Petrology,mineralogy, porosity,and cosmic‐ray exposure history of Huaxi ordinary chondrite

A meteorite fall was heard and collected on July 13, 2010 at about 18:00 (local time) in the Shibanjing village of the Huaxi district of Guiyang, Guizhou province, China. The total mass of the fall is estimated to be at least 1.6 kg; some fragments are missing. The meteorite consists mainly of olivine, low‐Ca pyroxene, high‐Ca pyroxene, plagioclase, kamacite, taenite, and troilite. Minor phases include chromite and apatite. Various textural types of chondrules exist in this meteorite: most chondrule textures can be easily defined. The grain sizes of secondary plagioclase in this meteorite range from 2 to 50 μm. The chemical composition of olivine and low‐Ca pyroxene are uniform; Fa in olivine and Fs in low‐Ca pyroxene are, respectively, 19.6 ± 0.2 and 17.0 ± 0.3 (mole%). Huaxi has been classified as an H5 ordinary chondrite, with a shock grade S2, and weathering W0. The weak shock features, rare fractures, and the high porosity (17.6%) indicates that Huaxi is a less compacted meteorite. The preatmospheric radius of Huaxi is ~11 cm, corresponding to ~21 kg. The meteorite experienced a relatively short cosmic‐ray exposure of about 1.6 ± 0.1 Ma. The ⁴He and ⁴⁰Ar retention ages are older than 4.6 Ga implying that Huaxi did not degas after thermal metamorphism on its parent body.     

Noble gases in the Xinjiang (Armanty) iron meteorite––A big object with a short cosmic‐ray exposure age

Abstract– We measured the concentrations and isotopic ratios of the cosmogenic noble gases He, Ne, and Ar in the very large iron meteorite Xinjiang (IIIE). The ³He and ⁴He data indicate that a significant portion of the cosmogenic produced helium has been lost via diffusion or in a recent impact event. High ²²Ne/²¹Ne ratios indicate that contributions to the cosmogenic ²¹Ne from sulfur and/or phosphorous are significant. By combining the measured nuclide concentrations with model calculations for iron meteorites we were able to determine the preatmospheric diameter of Xinjiang to 260–320 cm, which corresponds to a total mass of about 70–135 tons. The cosmic‐ray exposure age of Xinjiang is 62 ± 16 Ma, i.e., relatively short compared to most of the other iron meteorites. With the current database we cannot firmly determine whether Xinjiang experienced a complex irradiation history. The finding of ³He and ⁴He losses might argue for a recent impact event and therefore for a complex exposure.     

The cosmic‐ray exposure history of the Twannberg iron meteorite (IIG)

The Twannberg iron meteorite is one out of only six members of the group IIG. The combined noble gas and radionuclide data obtained in this new systematic study indicate that Twannberg with its ~570 recently recovered specimens was a large object with a preatmospheric radius in the range of ~2 m, which corresponds to ~250 × 10³ kg. The cosmic‐ray exposure age for Twannberg is 182 ± 45 Ma. The most surprising result is the long terrestrial age of T_terr =  ka, which is unexpected considering the humid conditions in Switzerland. However, this age is in accord with glaciation events, indicating that the less shielded samples from Mt. Sujet were found close to the position of the original strewn field, whereas the samples from Gruebmatt and Twannbach, which are from more shielded positions, were glacially transported to the east–northeast during the second last ice age (185–130 ka ago) from an original position west of Mt. Sujet.     

81Kr‐Kr age and multiple cosmic‐ray exposure history of the Vaca Muerta mesosiderite

Abstract– Noble gas isotopic compositions were measured for a eucritic pebble and bulk material of a silicate–metal mixture from the Vaca Muerta mesosiderite as well as pyroxene and plagioclase separated from the eucritic pebble by total melting and stepwise heating methods. Trapped noble gases were degassed completely by a high‐temperature thermal event, probably at the formation of the Vaca Muerta parent body (VMPB). The presence of fissiogenic Xe isotopes from extinct ²⁴⁴Pu in the bulk samples might be a result of rapid cooling from an early high‐temperature metamorphism. High concentrations of cosmogenic noble gases enabled us to determine precise isotopic ratios of cosmogenic Kr and Xe. Spallogenic Ne from Na and unique Ar isotopic compositions were observed. The ⁸¹Kr‐Kr exposure age of 168 ± 8 Myr for the silicate pebble is distinctly longer than the age of 139 ± 8 Myr for the bulk samples. The precursor of the pebble had been irradiated on the surface of the VMPB for more than 60 Myr (first stage irradiation), with subsequent incorporation into bulk materials approximately 4 Gyr ago. The Vaca Muerta meteorite was excavated from the VMPB 140 Myr ago (second stage irradiation). Relative diffusion rates among the cosmogenic Ar, Kr, and Xe based on data obtained by stepwise heating indicate that Kr and Xe can be partially retained in pyroxene and plagioclase under the condition that resets the K‐Ar system. This result supports the presence of fission Xe and of excess concentration of cosmogenic Kr, which could have survived the thermal event approximately 3.8 Gyr ago.     

Solar and galactic cosmic‐ray records of the Fermo (H) chondrite regolith breccia

Abstract— ‐We demonstrate the presence of solar flare as well as neutron capture effects in the isotopic composition of rare gases in the Fermo regolith breccia acquired on its parent body based on the measurements of tracks, rare gases and radionuclides. The track density along a 3.2 cm long core decreases by a factor of about 6 and by more than a factor of 13 within the meteorite, indicating small (2–9 cm) and asymmetrical ablation. Rare gases show a large trapped component; the isotopic ratios, particularly ²⁰Ne/²²Ne ? 11 and ²⁰Ne/³⁶Ar = 10 are indicative of a solar component. The galactic cosmic‐ray exposure age is determined to be 8.8 Ma. Activities of a dozen radionuclides ranging in half‐life from 16 day ⁴⁸V to 0.73 Ma ²⁶Al are consistent with their expected production rates. Track, rare gas and radionuclide data show that the meteoroid was a small body (≤ 120 kg) and had a simple, one‐stage exposure history to cosmic rays in the interplanetary space. However, ⁸²Kr and ¹²⁸Xe show an excess due to neutron irradiation on the parent body of the meteorite. The presence of solar gases and the neutron capture effects indicate several stages of irradiation on the parent asteroid. The chemical composition of Fermo confirms that it belongs to the H group of ordinary chondrites with lithic clasts having varying compositions. δ¹⁵N is found to be 8.3 ± 1.2%0, close to the typical values observed in H chondrites.     

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.     

The production of cosmogenic nuclides by galactic cosmic‐ray particles for 2π exposure geometries

Abstract— We present a purely physical model for the calculation of depth‐dependent production rates in 2π exposure geometries by galactic cosmic rays (GCR). Besides the spectra of primary and secondary particles and the excitation functions of the underlying nuclear reactions, the model is based on the integral number of GCR particles in the lunar orbit. We derived this value from adjusting modeled depth profiles for ¹⁰Be, ²⁶Al, and ⁵³Mn to measured data from the Apollo 15 drill core. The J_0,GCR value of 4.54 cm^?2 s^?1 and the solar modulation parameter of M = 490 MeV determined this way for 1 AU is in reasonable agreement with the J_0,GCR value derived recently for the meteoroid orbits (Leya et al., 2000b). We also show that the mean GCR proton spectrum in the lunar orbit has not changed substantially over about the last 10 Ma. For the major target elements we present depth‐dependent production rates for ¹⁰Be, ¹⁴C, ²⁶Al, ³⁶Cl and ⁵³Mn, as well as for the rare gas isotopes ^20,21,22Ne. In addition we present production rates for ^36,38Ar from Fe and Ni. The new results are consistent with the data for stony meteoroids presented recently by our group (Leya et al., 2000b), but for the rare gas isotopes the new production rates sometimes differ significantly from earlier estimates. The applicability of the ²²Ne/²¹Ne ratio as a shielding parameter is also discussed.     

Noble gas record,collisional history,and pairing of CV,CO, CK,and other carbonaceous chondrites

Abstract— Concentration and isotopic composition of the light noble gases as well as of ⁸⁴Kr, ¹²⁹Xe, and ¹³²Xe have been measured in bulk samples of 60 carbonaceous chondrites; 45 were measured for the first time. Solar noble gases were found in nine specimens (Arch, Acfer 094, Dar al Gani 056, Graves Nunataks 95229, Grosnaja, Isna, Mt. Prestrud 95404, Yamato (Y) 86009, and Y 86751). These meteorites are thus regolith breccias. The CV and CO chondrites contain abundant planetary‐type noble gases, but not CK chondrites. Characteristic features of CK chondrites are high ¹²⁹Xe/¹³²Xe ratios. The petrologic type of carbonaceous chondrites is correlated with the concentration of trapped heavy noble gases, similar to observations shown for ordinary chondrites. However, this correlation is disturbed for several meteorites due to a contribution of atmospheric noble gases, an effect correlated to terrestrial weathering effects. Cosmic‐ray exposure ages are calculated from cosmogenic ²¹Ne. They range from about 1 to 63.5 Ma for CO, CV, and CK classes, which is longer than exposure ages reported for CM and CI chondrites. Only the CO3 chondrite Isna has an exceptionally low exposure age of 0.15 Ma. No dominant clusters are observed in the cosmic‐ray exposure age distribution; only for CV and CK chondrites do potential peaks seem to develop at ～9 and ～29 Ma. Several pairings among the chondrites from hot deserts are suggested, but 52 of the 60 investigated meteorites are individual falls. In general, we confirm the results of Mazor et al. (1970) regarding cosmic‐ray exposure and trapped heavy noble gases. With this study, a considerable number of new carbonaceous chondrites were added to the noble gas data base, but this is still not sufficient to obtain a clear picture of the collisional history of the carbonaceous chondrite groups. Obviously, the exposure histories of CI and CM chondrites differ from those of CV, CO, and CK chondrites that have much longer exposure ages. The close relationship among the latter three is also evident from the similar cosmic‐ray exposure age patterns that do not reveal a clear picture of major breakup events. The CK chondrites, however, with their wide range of petrologic types, form the only carbonaceous chondrite group which so far lacks a solar‐gas‐bearing regolith breccia. The CK chondrites contain only minute amounts of trapped noble gases and their noble gas fingerprint is thus distinguishable from the other groups. In the future, more analyses of newly collected CK chondrites are needed to unravel the genetic and historic evolution of this group. It is also evident that the problems of weathering and pairing have to be considered when noble gas data of carbonaceous chondrite are interpreted.     

Devgaon (H3) chondrite: Classification and complex cosmic ray exposure history

Abstract— The Devgaon meteorite fell in India on February 12, 2001 and was immediately collected. It is an ordinary chondrite having a number of SiO₂‐rich objects and some Ca, Al‐rich inclusions. Olivines (Fa_17–19) are fairly equilibrated, while pyroxenes (Fs_4–20) are unequilibrated. Occasionally, shock veins are visible, but the bulk rock sample is very weakly shocked (S2). Chondrules and chondrule fragments are abundant. Based on chemical and petrological features, Devgaon is classified as an H3.8 group chondrite. Several cosmogenic radionuclides ranging in half‐lives from 5.6 d (⁵²Mn) to 7.3 times 10⁵ yr (²⁶Al), noble gases (He, Ne, Ar, Kr, and Xe), and particle track density have been measured. The track density in olivines from five spot samples varies between (4.6 to 9) × 10⁶ cm^?2 showing a small gradient within the meteorite. The light noble gases are dominated by cosmogenic and radiogenic components. Large amounts of trapped gases (Ar, Kr, and Xe) are present. In addition, (n, γ) products from Br and I are found in Kr and Xe, respectively. The average cosmic ray exposure age of 101 ± 8 Ma is derived based on cosmogenic ³⁸Ar, ⁸³Kr, and ¹²⁶Xe. The track production rates correspond to shielding depths of about 4.9 to 7.8 cm, indicating that the stone suffered type IV ablation. Low ⁶⁰Co, high (²²Ne/²¹Ne)_c, and large neutron produced excesses at ⁸⁰Kr, ⁸²Kr, and ¹²⁸Xe indicate a complex exposure history of the meteoroid. In the first stage, a meter‐sized body was exposed for nearly 10⁸ yr in the interplanetary space that broke up in ?50 cm‐sized fragments about a million years ago (stage 2), before it was captured by the Earth.     

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.     

Measuring ΩM and ΩΛ with long‐duration gamma‐ray bursts

Gamma‐ray bursts (GRBs) are one of the most luminous events in the Universe. In addition, the Universe itself is almost transparent to γ ‐rays, making GRBs detectable up to very high redshifts. As a result, GRBs are very suitable to probe the cosmological parameters. This work shows the potential of long‐duration GRBs for measuring the cosmological parameters Ω_M and Ω_Λ by comparing the observed log N ‐log P distribution with the theoretical one. Provided that the GRBs rate and luminosity function are well determined, the best values and 1σ confidence intervals obtained are Ω_M = 0.22^+0.05_–0.03 and Ω_Λ = 1.06^+0.05_–0.10. Finally, a set of simulations show the ability of the method to measure Ω_M and Ω_Λ (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Noble gases and cosmogenic radionuclides in the Gold Basin L4 chondrite shower: Thermal history,exposure history,and pre‐atmospheric size

Abstract— We measured the concentrations of the cosmogenic radionuclides ¹⁰Be, ²⁶Al, ³⁶Cl, and ⁴¹Ca in the stone and metal fractions of 15 fragments of the Gold Basin L4 chondrite shower, as well as noble gases in 18 Gold Basin fragments. A comparison of ¹⁰Be, ²⁶Al, and ⁴¹Ca concentrations with calculated production rates from two different models indicates that the Gold Basin samples came from depths of about 10 cm to more than 150 cm in an object with a radius of 3–5 m. As was predicted by recent model calculations, the noble gases show a reversal of the ²²Ne/²¹Ne ratio at very high shielding. The ²¹Ne/¹⁰Be and ²¹Ne/²⁶Al ratios in most samples are constant and correspond to a 4π exposure age of 18 ± 2 Myr. However, three Gold Basin samples show a 30–120% excess of ²¹Ne implying that they were previously exposed close to the surface of the parent body, whereas the other samples were buried several meters deeper. Concentrations of neutron‐capture ³⁶Ar in most samples are consistent with measured concentrations of neutron‐capture ³⁶Cl and an exposure age of 18 Myr. Large excesses of neutron‐capture ³⁶Ar were found in those samples with an excess of ²¹Ne, providing additional evidence of a first‐stage exposure on the parent body. The excess of spallation‐produced ²¹Ne and neutron‐capture‐produced ³⁶Ar in these samples indicate a first‐stage exposure of 35–150 Myr on the parent body. The radiogenic ⁴He and ⁴⁰Ar concentrations indicate a major impact on the parent body between 300 and 400 Myr ago, which must have preceded the impacts that brought the Gold Basin meteoroid to the surface of the parent body and then expelled it from the parent body 18 Myr ago.     

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.     

Petrology,geochemistry, and cosmic‐ray exposure age of Iherzolitic shergottite Northwest Africa 1950

Northwest Africa (NWA) 1950 is a new member of the lherzolitic shergottite clan of the Martian meteorites recently found in the Atlas Mountains. The petrological, mineralogical, and geochemical data are very close to those of the other known lherzolitic shergottites. The meteorite has a cumulate gabbroic texture and its mineralogy consists of olivine (Fo₆₆ to Fo₇₅), low and high‐Ca pyroxenes (En₇₈Fs₁₉Wo₂‐En₆₀Fs₂₆W₁₄; En₅₃Fs₁₆Wo₃₁‐En₄₅Fs₁₄Wo₄₁), and plagioclase (An₅₇Ab₄₁Or₁ to An₄₀Ab₅₇Or₃; entirely converted into maskelynite during intense shock metamorphism). Accessory minerals include phosphates (merrillite), chromite and spinels, sulfides, and a glass rich in potassium. The oxygen isotopic values lie on the fractional line defined by the other SNC meteorites (Δ¹⁷O = 0.312 %o). The composition of NWA 1950 is very similar to the other lherzolitic shergottites and suggests an origin from the same magmatic system, or at least crystallization from a close parental melt. Cosmogenic ages indicate an ejection age similar to those of the other lherzolitic shergottites. The intensity of the shock is similar to that observed in other shergottites, as shown by the occurrence of small melt pockets containing glass interwoven with stishovite.     

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.     

Cosmic‐ray exposure history of the Norton County enstatite achondrite

Abstract– We report measurements of cosmogenic nuclides in up to 11 bulk samples from various depths in Norton County. The activities of ³⁶Cl, ⁴¹Ca, ²⁶Al, and ¹⁰Be were measured by accelerator mass spectrometry; the concentrations of the stable isotopes of He, Ne, Ar, and Sm were measured by electron and thermal ionization mass spectrometry, respectively. Production rates for the nuclides were modeled using the LAHET and the Monte Carlo N‐Particle codes. Assuming a one‐stage irradiation of a meteoroid with a pre‐atmospheric radius of approximately 50 cm, the model satisfactorily reproduces the depth profiles of ¹⁰Be, ²⁶Al, and ⁵³Mn (<6%) but overestimates the ⁴¹Ca concentrations by about 20%. ³He, ²¹Ne, and ²⁶Al data give a one‐stage cosmic‐ray exposure (CRE) age of 115 Ma. Argon‐36 released at intermediate temperatures, ³⁶Ar_n, is attributed to production by thermal neutrons. From the values of ³⁶Ar_n, an assumed average Cl concentration of 4 ppm, and a CRE age of 115 Ma, we estimate thermal neutron fluences of 1–4 × 10¹⁶ neutrons cm^?2. We infer comparable values from ε¹⁴⁹Sm and ε¹⁵⁰Sm. Values calculated from ⁴¹Ca and a CRE age of 115 Ma, 0.2–1.4 × 10¹⁶ neutrons cm^?2, are lower by a factor of approximately 2.5, indicating that nearly half of the ¹⁴⁹Sm captures occurred earlier. One possible irradiation history places the center of proto‐Norton County at a depth of 88 cm in a large body for 140 Ma prior to its liberation as a meteoroid with a radius of 50 cm and further CRE for 100 Ma.     

Noble gases in interplanetary dust particles,II: Excess helium‐3 in cluster particles and modeling constraints on interplanetary dust particle exposures to cosmic‐ray irradiation

Abstract— Measurements of He isotopes in cluster interplanetary dust particles (IDPs) from stratospheric dust collector L2009 reveal anomalous ³He/⁴He ratios comparable to those seen earlier, up to ～40x the solar wind ratio, in particles from the companion collector L2011. These overabundances of ³He in the L2009 samples are masked by much higher ⁴He contents compared to the L2011 particles, and are visible only in minor gas fractions evolved by stepwise heating at high temperatures. Cosmic‐ray induced spallogenic reactions are efficient producers of ³He. The majority of this paper is devoted to a detailed assessment of the possible role of spallation in generating the ³He excesses in these and other cluster IDPs. A model of collisional erosion and fragmentation during inward transit through the interplanetary dust environment is used to estimate space lifetimes of particles from asteroidal and Edgeworth–Kuiper Belt sources. Results of the modeling indicate that Poynting–Robertson orbital evolution timescales of IDPs small enough to elude destruction on their way to Earth from either location are far shorter than the cosmic‐ray exposure ages required to account for observed ³He overabundances. Grains large enough to have sufficiently long space residence times are fragmented close to their sources. An alternative to long in‐space exposure could be prolonged irradiation of particles buried in parent body regoliths prior to their ejection as IDPs. A qualitative calculation suggests, however, that collisional erosion of asteroidal upper‐regolith materials is likely to occur on timescales shorter than the > 1 Ga burial times needed for accumulation of spallogenic ³He to the levels seen in several cluster particles. In contrast, regoliths on Edgeworth–Kuiper Belt objects may be stable enough to account for the ³He excesses, and delivery of heavily pre‐irradiated IDPs to the inner solar system by short‐period Edgeworth–Kuiper Belt comets remains a possibility. A potential problem is that the expected associated abundances of spallation‐produced ²¹Ne appear to be absent, although here the present IDP data base is too sparse and for the most part too imprecise to rule out a spallogenic origin. Relatively short periods of pre‐ejection residence in asteroidal regoliths may be responsible for the curiously broad exposure age distributions reported for micrometeorites extracted from Greenland and sea‐floor sediments.     

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 exposure age and heliocentric distance of the parent body of the Rumuruti chondrite PRE 95410

We measured concentrations and isotopic ratios of noble gases in the Rumuruti (R) chondrite Mount Prestrud (PRE) 95410, a regolith breccia exhibiting dark/light structures. The meteorite contains solar and cosmogenic noble gases. Based on the solar and cosmogenic noble gas compositions, we calculated a heliocentric distance of its parent body, a cosmic‐ray exposure age on the parent body regolith (parent body exposure age), and a cosmic‐ray exposure age in interplanetary space (space exposure age) of the meteorite. Assuming a constant solar wind flux, the estimated heliocentric distance was smaller than 1.4 ± 0.3 au, suggesting inward migration from the asteroid belt regions where the parent body formed. The largest known Mars Trojan 5261 Eureka is a potential parent body of PRE 95410. Alternatively, it is possible that the solar wind flux at the time of the parent body exposure was higher by a factor of 2–3 compared to the lunar regolith exposure. In this case, the estimated heliocentric distance is within the asteroid belt region. The parent body exposure age is longer than 19.1 Ma. This result indicates frequent impact events on the parent body like that recorded for other solar‐gas‐rich meteorites. Assuming single‐stage exposure after an ejection event from the parent body, the space exposure age is 11.0 ± 1.1 Ma, which is close to the peak of ~10 Ma in the exposure age distribution for the solar‐gas‐free R chondrites.     

40Ar‐39Ar and cosmic‐ray exposure ages of nakhlites—Nakhla,Lafayette, Governador Valadares—and Chassigny

Abstract– We present ⁴⁰Ar‐³⁹Ar dating results of handpicked mineral separates and whole‐rock samples of Nakhla, Lafayette, and Chassigny. Our data on Nakhla and Lafayette and recently reported ages for some nakhlites and Chassigny ( Misawa et al. 2006 ; Park et al. 2009 ) point to formation ages of approximately 1.4 Ga rather than 1.3 Ga that is consistent with previous suggestions of close‐in‐time formation of nakhlites and Chassigny. In Lafayette mesostasis, we detected a secondary degassing event at approximately 1.1 Ga, which is not related to iddingsite formation. It may have been caused by a medium‐grade thermal event resetting the mesostasis age but not influencing the K‐Ar system of magmatic inclusions and the original igneous texture of this rock. Cosmic‐ray exposure ages for these meteorites and for Governador Valadares were calculated from bulk rock concentrations of cosmogenic nuclides ³He, ²¹Ne, and ³⁸Ar. Individual results are similar to literature data. The considerable scatter of T₃, T₂₁, and T₃₈ ages is due to systematic uncertainties related to bulk rock and target element chemistry, production rates, and shielding effects. This hampers efforts to better constrain the hypothesis of a single ejection event for all nakhlites and Chassigny from a confined Martian surface terrain ( Eugster 2003 ; Garrison and Bogard 2005 ). Cosmic‐ray exposure ages from stepwise release age spectra using ³⁸Ar and neutron induced ³⁷Ar from Ca in irradiated samples can eliminate errors induced by bulk chemistry on production rates, although not from shielding conditions.