomogeneous distribution of alkalis in mesostasis also increases with petrologic type. TEM shows this to be the result of crystallization of albite. Residual glass compositions systematically change during metamorphism, becoming increasingly rich in K. Glass in type I chondrules also gains alkalis during metamorphism. Both types of chondrules were open to an exchange of alkalis with opaque matrix and other chondrules. The matrix in the least metamorphosed chondrites is rich in S and Na. The S is lost from the matrix at the earliest stages of metamorphism due to coalescence of minute grains. Progressive heating also results in the loss of sulfides from chondrule rims and increases sulfide abundances in coarse matrix assemblages as well as inside chondrules. Alkalis initially leave the matrix and enter chondrules during early metamorphism. Feldspar subsequently nucleates in the matrix and Na re‐enters from chondrules. These metamorphic trends can be used to refine classification schemes for chondrites. Cr distributions in olivine are a highly effective tool for assigning petrologic types to the most primitive meteorites and can be used to subdivide types 3.0 and 3.1 into types 3.00 through 3.15. On this basis, the most primitive ordinary chondrite known is Semarkona, although even this meteorite has experienced a small amount of metamorphism. Allan Hills (ALH) A77307 is the least metamorphosed CO chondrite and shares many properties with the ungrouped carbonaceous chondrite Acfer 094. Analytical problems are significant for glasses in type II chondrules, as Na is easily lost during microprobe analysis. As a result, existing schemes for chondrule classification that are based on the alkali content of glasses need to be revised.     

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).     

Chondrule thermal history from unequilibrated H chondrites: A transmission and analytical electron microscopy study

Abstract— Sixteen texturally different (porphyritic, barred, radial, cryptocrystalline) FeO‐rich chondrules from the unequilibrated ordinary chondrites Brownfield, Frontier Mountain (FRO) 90003 and FRO 90032 were characterized by optical and scanning electron microscopy and then thoroughly studied by transmission and analytical electron microscopy. Nanotextural and nanochemical data indicate similar thermal evolution for chondrules of the same textural groups; minor, yet meaningful differences occur among the different groups. Olivine is the earliest phase formed and crystallizes between 1500 and 1400 °C. Protoenstatite crystallizes at temperatures higher than 1350–1200 °C; it later inverts to clinoenstatite in the 1250–1200 °C range. Enstatite is surrounded by pigeonitic or (less frequently) augitic rims; the minimal crystallization temperature for the rims is 1000 °C; high pigeonite later inverts to low pigeonite, between 935 and 845 °C. The outer pigeonitic or augitic rims are constantly exsolved, producing sigmoidal augite or enstatite precipitates; sigmoidal precipitates record exsolution temperatures between 1000 and 640 °C. Cooling rate (determined using the speedometer based upon ortho‐clinoenstatite intergrowth) was in the order of 50–3000 °C/h at the clinoenstatite‐orthoenstatite transition temperature (close to 1250–1200 °C), but decreased to 5–10 °C/h or slower at the exsolution temperature (between 1000 and 650 °C), thus revealing nonlinear cooling paths. Nanoscale observations indicate that the individual chondrules formed and cooled separately from 1500 °C down to at least 650 °C. Accretion into chondritic parent body occurred at temperatures lower than 650 °C.     

Accessory phases in aubrites: spectral properties and implications for asteroid 44 Nysa

The reflectance spectra of meteoritic metal, meteoritic troilite and the CR carbonaceous chondrite EET87770 have been measured in order to investigate the causes of the spectral differences between the surface of the E-class asteroid 44 Nysa and the opaque free fraction of the Happy Canyon aubrite meteorite. The data indicate that the spectral differences require the presence on Nysa's surface of a small amount of a spectrally red sloped material, of which metal and troilite are the most reasonable candidates, and a material possessing absorption bands near 0.9m and 1.8m. A material similar to the carbonaceous chondrite inclusions found in some aubrites can provide a match to the 0.9m feature and perhaps the 1.8m feature. The required abundances of these components depends on whether they are areally distributed or intimately mixed with an enstatite rich material. Based on the petrologic associations seen in aubrites and a series of simulated mineral mixtures, an intimate mixture of 69–92% enstatite and 1–11% metal + troilite and an areal component of 7–20% carbonaceous chondrite type material can provide a reasonable match to the 0.3–2.6m spectrum of Nysa.     

Black ordinary chondrites: An analysis of abundance and fall frequency

Abstract— Black ordinary chondrite meteorites sample the spectral effects of shock on ordinary chondrite material in the space environment. Since shock is an important regolith process, these meteorites may provide insight into the spectral properties of the regoliths on ordinary chondrite parent bodies. To determine how common black chondrites are in the meteorite collection and, by analogy, the frequency of shock-alteration in ordinary chondrites, several of the world's major meteorite collections were examined to identify black chondrites. Over 80% of all catalogued ordinary chondrites were examined and, using an optical definition, 61 black chondrites were identified. Black chondrites account for approximately 13.7% of ordinary chondrite falls. If the optically altered gas-rich ordinary chondrites are included, the proportion of falls that exhibit some form of altered spectral properties increases to 16.7%. This suggests that optical alteration of asteroidal material in the space environment is a relatively common process.     

Barred olivine chondrules in ordinary chondrites: Constraints on chondrule formation

In general, barred olivine (BO) chondrules formed from completely melted precursors. Among BO chondrules in unequilibrated ordinary chondrites, there are significant positive correlations among chondrule diameter, bar thickness, and rim thickness. In the nebula, smaller BO precursor droplets cooled faster than larger droplets (due to their higher surface area/volume ratios) and grew thinner bars and rims. There is a bimodal distribution in the olivine FeO content in BO chondrules, with a hiatus between 11 and 19 wt% FeO. The ratio of (FeO rich)/(FeO poor) BO chondrules decreases from 12.0 in H to 1.6 in L to 1.3 in LL. This is the opposite of the case for porphyritic chondrules: the mean (FeO rich)/(FeO poor) modal ratio increases from 0.8 in H to 1.8 in L to 2.8 in LL. During H chondrite agglomeration, most precursor dustballs were small with low bulk FeO/(FeO + MgO) ratios and moderately high melting temperatures. The energy available for chondrule melting from flash heating was relatively low, capable of completely melting many ferroan dusty precursors (to form FeO-rich BO chondrules), but incapable of completely melting many magnesian dusty precursors (to form FeO-poor BO chondrules). When L and LL chondrites agglomerated somewhat later, significant proportions of precursor dustballs were relatively large and had moderately high bulk FeO/(FeO + MgO) ratios. The energy available from flash heating was higher, capable of completely melting higher proportions of magnesian dusty precursors to form FeO-poor BO chondrules. These differences may have resulted from an increase in the amplitude of lightning discharges in the nebula caused by enhanced charge separation.     

Chemical and oxygen isotopic properties of ordinary chondrites (H5, L6) from Oman: Signs of isotopic equilibrium during thermal metamorphism

Mean bulk chemical data of recently found H5 and L6 ordinary chondrites from the deserts of Oman generally reflect isochemical features which are consistent with the progressive thermal metamorphism of a common, unequilibrated starting material. Relative differences in abundances range from 0.5–10% in REE (Eu = 14%), 6–13% in siderophile elements (Co = 48%), and >10% in lithophile elements (exceptions are Ba, Sr, Zr, Hf, U = >30%) between H5 and L6 groups. These differences may have accounted for variable temperature conditions during metamorphism on their parent bodies. The CI/Mg‐normalized mean abundances of refractory lithophile elements (Al, Ca, Sm, Yb, Lu, V) show no resolvable differences between H5 and L6 suggesting that both groups have experienced the same fractionation. The REE diagram shows subtle enrichment in LREE with a flat HREE pattern. Furthermore, overall mean REE abundances are ~0.6 × CI with enriched La abundance (~0.9 × CI) in both groups. Precise oxygen isotope compositions demonstrate the attainment of isotopic equilibrium by progressive thermal metamorphism following a mass‐dependent isotope fractionation trend. Both groups show a ~slope‐1/2 line on a three‐isotope plot with subtle negative deviation in ?¹⁷O associated with δ¹⁸O enrichment relative to δ¹⁷O. These deviations are interpreted as the result of liberation of water from phyllosilicates and evaporation of a fraction of the water during thermal metamorphism. The resultant isotope fractionations caused by the water loss are analogous to those occurring between silicate melt and gas phase during CAI and chondrule formation in chondrites and are controlled by cooling rates and exchange efficiency.