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1.
The Sulagiri meteorite fell in India on 12 September 2008,LL6 chondrite class is the largest among all the Indian meteorites.Isotopic compositions of noble gases(He,Ne,Ar,Kr and Xe) and nitrogen in the Sulagiri meteorite and cosmic ray exposure history are discussed.Low cosmogenic(~(22)Ne/~(21)Ne)_c ratio is consistent with irradiation in a large body.Cosmogenic noble gases indicate that Sulagiri has a 4πcosmic-ray exposure(CRE) age of 27.9 ± 3.4 Ma and is a member of the peak of CRE age distribution of IX chondrites.Radiogenic ~4He and ~(40)Ar concentrations in Sulagiri yields the radiogenic ages as 2.29 and4.56 Ca,indicating the loss of He from the meteorite.Xenon and krypton are mixture of Q and spallogenic components.  相似文献   

2.
Mineralogical, chemical, textural, and isotopic studies of the abundant carbonaceous inclusions in the Jodzie howardite are consistent with CM characteristics. These CM xenoliths show regolith alteration on a level comparable to the Murray and Murchison meteorites but less than Nogoya, flow-oriented development of phyllosilicates and ‘poorly characterized phases’, and partial oxidation of sulfides. Temperature-programmed pyrolysis mass spectrometry (25°–1400°C) indicates that gas release patterns of volatiles and hydrocarbon components and percent contents of N(0.15), C(2.3) and S(2.4) are typical of CM meteorites. Release of significant amounts of SO2 is attributed to the thermal breakdown of ‘poorly characterized phases’ (Fe-Ni-C-S-O) that formed during low temperature aqueous alteration in the CM parent body.Noble gas abundances are well within the reported range of CM meteorites. The fact that the Ne composition is typical for ‘solar’ values and the isotopic structure of Xe is ‘planetary’ argues that these gases were entrapped by different mechanisms. Cosmic ray exposure ages for the xenoliths (3He, 5 × 106; 21Ne, 6.7 × 106; 38Ar, 6.9 × 106 yr) agree with the reported exposure age for the eucritic host. Volatile abundances, presence of intact organic molecules, and phyllosilicates in the CM xenoliths preclude regolith temperatures in excess of 200°C after CM incorporation. Mixing of the host and xenoliths probably occurred during a low-velocity collision of main belt asteroids.  相似文献   

3.
Based on the analysis of data in [1, 2] on the concentrations of noble gases and the cosmic ray exposure age (CREA) of chromite grains in fossil meteorites, it was demonstrated in [3] that the distributions of gas concentrations and cosmic ray exposure ages can be explained under the assumption of the fall of a single meteorite in the form of a meteorite shower in southern Sweden less than 0.2 Ma after the catastrophic destruction of the parental body (asteroid) of L chondrites in space at approximately 470 Ma. This assumption differs from the conclusion in [1, 2, 4] about the long-lasting (for 1–2 Ma) delivery of L chondrites to the Earth, with the intensity of the flux of this material one to two orders of magnitude greater than now. The analysis of newly obtained data on samples from the Brunflo fossil meteorite [5] corroborates the hypothesis of a meteorite shower produced by the fall of a single meteorite. The possible reason for the detected correlations between the cosmic ray exposure ages of meteorites and the masses of the samples with the 20Ne concentrations can be the occurrence of Ne of anomalous isotopic composition in the meteorites.  相似文献   

4.
Enstatite achondrites (aubrites) when compared to other stone meteorites have unusually long cosmic-ray exposure (CRE) ages. We report here the 150Sm/149Sm and 158Gd/157Gd ratios in six different structural phases, i.e., light and dark (shocked) grains and in matrix materials of Pesyanoe, in three different fragments from Pena Blanca Spring, and in one from Norton County, Shallowater, and Khor Temiki, to investigate the regolith history on the aubrite parent body. The results from phases components of Pesyanoe confirm earlier reported evidence for regolith irradiation of several aubrites. The inferred neutron fluences for six Pesyanoe separates vary between (2.13 and 2.82) × 1016 n cm−2. The fluences also significantly exceed those expected from cosmic-ray irradiation during transit to Earth and approach those observed in the lunar regolith. These observations confirm that the brecciated Pesyanoe meteorite, which contains solar wind (SW) gases only in dark phases, was processed in a regolith and that structural phases were differentially irradiated before compaction. On the other hand, in some aubrites (Mt. Egerton, Shallowater, Pena Blanca Spring, Norton County) neutron capture effects may entirely be due to space irradiation.  相似文献   

5.
Noble gas measurements were performed for nine aubrites: Bishopville, Cumberland Falls, Mayo Belwa, Mount Egerton, Norton County, Peña Blanca Spring, Shallowater, ALHA 78113 and LAP 02233. These data clarify the origins and histories, particularly cosmic-ray exposure and regolith histories, of the aubrites and their parent body(ies). Accurate cosmic-ray exposure ages were obtained using the 81Kr-Kr method for three meteorites: 52 ± 3, 49 ± 10 and 117 ± 14 Ma for Bishopville, Cumberland Falls and Mayo Belwa, respectively. Mayo Belwa shows the longest cosmic-ray exposure age determined by the 81Kr-Kr method so far, close to the age of 121 Ma for Norton County. These are the longest ages among stony meteorites. Distribution of cosmic-ray exposure ages of aubrites implies 4-9 break-up events (except anomalous aubrites) on the parent body. Six aubrites show “exposure at the surface” on their parent body(ies): (i) neutron capture 36Ar, 80Kr, 82Kr and/or 128Xe probably produced on the respective parent body (Bishopville, Cumberland Falls, Mayo Belwa, Peña Blanca Spring, Shallowater and ALHA 78113); and/or (ii) chondritic trapped noble gases, which were likely released from chondritic inclusions preserved in the aubrite hosts (Cumberland Falls, Peña Blanca Spring and ALHA 78113). The concentrations of 128Xe from neutron capture on 127I vary among four measured specimens of Cumberland Falls (0.5-76 × 10−14 cm3STP/g), but are correlated with those of radiogenic 129Xe, implying that the concentrations of (128Xe)n and (129Xe)rad reflect variable abundances of iodine among specimens. The ratios of (128Xe)n/(129Xe)rad obtained in this work are different for Mayo Belwa (0.045), Cumberland Falls (0.015) and Shallowater (0.001), meaning that neutron fluences, radiogenic 129Xe retention ages, or both, are different among these aubrites. Shallowater contains abundant trapped Ar, Kr and Xe (2.2 × 10−7, 9.4 × 10−10 and 2.8 × 10−10 cm3STP/g, respectively) as reported previously (Busemann and Eugster, 2002). Isotopic compositions of Kr and Xe in Shallowater are consistent with those of Q (a primordial noble gas component trapped in chondrites). The Ar/Kr/Xe compositions are somewhat fractionated from Q, favoring lighter elements. Because of the unbrecciated nature of Shallowater, Q-like noble gases are considered to be primordial in origin. Fission Xe is found in Cumberland Falls, Mayo Belwa, Peña Blanca Spring, ALHA 78113 and LAP 02233. The majority of fission Xe is most likely 244Pu-derived, and about 10-20% seems to be 238U-derived at 136Xe. The observed (136Xe)Pu corresponds to 0.019-0.16 ppb of 244Pu, from which the 244Pu/U ratios are calculated as 0.002-0.009. These ratios resemble those of chondrites and other achondrites like eucrites, suggesting that no thermal resetting of the Pu-Xe system occurred after ∼4.5 Ga ago. We also determined oxygen isotopic compositions for four aubrites with chondritic noble gases and a new aubrite LAP 02233. In spite of their chondritic noble gas signatures, oxygen with chondritic isotopic compositions was found only in a specimen of Cumberland Falls (Δ17O of ∼0.3‰). The other four aubrites and the other two measured specimens of Cumberland Falls are concurrent with the typical range for aubrites.  相似文献   

6.
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   

7.
Abundances of cosmic ray-produced noble gases and 26Al, including some new measurements, have been compiled for some 23 stone meteorites with exposure ages of < 3 × 106 yr. Concentrations of cosmogenic He, Ne, and Ar in these meteorites have been corrected for differences in target element abundances by normalization to L-chondrite chemistry. Combined noble gas measurements in depth samples of the Keyes and St. Séverin chondrites are utilized to derive equations for normalizing the production rates of cosmogenic 3He, 21Ne, and 38Ar in chondrites to an adopted ‘average’ shielding: 22Ne21Ne = 1.114. The measured unsaturated 26Al concentrations and the calculated equilibrium 26Al for these meteorites are combined to estimate exposure ages. These exposure ages are statistically compared with chemistry- and shielding-corrected concentrations of cosmogenic He, Ne, and Ar to derive absolute production rates for these nuclides. For L-chondrites, at ‘average’ shielding, these production rates (in 10?8 cm3/g 106 yr) are: 3He = 2.45,21Ne = 0.47, and 38Ar = 0.069, which are ~ 25% higher than production rates used in the past. From these production rates and relative chemical correction factors, production rates for other classes of stone meteorites are derived.  相似文献   

8.
We determined the He, Ne, and Ar isotopic abundances in the four acapulcoites Dhofar (DHO) 125, DHO 290, DHO 312, and Graves Nunataks 98028, the metal-rich diogenite Northwest Africa (NWA) 1982, and a unique achondrite, NWA 1058, that resembles the acapulcoites in its chemical composition. The noble gases in these meteorites consist of three components: trapped gases, cosmic-ray produced nuclides, and nuclides produced by K, Th, and U decay. The four acapulcoites yield cosmic-ray exposure (CRE) ages in the range of 5.0-5.7 Ma and confirm earlier conclusions concerning break-up of all acapulcoites from a common S-type parent asteroid, possibly in three events 4.9, 5.9, and 14.8 Ma ago. We also discuss the other characteristics (mineralogy, chemistry, formation ages, and oxygen and trapped noble gas isotopes) of all other acapulcoites and their relatives, the lodranites. We propose that the acapulcoite/lodranite parent asteroid had a shell structure similar to that of the H chondrites: The less metamorphosed acapulcoites correspond to the H3 and H4 chondrites and originate from the exterior layers, whereas the more severely metamorphosed lodranites, similar to the H5 and H6 chondrites, represent the inner regions of their parent body. Ungrouped achondrite NWA 1982, probably a diogenite, shows a CRE age of 18.9 ± 2.0 Ma that falls on the major exposure age cluster of the diogenites. The unique achondrite NWA 1058 differs in cosmic-ray exposure age (38.9 ± 4.0 Ma) and in oxygen-isotopic composition from the acapulcoites and lodranites and is probably a winonaite.  相似文献   

9.
The Antarctic lunar meteorite Meteorite Hills (MET) 01210 is a polymict regolith breccia, dominantly composed of mare basalt components. One relatively large (2.7 × 4.7 mm) basalt clast in MET 01210 (MET basalt) shows remarkable mineralogical similarities to the lunar-meteorite crystalline mare basalts Yamato (Y)-793169, Asuka (A)-881757, and Miller Range (MIL) 05035. All four basalts have similar rock texture, mineral assemblage, mineral composition, pyroxene crystallization trend, and pyroxene exsolution lamellae. The estimated TiO2 contents (∼2.0 wt%) of the MET basalt and MIL 05035 are close to the bulk-rock TiO2 contents of Y-793169 and A-881757. These similarities suggest that Y-793169, A-881757, MIL 05035, and the MET basalt came from the same basalt flow, which we designate the YAMM basalt. The source-basalt pairing of the YAMM is also supported by their similar REE abundances, crystallization ages (approx. 3.8-3.9 Ga), and isotopic compositions (low U/Pb, low Rb/Sr, and high Sm/Nd). The pyroxene exsolution lamellae, which are unusually coarse (up to a few microns) by mare standards, imply a relatively slow cooling in an unusually thick lava and/or subsequent annealing within a cryptomare. Reported noble gas and CRE data with close launch ages (∼1 Ma) and ejection depths (deeper than several meters) among the four meteorites further indicate their simultaneous ejection from the moon. Despite the marginally close terrestrial ages, pairing in the conventional Earth-entry sense seems unlikely because of the remote recovery sites among the YAMM meteorites.The high abundance (68%) of mare components in MET 01210 estimated from a two-component mixing model calculation could have resulted from either lateral mixing at a mare-highland boundary or vertical mixing in a cryptomare. The proportion of mare materials in MET 01210 is greater than in Apollo core samples at the mare-highland boundary. The burial depth (>several meters deep) inferred from the lack of surface irradiation of MET 01210 exceeds the typical mare regolith thickness (a few meters). Thus, the source of the YAMM meteorites is likely a terrain of locally high mare-highland mixing within a cryptomare. We searched for a possible source crater of the YAMM meteorites within the well-defined cryptomare, based on the multiple constraints obtained from this study and published data. An unnamed 1.4 km-diameter crater (53°W, 44.5°S) on the floor of the Schickard crater is the most suitable source for the YAMM meteorites.The 238U/204Pb (μ) value of the YAMM basalts is extremely low, relative to those of the Apollo mare basalts, but comparable to those of the Luna 24 very low-Ti basalts. The low-μ source indicates a derivation from a less differentiated mantle with a lack of KREEP components. Although the chemical sources of materials and heat source of melting might be independent, the heat source that generated the source magma of the YAMM and Luna 24 basalts may not be related to KREEP, unlike the case of the Apollo basalts. The distinct chemical and isotopic compositions of mantle sources between the Apollo basalts and the YAMM/Lunar 24 basalts imply differences in mantle composition and thermal evolution between the Procellarum KREEP Terrane (PKT) and non-PKT regions of the nearside.  相似文献   

10.
Two examined fragments of the Kaidun meteorite principally differ in the concentrations of isotopes of noble gases and are very heterogeneous in terms of the isotopic composition of the gases. Because these fragments belong to two basically different types of meteoritic material (EL and CR chondrites), these characteristics of noble gases could be caused by differences in the cosmochemical histories of the fragments before their incorporation into the parent asteroid. As follows from the escape kinetics of all gases, atoms of trapped and cosmogenic noble gases are contained mostly in the structures of two carrier minerals in the samples. The concentrations and proportions of the concentrations of various primary noble gases in the examined fragments of Kaidun are obviously unusual compared to data on most currently known EL and CR meteorites. In contrast to EL and CR meteorites, which contain the primary component of mostly solar provenance, the elemental ratios and isotopic composition of Ne and He in the fragments of Kaidun correspond to those typical of the primary components of A and Q planetary gases. This testifies to the unique conditions under which the bulk of the noble gases were trapped from the early protoplanetary nebula. The apparent cosmic-ray age of both of the Kaidun fragments calculated based on cosmogenic isotopes from 3He to 126Xe varies from 0.027 to 246 Ma as a result of the escape of much cosmogenic isotopes at relatively low temperatures. The extrapolated cosmic-ray age of the Kaidun meteorite, calculated from the concentrations of cosmogenic isotopes of noble gases, is as old as a few billion years, which suggests that the material of the Kaidun meteorite could be irradiated for billions of years when residing in an unusual parent body.  相似文献   

11.
Meteorite “finds” from the terrestrial hot deserts have become a major contributor to the inventory of Martian meteorites. In order to understand their nitrogen and noble gas components, we have carried out stepped heating experiments on samples from two Martian meteorites collected from hot deserts. We measured interior and surface bulk samples, glassy and non-glassy portions of Dar al Gani 476 and Sayh al Uhaymir 005. We have also analyzed noble gases released from the Antarctic shergottite Lewis Cliff 88516 by crushing and stepped heating. For the hot desert meteorites significant terrestrial Ar, Kr, Xe contamination is observed, with an elementally fractionated air (EFA) component dominating the low temperature releases. The extremely low Ar/Kr/Xe ratios of EFA may be the result of multiple episodes of trapping/loss during terrestrial alteration involving aqueous fluids. We suggest fractionation processes similar to those in hot deserts to have acted on Mars, with acidic weathering on the latter possibly even more effective in producing elementally fractionated components. Addition from fission xenon is apparent in DaG 476 and SaU 005. The Ar-Kr-Xe patterns for LEW 88516 show trends as typically observed in shergottites - including evidence for a crush-released component similar to that observed in EETA 79001. A trapped Ne component most prominent in the surface sample of DaG 476 may represent air contamination. It is accompanied by little trapped Ar (20Ne/36Ar > 50) and literature data suggest its presence also in some Antarctic finds. Data for LEW 88516 and literature data, on the other hand, suggest the presence of two trapped Ne components of Martian origin characterized by different 20Ne/22Ne, possibly related to the atmosphere and the interior. Caution is recommended in interpreting nitrogen and noble gas isotopic signatures of Martian meteorites from hot deserts in terms of extraterrestrial sources and processes. Nevertheless our results provide hope that vice-versa, via noble gases and nitrogen in meteorites and other relevant samples from terrestrial deserts, Martian secondary processes can be studied.  相似文献   

12.
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   

13.
The rates of production of21Ne and22Ne in spallation reactions, both due to solar as well as galactic cosmic rays, in some major meteoritic minerals, e.g. olivines, feldspars and pyroxenes, are calculated using their energy spectra and excitation functions. The production profiles of21Ne and22Ne due to galactic cosmic rays, and the22Ne/21Ne ratio depend upon the size of the meteoroid. The22Ne/21Ne ratio is very sensitive to the abundance of sodium and consequently its depth profile is distinctly different in feldspars, the ratio increasing with depth rather than decreasing as in pyroxenes and olivines. In the near-surface regions, up to a depth of 2cm, production due to solar flare protons dominates, giving rise to a steep gradient in isotopic production as well as in the22Ne/21Ne ratio. Composite production profiles are given and compared with measurements in some meteorites.  相似文献   

14.
Study of several cosmic ray effects, such as VH track density, spallogenic26Al and53Mn activity,21Ne and22Ne/21Ne ratio, made in the same sample or in cores taken from different meteorites can identify parameters related to the exposure history of meteorites and cosmic ray flux variations. Meteorites with single or multiple exposure can be distinguished from a track production rate —22Ne/21Ne correlation diagram and cosmic ray flux variations over 106–107 years can be deduced from a three-isotope correlation diagram of26Al,53Mn and21Ne. Isotopic data based on chondrites with simple, one-stage exposure are consistent with the same average galactic cosmic ray intensity over the past 2 million years as that during the past 107 years.  相似文献   

15.
Enstatite meteorites include the undifferentiated enstatite chondrites and the differentiated enstatite achondrites (aubrites). They are the most reduced group of all meteorites. The oxygen isotope compositions of both enstatite chondrites and aubrites plot along the terrestrial mass fractionation line, which suggests some genetic links between these meteorites and the Earth as well.For this study, we measured the Zn isotopic composition of 25 samples from the following groups: aubrites (main group and Shallowater), EL chondrites, EH chondrites and Happy Canyon (impact-melt breccia). We also analyzed the Zn isotopic composition and elemental abundance in separated phases (metal, silicates, and sulfides) of the EH4, EL3, and EL6 chondrites. The different groups of meteorites are isotopically distinct and give the following values (‰): aubrite main group (−7.08 < δ66Zn < −0.37); EH3 chondrites (0.15 < δ66Zn < 0.31); EH4 chondrites (0.15 < δ66Zn < 0.27); EH5 chondrites (δ66Zn = 0.27 ± 0.09; n = 1); EL3 chondrites (0.01 < δ66Zn < 0.63); the Shallowater aubrite (1.48 < δ66Zn < 2.36); EL6 chondrites (2.26 < δ66Zn < 7.35); and the impact-melt enstatite chondrite Happy Canyon (δ66Zn = 0.37).The aubrite Peña Blanca Spring (δ66Zn = −7.04‰) and the EL6 North West Forrest (δ66Zn = 7.35‰) are the isotopically lightest and heaviest samples, respectively, known so far in the Solar System. In comparison, the range of Zn isotopic composition of chondrites and terrestrial samples (−1.5 < δ66Zn < 1‰) is much smaller ( [Luck et al., 2005] and [Herzog et al., 2009]).EH and EL3 chondrites have the same Zn isotopic composition as the Earth, which is another example of the isotopic similarity between Earth and enstatite chondrites. The Zn isotopic composition and abundance strongly support that the origin of the volatile element depletion between EL3 and EL6 chondrites is due to volatilization, probably during thermal metamorphism. Aubrites show strong elemental depletion in Zn compared to both EH and EL chondrites and they are enriched in light isotopes (δ66Zn down to −7.04‰). This is the opposite of what would be expected if Zn elemental depletion was due to evaporation, assuming the aubrites started with an enstatite chondrite-like Zn isotopic composition. Evaporation is therefore not responsible for volatile loss from aubrites. On Earth, Zn isotopes fractionate very little during igneous processes, while differentiated meteorites show only minimal Zn isotopic variability. It is therefore very unlikely that igneous processes can account for the large isotopic fractionation of Zn in aubrites. Condensation of an isotopically light vapor best explains Zn depletion and isotopically light Zn in these puzzling rocks. Mass balance suggests that this isotopically light vapor carries Zn lost by the EL6 parent body during thermal metamorphism and that aubrites evolved from an EL6-like parent body. Finally, Zn isotopes suggest that Shallowater and aubrites originate from distinct parent bodies.  相似文献   

16.
An integrated study of the cosmic ray exposure history of the San Juan Capistrano meteorite was carried out using measurements of rare gas isotopic abundances, particle track densities and radioisotope concentrations. Spallation systematics determined for Kr isotopes in lunar samples are shown to be valid also for the San Juan Capistrano and St. Severin meteorites, thus allowing us to determine a reliable 81Kr/83Kr production ratio as needed for applying the 81Kr-Kr dating method. The 81Kr-Kr age of SJC is 28.7 ± 2.0 Myr, about 35% longer than ages determined by spallation He or Ne. The minimum observed track production rate (2.6 × 105 tracks/cm2 · Myr) sets a minimum of 8 cm for the preatmospheric radius of an assumed spherical body. Track density gradients and the low 60Co activity (<2.9 dpm/g Co) both set an upper limit of 10 cm to the radius. Track results show that ablation losses have averaged 6cm. The relative spallation yields of 78Kr and 83Kr, and the ratios 3He/21Ne and 22Ne/21Ne are all compatible with a hard irradiation as would be experienced by a sample depth of about 6 cm in a body of 8–10 cm. The low activities of 54Mn, 22Na and 26Al are also consistent with these irradiation conditions.  相似文献   

17.
Solar-type helium (He) and neon (Ne) in the Earths mantle were suggested to be the result of solar-wind loaded extraterrestrial dust that accumulated in deep-sea sediments and was subducted into the Earths mantle. To obtain additional constraints on this hypothesis, we analysed He, Ne and argon (Ar) in high pressure–low temperature metamorphic rocks representing equivalents of former pelagic clays and cherts from Andros (Cyclades, Greece) and Laytonville (California, USA). While the metasediments contain significant amounts of 4He, 21Ne and 40Ar due to U, Th and K decay, no solar-type primordial noble gases were observed. Most of these were obviously lost during metamorphism preceding 30 km subduction depth. We also analysed magnetic fines from two Pacific ODP drillcore samples, which contain solar-type He and Ne dominated by solar energetic particles (SEP). The existing noble gas isotope data of deep-sea floor magnetic fines and interplanetary dust particles demonstrate that a considerable fraction of the extraterrestrial dust reaching the Earth has lost solar wind (SW) ions implanted at low energies, leading to a preferential occurrence of deeply implanted SEP He and Ne, fractionated He/Ne ratios and measurable traces of spallogenic isotopes. This effect is most probably caused by larger particles, as these suffer more severe atmospheric entry heating and surface ablation. Only sufficiently fine-grained dust may retain the original unfractionated solar composition that is characteristic for the Earths mantle He and Ne. Hence, in addition to the problem of metamorphic loss of solar noble gases during subduction, the isotopic and elemental fractionation during atmospheric entry heating is a further restriction for possible subduction hypotheses.  相似文献   

18.
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19.
Analyses of meteorites for B abundances have shown that many chondrites are contaminated with terrestrial B, producing erroneously high meteoritic abundances of this element. Boron concentrations in freshly prepared interior samples are significantly lower than they are in samples with unknown or unspecified terrestrial histories. An estimate of the cosmic abundance based upon the analyses of 8 interior samples of 2 carbonaceous chondrites and 1 interior sample of each of 8 ordinary chondrites is a factor of 6.7 less than the previous low estimate. Our revised value, 3.0 B/1010H, is in excellent agreement with estimates based on observations of the solar photosphere. There is no longer a need to consider processes that enrich B in carbonaceous chondrites or deplete it in the sun. Relative meteoritic abundances of Li, Be and B are now in general agreement with models of nucleosynthesis of these light elements by galactic cosmic ray induced spallation.  相似文献   

20.
Total carbon determinations on the Haverö, Dingo Pup Donga, and North Haig ureilites yield values of 2.07, 3.17, and 5.58 wt.%, respectively. Haverö and Dingo Pup Donga contain relatively large amounts of trapped Ar, Kr and Xe, which like the carbon content varies with grain size for Haverö. These two meteorites also contain dominant cosmic rayproduced He and Ne, and show 3He exposure ages of ~23 m.y. and ~7 m.y., respectively. North Haig contains much smaller amounts of trapped gases and spallogenic gases, which may result from loss due to terrestrial weathering. The isotopic composition of Xe in five grain size analyses of Haverö and a whole rock analysis of Dingo Pup Donga shows the presence of a major solar-like Xe component. The presence of this solar component adds an additional complication to the concept of forming ureilites from carbonaceous chondrites.  相似文献   

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