Application of the sphalerite cosmobarometer to the enstatite chondrites

Abstract

Sphalerite is a minor constituent of a number of enstatite chondrites. The assemblage sphalerite-troilite-kamacite buffers the iron content of sphalerite so that it is a function of temperature and pressure only. High iron contents in sphalerite in enstatite chondrites are consistent with equilibration at low pressures. If nebular conditions are assumed and pressure is taken as zero, it is then possible to calculate blocking temperatures for diffusion of iron T(B) and cooling rates for the meteorites studied. The variable degree of equilibration of EH3 chondrites is correlated with calculated pressures, assuming that the least equilibrated, Y69001, formed at zero pressure. Parent-body radii calculated from these pressures are probably too large, however. Inhomogeneities and minor elements in sphalerite, as well as the assumptions made in the calculation may have had an adverse effect on the calculations. The EH4 chondrite Indarch appears to have cooled at an extremely rapid rate and has an unreasonably high T(B). Inhomogeneities and minor element content may again have affected results. The data for the EL6 chondrites are consistent with relatively slow cooling ca. 773 K, whereupon rapid cooling occurred. Cooling rates at 773 K are too fast by a factor of 102 to 103 in comparison with metallographically determined rates.