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1.
《Geochimica et cosmochimica acta》1999,63(13-14):2145-2162
We have applied a stepwise pyrolytic extraction technique to eleven individual lunar regolith grains to investigate the compositions of light noble gases embedded in grain surfaces by solar wind irradiation, with emphasis on the rather poorly known isotopic composition of solar-wind argon. Results are intriguing: average 20Ne/22Ne ratios observed in early pyrolytic releases from ilmenite grains separated from lunar soils 71501, 79035 and 10084 agree very well with both direct measures of the solar wind neon composition in the Apollo foils and with values obtained in first releases from acid-etched ilmenites by the Zürich laboratory, whereas these same pyrolytic and acid-etch fractions carry argon isotopic signatures that significantly disagree—average 36Ar/38Ar ratios near 5.8 for thermal extraction compared to 5.4–5.5 for chemical etching at Zürich. Consideration of the isotopic and elemental data from these grains in the context of first-order diffusive modeling calculations points to gas release at low temperatures, without significant isotopic or elemental fractionation, from isolated grain-surface reservoirs of solar wind composition. The physical nature of these reservoirs is presently unknown. In this interpretation the preferred solar wind 20Ne/22Ne and 21Ne/22Ne ratios deduced from this study are respectively 13.81 ± 0.08 and 0.0333 ± 0.0003, both within error of the Zürich acid-etch values, and 36Ar/38Ar = 5.77 ± 0.08. It may be possible to reconcile the discrepancy between the acid-etch and pyrolytic estimates for the solar wind 36Ar/38Ar ratio in the context of arguments originally advanced by Benkert et al. (1993) to account for their He and Ne isotopic compositions. At the other, high-temperature end of the release profile from one of these grains there are clear isotopic indications of the presence of a Ne constituent with 20Ne/22Ne close to the 11.2 ratio found at Zürich and attributed by these workers to a deeply-sited component implanted by solar energetic particles.  相似文献   

2.
Solar wind (SW) helium, neon, and argon trapped in a bulk metallic glass (BMG) target flown on NASA’s Genesis mission were analyzed for their bulk composition and depth-dependent distribution. The bulk isotopic and elemental composition for all three elements is in good agreement with the mean values observed in the Apollo Solar Wind Composition (SWC) experiment. Conversely, the He fluence derived from the BMG is up to 30% lower than values reported from other Genesis bulk targets or in-situ measurements during the exposure period. SRIM implantation simulations using a uniform isotopic composition and the observed bulk velocity histogram during exposure reproduces the Ne and Ar isotopic variations with depth within the BMG in a way which is generally consistent with observations. The similarity of the BMG release patterns with the depth-dependent distributions of trapped solar He, Ne, and Ar found in lunar and asteroidal regolith samples shows that also the solar noble gas record of extraterrestrial samples can be explained by mass separation of implanted SW ions with depth. Consequently, we conclude that a second solar noble gas component in lunar samples, referred to as the “SEP” component, is not needed. On the other hand, a small fraction of the total solar gas in the BMG released from shallow depths is markedly enriched in the light isotopes relative to predictions from implantation simulations with a uniform isotopic composition. Contributions from a neutral solar or interstellar component are too small to explain this shallow sited gas. We tentatively attribute this superficially implanted gas to low-speed, current-sheet related SW, which was fractionated in the corona due to inefficient Coulomb drag. This fractionation process could also explain relatively high Ne/Ar elemental ratios in the same initial gas fraction.  相似文献   

3.
Since about half a century samples from the lunar and asteroidal regoliths been used to derive information about elemental and isotopic composition and other properties of the present and past solar wind, predominantly for the noble gases and nitrogen. Secular changes of several important compositional parameters in the solar wind were proposed, as was a likely secular decrease of the solar wind flux. In 2004 NASA’s Genesis mission returned samples which had been exposed to the solar wind for almost 2.5 years. Their analyses resulted in an unprecendented accuracy for the isotopic and elemental composition of several elements in the solar wind, including noble gases, O and N. The Genesis data therefore also allow to re-evaluate the lunar and meteorite data, which is done here. In particular, claims for long-term changes of solar wind composition are reviewed.Outermost grain layers from relatively recently irradiated lunar regolith samples conserve the true isotopic ratios of implanted solar wind species. This conclusion had been made before Genesis based on the agreement of He and Ne isotopic data measured in the aluminum foils exposed to the solar wind on the Moon during the Apollo missions with data obtained in the first gas release fractions of stepwise in-vacuo etch experiments. Genesis data allowed to strengthen this conclusion and to extend it to all five noble gases. Minor variations in the isotopic compositions of implanted solar noble gases between relatively recently irradiated samples (<100 Ma) and samples irradiated billions of years ago are very likely the result of isotopic fractionation processes that happened after trapping of the gases rather than indicative of true secular changes in the solar wind composition. This is particularly important for the 3He/4He ratio, whose constancy over billions of years indicates that hardly any 3He produced as transient product of the pp-chains has been mixed from the solar interior into its outer convective zone. The He isotopic composition measured in the present-day solar wind therefore is identical to the (D + 3He)/4He ratio at the start of the suns’s main sequence phase and hence can be used to determine the protosolar D/H ratio.Genesis settled the long-standing controversy on the isotopic composition of nitrogen in lunar regolith samples. The 15N/14N ratio in the solar wind as measured by Genesis is lower than in any lunar sample. This proves that nitrogen in regolith samples is dominated by non-solar sources. A postulated secular increase of 15N/14N by some 30% over the past few Ga is not tenable any longer. Genesis also provided accurate data on the isotopic composition of oxygen in the solar wind, invaluable for cosmochemisty. These data superseded but essentially confirmed one value – and disproved a second one – derived from lunar regolith samples shortly prior to Genesis.Genesis also confirmed prior conclusions that lunar regolith samples essentially conserve the true elemental ratios of the heavy noble gases in the solar wind (Ar/Kr, Kr/Xe). Several secular changes of elemental abundances of noble gases in the solar wind had been proposed based on lunar and meteoritic data. I argue here that lunar data – in concert with Genesis – provide convincing evidence only for a long-term decrease of the Kr/Xe ratio by almost a factor of two over the past several Ga. It appears that the enhancement of abundances of elements with a low first ionisation potential in the solar wind (FIP effect) changed with time.Finally, Genesis allows a somewhat improved comparison of the present-day flux of solar wind Kr and Xe with the total amount of heavy solar wind noble gases in the lunar regolith. It remains unclear whether the past solar wind flux has been several times higher on average than it is today.  相似文献   

4.
High-resolution stepped heating has been used to extract light noble gases implanted in a suite of 13 individual lunar ilmenite and iron grains and in the Kapoeta howardite by solar wind (SW) and solar energetic particle (SEP) irradiation. Isotopic analyses of gases evolved at low temperatures from the lunar grains confirm the neon and argon compositions obtained by Pepin et al. (Pepin R. O., Becker R. H., and Schlutter D. J., “Irradiation records in regolith materials, I: Isotopic compositions of solar-wind neon and argon in single lunar regolith grains”, Geochim. Cosmochim. Acta63, 2145-2162, 1999) in an initial study of 11 regolith grains, primarily ilmenites. Combination of the data sets from both investigations yields 20Ne/22Ne = 13.85 ± 0.04, 21Ne/22Ne = 0.0334 ± 0.0003, and 36Ar/38Ar = 5.80 ± 0.06 for the lunar samples; the corresponding 36Ar/38Ar ratio in Kapoeta is 5.74 ± 0.06. The neon ratios agree well with those measured by Benkert et al. (Benkert J.-P., Baur H., Signer P., and Wieler R., “He, Ne, and Ar from the solar wind and solar energetic particles in lunar ilmenites and pyroxenes”, J. Geophys. Res. (Planets)98, 13147-13162, 1993) in gases extracted from bulk lunar ilmenite samples by stepped acid etching and attributed by them to the SW. The 36Ar/38Ar ratios, however, are significantly above both Benkert et al.’s (1993) proposed SW value of 5.48 ± 0.05 and a later estimate of 5.58 ± 0.03 from an acid-etch analysis of Kapoeta (Becker R. H., Schlutter D. J., Rider P. E., and Pepin R. O., “An acid-etch study of the Kapoeta achondrite: Implications for the argon-36/argon-38 ratio in the solar wind”, Meteorit. Planet. Sci.33, 109-113, 1998). We believe, for reasons discussed here and in our earlier report, that 5.80 ± 0.06 ratio most nearly represents the wind composition. The 3He/4He ratio in low-temperature gas releases, not measured in the first particle suite, is found in several grains to be indistinguishable from Benkert et al.’s (1993) SW estimate. Elemental ratios of He, Ne, and Ar initially released from grain-surface SW implantation zones are solar-like, as found earlier by Pepin et al. (1999). Gases evolved from these reservoirs at higher temperatures show evidence for perturbations from solar elemental compositions by prior He loss, thermal mobilization of excess Ne from fractionated SW components, or both.Attention in this second investigation was focused on estimating the isotopic compositions of both the SW and the more deeply sited SEP components in regolith grains. Several high-temperature “isotopic plateaus”—approximately constant isotopic ratios in gas fractions released over a number of consecutive heating steps—were observed in the close vicinities of the SEP ratios for He, Ne, and Ar reported by Benkert et al. (1993). Arguments presented in the text suggest that these plateaus are relatively free of interferences from multicomponent mixing artifacts that can mimic pure component signatures. Average SEP compositions derived from the stepped-heating plateau measurements are in remarkable agreement with the Zürich acid-etch values for all three gases.  相似文献   

5.
Analytical techniques of improved sensitivity have revealed details of the concentrations and isotopic compositions of light elements for a comprehensive suite of samples from the Apollo 12 regolith. These samples show a wide spread in maturity, although maximum contents observed for solar wind elements are less than observed at other sites, possibly reflecting relative recency of craters at the Apollo 12 site. Isotopic composition of nitrogen is consistent with the idea that 15N/14N in the solar wind has increased with time, at least a major part of this increase having occurred in the past 3.1 Gyr. Sulfur isotope systematics support a model in which sulfur is both added to the regolith, by meteoritic influx and lost, by an isotopically selective process. Most soils from this site are heavily contaminated with terrestrial carbon.  相似文献   

6.
We present the elemental and isotopic composition of noble gases in the bulk solar wind collected by the NASA Genesis sample return mission. He, Ne, and Ar were analyzed in diamond-like carbon on a silicon substrate (DOS) and 84,86Kr and 129,132Xe in silicon targets by UV laser ablation noble gas mass spectrometry. Solar wind noble gases are quantitatively retained in DOS and with exception of He also in Si as shown by a stepwise heating experiment on a flown DOS target and analyses on other bulk solar wind collector materials. Solar wind data presented here are absolutely calibrated and the error of the standard gas composition is included in stated uncertainties. The isotopic composition of the light noble gases in the bulk solar wind is as follows: 3He/4He: (4.64 ± 0.09) × 10−4, 20Ne/22Ne: 13.78 ± 0.03, 21Ne/22Ne: 0.0329 ± 0.0001, 36Ar/38Ar 5.47 ± 0.01. The elemental composition is: 4He/20Ne: 656 ± 5, and 20Ne/36Ar 42.1 ± 0.3. Genesis provided the first Kr and Xe data on the contemporary bulk solar wind. The preliminary isotope and elemental composition is: 86Kr/84Kr: 0.302 ± 0.003, 129Xe/132Xe: 1.05 ± 0.02, 36Ar/84Kr 2390 ± 150, and 84Kr/132Xe 9.5 ± 1.0. The 3He/4He and the 4He/20Ne ratios in the Genesis DOS target are the highest solar wind values measured in exposed natural and artificial targets. The isotopic composition of the other noble gases and the Kr/Xe ratio obtained in this work agree with data from lunar samples containing “young” (∼100 Ma) solar wind, indicating that solar wind composition has not changed within at least the last 100 Ma. Genesis could provide in many cases more precise data on solar wind composition than any previous experiment. Because of the controlled exposure conditions, Genesis data are also less prone to unrecognized systematic errors than, e.g., lunar sample analyses. The solar wind is the most authentic sample of the solar composition of noble gases, however, the derivation of solar noble gas abundances and isotopic composition using solar wind data requires a better understanding of fractionation processes acting upon solar wind formation.  相似文献   

7.
We have analyzed by thermal ionization mass spectrometry (TIMS) the isotopic composition of Cr in five progressive etches of size-sorted plagioclase grains separated from lunar soils 60601 and 62281. Aliquots of the etch solutions were spiked for isotopic dilution (ID) analysis of Cr and Ca. The Ca ID data indicate that the initial etch steps represent dissolution of an average 0.1 to 0.2 μm depth from the grain surfaces, the approximate depth expected for implanted solar wind. The Cr/Ca ratio in the initial etches is several fold higher than that expected for bulk plagioclase composition, but in subsequent etches decreases to approach the bulk value. This indicates a source of Cr extrinsic to the plagioclase grains, surface-correlated and resident in the outermost fraction of a μm, which we provisionally identify as solar wind Cr. The surface-correlated Cr is isotopically anomalous and by conventional TIMS data reduction has approximately 1 permil excess 54Cr and half as great excess 53Cr. In successive etches, as the Cr/Ca ratio decreases and approaches the bulk plagioclase value, the magnitude of the apparent anomalies decreases approaching normal composition. If these results do indeed characterize the solar wind, then either the solar wind is enriched in Cr due to spallation in the solar atmosphere, or the Earth and the various parent bodies of the meteorites are isotopically distinct from the Sun and must have formed from slightly different mixes of presolar materials. Alternative interpretations include the possibility that the anomalous Cr is meteoritic rather than solar or that the observed (solar) Cr is normal except for a small admixture of spallation Cr generated on the Moon. We consider these latter possibilities less likely than the solar wind interpretation. However, they cannot be eliminated and remain working hypotheses.  相似文献   

8.
The lunar regolith is exposed to irradiation from the solar wind and to bombardment by asteroids, comets and inter-planetary dust. Fragments of projectiles in the lunar regolith can potentially provide a direct measure of the sources of exogenous material being delivered to the Moon. Constraining the temporal flux of their delivery helps to address key questions about the bombardment history of the inner Solar System.Here, we use a revised antiquity calibration (after Eugster et al., 2001) that utilises the ratio of trapped 40Ar/36Ar (‘parentless’ 40Ar derived from radioactive decay of 40K, against solar wind derived 36Ar) to semi-quantitatively calculate the timing of the assembly of the Apollo 16 regolith breccias. We use the trapped 40Ar/36Ar ratios reported by McKay et al. (1986). Our model indicates that the Apollo 16 ancient regolith breccia population was formed between ∼3.8 and 3.4 Ga, consistent with regoliths developed and assembled after the Imbrium basin-forming event at ∼3.85 Ga, and during a time of declining basin-forming impacts. The material contained within the ancient samples potentially provides evidence of impactors delivered to the Moon in the Late-Imbrian epoch. We also find that a young regolith population was assembled, probably by local impacts in the Apollo 16 area, in the Eratosthenian period between ∼2.5 and 2.2 Ga, providing insights to the sources of post-basin bombardment. The ‘soil-like’ regolith breccia population, and the majority of local Apollo 16 soils, were likely closed in the last 2 Ga and, therefore, potentially provide an archive of projectile types in the Eratosthenian and Copernican periods.  相似文献   

9.
Pronounced variations in abundances and isotopic compositions of some light elements in soils from the Apollo 16 site are interpreted in terms of differing degrees of solar wind exposure for an originally, and approximately, homogeneous regolith. Carbon abundances in soils are compatible with a model in which equilibrium is established, after 104-105 yr, between solar wind input and loss by H stripping. However, this model does not explain the observed C isotopic distribution, suggesting that other sources of C or other processes, or both, are also important. Carbon abundances in rocks from Apollo 16 are higher (average 40 ppm) than at other landing sites although their isotopic compositions, ?35 < δ13C < ?16%. PDB, are normal. Abundances of N and, to a less extent, He and H in soils correlate with C as does a fraction of metallic Fe attributed to in situ reduction of indigenous Fe2+ by solar wind H.Fillet soil 67461 apparently contains solar wind C and N in a relatively unfractionated form, yielding an upper limit to solar wind (δ13C of ?16%., PDB and a value of 3.4 for CN in the solar wind.Sulfur at the Apollo 16 site represents a paradox in that, although abundances in soils are apparently controlled by local rock S contents, they also correlate, for all but one sample, with δ34S, which itself is apparently controlled by surface exposure age. A complex lunar S cycle is suggested.  相似文献   

10.
He, Ne, Ar, Kr and Xe concentrations and isotopic abundances were measured in three bulk grain size fractions prepared from sample L-16-19, No. 120 (C level, 20–22 cm depth) returned by the Luna 16 mission. The expected anticorrelation between the concentrations of trapped solar wind noble gases and grain size is observed. Elemental abundances of solar wind trapped noble gases are similar to those previously found in corresponding grain size fractions of the Apollo 11 and 12 fines. The trapped ratio 4He20Ne varies in the soils from different lunar maria due to diffusion losses. A rough correlation of 4He20Ne with the proportion of ilmenite in these samples is apparent. The elemental and isotopic ratios of the surface correlated noble gases in Luna 16 resemble those previously found in Apollo fines. Based on 21Ne, 78Kr and 126Xe a cosmic ray exposure age of 360 my was determined. This age is similar to those obtained for the soils from other lunar maria.  相似文献   

11.
Metallic spherules selected from the Apollo 11, 12, 14, 15 and 16 sites were studied by optical techniques as well as the electron probe and scanning electron microscope. In addition, metallic spherules of similar composition were produced experimentally. The structure of the metallic lunar spherules indicates an origin by solidification of molten globules of metal. The experimentally produced spherules have external morphologies, metallographic structures and solidification rates (7 × 102 to 106 ° C/sec) similar to the lunar spherules which have rapidly solidified. The majority of the lunar spherules are, however, either more slowly cooled or have been reheated in place with the lunar fragmental rocks, glass or soil. The heavy meteorite bombardment of the highlands is strongly reflected by the evidence of reheating and/or slow cooling of a majority of Apollo 14 and 16 spherules.The metallic spherules are probably produced from both lunar and meteoritic sources. Impact processes cause localized shock melting of metallic (and non-metallic) constituents at metal-sulfide phase interfaces in surface rocks and in the meteoritic projectile. The major source of metallic spherules is the metal phase present in the lunar rocks and soil. The large variation in spherule bulk compositions is attributed to the different meteoritic projectiles bombarding the Moon, metal phases of differing compositions in the lunar soils and rocks and to the experimental results which indicate that high S, high P alloys form two immiscible liquids when melted.  相似文献   

12.
The rare gases argon and xenon were studied intensively in lunar breccia 14318, one of a family of three Apollo 14 breccias exhibiting similarities, including substantial amounts of ‘parentless’ xenon from the spontaneous fission of extinct 244Pu. We made stepwise heatings on both unirradiated and pile-irradiated specimens. The isotopic composition of the xenon from fission was determined by a new method which invokes a minimum of assumptions; it is shown to be from 244Pu and almost certainly parentless. For example, the fission component, although not appreciably fractionated with respect to the trapped component during stepwise heating, has a low temperature character so that, relatively speaking, it appears to be more surficial than xenon emanating from uranium sites in the irradiated sample. We demonstrate that this effect is not an artifact of the neutron irradiation. The breccia contains abundant trapped argon with a high 40Ar36Ar ratio for lunar material—~14. Otherwise the argon is radiogenic and gives a convincing K-Ar age of 3.69 ± 0.09 b.y. by the stepwise 40Ar-39Ar method, nearly in agreement with ages for other Apollo 14 breccias obtained in our laboratory and elsewhere. Rock 14301, another of the family of breccias and one which has been studied in other laboratories, contains similar trapped argon and parentless xenon. Unlike 14318 it also contains a conspicuous excess of 129Xe from the radioactive decay of extinct 129I. Implications of the parentless xenon from extinct sources, as seen in these different rocks, depend upon the model adopted for its evolution and storage. We present four different models, all of which are unsatisfactory in some respects, so that we are presently unable to narrow the question. We must stress that other Apollo 14 breccias, such as 14321, contain fission xenon from 244Pu which was apparently produced in situ.  相似文献   

13.
The laser 40Ar/39Ar dating technique has been applied to five Luna 16 basalt fragments and one impact glass, and nine Luna 24 basalt fragments and one breccia. The textures of these basalts are fine-grained ophitic and coarse-grained basalts. The samples contain high levels of solar and lunar atmospheric argon acquired during their residence on the lunar surface. These trapped argon components are predominantly released at low temperature steps and can be distinguished from radiogenic and cosmogenic released at intermediate and high temperature steps. The apparent ages obtained for Luna 16 samples span a narrow range of 3.29 to 3.38 Ga. A young age of 0.988 Ga was obtained for a basaltic impact glass indicating the age of an impact event in the vicinity of Luna 16 landing site. The ages obtained for Luna 24 samples suggest the existence of at least three periods of volcanism occurring over a protracted interval of between 3.45 and 2.52 Ga. The long period of volcanism suggested for the Mare Crisium was likely due to a combination of geophysical and geochemical features in the surrounding and underlying areas of the Crisium Basin. Attempts at dating three Luna 20 samples were inconclusive due to their high trapped argon contents.  相似文献   

14.
We present bulk solar wind isotopic and elemental ratios for Ar, Kr, and Xe averaged from up to 14 individual analyses on silicon targets exposed to the solar wind for ∼2.3 years during NASA’s Genesis mission. All averages are given with 1σ standard errors of the means and include the uncertainties of our absolute calibrations. The isotopic ratios 86Kr/84Kr and 129Xe/132Xe are 0.303 ± 0.001 and 1.06 ± 0.01, respectively. The elemental ratios 36Ar/84Kr and 84Kr/132Xe are 2390 ± 120 and 9.9 ± 0.3, respectively. Average fluxes of 84Kr and 132Xe in the bulk solar wind in atoms/(cm2 s) are 0.166 ± 0.009 and 0.017 ± 0.001, respectively. The flux uncertainties also include a 2% uncertainty for the determination of the extracted areas. The bulk solar wind 36Ar/38Ar ratio of 5.50 ± 0.01 and the 36Ar flux of 397 ± 11 atoms/(cm2 s) determined from silicon targets agree well with the 36Ar/38Ar ratio and the 36Ar flux determined earlier on a different type of target by Heber et al. (2009). A comparison of the solar wind noble gas/oxygen abundance ratios with those in the solar photosphere revealed a slight enrichment of Xe and, within uncertainties a roughly uniform depletion of Kr-He in the solar wind, possibly related to the first ionization potentials of the studied elements. Thus, the solar wind elemental abundances He-Kr display within uncertainties roughly photospheric compositions relative to each other. A comparison of the Genesis data with solar wind heavy noble gas data deduced from lunar regolith samples irradiated with solar wind at different times in the past reveals uniform 36Ar/84Kr ratios over the last 1-2 Ga but an increase of the 84Kr/132Xe ratio of about a factor of 2 during the same time span. The reason for this change in the solar wind composition remains unknown.  相似文献   

15.
Noble gas isotopes are widely used to elucidate the history of the rocks in which they have been trapped, either from distinct reservoirs or by accumulation following radioactive decay. To extract noble gases from their host rocks, stepwise heating is the most commonly used technique to deconvolve isotopically different components, e.g., atmospheric, in situ radiogenic, or excess radiogenic from mantle or crustal reservoirs. The accurate determination of the isotopic composition of these different components is of crucial importance, e.g., for ages obtained by 40Ar-39Ar stepheating plateaus. However, diffusion theory-based model calculations predict that the stepwise thermal extraction process from mineral phases induces isotope fractionation and, hence, adulterates the original composition. Such effects are largely unconsidered, as they are small and a compelling experimental observation is lacking. We report the first unequivocal evidence for significant mass fractionation of argon isotopes during thermal extraction, observed on shungite, a carbon-rich Precambrian sedimentary rock. The degree of fractionation, as monitored by 38Ar/36Ar and 40Ar/36Ar ratios, very well agrees with theoretical predictions assuming an inverse square root dependence of diffusion coefficient and atomic mass, resulting in easier extraction of lighter isotopes. Hence, subatmospheric 40Ar/36Ar ratios obtained for argon extracted at low temperatures may not represent paleoatmospheric argon. Shungite argon resembles modern atmospheric composition, but constraints on the timing of trapping appear difficult to obtain, as shungites are multicomponent systems.In 40Ar-39Ar stepwise heating, the isotope fractionation effect could cause systematic underestimations of plateau ages, between 0.15 and 0.4% depending on age, or considerably higher if samples contain appreciable atmospheric Ar. The magnitude of this effect is similar to the presently achieved uncertainties of this increasingly precise dating technique. Our results also indicate the importance of thermally activated diffusion as a possible fractionation mechanism, e.g., for hydrothermal gas exhalations, or for carbonaceous carrier phases such as “Q” in meteorites that have been suggested as carriers of highly fractionated noble gas residues from the early solar nebula.  相似文献   

16.
Geochemical and 40Ar/39Ar data on nine impact glasses from the Apollo 14, 16, and 17 landing sites indicate at least seven distinct impact events with ages ∼800 Ma. Rock fragments analyzed by Barra et al. [Barra F., Swindle T. D., Korotev R. L., Jolliff B. L., Zeigler R. A., and Olsen E. (2006) 40Ar-39Ar dating of Apollo 12 regolith: implications for the age of Copernicus and the source of nonmare materials, Geochim. Cosmochim. Acta,70, 6016-6031] from the Apollo 12 landing site and some Apollo 12 spherules reported by Levine et al. [Levine J., Becker T. A., Muller R. A., Renne P. R. (2005) 40Ar/39Ar dating of Apollo 12 impact spherules, Geophys. Res. Let., 32, L15201, doi: 10.1029/2005GL022874.] show ∼800 Ma ages, close to the accepted age of the Copernicus event, 800 ± 15 Ma [Bogard D. D., Garrison D. H., Shih C. Y., and Nyquist L. E. (1994) 39Ar-40Ar dating of two lunar granites: The age of Copernicus, Geochim. Cosmochim. Acta, 58, 3093-3100]. These Apollo 12 samples are thought to have been affected by material from the Copernicus event since there is a Copernicus ray going through the Apollo 12 landing site. When all of these data are viewed collectively, including an Apollo 16 glass bomb [Borchardt R., Stöffler D., Spettel B., Palme H. and Wänke H. (1986) Composition, structure, and age of the Apollo 16 subregolith basement as deduced from the chemistry of post-Imbrium melt bombs. In Proceedings, 17th Lunar and Planetary Science Conference, pp. E43-E54], and in the context of diverse compositional range and sample location, there is a suggestion that there may have been a transient increase in the global lunar impact flux at ∼800 Ma. Therefore, the Copernicus impact event could have been one of many. If correct, there should be evidence for this increased impact flux around 800 Ma ago in the age statistics of terrestrial impact samples.  相似文献   

17.
The cosmic ray exposure (CRE) ages of aubrites are among the longest of stone meteorites. New aubrites have been recovered in Antarctica, and these meteorites permit a substantial extension of the database on CRE ages, compositional characteristics, and regolith histories. We report He, Ne, and Ar isotopic abundances of nine aubrites and discuss the compositional data, the CRE ages, and regolith histories of this class of achondrites. A Ne three-isotope correlation reveals a solar-type ratio of 20Ne/22Ne = 12.1, which is distinct from the present solar wind composition and lower than most ratios observed on the lunar surface. For some aubrites, the cosmic ray-produced noble gas abundances include components produced on the surface of the parent object. The Kr isotopic systematics reveal significant neutron-capture-produced excesses in four aubrites, which is consistent with Sm and Gd isotopic anomalies previously documented in some aubrites. The nominal CRE ages confirm a non-uniform distribution of exposure times, but the evidence for a CRE age cluster appears doubtful. Six meteorites are regolith breccias with solar-type noble gases, and the observed neutron effects indicate a regolith history. ALH aubrites, which were recovered from the same location and are considered to represent a multiple fall, yield differing nominal CRE ages and, if paired, document distinct precompaction histories.  相似文献   

18.
To investigate the formation and early evolution of the lunar mantle and crust we have analysed the oxygen isotopic composition, titanium content and modal mineralogy of a suite of lunar basalts. Our sample set included eight low-Ti basalts from the Apollo 12 and 15 collections, and 12 high-Ti basalts from Apollo 11 and 17 collections. In addition, we have determined the oxygen isotopic composition of an Apollo 15 KREEP (K - potassium, REE - Rare Earth Element, and P - phosphorus) basalt (sample 15386) and an Apollo 14 feldspathic mare basalt (sample 14053). Our data display a continuum in bulk-rock δ18O values, from relatively low values in the most Ti-rich samples to higher values in the Ti-poor samples, with the Apollo 11 sample suite partially bridging the gap. Calculation of bulk-rock δ18O values, using a combination of previously published oxygen isotope data on mineral separates from lunar basalts, and modal mineralogy (determined in this study), match with the measured bulk-rock δ18O values. This demonstrates that differences in mineral modal assemblage produce differences in mare basalt δ18O bulk-rock values. Differences between the low- and high-Ti mare basalts appear to be largely a reflection of mantle-source heterogeneities, and in particular, the highly variable distribution of ilmenite within the lunar mantle. Bulk δ18O variation in mare basalts is also controlled by fractional crystallisation of a few key mineral phases. Thus, ilmenite fractionation is important in the case of high-Ti Apollo 17 samples, whereas olivine plays a more dominant role for the low-Ti Apollo 12 samples.Consistent with the results of previous studies, our data reveal no detectable difference between the Δ17O of the Earth and Moon. The fact that oxygen three-isotope studies have been unable to detect a measurable difference at such high precisions reinforces doubts about the giant impact hypothesis as presently formulated.  相似文献   

19.
The total nitrogen contents of a number of lunar samples from Apollo 16 and 17 missions are reported. Solar wind is the main source for the observed excess nitrogen in most fines. Total nitrogen in the soils is found to be proportional to the solar wind rare gases Ar36 and Xe132. Linear correlations are also noted between the agglutinate contents of the soils and their carbon and nitrogen contents. Seventeen soils (Apollo 15, 16 and 17) have been sieved and nitrogen has been measured in various grain size fractions. An inverse correlation between the mean grain diameter and the nitrogen contents is seen, showing that a large fraction of the solar wind nitrogen is surface correlated. An apparent volume component, due to the presence of agglutinates, is found in most soils.  相似文献   

20.
Oxygen isotopic analyses were performed in the surface layers of lunar metallic grains from lunar regolith samples 71501 and 79035, presumably exposed at the Moon surface at different times. We were able to reproduce the two extreme O components previously found [Hashizume K. and Chaussidon M. (2005) A non-terrestrial 16O-rich isotopic composition for the protosolar nebula. Nature434, 619-622; Ireland T. R., Holden P., Norman M. D. and Clarke J. (2006) Isotopic enhancements of 17O and 18O from solar wind particles in the lunar regolith. Nature440, 776-778], with a range observed of −12 ± 5 < Δ17O < +33 ± 3‰ (1σ). The relatively minor 16O-rich component corresponding to an end-member Δ17O value lower than −20‰ is likely the solar component. This comes from the fact that its concentration roughly agrees with the maximum solar wind abundance expected among the grains from the two samples. At variance the 16O-poor component is 5-10 times more abundant and thus likely non-solar. The δ18O range found for the 16O-poor component may reflect various processes such as isotope exchange reaction during oxidation of metallic iron and/or isotope fractionation by evaporation/condensation at the surface of the Moon or during implantation at depth in the lunar metallic grains. The present study suggests that planetary solid materials in bulk are systematically depleted in 16O relative to the solar isotopic composition, suggesting the existence of non-mass-dependent isotopic fractionations associated to the formation of solids in the accretion disk.  相似文献   

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