Re-Os Systematics in Meteorites

Abstract

Extensive results have been obtained on the Re-Os system in iron meteorites, stony irons, and in chondrites. This has required the development of reliable techniques, with the single-most critical analytical consideration being the complete isotope equilibrium of tracer and sample. The work on iron meteorites has concentrated on whole rock measurements. Whole rock samples for irons of groups lAB, IIA, liD, IliA, IVA and IVB have yielded a well defined isochron with deviations from the isochron for any single sample at the level of 2-3%o. The data yield a slope of 0.07863 __ 0.0003, corresponding to T = 4.615+_0.018 AE [L(lS7Re) = 1.64 x 10 -11 a -1] and init ial (187Os/lS8Os)o = 0.09560_+ 0.0002. There is a suggestion that group IVA irons are slightly older. Since the uncertainty in the aSTRe decay constant is ~3%, the isochron age can be renormalized to T/4.55 AE, corresponding to 9~(187Re) = 1.664 x 10-11 al, in order to obtain agreement with U-Pb, Sm-Nd, and Rb-Sr ages on silicates. The welldefined whole rock isochron depends on substantial observed variations in Re/Os, which reflect chemical fractionation during initial fractional crystallization of FeNi masses, including as pods and as cores of early formed planetesimals. For iron meteorites, 'whole rock' samples measured are typically cubes of metal, 0.25 to lg, several mm on the side. Given that the crystal size for metal phases in iron meteorites is similar, the close adherence of the data to a single isochron requires very limited fractionation of Re-Os after initial FeNi crystallization and during the later exsolution of the low-Ni phase (kamacite) from the initial taenite, at lower temperatures. Furthermore, since the whole rock samples do not show a hint of a Re-Os secondary isochron at times younger than the age of the primary isochron, then any melting of the FeNi masses at younger times must have also been characterized by limited Re/Os fractionation and thus very limited further fractional crystallization. We have also determined Re-Os in FeS and in massive schreibersites in irons. Apparent, single-stage distribution coefficients are D(FeNi/Sulphide) = (2-4) x 102 for Re and 1.5 x 103 for Os. Similarly, D(FeNi/ Schreibersite) = 7-14 for Re and 30-54 for Os. The distribution coefficients for sulphide are sufficiently large that the segregation of immiscible sulphide would not affect the Re-Os systematics in the FeNi whole rocks. Similarly, based also on the abundance of P and, consequently, the limited abundance of schreibersite, the formation of the schreibersite would also be of minor importance for the whole-rock Re and Os mass balance. Measurements of PGE adjacent to massive schreibersites show gradients in Re and in Os abundances (up to 25%) and in Re/Os (up to 10%) for distances 1-2 mm away from the schreibersite. The schreibersites themselves show low Re and Os abundances but large enrichments in Re/Os, highly radiogenic 187Os/lSSOs and a range of model ages from 4.5 AE down to 3.5 AE. These data clearly indicate that the minor phases in iron meteorites show an extensive evolution, including slow cooling of the metal and diffusion of minor and trace elements for extended times, after the time of primary crystallization of the FeNi recorded by the whole rock isochron. Given the distribution coefficients and the abundances of sulphides and schreibersites, it is clear that these processes are not reflected in the Re-Os systematics of whole rock iron meteorite samples. The Re-Os narrow time evolution for irons is consistent with the systematics of the short lived chronometers Pd-Ag and Hf-W. We have determined Re-Os systematics also in pallasites. The data for whole-rock FeNi samples show a wide range in Re/Os and permit the determination of a metal whole rock isochron which is consistent with the measurements on iron meteorites. The data include measurements on Eagle Station, a meteorite with silicates preserving a unique oxygen isotope composition. While pallasites have been viewed as originating in core-mantle boundaries, in differentiated planetesimals, the Re-Os data on the FeNi phases are consistent with early formation and fast cooling by fractional crystallization, within 3%0 in age, or 14 Ma. The data are not consistent with slow cooling of the initial molten FeNi. Observed, slow metallographic cooling rates