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 Φ _Cr ∼ Φ _Fe, 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 Φ _Cr ∼ Φ _Fe ∼0, may belong to a different, possibly terrestrial, population. A seventh spherule with "chondritic" Cr/Fe, ∼17 × 10 ^-3 and subaverage Φ _Cr and Φ _Fe, 8-10‰/AMU, may represent still another source of particles. Because the higher vapor pressure of pure Cr should lead to Φ _Cr > Φ _Fe 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 Φ _Cr and Φ _Fe can be expressed in the form Φ _Cr = 0.31 × Φ _Fe^1.47, although the data also are adequately fit by a linear regression. Correlated variation of Φ _Ni and Φ _Fe can be fit by the empirical relationship Φ _Ni = 0.016 × Φ _Fe^2.58. For low Φ _Fe, we find Φ _Ni < Φ _Fe, which shows that Fe evaporates more rapidly than Ni at first, probably because of the lower vapor pressure of pure Ni and lower activity coefficient of Ni in the melt. For high Φ _Fe, we find Φ _Ni > Φ _Fe, 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 wüstite and magnetite. Differences between Φ _Ni and Φ _Fe 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 (σ _mean). 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.