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NASA NTRS1984

Ca isotopes in refractory inclusions

The present absolute isotope abundance measurements of Ca in Ca-Al-rich inclusions from the Allende and Leoville meteorites show the extreme rarity of nonlinear isotope effects in Ca, in sharp contrast to the endemic effects in Ti. The Ca in the Ca-Al-rich inclusions shows mass-dependent isotope fractionation effects whose range is a factor of 20 wider than the range previously established for bulk meteorites and for terrestrial and lunar samples. A correlation is found between Ca and Mg isotope fractionation effects and inclusion type, and a possible correlation between isotope fractionation and rare earth element abundance patterns is discussed.

Niederer, F. R.↗

NASA NTRS1984

Oxygen isotopes in deep sea spherules

The determination of the genetic relationships between the dust and small particles in the solar system, and the meteorites and larger bodies are examined. Oxygen isotopes proved useful in the identification of such relationships between one meteorite group and another. Of the various samples of submillimeter extraterrestrial particles available for laboratory study, only the deep sea spherules are abundant enough for precise oxygen isotope analysis using existing techniques. Complications arise in interpretation of the isotopic data, since these particles were melted during passage through the Earth's atmosphere, and have been in contact with seawater for prolonged periods. Spherules that were originally silicates are considered with the originally metallic ones to deduce their preterrestrial isotopic compositions. The type 1 spherules which enter the atmosphere as metallic particles, contain only atmospheric oxygen. The type S spherules contain a mixture of atmospheric oxygen and their original extraterrestrial oxygen. It is suggested that the Earth's mesosphere is strongly enriched in heavy isotopes of oxygen at altitudes near 90 km at which the iron particles are oxidized. Fractionation due to the combined diffusion of O atoms and O2 molecules may be responsible.

Mayeda, T. K.↗

NASA NTRS1984

Ca isotope variations in Allende

Ca-isotope measurements of Allende Ca-Al-rich inclusions (CAIs), together with those on an apatite-enriched fraction from Orgueil, indicate the existence of widespread excesses on the neutron-rich isotope Ca-48. Isotopic anomalies are noted in 7 out of 11 CAIs analyzed. This abundance of isotopic excesses places Ca alongside Ti and O, although no clear correlation has yet been found between Ca-48 and Ti-50, which are thought to be coproduced by neutron-rich nucleosynthetic processes within stars. It is suggested that the higher volatility of Ca, by comparison with Ti compounds, 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.

Jungck, M. H. A.↗

NASA NTRS1983

Oxygen isotopic compositions of chondrules in Allende and ordinary chondrites

The ferromagnesian chondrules in Allende follow a trend in the oxygen three-isotope plot that diverges significantly from the 16-O mixing line defined by light and dark inclusions and the matrix of the meteorite. The trend probably results from isotopic exchange with an external gaseous reservoir during the process of chondrule formation sometime after the establishment of the isotopic compositions of the inclusions and matrix. The Allende chondrules approach, but do not reach, the isotopic compositions of chondrules in unequilibrated ordinary chondrites, implying exchange with a similar ambient gas, but isotopically different solid precursors for the two types of meteorite.

Clayton, R. N.↗

NASA NTRS1985

Constraints on chondrule origin from petrology of isotopically characterized chondrules in the Allende meteorite

The petrologic and chemical properties of the ferromagnesian chondrules in the Allende carbonaceous chondrite were examined in terms of the isotopic composition and the correlations between isotopic patterns. Areas of thin sections were studied with a SEM and bulk chemical fractions of 12 constituents were quantified to calculate correlations with petrologic features. A possible correlation between (CaO + Al2O2)/MgO and oxygen isotopes imply the formation of oxygen isotopic compositions in the chondrules by exchanges between isotopically heavy nebular gases and O-16 enriched solids. Different rates of gaseous exchange occurred with the various types of chondrules. Factors which may have controlled the exchanges are discussed.

Mcsween, H. Y., Jr.↗

NASA NTRS1987

Rare earth element transport in the western North Atlantic inferred from Nd isotopic observations

The relationship between the Nd isotopic composition in the Atlantic waters and the origin and circulation of the water masses was investigated. Samples were collected in the western North Atlantic between 7 and 54 deg N. The isotopic composition (Nd-143/Nd-144 ratios) showed extensive vertical structure at all locations. In regions where a thermocline was well-developed, large isotopic shifts were observed across the base of the thermocline, while regions without a thermocline were characterized by much more gradual shifts in isotopic composition with depth. The data reveal an excellent correlation between the Nd isotopic distribution in the western North Atlantic water column and the distribution of water masses identified from temperature and salinity measurements.

Piepgras, D. J.↗

NASA NTRS1987

The effect of pressure and excitation energy on the isotopic fractionation in the formation of ozone by discharge of O2

Isotopic fractionation processes in the formation of O3 by discharge (at 20 kV and 3 MHz) of O2 in a fused silica container at 77 K are investigated experimentally, with a focus on the effects of changes in the isotopic abundances in the predischarge O2, the O2 pressure, and the excitation energy on the isotope abundances in the postdischarge O2 and O3. The results are presented in extensive tables and graphs and characterized in detail, and good general agreement is found with data obtained using 60-kHz excitation (Yang and Epstein, 1987). It is shown that O-17 and O-18 enrichment of the residual O2 (when the predischarge gas is enriched) or of the O3 (when atmospheric isotope abundances are used) can occur independently of one another. It is inferred that the rate of anharmonic predissociation of vibrationally hot O3 depends on the symmetry and mass of the O3 molecules. The significance of the present findings for studies of anomalous oxygen isotope abundances in meteorites is indicated.

Yang, Jongmann↗

NASA NTRS1987

O-16 excesses in Murchison and Murray hibonites - A case against a late supernova injection origin of isotopic anomalies in O, Mg, Ca, and Ti

Ion probe measurements of the oxygen isotopic composition of seven hibonite samples from the CM chondrites Murchison and Murray are reported. All samples show large O-16 excesses relative to terrestrial oxygen. The data for all samples plot along the carbonaceous chondrite O-16-rich mixing line and show no evidence for isotopic mass fractionation effects characteristic of FUN inclusions. These hibonites have the largest Ca-48 and Ti-50 isotopic anomalies found to date; thus there is no intrinsic relationship between anomalies of a nucleosynthetic origin and isotopic mass fractionation effects. The large O-16 excess seen in the sample with the largest measured Ca-48 and Ti-50 depletions argues against a late injection of exotic material from a nearby supernova as a source for the isotopic anomalies.

Fahey, A. J.↗

NASA NTRS1988

Isotopic investigations in the area of the Tunguska catastrophe in 1908 year

The hypotheses of the annihilation and thermonuclear character of the Tunguska explosion have been tested by measuring inductive Ar-39 radioactivity from K and Ca in rocks and soil under the explosion epicentrum. Ar-39 was not detected though its estimated radioactivity was expected to be 100 times higher than the radio-metrical plant sensitivity. These results testify against the nuclear nature of the Tunguska explosion. The contents of 11 elements in the ultrasmall quantity of matter of the silicate microspherules isolated from catastrophe peat layer at the explosion site were measured by method of neutron activation analysis. It was demonstrated that the enrichment of microspherules by light and volatile elements (Al, Na, Zn, Cs) and the impoverishment by more heavy and hard volatile ones (Fe, Co, Sc). It was shown that the microspherules were not the product of differentiation of the terrestrial soil or of an ordinary meteorite material. It was demonstrated that Pb isotopic content in catastrophe peat layer had more Pb-204, Pb-207, Pb-208 than Pb-206 as compared with Pb isotopic content of other peat layers and common Pb in this area. In the peat column taken at the Ostraya hill area in three nearcatastrophe layers, small increases of isotopic C-13 was determined. Isotopic C effect in the peat layer is confirmed also for the North peatbog. Observed isotopic changes are not accounted for by climatic changes or other physico-chemical reasons. They seem to be related to preservation in the peat of matter resembling carbonaceous chrondrites of the Cl type or more probably of cometary matter enriched much more in volatile elements.

Kolesnikov, E. M.↗

NASA NTRS1988

Isotopic anomalies - Chemical memory of Galactic evolution

New mechanisms for the chemical memory of isotopic anomalies are proposed which are based on the temporal change during the chemical evolution of the Galaxy of the isotopic composition of the mean ejecta from stars. Because of the differing temporal evolution of primary and secondary products of nucleosynthesis, the isotopic composition of the bulk interstellar medium changes approximately linearly with time, and thus any dust component having an age different from that of average dust will be isotopically anomalous. Special attention is given to C, O, Mg, Si, and isotopically heavy average-stellar condensates of SiC.

Clayton, Donald D.↗

NASA NTRS1990

Oxygen isotope fractionation between analcime and water - An experimental study

The oxygen isotope fractionation between analcime and water is studied to test the feasibility of using zeolites as low-temperature thermometers. The fractionation of oxygen isotopes between natural analcime and water is determined at 300, 350, and 400 C, and at fluid pressures ranging from 1.5 to 5.0 kbar. Also, isotope ratios for the analcime framework, the channel water, and bulk water are obtained. The results suggest that the channel water is depleted in O-18 relative to bulk water by a constant value of about 5 percent, nearly independent of temperature. The analcime-water fractionation curve is presented, showing that the exchange has little effect on grain morphology and does not involve recrystallization. The exchange is faster than any other observed for a silicate. The exchange rates suggest that zeolites in active high-temperature geothermal areas are in oxygen isotopic equilibrium with ambient fluids. It is concluded that calibrated zeolites may be excellent low-temperature oxygen isotope geothermometers.

Karlsson, Haraldur R.↗

NASA NTRS1991

Oxygen isotope fractionation in stratospheric CO2

A new cryogenic collection system has been flown on board a balloon gondola to obtain separate samples of ozone and carbon dioxide without entrapping major atmospheric gases. Precision laboratory isotopic analysis of CO2 samples collected between 26 and 35.5 km show a mass-independent enrichment in both O-17 and O-18 of about 11 per mil above tropospheric values. Ozone enrichment in its heavy isotopes was 9 to 16 percent in O3-50 and 8 to 11 percent in O3-49, respectively (Schueler et al., 1990). A mechanism to explain the isotope enrichment in CO2 has been recently proposed by Yung et al. (1991). The model is based on the isotope exchange between CO2 and O3 via O(1D), resulting in a transfer of the ozone isotope enrichment to carbon dioxide. Predicted enrichment and measured values agree well.

Thiemens, M. H.↗

NASA NTRS1991

Correlated Si isotope anomalies and large C-13 enrichments in a family of exotic SiC grains

A hypothesis is presented to the effect that the distinctive morphological characteristics and comparatively simple Si isotope systematics identify the platy SiC crystals as a genetically related family, formed around a single isotopically heterogeneous presolar star on an association of related stars. The enrichments in C-13 and the Si isotope systematics of the platy SiC are broadly consistent with theoretical models of nucleosynthesis in low-mass, carbon stars on the ASG. The Si isotope array most plausibly reflects mixing between (Si-28)-rich material, inherited from a previous generation of stars, and material enriched in Si-29 and Si-30, produced in intershell regions by neutron capture during He-burning. The absence of a correlation between the Si and C isotopic compositions of the SiC suggests either episodic condensation of SiC, extending over several thermal pulses, in the atmosphere of a single star, or the derivation of the SiC from several stars characterized by different rates of C-13 production.

Stone, J.↗

NASA NTRS1992

Solar-type xenon - Isotopic abundances in Pesyanoe

Elemental and isotopic abundances of Ar and Xe were measured in three grain-size separates in the dark phase of the enstatite achondrite Pesyanoe by stepwise heating, using a combination of pyrolysis and combustion steps. The data reveal a low-temperature gas fraction with Ar/Xe ratios close to the solar ratio and isotopically similar to solar-type Ar and Xe observed in lunar samples. The gas released at intermediate temperature steps shows that Ar and Xe are isotopically fractionated, compared to the low-temperature components. Pesyanoe does not have excess Ar-40 and fission Xe, which means its parent body was too small for ion reimplantation. It is concluded that solar-type Xe isotopic abundances can be inferred from the low-temperature component. A comparison of Pesyanoe and lunar data shows that isotopic signatures of solar-wind Xe, as sampled at two different points in solar system space and time, are identical within experimental error.

Kim, J. S.↗

NASA NTRS1992

Age and isotopic relationships among the angrites Lewis Cliff 86010 and Angra dos Reis

The paper presents results of a wide-ranging isotopic investigation of the the Antarctic angrite LEW-86010 (LEW), and reassesses the type angrite Angra dos Reis (ADOR) in order to obtain precise radiometric ages and initial Sr isotopic compositions, and to search for the erstwhile presence of the short-lived nuclei Sm-146 and Al-26 via their daughter products. The isotopic compositions of Sm, U, Ca, and Ti were measured to allow a detailed appraisal to be made of the relations between, and the geneology of, these two angrites. LEW proves to be severely contaminated with modern terrestrial Pb, which is shown to result from terrestrial weathering. Concordant Pb-Pb model ages of pyroxene separates are obtained; uranium isotopic compositions are normal within error. Overall, striking age and isotopic similarities between LEW and ADOR were found, suggesting almost simultaneous production on the same asteroid, even though recent experimental studies imply that the two are not comagmatic.

Lugmair, G. W.↗

NASA NTRS1992

Isotopic, optical, and trace element properties of large single SiC grains from the Murchison meteorite

Ion probe mass spectrometry is used to analyze 41 large SiC grains from the Murchison CM2 chondrite, ranging up to 15 x 26 microns, for the isotopic compositions of C, N, Mg, and Si, and the concentrations of Al, Ti, V, Fe, Zr, and Ba. The majority were found to have large isotopic anomalies. Only two grains, characterized by extremely heavy carbon, give evidence for fossil Mg-26. On the basis of C and Si isotopic composition, 29 of the grains fall into three compact clusters, presumably from three discrete sources. The clustering of coarse-grained SiC stands in sharp contrast to the quasi-continuous distribution of finer-grained SiC and suggests that the top 0.1 percent of the mass distribution is a distinct population. The C and N isotopic compositions of the anomalous grains are not very diagnostic, being consistent with H-burning in the CNO cycle. The Si-isotopic compositions qualitatively show the signature of neutron capture in He-burning shells of highly evolved stars.

Virag, Alois↗

NASA NTRS1993

Interstellar graphite in Murchison: Continued search for isotopically distinct components

We have measured C-, N-, and O-isotopic ratios of individual graphite grains from the Murchison density fractions KFB1 (2.1-2.15 g/cc) and KFC1 (2.15 -2.20 g/cc). According to the C-isotopic composition, KFC1 consists of two populations: one has light C while the other has heavy C. In addition to these two populations, KFB1 also contains grains whose C-isotopic ratios are approximately normal. In view of these isotopic distributions, it is difficult to understand why the Kr-isotopic compositions of these two separates are so different. In contrast to the lighter density fractions KE1 (1.6-2.05 g/cc) and KFA1 (2.05-2.10 g/cc), the separates KFB1 and KFC1 do not seem to contain many grains with significant O-18 excess.

Amari, Sachiko↗

NASA NTRS1994

First observation of a mass independent isotopic fractionation in a condensation reaction

Thiemens and Heidenreich (1983) first demonstrated that a chemically produced mass independent isotopic fractionation process could produce an isotopic composition which is identical to that observed in Allende inclusions. This raised the possibility that the meteoritic components could be produced by chemical, rather than nuclear processes. In order to develop a mechanistic model of the early solar system, it is important that relevant reactions be studied, particularly, those which may occur in the earliest condensation reactions. The isotopic results for isotopic fractionations associated with condensation processes are reported. A large mass independent isotopic fractionation is observed in one of the experiments.

Thiemens, M. H.↗