OXIDATION DURING METAMORPHISM OF THE ORDINARY CHONDRITES

Subtle but progressive changes in the compositions and relative abundances of olivine, pyroxene, and metal with increasing metamorphic grade in equilibrated (types 4-6) ordinary chondrites indicate that these meteorites experienced oxidation of metallic Fe during heating. Oxygen fugacities calculated from these mineral assemblages increase with petrologic type. The hypothesis of oxidation during chondrite metamorphism differs from a commonly accepted idea that chondritic Fe was reduced by graphite. The mineralogies of unequilibrated (type 3) chondrites do not conform to the progressions in equilibrated chondrites, allowing the possibility that Fe may first have been reduced through reaction with graphite at the onset of metamorphism, although there are conflicting observations supporting both oxidation and reduction in these meteorites. As temperatures increased to levels appropriate for types 4-6 chondrites, the activity of graphite may have been lowered by its dissolution in taenite; and, in any case, oxygen fugacities appear to have been well within the stability field of graphite, precluding its reaction. At these temperatures oxidation state was largely controlled by equilibrium between ferromagnesian silicates and metal. Oxygen fugacities calculated from chondrite mineral equilibria are 2-3 log units below previously reported intrinsic f(O2) measurements. We propose that progressive oxidation during metamorphism was promoted by interaction with small amounts of an oxidizing vapor, derived by heating ices originally accreted into the parent asteroids. Assuming that this vapor was pure H2O, the water:rock weight ratio required to account for the observed oxidation of Fe in H and L chondrites is very modest, less than several per mil. Progressive oxidation in the metamorphic sequence resulted from different degrees of reaction with such a fluid, possibly caused by temperature variations or evolution of the composition of the fluid as it permeated through the body. The presence of even minor amounts of a vapor phase during metamorphism affects interpretations of the volatile trace-element and oxygen isotopic fractionations observed in ordinary chondrites and inferred to have formed by nebular or parent-body processes.