Chemical fractionations in meteorites—II. Abundance patterns and their interpretation

Potassium isotope cosmochemistry: Genetic implications of volatile element depletion

“…Removal of volatiles by evaporation from chondrules and melt droplets has been proposed by Larimer and Anders ( 1967), Wolf and Anders ( 1980), and W&&e et al ( 1981, 1984). Experimental investigations of the chemical changes taking place during evaporation of Murchison and Allende chondrites by Notsu et al ( 1978), Hashimoto et al ( 1979), and Hashimoto (1983) have shown that evaporation can produce chemical fractionations similar to those produced by condensation.…”

“…This is not detectable by standard chemical techniques, which can resolve 5-20% changes in the K/La and K/U ratios. The two-component models proposed by Larimer and Anders ( 1967) and by Wtike et al ( 1984) invoke vaporization of alkalis which is not supported by the potassium isotope results. The chemical depletion of alkalis and other volatiles must have preceded the processes of chondrule, chondrite, and planetary formation, and occurred during the condensation of precursor dust, probably from a hot stage in the solar nebula.…”

“…Two kinds of mechanism can be proposed to account for the chemical fractionations observed in chondrites and planets: those involving incomplete condensation of volatiles, e.g., from a hot nebula (Larimer and Anders, 1967;Grossman and Larimer, 1974) and those involving the evaporation of Cllike matter accreted from a cold solar nebula (Ringwood, 1966). The mechanism of volatile element depletion holds the key to whether planetary and chondritic material formed from a hot solar nebula or a cold nebula, and is, therefore, an independent constraint on models of the thermal structure of the nebula derived from calculations of protostellar origin (Ruden and Lin, 1986;Cameron, 1988;Boss, 1993).…”

“…There have been many proposed means of accomplishing this, including loss from heated grains (Hashimoto et al, 1979;Hashimoto, 1983), loss from molten chondrules (Larimer and Anders, 1967;Ganapathy and Anders, 1974;Wolf and Anders, 1980), and loss from impact-disrupted planetesimals (Zook, 198 1;Wgnke et al, 198 1, 1984;cf. Anders, 1964, for earlier versions), which is sometimes taken to be identical to chondrule formation (Zook, 1981).…”

The case for an unfractionated244Pu/238U ratio in high-temperature condensates

“…Larimer and Anders [31 ] and Clark et al [32] showed that the cooling time of the solar nebula, and presumably condensate particles immersed in it, was less than 106 years. Thus, it is likely that the inclusions began retention of fissiogenic Xe from decay of 244pu within an interval of time after the beginning of condensation which was extremely short relative to the half-life of 244Pu.…”