Complex exposure histories for meteorites with “short” exposure ages

Abstract— We report measurements of ²⁶Al and ¹⁰Be activities in nine ordinary chondrites and of the light noble gas concentrations and ³⁶Cl and ⁴¹Ca activities in subsets of those meteorites. All but Murray have low ²¹Ne concentrations (<1.0 × 10^?8cm³STP/g) and have previously been used to estimate ²¹Ne production rates. Ladder Creek, Murchison, Sena, and Timochin have inventories of cosmogenic radionuclides that are compatible with a single stage of irradiation and give ²¹Ne production rates that are consistent with the standard L-chondrite value of 0.33 × 10^?8cm³STP/g/Ma. In contrast, Cullison, Guenie, Shaw, and Tsarev experienced complex irradiation histories. They and several other meteorites with low nominal exposure ages also have lower ³He/²¹Ne ratios than expected based on their ²²Ne/²¹Ne ratios. A general association between low ²¹Ne contents and ³He losses suggests that meteorites with short lifetimes often occupy orbits with small perihelia. However, meteorites with low ²¹Ne contents, one-stage exposure histories, and losses of cosmogenic ³He are rare. Possible explanations for the scarcity are (1) statistical, (2) that it is harder for more deeply buried protometeoroids to lose gas in a liberating collision, and (3) that it is harder to insert more deeply buried protometeoroids directly into orbits with small perihelia.     

The L3–6 chondritic regolith breccia Northwest Africa (NWA) 869: (II) Noble gases and cosmogenic radionuclides

Abstract– We measured cosmogenic radionuclides and noble gases in the L3–6 chondrite breccia Northwest Africa (NWA) 869, one of the largest meteorite finds from the Sahara. Concentrations of ¹⁰Be, ²⁶Al, and ³⁶Cl in stone and metal fractions of six fragments of NWA 869 indicate a preatmospheric radius of 2.0–2.5 m. The ¹⁴C and ¹⁰Be concentrations in three fragments yield a terrestrial age of 4.4 ± 0.7 kyr, whereas two fragments show evidence for a recent change in shielding, most likely due to a recent impact on the NWA meteoroid, approximately 10⁵ yr ago, that excavated material up to approximately 80 cm deep and exposed previously shielded material to higher cosmic‐ray fluxes. This scenario is supported by the low cosmogenic ³He/²¹Ne ratios in these two samples, indicating recent loss of cosmogenic ³He. Most NWA samples, except for clasts of petrologic type 4–6, contain significant amounts of solar Ne and Ar, but are virtually free of solar helium, judging from the trapped ⁴He/²⁰Ne ratio of approximately 7. Trapped planetary‐type Kr and Xe are most clearly present in the bulk and matrix samples, where abundances of ¹²⁹Xe from decay of now extinct ¹²⁹I are highest. Cosmogenic ²¹Ne varies between 0.55 and 1.92 × 10^?8 cm³ STP g^?1, with no apparent relationship between cosmogenic and solar Ne contents. Low cosmogenic (²²Ne/²¹Ne)_c ratios in solar gas free specimens are consistent with irradiation in a large body. Combined ¹⁰Be and ²¹Ne concentrations indicate that NWA 869 had a 4π cosmic‐ray exposure (CRE) age of 5 ± 1 Myr, whereas elevated ²¹Ne concentrations in several clasts and bulk samples indicate a previous CRE of 10–30 Myr on the parent body, most probably as individual components in a regolith. Unlike many other large chondrites, NWA 869 does not show clear evidence of CRE as a large boulder near the surface of its parent body. Radiogenic ⁴He concentrations in most NWA 869 samples indicate a major outgassing event approximately 2.8 Gyr ago that may have also resulted in loss of solar helium.     

Noble gases in metal and schreibersite of the Acuña (IIIAB) iron meteorite

Abstract— We measured abundances and isotopic compositions of noble gases in metal and schreibersite of the Acuña (IIIAB) iron meteorite. The concentrations of noble gases in Acuña metal are very low compared to those reported so far for other iron meteorites. The isotopic ratios of He, Ne and Ar indicate that they are mostly of cosmogenic origin. Cosmogenic components are even present in Kr and Xe, which could not have been produced from Fe, Ni and P and are probably due to the spallation of trace elements of higher masses. The high ⁴He/²¹Ne ratio of 420 in Acuña metal indicates that the samples were at a deep position within a very large meteoroid. The exposure ages of Acuña were estimated to be 50–200 Ma from ³He, ²¹Ne and ³⁸Ar abundances and by utilizing the diagrams of production rates vs. the ⁴He/²¹Ne ratio based on the Signer-Nier model. The low exposure age of Acuña may indicate a history different from that of other IIIAB irons whose exposure ages cluster at ～670 Ma. Otherwise, Acuña may be one of the samples with the low production rate, which can not be estimated from the diagrams of the Signer-Nier model.     

Calibration of cosmogenic noble gas production in ordinary chondrites based on 36Cl‐36Ar ages. Part 1: Refined produced rates for cosmogenic 21Ne and 38Ar

We measured the concentrations and isotopic compositions of He, Ne, and Ar in bulk samples and metal separates of 14 ordinary chondrite falls with long exposure ages and high metamorphic grades. In addition, we measured concentrations of the cosmogenic radionuclides ¹⁰Be, ²⁶Al, and ³⁶Cl in metal separates and in the nonmagnetic fractions of the selected meteorites. Using cosmogenic ³⁶Cl and ³⁶Ar measured in the metal separates, we determined ³⁶Cl‐³⁶Ar cosmic‐ray exposure (CRE) ages, which are shielding‐independent and therefore particularly reliable. Using the cosmogenic noble gases and radionuclides, we are able to decipher the CRE history for the studied objects. Based on the correlation ³He/²¹Ne versus ²²Ne/²¹Ne, we demonstrate that, among the meteorites studied, only one suffered significant diffusive losses (about 35%). The data confirm that the linear correlation ³He/²¹Ne versus ²²Ne/²¹Ne breaks down at high shielding. Using ³⁶Cl‐³⁶Ar exposure ages and measured noble gas concentrations, we determine ²¹Ne and ³⁸Ar production rates as a function of ²²Ne/²¹Ne. The new data agree with recent model calculations for the relationship between ²¹Ne and ³⁸Ar production rates and the ²²Ne/²¹Ne ratio, which does not always provide unique shielding information. Based on the model calculations, we determine a new correlation line for ²¹Ne and ³⁸Ar production rates as a function of the shielding indicator ²²Ne/²¹Ne for H, L, and LL chondrites with preatmospheric radii less than about 65 cm. We also calculated the ¹⁰Be/²¹Ne and ²⁶Al/²¹Ne production rate ratios for the investigated samples, which show good agreement with recent model calculations.     

Neon produced by solar cosmic rays in ordinary chondrites

Solar‐cosmic‐ray‐produced Ne (SCR‐Ne), in the form of low cosmogenic ²¹Ne/²²Ne ratios (²¹Ne/²²Ne_cos <0.8), is more likely to be found in rare meteorite classes, like Martian meteorites, than in ordinary chondrites. This may be the result of a sampling bias: SCR‐Ne is better preserved in meteorites with small preatmospheric radii and these specimens are often only studied if they belong to unusual or rare classes. We measured He and Ne isotopic concentrations and nuclear tracks in 25 small unpaired ordinary chondrites from Oman. Most chondrites have been intensively heated during atmospheric entry as evidenced by the disturbed track records, the low ³He/²¹Ne ratios, the low ⁴He concentrations, and the high peak release temperatures. Concentration depth profiles indicate significant degassing; however, the Ne isotopes are mainly undisturbed. Remarkably, six chondrites have low ²¹Ne/²²Ne_cos in the range 0.711–0.805. Using a new physical model for the calculation of SCR production rates, we show that four of the chondrites contain up to ~20% of SCR‐Ne; they are analyzed in terms of preatmospheric sizes, cosmic ray exposure ages, mass ablation losses, and orbits. We conclude that SCR‐Ne is preserved, regardless of the meteorite class, in specimens with small preatmospheric radii. Sampling bias explains the predominance of SCR‐Ne in rare meteorites, although we cannot exclude that SCR‐Ne is more common in Martian meteorites than it is in small ordinary chondrites.     

Noble gases in Grant and Carbo and the influence of S‐ and P‐rich mineral inclusions on the 41K‐40K dating system

Abstract— Cosmogenic He, Ne, and Ar were measured in the iron meteorites Grant (IIIAB) and Carbo (IID) to re‐determine their preatmospheric geometries and exposure histories. We also investigated the influence of sulphur‐ and/or phosphorus‐rich inclusions on the production rates of cosmogenic Ne. Depth profiles measured in Grant indicate a preatmospheric center location 117 mm left from the reference line and 9 mm below bar B, which is clearly different (?10 cm) from earlier results (?165 mm left from the reference line on bar F). For Carbo the preatmospheric center location was found to be 120 mm right of the reference line and 15 mm above bar J, which is in agreement with literature data. The new measurements indicate a spherical preatmospheric shape for both meteorites and, based on literature ³⁶C1 data, the radii were estimated to be about 32 cm and 70 cm for Grant and Carbo, respectively. We demonstrate that minor elements like S and P have a significant influence on the production rates of cosmogenic Ne. In our samples, containing on average 0.5% S and/or P, about 20% of ²¹Ne was produced from these minor elements. Using measured ²¹Ne concentrations and endmember ²²Ne/²¹Ne ratios for Fe + Ni and S + P, respectively, we show that it is possible to correct for ²¹Ne produced from S and/or P. The thus corrected data are then used to calculate new ⁴¹K‐⁴⁰K exposure ages—using published K data—which results in 564 ± 78 Ma for Grant and 725 ± 100 Ma for Carbo. The correction always lowers the ²¹Ne concentrations and consequently decreases the ⁴¹K‐⁴⁰K exposure ages. The discrepancies between ³⁶Cl‐³⁶Ar and ⁴¹K‐⁴⁰K ages are accordingly reduced. The existence of a significant long‐term variation of the GCR, which is based on a former 30–50% difference between ⁴¹K‐⁴⁰K and ³⁶Cl‐³⁶Ar ages, may warrant re‐investigation.     

Cosmic‐ray exposure age and preatmospheric size of the Bunburra Rockhole achondrite

Abstract– Bunburra Rockhole is the first meteorite fall photographed and recovered by the Desert Fireball Network in Australia. It is classified as an ungrouped achondrite similar in mineralogical and chemical composition to eucrites, but it has a distinct oxygen isotope composition. The question is if achondrites like Bunburra Rockhole originate from the same parent body as the howardite‐eucrite‐diogenite (HED) meteorites or from several separate, differentiated parent bodies. To address this question, we measured cosmogenic radionuclides and noble gases in the Bunburra Rockhole achondrite. The short‐lived radionuclides ²²Na and ⁵⁴Mn confirm that Bunburra Rockhole is a recent fall. The concentrations of ¹⁰Be, ²⁶Al and ³⁶Cl as well as the ²²Ne/²¹Ne ratio indicate that Bunburra Rockhole was a relatively small object (R approximately 15 cm) in space, consistent with the photographic fireball observations. The cosmogenic ³⁸Ar concentration yields a cosmic‐ray exposure (CRE) age of 22 ± 3 Myr, whereas ²¹Ne and ³He yield approximately 30% and approximately 60% lower ages, respectively, due to loss of cosmogenic He and Ne, mainly from plagioclase. With a CRE age of 22 Myr, Bunburra Rockhole is the first anomalous eucrite that overlaps with the main CRE peak of the HED meteorites. The radiogenic K‐Ar age of 4.1 Gyr is consistent with the U‐Pb age, while the young U,Th‐He age of approximately 1.4 Gyr indicates that Bunburra Rockhole lost radiogenic ⁴He more recently.     

Noble gases in ten Nullarbor chondrites: Exposure ages,terrestrial ages,and weathering effects

Abstract— We present concentration and isotopic composition of He, Ne, and Ar in ten chondrites from the Nullarbor region in Western Australia as well as the concentrations of ⁸⁴Ke, ¹²⁹Xe, and ¹³²Xe. From the measured cosmogenic ¹⁴C concentrations (Jull et al. 1995), shielding‐corrected production rates of ¹⁴C are deduced using cosmogenic ²²Ne/²¹Ne ratios. For shielding conditions characterized by ²²Ne/²¹Ne >1.10, this correction becomes significant and results in shorter terrestrial ages. The exposure ages of the ten Nullarbor chondrites are in the range of values usually observed in ordinary chondrites. Some of the meteorites have lost radiogenic gases as well as cosmogenic ³He. Most of the analyzed specimens show additional trapped Ar, Kr, and Xe of terrestrial origin. The incorporation of these gases into weathering products is common in chondrites from hot deserts.     

The complex exposure history of the Jiddat al Harasis 073 L‐chondrite shower

Abstract— We measured the concentrations and isotopic compositions of He, Ne, and Ar in 29 bulk samples from 11 different strewn field fragments of the large Jiddat al Harasis (JaH) 073 L6 chondrite shower, including 7 samples from known locations within the main mass. In addition, we measured the concentrations of cosmogenic ¹⁰Be, ²⁶Al, ³⁶Cl, and ⁴¹Ca in 10 samples. All fragments of this shower are characterized by low ¹⁰Be concentrations (7.6–12.8 dpm/kg), high ²⁶Al/¹⁰Be ratios (3.5‐5), large contributions of neutron capture ⁴¹Ca (200–1800 dpm/kgCa), low ³He/²¹Ne ratios (1.5‐3.0), large variations in cosmogenic ²¹Ne (1.2–12) × 10^?8cm³STP/g, and significant contributions of neutron‐capture ³⁶Ar. Stepwise heating experiments show that neutron‐capture produced ³⁶Ar is predominantly released between 1000–1200 °C. All these results are consistent with a first‐stage exposure of ?65 Ma within ?20 cm of the surface of the L‐chondrite parent body, followed by ejection of a 1.5‐2 m large object, which was then delivered to Earth within about 0.5 and 0.7 Ma. The cosmogenic nuclide data in JaH 073 thus corroborate the trend that many of the large chondrites studied so far experienced a complex exposure history. The observed ³He/²¹Ne ratios of 2.5‐3.0 in the most shielded samples (including those of the main mass) are lower than predicted by model calculations, but similar to the lowest values found in the large Gold Basin L‐chondrite shower. The Bern plot, which gives a linear correlation for ³He/²¹Ne versus ²²Ne/²¹Ne, is evidently not valid for very high shielding. Some of our measured ²²Ne/²¹Ne ratios in JaH 073 are lower than 1.06, which is not well understood, but might be explained by loss of cosmogenic neon from shocked sodium‐rich plagioclase during terrestrial weathering. The amount of trapped atmospheric argon in the JaH 073 fragments varies by almost two orders of magnitude and shows only a weak correlation with the size of the fragments, which range from <100 g to >50 kg. Finally, low concentrations of radiogenic ⁴He and ⁴⁰Ar indicate incomplete degassing < 1 Ga ago, probably at the main collision event on the L‐chondrite parent body ?480 Ma ago.     

Cosmic‐ray prdocution rates of helium,neon and argon isotopes in H chondrites based on chlorine‐36/argon‐36 ages

Abstract— We present the concentrations and isotopic compositions of He, Ne, and Ar for nonmagnetic fractions and bulk samples of 17 H chondrites which were recently investigated for their ³⁶Cl‐³⁶Ar cosmic‐ray exposure ages (Graf et al., 2001). All selected meteorites are observed falls with cosmic‐ray exposure ages close to the 7 Ma peak. The rare gas data are consistent with ¹⁰Be and ³⁶C1 production rates in the metal phase. Remarkably, only 1 out of the 17 H chondrites, Bath, shows clear indications for a complex exposure history. Based on rare gas concentrations and ³⁶Cl‐³⁶Ar exposure ages, ²¹Ne production rates as a function of ²²Ne/²¹ Ne and a mean ³⁸Ar production rate are determined. The results confirm model calculations which predict that the relationship between ²¹Ne production rates and ²²Ne/²¹Ne is ambiguous for high shielding. Besides the mean ³⁸Ar production rate we also give production rate ratios P(³⁸Ar from Ca)/P(³⁸Ar from Fe). They vary between 10 and 77, showing no significant correlation with ³⁸Ar concentrations or ²²Ne/²¹Ne. By investigating the metal separates, Graf et al. (2001) found significant ³He deficits for 6 out of the 17 meteorites. For the nonmagnetic fractions and bulk samples investigated here, the data points in a ³He/²¹Ne vs. ²²Ne/²¹Ne diagram plot in the area defined by most of the H chondrites. This means that ³He deficits in the metal phase are much more pronounced than in silicate minerals and we will argue that ³H diffusive losses in meteorites should be the rule rather than the exception. The ²¹Ne exposure ages, calculated on the basis of modeled ²¹Ne production rates, confirm the assumption by Graf et al. (2001) that the H5 chondrites with low ³He/³⁸Ar in the metal formed in a separate event than those with normal ³He/³⁸Ar ratios. The data can best be interpreted by assuming that the prominent 7 Ma exposure age peak of the H chondrites is due to at least two events about 7.0 and 7.6 Ma ago.     

Solar-proton-produced neon in shergottite meteorites and implications for their origin

Abstract— Measured Ne isotopes in samples of shergottite ALHA77005 show variations in ²¹Ne/²²Ne ratios and ²¹Ne abundances that are consistent with the presence of two cosmogenic components: a component produced by nuclear interactions of galactic cosmic rays (GCR) and a component produced at shallow shielding depths (～0–3 cm) by energetic solar flare protons (SCR). We suggest that the ²¹Ne/²²Ne ratio generally can be used to distinguish between SCR and GCR components in many meteorite types. Analysis of cosmogenic Ne produced in chondrite mineral separates, eucrites, and anorthositic lunar rocks, all having diverse major element compositions, indicate that the GCR ²¹Ne/²²Ne ratio increases modestly with relative Mg content. Data for hundreds of chondrite analyses suggest that SCR Ne is present in no more than a very small fraction of chondrites. Examination of literature data for other shergottites, however, indicate that all of these meteorites contain SCR Ne but that it is apparently absent in other SNC meteorites. The ubiquitous presence of SCR Ne in shergottites, in contrast to most other types of meteorites, suggests that the martian origin of shergottites gave them different orbital parameters compared to other meteorites. This in turn may have contributed to slower entry velocities and lesser surface ablation in the atmosphere or even to higher SCR production rates.     

Mineralogical and chemical composition and cosmic‐ray exposure history of two mesosiderites and two iron meteorites

Abstract— We performed a comprehensive study of the noble gas isotopic abundances, radionuclide activities, and mineralogical and chemical composition of two mesosiderites and two iron meteorites. For the mesosiderites Dong Ujimqin Qi and Weiyuan, the silicate and the metal phases were studied. The anomalous ataxite Rafrüti is not chemically related to any other meteorite class, whereas Ningbo is a type IVA octahedrite. The mineralogy and major and trace element abundances of the silicate phases of Dong Ujimqin Qi and Weiyuan are similar to those of other mesosiderites and distinct from those of the howardites. The cosmic‐ray exposure history was studied based on the concentrations of the cosmogenic noble gas nuclei and radionuclide activities. For the iron meteorites, cosmic‐ray exposure ages were calculated from the pairs ¹⁰Be‐²¹Ne, ²⁶Al‐²¹Ne, and ³⁶Cl‐³⁶Ar. Rafrüti yields the youngest exposure age of all ataxites (6.8 ± 1.7 Ma), whereas that of Ningbo with 107 ± 15 Ma falls within the range observed for the other octahedrites. The parent body break‐up times of the mesosiderites Dong Ujimqin Qi and Weiyuan are 252 ± 50 and 25.9 ± 5.0 Ma, respectively. We find no evidence for a common break‐up event for the mesosiderites and the howardites.     

Noble gas study of the Saratov L4 chondrite

Abstract– We have determined the elemental abundances and the isotopic compositions of noble gases in a bulk sample and an HF/HCl residue of the Saratov (L4) chondrite using stepwise heating. The Ar, Kr, and Xe concentrations in the HF/HCl residue are two orders of magnitude higher than those in the bulk sample, while He and Ne concentrations from both are comparable. The residue contains only a portion of the trapped heavy noble gases in Saratov; 40 ± 9% for ³⁶Ar, 58 ± 12% for ⁸⁴Kr, and 48 ± 10% for ¹³²Xe, respectively. The heavy noble gas elemental pattern in the dissolved fraction is similar to that in the residue but has high release temperatures. Xenon isotopic ratios of the HF/HCl residue indicate that there is no Xe‐HL in Saratov, but Ne isotopic ratios in the HF/HCl residue lie on a straight line connecting the cosmogenic component and a composition between Ne‐Q and Ne‐HL. This implies that the Ne isotopic composition of Q has been changed by incorporating Ne‐HL (Huss et al. 1996) or by being mass fractionated during the thermal metamorphism. However, it is most likely that the Ne‐Q in Saratov is intrinsically different from this component in other meteorites. The evidence of this is a lack of correlation between the isotopic ratio of Ne‐Q and petrologic types of meteorites (Busemann et al. 2000). A neutron capture effect was observed in the Kr isotopes, and this process also affected the ¹²⁸Xe/¹³²Xe ratio. The ³He and ²¹Ne exposure ages for the bulk sample are 33 and 35 Ma, respectively.     

Cosmic-ray exposure ages of diogenites and the recent collisional history of the howardite,eucrite and diogenite parent body/bodies

Abstract— We determined the cosmic-ray exposure age of 20 diogenites from measured cosmogenic noble gas isotopes and calculated production rates of ³He, ²¹Ne and ³⁸Ar. The production rates were calculated on the basis of the measured chemical composition and the cosmogenic ²²Ne/²¹Ne ratio of each sample. The shielding conditions of each sample were also checked on the basis of the measured ¹⁰Be and ²⁶AI concentrations. The exposure ages range from 6 to 50 Ma but do not form a continuous distribution: ten ages cluster at 21–25 Ma and four at 35–42 Ma. The two diogenite clusters coincide with the 22 Ma and 38 Ma peaks in the exposure age distribution of eucrites and howardites. After the selection from literature data of 32 eucrites and 11 howardites with reliable ages, we find a total of 23 howardite, eucrite and diogenite (HED) group meteorites at 20–25 Ma and 10 at 35–42 Ma. The shape of the two peaks is consistent with single impact events, and random number statistics show that they are statistically significant at the 99% level. Altogether, this provides strong evidence for two major impact events 22 Ma and 39 Ma ago. Although these two events can explain more than half of all HED exposure ages, it takes at least five impact events to explain all ages <50 Ma. An impact frequency of one per 10 Ma corresponds to projectiles of at least 2–4 km in diameter for Vesta and of 60–300 m for the 100× smaller Vesta-derived “vestoids.” Based on the HED exposure-age distribution, the size distribution of the main-belt asteroids and the difference in size between Vesta and the kilometer size vestoids, we favor Vesta as the major source of HED meteorites, although some of the meteorites may have been ejected from the vestoids rather than directly from Vesta.     

Ordinary Chondrites: 2. Diffusion Losses of Rare Gases

Data on the isotopic abundances and ratios of light rare gases (He and Ne) in 600 ordinary chondrites are analyzed. The ratio of cosmogenic isotopes (³He/²¹Ne) _c in 20% of the ordinary chondrites has been found to lie well below the correlation line that represents the dependence of (³He/²¹Ne) _c on (²²Ne/²¹Ne) _c . This effect shows up most clearly in ⁴He _r -chondrites, particularly in meteorites with diffusion losses of radiogenic ²¹Ne _c , and is most likely attributable to the predominant (compared to ³He _c ) diffusion losses of cosmogenic ³He_cthrough the solar heating of meteorites in orbits with small perihelion distances. This effect is enhanced by periodic variations in orbital parameters (including the perihelion distance) of meteorites throughout their exposure histories. Thermoluminescence data for ordinary chondrites confirm this scenario. The (³He/²¹Ne) _c ratio for 15% of the chondrites was significantly overestimated, which may stem from the fact that such meteorites were heavily shielded in preatmospheric bodies.     

Cosmic‐ray exposure histories of the lunar meteorites AaU 012 and Shişr 166

We measured the concentrations and isotopic compositions of the stable isotopes of He, Ne, Ar, Kr, and Xe in the two lunar impact‐melt breccias Abar al’ Uj (AaU) 012 and Shi?r 166 to obtain information on their cosmic‐ray exposure histories and possible launch pairing; the latter was suggested because of their similar chemical composition. AaU 012 has higher gas concentrations than Shi?r 166 and clearly contains implanted solar wind gases, indicating a shallow to moderate shielding for this meteorite in the lunar regolith. The maximum shielding depth of AaU 012 was most likely ≤310 g cm^?2 and its lunar regolith residence time was ≥420 ± 70 Ma. Our results indicate that in Shi?r 166 the trapped component is a mixture of air and solar wind. The low concentration of cosmogenic and solar wind gases indicate substantial diffusive gas loss and a shielding depth of <700 g cm^?2 on the Moon for Shi?r 166. All differences seen in the concentrations and isotopic compositions of the noble gases suggest that AaU 012 and Shi?r 166 are most likely not launch pairs, although a different exposure history on the Moon does not exclude the possibility that the two meteorites were ejected by a single, large impact event.     

Ordinary Chondrites: 1. Short Life of Small Meteorites

The data on the concentration of cosmogenic neon isotopes and the density of cosmic-ray tracks in about 600 ordinary chondrites were analyzed. For ordinary chondrites of all chemical classes, the dispersion of the ratios between cosmogenic isotopes of neon, (²²Ne/²¹Ne) _c , and a fraction of meteorites with high rates of cosmic-ray-track formation were demonstrated to decrease with increasing cosmic-ray-exposure age. Most likely, these effects are related to the fact that chondrites of small exposure ages are more frequent among meteorites with low degrees of shielding (small sizes), probably because smaller meteorites are placed into Earth-crossing orbits faster than larger ones. This, in turn, is attributed to more effective insertion of small asteroid-belt bodies into resonances, most likely due to the diurnal Yarkovsky effect.     

Energetic proton irradiation history of the howardite parent body regolith and implications for ancient solar activity

Abstract— Previous studies have shown that the Kapoeta howardite, as well as several other meteorites, contains excess concentrations of cosmogenic Ne in the darkened, solar-irradiated phase compared to the light, non-irradiated phase. The two explanations offered for the nuclear production of these Ne excesses in the parent body regolith are either from galactic cosmic-ray proton (GCR) irradiation or from a greatly enhanced flux of energetic solar “cosmic-ray” protons (SCR), as compared to the recent solar flux. Combining new isotopic data we obtained on acid-etched, separated feldspar from Kapoeta light and dark phases with literature data, we show that the cosmogenic ²¹Ne/²²Ne ratio of light phase feldspar (0.80) is consistent with only GCR irradiation in space for ～3 Ma. However, the ²¹Ne/²²Ne ratio (0.68) derived for irradiation of dark phase feldspar in the Kapoeta regolith indicates that cosmogenic Ne was produced in roughly equal proportions from galactic and solar protons. Considering a simple model of an immature Kapoeta parent body regolith, the duration of this early galactic exposure was only ～3–6 Ma, which would be an upper limit to the solar exposure time of individual grains. Concentrations of cosmogenic ²¹Ne in pyroxene separates and of cosmogenic ¹²⁶Xe in both feldspar and pyroxene are consistent with this interpretation. The near-surface irradiation time of individual grains in the Kapoeta regolith probably varied considerably due to regolith mixing to an average GCR irradiation depth of ～10 cm. Because of the very different depth scales for production of solar ～Fe tracks, SCR Ne, and GCR Ne, the actual regolith exposure times for average grains probably differed correspondingly. However, both the SCR ²¹Ne and solar track ages appear to be longer because of enhanced production by early solar activity. The SCR/GCR production ratio of ²¹Ne inferred from the Kapoeta data is larger by a at least a factor of 10 and possibly as much as a factor of ～50 compared to recent solar particle fluxes. Thus, this study indicates that our early Sun was much more active and emitted a substantially higher flux of energetic (>10 MeV/nucleon) protons.     

Cosmogenic nuclides in core samples of the Chico L6 chondrite: Evidence for irradiation under high shielding

Abstract— Core samples were obtained from various locations of the ～ 105-kg Chico, NM, L6 chondrite in order to study the effects of large shielding on the production rates of cosmic-ray-produced nuclides. Relations between measured abundances of cosmogenic nuclides (¹⁰Be, ²⁶Al, and stable isotopes of He, Ne, and Ar) and the cosmogenic ²²Ne/²¹Ne ratio were determined and compared with recent model predictions of production rates. The measured ²²Ne/²¹Ne ratios (1.06-1.08) and significant variations observed in concentrations of cosmogenic ²¹Ne and ³He suggest an ～40-cm shielding gradient across Chico and irradiation within a large object (> 100-cm radius). Noble gas data indicate that Chico experienced greater shielding than chondrites Knyahinya or Keyes and similar to Jilin. Values of ¹⁰Be (average = 20.7 dpm/kg) and ²⁶Al (average = 71.1 dpm/kg) are nearly constant, however, and show no correlation with either ²²Ne/²¹Ne or ²¹Ne. Activities of ¹⁰Be and ²⁶Al suggest irradiation in a smaller object (～40–80 cm radius). The ²⁶Al activity and the ²⁶Al/¹⁰Be ratio (average value = 3.42) are both significantly larger than values for most other chondrites. These results could indicate a two-stage irradiation with t₁ ～ 104 Ma and t₂ ～ 4 Ma and a second-stage body the size of Knyahinya. The single stage, ¹⁰Be/²¹Ne exposure age for Chico is 65 Ma. The ²²Ne/²¹Ne ratio apparently becomes insensitive to shielding for objects the size of Chico. No substantial evidence exists for chondrites with ²²Ne/²¹Ne ratios significantly less than ～ 1.055.     

Cosmogenic nuclides in the solar gas‐rich H3–6 chondrite breccia Frontier Mountain 90174

Abstract— We re‐evaluated the cosmic‐ray exposure history of the H3‐6 chondrite shower Frontier Mountain (FRO) 90174, which previously was reported to have a simple exposure history, an irradiation time of about 7 Ma, and a pre‐atmospheric radius of 80–100 cm (Welten et al. 2001). Here we measured the concentrations and isotopic compositions of He, Ne, and Ar in 8 aliquots of 6 additional fragments of this shower, and ¹⁰Be and ²⁶Al in the stone fractions of seven fragments. The radionuclide concentrations in the stone fractions, combined with those in the metal fractions, confirm that all samples are fragments of the FRO 90174 shower. Four of the fragments contain solarwind‐implanted noble gases with a solar ²⁰Ne/²²Ne ratio of ?12.0, indicating that FRO 90174 is a regolith breccia. The concentrations of solar gases and cosmogenic ²¹Ne in the samples analyzed by us and by Welten et al. (2001) overlap with those of the FRO H‐chondrites from the 1984 season, suggesting that many of these samples are also part of the large FRO 90174 chondrite shower. The cosmogenic ²¹Ne concentrations in FRO 90174 show no simple correlation with ¹⁰Be and ²⁶Al activities. We found ²¹Ne excesses between 0.3‐1.1 × 10^?8cm³STP/g in 6 of the 17 samples. Since excess ²¹Ne and trapped solar gases are not homogeneously distributed, i.e., we found in one fragment aliquots with and without excess ²¹Ne and solar ²⁰Ne, we conclude that excess ²¹Ne is due to GCR irradiation of the regolith before compaction of the FRO 90174 object. Therefore, the chondrite shower FRO 90174 did not simply experience an exposure history, but some material was already irradiated at the surface of an asteroid leading to excess ²¹Ne. This excess ²¹Ne is correlated to implanted solar gases, clearly indicating that both processes occurred on the regolith.