Shi Tong Xue

Isotopic and elemental composition of iron, nickel, and chromium in type I deep-sea spherules: implications for origin and composition of the parent micrometeoroids

We report elemental and isotopic analyses of Fe, Ni, and Cr in type I deep-sea spherules with masses ranging from 43 to 256 μg. We measured (1) the isotopic compositions of Fe and Cr by thermal ionization mass spectrometry; and (2) the elemental concentrations of Fe, Ni, and Cr and the isotopic compositions of Ni by inductively-coupled plasma mass spectrometry. Evaporation of Fe, Ni, and Cr during atmospheric entry led to large and similar average degrees of mass-dependent fractionation, Φ, in most spherules. The average value, ∼16‰/AMU, corresponds to mass losses of 80–85%, assuming open-system evaporation of the atoms. We find ΦCr ∼ ΦFe, in seven spherules. This observation implies similar evaporation rates for Cr and Fe and that the measured Cr/Fe ratios (mass/mass) are close to those of the progenitors. Four spherules have Cr/Fe ∼0.003; two others with low Cr/Fe, ∼8 × 10−4, high Fe/Ni, ∼2000, and ΦCr ∼ ΦFe ∼0, may belong to a different, possibly terrestrial, population. A seventh spherule with “chondritic” Cr/Fe, ∼17 × 10−3 and subaverage ΦCr and ΦFe, 8–10‰/AMU, may represent still another source of particles. Because the higher vapor pressure of pure Cr should lead to ΦCr > ΦFe we infer either that Cr has a low activity coefficient in liquid Fe or that it forms a relatively involatile species there. A best fit correlation between ΦCr and ΦFe can be expressed in the form ΦCr = 0.31 × ΦFe1.47, although the data also are adequately fit by a linear regression. Correlated variation of ΦNi and ΦFe can be fit by the empirical relationship ΦNi = 0.016 × ΦFe2.58. For low ΦFe, we find ΦNi ΦFe, which probably reflects the increase with temperature of the vapor pressure of pure Ni, changes in activity coefficients of Fe and Ni, and the formation of relatively involatile wustite and magnetite. Differences between ΦNi and ΦFe in many samples mean that measured Fe/Ni ratios may differ appreciably from pre-atmospheric values. After compensating for evaporation by using the Rayleigh law, we estimate an average pre-atmospheric Fe/Ni ratio (by mass) in type I spherules of 19 ± 4 (σmean). Similarly, by assuming Ir is involatile, we obtain a preatmospheric ratio of Ir/Ni = 3 × 10−5, which is about 10 times smaller than the average measured value, but similar to the cosmic (CI) abundance ratio of 4 × 10−5. Cosmogenic nuclides have been detected in some Type I spherules at levels indicating irradiation as metal in space. Among conventional meteorites, the best matches to both the Cr/Fe and Fe/Ni ratios inferred for type I progenitors are metal from CO, CV, and CR chondrites and from unequilibrated ordinary chondrites. The match with metal from CM chondrites is acceptable but somewhat poorer. Iron meteorites, because of their low Cr/Fe ratios and low flux to Earth, make unlikely progenitors for type I spherules. We propose that most type I spherules derive from metal grains in carbonaceous-chondrite-like objects that were freed by comminution in space, or, less likely, that collisions of large objects formed droplets rich in metal.

Nickel isotope abundances of type I deep-sea spheres and of iron-nickel spherules from sediments in Alberta, Canada

Nickel isotope abundances were measured by ICP-MS in twenty-one whole, type I deep-sea spheres, in Ni-rich cores and oxide shells separated from three others, and in Fe-Ni alloy spherules from Alberta, Canada. The nickel isotopes in the whole deep-sea spheres are mass fractionated from 0.4 to 2.4%/ AMU. These: values correspond to open system vaporization losses of Ni as high as 94% (relative). The degree of mass fractionation correlates well with bulk nickel content in most cases. Taken together with published iron isotope data, the nickel isotope results indicate a pre-loss FeNi ratio of about 12 for many spheres. Similar ratios are observed in the following types of meteoritic material: EL-chondrite metal; IA, IIE, IIIA, and IVA iron meteorites; and metal from pallasites and mesosiderites. Metal cores separated from three deep-sea spheres contain between 40 and 52% Ni, with mass fractionations ranging from undetectable to a high of 0.8%/AMU. Within experimental error, the degree of Ni mass fractionation in each oxide shell was the same as that in the corresponding core. No mass-dependent isotopic fractionation of nickel was observed in Ni-rich spherules recovered from Alberta sands of Pleistocene age. In general, Ni-rich samples have low degrees of isotopic fractionation which suggests that the most rapid vaporization of Ni occurs when both Fe and Ni have been oxidized.

Germanium isotopic compositions in Canyon Diablo spheroids

By using ICP-MS, we have measured the concentrations and isotopic abundances of Ge in the following samples: (1) the iron meteorites Camp Verde, Toluca, Picacho, and Canyon Diablo; (2) Canyon Diablo spheroids; and (3) oxide rims and metallic cores obtained by grinding Canyon Diablo spheroids. Whole Canyon Diablo spheroids contain appreciably more Ge than does the bulk meteorite. Germanium is enriched in the metallic cores of the spheroids where concentrations may exceed 500 ppm. It is depleted in the oxide rims compared to the metallic cores and to the bulk meteorite. The major isotope ratios, 72Ge70Ge, 73Ge70Ge, and 74Ge70Geea Ge, were determined with an uncertainty of ∼0.3% (1σ,). The isotopic abundances of Ge in the bulk iron meteorites and the metallic cores of spheroids match terrestrial values. However, the oxide rims of spheroids contain mass-fractionated Ge (4 ±‰/AMU), providing the first direct evidence for evaporative loss from these objects. Assuming Rayleigh distillation and GeO as the evaporating species, we estimate that about 50% of the Ge in the precursor of the oxide rims remains there.

Stable magnesium isotopes, 26Al, 10Be, and 26Mg/26Al exposure ages of iron meteorites

Abstract Concentrations of magnesium isotopes in the iron meteorites Charlotte, Picacho, Tlacotepec, and Grant between 0.15 and 5 ppbw have been measured by glow discharge mass spectrometry, typically with a precision of 15%. The 25 Mg/ 24 Mg and 26 Mg/ 24 Mg ratios are well above terrestrial values in most samples indicating the presence of a component produced by cosmic rays. Inferred concentrations of cosmogenic 26 Mg range from barely detectable to 0.255 ppb by mass. 26 Al and 10 Be activities were measured in the same four and other iron meteorites by accelerator mass spectrometry. Cosmic ray exposure ages based on the cosmogenic 26 Mg/ 26 Al ratios are comparable to, but not precise enough to distinguish between discordant, published ages obtained from other isotopes. Measured primordial or common magnesium contents are in the range between 0.2 and 4.6 ppb.

10Be in bauxite and commercial aluminum

Abstract Five different samples of commercial aluminum have 10Be concentrations that range from a low of 40 × 106 to a high of 100 × 106 (atom 10Be)/(g Al). The beryllium-10 is probably produced in the atmosphere and introduced into aluminum ore deposits in varying amounts by rainwater during ore genesis. One modern ore, a bauxite from Haiti, contains ~ 6 × 109 atom 10Be/(g sample) or 5.7 × 1010 atom 10Be/(g Al). Geologically older, allocthonous bauxite from Arkansas contains considerably less 10Be; this observation suggests that 10Be can be used to constrain the age of the deposit. The presence of 10Be in commercial aluminum makes it inadvisable to add modern Al to small samples in which very low levels of 10Be are to be determined.