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A carbonaceous chondrite was recovered immediately after the fall near the village of Diepenveen in the Netherlands on October 27, 1873, but came to light only in 2012. Analysis of sodium and poly-aromatic hydrocarbon content suggests little contamination from handling. Diepenveen is a regolith breccia with an overall petrology consistent with a CM classification. Unlike most other CM chondrites, the bulk oxygen isotopes are extremely O-16 rich, apparently dominated by the signature of anhydrous minerals, distributed on a steep slope pointing to the domain of intrinsic CM water. A small subset plots closer to the normal CM regime, on a parallel line 2 parts per thousand lower in delta O-17. Different lithologies in Diepenveen experienced varying levels of aqueous alteration processing, being less aqueously altered at places rather than more heated. The presence of an agglutinate grain and the properties of methanol-soluble organic compounds point to active impact processing of some of the clasts. Diepenveen belongs to a CM clan with similar to 5Ma CRE age, longer than most other CM chondrites, and has a relatively young K-Ar resetting age of similar to 1.5Ga. As a CM chondrite, Diepenveen may be representative of samples soon to be returned from the surface of asteroid (162173) Ryugu by the Hayabusa2 spacecraft.

We calibrated the Kr 81 Kr dating system for ordinary chondrites of different sizes using independent shielding corrected Cl 36 Ar 36 ages. Krypton concentrations and isotopic compositions were measured in bulk samples from 14 ordinary chondrites of high petrologic type and the cosmogenic Kr component was obtained by subtracting trapped Kr from phase Q. The thus determined average cosmogenic Kr 78/Kr 83 Kr 80/Kr 83 Kr 82/Kr 83 and Kr 84/Kr 83 ratios are slightly different from earlier estimates (Lavielle and Marti 1988; Wieler 2002). The cosmogenic Kr 78/Kr 83 ratio is correlated with the cosmogenic Ne 22/Ne 21 ratio confirming that Kr 78/Kr 83 is a reliable shielding indicator. Previously Kr 81 Kr ages have been determined by assuming the cosmogenic production rate of Kr 81 P(Kr 81)(c) to be 0.95 times the average of the cosmogenic production rates of Kr 80 and Kr 82; the factor Y=0.95 therefore accounts for the unequal production of the various Kr isotopes (Marti 1967a). However Y should be regarded as an empirical adjustment. For samples whose Kr 80 and Kr 82 concentrations may be affected by neutron capture reactions the shielding dependent cosmogenic (Kr 78/Kr 83)(c) ratio has been used instead to calculate P(Kr 81)/P(Kr 83) as for some lunar samples this ratio has been shown to linearly increase with (Kr 78/Kr 83)(c) (Marti and Lugmair 1971). However the Kr 81 Kr ages of our samples calculated with these methods are on average similar to 30 higher than their Cl 36 Ar 36 ages indicating that most if not all the Kr 81 Kr ages determined so far are significantly too high. We therefore re evaluated both methods to determine P(Kr 81)(c)/P(Kr 83)(c). Our new Y value of 0.70 +/ 0.04 is more than 25 lower than the value of 0.95 used so far. Furthermore together with literature data our data indicate that for chondrites P(Kr 81)(c)/P(Kr 83)(c) is rather constant at 0.43 +/ 0.02 at least for the shielding range covered by our samples ([Kr 78/Kr 83](c)=0.119 0.185; [Ne 22/Ne 21](c)=1.083 1.144) in contrast to the observations on lunar samples. As expected considering the method used Kr 81 Kr ages calculated either directly with this new P(Kr 81)(c)/P(Kr 83)(c) value or with our new Y value both agree with the corresponding Cl 36 Ar 36 ages. However the average deviation of 2 indicates the accuracy of both new Kr 81 Kr dating methods and the precision of the new dating systems of similar to 10 is demonstrated by the low scatter in the data. Consequently this study indicates that the Kr 81 Kr ages published so far are up to 30 too high.

The Almahata Sitta strewn field is dominated by ureilites, but contains a large fraction of chondritic fragments of various types. We analyzed stable isotopes of He, Ne, Ar, Kr, and Xe, and the cosmogenic radionuclides 10Be, 26Al, and 36Cl in six chondritic Almahata Sitta fragments (EL6 breccia, EL6, EL3-5, CB, LL4/5, R-like). The cosmic-ray exposure (CRE) ages of five of the six samples have an average of 19.2 ± 3.3 Ma, close to the average of 19.5 ± 2.5 Ma for four ureilites. The cosmogenic radionuclide concentrations in the chondrites indicate a preatmospheric size consistent with Almahata Sitta. This corroborates that Almahata Sitta chondrite samples were part of the same asteroid as the ureilites. However, MS-179 has a lower CRE age of 11.0 ± 1.4 Ma. Further analysis of short-lived radionuclides in fragment MS-179 showed that it fell around the same time, and from an object of similar size as Almahata Sitta, making it almost certain that MS-179 is an Almahata Sitta fragment. Instead, its low CRE age could be due to gas loss, chemical heterogeneity that may have led to an erroneous 21Ne production-rate, or, perhaps most likely, MS-179 could represent the true 4π exposure age of Almahata Sitta (or an upper limit thereof), while all other samples analyzed so far experienced exposure on the parent body of similar lengths. Finally, MS-179 had an extraordinarily high activity of neutron-capture 36Cl, ~600 dpm kg−1, the highest activity observed in any meteorite to date, related to a high abundance of the Cl-bearing mineral lawrencite.

The Sutter's Mill (SM) carbonaceous chondrite fell in California on April 22, 2012. The cosmogenic radionuclide data indicate that Sutter's Mill was exposed to cosmic rays for 0.082 ± 0.008 Myr, which is one of the shortest ages for C chondrites, but overlaps with a small cluster at approximately 0.1 Myr. The age is significantly longer than proposed ages that were obtained from cosmogenic noble gas concentrations, which have large uncertainties due to trapped noble gas corrections. The presence of neutron-capture 60Co and 36Cl in SM indicates a minimum preatmospheric radius of approximately 50 cm, and is consistent with a radius of 1–2 m, as derived from the fireball observations. Although a large preatmospheric size was proposed, one fragment (SM18) contains solar cosmic ray–produced short-lived radionuclides, such as 56Co and 51Cr. This implies that this specimen was less than 2 cm from the preatmospheric surface of Sutter's Mill. Although this conclusion seems surprising, it is consistent with the observation that the meteoroid fragmented high in the atmosphere. The presence of SCR-produced nuclides is consistent with the high SCR fluxes observed during the last few months before the meteorite's fall, when its orbit was less than 1 AU from the Sun.

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.

The Mifflin meteorite fell on the night of April 14, 2010, in southwestern Wisconsin. A bright fireball was observed throughout a wide area of the midwestern United States. The petrography, mineral compositions, and oxygen isotope ratios indicate that the meteorite is a L5 chondrite fragmental breccia with light/dark structure. The meteorite shows a low shock stage of S2, although some shock-melted veins are present. The U,Th-He age is 0.7Ga, and the K-Ar age is 1.8Ga, indicating that Mifflin might have been heated at the time of the 470Ma L-chondrite parent body breakup and that U, Th-He, and K-Ar ages were partially reset. The cosmogenic radionuclide data indicate that Mifflin was exposed to cosmic rays while its radius was 3065cm. Assuming this exposure geometry, a cosmic-ray exposure age of 25 +/- 3Ma is calculated from cosmogenic noble gas concentrations. The low 22Ne/21Ne ratio may, however, indicate a two-stage exposure with a longer first-stage exposure at high shielding. Mifflin is unusual in having a low radiogenic gas content combined with a low shock stage and no evidence of late stage annealing; this inconsistency remains unexplained. (Less)

We present the results of a noble gas (He, Ne, Ar) and cosmogenic radionuclide (10Be, 26Al, 36Cl) analysis of two chondritic fragments (#A100, L4 and #25, H5) found in the Almahata Sitta strewn field in Sudan. We confirm their earlier attribution to the same fall as the ureilites dominating the strewn field, based on the following findings: (1) both chondrite samples indicate a preatmospheric radius of approximately 300 g cm-2, consistent with the preatmospheric size of asteroid 2008 TC3 that produced the Almahata Sitta strewn field; (2) both have, within error, a 21Ne/26Al-based cosmic ray exposure age of approximately 20 Ma, identical to the reported ages of Almahata Sitta ureilites; (3) both exhibit hints of ureilitic Ar in the trapped component. We discuss a possible earlier irradiation phase for the two fragments of approximately 1020 Ma, visible only in cosmogenic 38Ar. We also discuss the approximately 3.8 Ga (4He) and approximately 4.6 Ga (40Ar) gas retention ages, measured in both chondritic fragments. These imply that the two chondrite fragments were incorporated into the ureilite host early in solar system evolution, and that the parent asteroid from which 2008 TC3 is derived has not experienced a large break-up event in the last 3.8 Ga. (Less)

The collection of approximately 3300 meteorites from the Queen Alexandra Range (QUE) area, Antarctica, is dominated by more than 2000 chondrites classified as either L5 or LL5. Based on concentrations of the cosmogenic radionuclides 10 Be, 26 Al, 36 Cl, and 41 Ca in the metal and stone fraction of 16 QUE L5 or LL5 chondrites, we conclude that 13 meteorites belong to a single meteorite shower, QUE 90201, with a large preatmospheric size and a terrestrial age of 125 kyr. Members of this shower have properties typical of L (e.g., pyroxene composition) and LL chondrites (e.g., metal abundance and composition), as well as properties intermediate between the L and LL groups (e.g., olivine composition), and is thus best described as an L ⁄ LL5 chondrite. Based on comparison with model calculations, the measured radionuclide concentrations in the metal and stone fractions of QUE 90201 indicate irradiation in an object with a preatmospheric radius of approximately 150 cm, representing one of the largest chondrites known so far. Based on the abundance of small L5 and LL5 chondrites at QUE and their distinct mass distribution, we conclude that the QUE 90201 shower includes up to 2000 fragments with a total recovered mass of 60-70 kg

– The isotopic compositions of Sm and/or Gd of nine documented drill core samples of the Jilin H5 chondrite were determined to study the neutron capture records of individual meteorite samples. All the samples showed significant isotopic shifts of 150Sm/149Sm and/or 158Gd/157Gd corresponding to neutron fluences of (1.3–1.7) × 1015 n cm−2. Considering the short 4π irradiation age of 0.32 Ma during the second stage, the 2π irradiation of 7 Ma during the first stage is the main influence on the Sm and Gd isotopic shifts of the Jilin chondrite. Although a depth dependence of the neutron capture effects was expected from the isotopic variations of 150Sm/149Sm and 158Gd/157Gd in the Jilin chondrite core samples that were possibly drilled perpendicular to the surface of the large object with a radius of >10 m in the 2π-geometry, no clear evidence was observed in this study. The data from the combination of the isotopic shifts between Sm and Gd defined as eSm/eGd suggest that the neutrons produced in the 2π-geometry of the Jilin chondrite follow the similar energy spectra as the neutrons in lunar samples, although the present analytical quality is not enough to discuss a critical discussion for the thermalization of the neutron energy levels.

Two silicate-rich dust layers were found in the Dome Fuji ice core in East Antarctica, at Marine Isotope Stages 12 and 13. Morphologies, textures, and chemical compositions of constituent particles reveal that they are high-temperature melting products and are of extraterrestrial origin. Because similar layers were found ∼ 2000 km east of Dome Fuji, at EPICA (European Project for Ice Coring in Antarctica)-Dome C, particles must have rained down over a wide area 434 and 481 ka. The strewn fields occurred over an area of at least 3 × 106 km2. Chemical compositions of constituent phases and oxygen isotopic composition of olivines suggest that the upper dust layer was produced by a high-temperature interaction between silicate-rich melt and water vapor due to an impact explosion or an aerial burst of a chondritic meteoroid on the inland East Antarctic ice sheet. An estimated total mass of the impactor, on the basis of particle flux and distribution area, is at least 3 × 109 kg. A possible parent material of the lower dust layer is a fragment of friable primitive asteroid or comet. A hypervelocity impact of asteroidal/cometary material on the upper atmosphere and an explosion might have produced aggregates of sub-μm to μm-sized spherules. Total mass of the parent material of the lower layer must exceed 1 × 109 kg. The two extraterrestrial horizons, each a few millimeters in thickness, represent regional or global meteoritic events not identified previously in the Southern Hemisphere.

The isotopic compositions of Sm and Gd in twelve Martian meteorites, ALH 77005, ALH 84001, DaG 735, Dhofar 019, EET 79001, Lafayette, Los Angeles, Nakhla, SaU 005, Y 000593, Y 000749 and Zagami, were determined to quantify the neutron capture records of individual meteorite specimens. Seven of these twelve samples, ALH 84001, Y 000749, DaG 735, Dhofar 019, EET 79001, SaU 005 and Zagami, showed significant isotopic shifts of 150 Sm/ 149 Sm and/or 158 Gd/ 157 Gd corresponding to neutron fluences of (0.7–3.4) × 10 15 n cm − 2 . Among these seven meteorites, the neutron fluences of ALH 84001, Y 000749, and Dhofar 019 apparently correlated with their cosmic-ray exposure ages, indicating that most of the irradiation took place while the meteoroids were small bodies in space after the ejection from Mars. However, our results suggest an accumulation of their inherited irradiation occurred on Mars. On the other hand, the exposure histories of the other four meteorites (basaltic shergottites), DaG 735, EET 79001, SaU 005, and Zagami, cannot be explained as single- or multistage irradiations in space, or as a single irradiation on the Martian surface. The mixing between basaltic lava with a significantly irradiated Martian regolith is a reasonable interpretation of the excess neutron capture records observed in these four basaltic shergottites.

Cosmic ray produced 10Be (half-life = 1.36 × 106 yr), 26Al (7.05 × 105 yr), and 36Cl (3.01 × 105 yr) were measured in a depth profile of 19 carefully-ground samples from the glass-coated lunar surface rock 64455. The solar cosmic ray (SCR) produced 26Al and 36Cl in this rock are present in high concentrations, which in combination with the low observed erosion rate

We re-evaluated the cosmic-ray exposure history of the H36 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 x 10^(-8) cm3 STP/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.

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 (3.55), 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^(-8) cm^3 STP/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 1.52 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 2.53.0 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 50 kg. Finally, low concentrations of radiogenic 4He and 40Ar indicate incomplete degassing

The concentrations of cosmogenic radionuclides and noble gases in Pitts (IAB) and Horse Creek (ungrouped) provide unambiguous evidence that both irons have a complex exposure history with a first-stage irradiation of 100-600 Myr under high shielding, followed by a second-stage exposure of ?1 Myr as small objects. The first-stage exposure ages of ~100 Myr for Horse Creek and ~600 Myr for Pitts are similar to cosmic-ray exposure ages of other iron meteorites, and most likely represent the Yarkovsky orbital drift times of irons from their parent bodies in the main asteroid belt to one of the nearby chaotic resonance zones. The short second-stage exposure ages indicate that collisional debris from recent impact events on their precursor objects was quickly delivered to Earth. The short delivery times suggests that the recent collision events occurred while the precursor objects of Horse Creek and Pitts were either very close to the chaotic resonance zones or already in Earthcrossing orbits. Since the cosmogenic noble gas records of Horse Creek and Pitts indicate a minimum radius of a few meters for the precursor objects, but do not exclude km-sized objects, we conclude that these irons may represent fragments of two near-Earth asteroids, 3103 Eger and 1986 DA, respectively. Finally, we used the cosmogenic nuclide concentrations in Horse Creek, which contains 2.5 wt% Si, to test current model calculations for the production of cosmogenic 10Be, 26Al, and neonisotopes from iron, nickel, and silicon.

We report concentrations of cosmogenic 10Be, 26Al, 36Cl, and 41Ca in the metal phase of 26 ordinary chondrites from Frontier Mountain (FRO), Antarctica, as well as cosmogenic 14C in eight and noble gases in four bulk samples. Thirteen out of 14 selected H chondrites belong to two previously identified pairing groups, FRO 90001 and FRO 90174, with terrestrial ages of ~40 and ~100 kyr, respectively. The FRO 90174 shower is a heterogeneous H3-6 chondrite breccia that probably includes more than 300 individual fragments, explaining the high H/L chondrite ratio (3.8) at Frontier Mountain. The geographic distribution of 19 fragments of this shower constrains ice fluctuations over the past 50-100 kyr to less than 40 m, supporting the stability of the meteorite trap over the last glacial cycle. The second H-chondrite pairing group, FRO 90001, is much smaller and its geographic distribution is mainly controlled by wind-transport. Most L-chondrites are younger than 50 kyr, except for the FRO 93009/01172 pair, which has a terrestrial age of ~500 kyr. These two old L chondrites represent the only surviving members of a large shower with a similar preatmospheric radius (~80 cm) as the FRO 90174 shower. The find locations of these two paired L-chondrite fragments on opposite sides of Frontier Mountain confirm the general glaciological model in which the two ice flows passing both ends of the mountain are derived from the same source area on the plateau. The 50 FRO meteorites analyzed so far represent 21 different falls. The terrestrial ages range from 6 kyr to 500 kyr, supporting the earlier proposed concentration mechanism.

We have used cosmogenic nuclides to estimate limits on the surface exposure durations and erosion rates of alluvial fans and bedrock surfaces in the Atacama Desert in Chile. The oldest landforms we studied are extensive alluvial fans referred to as “Atacama gravels”. With the exception of samples collected in Antarctica, the cobbles collected on these alluvial surfaces have the lowest erosion rates of any samples, as determined by cosmogenic nuclides, analyzed to date. The oldest cobble has a model surface exposure age of 9 Myr, based on combined measurements of cosmogenic 10Be, 26Al, and 21Ne concentrations. Cobbles from the alluvial fans are eroding slower than the surrounding steep mountainous bedrock surfaces. Maximum erosion rates for cobbles on alluvial surfaces are uniformly

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

The Hera mission is a proposed Discovery class mission to collect three samples from each of three near-Earth asteroids. Returned samples would have information on geological context and possibly stratigraphy, would provide fresh regolith, and conceivably samples that would not naturally reach the Earth. During the development of a simple touch-and-go sample collector, questions arose concerning the nature of the samples to be collected, their maximum science return, and the simplest engineering designs. This article reports the results of a small workshop convened to discuss this topic. It is argued that the maximum science return for the Hera samples would be obtained if asteroids of different major spectral classes were visited, samples were disturbed as little as possible during collection, and samples from the very surface were obtained. Surface samples would have the utmost value in interpreting links between asteroids and meteorites, would yield maximum information in solar exposure, would avoid planetary protection concerns, and would produce material not reaching Earth as meteorites. At the same time, they would be simpler to sample than subsurface samples. � 2004 COSPAR. Published by Elsevier Ltd. All rights reserved.

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 3­5 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 4 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.

Cosmic-ray-produced (cosmogenic) nuclides were studied in fragments of the Brenham pallasite, a large stony iron meteorite. The contents of light noble gases (He, Ne, and Ar) and long-lived radionuclides (10Be, 26Al, 36Cl, and 53Mn), produced by nuclear reactions with cosmic rays, were measured in the separated metal and olivine phases from numerous samples representing a wide range of shielding conditions in the meteoroid. The distribution of cosmogenic nuclide concentrations in the metal follows patterns similar to that observed in large iron meteorites. Shielding effects were estimated from the relative proportions of low- and high-energy reaction products. The production rates varied, from surface to interior, by a factor of more than several hundred. The 36Cl-36Ar cosmic-ray exposure age of Brenham is 156 ± 8 Myr. This determination is based on a multiple nuclide approach that utilizes cosmogenic nuclide pairs. This approach not only yields a "shielding independent" exposure age but also demonstrates that the production of cosmogenic nuclides occurred in a single stage. The depth profiles of 10Be in the stone phase and 53Mn in the metal phase are shown superimposed on corresponding profiles from the Apollo 15 long drill core. Surprisingly low abundances of lithophile elements, such as K, U, and Th, provided a unique opportunity to examine the production systematics of those nuclides whose inventories typically have significant contributions from non-cosmogenic sources, particularly radiogenic contributions. The U and Th contents of the olivine samples are extremely low, allowing detection of cosmogenic 4He production from oxygen, magnesium, silicon, and iron.

— 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.

The production of 3 He, 21 Ne, and 22 Ne in meteoroids of various sizes and in the lunar surface was investigated. The LAHET code system, a purely physical model for calculating cosmic- ray particle fluxes, was used to simulate cosmic-ray particle interactions with extraterrestrial matter. We discuss the depth and size dependence of the shielding parameter 22 Ne/ 21 Ne, which is used for reconstruction of pre-atmospheric sizes, depth, and exposure histories. The 22 Ne/ 21 Ne ratio decreases with increasing depth or pre-atmospheric size but then increases with depth in very large objects. This increase with depth in the 22 Ne/ 21 Ne ratio means that this ratio is a poor indicator of shielding in some large objects. The dependence of 3 He/ 21 Ne as function of 22 Ne/ 21 Ne was also calculated, and differences between the calculations and the Bern line are discussed.

— The cosmogenic radionuclides, 10Be, 26Al, 36Cl, and 53Mn were measured in selected clasts and matrix samples from the howardite Kapoeta. Previous measurements of cosmogenic 21Ne indicate higher cosmic-ray exposure ages for bulk samples than for some separated clasts or mineral separates. A possible interpretation for this difference in apparent exposure ages is a complex recent exposure history for Kapoeta. In this scenario some constituents are exposed to cosmic rays in a 2π geometry as part of a larger body immediately preceding its 4π exposure in a smaller body. To test this scenario we measured cosmogenic radionuclides in several clasts from Kapoeta. These measurements are consistent with a simple single-stage 4π exposure history during which the entire inventory of cosmogenic radionuclides was produced. Taken together, these data are most consistent with a single-stage 4π exposure lasting ∼3 Ma. This scenario is nevertheless consistent with models in which the exposure of some constituents of Kapoeta to energetic particles occurred at an earlier time, as is indicated by 21Ne measurements. However, from our data we conclude that insubstantial quantities of cosmogenic radionuclides were inherited from this earlier irradiation; this earlier exposure to energetic particles must have predated the recent exposure by at least ∼10 Ma to allow for the decay of the long half-life cosmogenic radionuclides.

We measured the concentrations of IoBe, 26A1,36Cl, 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 x 105 years) and 41Ca (1.04 x lo5 years) in the stone fraction. The measured activities of neutron-capture 36Cl and 41Ca, as well as spallation produced 1OBe and 26A1, were compared with Monte Carlo-based model calculations. The largest shower, FRO 901 74, includes eight fragments with an average terrestrial age of (1 00 & 30) x 103 years; the neutron- capture saturation activities extend to 27 dpmkg stone for 36C1 and 19 dpmkg stone for 41Ca. The concentrations of spallation produced loge, 26A1 and 36Cl constrain the radius (R) to 80-100 cm, while the neutron-capture 4lCa activities indicate that the samples originated from the outer 25 cm. With a pre-atmospheric radius of 80-1 00 cm, FRO 90 174 is among the largest of the Antarctic stony meteorites. The large pre-atmospheric size supports our hypothesis that at least 50 of the -1 50 classified HS/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 9000 1, includes four fragments with an average terrestrial age of (40 2 10) x 103 years; they contain small contributions of neutron-capture 36C1, but no excess 0f41Ca. FRO 9000 1 experienced a complex exposure history with high shielding conditions in the first stage (1 50 < R < 300 cm) and much lower shielding in the second stage (R < 30 cm), the latter starting -1 .O million years (Ma) ago. Based on the measured 1oBe/2lNe and 26A1/21Ne ratios, the cosmic-ray exposure ages of the two showers are 7.2 k 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.

This is the first contribution towards a reevaluation of exposure histories of iron meteorites and of the constancy of the cosmic ray flux over the last billion years, as recorded in these fossil detectors. We have performed new 36Cl, 26Al, 10Be, and noble gas measurements, including determination of the shielding parameter, S = 4He/21Ne, in samples with published K data. The K isotopic data, coupled to 36Ar and 36Cl concentrations permit selection of meteorites which have only experienced simple (constant geometry) irradiation histories. These objects can be used for the calibration of shielding-dependent production rates within these metallic detectors. In order to carry out production rate calibrations based on 40K–41K data, we assume constancy of the cosmic flux during the interval 150 to 700 My ago. We note that meteorites with very old potassium ages cannot be included in this calibration, as these meteorites require distinct parameter sets. A calibration data set representing a total of 13 meteorites was used to compute long-term (0.5 Gy) average production rates. These average production rates of 36Cl from this particular calibration set are significantly (28%) lower than those determined for the recent (≤10 My) cosmic ray flux. We also document here the quality of the resulting potassium production rate parameter M0(S) with a calculated isochron for irons of group IVA.

A nearly continuous record of 10Be (half-life of 1.5×106 years) concentrations is reported in the Greenland Ice Sheet Project 2 (GISP2) ice core for the time period between 3288 and 40,055 years B.P. The resolution is between 20 and 50 years in the Holocene. During the Pleistocene, sampling was coarser, with the resolution ranging between 50 and 200 years. Both concentrations and fluxes are reported. Concentrations of 10Be are observed to correlate strongly with δ18O and more weakly with snow accumulation rates. Beryllium 10 fluxes show less dependence than do concentrations on climate-related parameters. A good correlation exists between Δ14C and 10Be for centennial scale variations, which are most likely due to heliomagnetic modulation of the 10Be production rate in the atmosphere. There is no evidence of a long-term geomagnetic effect on the 10Be flux at GISP2. The interpretation of the record depends strongly on the model one uses to infer atmospheric 10Be concentrations from the measured concentrations in snow.

Systematic measurements of the concentrations of cosmogenic 41 Ca (half-life = 1.04 × 10 5 yr) in the Apollo 15 long core 15001–15006 were performed by accelerator mass spectroscopy. Earlier measurements of cosmogenic 10 Be, 14 C, 26 Al, 36 Cl, and 53 Mn in the same core have provided confirmation and improvement of theoretical models for predicting production profiles of nuclides by cosmic ray induced spallation in the Moon and large meteorites. Unlike these nuclides, 41 Ca in the lunar surface is produced mainly by thermal neutron capture reactions on 40 Ca. The maximum productions of 41 Ca, about 1 dpm/g Ca, was observed at a depth in the Moon of about 150 g/cm 2 . For depths below about 300 g/cm 2 , 41 Ca production falls off exponentially with an e-folding length of 175 g/cm 2 . Neutron production in the Moon was modeled with the Los Alamos High Energy Transport Code System, and yields of nuclei produced by low-energy thermal and epithermal neutrons were calculated with the Monte Carlo N-Particle code. The new theoretical calculations using these codes are in good agreement with our measured 41 Ca concentrations as well as with 60 Co and direct neutron fluence measurements in the Moon.

Cosmic-ray produced C-14 (t(sub 1/2) = 5730 years), 36Cl (3.01 x 10(exp 5 years), Al-26 (7.05 x 10(exp 5 years), and Be-10 (1.5 x 10(exp 6 years) in the recently discovered lunar meteorites Queen Alexandra Range 93069 (QUE 93069) and 94269 (QUE 94269) were measured by accelerator mass spectrometry. The abundance pattern of these four cosmogenic radionuclides and of noble gases indicates QUE 93069 and QUE 94269 were a paired fall and were exposed to cosmic rays near the surface of the Moon for at least several hundred million years before ejection. After the meteorite was launched from the Moon, where it had resided at a depth of 65-80 g/cm square, it experienced a short transition time, approximately 20-50 ka, before colliding with the Earth. The terrestrial age of the meteorite is 5-10 ka. Comparison ofthe cosmogenic nuclide concentrations in QUE 93069/94269 and MAC 88104/88105 clearly shows that these meteorites were not ejected by a common event from the Moon.

We show that meteorites can provide chronological constraints upon the age of the ice cropping out at the Frontier Mountain meteorite trap (Antarctica) when their terrestrial age is placed in a glaciological context. Amongst the over 700 meteorites found so far, Frontier Mountain (FRO) 84001, 99028, 93005 and 93054 were most likely not wind-drifted across the ice field, since their masses (772–1665 g) are much heavier than the local ∼ 200 g wind transport threshold. The four meteorites were found along a stretch of ice where a representative section of the Frontier Mountain blue ice crops out. Based on the bedding of englacial tephra layers, the structure of the ice along the section appears to be essentially an up-glacier dipping monocline. The 14 C terrestrial age of FRO 8401, 99028 and 93005 are 13 ± 2, 21 ± 3 and 27 ± 2 ky, respectively; the 41 Ca/ 36 Cl age of FRO 93054 is 40 ± 10 ky. The terrestrial ages of the four meteorites increase from the top to the bottom layers of the monocline. This geographic distribution is best explained by delivery of meteorites at the ice surface through the “ice-flow model” (i.e., englacial transport from the snow accumulation zone and exhumation in the blue ice area through ablation) rather than direct fall. Since the effect of ablation in decoupling terrestrial ages of meteorites and the age of the ice on which they sit must have been minor (most likely ≤ 7 ky) based on the local ice dynamics, we conclude that the age of the bulk of the ice body currently under ablation at Frontier Mountain is up to ∼ 50 ky old. This result has implications on both the meteorite concentrations mechanism at Frontier Mountain and the regional ice dynamics.

Cosmic-ray-produced 53 Mn (t ½ = 3.7 × 10 6 years) has been determined by neutron activation analysis in twenty-two chondrites including three Antarctic meteorites: Yamato-7301 (j), -7305 (k) and -7304 (m). 53 Mn was also measured in four mesosiderites, three iron meteorites, Bencubbin (unique) and Udei Station (iron with silicate inclusions). In addition, preliminary results for 10 Be (1.6 × 10 6 years) were obtained in the Yamato meteorites using a low-background needle GM counter. Based on published values of rare gas ages, corrections were made for undersaturation; the average specific saturation activities of 53 Mn were found in the range 450 ± 90dpm 53 Mn/kg Fe in most of the chondrites and 490 ± 75dpm 53 Mn/kg Fe in the mesosiderites. Two meteorites had extremely low contents of 53 Mn: 102 ± 6 in Yamato-7301 and 48 ± 3dpm 53 Mn/kg Fe in Bondoc. The Bondoc mesosiderite was already known to have a low concentration of cosmogenic radionuclides due to its large pre-atmospheric size. Several possible mechanisms are discussed to explain the low 53 Mn activity in Yamato-7301: (1) long terrestrial age of about 7 m.y.; (2) low production rate of 53 Mn due to heavy pre-atmospheric shielding (>70cm); (3) multi-stage irradiation history resulting in an undersaturation of 53 Mn; and (4) a mechanism in which two or three of the above factors are combined. The ratio of 53 Mn production rate in Ni to that in Fe has been estimated to be 1/3, based on the measurements of 53 Mn in the metallic and silicate phases of St. Severin meteorite, as well as on published results of some high-energy bombardment experiments.

Cosmogenic nuclides have been studied in the Kirin chondrite (H5). Some experimental results obtained in samples sent to us (Nos. 1 and 4 fragments) are described. We measured the gamma rays of 50Mn, 26Al, 22Na and 60Co, using Ge(Li) detector systems. 53Mn was determine by an activation method based on 53Mn(n, γ)54Mn. The light rare gases were also measured by a mass spectrometer in one of the specimens. We compare our data with others and conclude that it is possible to read out the fossil records in the meteorite and interpret them by, 1) Multiple exposure history and 2) Depth effects in the production of cosmogenic nuclides in different geometries.

Descriptions are given of the experimental methods used in determining Al-26 and Mn-53 activities and of the possible improvements that can be expected in the near future. The results obtained for the seven lunar core sections where both Al-26 and Mn-53 are measured with good depth definition are discussed. It is then shown how meteoritic impacts and the secondary effects they induce modify the Al-26 and Mn-53 activities in a core. Using a Monte Carlo method, 150 possible depositional histories for a core are generated; from these the resulting activity profiles for both Al-26 and Mn-53 are computed, using several possible SCR energy spectra. These model-generated profiles are then compared with the experimental profiles in the seven cores, and plausible scenarios for the depositional history of each of the cores are tentatively inferred.

Mn-53 data are presented for four Apollo cores with an emphasis on the past 10-to-the-7th-year history of these samples: (1) Core 15011 has excess Mn-53 down to a depth of at least 9 cm, which is probably due to the continuous addition of near-surface irradiated material. (2) The Mn-53 profile for 64002 above 12 cm is relatively flat and in excess of that calculated for a static core, which suggests that the upper part of this core was deposited rapidly about 2 m.y. ago. (3) Results for the Apollo 17 drill stem show a large excess of Mn-53 from at least 3 cm downwards. Some magnetic separates obtained from this core have very high Mn-53 activities, probably due to the presence of agglutinates that were recently irradiated very near the surface. (4) The Mn-53 content of the upper part of 74002 is similar to that of trench samples collected nearby and indicates that the gray, orange, and black soils at this site have been exposed together near the surface for at least the last 10 m.y.