PRIMORDIAL CONDENSATION OF METEORITE COMPONENTS - EXPERIMENTAL-EVIDENCE OF THE STATE OF THE SOURCE MEDIUM

Mineral grains and grain aggregates in meteorites carry potential information on the conditions in the environment where they formed. To avoid model-dependent interpretations it is necessary to develop experimental criteria that uniquely reflect the environmental parameters of interest. These parameters include the various temperatures of the source medium and the temperature of grains at growth, all of which are observed to be highly differentiated in the space medium in accordance with the radiation laws. Independent tests are also necessary to establish the chemical composition of the source medium which appears from the meteorite record to have been variable. Another important variable is the number density which is observed to span over more than twelve orders of magnitude in the domains in space where dust is being modified by condensation or evaporation. Metal diffusion couples in the interior of refractory mineral grains (melilite and spinel) indicate that these couples and their refractory host grains may have existed up to a year at 780 K. They could have supported temperatures lower than 900 K more than a day but cannot have formed at grain temperatures of the order of 1700 K as is assumed in some theories. The temperature parameters of the source medium are more difficult to determine from the record in meteorite minerals; assessments, in order to be realistic, have to be made within the bounds of the observed properties of the space medium. Kinetic isotope effects, imprinted together with nuclear effects on meteorite solids, provide a promising source of information on fractionation processes and on the state of excitation in the source medium. Theoretical and experimental verification of the isotope fractionation mechanisms, which may be responsible for the observed effects, are thus of importance. © 1979 D. Reidel Co.