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1.
The isotopic composition of Ag and the concentration of Ag and Pd have been determined in Canyon Diablo (IA), Grant (IIIB), Hoba, Santa Clara, Tlacotepec and Warburton Range (IVB), Piñon and Deep Springs (anom.). Troilite from Grant and Santa Clara have also been analyzed. All of these meteorites, with the exception of Canyon Diablo, give 107Ag109Ag in the metal phase that is greater than the terrestrial value with the enrichments of 107Ag ranging from ~2% to 212%. These data show that Ag of anomalous isotopic composition is common to all IVB and anomalous meteorites. The results on Grant suggest that the anomalies may be widespread including more common meteorite groups. There is a general correlation of 107Ag109Ag with PdAg except for the data from FeS of Santa Clara. It is concluded that the excess 107Ag is the result of decay of 107Pd, a nuclide that is extinct at present with an abundance of 107Pd108Pd of about 3 × 10?5. The troilite in Grant exhibits normal 107Ag109Ag to within errors, a high Ag concentration and a low ratio of 108Pd109Ag ~0.17. Grant metal has 107Ag109Ag that is ~2% greater than normal and a high ratio of 108Pd109Ag ~ 103. The data from Grant appear to represent a 107Pd-107Ag isochron and indicate that the cooling rate at elevated temperatures was sufficiently rapid to preserve substantial isotopic differences between metal and troilite. Troilite in Santa Clara was found to contain Ag with a very high 107Ag109Ag ratio (108% above normal), an Ag concentration only a factor of three above the metal and a high value of 108Pd109Ag ~1.3 × 104. The troilite has a higher 107Ag109Ag than the metal. These data are not compatible with a simple model of in situ decay and subsequent local Ag redistribution between metal and troilite during cooling. These data suggest that Ag in Santa Clara and possibly other IVB meteorites is made up of almost pure 107Ag produced from 107Pd decay and 109Ag produced by nuclear reactions with only a small amount of “normal” Ag. This indicates an intense energetic particle bombardment history in the early solar system (~1020 p/m2) which occurred after the formation of small planetary bodies. We infer that a T-Tauri activity by the early sun contributed to some late stage “nucleosynthesis” and the heating of a dust cloud. In addition, implications on the early thermal evolution of iron meteorites are presented based on 107Pd decay and models of the cooling history.  相似文献   

2.
3.
Hydrogen which is highly enriched in deuterium is present in organic matter in a variety of meteorites including non-carbonaceous chondrites. The concentrations of this hydrogen are quite large. For example Renazzo contains 140 μmoles/g of the 10,000‰ δD hydrogen. The DH ratios of hydrogen in the organic matter vary from 8 × 10?5 to 170 × 10?5 (δD ranges from ? 500‰ to 10,000‰) as compared to 16 × 10?5 for terrestrial hydrogen and 2 × 10?5 for cosmic hydrogen. The majority of the unequilibrated primitive meteorites contain hydrogen whose DH ratios are greater than 30 × 10?5. If the DH ratios in these compounds were due to enrichment relative to cosmic hydrogen by isotope exchange reactions, it would require that these reactions take place below 150 K. In addition the organic compounds having DH ratios above 50 × 10?5 would require temperatures of formation of < 120 K. These types of deuterium enrichments must take place by ion-molecule reactions in interstellar clouds where both ionization and low temperatures exist. Astronomically observed DH ratios in organic compounds in interstellar clouds are typically 180 × 10?5 and range between about 40 × 10?5 and 5000 × 10?5. The DH values we have determined are the lower limits for the organic compounds derived from interstellar molecules because all processes subsequent to their formation, including terrestrial contamination, decrease their DH ratios.In contrast, the DH ratios of hydrogen associated with hydrated silicates are relatively uniform for the meteorites we have analyzed with an average value of 14 × 10?5; very similar to the terrestrial value. These phyllosilicates values suggest equilibration of H2O with H2 in the solar nebula at temperatures of about 200 K and higher.The 13C12C ratios of organic matter, irrespective its DH ratio, lie well within those observed for the earth. If organic matter originated in the interstellar medium, our data would indicate that the 13C12C ratio of interstellar carbon five billion years ago was similar to the present terrestrial value.Our findings suggest that other interstellar material, representing various inputs from various stars, in addition to the organic matter is preserved and is present in the meteorites which contain the high DH ratios. We feel that some elements existing in trace quantities which possess isotopic anomalies in the meteorites may very well be such materials.  相似文献   

4.
The redox potential of ZoBell's solution, consisting of 3.33 × 10?3 molar K4Fe(CN)6, 3.33 × 10?3 molar K3Fe(CN)6 and 0.10 molar KCl, has been measured by a polished platinum electrode vs a saturated KCl, Ag/AgCl reference electrode. Measurements in the temperature range 8–85°C fit the equation E(volts) = 0.23145 ? 1.5220 × 10?3 (t ? 25) ? 2.2449 × 10?6(t ? 25)2 where t is in degrees Celsius. Evaluation of literature data was necessary to obtain a reliable value for the Ag/AgCl half-cell reference potential as a function of temperature. Combining the measurements from this study with the literature evaluation of the Ag/AgCl reference potential yields the temperature dependent potential for ZoBell's solution: E(volts) = 0.43028 ? 2.5157 × 10?3 (t ? 25) ? 3.7979 × 10?6 (t ? 25)2 relative to the standard hydrogen potential. From these data the enthalpy, entropy, free energy and heat capacity for the ferro-ferricyanide redox couple have been calculated. The temperature equation for the potential of ZoBell's solution may be used for checking potentiometric equipment in the determination of the redox potential of natural waters.  相似文献   

5.
The 3He4He ratios measured in 27 Southern Africa diamond stones, four from Premier Mine and the rest of unidentified origin, range from 4.2 × 10?8 to 3.2 × 10?4, with three stones above 1 × 10?4. We conclude that the initial helium isotopic ratio (3He4He)0 in the earth was significantly higher than that of the planetary helium-A (3He4He = 1.42 × 10?4), but close to the solar helium (3He4He ? 4 × 10?4).The apparent K-Ar ages for the twelve diamonds of unidentified origin show enormously old age, indicating excess argon-40. 3He4He evolution in diamonds suggests that the diamonds with the high 3He4He ratio (>2 × 10?4) may be as old as the earth.Noble gas elemental abundance in the diamonds relative to the air noble gas abundance shows monotonie decrease with a decreasing mass number.This paper discusses the implications of these observations on the early solar system and the origin of diamonds.  相似文献   

6.
Determinations of 40Ar39Ar ages are reported for seven severely shock-heated chondrites. Shaw gives a plateau age of 4.29 Gyr. Louisville, Farmington, and Wickenburg give well-defined intercept ages of 0.5–0.6 Gyr. Orvinio, Arapahoe, and Lubbock show complex 40Ar39Ar release curves, with age minima of 0.7–1.0 Gyr. Degassing times of 0.5–1.0 Gyr are suggested for these meteorites. Most severely shocked chondrites were apparently not totally degassed of 40Ar by the event, but retained from ~ 2 to ~45% of their 40Ar. When calculated values of the diffusion parameter, Da2, for Ar are examined in Arrhenius plots, they show two distinct linear relationships, which apparently correspond to the degassing of different mineral phases with distinct KCa ratios and different average temperatures for Ar release. The experimentally determined values of Da2 for the high temperature phase of several severely shocked chondrites are ~10?7 to 10?5sec?1 for their determined shock-heating temperatures of ~950°C to ~ 1200°C. The inferred reheating temperatures, Da2 values, and fraction of 40Ar loss during the reheating event for these seven chondrites suggest post-shock cooling rates and burial depth of ~ 10?2 10?4°C/sec and ~0.5–2m, respectively. For three chondrites these cooling rates agree with those determined from Ni diffusion in metal grains: for five chondrites the cooling rates derived from 40Ar and Ni disagree by a factor of ~105. It is suggested that five of these severely shocked chondrites were part of large ejecta blankets containing hot material and cold clasts with a distribution of sizes and that the cooling rate of this ejecta appreciably decreased as a function of time.  相似文献   

7.
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   

8.
Radioactivity measurements have shown evidence for long-lived 92Nb and 2.03 × 104yr 94Nb in natural niobium. The specific activity of 94Nb was observed to be 0.32 ± 0.03 dis/min. kg Nb and that of 92Nb to be 0.058 ± 0.035 dis/min. kg Nb. With t12 taken as ≈ 1.7 × 108yr, the isotopic abundance of 98Nb is 1.2 × 10?10 per cent.  相似文献   

9.
A black inclusion from the Krymka LL3 chondrite was analyzed for 20 trace elements and five noble gases, by radiochemical neutron activation and mass spectrometry. The trace element pattern somewhat resembles that of C1 or C2 chondrites, but with several unique features. Elements of nebular condensation T ? 1000 K (U, Re, Os, Ir, Ni, Pd, Au, Sb and Ge) are essentially undepleted, as in C1 chondrites, but ReIr is 1.49 × higher than the characteristic Cl value. Among elements condensing below 1000 K, Cs, Se, Te, and In are depleted to approximately C2 levels (~0.6 × C1), whereas Ag, Bi, Tl are enriched to ~ 1.6 × C1. Such enrichments are thought to be characteristic of late nebular condensates.The noble-gas pattern also is unique. Gas contents are higher than in C1s, by factors of 2.6 to 19 for Ne through Xe. The Ar36Xe132 ratio of 500 is higher than mean values for C1s or C2s (109 or 89) and exceeds even the highest value seen in C3Os, 420, whereas the He4Ne20 ratio of 62 is much lower than the values for C1s and C2s (200–370). The Xe129Xe132 and Xe136Xel32 ratios of 1.040 and 0.320 resemble those of C1 chondrites, and seem to imply typical proportions of radiogenic Xe129 and ‘fissiogenic’ xenon.It appears that the inclusion represents a new primitive meteorite type, similar to C-chondrites, but probably a late condensate from a region of higher nebular pressure.  相似文献   

10.
He3, He4, Ne21 and Ar38 contents were determined in 18 metal, troilite, sehreibersite and graphite inclusions of 9 iron meteorites, by total outgassing and stepwise heating. The He4He3 ratio in metal phase ranges from 3.85 to 4.65, but in non-metallic samples, from 6.70 to 30.5. The results for cosmogenic isotopes of helium, neon and argon disagree appreciably with data on accelerator-irradiated targets. It should be noted, however, that some inclusions have lost considerable amounts of gas by diffusion.Uranium contents of 22 troilite and sehreibersite samples were determined by the fission track technique. The average uranium content of troilite is 0.4-0.7 ppb. Excess He4 of unknown origin was observed in troilite inclusions. If one assumes that the excess He4 was produced by uranium decay in situ, then the apparent U-He4 age is at least 5.9 × 109 yr.  相似文献   

11.
Metal and silicate portions from 13 mesosiderites, one pallasite, Bencubbin (“unique”) and Udei Station (‘iron with silicate inclusions’) have been analysed for their content of He, Ne and Ar; in most cases 36Cl could be determined as well. 36Cl-36Ar cosmic ray exposure ages fall between 10 and 160 Myr. Half of the metal samples show a deficit of spallogenic 3He (up to 30%) which we ascribe to a loss of tritium. The observed depletion of 3He in the silicates is correlated with their mineralogical composition: feldspar has lost its 3He in all cases, pyroxene definitely in one and possibly in five others, while olivine has been affected in only two meteorites. The thermal histories during their exposure to the cosmic radiation have been different for different meteoroids. Nevertheless, with the exception of Veramin, the data are compatible with the assumption of a continuous diffusion loss during a considerable fraction of the exposure era. For Veramin, however, an episodic event late in the exposure history is required. The exceptionally high 39Ar36Cl ratio in the metal, which is due to a high 39Ar activity, indicates that the event occurred during the last 500,000 years or so and resulted in an extremely excentric orbit (large aphelion).Production rates of 38,39Ar from Ca and 21,22Ne from Mg are given. The ratio P38CaP21Mg is close to unity. The ratios P38CaP38Fe vary between 20 and 50, and are not correlated with the absolute production rate of 38Ar from metal. The 22Ne21Ne production ratio from Mg is found to be close to but below unity.Of the mesosiderites only Veramin shows unambiguous evidence for primordial rare gases with larger amounts and a higher 20Ne36Ar ratio in the olivine, suggesting in situ fractionation to have at least been partly responsible for the abundance pattern found. Bencubbin contains large amounts of strongly fractionated primordial gases, but again part of the fractionation may have occurred in situ. Udei Station shows an excess of (3.5 ± 0.6) × 10?10 cm3 STP 129Xe/g in the non-magnetic portion.  相似文献   

12.
Cyclic voltammetry has been done for Ni2+, Co2+, and Zn2+ in melts of diopside composition in the temperature range 1425 to 1575°C. Voltammetric curves for all three ions excellently match theoretical curves for uncomplicated, reversible charge transfer at the Pt electrode. This implies that the neutral metal atoms remain dissolved in the melt. The reference electrode is a form of oxygen electrode. Relative to that reference assigned a reduction potential of 0.00 volt, the values of standard reduction potential for the ions are E1 (Ni2+Ni0, diopside, 1500°C) = ?0.32 ± .01 V, E1 (Co2+Co0, diopside, 1500°C) = ?0.45 ± .02 V, and E1 (Zn2+Zn0, diopside, 1500°C) = ?0.53 ± .01 V. The electrode reactions are rapid, with first order rate constants of the order of 10?2 cm/sec. Diffusion coefficients were found to be 2.6 × 10?6 cm2/sec for Ni2+, 3.4 × 10?6 cm2/sec for Co2+, and 3.8 × 10?6 cm2/sec for Zn2+ at 1500°C. The value of E1 (Ni2+Ni0, diopside) is a linear function of temperature over the range studied, with values of ?0.35 V at 1425°C and ?0.29 V at 1575°C. At constant temperature the value of E1 (Ni2+Ni0, 1525°C) was not observed to vary with composition over the range CaO · MgO · 2SiO2 to CaO·MgO·3SiO2 or from 1.67 CaO·0.33MgO·2SiO2 to 0.5 CaO·1.5MgO·2SiO2. The value for the diffusion coefficient for Ni2+ decreased by an order of magnitude at 1525°C over the compositional range CaO · MgO · 1.25SiO2 to CaO · MgO · 3SiO2. This is consistent with a mechanism by which Ni2+ ions diffuse by moving from one octahedral coordination site to another in the melt, with the same Ni2+ species discharging at the cathode regardless of the SiO2 concentration in the melt.  相似文献   

13.
The long-lived nuclide 92Nb together with 91Nb and 94Nb were produced from Mo by neutron irradiation. The activities of the nuclides were determined by γ spectroscopy and the atomic abundances were determined by mass spectrometry. Intercomparison of 92Nb and 94Nb activities and abundances yielded a value for half-life (92Nb) = (3.3 ± 0.5) × 107 yr based on a value for half-life (94Nb) = 2.0 × 104 yr. The same value was obtained within errors from absolute counting data and dilution analysis.  相似文献   

14.
The inert gases have been measured in six size fractions covering the range below 500 μm, in a single feldspathic fragment weighing 523 μg, and in an agglutinate particle weighing 465 μg. The two size fractions between 125 and 250 μm as well as 250 and 500 μm were separated into magnetic and non-magnetic portions, which were measured separately. Like the Apollo and Luna 16 fines, the terra fines represented by Luna 20 are very rich in trapped solar-wind gases, but they contain relatively less He4 and Ne20, which is revealed by their average He4Ne20 ratio of 35 and Ne20Ar36 ratio of 2.9. Obviously the terra materials are less retentive for solar-wind He and Ne than typical mare fines such as 10084. Whether this is due to the relatively small TiO2 or the relatively large plagioclase content of the former is not resolved. (Ar36Kr84)trapped and (Ar36Xe132)trapped ratios are relatively large; the average values are 2800 and 14400, respectively. The apparent Ne21 radiation ages of all the size fractions are in the range 209–286 × 106 yr; the average is 260 × 106 yr. This is in the range of values known for the Apollo and Luna 16 fines. The feldspathic fragment has a much greater apparent Nec21 age of 780 × 106 yr. The Ar40-Ar36 systematic reveals the presence of two Ar40 components, because Ar40 = (1.41 ± 0.076)Ar36 + (0.490 ± 0.130) × 10?4 (cm3 STP/g). The Ar40Ar36 slope of 1.41 is not inconsistent with an origin of the sample from a relatively old terra region.  相似文献   

15.
The performance characteristics of PANURGE, a modified CAMECA IMS3F ion microprobe, have been studied at a mass resolving power of 5000 for the purpose of determining isotopic ratios at a precision level approaching that of counting statistics using beam switching. The techniques used for this type of measurement are described. Using this approach, the isotopic composition of Mg and Si and the atomic ratio of AlMg in minerals from the Allende inclusion WA and the Allende FUN inclusion Cl have been measured with the ion microprobe at high mass resolving power. Enrichments in 26Mg of up to 260%. have been found. Mg and AlMg measurements on cogenetic spinel inclusions and host plagioclase crystals yield Mg-Al isochrons in excellent agreement with precise mineral isochrons determined by thermal emission mass spectrometry. The measurements confirm the presence of substantial excess 26Mg in WA (26Mg127Al = 5 × 10?5) and its near absence in Cl (26Mg127Al < 4 × 10?6). In WA plagioclase, data for which 27Al24Mg = 300 to 1000 define a linear array with 26Mg127Al = 3 × 105 and with initial 26Mg24Mg composition 30%. greater than in high Mg phases. This suggests a metamorphic reequilibration of Mg in Allende plagioclase at least 0.6 my after WA formation. There were no variations in detected 26Mg127Al in WA plagioclase associated with concentration of 26Mg1 into isolated clusters. We have confirmed by ion probe measurements that the Mg composition in Allende Cl is highly fractionated and is uniform among pyroxene, melilite, plagioclase, spinel crystals and spinel included in melilite and plagioclase crystals. Likewise, the Si composition is mass fractionated and is the same in pyroxene, melilite and plagioclase.  相似文献   

16.
The diffusivity of oxygen was determined in melts of Jadeite (NaAlSi2O6) and diopside (CaMgSi2O6) compositions using diffusion couples with 18O as a tracer. In the Jadeite melt, the diffusivity of oxygen increases from 6.87?0.25+0.28 × 10?10cm2/sec at 5 Kb to 1.32 ± 0.08 × 10?9cm2/sec at 20 Kb at constant temperature (1400°C), whereas in the diopside melt at 1650°C, the diffusivity decreases from 7.30?0.180.29 × 10?7cm2/sec at 10 Kb to 5.28?0.55+0.60 × 10?7cm2/sec at 17 Kb. These results demonstrate that the diffusivity is inversely correlated with the viscosity of the melt. For the jadeite melt, in particular, the inverse correlation is very well approximated by the Eyring equation using the diameter of oxygen ions as a unit distance of translation, suggesting that the viscous flow is rate-limited by the diffusion of individual oxygen ions. In the diopside melt, the activation volume is slightly greater than the molar volume of oxygen ion, indicating that the individual oxygen ion is the diffusion unit. The negative activation volume obtained for the jadeite melt is interpreted as the volume decrease associated with a diffusive jump of an oxygen ion due to local collapse of the network structure.  相似文献   

17.
Xanthates are used in the flotation of sulfide ores although their aqueous solutions are not stable under certain conditions. Their stability in acidic and weakly acidic aqueous solutions was therefore investigated, as these media are required for some processes.The peak absorbances of ethylxanthate ion and carbon disulfide were first determined in aqueous solution. The decomposition of ethylxanthate ion was analyzed by measuring variations in absorbance (at 301 nm) and pH with respect to time. A pH regulation system was then used while measuring variations in absorbance and productions of protons caused by xanthate decomposition.The results concerning xanthate half-lives show good agreement with the literature, but the kinetic results deviate substantially. The following relation was obtained for half-life:
T12=9.67×10?6(pH)11;4?7;T12in seconds
We established that ethylxanthate decomposition at pH 4 is a first order reaction with respect to ethylxanthate concentration, and postulating this order to the other pH values, the following kinetic relation was found:
v= ?(1.22×104[H+]?1.36×10?2)([EtX?]) (4?pH?7)
where v is the rate of decomposition (mol l?1 min?1), and [EtX?] is the ethylxanthate concentration when the decomposition equilibria are reached (mol l?1). The better concentration was found to obey the law:
[EtX?]=3.142×10?5 pH ? 1.255 × 10?4 (4?pH?6)
  相似文献   

18.
The decay constant 87Rb has been redetermined by measuring the amount of radiogenic 87Sr produced over a period of 19 years, in 20 g samples of purified RbClO4, using isotope dilution techniques. The rubidium sample was spiked with 84Sr and the nanogram quantities of strontium separated by coprecipitation with Ba(NO3)2. Analyses were carried out on a 25cm, 90° sector mass spectrometer equipped with a Spiraltron electron multiplier. Measurement of three independent ratios permitted continuous monitoring of the ion beam fractionation. The average of nine determinations gives a value for the decay constant of 1.419(±0.012) × 10?11 yr?1 (2σ). [τ12 = 4.89(±0.04) × 1010yr.]  相似文献   

19.
The geochemical history of Lake Lisan, the Pleistocene precursor of the Dead Sea, has been studied by geological, chemical and isotopic methods.Aragonite laminae from the Lisan Formation yielded (equivalent) Sr/Ca ratios in the range 0.5 × 10?2?1 × 10?2, Na/Ca ratios from 3.6 × 10?3 to 9.2 × 10?3, δ18OPDB values between 1.5 and 7%. and δ13CPDB from ?7.7 to 3.4%..The distribution coefficient of Na+ between aragonite and aqueous solutions, λANa, is experimentally shown to be very sensitive to salinity and nearly temperature independent. Thus, Na/Ca in aragonite serves as a paleosalinity indicator.Sr/Ca ratios and δ18O values in aragonite provide good long-term monitors of a lake's evolution. They show Lake Lisan to be well mixed, highly evaporated and saline. Except for a diluted surface layer, the salinity of the lake was half that of the present Dead Sea (15 vs 31%).Lake Lisan evolved from a small, yet deep, hypersaline Dead Sea-like, water body. This initial lake was rapidly filled-up to its highest stand by fresh waters and existed for about 40,000 yr before shrinking back to the present Dead Sea. The chemistry of Lake Lisan at its stable stand represented a material balance between a Jordan-like input, an original large mass of salts and a chemical removal of aragonite. The weighted average depth of Lake Lisan is calculated, on a geochemical basis, to have been at least 400, preferably 600 m.The oxygen isotopic composition of Lake Lisan water, which was higher by at least 3%. than that of the Dead Sea, was probably dictated by a higher rate of evaporation.Na/Ca ratios in aragonite, which correlate well with δ13C values, but change frequently in time, reflect the existence of a short lived upper water layer of varying salinity in Lake Lisan.  相似文献   

20.
Isotopic concentrations of the noble gases have been measured in several different phases of Elephant Moraine A79001 and in whole rock samples of Zagami and Allan Hills A77005, three meteorites which belong to the rare group of SNC achondrites that may have originated from the planet Mars. Shocked phases of EETA79001 contain a trapped Ar, Kr, and Xe component characterized by 84Kr132Xe ~15, 40Ar36Ar > 2000, 129Xe132Xe ≥ 2, and 4He40Ar ≤ 0.1. These elemental and isotopic ratios are unlike those for any other noble gas component except analyses of the Martian atmosphere made by Viking spacecraft. The isotopic composition of the trapped Kr shows an approximate 1% per mass unit enrichment of lighter isotopes compared to terrestrial Kr, and the traped Xe may show either a fission component or a fractionated enrichment of heavier isotopes compared to terrestrial Xe. It is hypothesized that these gases represent a portion of the Martian atmosphere which was shock-implanted into EETA79001, and that they constitute direct evidence of a Martian origin for the shergottite meteorites. Cosmic ray-produced gases in the eight known SNC meteorites form three distinct groups with exposure ages of ~11 MY (Chassigny and the nakhlites), ~2.6 MY (Shergotty, Zagami, and ALHA77005), and ~0.5 MY (EETA79001). These ages suggest three distinct events and cannot have been produced by irradiation for a common time under greatly different shielding. Comparison of cosmogenic 3He21Ne measured in EETA79001 with two independent models for the production of this ratio as a function of shielding indicates that this meteorite was irradiated in space as a relatively small object. If the SNC meteorites were ejected from Mars ~ 180 My ago, the shock age of the shergottites, they must have been relatively large objects (>6 meters diameter) which experienced at least three space collisions to initiate cosmic ray exposure. Ejection from Mars by three events at the times of initiation of cosmic ray exposure would permit the ejected objects to have been much smaller (<1 meter diameter), but would require three such events on 1.3 Gy Martian terraine in the past ~10 MY and would not explain the common 180 MY shock age seen in all four shergottites.  相似文献   

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