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1.
Abstract— The noble gases He, Ne, Ar, Kr, and Xe were measured in 27 individual Antarctic micrometeorites (AMMs) in the size range 60 to 250 μm that were collected at the Dome Fuji Station. Eleven of the AMMs were collected in 1996 (F96 series) and 16 were collected in 1997 (F97 series). One of the F97 AMMs is a totally melted spherule, whereas all other particles are irregular in shape. Noble gases were extracted using a Nd‐YAG continuous wave laser with an output power of 2.5‐3.5 W for ?5 min. Most particles released measurable amounts of noble gases. 3He/4He ratios are determined for 26 AMMs ((0.85‐9.65) × 10?4). Solar energetic particles (SEP) are the dominant source of helium in most AMMs rather than solar wind (SW) and cosmogenic He. Three samples had higher 3He/4He ratios compared to that of SW, showing the presence of spallogenic 3He. The Ne isotopic composition of most AMMs resembled that of SEP as in the case of helium. Spallogenic 21Ne was detected in three samples, two of which had extremely long cosmic‐ray exposure ages (> 100 Ma), calculated by assuming solar cosmic‐ray (SCR) + galactic cosmic‐ray (GCR) production. These two particles may have come to Earth directly from the Kuiper Belt. Most AMMs had negligible amounts of cosmogenic 21 Ne and exposure ages of <1 Ma. 40Ar/36Ar ratios for all particles (3.9–289) were lower than that of the terrestrial atmosphere (296), indicating an extraterrestrial origin of part of the Ar with a very low 40Ar/36Ar ratio plus some atmospheric contamination. Indeed, 40Ar/36Ar ratios for the AMMs are higher than SW, SEP, and Q‐Ar values, which is explained by the presence of atmospheric 40Ar. The average 38Ar/36Ar ratio of 24 AMMs (0.194) is slightly higher than the value of atmospheric or Q‐Ar, suggesting the presence of SEP‐Ar which has a relatively high 38Ar/36Ar ratio. According to the elemental compositions of the heavy noble gases, Dome Fuji AMMs can be classified into three groups: chondritic (eight particles), air‐affected (nine particles), and solar‐affected (eight particles). The eight AMMs classified as chondritic preserve the heavy noble gas composition of primordial trapped component due to lack of atmospheric adsorption and solar implantation. The average of 129Xe/132Xe ratio for the 16 AMMs not affected by atmospheric contamination (1.05) corresponds to the values in matrices of carbonaceous chondrites (?1.04). One AMM, F96DK038, has high 129Xe/132Xe in excess of this ratio. Our results imply that most Dome Fuji AMMs originally had chondritic heavy noble gas compositions, and carbonaceous chondrite‐like objects are appropriate candidate sources for most AMMs.  相似文献   

2.
Abstract— The HF/HCI‐resistant residues of the chondrites CM2 Cold Bokkeveld, CV3 (ox.) Grosnaja, CO3.4 Lancé, CO3.7 Isna, LL3.4 Chainpur, and H3.7 Dimmitt have been measured by closed‐system stepped etching (CSSE) in order to better characterise the noble gases in “phase Q”, a major carrier of primordial noble gases. All isotopic ratios in phase Q of the different meteorites are quite uniform, except for (20Ne/22Ne)Q. As already suggested by precise earlier measurements (Schelhaas et al., 1990; Wieler et al., 1991, 1992), (20Ne/22Ne)Q is the least uniform isotopic ratio of the Q noble gases. The data cluster ～10.1 for Cold Bokkeveld and Lancé and 10.7 for Chainpur, Grosnaja, and Dimmitt, respectively. No correlation of (20Ne/22Ne)Q with the classification or the alteration history of the meteorites has been found. The Ar, Kr, and Xe isotopic ratios for all six samples are identical within their uncertainties and similar to earlier Q determinations as well as to Ar‐Xe in ureilites. Thus, an unknown process probably accounts for the alteration of the originally incorporated Ne‐Q. The noble gas elemental compositions provide evidence that Q consists of at least two carbonaceous carrier phases “Q1” and “Q2” with slightly distinct chemical properties. Ratios (Ar/Xe)Q and (Kr/Xe)Q reflect both thermal metamorphism and aqueous alteration. These parent‐body processes have led to larger depletions of Ar and Kr relative to Xe. In contrast, meteorites that suffered severe aqueous alteration, such as the CM chondrites, do not show depletions of He and Ne relative to Ar but rather the highest (He/Ar)Q and (Ne/Ar)Q ratios. This suggests that Q1 is less susceptible to aqueous alteration than Q2. Both subphases may well have incorporated noble gases from the same reservoir, as indicated by the nearly constant, though very large, depletion of the lighter noble gases relative to solar abundances. However, the elemental ratios show that Q1 and Q2 must have acquired (or lost) noble gases in slightly different element proportions. Cold Bokkeveld suggests that Q1 may be related to presolar graphite. Phases Q1 and Q2 might be related to the subphases that have been suggested by Gros and Anders (1977). The distribution of the 20Ne/22Ne ratios cannot be attributed to the carriers Q1 and Q2. The residues of Chainpur and Cold Bokkeveld contain significant amounts of Ne‐E(L), and the data confirm the suggestion of Huss (1997) that the 22Ne‐E(L) content, and thus the presolar graphite abundances, are correlated with the metamorphic history of the meteorites.  相似文献   

3.
Carbon and oxygen isotopes show no large anomalies on Venus (10–15%) or Mars (<5%); the high value of15N/14N found on Mars is explained by non-thermal escape of nitrogen. The isotopes of non-radiogenic noble gases in the atmosphere of Mars exhibit abundance patterns similar to those in the primordial component of meteoritic gases and in the Earth's atmosphere. This implies that gas fractionation took place in the inner solar nebula prior to planet formation. The relatively high value of129Xe on Mars emphasizes its deficiency on Earth, implying a difference in accretion histories of volatiles for the two planets. In the outer solar system, we find normal isotope ratios for nitrogen and carbon on Jupiter, and for carbon on Saturn, but precision is low (±15% at best). Controversy exists about the correct value of D/H, with current estimates ranging from 2.3±1.1 to 5.1±0.7×10–5. Planetary missions planned for the next few years should add considerably to the quantity and quality of these data.Paper presented at the Conference on Protostars and Planets, held at the Planetary Science Institute, University of Arizona, Tucson, Arizona, between January 3 and 7, 1978.  相似文献   

4.
Abstract— Ar‐rich noble gases, the so‐called “subsolar” noble gases, are a major component of heavy primordial noble gases in unequilibrated ordinary chondrites and some classes of anhydrous carbonaceous chondrites, whereas they are almost absent in hydrous carbonaceous chondrites that suffered extensive aqueous alteration. To understand the effects of aqueous alteration on the abundance of Ar‐rich noble gases, we performed an aqueous alteration experiments on the Ningqiang type 3 carbonaceous chondrite that consists entirely of anhydrous minerals and contains Ar‐rich noble gases. Powdered samples and deionized neutral water were kept at 200 °C for 10 and 20 days, respectively. Mineralogical analyses show that, during the 10‐day alteration, serpentine and hematite formed at the expense of olivine, low‐Ca pyroxene, and sulfide. Noble gas analyses show that the 10‐day alteration of natural Ningqiang removed 79% of the primordial 36Ar, 68% of the 84Kr, and 60% of the 132Xe, but only 45% of the 4He and 53% of the primordial 20Ne. Calculated elemental ratios of the noble gases removed during the 10‐day alteration are in the range of those of Ar‐rich noble gases. These results indicate that Ar‐rich noble gases are located in materials that are very susceptible to aqueous alteration. In contrast, heavy primordial noble gases remaining in the altered samples are close to Q gas in elemental and isotope compositions. This indicates that phase Q is much more resistant to aqueous alteration than the host phases of Ar‐rich noble gases. In the 20‐day sample, the mineralogical and noble gas signatures are basically similar to those of the 10‐day sample, indicating that the loss of Ar‐rich noble gases was completed within the 10‐day alteration. Our results suggest that almost all of the Ar‐rich noble gases were lost from primitive asteroids during early, low‐temperature aqueous alteration.  相似文献   

5.
Abstract— We studied the elemental and isotopic abundances of noble gases (He, Ne, Ar in most cases, and Kr, Xe also in some cases) in individual chondrules separated from six ordinary, two enstatite, and two carbonaceous chondrites. Most chondrules show detectable amounts of trapped 20Ne and 36Ar, and the ratio (36Ar/20Ne)t (from ordinary and carbonaceous chondrites) suggests that HL and Q are the two major trapped components. A different trend between (36Ar/20Ne)t and trapped 36Ar is observed for chondrules in enstatite chondrites indicating a different environment and/or mechanism for their formation compared to chondrules in ordinary and carbonaceous chondrites. We found that a chondrule from Dhajala chondrite (DH‐11) shows the presence of solar‐type noble gases, as suggested by the (36Ar/20Ne)t ratio, Ne‐isotopic composition, and excess of 4He. Cosmic‐ray exposure (CRE) ages of most chondrules are similar to their host chondrites. A few chondrules show higher CRE age compared to their host, suggesting that some chondrules and/or precursors of chondrules have received cosmic ray irradiation before accreting to their parent body. Among these chondrules, DH‐11 (with solar trapped gases) and a chondrule from Murray chondrite (MRY‐1) also have lower values of (21Ne/22Ne)c, indicative of SCR contribution. However, such evidences are sporadic and indicate that chondrule formation event may have erased such excess irradiation records by solar wind and SCR in most chondrules. These results support the nebular environment for chondrule formation.  相似文献   

6.
Abstract— We present an isotope study of noble gases in Divnoe, an anomalous meteorite, and also Rb-Sr and K-Ar dating of this meteorite. The relatively young Rb-Sr age obtained (3.39 Ga) seems doubtful and, most probably, results from weathering or contamination. The ancient K-Ar age (4.67+0.20–0.40), together with clear excess of 129Xe, allows the suggestion of very early formation of the Divnoe meteorite. Concentrations and isotope ratios of noble gases in Divnoe are: 17.9 ≤ 3He ≤ 29.0 × 10?8; 20Ne = 6.22 × 10?8; 2.44 ≤ 36Ar ≤ 5.10 × 10?8; 130Xe = 41.3 × 10?12 cm3/g; 0.079 ≤ 3He/4He ≤ 0.193; 20Ne/22Ne = 0.860; 21Ne/22Ne = 0.927; 3.47 ≤ 40Ar/36Ar ≤ 9.47; 2.22 ≤ 36Ar/38Ar ≤ 3.27; 129Xe/132Xe = 1.09. The exposure age calculated from cosmogenic 3He, 21Ne, and 38Ar is 17.9 ± 0.9 Ma. On the basis of the isotope data for the noble gases and O, and abundances of K, Rb, and Sr, an attempt was made to estimate the relationship of Divnoe to other meteorite types. The O-isotope characteristics of Divnoe are clearly distinct from those of ordinary chondrites, acapulcoites/lodranites, and SNC meteorites (Petaev et al., 1994, Clayton, 1993). In plots of 136Xe vs. 129Xe/130Xe, the Divnoe data fall outside of the data fields for carbonaceous and enstatite chondrites. The light noble gas data, especially the 40Ar/38Ar ratio, and the 40Ar, 38Ar, 3He, and 4He contents of Divnoe differ significantly from those of all meteorite types except diogenites. The K, Rb, and Sr abundances in Divnoe are substantially lower than in most other meteorites. In the concentrations of these elements, as well as in the REE pattern, the Divnoe meteorite is similar only to diogenites. Divnoe probably should be treated as a restite remaining after partial melting of the chondritic mantle of a parent asteroid body.  相似文献   

7.
The presence and compositions of atmospheres on the terrestrial planets do not follow directly from condensation models which would have Earth accreting near 500°K. No single mechanism yet proposed adequately accounts for the abundances of noble gases and carbon and nitrogen in the atmospheres. We show that the composition of clathrates forming at low temperatures in cold regions of the nebula can be predicted. Addition of about 1 ppm clathrate material to the Earth can explain observed abundances of Ar, Kr, and Xe. Condensation and adsorption processes occuring at 400–500°K are necessary to explain the observed abundances of Ne, H2O, C, and N. Possible sources of clathrates could be cometary bodies formed in the outer solar system.  相似文献   

8.
Abstract— Noble gases in two ureilites, Kenna and Allan Hills (ALH) 78019, were measured with two extraction methods: mechanical crushing in a vacuum and heating. Large amounts of noble gases were released by crushing, up to 26.5% of 132Xe from ALH 78019 relative to the bulk concentration. Isotopic ratios of the crush‐released Ne of ALH 78019 resemble those of the trapped Ne components determined for some ureilites or terrestrial atmosphere, while the crush‐released He and Ne from Kenna are mostly cosmogenic. The crush‐released Xe of ALH 78019 and Kenna is similar in isotopic composition to Q gas, which indicates that the crush‐released noble gases are indigenous and not caused by contamination from terrestrial atmosphere. In contrast to the similarities in isotopic composition with the bulk samples, light elements in the crush‐released noble gases are depleted relative to Xe and distinct from those of each bulk sample. This depletion is prominent especially in the 20Ne/132Xe ratio of ALH 78019 and the 36Ar/132Xe ratio of Kenna. The values of measured 3He/21Ne for the gases released by crushing are significantly higher than those for heating‐released gases. This suggests that host phases of the crush‐released gases might be carbonaceous because cosmogenic Ne is produced mainly from elements with a mass number larger than Ne. Based on our optical microscopic observation, tabular‐foliated graphite is the major carbon mineral in ALH 78019, while Kenna contains abundant polycrystalline graphite aggregates and diamonds along with minor foliated graphite. There are many inclusions at the edge and within the interior of olivine grains that are reduced by carbonaceous material. Gaps can be seen at the boundary between carbonaceous material and silicates. Considering these petrologic and noble gas features, we infer that possible host phases of crush‐released noble gases are graphite, inclusions in reduction rims, and gaps between carbonaceous materials and silicates. The elemental ratios of noble gases released by crushing can be explained by fractionation, assuming that the starting noble gas composition is the same as that of amorphous carbon in ALH 78019. The crush‐released noble gases are the minor part of trapped noble gases in ureilites but could be an important clue to the thermal history of the ureilite parent body. Further investigation is needed to identify the host phases of the crush‐released noble gases.  相似文献   

9.
We review the origin and evolution of the atmospheres of Earth, Venus and Mars from the time when their accreting bodies were released from the protoplanetary disk a few million years after the origin of the Sun. If the accreting planetary cores reached masses \(\ge 0.5 M_\mathrm{Earth}\) before the gas in the disk disappeared, primordial atmospheres consisting mainly of H\(_2\) form around the young planetary body, contrary to late-stage planet formation, where terrestrial planets accrete material after the nebula phase of the disk. The differences between these two scenarios are explored by investigating non-radiogenic atmospheric noble gas isotope anomalies observed on the three terrestrial planets. The role of the young Sun’s more efficient EUV radiation and of the plasma environment into the escape of early atmospheres is also addressed. We discuss the catastrophic outgassing of volatiles and the formation and cooling of steam atmospheres after the solidification of magma oceans and we describe the geochemical evidence for additional delivery of volatile-rich chondritic materials during the main stages of terrestrial planet formation. The evolution scenario of early Earth is then compared with the atmospheric evolution of planets where no active plate tectonics emerged like on Venus and Mars. We look at the diversity between early Earth, Venus and Mars, which is found to be related to their differing geochemical, geodynamical and geophysical conditions, including plate tectonics, crust and mantle oxidation processes and their involvement in degassing processes of secondary \(\hbox {N}_2\) atmospheres. The buildup of atmospheric \(\hbox {N}_2\), \(\hbox {O}_2\), and the role of greenhouse gases such as \(\hbox {CO}_2\) and \(\hbox {CH}_4\) to counter the Faint Young Sun Paradox (FYSP), when the earliest life forms on Earth originated until the Great Oxidation Event \(\approx \) 2.3 Gyr ago, are addressed. This review concludes with a discussion on the implications of understanding Earth’s geophysical and related atmospheric evolution in relation to the discovery of potential habitable terrestrial exoplanets.  相似文献   

10.
Abstract— We have carried out noble gas measurements on graphite from a large graphite‐metal inclusion in Canyon Diablo. The Ne data of the low‐temperature fractions lie on the mixing line between air and the spallogenic component, but those of high temperatures seem to lie on the mixing line between Ne‐HL and the spallogenic component. The Ar isotope data indicate the presence of Q in addition to air, spallogenic component and Ar‐HL. As the elemental concentration of Ne in Q is low, we could not detect the Ne‐Q from the Ne data. On the other hand, we could not observe Xe‐HL in our Xe data. As the Xe concentration and the Xe/Ne ratio in Q is much higher than that in the HL component, it is likely that only the contribution of Q is observed in the Xe data. Xenon isotopic data can be explained as a mixture of Q, air, and “El Taco Xe.” The Canyon Diablo graphite contains both HL and Q, very much like carbonaceous chondrites, retaining the signatures of various primordial noble gas components. This indicates that the graphite was formed in a primitive nebular environment and was not heated to high, igneous temperatures. Furthermore, a large excess of 129Xe was observed, which indicates that the graphite was formed at a very early stage of the solar system when 129I was still present. The HL/Q ratios in the graphite in Canyon Diablo are lower than those in carbonaceous chondrites, indicating that some thermal metamorphism occurred on the former. We estimated the temperature of the thermal metamorphism to about 500–600 °C from the difference of thermal retentivities of HL and Q. It is also noted that “El Taco Xe” is commonly observed in many IAB iron meteorites, but its presence in carbonaceous chondrites has not yet been established.  相似文献   

11.
Abstract— Chondrules are generally believed to have lost most or all of their trapped noble gases during their formation. We tested this assumption by measuring He, Ne, and Ar in chondrules of the carbonaceous chondrites Allende (CV3), Leoville (CV3), Renazzo (CR2), and the ordinary chondrites Semarkona (LL3.0), Bishunpur (LL3.1), and Krymka (LL3.1). Additionally, metalsulfide‐rich chondrule coatings were measured that probably formed from chondrule metal. Low primordial 20Ne concentrations are present in some chondrules, while even most of them contain small amounts of primordial 36Ar. Our preferred interpretation is that‐in contrast to CAIs‐the heating of the chondrule precursor during chondrule formation was not intense enough to expel primordial noble gases quantitatively. Those chondrules containing both primordial 20Ne and 36Ar show low presolar‐diamond‐like 36Ar/20Ne ratios. In contrast, the metal‐sulfide‐rich coatings generally show higher gas concentrations and Q‐like 36Ar/20Ne ratios. We propose that during metalsilicate fractionation in the course of chondrule formation, the Ar‐carrying phase Q became enriched in the metal‐sulfide‐rich chondrule coatings. In the silicate chondrule interior, only the most stable Ne‐carrying presolar diamonds survived the melting event leading to the low observed 36Ar/20Ne ratios. The chondrules studied here do not show evidence for substantial amounts of fractionated solar‐type noble gases from a strong solar wind irradiation of the chondrule precursor material as postulated by others for the chondrules of an enstatite chondrite.  相似文献   

12.
We used chemical equilibrium and chemical kinetic calculations to model chemistry of the volatiles released by heating different types of carbonaceous, ordinary and enstatite chondritic material as a function of temperature and pressure. Our results predict the composition of atmospheres formed by outgassing during accretion of the Earth and other terrestrial planets. Outgassing of CI and CM carbonaceous chondritic material produces H2O-rich (steam) atmospheres in agreement with the results of impact experiments. However, outgassing of other types of chondritic material produces atmospheres dominated by other gases. Outgassing of ordinary (H, L, LL) and high iron enstatite (EH) chondritic material yields H2-rich atmospheres with CO and H2O being the second and third most abundant gases. Outgassing of low iron enstatite (EL) chondritic material gives a CO-rich atmosphere with H2, CO2, and H2O being the next most abundant gases. Outgassing of CV carbonaceous chondritic material gives a CO2-rich atmosphere with H2O being the second most abundant gas. Our results predict that the atmospheres formed during accretion of the Earth and Mars were probably H2-rich unless the accreted material was dominantly CI and CM carbonaceous chondritic material. We also predict significant amounts of S, P, Cl, F, Na, and K in accretionary atmospheres at high temperatures (1500-2500 K). Finally, our results may be useful for interpreting spectroscopic observations of accreting extrasolar terrestrial planets.  相似文献   

13.
No meteorites from Mercury and Venus have been conclusively identified so far. In this study, we develop an original approach based on extensive Monte Carlo simulations and diffusion models to explore the radiogenic argon (40Ar*) and helium (4He*) loss behavior and the range of 40Ar/39Ar and (U‐Th)/He age signatures expected for a range of crystals if meteorites from these planets were ever to be found. We show that we can accurately date the crystallization age of a meteorite from both Mercury and Venus using the 40Ar/39Ar technique on clinopyroxene (± orthopyroxene) and that its 40Ar/39Ar age should match the Pb‐Pb age. At the surface of Mercury, phases like albite and anorthite will exhibit a complete range of 40Ar* loss ranging from 0% to 100%, whereas merrillite and apatite will show 100% 4He* loss. By measuring the crystal size and diffusion parameters of a series of plagioclase crystals, one can inverse the 40Ar* loss value to estimate the maximum temperature experienced by a rock, and narrow down the possible pre‐ejection location of the meteorite at the surface of Mercury. At the surface of Venus, plagioclase and phosphate phases will only record the age of ejection. The (U‐Th)/He systematics of merrillite and apatite will be, respectively, moderately and strongly affected by 4He* loss during the transit of the meteorite from its host planet to Earth. Finally, meteorites from Mercury or Venus will each have their own 40Ar/39Ar and (U‐Th)/He isotopic age and 38Arc cosmic ray exposure age signatures over a series of different crystal types, allowing to unambiguously recognize a meteorite for any of these two planets using radiogenic and cosmogenic noble gases.  相似文献   

14.
Lunar basalt 15016 (~3.3 Ga) is among the most vesicular (50% by volume) basalts recovered by the Apollo missions. We investigated the possible occurrence of indigenous lunar nitrogen and noble gases trapped in vesicles within basalt 15016, by crushing several cm-sized chips. Matrix/mineral gases were also extracted from crush residues by fusion with a CO2 laser. No magmatic/primordial component could be identified; all isotope compositions, including those of vesicles, pointed to a cosmogenic origin. We found that vesicles contained ~0.2%, ~0.02%, ~0.002%, and ~0.02% of the total amount of cosmogenic 21Ne, 38Ar, 83Kr, and 126Xe, respectively, produced over the basalt's 300 Myr of exposure. Diffusion/recoil of cosmogenic isotopes from the basaltic matrix/minerals to intergrain joints and vesicles is discussed. The enhanced proportion of cosmogenic Xe isotopes relative to Kr detected in vesicles could be the result of kinetic fractionation, through which preferential retention of Xe isotopes over Kr within vesicles might have occurred during diffusion from the vesicle volume to the outer space through microleaks. This study suggests that cosmogenic loss, known to be significant for 3He and 21Ne, and to a lesser extent for 36Ar (Signer et al. 1977 ), also occurs to a negligible extent for the heaviest noble gases Kr and Xe.  相似文献   

15.
108 +/- 11 photons of the martian He 584-angstroms airglow detected by the Extreme Ultraviolet Explorer satellite during a 2-day exposure (January 22-23, 1993) correspond to the effective disk average intensity of 43 +/- 10 Rayleigh. Radiative transfer calculations, using a model atmosphere appropriate to the conditions of the observation and having an exospheric temperature of 210 +/- 20 K, result in a He mixing ratio of 1.1 +/- 0.4 ppm in the lower atmosphere. Nonthermal escape of helium is due to electron impact ionization and pickup of He+ by the solar wind, to collisions with hot oxygen atoms, and to charge exchange with molecular species with corresponding column loss rates of 1.4 x 10(5), 3 x 10(4), and 7 x 10(3) cm-2 sec-1, respectively. The lifetime of helium on Mars is 5 x 10(4) years. The He outgassing rate, coupled with the 40Ar atmospheric abundance and with the K:U:Th ratio measured in the surface rocks, is used as input to a single two-reservoir degassing model which is applied to Mars and then to Venus. A similar model with known abundances of K, U, and Th is applied to Earth. The models for Earth and Mars presume loss of all argon accumulated in the atmospheres during the first billion years by large-scale meteorite and planetesimal impacts. The models show that the degassing coefficients for all three planets may be approximated by function delta = delta (0)(t(0)/t)1/2 with delta (0) = 0/1, 0.04, and 0.0125 Byr-1 for Earth, Venus, and Mars, respectively. After a R2 correction this means that outgassing processes on Venus and Mars are weaker than on Earth by factors of 3 and 30, respectively. Mass ratios of U and Th are almost the same for all three planets, while potassium is depleted by a factor of 2 in Venus and Mars. Mass ratios of helium and argon are close to 5 x 10(-9) and 2 x 10(-8) g/g in the interiors of all three planets. The implications of these results are discussed.  相似文献   

16.
Abstract— From November 1998 to January 1999, the 39th Japanese Antarctic Research Expedition (JARE) conducted a large‐scale micrometeorite collection at 3 areas in the meteorite ice field around the Yamato Mountains, Antarctica. The Antarctic micrometeorites (AMMs) collected were ancient cosmic dust particles. This is in contrast with the Dome Fuji AMMs, which were collected previously from fresh snows in 1996 and 1997 and which represent modern micrometeorites. To determine the noble gas concentrations and isotopic compositions of individual AMMs, noble gas analyses were carried out using laser‐gas extraction for 35 unmelted Yamato Mountains AMMs and 3 cosmic spherules. X‐ray diffraction analyses were performed on 13 AMMs before the noble gas measurement and mineral compositions were determined. AMMs are classified into 4 main mineralogical groups, defined from the heating they suffered during atmospheric entry. Heating temperatures of AMMs, inferred from their mineral compositions, are correlated with 4He concentrations and reflect the effect of degassing during atmospheric entry. Jarosite, an aqueous alteration product, is detected for 4 AMMs, indicating the aqueous alteration during long‐time storage in Antarctic ice. Jarosite‐bearing AMMs have relatively low concentrations of 4He, which is suggestive of loss during the alteration. High 3He/4He ratios are detected for AMMs with high 20Ne/4He ratios, showing both cosmogenic 3He and preferential He loss. SEP (solar energetic particles)‐He and Ne, rather than the solar wind (SW), were dominant in AMMs, presumably showing a preferential removal of the more shallowly implanted SW by atmospheric entry heating. The mean 20Ne/22Ne ratio is 11.27 ± 0.35, which is close to the SEP value of 11.2. Cosmogenic 21Ne is not detected in any of the particles, which is probably due to the short cosmic ray exposure ages. Ar isotopic compositions are explained by 3‐component mixing of air, Q, and SEP‐Ar. Ar isotopic compositions can not be explained without significant contributions of Q‐Ar. SEP‐Ne contributed more than 99% of the total Ne. As for 36Ar and 38Ar, the abundance of the Q component is comparable to that of the SEP component. 84Kr and 132Xe are dominated by the primordial component, and solar‐derived Xe is almost negligible.  相似文献   

17.
Effects of aqueous alteration on primordial noble gas carriers were investigated by analyzing noble gases and determining presolar SiC abundances in insoluble organic matter (IOM) from four Tagish Lake meteorite (C2‐ung.) samples that experienced different degrees of aqueous alteration. The samples contained a mixture of primordial noble gases from phase Q and presolar nanodiamonds (HL, P3), SiC (Ne‐E[H]), and graphite (Ne‐E[L]). The second most altered sample (11i) had a ~2–3 times higher Ne‐E concentration than the other samples. The presolar SiC abundances in the samples were determined from NanoSIMS ion images and 11i had a SiC abundance twice that of the other samples. The heterogeneous distribution of SiC grains could be inherited from heterogeneous accretion or parent body alteration could have redistributed SiC grains. Closed system step etching (CSSE) was used to study noble gases in HNO3‐susceptible phases in the most and least altered samples. All Ne‐E carried by presolar SiC grains in the most altered sample was released during CSSE, while only a fraction of the Ne‐E was released from the least altered sample. This increased susceptibility to HNO3 likely represents a step toward degassing. Presolar graphite appears to have been partially degassed during aqueous alteration. Differences in the 4He/36Ar and 20Ne/36Ar ratios in gases released during CSSE could be due to gas release from presolar nanodiamonds, with more He and Ne being released in the more aqueously altered sample. Aqueous alteration changes the properties of presolar grains so that they react similar to phase Q in the laboratory, thereby altering the perceived composition of Q.  相似文献   

18.
Geological exploration of the solar system shows that solid-surfaced planets and satellites are subject to endogenic processes (volcanism and tectonism) and exogenic processes (impact cratering and gradation). The present appearance of planetary suffaces is the result of the complex interplay of these processes and is the linked to the evolution of planets and their environments. Terrestrial planets that have dynamic atmospheres are Earth, Mars, and Venus. Atmospheric interaction with the surfaces of these planets, oraeolian activity, is a form of gradation. The manifestation of aeolian activity is the weathering and erosion of rocks into sediments, transportation of the weathered debris (mostly sand and dust) by the wind, and deposition of windblown material. Wind-eroded features include small-scale ventifacts (wind-sculptured rocks) and large-scale landforms such as yardangs. Wind depositional features include dunes, drifts, and mantles of windblown sediments. These and other aeolian features are observed on Earth, Mars, and Venus.  相似文献   

19.
Abstract– We have determined the elemental abundances and the isotopic compositions of noble gases in a bulk sample and an HF/HCl residue of the Saratov (L4) chondrite using stepwise heating. The Ar, Kr, and Xe concentrations in the HF/HCl residue are two orders of magnitude higher than those in the bulk sample, while He and Ne concentrations from both are comparable. The residue contains only a portion of the trapped heavy noble gases in Saratov; 40 ± 9% for 36Ar, 58 ± 12% for 84Kr, and 48 ± 10% for 132Xe, respectively. The heavy noble gas elemental pattern in the dissolved fraction is similar to that in the residue but has high release temperatures. Xenon isotopic ratios of the HF/HCl residue indicate that there is no Xe‐HL in Saratov, but Ne isotopic ratios in the HF/HCl residue lie on a straight line connecting the cosmogenic component and a composition between Ne‐Q and Ne‐HL. This implies that the Ne isotopic composition of Q has been changed by incorporating Ne‐HL (Huss et al. 1996) or by being mass fractionated during the thermal metamorphism. However, it is most likely that the Ne‐Q in Saratov is intrinsically different from this component in other meteorites. The evidence of this is a lack of correlation between the isotopic ratio of Ne‐Q and petrologic types of meteorites (Busemann et al. 2000). A neutron capture effect was observed in the Kr isotopes, and this process also affected the 128Xe/132Xe ratio. The 3He and 21Ne exposure ages for the bulk sample are 33 and 35 Ma, respectively.  相似文献   

20.
Abstract— The Devgaon meteorite fell in India on February 12, 2001 and was immediately collected. It is an ordinary chondrite having a number of SiO2‐rich objects and some Ca, Al‐rich inclusions. Olivines (Fa17–19) are fairly equilibrated, while pyroxenes (Fs4–20) are unequilibrated. Occasionally, shock veins are visible, but the bulk rock sample is very weakly shocked (S2). Chondrules and chondrule fragments are abundant. Based on chemical and petrological features, Devgaon is classified as an H3.8 group chondrite. Several cosmogenic radionuclides ranging in half‐lives from 5.6 d (52Mn) to 7.3 times 105 yr (26Al), noble gases (He, Ne, Ar, Kr, and Xe), and particle track density have been measured. The track density in olivines from five spot samples varies between (4.6 to 9) × 106 cm?2 showing a small gradient within the meteorite. The light noble gases are dominated by cosmogenic and radiogenic components. Large amounts of trapped gases (Ar, Kr, and Xe) are present. In addition, (n, γ) products from Br and I are found in Kr and Xe, respectively. The average cosmic ray exposure age of 101 ± 8 Ma is derived based on cosmogenic 38Ar, 83Kr, and 126Xe. The track production rates correspond to shielding depths of about 4.9 to 7.8 cm, indicating that the stone suffered type IV ablation. Low 60Co, high (22Ne/21Ne)c, and large neutron produced excesses at 80Kr, 82Kr, and 128Xe indicate a complex exposure history of the meteoroid. In the first stage, a meter‐sized body was exposed for nearly 108 yr in the interplanetary space that broke up in ?50 cm‐sized fragments about a million years ago (stage 2), before it was captured by the Earth.  相似文献   

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