Ernst K. Zinner

A method for the quantitative measurement of rare earth elements in the ion microprobe

Abstract An ion probe method for the quantitative measurement of rare earth elements (REE) in phosphates is presented. The measurements are made with a Cameca IMS 3F ion microprobe. All complex molecular interferences are removed effectively with moderate energy filtering and the remaining mass spectrum is deconvoluted into contributions from REE and their monoxides only. Corrections for fluorides, which are present in fluorapatites, can be made. Sensitivity factors relative to Ca are determined from a terrestrial apatite standard. Comparison with other standards of known REE concentrations shows that working curves are linear over a wide range of concentrations. The method allows the measurements of all REE in spots of 5–20 μm in diameter. Detection limits depend on the overall REE pattern but are generally ⩽ 50 p.p.b. for the light REE and ∼ 200 p.p.b. for the heavy REE. Preliminary measurements on oxides show that this technique can be extended successfully to other phases.

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.

Impact Features on Stardust: Implications for Comet 81P/Wild 2 Dust

Particles emanating from comet 81P/Wild 2 collided with the Stardust spacecraft at 6.1 kilometers per second, producing hypervelocity impact features on the collector surfaces that were returned to Earth. The morphologies of these surprisingly diverse features were created by particles varying from dense mineral grains to loosely bound, polymineralic aggregates ranging from tens of nanometers to hundreds of micrometers in size. The cumulative size distribution of Wild 2 dust is shallower than that of comet Halley, yet steeper than that of comet Grigg-Skjellerup.

CARBON, NITROGEN, MAGNESIUM, SILICON, AND TITANIUM ISOTOPIC COMPOSITIONS OF SINGLE INTERSTELLAR SILICON CARBIDE GRAINS FROM THE MURCHISON CARBONACEOUS CHONDRITE

Seven hundred and twenty SiC grains from the Murchison CM2 chondrite, ranging in size from 1 to 10 micrometers, were analyzed by ion microprobe mass spectrometry for their C-isotopic compositions. Subsets of the grains were also analyzed for N (450 grains), Si (183 grains), Mg (179 grains), and Ti (28 grains) isotopes. These results are compared with previous measurements on 41 larger SiC grains (up to 15 x 26 micrometers) from a different sample of Murchison analyzed by Virag et al. (1992) and Ireland, Zinner, & Amari (1991a). All grains of the present study are isotopically anomalous with C-12/C-13 ratios ranging from 0.022 to 28.4 x solar, N-14/N-15 ratios from 0.046 to 30 x solar, Si-29/Si-28 from 0.54 to 1.20 x solar, Si-30/Si-28 from 0.42 to 1.14 x solar, Ti-49/Ti-48 from 0.96 to 1.95 x solar, and Ti-50/Ti-48 from 0.94 to 1.39 x solar. Many grains have large Mg-26 excesses from the decay of Al-26 with inferred Al-26/Al-27 ratios ranging up to 0.61, or 12,200 x the ratio of 5 x 10(exp -5) inferred for the early solar system. Several groups can be distinguished among the SiC grains. Most of the grains have C-13 and N-14 excesses, and their Si isotopic compositions (mostly excesses in Si-29 and Si-30) plot close to a slope 1.34 line on a Delta Si-29/Si-28 versus Delta Si-30/Si-28 three-isotope plot. Grains with small C-12/C-13 ratios (less than 10) tend to have smaller or no N-14 excesses and high Al-26/Al-27 ratios (up to 0.01). Grains with C-12/C-13 greater than 150 fall into two groups: grains X have N-15 excesses and Si-29 and Si-30 deficits and the highest (0.1 to 0.6) Al-26/Al-27 ratios; grains Y have N-14 excesses and plot on a slope 0.35 line on a Si three-isotope plot. In addition, large SiC grains of the Virag et al. (1992) study fall into three-distinct clusters according to their C-, Si-, and Ti-isotopic compositions. The isotopic diversity of the grains and the clustering of their isotopic compositions imply distinct and multiple stellar sources. The C- and N-isotopic compositions of most grains are consistent with H-burning in the CNO cycle. These and s-process Kr, Xe, Ba, and Nd suggest asymptotic giant branch (AGB) or Wolf-Rayet stars as likely sources for the grains, but existing models of nucleosynthesis in these stellar sites fail to account in detail for all the observed isotopic compositions. Special problems are posed by grains with C-12/C-13 less than 10 and almost normal and heavy N-isotopic compositions. Also the Si- and Ti-isotopic compositions, with excesses in Si-29 and Si-30 relative to Si-28 and excesses in all Ti isotopes relative to Ti-48, do not precisely conform with the compositions predicted for slow neutron capture. Additional theoretical efforts are needed to achieve an understanding of the isotopic composition of the SiC grains and their stellar sources.

Elemental compositions of comet 81P/Wild 2 samples collected by Stardust

We measured the elemental compositions of material from 23 particles in aerogel and from residue in seven craters in aluminum foil that was 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 ng). The mean elemental composition of this Wild 2 material is consistent with the CI meteorite composition, which is thought to represent the bulk composition of the solar system, for the elements Mg, Si, Mn, Fe, and Ni to 35%, and for Ca and Ti to 60%. The elements Cu, Zn, and Ga appear enriched in this Wild 2 material, which suggests that the CI meteorites may not represent the solar system composition for these moderately volatile minor elements.

26Al, 244Pu, 50Ti, REE, and trace element abundances in hibonite grains from CM and CV meteorites

The ion microprobe was used to measure Ti and Mg isotopes as well as rare earth and other trace elements in ten hibonites from the CM carbonaceous chondrites Murchison, Murray, and Cold Bokkeveld and in two hibonites and Ti-rich pyroxene from the CV chondrite Allende. In hibonites from Murchison and Murray fission track densities were also measured, as were Th and U concentrations. Eight of the hibonites, from all four meteorites, exhibit large Ti isotopic anomalies, particularly in 50Ti. Two grains from Murray have 50Ti excesses of ~ 10%. At least four nucleosynthetic components are required to account for all the Ti isotopic data. Neutron-rich nuclear statistical equilibrium nucleosynthesis is the most likely process to account for a 50Ti-rich component (with 50Ti49Ti

Ion microprobe isotopic measurements of individual interplanetary dust particles

20). The ion probe Ti isotopic measurements confirm that the solar nebula was isotopically heterogeneous on a small spatial scale and argue for a chemical memory origin of the Ti isotopic anomalies which were probably carried into the solar system in the form of refractory dust grains. However, there is no experimental evidence that such interstellar grains survived the formation of the hibonites. The REE and trace element patterns of the hibonites are similar to those seen in CAIs and can be interpreted in terms of fractionation effects during condensation from a gas of solar composition, thus arguing for a solar system origin of the hibonites. Additional evidence for such an origin is provided by the PuTh ratios, which are ~ 10−4, and by the Mg isotopic compositions which are normal except for 26Mg∗ due to 26Al. Only three out of ten hibonites exhibit 26Mg∗, consistent with previous studies which demonstrated the paucity of 26Mg∗ in hibonites. Because of the refractory nature of hibonite and the presence of large Ti isotopic effects, we conclude that a heterogeneous distribution of 26Al in the early solar system is the most likely reason. In particular, our observations of δ50Ti = 15%. and of an isochron with (26Al27Al)0 = 5 × 10−8 in the FUN inclusion HAL are evidence against both late formation and Mg redistribution to explain the lack of 26Al in hibonites. There are no obvious correlations between the Ti isotopic compositions, the presence of 26Mg∗, the presence of 244Pu, and the REE and trace element patterns in individual hibonites. This indicates that the anomalous 50Ti, as well as 26A1 and 244Pu, were not co-produced in a single astrophysical source, and/or that these nuclides were introduced into the solar nebula by different carriers before being incorporated into the hibonites.

Interstellar SiC with unusual isotopic compositions - Grains from a supernova?

Ion microprobe measurements of D/H ratios in individual fragments of eight stratospheric dust particles give δD values ranging from −386 to +2534‰ relative to SMOW. The δD values in five particles far exceed those in terrestrial samples and prove that the samples are interplanetary dust particles (IDPs). The hydrogen isotopic composition is heterogeneous on a scale of a few microns demonstrating that the dust is unequilibrated. Measurements of D/H ratios in conjunction with elemental and molecular ion signals in different fragments of individual IDPs show that a carbonaceous phase, not water, is the carrier of the D enrichments. Previous infrared transmission measurements have shown that IDPs fall into three main spectral classes. Particles from two of those three IR classes show large D/H ratios. Two particles studied from the third class do not. However, one of these contains solar flare tracks and is extraterrestrial. Thus, most, but not all, IDPs contain hydrogen with a non-terrestrial isotopic composition. Carbon isotopic measurements on fragments of three IDPs give ratios similar to terrestrial values and show a largely uniform isotopic composition for a given particle. Small, but significant, differences in δ13C of ~40‰ between particles are seen. No correlations between the hydrogen and carbon isotopic compositions are observed. The magnesium and silicon isotopic compositions of fragments of three IDPs are found to be normal within measurement errors.

Interstellar grains within interstellar grains

Results are presented from an ion microprobe mass spectrometric analyses of five SiC grains from the Murchison carbonaceous meteorite. Unlike most interstellar SiC grains from primitive meteorites, the five grains from the Murchison meteorite show large excesses of C-12 (up to 28 times solar) and N-15 (up to 22 times solar), depletion in Si-29 and Si-30 (up to 59 percent), Al-26/Al-27 ratios between 0.1 and 0.6, and Ti-49 excesses up to 95 percent; in addition, one grain has a large Ca-44 excess (300 percent). The Ca and Ti anomalies point toward explosive nucleosynthesis in supernovae and the in situ decay of the radioactive precursors Ti-44 and V-49 in SiC grains formed in supernova ejecta. However, there is no simple formation scenario that can give a consistent explanation for the isotopic compositions of these grains.

Low-Density Graphite Grains and Mixing in Type II Supernovae

Five interstellar graphite spherules extracted from the Murchison carbonaceous meteorite are studied. The isotopic and elemental compositions of individual particles are investigated with the help of an ion microprobe, and this analysis is augmented with structural studies of ultrathin sections of the grain interiors by transmission electron microscopy. As a result, the following procedure for the formation of the interstellar graphite spherule bearing TiC crystals is inferred: (1) high-temperature nucleation and rapid growth of the graphitic carbon spherule in the atmosphere of a carbon-rich star, (2) nucleation and growth of TiC crystals during continued growth of the graphitic spherule and the accretion of TiC onto the spherule, (3) quenching of the graphite growth process by depletion of C or by isolation of the spherule before other grain types could condense.