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1.
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.  相似文献   

2.
Abstract— We present the concentrations and isotopic compositions of He, Ne, and Ar for nonmagnetic fractions and bulk samples of 17 H chondrites which were recently investigated for their 36Cl‐36Ar cosmic‐ray exposure ages (Graf et al., 2001). All selected meteorites are observed falls with cosmic‐ray exposure ages close to the 7 Ma peak. The rare gas data are consistent with 10Be and 36C1 production rates in the metal phase. Remarkably, only 1 out of the 17 H chondrites, Bath, shows clear indications for a complex exposure history. Based on rare gas concentrations and 36Cl‐36Ar exposure ages, 21Ne production rates as a function of 22Ne/21 Ne and a mean 38Ar production rate are determined. The results confirm model calculations which predict that the relationship between 21Ne production rates and 22Ne/21Ne is ambiguous for high shielding. Besides the mean 38Ar production rate we also give production rate ratios P(38Ar from Ca)/P(38Ar from Fe). They vary between 10 and 77, showing no significant correlation with 38Ar concentrations or 22Ne/21Ne. By investigating the metal separates, Graf et al. (2001) found significant 3He deficits for 6 out of the 17 meteorites. For the nonmagnetic fractions and bulk samples investigated here, the data points in a 3He/21Ne vs. 22Ne/21Ne diagram plot in the area defined by most of the H chondrites. This means that 3He deficits in the metal phase are much more pronounced than in silicate minerals and we will argue that 3H diffusive losses in meteorites should be the rule rather than the exception. The 21Ne exposure ages, calculated on the basis of modeled 21Ne production rates, confirm the assumption by Graf et al. (2001) that the H5 chondrites with low 3He/38Ar in the metal formed in a separate event than those with normal 3He/38Ar ratios. The data can best be interpreted by assuming that the prominent 7 Ma exposure age peak of the H chondrites is due to at least two events about 7.0 and 7.6 Ma ago.  相似文献   

3.
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.  相似文献   

4.
Abstract— 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.  相似文献   

5.
Abstract— We present a purely physical model for the calculation of depth‐ and size‐dependent production rates of cosmogenic nuclides by galactic cosmic‐ray (GCR) particles. besides the spectra of primary and secondary particles and the excitation functions of the underlying nuclear reactions, the model is based on only one free parameter—the integral number of gcr particles in the meteoroid orbits. We derived this value from analysis of radionuclide data in Knyahinya. We also show that the mean GCR proton spectrum in the meteoroid orbits has been constant over about the last 10 Ma. For the major target elements in stony meteoroids, we present depth‐ and size‐dependent production rates for 10Be, 14C, 26Al, 36Cl, and 53Mn as well as for the rare gas isotopes 3He, 20Ne, 21Ne, 22Ne, 36Ar, and 38Ar. The new data differ from semi‐empirical estimates by up to a factor of 4 but agree within ～20% with results obtained by earlier parametric or physical approaches. The depth and size dependence of the shielding parameter 22Ne/21Ne and the correlations 26Al vs. 10Be, 26Al vs. 53Mn, 10Be/21Ne vs. 22Ne/21Ne, and 36Ar vs. 36Cl for deciphering preatmospheric sizes, shielding depths, terrestrial residence times, and exposure histories are also discussed.  相似文献   

6.
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.  相似文献   

7.
Abstract— We determined He, Ne, Ar, 10Be, 26Al, 36Cl, and 14C concentrations, as well as cosmic-ray track densities and halogen concentrations in different specimens of the H6 chondrite Torino, in order to constrain its exposure history to cosmic radiation. The Torino meteoroid had a radius of ～20 cm and travelled in interplanetary space for 2.5–10 Ma. Earlier, Torino was part of a larger body. The smallest possible precursor had a radius of 55 cm and a journey through space longer than ～65 Ma. If the first-stage exposure took place in a body with a radius of >3 m or in the parent asteroid, then it lasted nearly 300 Ma. The example of Torino shows that it is easy to underestimate first-stage exposure ages when constructing two-stage histories.  相似文献   

8.
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.  相似文献   

9.
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.  相似文献   

10.
Abstract— We studied the elemental and isotopic abundances of noble gases (He, Ne, Ar in most cases, and Kr, Xe also in some cases) in individual chondrules separated from six ordinary, two enstatite, and two carbonaceous chondrites. Most chondrules show detectable amounts of trapped 20Ne and 36Ar, and the ratio (36Ar/20Ne)t (from ordinary and carbonaceous chondrites) suggests that HL and Q are the two major trapped components. A different trend between (36Ar/20Ne)t and trapped 36Ar is observed for chondrules in enstatite chondrites indicating a different environment and/or mechanism for their formation compared to chondrules in ordinary and carbonaceous chondrites. We found that a chondrule from Dhajala chondrite (DH‐11) shows the presence of solar‐type noble gases, as suggested by the (36Ar/20Ne)t ratio, Ne‐isotopic composition, and excess of 4He. Cosmic‐ray exposure (CRE) ages of most chondrules are similar to their host chondrites. A few chondrules show higher CRE age compared to their host, suggesting that some chondrules and/or precursors of chondrules have received cosmic ray irradiation before accreting to their parent body. Among these chondrules, DH‐11 (with solar trapped gases) and a chondrule from Murray chondrite (MRY‐1) also have lower values of (21Ne/22Ne)c, indicative of SCR contribution. However, such evidences are sporadic and indicate that chondrule formation event may have erased such excess irradiation records by solar wind and SCR in most chondrules. These results support the nebular environment for chondrule formation.  相似文献   

11.
Abstract– 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 10–20 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.  相似文献   

12.
We analyzed He and Ne in chromite grains from the regolith breccia Ghubara (L5), to compare it with He and Ne in sediment‐dispersed extraterrestrial chromite (SEC) grains from mid‐Ordovician sediments. These SEC grains arrived on Earth as micrometeorites in the aftermath of the L chondrite parent body (LCPB) breakup event, 470 Ma ago. A significant fraction of them show prolonged exposure to galactic cosmic rays for up to several 10 Ma. The majority of the cosmogenic noble gases in these grains were probably acquired in the regolith of the LCPB (Meier et al. 2010 ). Ghubara, an L chondritic regolith breccia with an Ar‐Ar shock age of 470 Ma, is a sample of that regolith. We find cosmic‐ray exposure ages of up to several 10 Ma in some Ghubara chromite grains, confirming for the first time that individual chromite grains with such high exposure ages indeed existed in the LCPB regolith, and that the >10 Ma cosmic‐ray exposure ages found in recent micrometeorites are thus not necessarily indicative of an origin in the Kuiper Belt. Some Ghubara chromite grains show much lower concentrations of cosmogenic He and Ne, indicating that the 4π (last‐stage) exposure age of the Ghubara meteoroid lasted only 4–6 Ma. This exposure age is considerably shorter than the 15–20 Ma suggested before from bulk analyses, indicating that bulk samples have seen regolith pre‐exposure as well. The shorter last‐stage exposure age probably links Ghubara to a small peak of 40Ar‐poor L5 chondrites of the same exposure age. Furthermore, and quite unexpectedly, we find a Ne component similar to presolar Ne‐HL in the chromite grains, perhaps indicating that some presolar Ne can be preserved even in meteorites of petrologic type 5.  相似文献   

13.
We report light noble gas (He, Ne, and Ar) concentrations and isotopic ratios in 11 achondrites, Tafassasset (unclassified primitive achondrite), Northwest Africa (NWA) 12934 (angrite), NWA 12573 (brachinite), Jiddat al Harasis (JaH) 809 (ureilite), NWA 11562 (ungrouped achondrite), four lodranites (NWA 11901, NWA 7474, NWA 6685, and NWA 6484), NWA 2871 (acapulcoite), and Sahara 02029 (winonaite), most of which have not been previously studied for noble gases. We discuss their noble gas isotopic composition, determine their cosmogenic nuclide content, and systematically calculate their cosmic ray exposure (CRE) and gas retention ages. In addition, we estimate their preatmospheric radii and preatmospheric masses based on the shielding parameter (22Ne/21Ne)cos. None of the studied meteorites shows evidence of contribution from solar cosmic rays (SCRs). JaH 809 and NWA 12934 show evidence of 3He diffusive losses of >90% and 40%, respectively. The winonaite Sahara 02029 has lost most of its noble gases, either during or before analysis. The average CRE age of Tafassasset of ~49 Ma is lower than that reported by Patzer et al. (2003), but is consistent with it within the uncertainties; this confirms that Tafassasset and CR chondrites are not source paired, CR chondrites having CRE ages from 1 to 25 Ma (Herzog & Caffee, 2014). The ureilite JaH 809 has a CRE age of ~5.4 Ma, which falls into the typical range of exposure ages for ureilites; the angrite NWA 12934 has a CRE age of ~49 Ma, which is within the main range of exposure ages reported for angrites (0.2–56 Ma). We calculate a CRE age of ~2.4 Ma for the brachinite NWA 12573, which falls into a possible “cluster” in the brachinite CRE age histogram around ~3 Ma. Three lodranites (NWA 11901, NWA 7474, and NWA 6685) have CRE ages higher than the average CRE ages of lodranites measured so far, NWA 11901 and NWA 6685 having CRE ages far higher than the CRE age already reported by Li et al. (2019) on NWA 8118. The measured 40K-40Ar gas retention ages fit well into established systematics. The gas retention age of Tafassasset is consistent, within respective uncertainties, with that previously calculated by Patzer et al. (2003). Our study indicates that Tafassasset originates from a meteoroid with a preatmospheric radius of ~20 cm, however discordant with the radius of ~85 cm inferred in a previous study (Patzer et al., 2003).  相似文献   

14.
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.  相似文献   

15.
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.  相似文献   

16.
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17.
We present noble gas data for 16 shergottites, 2 nakhlites (NWA 5790, NWA 10153), and 1 angrite (NWA 7812). Noble gas exposure ages of the shergottites fall in the 1–6 Ma range found in previous studies. Three depleted olivine‐phyric shergottites (Tissint, NWA 6162, NWA 7635) have exposure ages of ~1 Ma, in agreement with published data for similar specimens. The exposure age of NWA 10153 (~12.2 Ma) falls in the range of 9–13 Ma reported for other nakhlites. Our preferred age of ~7.3 Ma for NWA 5790 is lower than this range, and it is possible that NWA 5790 represents a distinct ejection event. A Tissint glass sample contains Xe from the Martian atmosphere. Several samples show a remarkably low (21Ne/22Ne)cos ratio < 0.80, as previously observed in a many shergottites and in various other rare achondrites. This was explained by solar cosmic ray‐produced Ne (SCR Ne) in addition to the commonly found galactic cosmic ray‐produced Ne, implying very low preatmospheric shielding and ablation loss. We revisit this by comparing measured (21Ne/22Ne)cos ratios with predictions by cosmogenic nuclide production models. Indeed, several shergottites, acalpulcoites/lodranites, angrites (including NWA 7812), and the Brachina‐like meteorite LEW 88763 likely contain SCR Ne, as previously postulated for many of them. The SCR contribution may influence the calculation of exposure ages. One likely reason that SCR nuclides are predominantly detected in meteorites from rare classes is because they usually are analyzed for cosmogenic nuclides even if they had a very small (preatmospheric) mass and hence low ablation loss.  相似文献   

18.
We measured the He, Ne, and Ar isotopic concentrations and the 10Be, 26Al, 36Cl, and 41Ca concentrations in 56 iron meteorites of groups IIIAB, IIAB, IVA, IC, IIA, IIB, and one ungrouped. From 41Ca and 36Cl data, we calculated terrestrial ages indistinguishable from zero for six samples, indicating recent falls, up to 562 ± 86 ka. Three of the studied meteorites are falls. The data for the other 47 irons confirm that terrestrial ages for iron meteorites can be as long as a few hundred thousand years even in relatively humid conditions. The 36Cl‐36Ar cosmic ray exposure (CRE) ages range from 4.3 ± 0.4 Ma to 652 ± 99 Ma. By including literature data, we established a consistent and reliable CRE age database for 67 iron meteorites. The high quality of the CRE ages enables us to study structures in the CRE age histogram more reliably. At first sight, the CRE age histogram shows peaks at about 400 and 630 Ma. After correction for pairing, the updated CRE age histogram comprises 41 individual samples and shows no indications of temporal periodicity, especially not if one considers each iron meteorite group separately. Our study contradicts the hypothesis of periodic GCR intensity variations (Shaviv 2002, 2003), confirming other studies indicating that there are no periodic structures in the CRE age histogram (e.g., Rahmstorf et al. 2004; Jahnke 2005). The data contradict the hypothesis that periodic GCR intensity variations might have triggered periodic Earth climate changes. The 36Cl‐36Ar CRE ages are on average 40% lower than the 41K‐K CRE ages (e.g., Voshage 1967). This offset can either be due to an offset in the 41K‐K dating system or due to a significantly lower GCR intensity in the time interval 195–656 Ma compared to the recent past. A 40% lower GCR intensity, however, would have increased the Earth temperature by up to 2 °C, which seems unrealistic and leaves an ill‐defined 41K‐K CRE age system the most likely explanation. Finally, we present new 26Al/21Ne and 10Be/21Ne production rate ratios of 0.32 ± 0.01 and 0.44 ± 0.03, respectively.  相似文献   

19.
Abstract– The interior texture and chemical and noble gas composition of 99 cosmic spherules collected from the meteorite ice field around the Yamato Mountains in Antarctica were investigated. Their textures were used to classify the spherules into six different types reflecting the degree of heating: 13 were cryptocrystalline, 40 were barred olivine, 3 were porphyritic A, 24 were porphyritic B, 9 were porphyritic C, and 10 were partially melted spherules. While a correlation exists between the type of spherule and its noble gas content, there is no significant correlation between its chemical composition and noble gas content. Fifteen of the spherules still had detectable amounts of extraterrestrial He, and the majority of them had 3He/4He ratios that were close to that of solar wind (SW). The Ne isotopic composition of 28 of the spherules clustered between implantation‐fractionated SW and air. Extraterrestrial Ar, confirmed to be present because it had a 40Ar/36Ar ratio lower than that of terrestrial atmosphere, was found in 35 of the spherules. An enigmatic spherule, labeled M240410, had an extremely high concentration of cosmogenic nuclides. Assuming 4π exposure to galactic and solar cosmic rays as a micrometeoroid and no exposure on the parent body, the cosmic‐ray exposure (CRE) age of 393 Myr could be computed using cosmogenic 21Ne. Under these model assumptions, the inferred age suggests that the particle might have been an Edgeworth‐Kuiper Belt object. Alternatively, if exposure near the surface of its parent body was dominant, the CRE age of 382 Myr can be estimated from the cosmogenic 38Ar using the production rate of the 2π exposure geometry, and implies that the particle may have originated in the mature regolith of an asteroid.  相似文献   

20.
Abstract— 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.  相似文献   

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