High temperature heating of the Allende meteorite

Abstract

Allende bulk samples were heated up to 2000°C by DC arc and resistance heating. The chemical composition of evaporation residues was measured by instrumental neutron activation analysis (INAA) and electron probe microanalyser (EPMA). The evaporation sequence of elements observed was: (Na)-(Fe, Ni, Co, Au, Ir)—(Fe, Mn, Cr) (Mg, Si)-(Al, Ca, Ti, Sc, REE). Chemical compositions of 65% and 4% residual fractions were in agreement with those of a chondrule and a Ca, Al-rich inclusion of the Allende meteorite, respectively. Iridium was lost from residual solid in the early stage of heating. The early loss of the refractory siderophile elements may provide a clue to distinguish between two possibilities for the origin of Allende Ca, Al-rich inclusions: early condensates vs. evaporation residue. Ca, Al-rich inclusions (CAI) in the Allende meteorite may provide information on the physical and chemical conditions prevailing in the early solar system. So far we have two possibilities for the origin of CAI: 1. (1) High temperature condensates from a “hot” solar nebula (GROSSMAN, 1972, 1973; WANKE et al., 1974); 2. (2) Evaporation residues of the pre-existing primordial dust in the early solar system (KURAT, 1970; TANAKA and MASUDA, 1973; CHOU et al., 1976; CLAYTON, 1977). A condensation origin has been supported by theoretical considerations of equilibrium condensation from a cooling nebula, while an evaporation origin has received no endorsement from either theoretical or experimental approaches. We have heated bulk Allende samples to determine whether or not the chemical composition of CAI can be derived in the course of fractional vaporization of pre-solar matter which, in this experiment, is approximated by the bulk Allende composition.