Impact ages of meteorites: A synthesis

Abstract— Isotopic ages of meteorites that indicate chronometer resetting due to impact heating are summarized. Most of the ages were obtained by the ³⁹Ar-⁴⁰Ar technique, but several Rb-Sr, Pb-Pb, and Sm-Nd ages also suggest some degree of impact resetting. Considerations of experimental data on element diffusion in silicates suggest that various isotopic chronometers ought to differ in their ease of resetting during shock heating in the order K-Ar (easiest), Rb-Sr, Pb-Pb, and Sm-Nd, which is approximately the order observed in meteorites. Partial rather than total chronometer resetting by impacts appears to be the norm; consequently, interpretation of the event age is not always straightforward. Essentially all ³⁹Ar-⁴⁰Ar ages of eucrites and howardites indicate partial to total resetting in the relatively narrow time interval of 3.4–4.1 Ga ago (1 Ga = 10⁹ years). Several disturbed Rb-Sr ages appear consistent with this age distribution. This grouping of ages and the brecciated nature of many eucrites and all howardites argues for a large-scale impact bombardment of the HED parent body during the same time period that the Moon received its cataclysmic bombardment. Other meteorite parent bodies such as those of mesosiderites, some chondrites, and IIE irons also may have experienced this bombardment. These data suggest that the early bombardment was not lunar specific but involved much of the inner Solar System, and may have been caused by breakup of a larger planetismal. Although a few chondrites show evidence of age resetting ～3.5–3.9 Ga ago, most impact ages of chondrites tend to fall below 1.3 Ga in age. A minimum of ～4 impact events, including events at 0.3, 0.5, 1.2, and possibly 0.9 Ga appear to be required to explain the younger ages of H, L, and LL chondrites, although additional events are possible. Most L chondrites show evidence of shock, and the majority of ³⁹Ar-⁴⁰Ar ages of L chondrites fall near 0.5 Ga. The L chondrite parent body apparently experienced a major impact at this time, which may have disrupted it. The observations (1) that lunar highland rocks experienced major impact resetting of various isotopic chronometers ～3.7–4.1 Ga ago; (2) that the HED parent body experienced widespread impact resetting of the K-Ar chronometer but only modest disturbance of other isotopic systems, during a similar time period; (3) that ordinary chondrite parent bodies show much more recent and less extensive impact resetting; and (4) that impacts, which initiated cosmic-ray exposure of most stone meteorites almost never reset isotopic chronometers, may all be a consequence of relative parent body size. Greater degrees of isotopic chronometer resetting occur in larger and warmer impact ejecta deposits that cool slowly. The relatively greater size of bodies like the Moon and Vesta (assumed to be the parent asteroid of HED meteorites) both permit such favorable ejecta deposits to occur more easily compared to smaller parent bodies (generally assumed for chondrites) and also protect parent objects from collisional disruption. Thus, impacts on larger bodies would tend to more easily reset chronometers, consistent with the observed relative ease of resetting of Moon (easiest), HED, chondrites and of K-Ar (easiest), Rb-Sr, other chronometers. In contrast, the more recent impact ages of chondrites are postulated to represent collisional disruption of smaller parent objects whose fragments are more readily removed from the meteorite source reservoirs. Impacts that initiate cosmic-ray exposure are mostly small in scale and produce little heating.     

Restriction of parent body heating by metal‐troilite melting: Thermal models for the ordinary chondrites

Ordinary chondrite meteorites contain silicates, Fe,Ni‐metal grains, and troilite (FeS). Conjoined metal‐troilite grains would be the first phase to melt during radiogenic heating in the parent body, if temperatures reached over approximately 910–960 °C (the Fe,Ni‐FeS eutectic). On the basis of two‐pyroxene thermometry of 13 ordinary chondrites, we argue that peak temperatures in some type 6 chondrites exceeded the Fe,Ni‐FeS eutectic and thus conjoined metal‐troilite grains would have begun to melt. Melting reactions consume energy, so thermal models were constructed to investigate the effect of melting on the thermal history of the H, L, and LL parent asteroids. We constrained the models by finding the proportions of conjoined metal‐troilite grains in ordinary chondrites using high‐resolution X‐ray computed tomography. The models show that metal‐troilite melting causes thermal buffering and inhibits the onset of silicate melting. Compared with models that ignore the effect of melting, our models predict longer cooling histories for the asteroids and accretion times that are earlier by 61, 124, or 113 kyr for the H, L, and LL asteroids, respectively. Because the Ni/Fe ratio of the metal and the bulk troilite/metal ratio is higher in L and LL chondrites than H chondrites, thermal buffering has the greatest effect in models for the L and LL chondrite parent bodies, and least effect for the H chondrite parent. Metal‐troilite melting is also relevant to models of primitive achondrite parent bodies, particularly those that underwent only low degrees of silicate partial melting. Thermal models can predict proportions of petrologic types formed within an asteroid, but are systematically different from the statistics of meteorite collections. A sampling bias is interpreted to explain these differences.     

Peak metamorphic temperatures in type 6 ordinary chondrites: An evaluation of pyroxene and plagioclase geothermometry

Abstract— Quantifying the peak temperatures achieved during metamorphism is critical for understanding the thermal histories of ordinary chondrite parent bodies. Various geothermometers have been used to estimate equilibration temperatures for chondrites of the highest metamorphic grade (type 6), but results are inconsistent and span hundreds of degrees. Because different geothermometers and calibration models were used with different meteorites, it is unclear whether variations in peak temperatures represent actual ranges of metamorphic conditions within type 6 chondrites or differences in model calibrations. We addressed this problem by performing twopyroxene geothermometry, using QUILF95, on the same type 6 chondrites for which peak temperatures were estimated using the plagioclase geothermometer (Nakamuta and Motomura 1999). We also calculated temperatures for published pyroxene analyses from other type 6 H, L, and LL chondrites to determine the most representative peak metamorphic temperatures for ordinary chondrites. Pyroxenes record a narrow, overlapping range of temperatures in H6 (865–926 °C), L6 (812–934 °C), and LL6 (874–945 °C) chondrites. Plagioclase temperature estimates are 96–179 °C lower than pyroxenes in the same type 6 meteorites. Plagioclase estimates may not reflect peak metamorphic temperatures because chondrule glass probably recrystallized to plagioclase prior to reaching the metamorphic peak. The average temperature for H, L, and LL chondrites (～900 °C), which agrees with previously published oxygen isotope geothermometry, is at least 50 °C lower than the peak temperatures used in current asteroid thermal evolution models. This difference may require minor adjustments to thermal model calculations.     

Ar‐Ar ages and thermal histories of enstatite meteorites

Abstract– Compared with ordinary chondrites, there is a relative paucity of chronological and other data to define the early thermal histories of enstatite parent bodies. In this study, we report ³⁹Ar‐⁴⁰Ar dating results for five EL chondrites: Khairpur, Pillistfer, Hvittis, Blithfield, and Forrest; five EH chondrites: Parsa, Saint Marks, Indarch, Bethune, and Reckling Peak 80259; three igneous‐textured enstatite meteorites that represent impact melts on enstatite chondrite parent bodies: Zaklodzie, Queen Alexandra Range 97348, and Queen Alexandra Range 97289; and three aubrites, Norton County, Bishopville, and Cumberland Falls Several Ar‐Ar age spectra show unusual ³⁹Ar recoil effects, possibly the result of some of the K residing in unusual sulfide minerals, such as djerfisherite and rodderite, and other age spectra show ⁴⁰Ar diffusion loss. Few additional Ar‐Ar ages for enstatite meteorites are available in the literature. When all available Ar‐Ar data on enstatite meteorites are considered, preferred ages of nine chondrites and one aubrite show a range of 4.50–4.54 Ga, whereas five other meteorites show only lower age limits over 4.35–4.46 Ga. Ar‐Ar ages of several enstatite chondrites are as old or older as the oldest Ar‐Ar ages of ordinary chondrites, which suggests that enstatite chondrites may have derived from somewhat smaller parent bodies, or were metamorphosed to lower temperatures compared to other chondrite types. Many enstatite meteorites are brecciated and/or shocked, and some of the younger Ar‐Ar ages may record these impact events. Although impact heating of ordinary chondrites within the last 1 Ga is relatively common for ordinary chondrites, only Bethune gives any significant evidence for such a young event.     

39Ar‐40Ar chronology of R chondrites

Abstract— This study presents the first determinations of ³⁹Ar‐⁴⁰Ar ages of R chondrites for the purpose of understanding the thermal history of the R chondrite parent body. The ³⁹Ar‐⁴⁰Ar ages were determined on whole‐rock samples of four R chondrites: Carlisle Lakes, Rumuruti, Acfer 217, and Pecora Escarpment #91002 (PCA 91002). All samples are breccias except for Carlisle Lakes. The age spectra are complicated by recoil and diffusive loss to various extents. The peak ³⁹Ar‐⁴⁰Ar ages of the four chondrites are 4.35, ?4.47 ± 0.02, 4.30 ± 0.07 Ga, and 4.37 Ga, respectively. These ages are similar to Ar‐Ar ages of relatively unshocked ordinary chondrites (4.52–4.38 Ga) and are older than Ar‐Ar ages of most shocked ordinary chondrites («4.2 Ga). Because the meteorites with the oldest (Rumuruti, ?4.47 Ga) and the youngest (Acfer 217, ?4.30 Ga) ages are both breccias, these ages probably do not record slow cooling within an undisrupted asteroidal parent body. Instead, the process of breccia formation may have differentially reset the ages of the constituent material, or the differences in their age spectra may arise from mixtures of material that had different ages. Two end‐member type situations may be envisioned to explain the age range observed in the R chondrites. The first is if the impact(s) that reset the ages of Acfer 217 and Rumuruti was very early. In this case, the ?170 Ma maximum age difference between these meteorites may have been produced by much deeper burial of Acfer 217 than Rumuruti within an impact‐induced thick regolith layer, or within a rubble pile type parent body following parent body re‐assembly. The second, preferred scenario is if the impact that reset the age of Acfer 217 was much later than that which reset Rumuruti, then Acfer 217 may have cooled more rapidly within a much thinner regolith layer. In either scenario, the oldest age obtained here, from Rumuruti, provides evidence for relatively early (?4.47 Ga) impact events and breccia formation on the R chondrite parent body.     

Noble gases and mineralogy of meteorites from China and the Grove Mountains,Antarctica: A 0.05 Ma cosmic ray exposure age of GRV 98004

Abstract— We determined the mineralogical and chemical characteristics and the He, Ne, and Ar isotopic abundances of 2 meteorites that fell in China and of 2 meteorites that were recovered by the 15th Chinese Antarctic Research Expedition. Guangmingshan (H5), Zhuanghe (H5), and Grove Mountain (GRV) 98002 (L5) yield cosmic ray exposure (CRE) ages of 68.7 ± 10.0 Ma, 3.8 ± 0.6 Ma, and 17.0 ± 2.5 Ma, respectively. These ages are within the range typically observed for the respective meteorite types. GRV 98004 (H5) had an extremely short parent body‐Earth transfer time of 0.052 ± 0.008 Ma. Its petrography and mineral chemistry are indistinguishable from other typical H5 chondrites. Only 3 other meteorites exist with similarly low CRE ages: Farmington (L5), Galim (LL6), and ALH 82100 (CM2). We show that several asteroids in Earth‐crossing orbits, or in the main asteroid belt with orbits close to an ejection resonance, are spectrally matching candidates and may represent immediate precursor bodies of meteorites with CRE ages ≤0.1 Ma.     

FRAGMENTAL BRECCIAS AND THE COLLISIONAL EVOLUTION OF ORDINARY CHONDRITE PARENT BODIES

Our survey of type 4–6 ordinary chondrites indicates that gas-poor, melt-rock and/or exotic clast-bearing fragmental breccias constitute 5%, 22% and 23%, respectively, of H, L and LL chondrites. These abundances contrast with the percentages of solar-gas-rich regolith breccias among ordinary chondrites: H (14%), L (3%) and LL (8%) (Crabb and Schultz, 1981). Petrologic study of several melt-rock-clast-bearing fragmental breccias indicates that some acquired their clasts prior to breccia metamorphism and others acquired them after metamorphism of host material. In general, the melt-rock clasts in gas-poor H chondrite fragmental breccias were acquired after breccia metamorphism and were probably formed by impacts into boulders or exposed outcrops of H4-6 material in the H chondrite parent body regolith. In contrast, most of the melt-rock clasts in gas-poor L and LL fragmental breccias were acquired prior to breccia metamorphism. The low abundance of regolith breccias among L chondrites and evidence that at least two-thirds of the L chondrites suffered a major shock event 0.5 Gyr ago, suggest that the L parent body may have been disrupted by a major collision at that time and that the remaining parent body fragments were too small to develop substantial regoliths (e.g., Heymann, 1967; Crabb and Schultz, 1981). Such a disruption would have exposed a large amount of L chondrite bedrock, some of which would consist of fragmental breccias that acquired melt-rock clasts very early in solar system history, prior to metamorphism. The exposed bedrock would serve as a potential target for sporadic meteoroid impacts to produce a few fragmental breccias with unmetamorphosed melt-rock clasts. The high proportion of genomict brecciated LL chondrites reflects a complex collisional history, probably including several episodes of parent body disruption and gravitational reassembly. Differences in the abundances of different kinds of breccias among the ordinary chondrite groups are probably due to the stochastic nature of major asteroidal collisions.     

Young asteroid mixing revealed in ordinary chondrites: The case of NWA 5764, a polymict LL breccia with L clasts

Polymict chondritic breccias—rocks composed of fragments originating from different chondritic parent bodies—are of particular interest because they give insights into the mixing of asteroids in the main asteroid belt (occurrence, encounter velocity, transfer time). We describe Northwest Africa (NWA) 5764, a brecciated LL6 chondrite that contains a >16 cm³ L4 clast. The L clast was incorporated in the breccia through a nondestructive, low‐velocity impact. Identical cosmic‐ray exposure ages of the L clast and the LL host (36.6 ± 5.8 Myr), suggest a short transfer time of the L meteoroid to the LL parent body of 0.1 ± 8.1 Myr, if that meteoroid was no larger than a few meters. NWA 5764 (together with St. Mesmin, Dimmitt, and Glanerbrug) shows that effective mixing is possible between ordinary chondrite parent bodies. In NWA 5764 this mixing occurred after the peak of thermal metamorphism on the LL parent body, i.e., at least several tens of Myr after the formation of the solar system. The U,Th‐He ages of the L clast and LL host, identical at about 2.9 Ga, might date the final assembly of the breccia, indicating relatively young mixing in the main asteroid belt as previously evidenced in St. Mesmin.     

Parent bodies of L and H chondrites: Times of catastrophic events

Abstract— An analysis of the distribution of ³He and ⁴He in L and H chondrites has shown that the parent body of L chondrites underwent a catastrophic collision in space 340 ± 50 Ma ago. This age differs considerably from the collision age of 520 ± 60 Ma given previously (Heymann, 1967). The parent body of H chondrites may also have undergone local heating and degassing ～200 Ma ago. Data for L chondrites argue in favour of Antarctic and non-Antarctic meteorites having originated from a common parent body.     

Evidence for a late thermal event of unequilibrated enstatite chondrites: A Rb-Sr study of Qingzhen and Yamato 6901 (EH3) and Khairpur (EL6)

Abstract— The Rb-Sr whole rock and internal systematics of two EH3 chondrites, Qingzhen and Yamato 6901, and of one EL6 chondrite, Khairpur, were determined. Sulfides were separated using a stepwise dissolution technique. The mineral species in each fraction were estimated based on the chemical analyses of 12 major elements. The internal Rb-Sr systematics of the EH3 chondrites are highly disturbed. Fractions corresponding to sulfide phases show excess ⁸⁷Sr, while other fractions corresponding to silicate phases produce a linear trend on a Rb-Sr evolution diagram. If these linear relations are interpreted as isochrons, the ages of the silicate phases are 2.12 ± 0.23 Ga and 2.05 ± 0.33 Ga with the initial Sr isotopic ratios of 0.7112 ± 0.0018 and 0.7089 ± 0.0032, for Qingzhen and Yamato 6901, respectively. The process of the isotopic disturbance probably involved the breakdown of the major K-bearing sulfide (djerfisherite), and a lack of isotopic exchange between sulfide and silicate phases indicates moderate temperatures of reheating. Although a complete Sr isotopic re-homogenization among silicate phases was not attained, we interpret the Rb-Sr results as indicative of a late thermal event about 2 Ga ago on the parent bodies of these EH3 chondrites. These ages agree well with previously published K-Ar ages. An older isochron age of 4.481 ± 0.036 Ga with a low initial Sr isotopic ratio of 0.69866 ± 0.00038 was obtained for the data from silicate fractions of Khairpur, indicating early petrological equilibration on the parent body of EL6 chondrites.     

Phosphate and feldspar mineralogy of equilibrated L chondrites: The record of metasomatism during metamorphism in ordinary chondrite parent bodies

In ordinary chondrites (OCs), phosphates and feldspar are secondary minerals known to be the products of parent‐body metamorphism. Both minerals provide evidence that metasomatic fluids played a role during metamorphism. We studied the petrology and chemistry of phosphates and feldspar in petrologic type 4–6 L chondrites, to examine the role of metasomatic fluids, and to compare metamorphic conditions across all three OC groups. Apatite in L chondrites is Cl‐rich, similar to H chondrites, whereas apatite in LL chondrites has lower Cl/F ratios. Merrillite has similar compositions among the three chondrite groups. Feldspar in L chondrites shows a similar equilibration trend to LL chondrites, from a wide range of plagioclase compositions in petrologic type 4 to a homogeneous albitic composition in type 6. This contrasts with H chondrites which have homogeneous albitic plagioclase in petrologic types 4–6. Alkali‐ and halogen‐rich and likely hydrous metasomatic fluids acted during prograde metamorphism on OC parent bodies, resulting in albitization reactions and development of phosphate minerals. Fluid compositions transitioned to a more anhydrous, Cl‐rich composition after the asteroid began to cool. Differences in secondary minerals between H and L, LL chondrites can be explained by differences in fluid abundance, duration, or timing of fluid release. Phosphate minerals in the regolith breccia, Kendleton, show lithology‐dependent apatite compositions. Bulk Cl/F ratios for OCs inferred from apatite compositions are higher than measured bulk chondrite values, suggesting that bulk F abundances are overestimated and that bulk Cl/F ratios in OCs are similar to CI.     

Results of statistical analysis of primary components concentrations in bulk chemical composition of ordinary chondrite groups

We used two different methods of statistical analysis—cluster analysis and principal component analysis—to analyze the concentrations of principal chemical components (Si, Mg, Ca, Fe, Ni) and Co in ordinary chondrites. The analysis is based predominantly on published data (metadata). In total, chemical composition data from 646 ordinary chondrites were used in the statistical analysis. The aim of this analysis was to establish whether it would be possible or not to distinguish H, L, and LL chondrites based on the concentrations of major elements and Co in their bulk chemical compositions. It was also important to determine what conclusions such an analysis could enable to draw about matter differentiation in the formation environments of primordial parent bodies of particular ordinary chondrite groups (H, L, and LL). Another aim of the statistical analysis was to determine whether the distribution of Fe and Ni (with Co admixtures) is independent of petrographic types within particular groups of chondrites. This is of crucial importance for determining the distribution of FeNi(Co) ore occurrences in potential extraterrestrial deposits on modern asteroids—the sources of ordinary chondrites. The obtained results of statistical analyses confirmed that a clear-cut distinction between particular groups of ordinary chondrites is only possible for group H, while distinguishing L chondrites from LL chondrites is not always obvious. The results of the statistical analyses relating to the question of the possible existence of several primordial parent bodies (formation environments) of each group of ordinary chondrites are consistent with the results of contemporary astronomical spectroscopy research. What is particularly interesting is obtaining indications of the existence of common formation environments of the matter of L and LL chondrites, possibly on a few primordial parent bodies. The statistical analyses indicate that there is no correlation between the concentration of principal chemical components and the petrographic type of ordinary chondrites. This proves homogenous distributions of these elements within the parent bodies of each group of ordinary chondrites. Hence, the distribution of these elements in individual present-day asteroids is also homogenous.     

A coordinated spectral, mineralogical, and compositional study of ordinary chondrites

Mineral compositions and abundances derived from visible/near-infrared (VIS/NIR or VNIR) spectra are used to classify asteroids, identify meteorite parent bodies, and understand the structure of the asteroid belt. Using a suite of 48 equilibrated (types 4-6) ordinary (H, L, and LL) chondrites containing orthopyroxene, clinopyroxene, and olivine, new relationships between spectra and mineralogy have been established. Contrary to previous suggestions, no meaningful correlation is observed between band parameters and cpx/(opx + cpx) ratios. We derive new calibrations for determining mineral abundances (ol/(ol + px)) and mafic silicate compositions (Fa in olivine, Fs in pyroxene) from VIS/NIR spectra. These calibrations confirm that band area ratio (BAR) is controlled by mineral abundances, while Band I center is controlled by mafic silicate compositions. Spectrally-derived mineralogical parameters correctly classify H, L and LL chondrites in ∼80% of cases, suggesting that these are robust relationships that can be applied to S(IV) asteroids with ordinary chondrites mineralogies. Comparison of asteroids and meteorites using these new mineralogical parameters has the advantage that H, L and LL chemical groups were originally defined on the basis of mafic silicate compositions.     

Cosmic‐ray exposure age and heliocentric distance of the parent bodies of enstatite chondrites ALH 85119 and MAC 88136

Abstract— We measured concentrations and isotopic ratios of noble gases in enstatite (E) chondrites Allan Hills (ALH) 85119 and MacAlpine Hills (MAC) 88136. These two meteorites contain solar and cosmogenic noble gases. Based on the solar and cosmogenic noble gas compositions, we calculated heliocentric distances, parent body exposure ages, and space exposure ages of the two meteorites. The parent body exposure ages are longer than 6.7 Ma for ALH 85119 and longer than 8.7 Ma for MAC 88136. The space exposure ages are shorter than 2.2 Ma for ALH 85119 and shorter than 3.9 Ma for MAC 88136. The estimated heliocentric distances are more than 1.1 AU for ALH 85119 and 1.3 AU for MAC 88136. Derived heliocentric distances indicate the locations of parent bodies in the past when constituents of the meteorites were exposed to the Sun. From the mineralogy and chemistry of E chondrites, it is believed that E chondrites formed in regions within 1.4 AU from the Sun. The heliocentric distances of the two E chondrite parent bodies are not different from the formation regions of E chondrites. This may imply that heliocentric distances of E chondrites have been relatively constant from their formation stage to the stage of exposure to the solar wind.     

Testing accretion mechanisms of the H chondrite parent body utilizing nucleosynthetic anomalies

Planetary bodies a few hundred kilometers in radii are the precursors to larger planets but it is unclear whether these bodies themselves formed very rapidly or accreted slowly over several millions of years. Ordinary H chondrite meteorites provide an opportunity to investigate the accretion time scale of a small planetary body given that variable degrees of thermal metamorphism present in H chondrites provide a proxy for their stratigraphic depth and, therefore, relative accretion times. We exploit this feature to search for nucleosynthetic isotope variability of ⁵⁴Cr, which is a sensitive tracer of spatial and temporal variations in the protoplanetary disk's solids, between 17 H chondrites covering all petrologic types to obtain clues about the parent body accretionary rate. We find no systematic variability in the mass‐biased corrected abundances of ⁵³Cr or ⁵⁴Cr outside of the analytical uncertainties, suggesting very rapid accretion of the H chondrite parent body consistent with turbulent accretion. By utilizing the μ⁵⁴Cr–planetary mass relationship observed between inner solar system planetary bodies, we calculate that the H chondrite accretion occurred at 1.1 ± 0.4 or 1.8 ± 0.2 Myr after the formation of calcium‐aluminum‐rich inclusions (CAIs), assuming either the initial ²⁶Al/²⁷Al abundance of inner solar system solids determined from angrite meteorites or CAIs from CV chondrites, respectively. Notably, these ages are in agreement with age estimates based on the parent bodies’ thermal evolution when correcting these calculations to the same initial ²⁶Al/²⁷Al abundance, reinforcing the idea of a secular evolution in the isotopic composition of inner disk solids.     

Analysis of ordinary chondrites using powder X‐ray diffraction: 2. Applications to ordinary chondrite parent‐body processes

Abstract– We evaluate the chemical and physical conditions of metamorphism in ordinary chondrite parent bodies using X‐ray diffraction (XRD)‐measured modal mineral abundances and geochemical analyses of 48 type 4–6 ordinary chondrites. Several observations indicate that oxidation may have occurred during progressive metamorphism of equilibrated chondrites, including systematic changes with petrologic type in XRD‐derived olivine and low‐Ca pyroxene abundances, increasing ratios of MgO/(MgO+FeO) in olivine and pyroxene, mean Ni/Fe and Co/Fe ratios in bulk metal with increasing metamorphic grade, and linear Fe addition trends in molar Fe/Mn and Fe/Mg plots. An aqueous fluid, likely incorporated as hydrous silicates and distributed homogeneously throughout the parent body, was responsible for oxidation. Based on mass balance calculations, a minimum of 0.3–0.4 wt% H₂O reacted with metal to produce oxidized Fe. Prior to oxidation the parent body underwent a period of reduction, as evidenced by the unequilibrated chondrites. Unlike olivine and pyroxene, average plagioclase abundances do not show any systematic changes with increasing petrologic type. Based on this observation and a comparison of modal and normative plagioclase abundances, we suggest that plagioclase completely crystallized from glass by type 4 temperature conditions in the H and L chondrites and by type 5 in the LL chondrites. Because the validity of using the plagioclase thermometer to determine peak temperatures rests on the assumption that plagioclase continued to crystallize through type 6 conditions, we suggest that temperatures calculated using pyroxene goethermometry provide more accurate estimates of the peak temperatures reached in ordinary chondrite parent bodies.     

The Richfield LL3 chondrite

Abstract— Richfield is a moderately shocked (shock stage S4) LL3.7 genomict breccia find consisting mainly of light-colored recrystallized clasts and dark clasts exhibiting significant silicate darkening; a few impact-melt-rock clasts and LL5 chondrite clasts also occur. The cosmic-ray exposure age of 14.5 Ma is indistinguishable from the main exposure peak for LL chondrites (15 Ma). Although the exposure ages indicate little He loss, the gas-retention ages indicate high gas losses that must have occurred prior to or during ejection from the LL parent body.     

Chondrule cooling rates inferred from diffusive profiles in metal lumps from the Acfer 097 CR2 chondrite

Abstract— CR chondrites contain metal lumps (>300 μm) either attached to chondrule silicates or apparently isolated in the matrix. Here, laser ablation microanalysis of six metal lumps from a polished thin section of the Acfer 097 CR2 chondrite at 15 μm spatial resolution revealed zoning profiles for the volatile elements Cu and Ga. The mutual diffusivities of Cu and Ga were used to infer T ～ 1473 ± 100 K from the correlation of Cu versus Ga. The cooling rates of the metal lumps were calculated to be 0.5–50 K h^?1 for T_p ～ 1473 ± 100 K, with a maximum possible range of 0.1–400 K h^?1 for T_p ～ 1200–1800 K, overlapping the range of cooling rates inferred from petrological studies of type I chondrules (10–1000 K h^?1). Chondrule textures were established near the peak heating temperatures of chondrules (approximately 1900–2000 K), while the Cu and Ga diffusive profiles were established after solidification (T ～ 1500 K), consistent with nonlinear cooling. Furthermore, one chondrule (N2) has a more complex zoning profile that is modeled as a three‐stage cooling history involving initial cooling at approximately 1 K h^?1, followed by mild re‐heating (T ～ 1700 K) that re‐evaporated Cu and Ga from the outer approximately 100 μm of the metal lump and then cooled again at approximately 5 K h^?1. The thermal effects of parent body and other preaccretionary heating events on the Cu and Ga zoning profiles are examined. Although CR parent bodies have experienced aqueous alteration, the thermal effects of this process can neither produce nor erase the Cu and Ga diffusive profiles. Thus, metal lumps in CR chondrites record the solid‐state thermal history of chondrules as they travelled away from the chondrule‐forming region.     

Light noble gases in 12 meteorites from the Omani desert,Australia, Mauritania,Canada, and Sweden

We measured the concentrations and isotopic compositions of He, Ne, and Ar in 14 fragments from 12 different meteorites: three carbonaceous chondrites, six L chondrites (three most likely paired), one H chondrite, one R chondrite, and one ungrouped chondrite. The data obtained for the CV3 chondrites Ramlat as Sahmah (RaS) 221 and RaS 251 support the hypothesis of exposure age peaks for CV chondrites at approximately 9 Ma and 27 Ma. The exposure age for Shi?r 033 (CR chondrite) of 7.3 Ma is also indicative of a possible CR chondrite exposure age peak. The three L chondrites Jiddat al Harasis (JaH) 091, JaH 230, and JaH 296, which are most likely paired, fall together with Hallingeberg into the L chondrite exposure age peak of approximately 15 Ma. The two L chondrites Shelburne and Lake Torrens fall into the peaks at approximately 40 Ma and 5 Ma, respectively. The ages for Bassikounou (H chondrite) and RaS 201 (R chondrite) are approximately 3.5 Ma and 5.8 Ma, respectively. Six of the studied meteorites show clear evidence for ³He diffusive losses, the deficits range from approximately 17% for one Lake Torrens aliquot to approximately 45% for RaS 211. The three carbonaceous chondrites RaS 221, RaS 251, and Shi?r 033 all have excess ⁴He, either of planetary or solar origin. However, very high ⁴He/²⁰Ne ratios occur at relatively low ²⁰Ne/²²Ne ratios, which is unexpected and needs further study. The measured ⁴⁰Ar ages fit well into established systematics. They are between 2.5 and 4.5 Ga for the carbonaceous chondrites, older than 3.6 Ga for the L and H chondrites, and about 2.4 Ga for the R chondrite as well as for the ungrouped chondrite. Interestingly, none of our studied L chondrites has been degassed in the 470 Ma break‐up event. Using the amount of trapped ³⁶Ar as a proxy for noble gas contamination due to terrestrial weathering we are able to demonstrate that the samples studied here are not or only very slightly affected by terrestrial weathering (at least in terms of their noble gas budget).     

Incompletely compacted equilibrated ordinary chondrites

Abstract— 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.