Isotopic and elemental composition of iron, nickel, and chromium in type Ideep-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 mu g. 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 co mpositions of Ni by inductively-coupled plasma mass spectrometry. Evaporation of Fe, Ni, and Cr during atmospheric entry led to large and sim ilar average degrees of mass-dependent fractionation, Phi, in most spherule s. The average value, similar to 16 parts per thousand/AMU, corresponds to mass losses of 80-85%, assuming open-system evaporation of the atoms. We find Phi(Cr) similar to Phi(Fe) in seven spherules. This observation imp lies similar evaporation rates for Cr and Fe and that the measured Cr/Fe ra tios (mass/mass) are close to those of the progenitors. Four spherules have Cr/Fe similar to 0.003; two others with low Cr/Fe, similar to 8 x 10(-4), high Fe/Ni, similar to 2000, and Phi(Cr) similar to Phi(Fe) may belong to a different, possibly terrestrial, population. A seventh spherule with "chon dritic" Cr/Fe, similar to 17 x 10(-3) and subaverage Phi(Cr) and Phi(Fe), 8 -10 parts per thousand/AMU, may represent still another source of particles . Because the higher vapor pressure of pure Cr should lead to Phi(Cr) > Phi (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 correlatio n between Phi(Cr) and Phi(Fe) can be expressed in the form Phi(Cr) = 0.31 x Phi(Fe)(1.47), although the data also are adequately fit by a linear regre ssion. Correlated variation of Phi(Ni) and Phi(Fe) can be fit by the empirical rel ationship Phi(Ni) = 0.016 x Phi(Fe)(2.58). For low Phi(Fe), we find Phi(Ni) < Phi(Fe), which shows that Fe evaporates more rapidly than Ni at first, p robably because of the lower vapor pressure of pure Ni and lower activity c oefficient of Ni in the melt. For high Phi(Fe), we find Phi(Ni) > Phi(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 formati on of relatively involatile wustite and magnetite. Differences between Phi( Ni) and Phi(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 ra tio (by mass) in type I spherules of 19 +/- 4 (sigma(mean)). Similarly, by assuming Ir is involatile, we obtain a preatmospheric ratio of Ir/Ni = 3 x 10(-5), which is about 10 times smaller than the average measured value, bu t similar to the cosmic (CI) abundance ratio of 4 x 10(-5). Cosmogenic nuclides have been detected in some Type I spherules at levels i ndicating irradiation as metal in space. Among conventional meteorites, the best matches to both the Cr/Fe and Fe/Ni ratios inferred for type I progen itors are metal from CO, CV, and CR chondrites and from unequilibrated ordi nary 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-chondri te-like objects that were freed by comminution in space, or, less likely, t hat collisions of large objects formed droplets rich in metal. Copyright (C ) 1999 Elsevier Science Ltd.