This work reports noble gas isotopic analyses of samples of the. St. Severin meteorite. The purpose of this study is to investigate the radiometric chronologies of this meteorite as given by the parent-daughter systems of ('40)K -('40)Ar, ('129)I-('129)Xe, and ('244)Pu-('131-136)Xe. The St. Severin meteorite consists of two distinct lithologies (labeled light and dark) which were produced by impact brecciation early after the formation of St. Severin. The dark lithology consists of large (>(' )1 cm('3)) angular fragments held in a light colored matrix (the light lithology) which appears to consist of finely crushed dark material. These two lithologies provide the opportunity to examine the effects of these differences on the radiometric ages without having to consider genetic differences or differences in the thermal histories of the samples. For the light and dark lithologies respectively, the ('40)Ar retention ages are 4.37 and 4.42 x 10('9) yr. and the ('129)Xe retention ages are 9 and 36 x 10('6) yr. after the Bjurbole meteorite (i.e. (DELTA)T (TBOND) 0 for ('129)I/('127)I = 1.095 x 10('-4)). These age differences are well resolved with respect to the experimental uncertainties. Both the K-Ar and I-Xe ages are interpreted as dating the relaxation of metamorphic conditions. The event dated is described in terms of a closure temperature which defines the effective halt of isotopic homogenization. In this view, the age differences between the two lithologies, combined with the estimated differences in closure temperatures, imply slow cooling rates, (TURN)1-2(DEGREES)C/10('6) yr, for temperatures between 500 and 200(DEGREES)C. This result is in good agreement with cooling rates determined for St. Severin by fission track and metallographic techniques. The absolute ages combined with the estimated closure temperatures require appreciably greater cooling rates above 500 to 600(DEGREES)C, cooling rates of the order of 20 to 30(DEGREES)C/10('6) yr. These results thus favor models for meteorite parent bodies where peak metamorphic temperatures are reached in small bodies ((TURN)1 km in radius) which can cool rapidly, followed by accretion into larger bodies ((TURN)100 km in radius) on a time scale of (TURN)10('7) yr where the final slow cooling occurs. The study of plutonium-derived xenon in St. Severin shows that the presently accepted value of ('244)Pu/('238)U = .015 for the early solar system, must be revised downward by a factor of three to 0.005. This revision has substantial implications regarding the generally proposed age distribution of nuclei in the early solar system. The most important of these concerns ('129)I. It is shown that a significant fraction ((TURN) 1/2) of the solar system inventory of ('129)I was added to the presolar nebula only a few million years before the formation of macroscopic solid objects. The implications of this for I-Xe dating are clear. If half of all the ('129)I was added to the presolar nebula on a time scale of a million years (comparable to gravitational freefall times), just before the formation of solid objects, it seems unlikely that this component could be well mixed with the existing matter. Thus the case for inhomogeneity in the intial ('129)I/('127)I ratio in the early solar system must be given careful consideration. It thus seems quite likely that some of the apparent I-Xe age differences seen between meteorites may be due to the isotopic inhomogeneity of ('129)I/('127)I so that, in fact, xenon isotopic closure may well have been much more isochronous among meteorites than previously thought.
- Pub Date:
- Physics: Astronomy and Astrophysics, Geological Survey