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.     

Evidence for common breakup events of the acapulcoites‐lodranites and chondrites

Abstract— Acapulcoites and lodranites are believed to originate on a common parent body and to represent some of the earliest events in the differentiation of the chondritic asteroids. We have conducted isotopic studies of the noble gases He, Ne, Ar, Kr, and Xe, and determinations of the concentrations of the major elements and of the radionuclides ¹⁰Be, ²⁶Al, and ³⁶Cl in an attempt to constrain the cosmic‐ray exposure history of two members of the acapulcoite‐lodranite clan recovered in Antarctica: Frontier Mountain (FRO) 95029 and Graves Nunataks (GRA) 95209. From cosmic‐ray‐produced ³He, ²¹Ne, and ³⁸Ar and appropriate production rates, we derive parent‐body breakup times of 4.59 ± 0.60 and 6.82 ± 0.60 Ma for FOR 95029 and GRA 95209, respectively. These times are consistent with those obtained from the pairs ¹⁰Be‐²¹Ne and ²⁶Al‐²¹Ne; whereas the times inferred from the pair ³⁶Cl‐³⁶Ar are slightly longer, perhaps because the ³⁶Cl activities decreased as a result of decay on Earth. Terrestrial ages up to ～50 ka for the two meteorites are consistent with the measured ³⁶Cl activities of the metal phases. All acapulcoites and lodranites dated until now show cosmic‐ray exposure ages in the range of 4–10 Ma. This is the same range as that found for the major exposure age cluster of the H chondrites. As a common parent body is improbable on the basis of the O‐isotopic systematics, a common set of impactors might have affected the asteroid belt 4–10 Ma ago.     

Comparison of cosmic‐ray exposure ages and trapped noble gases in chondrule and matrix samples of ordinary,enstatite, and carbonaceous chondrites

Abstract— We performed a comprehensive study of the He, Ne, and Ar isotopic abundances and of the chemical composition of bulk material and components of the H chondrites Dhajala, Bath, Cullison, Grove Mountains 98004, Nadiabondi, Ogi, and Zag, of the L chondrites Grassland, Northwest Africa 055, Pavlograd, and Ladder Creek, of the E chondrite Indarch, and of the C chondrites Hammadah al Hamra 288, Acfer 059, and Allende. We discuss a procedure and necessary assumptions for the partitioning of measured data into cosmogenic, radiogenic, implanted, and indigenous noble gas components. For stone meteorites, we derive a cosmogenic ratio ²⁰Ne/²²Ne of 0.80 ± 0.03 and a trapped solar ⁴He/³He ratio of 3310 ± 130 using our own and literature data. Chondrules and matrix from nine meteorites were analyzed. Data from Dhajala chondrules suggest that some of these may have experienced precompaction irradiation by cosmic rays. The other chondrules and matrix samples yield consistent cosmic‐ray exposure (CRE) ages within experimental errors. Some CRE ages of some of the investigated meteorites fall into clusters typically observed for the respective meteorite groups. Only Bath's CRE age falls on the 7 Ma double‐peak of H chondrites, while Ogi's fits the 22 Ma peak. The studied chondrules contain trapped ²⁰Ne and ³⁶Ar concentrations in the range of 10^?6–10^?9 cm³ STP/g. In most chondrules, trapped Ar is of type Q (ordinary chondritic Ar), which suggests that this component is indigenous to the chondrule precursor material. The history of the Cullison chondrite is special in several respects: large fractions of both CR‐produced ³He and of radiogenic ⁴He were lost during or after parent body breakup, in the latter case possibly by solar heating at small perihelion distances. Furthermore, one of the matrix samples contains constituents with a regolith history on the parent body before compaction. It also contains trapped Ne with a ²⁰Ne/²²Ne ratio of 15.5 ± 0.5, apparently fractionated solar Ne.     

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.     

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).     

The Monahans chondrite and halite: Argon‐39/argon‐40 age,solar gases,cosmic‐ray exposure ages,and parent body regolith neutron flux and thickness

Abstract— The Monahans H‐chondrite is a regolith breccia containing light and dark phases and the first reported presence of small grains of halite. We made detailed noble gas analyses of each of these phases. The ³⁹Ar‐⁴⁰Ar age of Monahans light is 4.533 ± 0.006 Ma. Monahans dark and halite samples show greater amounts of diffusive loss of ⁴⁰Ar and the maximum ages are 4.50 and 4.33 Ga, respectively. Monahans dark phase contains significant concentrations of He, Ne and Ar implanted by the solar wind when this material was extant in a parent body regolith. Monahans light contains no solar gases. From the cosmogenic ³He, ²¹Ne, and ³⁸Ar in Monahans light we calculate a probable cosmic‐ray, space exposure age of 6.0 ± 0.5 Ma. Monahans dark contains twice as much cosmogenic ²¹Ne and ³⁸Ar as does the light and indicates early near‐surface exposure of 13–18 Ma in a H‐chondrite regolith. The existence of fragile halite grains in H‐chondrites suggests that this regolith irradiation occurred very early. Large concentrations of ³⁶Ar in the halite were produced during regolith exposure by neutron capture on ³⁵Cl, followed by decay to ³⁶Ar. The thermal neutron fluence seen by the halite was (2–4) × 10¹⁴ n/cm². The thermal neutron flux during regolith exposure was ～0.4‐0.7 n/cm²/s. The Monahans neutron fluence is more than an order of magnitude less than that acquired during space exposure of several large meteorites and of lunar soils, but the neutron flux is lower by a factor of ≤5. Comparison of the ³⁶Ar_n/²¹Ne_cos ratio in Monahans halite and silicate with the theoretically calculated ratio as a function of shielding depth in an H‐chondrite regolith suggests that irradiation of Monahans dark occurred under low shielding in a regolith that may have been relatively shallow. Late addition of halite to the regolith can be ruled out. However, irradiation of halite and silicate for different times at different depths in an extensive regolith cannot be excluded.     

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.     

Statistical Estimation of the Production of Cosmic-Ray-Induced Nuclides in Meteorites*

Abstract— Among iron meteorites there is a simple relationship estimating, with two parameters, the production rates of spallation products of mass A, that range in ΔA from 2 to 36 (Cr to Ne), where ΔA is 56 — A. This paper describes further applications of this relation, extending it to chondrites, in terms of the effective ΔA. It is possible to systematize the production of common products in chondrites such as ³He, ²⁶Al,²²Ne, ²¹Ne and ³⁸Ar. For this purpose, data on products determined in the metal phases of chondrites are useful in correlating stones to irons. For general application to stone meteorites, statistical treatments for multielement target systems are employed, except for the lowest energy reaction products. Statistical methods for spallation products are thus now applicable for all iron and stone meteorites.     

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.     

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.     

Cosmic‐ray exposure history of two Frontier Mountain H‐chondrite showers from spallation and neutron‐capture products

Abstract— We measured the concentrations of ¹⁰Be, ²⁶Al, ³⁶Cl, ⁴¹Ca and ¹⁴C in the metal and/or stone fractions of 27 Antarctic chondrites from Frontier Mountain (FRO), including two large H‐chondrite showers. To estimate the pre‐atmospheric size of the two showers, we determined the contribution of neutron‐capture produced ³⁶Cl (half‐life = 3.01 times 10⁵ years) and ⁴¹Ca (1.04 times 10⁵ years) in the stone fraction. The measured activities of neutron‐capture ³⁶Cl and ⁴¹Ca, as well as spallation produced ¹⁰Be and ²⁶Al, were compared with Monte Carlo‐based model calculations. The largest shower, FRO 90174, includes eight fragments with an average terrestrial age of (100 ± 30) × 10³ years; the neutron‐capture saturation activities extend to 27 dpm/kg stone for ³⁶Cl and 19 dpm/kg stone for ⁴¹Ca. The concentrations of spallation produced ¹⁰Be, ²⁶Al and ³⁶Cl constrain the radius (R) to 80–100 cm, while the neutron‐capture ⁴¹Ca activities indicate that the samples originated from the outer 25 cm. With a pre‐atmospheric radius of 80–100 cm, FRO 90174 is among the largest of the Antarctic stony meteorites. The large pre‐atmospheric size supports our hypothesis that at least 50 of the ～150 classified H5/H6‐chondrites from the Frontier Mountain stranding area belong to this single fall; this hypothesis does not entirely account for the high H/L ratio at Frontier Mountain. The smaller shower, FRO 90001, includes four fragments with an average terrestrial age of (40 ± 10) × 10³ years; they contain small contributions of neutron‐capture ³⁶Cl, but no excess of ⁴¹Ca. FRO 90001 experienced a complex exposure history with high shielding conditions in the first stage (150 < R < 300 cm) and much lower shielding in the second stage (R < 30 cm), the latter starting ～1.0 million years (Ma) ago. Based on the measured ¹⁰Be/²¹Ne and ²⁶Al/²¹Ne ratios, the cosmic‐ray exposure ages of the two showers are 7.2 ± 0.5 Ma for FRO 90174 and 8 ± 1 Ma for FRO 90001. These ages coincide with the well‐established H‐chondrite peak and corroborate the observation that the exposure age distribution of FRO H‐chondrites is similar to that of non‐Antarctic falls. In addition, we found that corrections for neutron‐capture ³⁶Ar (from decay of ³⁶Cl) result in concordant ²¹Ne and ³⁸Ar exposure ages.     

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.     

New model calculations for the production rates of cosmogenic nuclides in iron meteorites

Abstract— Here we present the first purely physical model for cosmogenic production rates in iron meteorites with radii from 5 cm to 120 cm and for the outermost 1.3 m of an object having a radius of 10 m. The calculations are based on our current best knowledge of the particle spectra and the cross sections for the relevant nuclear reactions. The model usually describes the production rates for cosmogenic radionuclides within their uncertainties; exceptions are ⁵³Mn and ⁶⁰Fe, possibly due to normalization problems. When an average S content of about 1 ± 0.5% is assumed for Grant and Carbo samples, which is consistent with our earlier study, the model predictions for ³He, ²¹Ne, and ³⁸Ar are in agreement. For ⁴He the model has to be adjusted by 24%, possibly a result of our rather crude approximation for the primary galactic α particles. For reasons not yet understood the modeled ³⁶Ar/³⁸Ar ratio is about 30–40% higher than the ratio typically measured in iron meteorites. Currently, the only reasonable explanation for this discrepancy is the lack of experimentally determined neutron induced cross sections and therefore the uncertainties of the model itself. However, the new model predictions, though not yet perfect, enable determining the radius of the meteoroid, the exposure age, the sulphur content of the studied sample as well as the terrestrial residence time. The determination of exposure ages is of special interest because of the still open question whether the GCR was constant over long time scales. Therefore we will discuss in detail the differences between exposure ages determined with different cosmogenic nuclides. With the new model we can calculate exposure ages that are based on the production rates (cm³STP/(gMa)) of noble gases only. These exposure ages, referred to as noble gas exposure ages or simply ^3,4He, ²¹Ne, or ^36,38Ar ages, are calculated assuming the current GCR flux. Besides calculating noble gas ages we were also able to improve the ⁴¹K‐⁴⁰K‐and the ³⁶Cl‐³⁶Ar dating methods with the new model. Note that we distinguish between ³⁶Ar ages (calculated via ³⁶Ar production rates only) and ³⁶Cl‐³⁶Ar ages. Exposure ages for Grant and Carbo, calculated with the revised ⁴¹K‐⁴⁰K method, are 628 ± 30 Ma and 841 ± 19 Ma, respectively. For Grant this is equal to the ages obtained using ³He, ²¹Ne, and ³⁸Ar but higher than the 3⁶Ar‐ and ³⁶Cl‐³⁶Ar ages by ?30%. For Carbo the ⁴¹K‐⁴⁰K age is ?40% lower than the ages obtained using ³He, ²¹Ne, and ³⁸Ar but equal to the ³⁶Ar age. These differences can either be explained by our still insufficient knowledge of the neutron‐induced cross sections or by a long‐term variation of the GCR.     

Noble gases and strontium isotopes in the unique meteorite Divnoe

Abstract— We present an isotope study of noble gases in Divnoe, an anomalous meteorite, and also Rb-Sr and K-Ar dating of this meteorite. The relatively young Rb-Sr age obtained (3.39 Ga) seems doubtful and, most probably, results from weathering or contamination. The ancient K-Ar age (4.67^+0.20_–0.40), together with clear excess of ¹²⁹Xe, allows the suggestion of very early formation of the Divnoe meteorite. Concentrations and isotope ratios of noble gases in Divnoe are: 17.9 ≤ ³He ≤ 29.0 × 10^?8; ²⁰Ne = 6.22 × 10^?8; 2.44 ≤ ³⁶Ar ≤ 5.10 × 10^?8; ¹³⁰Xe = 41.3 × 10^?12 cm³/g; 0.079 ≤ ³He/⁴He ≤ 0.193; ²⁰Ne/²²Ne = 0.860; ²¹Ne/²²Ne = 0.927; 3.47 ≤ ⁴⁰Ar/³⁶Ar ≤ 9.47; 2.22 ≤ 36Ar/³⁸Ar ≤ 3.27; ¹²⁹Xe/¹³²Xe = 1.09. The exposure age calculated from cosmogenic ³He, ²¹Ne, and ³⁸Ar is 17.9 ± 0.9 Ma. On the basis of the isotope data for the noble gases and O, and abundances of K, Rb, and Sr, an attempt was made to estimate the relationship of Divnoe to other meteorite types. The O-isotope characteristics of Divnoe are clearly distinct from those of ordinary chondrites, acapulcoites/lodranites, and SNC meteorites (Petaev et al., 1994, Clayton, 1993). In plots of ¹³⁶Xe vs. ¹²⁹Xe/¹³⁰Xe, the Divnoe data fall outside of the data fields for carbonaceous and enstatite chondrites. The light noble gas data, especially the ⁴⁰Ar/³⁸Ar ratio, and the ⁴⁰Ar, ³⁸Ar, ³He, and ⁴He contents of Divnoe differ significantly from those of all meteorite types except diogenites. The K, Rb, and Sr abundances in Divnoe are substantially lower than in most other meteorites. In the concentrations of these elements, as well as in the REE pattern, the Divnoe meteorite is similar only to diogenites. Divnoe probably should be treated as a restite remaining after partial melting of the chondritic mantle of a parent asteroid body.     

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).     

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.     

Ejection ages from krypton‐81‐krypton‐83 dating and pre‐atmospheric sizes of martian meteorites

Abstract— Cosmic‐ray exposure (CRE) ages and Mars ejection times were calculated from the radionuclide ⁸¹Kr and stable Kr isotopes for seven martian meteorites. The following ⁸¹Kr‐Kr CRE ages were obtained: Los Angeles = 3.35 ± 0.70 Ma; Queen Alexandra Range 94201 = 2.22 ± 0.35 Ma; Shergotty = 3.05 ± 0.50 Ma; Zagami = 2.98 ± 0.30 Ma; Nakhla = 10.8 ± 0.8 Ma; Chassigny = 10.6 ± 2.0 Ma; and Allan Hills 84001 = 15.4 ± 5.0 Ma. Comparison of these ages with previously obtained CRE ages from the stable noble gas nuclei ³He, ²¹Ne, and ³⁸Ar shows excellent agreement. This indicates that the method for the production rate calculation for the stable nuclei is reliable. In all martian meteorites we observe effects induced by secondary cosmic‐ray produced epithermal neutrons. Epithermal neutron fluxes, φ_n (30–300 eV), are calculated based on the reaction ⁷⁹Br(n, γβ)⁸⁰Kr. We show that the neutron capture effects were induced in free space during Mars‐Earth transfer of the meteoroids and that they are not due to a pre‐exposure on Mars before ejection of the meteoritic material. Neutron fluxes and slowing down densities experienced by the meteoroids are calculated and pre‐atmospheric sizes are estimated. We obtain minimum radii in the range of 22–25 cm and minimum masses of 150–220 kg. These results are in good agreement with the mean sizes reported for model calculations using current semiempirical data.     

On the Bur Gheluai H5 chondrite and other meteorites with complex exposure histories

Abstract— We present the ¹⁴C, ²⁶Al, ¹⁰Be, ³He, ⁴He, ²⁰Ne, ²¹Ne, ²²Ne, ³⁶Ar, ³⁸Ar, and ⁴⁰Ar concentrations and the track densities measured in up to 13 samples of the Bur Gheluai (H5) meteorite fall. Only a multi-stage exposure history can explain the data in a self-consistent way. Parameters for a model two-stage history obtained by simultaneous, least-squares fitting of the concentrations of ¹⁴C, ²⁶Al, ¹⁰Be, and ²¹Ne were: first stage duration ～10 Ma and radius >2 m; second stage duration ～0.6 Ma and radius 40–100 cm. Nominal one-stage ²¹Ne production rates (P₂₁) inferred from ²⁶Al in Bur Gheluai samples exceed those inferred from ¹⁰Be as expected for a meteorite with a complex history. Nonetheless, data for other meteorites indicate that multi-stage irradiations alone do not account for all the high reported values of P₂₁ based on ²⁶A***l: The equations describing the production of cosmogenic nuclides show that uncorrected shielding effects may also play a role. A compilation of ordinary, solar-gas-poor chondrites for which two-stage histories have been proposed includes many with short second stages but none with unambiguously long first stages (>0.2 Ga).     

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 enstatite chondrites I: Exposure ages,pairing, and weathering effects

Abstract— Cosmic‐ray exposure ages calculated from cosmogenic noble gas nuclides are reported for 57 enstatite (E) chondrites, 43 of them were measured for the first time. With a total of 62 individual E chondrites (literature and this data, corrected for pairing) the observed spectrum of ages ranges between 0.07 and 66 Ma. Three clusters seem to develop at about 3.5, 8, and 25 Ma, respectively. Since the uncertainty of ages is estimated to be ～20% (in contrast to 10 to 15% for ordinary chondrites) and the number of examined samples is still comparatively small, these peaks have to be confirmed by more measurements. Regarding the two subgroups, EH and EL chondrites, no systematic trend is apparent in the distribution of cosmic‐ray exposure ages. Several E chondrites yield significantly lower ³⁸Ar ages compared to those calculated from cosmogenic ³He and ²¹Ne. For these E chondrites, we suggest a reduction of cosmogenic ³⁸Ar as a result of weathering. In order to prove the possible influence of terrestrial alteration on the cosmogenic noble gas record of E‐chondritic material, we simulated terrestrial weathering in an experiment of 12 weeks duration. The treatment showed that a significant amount of cosmogenic ³⁸Ar is lost on Earth by the influence of water.