Rhenium-osmium isotope systematics of carbonaceous chondrites

Rhenium and osmium concentrations and Os isotopic compositions of eight carbonaceous chondrites, one LL3 ordinary chondrite, and two iron meteorites were determined by resonance ionization mass spectrometry. Iron meteorite (187)Re/(186)Os and (l87)Os/(l86)Os ratios plot on the previously determined iron meteorite isochron, but most chondrite data plot 1 to 2 percent above this meteorite isochron. This suggests either that irons have significantly younger Re-Os closure ages than chondrites or that chondrites were formed from precursor materials with different chemical histories from the precursors of irons. Some samples of Semarkona (LL3) and Murray (C2M) meteorites plot 4 to 6 percent above the iron meteorite isochron, well above the field delineated by other chondrites. Murray may have lost Re by aqueous leaching during its preterrestrial history. Semarkona could have experienced a similar loss of Re, but only slight aqueous alteration is evident in the meteorite. Therefore, the isotopic composition of Semarkona could reflect assembly of isotopically heterogeneous components subsequent to 4.55 billion years ago or Os isotopic heterogeneities in the primordial solar nebula.     

Oxygen isotope variation in stony-iron meteorites

Asteroidal material, delivered to Earth as meteorites, preserves a record of the earliest stages of planetary formation. High-precision oxygen isotope analyses for the two major groups of stony-iron meteorites (main-group pallasites and mesosiderites) demonstrate that each group is from a distinct asteroidal source. Mesosiderites are isotopically identical to the howardite-eucrite-diogenite clan and, like them, are probably derived from the asteroid 4 Vesta. Main-group pallasites represent intermixed core-mantle material from a single disrupted asteroid and have no known equivalents among the basaltic meteorites. The stony-iron meteorites demonstrate that intense asteroidal deformation accompanied planetary accretion in the early Solar System.     

Electrolytic dissolution of iron meteorites

When iron meteorites are dissolved anodically in neutral solution, nonmetallic inclusions are not attached and collect at the bottom of the anode compartment. When the meteorites contain both kamacite and taenite, the kamacite dissolves preferentially, revealing a three-dimensional Widmanst?tten pattern.     

Carbon isotope fractionation in the Fischer-tropsch synthesis and in meteorites

Carbon dioxide and organic compounds made by a Fischer-Tropsch reaction at 400 degrees K show a kinetic isotope fractionation of 50 to 100 per mil, similar to that observed in carbonaceous chondrites. This result supports the view that organic compounds in meteorites were produced by catalytic reactions between carbon monoxide and hydrogen in the solar nebula.     

Potassium: argon dating of iron meteorites

The potassium:argon age of the metal phase of Weekeroo Station iron meteorite, determined by neutronactivation analysis, is about 10(10) years; it is similar to ages previously measured for other iron meteorites, but distinctly disagrees with a strontium: rubidium age of 4.7 X 10(9) years measured by other workers on silicate inclusions in this meteorite.     

A striking nitrogen isotope anomaly in the bencubbin and weatherford meteorites

The stony-iron meteorites Bencubbin and Weatherford contain nitrogen with a ratio of nitrogen-15 to nitrogen-14 larger than normal by as much as a factor of 2. The excess nitrogen-15 may be due either to a nucleosynthetic origin or to extreme isotopic fractionation. In the former case, it may reflect failure to homogenize nitrogen-15 produced in nova explosions. In the latter case, it may reflect chemical processing at temperatures below 40 K in a presolar molecular cloud.     

Rapid methods of determining cooling rates of iron and stony iron meteorites

Two rapid and simple methods have been developed for determining the approximate cooling rates of iron and stony-iron meteorites in which kamacite formed by diffusion-controlled growth along planar fronts. The first method requires only measurements of the mean kamacite bandwidth and the bulk nickel content. The second method requires the determination of the nickel composition near the taenite-kamacite interface with an electron microprobe.     

Equilibration temperatures of iron and magnesium in chondritic meteorites

The distribution coefficients for Fe(++) and Mg(++) were calculated from new microprobe analyses of coexisting olivine, orthopyroxene, and calcic pyroxene in chondritic meteorites. Interpretation of the data shows that (i) the equilibration temperatures were of the order of 850 degrees C, and (ii) the olivineorthopyroxene partition does not reflect ideal behavior. This equilibration temperature is much lower than previous estimates.     

X-ray diffraction study of minerals from shocked iron meteorites

Diffraction analysis of minerals from iron meteorites indicates a pronounced shock-induced alteration in the minerals' crystallographic character. The extent of alteration seems to be dependent on the degree of shock and can therefore serve as a measure of shock intensity. The changes appear to be due to the minerals' direct recrystallization during passage of the shock wave.     

Iron isotope constraints on the Archean and Paleoproterozoic ocean redox state

The response of the ocean redox state to the rise of atmospheric oxygen about 2.3 billion years ago (Ga) is a matter of controversy. Here we provide iron isotope evidence that the change in the ocean iron cycle occurred at the same time as the change in the atmospheric redox state. Variable and negative iron isotope values in pyrites older than about 2.3 Ga suggest that an iron-rich global ocean was strongly affected by the deposition of iron oxides. Between 2.3 and 1.8 Ga, positive iron isotope values of pyrite likely reflect an increase in the precipitation of iron sulfides relative to iron oxides in a redox stratified ocean.     

Oxygen isotope constraints on the origin of impact glasses from the cretaceous-tertiary boundary

Laser-extraction oxygen isotope and major element analyses of individual glass spherules from Haitian Cretaceous-Tertiary boundary sediments demonstrate that the glasses fall on a mixing line between an isotopically heavy (delta(18)O = 14 per mil) high-calcium composition and an isotopically light (delta(18)O = 6 per mil) high-silicon composition. This trend can be explained by melting of heterogeneous source rocks during the impact of an asteroid (or comet) approximately 65 million years ago. The data indicate that the glasses are a mixture of carbonate and silicate rocks and exclude derivation of the glasses either by volcanic processes or as mixtures of sulfate-rich evaporate and silicate rocks.     

Mossbauer investigation of shocked and unshocked iron meteorites and fayalite

M?ssbauer spectra of several iron meteorites have been measured by a resonant scattering technique rather than by the conventional transmission method, thereby eliminating the necessity for the preparation of thin samples. No significant differences were observed in the spectra of specimens of mechanically deformed, shocked, and unshocked iron meteorites, nor in the absorption spectra of artificially shocked and unshocked fayalite.     

Oxygen isotope constraints on the sulfur cycle over the past 10 million years

Oxygen isotopes in marine sulfate (delta18O(SO4)) measured in marine barite show variability over the past 10 million years, including a 5 per mil decrease during the Plio-Pleistocene, with near-constant values during the Miocene that are slightly enriched over the modern ocean. A numerical model suggests that sea level fluctuations during Plio-Pleistocene glacial cycles affected the sulfur cycle by reducing the area of continental shelves and increasing the oxidative weathering of pyrite. The data also require that sulfate concentrations were 10 to 20% lower in the late Miocene than today.     

Rhenium-osmium and samarium-neodymium isotopic systematics of the stillwater complex

Isotopic data for the Stillwater Complex, Montana, which formed about 2700 Ma (million years ago), were obtained to evaluate the role of magma mixing in the formation of strategic platinum-group element (PGE) ore deposits. Neodymium and osmium isotopic data indicate that the intrusion formed from at least two geochemically distinct magmas. Ultramafic affinity (U-type) magmas had initial epsilon(Nd) of -0.8 to -3.2 and a chondritic initial (187)Os/(186)Os ratio of approximately 0.88, whereas anorthositic affinity (A-type) magmas had epsilon(Nd) of -0.7 to +1.7 and an initial (187)Os/(186)Os ratio of approximately -1.13. These data suggest that U-type magmas were derived from a lithospheric mantle source containing recycled crustal materials whereas A-type magmas originated either by crustal contamination of basaltic magmas or by partial melting of basalt in the lower crust. The Nd and Os isotopic data also suggest that Os, and probably the other PGEs in ore horizons such as the J-M Reef, was derived from A-type magmas. The Nd and Os isotopic heterogeneity observed in rocks below the J-M Reef also suggests that A-type magmas were injected into the Stillwater U-type magma chamber at several stages during the development of the Ultramafic series.     

Ungrouped iron meteorites in antarctica: origin of anomalously high abundance

Eighty-five percent of the iron meteorites collected outside Antarctica are assigned to 13 compositionaily and structurally defined groups; the remaining 15 percent are ungrouped. Of the 31 iron meteorites recovered from Antarctica, 39 percent are ungrouped. This major difference in the two sets is almost certainly not a stochastic variation, a latitudinal effect, or an effect associated with differences in terrestrial ages. It seems to be related to the median mass of Antarctic irons, which is about 1/100 that of non-Antarctic irons. During impacts on asteroids, smaller fragments tend to be ejected into space at higher velocities than larger fragments, and, on average, small meteoroids have undergone more changes in orbital velocity than large ones. As a result, the set of asteroids that contributes small meteoroids to Earth-crossing orbits is larger than the set that contributes large meteoroids. Most small iron meteorites may escape from the asteroid belt as a result of impact-induced changes in velocity that reduce their perihelia to values less than the aphelion of Mars.     

Comment on "Iron isotope constraints on the Archean and Paleoproterozoic ocean redox state"

Rouxel et al. (Reports, 18 February 2005, p. 1088) argued that changes in the iron isotopic composition of sedimentary sulfides reflect changes in the oxidation state of the atmosphere-ocean system between 2.3 and 1.8 million years ago. We show that misinterpretations of the origins of these minerals undermine their conclusions.     

Natural iron isotope variations in human blood

Isotopic analysis of human blood and liver and muscle tissue indicates that each individual bears a long-term iron (Fe) isotope signature in the blood. Blood and tissue differ slightly in isotopic composition and are depleted by up to 2.6 per mil in 56Fe relative to 54Fe when compared to dietary Fe. The 56Fe/54Fe isotope ratio in the blood of males is, on average, lower by 0.3 per mil than that of females. These results suggest that Fe isotope effects in the blood reflect differences in intestinal Fe absorption between individuals and genotypes.     

Potassium feldspar in weekeroo station, kodaikanal, and colomera iron meteorites

Sodium plagioclase and small amounts of potassium feldspar are common constituents of silicate inclusions in the Weekeroo Station and Colomera iron meteorites; flamboyant x-ray antiperthite is unique to Kodaikanal silicate inclusions. Enrichment of potassium, sodium, silicon, and aluminum in these inclusions indicates a higher degree of chemical differentiation than in other meteorites.