D. A. Papanastassiou

Ca isotope fractionation on the Earth and other solar system materials

It is demonstrated that differences in the ^(40)Ca/^(44)Ca ratio due to mass dependent isotope fractionation in nature are clearly resolvable to a level of 0.5‰. This precision is obtained (a) by using the double spike technique; (b) by using a mass-dependent law for correction of instrumental mass fractionation; and (c) by eliminating fractionation effects identified as due to differential elution of isotopes through ion exchange resins. We have determined the following uniform Ca isotopic composition after removing small natural fractionation effects: ^(40)Ca/^(44)Ca = 47.153 ± 3, ^(42)Ca/^(44)Ca = 0.31221 ± 2, ^(43)Ca/^(44)Ca = 0.06486 ± I, ^(46)Ca/^(44)Ca = 0.00152 ± I, ^(48)Ca/^(44)Ca = 0.08871 ± 2, where the errors correspond to the last figures shown. This yields an atomic weight of 40.076 ± 0.001. The data indicate the absence in the studied samples of detectable, distinct nuclear components in Ca similar to those observed for oxygen. In the samples studied, there is a distinct but small degree of Ca isotope fractionation. Overlapping ranges of fractionation of 2.5‰ for ^(40)Ca/^(44)Ca (four atomic mass units) are observed in meteorites, lunar, and terrestrial samples. Means by which isotope fractionation could arise for Ca are discussed, but the small range of effects and the lack of systematic variations do not permit at present the identification of the mechanisms responsible for the fractionation observed in the suite of samples. Ca in the biological cycle does not show fractionation effects larger than observed for non-biogenic samples. In contrast to these results, we have observed large effects of up to 13‰, for industrially purified Ca.

Negative thermal ion mass spectrometry of osmium, rhenium and iridium

We report on a technique for obtaining intense ion beams of negatively charged oxides of Os, Re and Ir by thermal ionization, in a conventional surface ionization mass spectrometer. It was found that the principal ion species of Os, Re and Ir produced are OsO_3^−, ReO_4^− and IrO_2^−. The sharp distinction in the masses of the dominant molecular species produced by this technique permits the measurement of isotopic compositions of each element from mixtures of platinum-group elements without significant isobaric interferences. For ^(187)Re-^(187)Os isotope studies, this technique offers the advantage of isotopic analyses without prior chemical separation of Re from Os, as no isobaric interference between the oxides of ^(187)Os and ^(187)Re exists under these conditions. For 4 ng Os, stable ion currents of 3 × 10^(−12) A can be maintained for over one hour, which allows determination of isotopic ratios with a Faraday collector to a precision of better than ±2‰ (2 σ_m. For 70 pg Os, isotopic ratios can be measured with a precision of better than ±5‰ using a secondary electron multiplier. The detection limit for Os is estimated to be below 10^(−14) g. Osmium isotopic ratios have also been determined by direct loading of natural iridosmine with a precision of ±0.5‰ or better. We have obtained ionization efficiencies of 2–6% for Os and >20% for Re; these are superior to those reported for other techniques available to date and demonstrate that negative thermal ion mass spectrometry will have widespread application to ^(187)Re-^(187)Os chronometry and to studies of the geochemistry and environmental chemistry of the platinum-group elements.

Mineralogy and Petrology of Comet 81P/Wild 2 Nucleus Samples

The bulk of the comet 81P/Wild 2 (hereafter Wild 2) samples returned to Earth by the Stardust spacecraft appear to be weakly constructed mixtures of nanometer-scale grains, with occasional much larger (over 1 micrometer) ferromagnesian silicates, Fe-Ni sulfides, Fe-Ni metal, and accessory phases. The very wide range of olivine and low-Ca pyroxene compositions in comet Wild 2 requires a wide range of formation conditions, probably reflecting very different formation locations in the protoplanetary disk. The restricted compositional ranges of Fe-Ni sulfides, the wide range for silicates, and the absence of hydrous phases indicate that comet Wild 2 experienced little or no aqueous alteration. Less abundant Wild 2 materials include a refractory particle, whose presence appears to require radial transport in the early protoplanetary disk.

Isotopic evidence for a terminal lunar cataclysm

Most highland total rock samples define a single UPb isochron which corresponds to a metamorphism age of ∼ 3.9AE. This age is also obtained for internal UPb isochrons for some of these samples. The data on 18 rock samples range from concordant samples with238U/206Pb∼ 1.2 to discordant ones with238U/206Pb∼ 0.02. This feature coupled with a correlated pattern of 238U/204Pb ratios, indicates that Pb was extensively mobilized at ∼ 3.9AE. The observed PbU fractionation is essentially due to Pb volatilization during the metamorphic events. Volatile Pb transport is not accompanied by similar effects in Rb and must therefore be attributed to a specific process. RbSr internal isochrons for the same rocks determine distinct metamorphic events in the interval 3.85–4.00 AE. We conclude that highland samples from widely separated areas bear the imprint of an event or series of events in a narrow time interval which can be identified with a cataclysmic impacting rate of the moon at ∼ 3.9AE, although differentiation by internal magma generation cannot be excluded. This cataclysm is associated with the Imbrium impact and very possibly the formation of Crisium and Orientale and possibly several other major basins in a narrow time interval (∼2 × 108yr or less). The UPb data indicate formation of the lunar crust at ∼ 4.4AE, which is distinctly younger than 4.6 AE, the time generally associated with planetary formation. If the lunar crust started being formed at ∼ 4.6AE, then this process must have continued until times significantly younger than 4.4 AE. RbSr data also indicate formation of the lunar crust around 4.5 AE with only minor additions of high Rb/Sr materials to the crust at times younger than 4.3 AE. Using the UPb systematics, K/U and the average U concentration of the moon as obtained from heat-flow measurements, we estimate the lunar concentrations: primordial Pb= 35ppb;Rb= 0.5ppm with Rb/Sr= 0.006.

Organics captured from comet 81P/Wild 2 by the Stardust spacecraft

Organics found in comet 81P/Wild 2 samples show a heterogeneous and unequilibrated distribution in abundance and composition. Some organics are similar, but not identical, to those in interplanetary dust particles and carbonaceous meteorites. A class of aromatic-poor organic material is also present. The organics are rich in oxygen and nitrogen compared with meteoritic organics. Aromatic compounds are present, but the samples tend to be relatively poorer in aromatics than are meteorites and interplanetary dust particles. The presence of deuterium and nitrogen-15 excesses suggest that some organics have an interstellar/protostellar heritage. Although the variable extent of modification of these materials by impact capture is not yet fully constrained, a diverse suite of organic compounds is present and identifiable within the returned samples.

Isotopic Compositions of Cometary Matter Returned by Stardust

Hydrogen, carbon, nitrogen, and oxygen isotopic compositions are heterogeneous among comet 81P/Wild 2 particle fragments; however, extreme isotopic anomalies are rare, indicating that the comet is not a pristine aggregate of presolar materials. Nonterrestrial nitrogen and neon isotope ratios suggest that indigenous organic matter and highly volatile materials were successfully collected. Except for a single 17O-enriched circumstellar stardust grain, silicate and oxide minerals have oxygen isotopic compositions consistent with solar system origin. One refractory grain is 16O-enriched, like refractory inclusions in meteorites, suggesting that Wild 2 contains material formed at high temperature in the inner solar system and transported to the Kuiper belt before comet accretion.

INITIAL STRONTIUM ISOTOPIC ABUNDANCES AND THE RESOLUTION OF SMALL TIME DIFFERENCES IN THE FORMATION OF PLANETARY OBJECTS.

It is shown that differences in the 87 Sr/ 86 Sr ratio corresponding to about 10 −2 % are clearly resolvable using improved instrumental techniques. Several basaltic achondrites having a total spread of 0.2% in 87 Sr/ 86 Sr were studied and appear to define an identical initial 87 Sr/ 86 Sr abundance corresponding to 0.698976 ± 0.000055 (maximum uncertainties). These samples are found to lie on a well defined isochron with a slope of 0.0629 ∓ 0.0037 and an age 4.39 ∓ 0.26 × 10 9 yr. for λ = 1.39 × 10 −11 yr −1 . The maximum deviation of a data point from the best fit line is 6 × 10 −3 %. This shows that if the samples were derived from material with chondritic or solar Rb/Sr abundance ratios, they were formed within a time period of 4 m.y. or 1.6 m.y. respectively. The application of such measurements to establish a refined early solar system chronology is discussed.

Aluminum-26 in the early solar system - Fossil or fuel

The isotopic composition of Mg was measured in different phases of a Ca-Al rich inclusion in the Allende meteorite. Large excesses of /sup 26/Mg of up to 10% were found. These excesses correlate strictly with the /sup 27/Al//sup 24/Mg for four coexisting phases with distinctive chemical compositions. Models of in situ decay of /sup 26/Al within the solar system and of mixing of interstellar dust grains containing fossil /sup 26/Al with normal solar system material are presented. The observed correlation provides definitive evidence for the presence of /sup 26/Al in the early solar system. This requires either injection of freshly synthesized nucleosynthetic material into the solar system immediately before condensation and planet formation, or local production within the solar system by intense activity of the early Sun. Planets promptly produced from material with the inferred /sup 26/Al//sup 27/Al would melt within approx.3 x 10/sup 5/ yr.

The identification of early condensates from the solar nebula

Abstract Calcium-aluminum-rich chondrules which are highly deficient in alkalis were extracted from the carbonaceous chondrite Allende and yield a range of compositions with the lowest measured isotopic composition of ( 87 Sr / 86 Sr ) ALL = 0.69877±0.00002 and identify this material as the earliest known condensate from the solar nebula. Other chondrules suggest the possible presence of even more primitive Sr in this meteorite. This result also shows that some chondritic material formed very near the earliest part of the condensation sequence. Using alkali-deficient planetary objects (Moon, basaltic achondrites, Angra dos Reis, Allende), the Sr data indicate a time interval for condensation of 10 m.y. (from ALL to BABI) if condensation occurred in a solar Rb/Sr environment. A variety of alkali-rich olivine chondrules and CaAl-rich aggregates from Allende fail to determine an isochron and indicate that the element distribution in this meteorite was disturbed later than 3.6ae, possibly recently, in a cometary nucleus. This disturbance requires that the determination of initial 87 Sr/ 86 Sr be done on essentially Rb-free phases. Strontium data from equilibrated chondrites and from an iron meteorite establish an interval for metamorphism or differentiation in protoplanetary objects which followed the condensation process by ≈80 mm.y. The chronology for condensation and early planetary evolution obtained for Sr is in disagreement with the 129 I chronology but can be brought into agreement, if it is assumed that the high temperature iodine containing phases have not been affected by the metamorphic events determined by Sr.

Lunar chronology and evolution from RbSr studies of Apollo 11 and 12 samples

Abstract Rb-Sr internal isochrons for a total of eight Apollo 12 crystalline rocks yield ages of 3.36 to 3.16 AE. The initial Sr compositions ( I ) are relatively primitive and range from 0.69918 to 0.69957 as compared to BABI= 0.69898 ± 3 . No clear groupings in I are observed, however, the wide range indicates that at least four different rock bodies were sampled. An Apollo 11 basalt (10024) yielded an age of 3.61 ± 0.07AE andI = 0.69935 ∓ 8 in agreement with previous results on other Apollo 11 high K rocks. Several Apollo 12 soil samples yield model ages which range from 4.4 to 4.6 AE and indicate that the special nature and older “age” of the lunar soil determined at the Apollo 11 site is a widespread phenomenon. Initial Sr compositions from Apollo 11 and Apollo 12 support our previous conclusions that the moon as a whole has a Rb/Sr much lower ( Rb/Sr≈ 0.008 ) than found in chondrites. We summarize the current status of Rb Sr lunar chronology and some implications regarding the melting and differentiation history of the moon.