Origin, age, and composition of meteorites

Abstract

There are perhaps as many opinions as to the origin of meteorites as there are students of meteorites. Probably in no other branch of natural science is there such a wealth of observational data coupled with such a lack of unanimity in interpretation." (WooD, 1963a) Abstract. This paper attempts to bring together and evaluate all significant evidence on the origin of meteorites.The iron meteorites seem to have formed at low pressures. Laboratory evidence shows that the absence of a Widmanst~itten pattern in meteorites with ~ 16 ~ Ni cannot be attributed to high pressures, but to supercooling or an unusually last cooling rate for these meteorites, which prevented the development of a pattern. The presence of tridymite in the Steinbach siderophyre provides further, direct proof that the Widmanst/itten pattern can form at pressures less than 3 kb. Neither diamond, nor cliftonite, nor cohenite are reliable pressure indicators in meteorites. Diamonds were formed by shock while cliftonite may have been derived from a cubic carbide such as Fe4C. Cohenite is apparently stabilized by kinetic rather than thermodynamic factors. Several lines of evidence suggest that the irons come from more than one parent body, perhaps as many as four.The frequency of pallasites is perfectly consistent with an origin in the transition zone between core and mantle of the parent body. "Hybrid" meteorites such as Brenham are not necessarily derived from the metal-silicate interface, but probably resulted from dendrite growth in the solidifying melt.Ordinary chondrites definitely are equilibrium assemblages rather than chance conglomerates. According to the best available evidence, Prior's rules seem to be valid. The metal particles in chondrites differentiated into kamacite and taenite in their present location, rather than in a remote earlier environment. Trace element abundances in ordinary and carbonaceous chondrites suggest that these meteorites accreted from two types of matter: an undepleted fraction that separated from its complement of gases at low temperatures, and a depleted fraction that lost its gases at high temperatures. These two fractions of primitive meteoritic matter are tentatively identified with the matrix and chondrules-phis-metal, respectively. New restrictive limits are placed on the iron-silicate fractionation in chondrites. No direct evolutionary path exists that connects the currently accepted solar abundances of Fe and Ni and the observed Fe/Si and Ni/Si ratios in chondrites. Apparently the solar abundance of iron is in error. The iron-silicate fractionation seems to have occurred while chondritic matter was in a more strongly reduced state than its present one.The U-He and K-At ages of hypersthene chondrites are systematically shorter than those of bronzite chondrites. Short ages are correlated with shock effects, and it seems that the hypersthene chondrites suffered reheating and partial-to-complete outgassing 0.4 AE ago. The cosmic-ray exposure ages of all classes of meteorite s cluster distinctly, indicating th...

Photometric and polarimetric properties of the bruderheim chondritic meteorite

“…In fact, there is strong evidence, both chemical (Anders, 1964) and orbital (Anders, 1971) that most meteorites are derived from a small number (probably less than a dozen) of parent bodies. An interesting possibility is that some of the Apollo asteroids, such as…”

“…Because there are many indirect arguments (Anders, 1964) which indicate that most meteorites are fragments of asteroid bodies, and since McCord, et al (1970) 'have successfully identified the spectral reflectance curve of one asteroid, Vesta, with that of a basaltic achondrite, laboratory measurments on meteorite samples should prove useful in an intrepretation of asteroid surfaces. We 'have chosen to study the spectral, photometric, and polarimetric properties of the Bruderheim olivine-hypersthene chondrite, which is representative of the most common meteoritic material recovered on Earth (Mason, 1962), and, hence, might also be the most common in interplanetary space.…”

Asteroid surface materials: Mineralogical characterizations from reflectance spectra

“…The major uncertainty in evaluating any particular asteroidal object as a source of some portion of the terrestrial meteorite flux lies in the incomplete understanding of the mechanism which converts the original orbit into one which intersects that of the earth. A variety of mechanisms have been proposed including close encounters with Mars (Anders, 1964;Arnold, 1965), secular perturbation (Kozai, 1962;Williams, 1969), resonant perturbations (Zimmerman and Wetherill, 1973), secular resonant accelerations (Williams, 1973) and non-gravitational accelerations (Peterson, 1976). Several authors predict that specific asteroids should be providing a significant contribution to the terrestrial flux if their mechanism is dominant.…”

Raman spectroscopic study of four Spanish shocked ordinary chondrites: Cañellas, Olmedilla de Alarcón, Reliegos and Olivenza

“…Anders 1964;Keil 1964;Heymann 1967;Neal et al 1981;Walsh & Lipschutz 1982;Walsh et al 1983;Friedrich et al 2001Friedrich et al , 2002Friedrich et al , 2008Grier et al 2004;Horz et al 2005;Friedrich 2006;Ferroir et al 2008;Hirata et al 2009). Consequently, prior to a chemical study, it is convenient to determine whether the chondrite has been affected by shock, and to what extent.…”