On the Mn-53 heterogeneity in the early solar system

摘要

It is well established that the prolonged and thorough mixing of numerous nucleosynthetic components that constitutes the matter in the solar nebula resulted in an essential isotopic homogeneity of the solar system material. This may or may not be true for the short-lived radionuclides which were injected into or formed within the solar nebula just prior to or during solar system formation. Distinguishing between their heterogeneous or homogeneous distribution is important because the short- lived radionuclides are now widely used for the relative chronology of various objects and processes in the early solar system and as constraints for models of nucleosynthesis. The recent studies of the Mn-53-Cr-53 isotope system (half life of Mn-53 is 3.7 Ma) in various solar system objects have shown that the relative abundance of radiogenic Cr-53 is consistent with essentially homogeneous distribution of Mn-53 in the asteroid belt. Thus, the relative Mn-53-Cr-53 chronometer can be directly used for dating samples which originated in the asteroid belt. Importantly, however, all meteorite groups studied so far indicate a clear excess of Cr-53 as compared to Earth and to a lunar sample, which exhibits also a terrestrial Cr-53/Cr-52 ratio. The results from the Martian (SNC) meteorites show that their Cr-53 excesses are less than half of those found in the asteroid belt bodies. Thus, the characteristic Cr-53/Cr-52 ratio of Mars is intermediate between that of the Earth-Moon system and those of the other meteorites. If these Cr-53 variations are viewed as a function of the heliocentric distance, the radial dependence of the relative abundances of radiogenic Cr-53 is indicated. This observed gradient can be explained by either an early, volatility controlled, Mn/Cr fractionation within the nebula or by an initial radial heterogeneous distribution of Mn-53. Although model calculations of the Mn/Cr ratios in the bulk terrestrial planets seem to be inconsistent with the volatility driven scenario, the precision of these calculations is inadequate for eliminating this possibility. In contrast, recent studies of the Mn-53-Cr-53 system in the enstatite chondrites indicate that, while their bulk Mn/Cr ratios are essentially the same as in ordinary chondrites, the Cr-53 excess in bulk enstatite chondrites is three times lower than that in the bulk ordinary chondrites. This difference cannot be explained by a Mn/Cr fractionation and, thus, strongly suggests that a radial heterogeneous distribution of Mn-53 must have existed in at least the early inner solar system. Using the observed gradient and the Cr-53/Cr-52 ratio of the bulk enstatite chondrites, their parent body(ies) formed at similar to 1.4 AU or somewhat closer to the Sun. [References: 38]