Lunar and Planetary Science XXIX 1801.pdf Consequences of Mass-Independent, Chemical, Oxygen-Isotopic Fractionation in the Solar Nebula Joseph A. Nuth III, As- trochemistry Branch, Code 691, NASA Goddard Space Flight Center, Greenbelt, MD 20771 Widespread, systematic, non-mass-dependent, oxygen- evaporates as an atom (e.g. Fe) or a simple diatomic molecule isotopic differences have been documented in analyses of (e.g. SiO), reacts with a large but reasonably dilute reservoir of many different meteorite classes (Thiemens, 1988; Clayton, oxygen (e.g. H2O) to form a triatomic molecule (such as SiO2) Mayeda and Molini-Velsko, 1985) in a variety of studies be- having a much lower vapor pressure than the diatomic species ginning after the initial discovery of this effect (Clayton, (Schick, 1960). This triatomic molecule then condenses on Grossman and Mayeda, 1973). Commonly held explanations any available surface at a rate proportional to its vapor-phase for these differences involve at least three, and sometimes concentration. Less refractory species (e.g. SiO) would re- four very different reservoirs of oxygen isotopes in the primi- main in the vapor until they too form trimers. If the isotopi- tive nebula (Thiemens, 1988). In the simplest model, exchange cally non-symmetric trimer is indeed more easily stabilized due occurs between a 16-0-rich solid reservoir (potentially carried to the larger number of vibrationally active modes available to by grains produced in a supernova), a 16-0-poor solid reser- it, then such a reaction sequence would "extract" 17-O and 18- voir of pre-existing molecular-cloud grains and a larger reser- O from the gas-phase reservoir via preferential reaction of the voir of oxygen-containing gas. These reservoirs never com- SiO dimer to form the (16-O)Si(17- or 18-O) trimer. The end pletely mix during a series of vaporization/recondensation result would be a gas phase that is enriched in 16-O and a steps: each meteorite group exchanged oxygen isotopes as solid phase that is non-mass-dependently enriched in 17-O the result of mixing and melting or hydrothermal exchange and 18-O in proportion to the number of vaporization- between solid reservoirs and a large gas-phase reservoir. recondensation sequences the material experienced. This Detailed explanations for each observed meteoritic component mechanism would have several consequences for models of are possible using such a model, but can get to be quite com- nebular evolution. plex: This is not in itself a major problem given the likely complexity of the nebula. Initial Oxygen Isotopic Abundance in the Solar Nebula: In- Thiemens & coworkers suggested that molecular-symmetry dividual grains in the presolar molecular cloud carry some induced stabilization of isotopically non-symmetric trimers record of their nucleosynthetic history. The oxygen isotopic such as (16-0)=Si=(18-0) could have produced non-mass- content of presolar corundum grains (Huss et al., 1994; Nittler dependent oxygen isotopic effects in the nebula. This sug- et al., 1994) is directly attributable to stellar processes and was gestion is still viable and has been experimentally demon- not reset in the solar nebula. Similarly, the elemental and iso- strated in a long series of experiments involving ozone and topic composition of presolar carbonaceous grains attests to molecular oxygen (Thiemens, 1988 and references therein). the fact that not all materials incorporated into meteorites were Unfortunately, this effect has never been convincingly dem- processed to a significant degree in the nebula (Huss, 1997). onstrated to occur in refractory species, although Nelson, et Therefore, the initial solid component of the pre-collapsed al. (1989) found some evidence that such processes might nebula would represent a very wide range of oxygen isotopic have occurred during condensation of refractory vapors. compositions resulting from processes as varied as conden- More recent work by Humayan and Clayton (1995) has con- sation in supernovae outflows (Clayton, 1982) to synthesis in vincingly demonstrated that large scale partial vaporization grain mantles (Greenberg, 1983) following ion-molecule reac- can not account for the chemical fractionation of moderately tions in cold clouds (Zinner, 1988). One expects the oxygen volatile elements observed in meteorites and larger planetary isotopic composition of the gas phase to be more uniform, bodies. It is possible to explain such fractionation effects via especially following the evaporation of ice mantles coating partial condensation from a hot vapor, but, this would make it grains in the inner portions of the nebula and turbulent mixing very difficult to maintain the very distinct reservoirs required of this gas re servoir. to explain the oxygen isotopic speciation observed in different Another aspect of this problem is that a nebula of solar com- meteorite classes. Although there is no doubt that vaporiza- position, is likely to contain at least an order of magnitude tion and recondensation played a significant role in the evo- more oxygen as gas than as solids. Generation of additional lution of solar system materials (Clayton, Mayeda and Molini- volatiles, e.g. CO via reaction of carbonaceous solids with Velsko, 1985, Boynton, 1988), the invariance of the potassium oxides in hotter portions of the nebula (Lodders and Fegley, isotopic ratio in planetary materials puts severe constraints on 1997) could augment oxygen in the gas-phase reservoir be- their processing hi story. yond that due to ice evaporation. In regions where refractory materials are vaporized, the gas-phase reservoir could easily Mechanism for the Fractionation of Oxygen Isotopes: Thie- contain 20-30 times more oxygen than the solid reservoirs. mens suggested a chemical means to fractionate oxygen iso- Therefore, even a very efficient enrichment process acting to topes in a non-mass-dependent fashion via molecular symme- partition oxygen isotopes between gas and solid would have try induced stabilization of non-symmetric refractory trimers a greater effect on the solid reservoir than on the gas. such as (16O)Si(18O) or (17O)Si(16O) as compared to the less If we consider only the vaporization and recondensation of stable (16O)Si(16O). Asymmetric trimers have access to many silica and alumina and possible exchange with a gas reservoir more vibrational and rotational levels than do symmetric spe- twenty times larger, it is relatively easy to calculate the magni- cies and this could result in increased stability of both the tude of the exchange process needed to provide the broad stable trimer as well as the reactive intermediate from which range of isotopic compositions observed in meteorites and the trimer forms. In this mechanism, the refractory entity their components. However, a more interesting consideration Lunar and Planetary Science XXIX 1801.pdf CONSEQUENCES OF MASS-INDEPENDENT OXYGEN FRACTIONATION, J. Nuth is the direction of fractionation. We assume for simplicity that a composition more 16-O-rich than (-40, -42) and should be the initial isotopic composition of the gas and solid reservoirs representative of the oxygen isotopic content of the pre-solar were identical and that silicates vaporize as SiO molecules, nebula, modified by the extraction of heavy oxygen isotopes react with an oxygen atom from the gas and condense as during the proces sing of solids. SiO2. If such a process preferentially extracts heavy oxygen isotopes from the gas-phase, then solid condensates will be Summary: If a chemical mechanism operated in the primitive enriched in 17-O and 18-O while the gas will become enriched solar nebula to fractionate oxygen isotopes, it will have very in 16-O. What does this tell us about the average composi- specific, predictable consequences for individual meteoritic tion of the initial nebular rese rvoir? components, for comets and even for planets. Comets must First, vaporization followed by recondensation will move sol- be extremely rich in 16-O relative to SMOW if they are repre- ids up a slope 1 mixing line and result in a gas-phase reservoir sentative of interstellar ices entering the nebula and even that is enriched in 16-O. Because the gas reservoir is much more 16-O enriched if representative of recondensed nebular larger than the solid, back reaction of the recondensed solids water vapor. The oxygen isotopes in CAI rims should be with the 16-O enriched gas will not return a solid to its original heavier than the average isotopic composition of the CAI starting composition. The process therefore produces a mo- itself. Higher concentrations of heavier oxygen isotopes in notonic increase in the heavy oxygen content of the solids both CAIs and chondrules should correlate with an increased together with a much smaller, but still monotonic increase in number of vaporization/recondensation cycles. More 16-O the 16-O fraction of the gas. rich meteorite classes should contain a higher percentage of Individual grains are likely to have had isotopic compositions primitive, unprocessed interstellar materials, some of which reflecting a wide range of nucleosynthetic environments and may have been lost when the parent body lost most of it's processing histories. However, aggregates large enough to primitive water. These predictions are a natural consequence form macroscopic components such as CAIs, chondrules or of the hypothesis that the formation of SiO2 from SiO or AIO2 planetesimals should represent an average of most such envi- from AIO extracts heavy oxygen isotopes from the gas-phase ronments. As an example, a single 1mm solid particle pro- and becomes incorporated into the recondensed solid. Pref- duced in the nebula would be "assembled" from a trillion in- erential extraction of heavy oxygen is due to the increased dividual "average" 0.1 micron presolar grains. This number is stability provided to unsymmetric isotopomers of SiO2 and even larger since the standard Mathis, Rumpl and Nordseick AIO2 trimers by the increase in the number of non-degenerate (1977) size distribution predicts many more smaller grains than vibrational states into which energy may be partitioned.