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Papanastassiou, D. A.

Publications and source records attributed to Papanastassiou, D. A..

At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2006

Elemental Compositions of Comet 81P/Wild 2 Samples Collected by Stardust

We measured the chemical compositions of material from 23 particles in aerogel and residue in 7 craters in aluminum foil, collected during passage of the Stardust spacecraft through the coma of Comet 81P/Wild 2. These particles are chemically heterogeneous at the largest size-scale analyzed, ~180 nanograms. The mean chemical composition of this Wild 2 material agrees with the CI meteorite composition for the refractory elements Mg, Si, Cr, Fe, and Ni to 35%, and for Ca and Mn to 50%. The data suggest the moderately volatile elements Cu, Zn, and Ga may be enriched in this Wild 2 material.

Flynn, G. J.↗
NASA NTRS2005

The Palladium Isotopic Composition in Iron Meteorites

Ru, Mo and Pd are very useful indicators for the identification of nucleosynthetic components. We have developed techniques for Pd isotopes, in an effort to check the extent of isotopic effects in this mass region and for a Pt-group element which is less refractory than Ru. Stable Pd isotopes are produced by the process only (102Pd), the s-process only (104Pd), the process only (Pd-110) and by both the r- and s-processes (Pd-105, Pd-106, Pd-108). Kelly and Wasserburg reported a hint of a shift in 102Pd (approx. 25(epsilon)u; 1(epsilon)u (triple bonds) 0.01%) in Santa Clara. Earlier searches for Mo and Ru isotopic anomalies were either positive or negative.

Chen, J. H.↗
NASA NTRS2005

The Mn-53-Cr-53 System in CAIs: An Update

High precision techniques have been developed for the measurement of Cr isotopes on the Triton mass spectrometer, at JPL. It is clear that multiple Faraday cup, simultaneous ion collection may reduce the uncertainty of isotope ratios relative to single Faraday cup ion collection, by the elimination of uncertainties from ion beam instabilities (since ion beam intensities for single cup collection are interpolated in time to calculate isotope ratios), and due to a greatly increased data collection duty cycle, for simultaneous ion collection. Efforts to measure Cr by simultaneous ion collection have not been successful in the past. Determinations on Cr-50-54Cr, by simultaneous ion collection on the Finnigan/ MAT 262 instrument at Caltech, resulted in large variations in extrinsic precision, for normal Cr, of up to 1% in Cr-53/Cr-52 (data corrected for mass fractionation, using Cr-50/Cr-52).

Papanastassiou, D. A.↗
NASA NTRS2004

More on Ru Endemic Isotope Anomalies in Meteorites

We reported last year on endemic isotope anomalies for Ru in iron meteorites, pallasites, ordinary chondrites, and on a whole-rock sample of Allende. We have extended the Ru measurements to more meteorites, to refractory Ca-Al-rich inclusions (CAI) from Allende, and to a whole rock sample of Murchison (CM2). In a companion abstract we report on new measurements for the Mo isotopes, in some of the same samples. There has been a renewed interest in searching for isotope anomalies in this nuclide region, as Ru and Mo include many isotopes from r-, s-, and p-process nucleosynhesis. Furthermore, the Ru and Mo p-process isotopes show atypically high abundances, which have been hard to explain through the standard nucleosynthetic processes. Effects are possible in Ru-98 and Ru-99 from Tc-98 (with a poorly known t(sub 1/2)=4.2 to 10Ma) and from Tc-99 (t(sub 1/2)=0.21Ma). Natural Tc is now extinct on Earth due to the short half-lives, but may have been present in the early solar system. Both radiogenic and general isotope anomalies are important in understanding the processes for the formation of the early solar system. The current emphasis on Ru and Mo is also the result of the development of Negative-ion Thermal Ionization Mass Spectrometry and of Multiple-Collector, Inductively-Coupled-Mass-Spectrometry. We have also developed specific chemical siparation techniques for Ru, which eliminated mass interference effects.

Papanastassiou, D. A.↗
NASA NTRS2004

Endemic Mo Isotopic Anomalies in Iron and Carbonaceous Meteorites

Mo in refractory interstellar grains shows large isotope anomalies. Recent Mo studies showed isotope effects in Allende and Murchison, and in iron meteorites, mesosiderites, and pallasites. Excesses of p- and r-process isotopes (or depletion of sprocess isotopes) of up to 3.5 epsilon units (epsilon u=parts in 10(exp 4)) were reported. We have reported on endemic isotope anomalies in Ru. Other workers have resolved no isotope anomalies for Mo or Ru and have claimed that the work by others is incorrect. Because Ru isotopes can interfere at Mo-96, Mo-98, Mo-100, we improved the chemical separations and eliminated interferences. For Mo work, we used the same solutions from which we separated and analyzed Ru. Three of the iron meteorites (Coahuila, Cape York, and Cape of Good Hope) were chosen for their large Mo isotopic effects. Mo was loaded on outgassed Re filaments, and then reduced; we used Ba(OH)2-NaOH as emitter, and measured Mo in static mode, as MoO3(-). We used Mo-98/Mo-96 for the mass fractionation correction (exponential law). No interferences from Ru or Zr isotopes were detected using the electron multiplier and no corrections were needed. For results on Mo standards we show 2 sigma(not 2 sigma mean) external precision better than: 0.7 epsilon u for Mo-94/Mo-96 and Mo-95/Mo-96; 1.0 epsilon u for Mo-92/Mo-96 and Mo-97/Mo-96; 1.4 epsilon u for Mo-100/Mo-96. Reproducibility for Mo standards is shown as contours (blue lines).

Chen, J. H.↗
NASA NTRS2003

Endemic Ru Isotopic Anomalies in Iron Meteorites and in Allende

Small variations for Mo isotopes have been observed recently in the Allende meteorite and in iron meteorites, mesosiderites, and pallasites, using ICPMS. Large effects for Mo have been reported for leaches of Orgueil and in SiC and graphite from Murchison. Variations for Mo in bulk Allende and in Murchison have also been presented by NTIMS. Effects in Ru isotopes can define further the preserved exotic r, s, and p contributions in this mass region, and possible effects in Ru-98 and Ru-99 from Tc-98 (4.2 Ma half-life) and Tc-99 (0.21 Ma half-life). Previous attempts at determination of Ru isotopes yielded no resolved effects. The present work represents a substantial improvement in precision over the earlier work. Chemical and mass spectrometric analytical techniques are presented to determine the Ru isotope compositions in terrestrial standards and in meteorites.

Chen, J. H.↗
NASA NTRS2003

No Nd-142 Excess in the Early Archean Isua Gneiss IE 715-28

There is abundant evidence for the existence of 146Sm in the early solar system and for preservation of effects in the 146Sm-142Nd system in differentiated meteorites ([1]; see recent discussion in Stewart et al. 1994). Information from the 182Hf-182W system, as revised by new careful work [2-3] also indicates that the Earth s core formed relatively early. It is in principle possible for early-formed crust and mantle reservoirs on Earth to have preserved evidence for 146Sm if such reservoirs were produced with high Sm/Nd fractionation and if they have remained isolated and closed since ~4.3 Ga. The mean life of 146Sm of 149 Ma is sufficiently long to make this an intriguing possibility.

Papanastassiou, D. A.↗
NASA NTRS2003

Rb-Sr and Sm-Nd Study of the D'Orbigny Angrite

D'Orbigny, is a relatively a new angrite find. Angrites are achondrites that show unique mineralogy and typically ancient crystallization ages. D Orbigny has been described extensively by. We have initiated a study of D Orbigny. A Pb-Pb model age of 4559 Ga has been reported. The presence of Pu-244 fission Xe in D Orbigny has also been reported. We present Sm-Nd and Rb-Sr results and a Mn-53-Cr-53 study is in progress. We had expected to find a relatively well-behaved Sm-Nd system, and distinct evidence for the Sm-146-Nd-142 system. Emphasis in this study was placed on the precise measurement of initial Sr-87/Sr-86, since primitive initial Sr-87/Sr-86 may be characteristic of the angrite parent body. Sr isotope measurements were obtained in the recently completed laboratories at JPL, using the ThermoFinnigan Triton mass spectrometer. Due to lower amounts of Nd and Sm, Sm-Nd data were obtained on the Lunatic I spectrometer at Caltech, due to established high ionization techniques, on this instrument using light rare earth oxide ions.

Tonui, E. K.↗
NASA NTRS2002

Cr Isotopes in Allende Ca-Al-rich Inclusions

We have determined Cr isotope compositions in minerals from Allende CAI in order to address the initial 53Mn (half-life 3.7 Ma) abundance in the solar system. Additional information is contained in the original extended abstract.

Bogdanovski, O.↗
NASA NTRS2001

Sr and Nd Data for Upper Eocene Spherule Layers

Sr and Nd data for clinopyroxene-bearing (cpx) spherules from three sites support conclusions that there is only one cpx spherule layer and that the source crater may be Popigai. Additional information is contained in the original extended abstract.

Liu, Shaobin↗
NASA NTRS2000

Martian Chronology: Goals for Investigations from a Recent Multidisciplinary Workshop

The absolute chronology of Martian rocks and events is based mainly on crater statistics and remains highly uncertain. Martian chronology will be critical to building a time scale comparable to Earth's to address questions about the early evolution of the planets and their ecosystems. In order to address issues and strategies specific to Martian chronology, a workshop was held, 4-7 June 2000, with invited participants from the planetary, geochronology, geochemistry, and astrobiology communities. The workshop focused on identifying: a) key scientific questions of Martian chronology; b) chronological techniques applicable to Mars; c) unique processes on Mars that could be exploited to obtain rates, fluxes, ages; and d) sampling issues for these techniques. This is an overview of the workshop findings and recommendations.

Nyquist, L.↗