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1.
Study of several cosmic ray effects, such as VH track density, spallogenic26Al and53Mn activity,21Ne and22Ne/21Ne ratio, made in the same sample or in cores taken from different meteorites can identify parameters related to the exposure history of meteorites and cosmic ray flux variations. Meteorites with single or multiple exposure can be distinguished from a track production rate —22Ne/21Ne correlation diagram and cosmic ray flux variations over 106–107 years can be deduced from a three-isotope correlation diagram of26Al,53Mn and21Ne. Isotopic data based on chondrites with simple, one-stage exposure are consistent with the same average galactic cosmic ray intensity over the past 2 million years as that during the past 107 years.  相似文献   

2.
An investigation on the low energy cosmic ray products 26Al and 53Mn in 18 chondrites encompassed several side aspects in meteoritics. The ratio of observed to calculated specific activity of 26Al in 13 chondrites with recovered masses ranging from ~ 3 to 2000 kg remains constant within ±15 per cent of a mean of 0.93. This is consistent with Fuse and Anders' observation of invariance of the 26Al-production rate with depth and size. A plot of the 53Mn specific production rate (dpm/kg Fe) versus recovered meteorite mass revealed that the compensation by the secondaries for the absorption of the primaries is also effective in the case of 53Mn. Thus, the 53Mn production rate remains rather constant within ±15 per cent of a mean of 520 dpm/kg Fe for most of the stones. Applying this information to two chondrites, Ladder Creek and Scurry, of known relatively short exposure and terrestrial ages, the 53Mn half-life (T53), computed from their 53Mnobs/26Alobs ratios, is found to be 3.85 ± 0.4 m.y. which agrees very well with the currently accepted value of T53 = 3.8 ± 0.6 m.y. From the observed 53Mn/54Mn activity ratio in Allende, relative to the production rate ratio of 0.98, an exposure age of tR = 4.2 m.y. is derived, in concordance with literature data. The result strongly suggests that this isotope pair holds promise as an index of absolute radiation ages of fresh falls.  相似文献   

3.
Recent results on cosmic ray interactions in lunar samples and meteorites resulting in production of stable and radionuclides, particle tracks and thermoluminescence are reviewed. A critical examination of26A1 depth profiles in lunar rocks and soil cores, together with particle track data, enables us to determine the long term average fluxes of energetic solar protons (>10 MeV) which can be represented by (J s,R o)=(125, 125). The lunar rock data indicate that this flux has remained constant for 5×105 to 2×106 years. Production rates of stable and radionuclides produced by galactic cosmic rays is given as a function of size and depth of the meteoroid. Radionuclide (53Mn,28Al) depth profiles in meteorite cores, whose preatmospheric depths are deduced from track density profiles are used to develop a general procedure for calculating isotope production rates as a function of meteoroid size. Based on the track density and22Ne/21Ne production rates, a criterion is developed to identify meteorites with multiple exposure history.22Ne/21Ne ratio <1·06 is usually indicative of deep shielded exposure. An examination of the available data suggests that the frequency of meteorites with multiple exposure history is high, at least 15% for LL, 27% for L and 31% for H chondrites. The epi-thermal and the thermal neutron density profiles in different meteorites are deduced from60Co and track density data in Dhajala, Kirin and Allende chondrites. The data show that the production profile depends sensitively on the size and the chemical composition of the meteoroid. Cosmic ray-induced thermoluminescence in meteorites of known preatmospheric sizes has been measured which indicates that its production profile is nearly flat and insensitive to the size of the meteoroid. Some new possibilities in studying cosmic ray implanted radionuclides in meteorites and lunar samples using resonance ionisation spectroscopy are discussed.  相似文献   

4.
An excellent 53Mn-53Cr isochron for bulk CI, CM, CO, CV, CB, and ungrouped C3 chondrites seems to suggest that each carbonaceous chondrite group acquired its Mn/Cr ratio 4568 ± 1 Myr ago. This age is indistinguishable from the age of Ca-Al-rich inclusions (CAIs), which is considered to be the start of the solar system (t0). However, carbonaceous chondrites were not assembled until at least 1.5-5 Myr after t0, to judge by the 207Pb-206Pb and 26Al-26Mg ages of the chondrules within them, and by the fact that they were not melted by heat from the decay of 26Al. Presumably, therefore, these meteorites inherited their bulk Mn-Cr isochron from precursor materials which experienced Mn-Cr fractionation at t0. As a possible physical mechanism for how the isochron was established initially, and later inherited by the carbonaceous chondrites, we propose the rapid formation at t0 of planetesimals that were variably depleted in moderately volatile elements, and hence had variably low Mn/Cr. The planetesimals and the undepleted (high Mn/Cr) primitive dust from which they were made shared the same initial ε53Cr, and therefore evolved on an isochron. We suggest that later impact-disruption of the planetesimals produced dusty debris, which became mixed, in various proportions, with unprocessed (high Mn/Cr) dust before accreting to the carbonaceous chondrite parent bodies. With mixing in a closed system, the isochron was unchanged. We infer that some debris-rich material was converted to chondrules prior to accretion. The chondrules could have been formed by flash melting of the mixed dust, or could instead have been made directly by the impact splashing of molten planetesimals, or by condensation from impact-generated vapor plumes.  相似文献   

5.
Noble gases were redetermined in six ordinary chondrites in order to clarify some discrepancies between cosmic-ray exposure ages from noble gas isotopes and 26Al activity. Generally, samples used for noble-gas analyses were from the same specimen which earlier had been used for 26Al determinations in this laboratory. The high 21Ne ages (> 3 × 106 y) for Dimmitt, Menow and Pierceville are consistent with 26A1 activity at saturation level. Variations in both 21Nec age and 26Al activity, in earlier measurements on Dimmitt, were due to “shielding”; very low noble-gas ages in earlier analyses on Menow and Pierceville were due to mis-labelling of samples. Earlier observation of very low noble-gas ages for Seres, which is inconsistent with its saturation activity of 26Al, is confirmed. Ladder Creek yields consistent ages both from noble gases and 26Al. Malotas contains a substantial excess of 21Ne, as revealed by a higher 21Ne age, compared to its 26Al age; the excess 21Ne is probably due to “pre-irradiation”. Isotopic compositions for trapped Kr and Xe in these chondrites, excepting Seres, are very similar to compositions determined earlier for ordinary chondrites. The Xe in Seres is enriched in light isotopes and is similar to solar Xe.  相似文献   

6.
The 26Al, light rare gas and major and minor element contents of Al-rich and poor samples separated from Allende. Bereba and Junivas have been measured. The production rate of 21Ne from Al (21PAl) is (1.9 ± 0.6) × 21PSi and 2221PAl = 1.4 ± 0.4. The 3He, 21Ne and 38Ar exposure ages of the eucritic pyroxenes agree suggesting complete cosmogenic gas retention. The eucritic feldspars have lost virtually all 3He and most radiogenic 4He. The equation 26Al = 0.42 ± 0.41 Mg + 2.74 ± 0.21 Si + 4.92 ± 0.51 Al + 1.33 S + 0.24 Ca + 0.03 Fe reproduces within 15% our 26Al measurements and the average values measured in ordinary chondrites without recourse to unusual cosmic-ray effects.  相似文献   

7.
Early Solar System chronology is usually built with the assumption that the distribution of short-lived radionuclides was homogeneous through the solar accretion disk. At present, there is no unambiguous evidence for a homogeneous distribution of short-lived radionuclides in the solar accretion disk, while some data point to a heterogeneous distribution of short-lived radionuclides. In this paper, we explore a possible chronology based on a heterogeneous distribution of 26Al and 53Mn in the accretion disk. Our basic assumption is that the different abundances of extinct short-lived radionuclides in calcium-aluminium-rich inclusions (CAIs) and chondrules are due to spatial rather than temporal differences. We develop a simple model where CAIs and chondrules form contemporaneously, in different spatial locations, and are characterised by distinct initial 26Al and 53Mn abundances. In this model, all evolved bodies are supposed to be originally chondritic, i.e., to be made of a mixture of CAIs, chondrules, and matrix. This mixture determines the initial content in 26Al and 53Mn of a chondritic parent-body as a function of its CAI and chondrule abundance fraction. This approach enables us to calculate coherent 26Al and 53Mn ages from the agglomeration of the parent-body precursors (CAIs and chondrules) until the isotopic closure of 26Al and 53Mn, thereafter called 26Al-53Mn age. We calculate such 26Al-53Mn ages for a diversity of evolved objects, with the constraint that they should be found for realistic chondritic parent-body precursors, i.e., objects having similar or identical petrograpy to the existing chondrite groups. The so defined age of the d’Orbigny angrite is 4.3 ± 1.1 Myr, for the Asuka-881394 eucrite 2.8 ± 1.0 Myr, for the H4 chondrite Sainte Marguerite ∼3 Myr, and for H4 Forest Vale ∼5 Myr. The calculated 26Al-53Mn ages give timescales for the evolution of the respective parent-bodies/meteorites that can be investigated in the light of further petrographic studies. We anchor the calculated relative chronology to an absolute chronology using absolute Pb-Pb ages and relative Hf-W ages of the objects under scrutiny. The precursors of Sainte Marguerite and Forest Vale agglomerated at the same time (∼4565.8 ± 1.2 Ma ago). The precursors of eucrites (Asuka-881394) agglomerated 4564.8 ± 1.2 Ma ago. The precursors of angrites agglomerated late (4561.5 ± 1.8 Ma ago). Our model provides a fully compatible Al-Mg/Mn-Cr/Pb-Pb chronology, and is shown to be robust to reasonable changes in the input parameters. The calculated initial 26Al/27Al ratios are high enough to have 26Al as a possible heat source for differentiation.  相似文献   

8.
The 182Hf-182W isotopic systematics of Ca-Al-rich inclusions (CAIs), metal-rich chondrites, and iron meteorites were investigated to constrain the relative timing of accretion of their parent asteroids. A regression of the Hf-W data for two bulk CAIs, various fragments of a single CAI, and carbonaceous chondrites constrains the 182Hf/180Hf and εW at the time of CAI formation to (1.07 ± 0.10) × 10−4 and −3.47 ± 0.20, respectively. All magmatic iron meteorites examined here have initial εW values that are similar to or slightly lower than the initial value of CAIs. These low εW values may in part reflect 182W-burnout caused by the prolonged cosmic ray exposure of iron meteorites, but this effect is estimated to be less than ∼0.3 ε units for an exposure age of 600 Ma. The W isotope data, after correction for cosmic ray induced effects, indicate that core formation in the parent asteroids of the magmatic iron meteorites occurred less than ∼1.5 Myr after formation of CAIs. The nonmagmatic IAB-IIICD irons and the metal-rich CB chondrites have more radiogenic W isotope compositions, indicating formation several Myr after the oldest metal cores had segregated in some asteroids.Chondrule formation ∼2-5 Myr after CAIs, as constrained by published Pb-Pb and Al-Mg ages, postdates core formation in planetesimals, and indicates that chondrites do not represent the precursor material from which asteroids accreted and then differentiated. Chondrites instead derive from asteroids that accreted late, either farther from the Sun than the parent bodies of magmatic iron meteorites or by reaccretion of debris produced during collisional disruption of older asteroids. Alternatively, chondrites may represent material from the outermost layers of differentiated asteroids. The early thermal and chemical evolution of asteroids appears to be controlled by the decay of 26Al, which was sufficiently abundant (initial 26Al/27Al >1.4 × 10−5) to rapidly melt early-formed planetesimals but could not raise the temperatures in the late-formed chondrite parent asteroids high enough to cause differentiation. The preservation of the primitive appearance of chondrites thus at least partially reflects their late formation rather than their early and primitive origin.  相似文献   

9.
The Sulagiri meteorite fell in India on 12 September 2008,LL6 chondrite class is the largest among all the Indian meteorites.Isotopic compositions of noble gases(He,Ne,Ar,Kr and Xe) and nitrogen in the Sulagiri meteorite and cosmic ray exposure history are discussed.Low cosmogenic(~(22)Ne/~(21)Ne)_c ratio is consistent with irradiation in a large body.Cosmogenic noble gases indicate that Sulagiri has a 4πcosmic-ray exposure(CRE) age of 27.9 ± 3.4 Ma and is a member of the peak of CRE age distribution of IX chondrites.Radiogenic ~4He and ~(40)Ar concentrations in Sulagiri yields the radiogenic ages as 2.29 and4.56 Ca,indicating the loss of He from the meteorite.Xenon and krypton are mixture of Q and spallogenic components.  相似文献   

10.
An integrated study of the cosmic ray exposure history of the San Juan Capistrano meteorite was carried out using measurements of rare gas isotopic abundances, particle track densities and radioisotope concentrations. Spallation systematics determined for Kr isotopes in lunar samples are shown to be valid also for the San Juan Capistrano and St. Severin meteorites, thus allowing us to determine a reliable 81Kr/83Kr production ratio as needed for applying the 81Kr-Kr dating method. The 81Kr-Kr age of SJC is 28.7 ± 2.0 Myr, about 35% longer than ages determined by spallation He or Ne. The minimum observed track production rate (2.6 × 105 tracks/cm2 · Myr) sets a minimum of 8 cm for the preatmospheric radius of an assumed spherical body. Track density gradients and the low 60Co activity (<2.9 dpm/g Co) both set an upper limit of 10 cm to the radius. Track results show that ablation losses have averaged 6cm. The relative spallation yields of 78Kr and 83Kr, and the ratios 3He/21Ne and 22Ne/21Ne are all compatible with a hard irradiation as would be experienced by a sample depth of about 6 cm in a body of 8–10 cm. The low activities of 54Mn, 22Na and 26Al are also consistent with these irradiation conditions.  相似文献   

11.
Origin and chronology of chondritic components: A review   总被引：1，自引：0，他引：1  
Mineralogical observations, chemical and oxygen-isotope compositions, absolute 207Pb-206Pb ages and short-lived isotope systematics (7Be-7Li, 10Be-10B, 26Al-26Mg, 36Cl-36S, 41Ca-41K, 53Mn-53Cr, 60Fe-60Ni, 182Hf-182W) of refractory inclusions [Ca,Al-rich inclusions (CAIs) and amoeboid olivine aggregates (AOAs)], chondrules and matrices from primitive (unmetamorphosed) chondrites are reviewed in an attempt to test (i) the x-wind model vs. the shock-wave model of the origin of chondritic components and (ii) irradiation vs. stellar origin of short-lived radionuclides. The data reviewed are consistent with an external, stellar origin for most short-lived radionuclides (7Be, 10Be, and 36Cl are important exceptions) and a shock-wave model for chondrule formation, and provide a sound basis for early Solar System chronology. They are inconsistent with the x-wind model for the origin of chondritic components and a local, irradiation origin of 26Al, 41Ca, and 53Mn. 10Be is heterogeneously distributed among CAIs, indicating its formation by local irradiation and precluding its use for the early solar system chronology. 41Ca-41K, and 60Fe-60Ni systematics are important for understanding the astrophysical setting of Solar System formation and origin of short-lived radionuclides, but so far have limited implications for the chronology of chondritic components. The chronological significance of oxygen-isotope compositions of chondritic components is limited. The following general picture of formation of chondritic components is inferred. CAIs and AOAs were the first solids formed in the solar nebula ∼4567-4568 Myr ago, possibly within a period of <0.1 Myr, when the Sun was an infalling (class 0) and evolved (class I) protostar. They formed during multiple transient heating events in nebular region(s) with high ambient temperature (at or above condensation temperature of forsterite), either throughout the inner protoplanetary disk (1-4 AU) or in a localized region near the proto-Sun (<0.1 AU), and were subsequently dispersed throughout the disk. Most CAIs and AOAs formed in the presence of an 16O-rich (Δ17O ∼ −24 ± 2‰) nebular gas. The 26Al-poor [(26Al/27Al)0 < 1 × 10−5], 16O-rich (Δ17O ∼ −24 ± 2‰) CAIs - FUN (fractionation and unidentified nuclear effects) CAIs in CV chondrites, platy hibonite crystals (PLACs) in CM chondrites, pyroxene-hibonite spherules in CM and CO chondrites, and the majority of grossite- and hibonite-rich CAIs in CH chondrites—may have formed prior to injection and/or homogenization of 26Al in the early Solar System. A small number of igneous CAIs in ordinary, enstatite and carbonaceous chondrites, and virtually all CAIs in CB chondrites are 16O-depleted (Δ17O > −10‰) and have (26Al/27Al)0 similar to those in chondrules (<1 × 10−5). These CAIs probably experienced melting during chondrule formation. Chondrules and most of the fine-grained matrix materials in primitive chondrites formed 1-4 Myr after CAIs, when the Sun was a classical (class II) and weak-lined T Tauri star (class III). These chondritic components formed during multiple transient heating events in regions with low ambient temperature (<1000 K) throughout the inner protoplanetary disk in the presence of 16O-poor (Δ17O > −5‰) nebular gas. The majority of chondrules within a chondrite group may have formed over a much shorter period of time (<0.5-1 Myr). Mineralogical and isotopic observations indicate that CAIs were present in the regions where chondrules formed and accreted (1-4 AU), indicating that CAIs were present in the disk as free-floating objects for at least 4 Myr. Many CAIs, however, were largely unaffected by chondrule melting, suggesting that chondrule-forming events experienced by a nebular region could have been small in scale and limited in number. Chondrules and metal grains in CB chondrites formed during a single-stage, highly-energetic event ∼4563 Myr ago, possibly from a gas-melt plume produced by collision between planetary embryos.  相似文献   

12.
The 10Be contents of 28 stony meteorites with known 21Ne contents range from 0.97 to 23 dpm/kg and give an average 21Ne production rate (P21) of (0.28 ± 0.02) × 108 cm3 STP/g-Myr for shielding conditions corresponding to 22Ne21Ne = 1.114 in an H-chondrite. Our P21(10Be) agrees with others' P21 based on 22Na, 81Kr and 53Mn but not on 26A1. Temporal variations in the cosmic ray flux do not explain the disagreement satisfactorily; major errors in the radionuclide half-lives are not indicated. The discrepancy seems rooted in the data selection and the difficulties of making accurate corrections for shielding, chemical composition and other sources of variability.  相似文献   

13.
Abundances of cosmic ray-produced noble gases and 26Al, including some new measurements, have been compiled for some 23 stone meteorites with exposure ages of < 3 × 106 yr. Concentrations of cosmogenic He, Ne, and Ar in these meteorites have been corrected for differences in target element abundances by normalization to L-chondrite chemistry. Combined noble gas measurements in depth samples of the Keyes and St. Séverin chondrites are utilized to derive equations for normalizing the production rates of cosmogenic 3He, 21Ne, and 38Ar in chondrites to an adopted ‘average’ shielding: 22Ne21Ne = 1.114. The measured unsaturated 26Al concentrations and the calculated equilibrium 26Al for these meteorites are combined to estimate exposure ages. These exposure ages are statistically compared with chemistry- and shielding-corrected concentrations of cosmogenic He, Ne, and Ar to derive absolute production rates for these nuclides. For L-chondrites, at ‘average’ shielding, these production rates (in 10?8 cm3/g 106 yr) are: 3He = 2.45,21Ne = 0.47, and 38Ar = 0.069, which are ~ 25% higher than production rates used in the past. From these production rates and relative chemical correction factors, production rates for other classes of stone meteorites are derived.  相似文献   

14.
He, Ne and Ar isotopes have been measured in six, and 26Al in two diogenites. Cosmic-ray exposure ages corrected for shielding effects using 22Ne/21Ne ratios are generally concordant. Five diogenites have a group age of 14 Myr and three others may have a group age of 24 Myr, implying that two collisions may have produced 8 of the 9 diogenites.  相似文献   

15.
We have found clear evidence of live 10Be in five normal Type A Calcium-aluminum-rich inclusions (CAIs), one normal Type B CAI, and one FUN Type A CAI, all from CV3 chondrites. The (10Be/9Be)0 ratios range from ∼0.36 × 10-3 to ∼0.77 × 10-3 and are similar to those found by previous workers. The (10Be/9Be)0 ratios do not correlate in a temporal fashion with (26Al/27Al)0, suggesting that 10Be and 26Al were produced by different mechanisms. An examination of possible sources for the short-lived radionuclides indicates that production of 10Be was almost certainly by particle irradiation, possibly within the solar system, and was probably accompanied by significant production of 41Ca and 53Mn. In contrast, all of the 60Fe, most of the 26Al, and some of the 53Mn were probably produced in stars and were imported into the solar system within presolar dust grains.  相似文献   

16.
Noble gas analyses of the Ni-Fe of 9 L, 5 H and 2 LL chondrites quantitatively support previous suggestions of radiogenic 4He recoil and 3He deficits. Furthermore, noble gases in the Ni-Fe show evidence for in situ produced radiogenic 4He and in some cases for recoil loss of 38Ar and gain of 21Ne.The ratio of spallogenic 21Ne and 38Ar in the metal phase is found to correlate strongly with 3He/21Ne and 22Ne/21Ne in bulk samples of these chondrites. This is proof of the dependence of these ratios on the irradiation hardness experienced by the meteoroid in space. ‘Hardness indices’ n = 1.9–2.2 are found, indicating that on the average the stone meteoroids from which the samples came were smaller in mass than iron meteoroids. The spallogenic 21Ne/38Ar ratio in metallic Ni-Fe can be used with the semi-empirical production model deduced from the Grant iron meteorite to calibrate spallogenic 3He/21Ne and 4Ne/21Ne in bulk samples of L, LL and H chondrites for meteoroid size and sample location allowing the estimation of minimal meteoroid masses. 3He and 21Ne production rates calculated from previously determined 36Ar/38Ar exposure ages for four L chondrites indicate that they are probably not single-valued functions of the 3He/21Ne ratio. The ratio of 3He in bulk samples to 38Ar in metal samples of the same meteorite is constant (= 20 ± 3) whereas the ratio of 21Ne in the bulk to 38Ar in the metal varies by as much as a factor of two in correlation with 3He/21Ne.  相似文献   

17.
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, <0.5 mm/Myr, provide well defined depth profiles characterizing the SCR component of the cosmic rays. In conjunction with new experimentally determined excitation functions, the 36Cl concentrations suggest a softer solar-proton spectral shape than that derived from most previous measurements. The fact that no SCR-produced 10Be activity could be detected in 64455 is in good agreement with observations in 68815 and also indicates a softer SCR spectrum. Comparison of observed SCR profiles in 64455 with theoretical calculations indicates that the average solar-proton spectrum over the past 2 Myr (based on 26Al) has an exponential rigidity parameter (R0) of about 90 MV with a proton flux (J) of 73 protons/cm2/s·4π above 10 MeV. Over the last ∼0.5 Myr (based on 36Cl) R0 is about 70 MV with a flux of ∼196 protons/cm2/s·4π above 10 MeV. These SCR fluxes are consistent with most previous work.  相似文献   

18.
CM chondrites contain carbonates and other secondary minerals such as phyllosilicates, sulfides, sulfates, oxides and hydroxides that are believed to have formed by aqueous alteration reactions on their parent asteroid. We report in situ Mn-Cr isotope measurements in the highly aqueously altered CM2.1 chondrites QUE 93005 and ALH 83100 using secondary ion mass spectrometry (Cameca ims-1270 ion microprobe). The 53Cr excesses are correlated with the 53Mn/55Mn ratio and result from the in situ decay of 53Mn, a short-lived radioisotope with a half-life of 3.7 Ma. If we assume that carbonate grains in samples QUE 93005 and ALH 83100 are cogenetic, then the excesses define initial 53Mn/55Mn ratios ((53Mn/55Mn)0) of (4.1 ± 1.2) × 10−6 and (5.1 ± 1.7) × 10−6, respectively. These values are comparable to those in carbonates from other CM chondrites as reported in the literature. Initial 53Mn/55Mn ratios for calculated model isochrones for individual carbonate grains range from (3.8 ± 1.4) × 10−6 to (4.8 ± 2.1) × 10−6 for QUE 93005 and from (3.1 ± 1.6) × 10−6 to (1.3 ± 0.5) × 10−5 for ALH 83100. A possible interpretation for the ranges in (53Mn/55Mn)0 could be that alteration in individual CM chondrites was episodic and occurred over an extended period of time. However, isochrones based on the entire set of carbonate grains in each of the CM chondrites imply that the degree of aqueous alteration is roughly correlated with the age of carbonate formation in CM chondrites of different subtypes and that alteration on the CM parent asteroid started contemporaneously with or shortly after CAI formation and lasted at least 4 Ma.  相似文献   

19.
Abundances and isotopic compositions of He, Ne, Ar, and Xe have been measured in eight recently fallen chondrites. Ratios of concentrations of cosmic ray-produced 3He, 21Ne, 22Ne and 38Ar indicate that all eight samples experienced less than average cosmic ray shielding. 3He and 21Ne exposure ages were calculated using shielding corrected chondritic production rates and the measured 22Ne21Ne. Exposure ages calculated from 22Na22Ne and 26Al21Ne ratios and constant relative production rates show a bias between the two ages due to variations in 22Na26Al. Arguments are presented that this bias is due to irradiation hardness differences, and therefore the use of constant values for both the 22Na22Ne and 26Al21Ne production ratios is not permitted. Dwaleni, Swaziland, was found to be an unusual gas-rich chondrite with high concentrations of solar-derived He and Ne and planetary-type Xe.  相似文献   

20.
The 26Al-26Mg isotope systematics in 33 petrographically and mineralogically characterized plagioclase-rich chondrules (PRCs) from 13 carbonaceous chondrites (CCs) - one ungrouped (Acfer 094), six CR, five CV, and one CO - reveal large variations in the initial 26Al/27Al ratio, (26Al/27Al)0. Well-resolved 26Mg excesses (δ26Mg) from the in situ decay of the short-lived nuclide 26Al (t1/2 ∼ 0.72 Ma) were found in nine chondrules, two from Acfer 094, five from the CV chondrites, Allende and Efremovka, and one each from the paired CR chondrites, EET 92147 and EET 92042, with (26Al/27Al)0 values ranging from ∼3 × 10−6 to ∼1.5 × 10−5. Data for seven additional chondrules from three CV and two CR chondrites show evidence suggestive of the presence of 26Al but do not yield well defined values for (26Al/27Al)0, while the remaining chondrules do not contain excess radiogenic 26Mg and yield corresponding upper limits of (11-2) × 10−6 for (26Al/27Al)0. The observed range of (26Al/27Al)0 in PRCs from CCs is similar to the range seen in chondrules from unequilibrated ordinary chondrites (UOCs) of low metamorphic grade (3.0-3.4). However, unlike the UOC chondrules, there is no clear trend between the (26Al/27Al)0 values in PRCs from CCs and the degree of thermal metamorphism experienced by the host meteorites. High and low values of (26Al/27Al)0 are found equally in PRCs from both CCs lacking evidence for thermal metamorphism (e.g., CRs) and CCs where such evidence is abundant (e.g., CVs). The lower (26Al/27Al)0 values in PRCs from CCs, relative to most CAIs, are consistent with a model in which 26Al was distributed uniformly in the nebula when chondrule formation began, approximately a million years after the formation of the majority of CAIs. The observed range of (26Al/27Al)0 values in PRCs from CCs is most plausibly explained in terms of an extended duration of ∼2-3 Ma for the formation of CC chondrules. This interval is in sharp contrast to most CAIs from CCs, whose formation appears to be restricted to a narrow time interval of less than 105 years. The active solar nebula appears to have persisted for a period approaching 4 Ma, encompassing the formation of both CAIs and chondrules present in CCs, and raising important issues related to the storage, assimilation and mixing of chondrules and CAIs in the early solar system.  相似文献   

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