POTASSIUM ISOTOPE COSMOCHEMISTRY - GENETIC-IMPLICATIONS OF VOLATILE ELEMENT DEPLETION

We report high precision (+/-0.5 parts per thousand) potassium isotopi c determinations on bulk chondrites, achondrites, and lunar samples, o n a separated chondrule, and two CAIs. We find that potassium shows a remarkable isotopic homogeneity in various solar system bodies, even t hough there are chemical depletions of a factor of about 30, between C 1 chondrites and eucrites and lunar samples. Theories that propose the evaporation of volatile elements from initially condensed (C1 chondri te) material to account for such chemical depletions, necessarily impl y the existence of large isotopic mass fractionations, e.g., about +40 parts per thousand for Earth, +90 parts per thousand for eucrites and lunar rocks. Volatile loss of potassium (and by implication Na, Rb, C s, and other elements of similar volatility) during chondrule formatio n is also ruled out. The high precision of the data place stringent li mits of less than or equal to 2% on the quantity of potassium that cou ld have been lost by partial volatilization. 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 Wanke et al. (1984) invoke vaporization of alkali s which is not supported by the potassium isotope results. The chemica l depletion of alkalis and other volatiles must have preceded the proc esses of chondrule, chondrite, and planetary formation, and occurred d uring the condensation of precursor dust, probably from a hot stage in the solar nebula.