A comparative study of experimental and meteoritic metal-sulfide assemblages

Sulfide formation via a gas-solid reaction between iron-nickel metal and H_2/H_2S gas mixtures was studied experimentally. This reaction produces distinctive chemical fractionations in both metal and sulfide that can help identify pristine nebular sulfide condensates in meteorites. The resulting sulfide morphology consists of a troilite scale divided into two distinct layers : an inner layer containing small, randomly oriented crystals and an outer layer consisting of large, columnar crystals. A thin band of metal surrounding the unreacted metal core and small metal blebs located in the inner sulfide layer are significantly enriched in nickel relative to the starting metal composition. The stoichiometry of the sulfide is nearly ideal {(Fe+Ni+Co)/S=1} at the metal-sulfide interface but the sulfur content increases with distance from the metal. A significant amount of nickel is present in the sulfide layer and increases in concentration across the sulfide layer. The nickel concentration gradient results from diffusion of nickel ions in the sulfide being faster than that of iron ions. Microprobe analyses on metal-sulfide assemblages in the LL3 unequilibrated ordinary chondrite Allan Hills-764 (ALH-764) do not show these chemical fractionations. Instead, textural and chemical evidence suggests that these meteoritic sulfides were altered during a post accretion heating event

Trace Element Zoning and Incipient Metamictization in a Lunar Zircon: Application of Three Microprobe Techniques

We have determined major (Si, Zr, Hf), minor (Al, Y, Fe, P), and trace element (Ca, Sc, Ti, Ba, REE, Th, U) concentrations and Raman spectra of a zoned, 200 microns zircon grain in lunar sample 14161,7069, a quartz monzodiorite breccia collected at the Apollo 14 site. Analyses were obtained on a thin section in situ with an ion microprobe, an electron microprobe, and a laser Raman microprobe. The zircon grain is optically zoned in birefringence, a reflection of variable (incomplete) metamictization resulting from zo- nation in U and Th concentrations. Variations in the concentrations of U and Th correlate strongly with those of other high-field-strength trace elements and with changes in Raman spectral parameters. Concentrations of U and Th range from 21 to 55 ppm and 6 to 31 ppm, respectively, and correlate with lower Raman peak intensities, wider Raman peaks, and shifted Si-O peak positions. Concentrations of heavy rare earth elements range over a factor of three to four and correlate with intensities of fluorescence peaks. Correlated variations in trace element concentrations reflect the original magmatic differentiation of the parental melt approx. 4 b.y. ago. Degradation of the zircon structure, as reflected by the observed Raman spectral parameters, has occurred in this sample over a range of alpha-decay event dose from approx. 5.2 x 10(exp 14) to 1.4 x 10(exp 15) decay events per milligram of zircon, as calculated from the U and Th concentrations. This dose is well below the approx. 10(exp 16) events per milligram cumulative dose that causes complete metamictization and indicates that laser Raman microprobe spectroscopy is an analytical technique that is very sensitive to the radiation-induced damage in zircon

The kinetics and mechanism of iron sulfide formation in the solar nebula

We summarize an experimental study of the kinetics and mechanism of FeS formation by the reaction of H_2S-H_2 gas mixtures with iron metal. Characterization of the reacted samples by optical microscopy, X-ray diffraction, electron microprobe analyses, and gravimetric analyses provided detailed information on the Fe/S ratio, microstructure and morphology, and formation kinetics of the iron sulfide layers. The Fe/S ratios of the iron sulfide layers varied from Fe_S to FeS with temperature and gas composition, in agreement with models of gas-solid equilibrium. The morphology, microstructure, and growth orientation of the sulfide layers also varied with temperature and gas composition. Typically, sulfide layer growth proceeded by the development of a compact, uniformly oriented scale which later cracked when it could no longer plastically deform. Further reaction led to the growth of a finer grained, randomly oriented, highly porous inner layer between the metal and original sulfide scale. Initially sulfide layers grew linearly with time with the kinetics controlled by chemical reactions at the gas-solid interface. However, upon reaching a critical thickness, diffusion through the sulfide scale became the rate limiting step and layer growth followed parabolic kinetics. The linear and parabolic rate constants for iron sulfide growth were determined and then used to constrain FeS formation in the solar nebula. FeS formation is rapid compared to estimated nebular lifetimes of 1-10 million years. Our results also imply that the variations in the sulfur content of chondritic material are due to removal of metal grains from contact with the gas (e. g., by accretion into larger bodies) at temperatures above 400K, where complete sulfur condensation occurs, rather than by kinetic inhibition of gas-solid equilibrium between H_2S gas and iron metal grains

Erratum to: Azimuthal asymmetries of charged hadrons produced in high-energy muon scattering off longitudinally polarised deuterons

The original version of this article unfortunately contained a mistake