Search

FUN (Fractionation and Unidentified Nuclear) calcium-, aluminum-rich inclusions (CAIs) have large mass-dependent fractionations of silicon, magnesium, and oxygen isotopes (up to δ29Si ∼15‰ and δ25Mg ∼40‰), and mass-independent isotopic anomalies in many elements. To test the proposition that the mass-fractionation effects of all three isotopic systems in FUN CAIs were the result of evaporation of at least partially molten precursors, we conducted a series of experiments in which two magnesium- and silicon-rich melts (FUN1 with 53.4wt% MgO and 41.3% SiO2, and FUN2 with 32.7% MgO and 38.7% SiO2, and Al2O3 and CaO in solar proportions) were evaporated into vacuum at 1900°C for various lengths of time. The chemical and isotopic compositions of the evaporation residues were measured and compared to two of the most highly mass-fractionated FUN CAIs, Vigarano 1623-5 and Allende C1. The isotopic composition of the evaporation residues was also used to determine the kinetic isotopic fractionation factors α 25,24 =0.98372±0.00041 for 25Mg/24Mg and α 17,16 =0.9883±0.0006 for 17O/16O for residues containing >15wt% MgO, and α 25,24 =0.98567±0.00046 and α 17,16 ∼0.994 for residues containing <15wt% MgO. The 29Si/28Si fractionation factor α 29,28 =0.9899±0.0004 was found to fit the data from the entire set of residues. Simple linear correlations were found for δ29Si, δ25Mg, and δ17O as a function of the fraction of magnesium or silicon remaining in the residues. The fact that the isotopic fractionations of magnesium, silicon and oxygen of C1 are in the same proportions as in the experimental evaporation residues suggests that the evaporation played a major role in the chemical evolution of this FUN inclusion. In the case of Vigarano 1623-5, the magnesium and oxygen isotopic fractionations are consistent with the experimental data, but fractionation of silicon isotopes relative to that of magnesium in 1623-5 is about a third less than in the experimental residues. Assuming that Allende C1 and Vigarano 1623-5 are evaporation residues that were produced in much the same way as our experimental residues (i.e., evaporation of completely molten droplets), the chemical compositions of their precursors were calculated using bulk chemical and isotopic compositions of C1 and 1623-5 together with the experimentally determined kinetic fractionation factors α 25,24 and α 29,28. It was found that the present chemical and isotopic compositions of C1 can be explained by evaporation of a precursor with a bulk composition close to that of a condensate from a solar composition gas. In the case of Vigarano 1623-5, however, the calculated precursor is significantly enriched in magnesium and depleted in silicon compared to plausible condensates from a solar composition gas. Among the possible reasons for such misfit could be uncertainties in bulk chemical and isotopic compositions measured in Vigarano 1623-5, or evaporation at lower temperatures from partially rather than completely molten precursors which could have different evaporation kinetics and isotopic fractionation factors. [ABSTRACT FROM AUTHOR]

- Evaporation rates of K2O, Na2O, and FeO from chondrule-like liquids and the associated potassium isotopic fractionation of the evaporation residues were measured to help understand the processes and conditions that affected the chemical and isotopic compositions of olivine-rich type IA and type IIA chondrules from Semarkona. Both types of chondrules show evidence of having been significantly or totally molten. However, these chondrules do not have large or systematic potassium isotopic fractionation of the sort found in the laboratory evaporation experiments. The experimental results reported here provide new data regarding the evaporation kinetics of sodium and potassium from a chondrule-like melt and the potassium isotopic fractionation of evaporation residues run under various conditions ranging from high vacuum to pressures of one bar of H2+CO2, or H2, or helium. The lack of systematic isotopic fractionation of potassium in the type IIA and type IA chondrules compared with what is found in the vacuum and one-bar evaporation residues is interpreted as indicating that they evolved in a partially closed system where the residence time of the surrounding gas was sufficiently long for it to have become saturated in the evaporating species and for isotopic equilibration between the gas and the melt. A diffusion couple experiment juxtaposing chondrule-like melts with different potassium concentrations showed that the diffusivity of potassium is sufficiently fast at liquidus temperatures (DK > 2 × 10−4cm2 s−1 at 1650 °C) that diffusion-limited evaporation cannot explain why, despite their having been molten, the type IIA and type IA chondrules show no systematic potassium isotopic fractionation. [ABSTRACT FROM AUTHOR]

Experiments exposing Type B calcium-, aluminum-rich inclusion (CAI)-like melts at high temperatures to high vacuum or reducing hydrogen-rich gas mixtures were used to determine the rates and consequences of elemental and isotopic fractionation by evaporation. Silicon and magnesium were found to evaporate much faster than calcium and aluminum, and the resulting residual liquid trajectories in composition space are reproduced via a thermodynamic model for the saturation vapor pressure of the evaporating species. Isotopic fractionations associated with evaporation were measured for magnesium. The resulting relationship between fraction of magnesium lost and enrichment of the residue in the heavy isotopes of magnesium follows a Rayleigh fractionation curve with a fractionation factor that is close to, but not exactly, the theoretically expected value. The rate of evaporation is found to be a strong function of temperature, oxygen fugacity, and melt composition, which can be understood and modeled in terms of the dependence of the saturation vapor pressures on these variables. The relationship between evaporation rate, which we measure, and calculated saturation vapor involves empirical evaporation coefficients that we find to be significantly less than one (∼0.1). Analytical and numerical models are used to characterize how diffusion in both the melt and in the surrounding gas affects evaporation rates and the degree of chemical and isotopic fractionation. The experimental data and theoretical considerations are combined to give a parameterization of the rates and consequences of evaporation of Type B CAI-like liquids, which is then used to translate the measured isotopic fractionation of Type B CAIs into constraints on their thermal history. Cooling rates of the order of 10°C per hour are indicated. [Copyright &y& Elsevier]