Thermal metamorphism of primitive meteorites

Noble gases, C and S are lost from Allende samples heated for 1 week at temperatures of 400 to 10000C in a low pressure environment. In the extreme, losses of 3He and 4He are approximately 100 x while for C, S and Ne, Ar and Kr isotopes and 132Xe, these are =< 10 x. Except for He, these losses are less severe than those of Bi or Tl from samples heated in the same runs. Significant He, Ne and Ar isotopic fractionation during heating indicates preferential outgassing of specific reservoirs. Apparent activation energies for all species generally indicate loss controlled by a diffusive process. Next to He, 40Ar is the most labile of those species considered here but still less so than Bi or Tl. L-group (but not H- or LL-group) chondrites may have lost mobile elements like Tl while being outgassed after late impact-associated heating. A less likely alternative possibility involving a collateral relation between condensation conditions and depth in a parent object may also explain the L-group trend. (author)