Nebular or parent body alteration of chondritic material: Neither or both?

— Discussions of meteorite properties often concern whether they are the result of “nebular” or “parent body” processes, but several decades of research have not resolved the issue. Part of the problem is that any gas‐phase reaction involving meteoritic materials thought to have occurred is automatically assumed to be nebular, even though the most primitive solar system objects are water‐ and volatile‐rich and could easily generate vapors. Reactions important in meteorite genesis are those involving (1) oxidation of Fe, (2) oxidation of other cations, (3) reduction, (4) olivine‐pyroxene equilibria, and (5) hydration of silicates. The P‐T‐x conditions required are almost invariably incompatible with standard models for the early solar nebula, but clearly many of these reactions occurred prior to final agglomeration. Most of the reactions require high pressures (>10 −3 atm) or, more importantly, major departures from cosmic composition, even though the final rocks are remarkably cosmic in elemental proportions. We suggest that most of these reactions occurred in a regolith rendered “dynamic” by the flow of copious volatiles. In such a scenario, liquid and gas phase reactions can occur at elevated temperatures and pressures relative to the nebula and with noncosmic gas phase compositions; but the system is “closed” to most components, so that cosmic proportions are essentially preserved.