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1.
We report measurements of the absolute isotope abundance of Ca in Ca-Al-rich inclusions from the Allende and Leoville meteorites. Improved high precision measurements are reported also for 46Ca. We find that nonlinear isotope effects in Ca are extremely rare in these inclusions. The absence of nonlinear effects in Ca, except for the effects in FUN inclusions, is in sharp contrast to the endemic effects in Ti. One fine-grained inclusion shows an excess of 46Ca of (7 ± 1)%., which is consistent with addition of only 46Ca or of an exotic (1) component with 46Ca1 ~ 48Ca1. FUN inclusion EK-1-4-1 shows a small 46Ca excess of (3.3 ± 1.0)%.; this confirms that the exotic Ca components in EK-1-4-1 were even more deficient in 46Ca relative to 48Ca than is the case for normal Ca. The Ca in the Ca-Al-rich inclusions shows mass dependent isotope fractionation effects (as deduced from the absolute ) which have a range from ?3.8 to +6.7%. per mass unit difference. This range is a factor of 20 wider than the range previously established for bulk meteorites and for terrestrial and lunar samples. Ca and Mg isotope fractionation effects in the Ca-Al-rich inclusions are common and attributed to kinetic isotope effects which imply the production of the inclusions by complex sequences of condensation, vaporization and recondensation. A correlation was found between Ca and Mg isotope fractionation effects and inclusion type. A possible correlation between isotope fractionation and rare earth element abundance patterns is discussed. 相似文献
2.
F.R. Niederer D.A. Papanastassiou G.J. Wasserburg 《Geochimica et cosmochimica acta》1981,45(7):1017-1031
We describe the analytical techniques developed for the precise measurement of the titanium isotope abundances using a TiO+ ion beam. Terrestrial, lunar, and bulk meteorite samples yield identical results. Using a normalization to for mass dependent isotope fractionation, we obtain the normal Ti composition: ; ; ; (2σ grand mean), taking and . Measurements on thirteen coarse-grained and fine-grained Ca-Al-Ti-rich inclusions from the Allende and Leoville meteorites show the presence of widespread, significant, nonlinear isotope anomalies in the Ti isotopes which were not used for normalization. The data require the addition of at least three exotic components. The distinct correlation of non-linear effects for the most neutron-rich isotopes of Ca and Ti and the absence of substantial effects at 46Ca in the FUN samples EK-1-4-1 and C-1 indicate that the effects reflect neutron-rich equilibrium or quasi-equilibrium nucleosynthetic processes in the outer layers of a supernova core. The results on Ca and Ti in conjunction with the isotopic effects on other elements (Mg, Sr, Ba, Nd, Sm) show that the samples represent mixtures of different nucleosynthetic components from distinctive processes (‘e’, ‘r’, ‘p’) which do not appear to be related to processes in the same stellar sites. 相似文献
3.
W.A. Russell D.A. Papanastassiou T.A. Tombrello 《Geochimica et cosmochimica acta》1978,42(8):1075-1090
4.
Robert Keith Moniot 《Geochimica et cosmochimica acta》1980,44(2):253-271
Acid-resistant residues were prepared by HCl-HF demineralization of three H-type ordinary chondrites: Brownfield 1937 (H3), Dimmitt (H3,4), and Estacado (H6). These residues were found to contain a large proportion of the planetary-type trapped Ar, Kr, and Xe in the meteorites. The similarity of these acid residues to those from carbonaceous chondrites and LL-type ordinary chondrites suggests that the same phase carries the trapped noble gases in all these diverse meteorite types. Because the H group represents a large fraction of all meteorites, this result indicates that the gas-rich carrier phase is as universal as the trapped noble-gas component itself. When treated with an oxidizing etchant, the acid residues lost almost all their complement of noble gases. In addition, the Xe in at least one oxidized residue, from Dimmitt, displayed isotopic anomalies of the type known as CCFX or DME-Xe, which is characterized by simultaneous excesses of both the lightest and heaviest isotopes. The anomaly in the Dimmitt sample differs from that observed in carbonaceous-chondrite samples, however, in the relative proportions of the light- and heavy-isotope excesses.The results of this study do not show an inverse correlation between trapped and trapped 36Ar abundance, as has been reported for acid-resistant residues from LL-chondrites. The results of this work therefore fail to support the hypothesis that meteoritic trapped noble gas abundances were established at the time of condensation. 相似文献
5.
Noble gas data are reported for 12 E-chondrites. Combined with literature data, they show that K-Ar ages are >4 Æ for 14 out of 18 meteorites, yet U, Th-He ages are often shorter, perhaps due to late, mild reheating. Cosmic-ray exposure ages differ systematically between types 4 and 6, with E4's mostly below 16 Myr and E6's above 30 Myr. This may mean that the E-chondrite parent body contains predominantly a single petrologic type on the (~ 1 km) scale of individual impacts, in contrast to the more thoroughly mixed parent bodies of the ordinary chondrites.The heavy noble gases consist of at least two primordial components: the usual planetary component () and a less fractionated, ‘subsolar’ component (). The latter is found in highest concentration in the E4 chondrite South Oman (36Ar = 760 × 10?8cc/g, ). The isotopic compositions of both components are similar to typical planetary values, indicating that some factor other than mass controlled the noble gas elemental ratios. The heavy Xe isotopes occasionally show some of the lowest and ratios measured in bulk chondrites, suggestive of nearly fission-free Xe (e.g. ). Amounts of planetary gas in E4 E6 chondrites fall in the range for ordinary chondrites of types 4–6, but, in contrast to the ordinary chondrites. fail to correlate with petrologic type or volatile trace element contents. Another unusual feature of E-chondrites is that primordial Ne is present even in most 4's and 5's (20Nep ~ 1 to 7 × 10?8cc/g). with an isotopic composition consistent with planetary Ne.Analyses of mineral separates show that the planetary gases are concentrated in an HF- and HCl-insoluble mineral similar to phase Q, the poorly characterized, HNO3-soluble carrier of primordial gases in carbonaceous and ordinary chondrites. The subsolar gases, on the other hand, are located in an HCl- and HNO3-resistant phase, possibly enstatite or a minor phase included in enstatite. Much of the 129Xer (50% for E4's, > 70% for E6's) is in HCl-resistant but HF-soluble sites, suggestive of a silicate.A similar subsolar component may be responsible for the high ratios of some C3's, unequilibrated ordinary chondrites, and the unique aubrite Shallowater. The planet Venus also has a high ratio, well above the planetary range, and hence may have acquired its noble gases from an E-chondrite-like material, similar to South Oman. 相似文献
6.
Determinations of ages are reported for seven severely shock-heated chondrites. Shaw gives a plateau age of 4.29 Gyr. Louisville, Farmington, and Wickenburg give well-defined intercept ages of 0.5–0.6 Gyr. Orvinio, Arapahoe, and Lubbock show complex release curves, with age minima of 0.7–1.0 Gyr. Degassing times of 0.5–1.0 Gyr are suggested for these meteorites. Most severely shocked chondrites were apparently not totally degassed of 40Ar by the event, but retained from ~ 2 to ~45% of their 40Ar. When calculated values of the diffusion parameter, , for Ar are examined in Arrhenius plots, they show two distinct linear relationships, which apparently correspond to the degassing of different mineral phases with distinct ratios and different average temperatures for Ar release. The experimentally determined values of for the high temperature phase of several severely shocked chondrites are ~10?7 to 10?5sec?1 for their determined shock-heating temperatures of ~950°C to ~ 1200°C. The inferred reheating temperatures, values, and fraction of 40Ar loss during the reheating event for these seven chondrites suggest post-shock cooling rates and burial depth of ~ 10?2 10?4°C/sec and ~0.5–2m, respectively. For three chondrites these cooling rates agree with those determined from Ni diffusion in metal grains: for five chondrites the cooling rates derived from 40Ar and Ni disagree by a factor of ~105. It is suggested that five of these severely shocked chondrites were part of large ejecta blankets containing hot material and cold clasts with a distribution of sizes and that the cooling rate of this ejecta appreciably decreased as a function of time. 相似文献
7.
G.L. Cumming 《Chemical Geology》1973,11(3):157-165
Computer simulation of random errors in the measurements for the double spike technique applied to Pb isotope analyses, indicates that normalized error multiplication factors for the calculated values of isotope ratios lie in the range 2 to 3. The optimum isotopic composition of the tracer is about and the optimum ratio of amount of tracer to amount of unknown is about P = 1.0. Determination of errors in the fractionation factor alone does not adequately describe the ultimate errors in the calculated isotope ratios except under special circumstances. Uncertainty in the actual composition of the tracer is closely proportional to uncertainty in the unknown composition. The purity of the spike is unimportant at least as long as the separated 207Pb and 204Pb isotopes are greater than 90% pure. 相似文献
8.
He3, He4, Ne21 and Ar38 contents were determined in 18 metal, troilite, sehreibersite and graphite inclusions of 9 iron meteorites, by total outgassing and stepwise heating. The ratio in metal phase ranges from 3.85 to 4.65, but in non-metallic samples, from 6.70 to 30.5. The results for cosmogenic isotopes of helium, neon and argon disagree appreciably with data on accelerator-irradiated targets. It should be noted, however, that some inclusions have lost considerable amounts of gas by diffusion.Uranium contents of 22 troilite and sehreibersite samples were determined by the fission track technique. The average uranium content of troilite is 0.4-0.7 ppb. Excess He4 of unknown origin was observed in troilite inclusions. If one assumes that the excess He4 was produced by uranium decay in situ, then the apparent U-He4 age is at least 5.9 × 109 yr. 相似文献
9.
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: 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
10.
New data on the U, Pu, and P distributions in less metamorphosed H-chondrites (type 3–5), coupled with literature results, permit a provisional picture to be assembled of the chemistry of these elements and for the rare earth elements in ordinary chondrites and the changes brought about by chondritic metamorphism. Preferential associations of phosphates with metals and/or sulndes in all chondrites strongly indicate an “initially” siderophile or conceivably chalcophile character for P in ordinary chondrite precursor materials with phosphate subsequently formed by oxidation. This oxidation occurred prior to or during chondritic metal-silicate fractionation. Uranium is initially concentrated in chondrule glass at ~ 100 ppb levels with phosphates (primarily merrillite) in H-3 chondrites being essentially U-free (<20 ppb). As chondrule glass devitrified during metamorphism, U migrated into phosphates reaching ~ 50 ppb in Nadiabondi (H-5) merrillite and 200–300 ppb in merrillite from equilibrated chondrites but “froze out” before total concentration in phosphates occurred. Relative 244Pu fission track densities in the outer 5 μm of olivine and pyroxene grains in contact with merrillite and with chondrule mesostasis in Bremervörde (H-3) give Pu(mesostasis)/Pu(merrillite) <0.01, implying total concentration of Pu in phosphates. Similarly, no detectable Pu (<0.1 ppb) was found in chondrule mesostasis in Tieschitz and Sharps; whereas, direct measurements of tracks in phosphates in H-3 chondrites are consistent with high (?10 ppb) Pu concentrations. Thus, a strong Pu-P correlation is indicated for ordinary chondrites. There is variable Pu/U fractionation in all chondritic phosphates reaching an extreme degree in the unequilibrated chondrites; therefore, the Pu/U ratio in phosphates appears relatively useless for relative meteorite chronology. Literature data indicate that the REE are located in chondrules in unequilibrated chondrites, most likely in glass; thus there may also be strong Pu/Nd fractionation within these meteorites. Like U, the REE migrate into phosphates during metamorphism but, unlike U, appear to be quantitatively concentrated in phosphates in equilibrated chondrites. Thus relative ages, based on Pu/Nd, may be possible for equilibrated chondrites, but the same chronological conclusions are probably obtainable from Pu concentrations in phosphates, i.e., on the Pu/P ratio. However, Pu/P chronology is possible only for ordinary chondrites; so there appears to be no universal reference element to cancel the effects of Pu chemical fractionation in all meteorites. Available data are consistent with — but certainly do not prove-that variations in Pu/P represent age differences, but if these age differences do not exist, then it is conceivable that the solar system ratio, important for cosmochronology, is still lower than the presently accepted value of 0.007. 相似文献
11.
Measurements of the isotopic composition of nitrogen in the solar system are summarized. We show that the 30% change, during the last 3 to 4 billion years, of in solar-wind-bearing lunar soils and breccias probably does not reflect changes in this ratio at the solar surface. Such changes, whether by spallation or thermonuclear reactions are ruled out by comparing the yields of 15N with those of other rare isotopes such as 9Be, 11B, 3He or 13C, even if an arbitrary degree of solar mixing is introduced. Moreover, we calculate that the solar activity required for producing significant amounts of 15N by spallation at the solar surface should have resulted in a particle bombardment of the Moon of an intensity that would have produced amounts of spallation isotopes (e.g.15N, 21Ne, 38Ar, 131Xe) several orders of magnitude in excess of what is actually found in the whole regolith.We argue that accretion of interstellar matter also does not work as a cause for a significant change of the photospheric ratio. Evidence is presented that the mixing depth at the solar surface on a time scale of ?109 years is (10?2 ?10?1) M⊙ Mixing to this depth renders accretion of interstellar matter as a source of compositional changes at the solar surface inefficient, even if allowance is made for the expected large difference in the accretion rates of condensed and gaseous matter. A quantitative treatment of several alternatives of solar accretion leads to serious contradictions (e.g. with the low Ne abundances in planetary atmospheres or with the amounts of nitrogen that should have been directly accreted by the Moon), and we conclude that accretion during the main sequence life of the Sun is an unlikely source of changes in at the solar surface.A ratio of is our best estimate for average solar system material and for the Sun. We propose that a rare, very light nitrogen component (called LPN) is admixed in varying amounts to planetary matter. Undiluted LPN has not been found in meteorites or planetary atmospheres, but we show that the combined effects of LPN admixture and isotope fractionation can in principle account for the variability of observed in the planetary system. Determination of the Jovian ratio with an accuracy of ~10% would crucially test our interpretation of the nitrogen isotope observations. 相似文献
12.
Metal and silicate portions from 13 mesosiderites, one pallasite, Bencubbin (“unique”) and Udei Station (‘iron with silicate inclusions’) have been analysed for their content of He, Ne and Ar; in most cases 36Cl could be determined as well. 36Cl-36Ar cosmic ray exposure ages fall between 10 and 160 Myr. Half of the metal samples show a deficit of spallogenic 3He (up to 30%) which we ascribe to a loss of tritium. The observed depletion of 3He in the silicates is correlated with their mineralogical composition: feldspar has lost its 3He in all cases, pyroxene definitely in one and possibly in five others, while olivine has been affected in only two meteorites. The thermal histories during their exposure to the cosmic radiation have been different for different meteoroids. Nevertheless, with the exception of Veramin, the data are compatible with the assumption of a continuous diffusion loss during a considerable fraction of the exposure era. For Veramin, however, an episodic event late in the exposure history is required. The exceptionally high ratio in the metal, which is due to a high 39Ar activity, indicates that the event occurred during the last 500,000 years or so and resulted in an extremely excentric orbit (large aphelion).Production rates of 38,39Ar from Ca and 21,22Ne from Mg are given. The ratio is close to unity. The ratios vary between 20 and 50, and are not correlated with the absolute production rate of 38Ar from metal. The production ratio from Mg is found to be close to but below unity.Of the mesosiderites only Veramin shows unambiguous evidence for primordial rare gases with larger amounts and a higher ratio in the olivine, suggesting in situ fractionation to have at least been partly responsible for the abundance pattern found. Bencubbin contains large amounts of strongly fractionated primordial gases, but again part of the fractionation may have occurred in situ. Udei Station shows an excess of (3.5 ± 0.6) × 10?10 cm3 STP 129Xe/g in the non-magnetic portion. 相似文献
13.
B.M. Kennedy M.A. Lynch J.H. Reynolds S.P. Smith 《Geochimica et cosmochimica acta》1985,49(5):1251-1261
The Roving Automated Rare Gas Analysis (RARGA) lab of Berkeley's Physics Department was deployed in Yellowstone National Park for a 19 week period commencing in June, 1983. During this time 66 gas and water samples representing 19 different regions of hydrothermal activity within and around the Yellowstone caldera were analyzed on site. Routinely, the abundances of five stable noble gases and the isotopic compositions of He, Ne, and Ar were determined for each sample. In a few cases the isotopes of Kr and Xe were also determined and found to be of normal atmospheric constitution.Correlated variations in the isotopic compositions of He and Ar can be explained within the precision of the measurements by mixing of only three distinct components. The first component is of magmatic origin and is enriched in the primordial isotope 3He with times the air value. This component also contains radiogenic 40Ar and possible 36Ar with , resulting in a ratio ≥ 41,000 times the air value. The second component is assumed to be purely radiogenic 4He and 40Ar (). This component is the probable carrier of observed excesses of 211Ne, attributed to the α,n reaction on 18O. Its radiogenic character implies a crustal origin in U. Th, and Krich aquifer rocks. The third component, except for possible mass fractionation, is isotopically indistinguishable from the noble gases in the atmosphere. This component originates largely from infiltrating run-off water saturated with atmospheric gases.In addition to exhibiting nucleogenic 211Ne, Ne data show anomalies in the ratio , which correlate roughly with the anomalies for the most part, but not as would occur from simple mass fractionation. Some exaggerated instances of the anomaly occur which could be explained by combined mass fractionation of Ne and Ar isotopes to a severe degree coupled with remixing with normally isotopic gases. Otherwise exotic processes have to be invoked to explain the 20Ne data.Relative abundances of the non-radiogenic and non-nucleogenic noble gases (22Ne, 36Ar, 84Kr, and 132Xe) are highly variable but strongly correlated. High Xe/Ar ratios are always accompanied by low Ne/ Ar ratios and vice versa. Except for water from the few cold (T < 20°C) springs analyzed, none of the samples have relative abundances consistent with air saturated water and the observed variations are not readily explained by the distillation of air saturated water.In characterizing each area of hydrothermal activity by the highest ratio found for that area, we find that within the caldera this parameter is somewhat uniform at ~7 ± 1 times the air value. There are exceptions, most notably at Mud Volcano, an area located along a crest of recent and rapid uplift. Here the maximum ratio is ~ 16 times the air value. Also noteworthy is Gibbon Basin which is in the vicinity of the most recent rhyolitic volcanism and exhibits a ratio ~ 13 times the air value. Immediately outside the caldera the maximum ratio decreases rapidly to values < ~3 times the air value. 相似文献
14.
We present Ca isotopic measurements on Allende Ca-Al-rich inclusions (CAIs) and an apatite enriched fraction from Orgueil. The results on CAIs show widespread excesses on the neutron-rich isotope 48Ca. All other isotopes agree with the normal within our presently obtainable precision. Seven out of 11 CAIs analyzed exhibit isotopic anomalies ranging up to ~ +6? units (1? = 1 part in 10,000). This abundant occurrence of isotopic excesses places Ca alongside Ti and O, elements which show isotopic anomalies in all Allende CAIs measured so far. However, at present no clear correlation can be found between excesses in 48Ca and 50Ti, the isotopes which are thought to be coproduced by neutron-rich nucleosynthetic processes within stars. We believe that the relatively higher volatility of Ca compared to Ti compounds could have led to a variable dilution with isotopically “normal” Ca in vaporization and recondensation processes in stellar envelopes, the interstellar medium and/or the solar nebula. High precision measurements of 46Ca limit possible anomalies in this 33 ppm abundance isotope to 10? units or less and, together with the observed 48Ca and 50Ti anomalies, constrain possible nucleosynthetic mechanisms capable of producing these neutron-rich nuclei. 相似文献
15.
Nine LL-chondrites were studied by a selective etching technique, to characterize the noblegas components in three mineral fractions: HF-HCl-solubles (silicates, metal, troilite, etc.; comprising ～ 99% of the meteorite), chromite and carbon (～ 0.3–0.7%) and Q (a poorly characterized mineral defined by its solubility in HNO3, comprising ～ 0.05% of the meteorite but containing most of the Ar, Kr, Xe and a neon component of ). The ratio in Q falls wi petrologic type and rising 36Ar content, as expected for condensation from a cooling solar nebula, but contrary to the trend expected for metamorphic losses. Chondrites of different petrologic types therefore cannot all be derived from the same volatile-rich ancestor, but must have formed over a range of temperatures, with correspondingly different intrinsic volatile contents.The CCFXe (carbonaceous chondrite fission) component varies systematically with petrologic type. The most primitive LL3s (Krymka, Bishunpur, Chainpur) contain substantial amounts of CCFXe in chromite-carbon, enriched relative to primordial Xe as shown by high (0.359–0.459, vs 0.310 for primordial Xe). These are accompanied by He and by Ne with and by variable amounts of a xenon component enriched in the light isotopes. The chromite in these meteorites is compositionally peculiar, containing substantial amounts of Fe(III). These meteorites, as well as Parnallee (LL3) and Hamlet (LL4) also contain CCFXe in phase Q, heavily diluted by primordial . On the other hand, LL5s and 6s (Olivenza, St. Séverin, Manbhoom and Dhurmsala) contain no CCFXe in either mineral. This deficiency must be intrinsic rather than caused by metamorphic loss, because Q in these meteorites still contains substantial amounts of primordial Ne.If CCFXe comes from a supernova, then its distribution in LL-chondrites requires three presolar carrier minerals of the right solubility properties, containing three different xenon components in certain combinations. These minerals must be appropriately distributed over the petrologic types, together with locally produced Q containing primordial gases, and they must be isotopically normal, in contrast to the gases they contain. On the other hand, if CCFXe comes from fission of a volatile superheavy element, then its decrease from LL3 to LL6 can be attributed to progressively less complete condensation from the solar nebula. Ad hoc assumptions must of the host phase Q, its association with ferrichromite and the origin of the associated xenon component enriched in the light isotopes.Soluble minerals in LL3s and LL4s contain a previously unobserved, solar xenon component, which, however, is not derived from the solar wind. Three types of ‘primordial’ xenon thus occur side-by-side in different minerals of the same meteorite: strongly fractionated Xe in ferrichromite and carbon, lightly fractionated Xe in phase Q, and ‘solar’ Xe in solubles. Because the first two can apparently be derived from the third by mass fractionation, it seems likely that all were trapped from the same solar nebula reservoir, but with different degrees of mass fractionation. 相似文献
16.
Silver in the metal phases of Cape York (IIIA) and Grant (IIIB) has been determined after an extensive surface cleaning process. The was found to be enriched over that found in terrestrial Ag by ~7%. to 19%., demonstrating the presence of excess 107Ag (107Ag1) in this class of meteorites. An effort was made to find schreibersite with a distinctive 108Pd/109Ag ratio in order to establish a three-point isochron, but the results are not markedly different from those obtained for the bulk metal. The Ag isotopic ratio of sulfides from the same meteorites were nearly normal in composition. These results demonstrate correlations of with between coexisting phases of two iron meteorites that are associated with planetary differentiation processes. The ratios were found to be 1.7 × 10?5 and 1.2 × 10?5 for Cape York and Grant, respectively. These observations are in support of the widespread presence of 107Pd in the early solar system. The difference in isotopic composition between metal and sulfide phases demonstrates that silver diffusion was small (over 6.5 × 106 y) indicating a cooling rate much greater than 150°C/my for meteorites which have been attributed to small planetary cores. Uranium determinations were carried out on the metal phases and concentrations of ~ g U/g and 2 × 10?10g U/g were found for Cape York and Grant, respectively. The Pb in these meteorites was determined using the improved cleaning procedures and chemical separations with low blank levels. The results confirm the presence of variable proportions of radiogenic Pb in both the metal and sulfide phases of iron meteorites. No simple explanation for the presence of radiogenic lead is apparent; while terrestrial contamination may appear to be the obvious explanation, it is possible that this effect could result from relatively recent metamorphism in the meteorite parent body. 相似文献
17.
G.F Herzog 《Geochimica et cosmochimica acta》1973,37(9):2125-2133
Al26 and noble gas contents of 6 ordinary chondrites with ratios above 6.0 or below 4.0 are used to infer the variability of the production rates of He3 and Ne21 (PHe3 and PNe21). The ratio of the observed Al26 content to a calculated, normal value is taken as a measure of the change of PNe21 from its normal value. The corresponding change in PHe3 is then computed from the observed ratio and an average value of PHe3.According to these calculations which exclude orbital effects, PHe3 will be near the average value in meteorites with high ratios, while PNe21 will be about 30 per cent below normal. In meteorites with low ratios, PHe3 may be depressed by as much as 25 per cent from normal while PNe21 may be 15–20 per cent above the average. 相似文献
18.
Jun-Ichi Matsuda Roy S. Lewis Hiroshi Takahashi Edward Anders 《Geochimica et cosmochimica acta》1980,44(11):1861-1874
Noble gases were measured in bulk samples of the C3V chondrites Grosnaja, Vigarano, and Leoville, and in HF,HCl-insoluble residues before and after etching with HNO3. The residues were characterized by INAA and SEM. Gas components were determined, directly or by subtraction, for the following fractions: HF,HCl-solubles (?98% of the meteorite), ‘sphase Q’, a poorly characterized trace mineral that is insoluble in HCl-HF but soluble in HNO3, and an insoluble residue, consisting of ferrichromite, carbonaceous matter, and spinel.Bulk meteorites show some correlation of the noble-gas pattern with McSween's subclasses: two ‘oxidized’ C3V's—Allende (LEWIS et al, 1975) and Grosnaja— have lower Ar/Xe but higher Ne/Xe ratios than the ‘reduced’ C3V's—Vigarano and Leoville—which are transitional to LL3's and C3O chondrites in both respects. An HCl-soluble mineral of high Ar/Xr ratio seems to be responsible. In other respects, the 3 C3V's of this study resemble Allende, with only moderate differences. Phase Q contains most of the Ar, Kr, Xe, but only small amounts of Ne; the etched residues contain planetary Ne () and the controversial CCFXe component, enriched in the heavy Xe isotopes (). The CCFXe is accompanied by an ‘L-Xe’ component that is enriched in the light Xe isotopes. The proportion of the two is virtually constant in C3V's. as in all other C-chondrites. in contrast to the ~ 2-fold variation in ordinary chondrites.C3V's have systematically higher ratios, and hence higher ratios of CCFXe to planetary Xe, than do other chondrite classes. This may reflect some peculiarity in their formation conditions, favoring uptake of CCFXe. 相似文献
19.
Abundances and isotopic compositions of He, Ne, Ar, and Xe have been measured in eight recently fallen chondrites. Ratios of concentrations of cosmic ray-produced 3He, 21Ne, 22Ne and 38Ar indicate that all eight samples experienced less than average cosmic ray shielding. 3He and 21Ne exposure ages were calculated using shielding corrected chondritic production rates and the measured . Exposure ages calculated from and ratios and constant relative production rates show a bias between the two ages due to variations in . Arguments are presented that this bias is due to irradiation hardness differences, and therefore the use of constant values for both the and production ratios is not permitted. Dwaleni, Swaziland, was found to be an unusual gas-rich chondrite with high concentrations of solar-derived He and Ne and planetary-type Xe. 相似文献
20.
A.V Hirner W Graf R Treibs A.N Melzer P Hahn-Weinheimer 《Geochimica et cosmochimica acta》1984,48(11):2179-2186
Eleven samples of crude oil from the Molasse Basin of Southern Germany were fractionated and their contents of sulfur and nitrogen as well as the stable isotope compositions of these elements ( and , resp.) investigated.According to the δ34S determinations, all crude oils from the Tertiary base of the Western and Eastern Molasse belong to one oil family and differ significantly from the Triassic and Liassic oils in the Western Molasse.An enrichment of 34S was observed with increasing polarity of crude oil fractions. The isotope distributions of sulfur in the polar constituents of the biodegraded oils from the sandstones of Ampfing, however, approach a homogeneous distribution.The nitrogen isotope distribution is rather uniform in Southern German oils. A regional differentiation can be recognized, although the overall isotopic variation is small. The δ15N values of the crudes and asphaltenes do not correlate. 相似文献