A spectral analysis of ordinary chondrites, S‐type asteroids, and their component minerals: Genetic implications

Abstract

Three salient features of visible and infrared reflectance spectra of ordinary chondrites (OCs) and S‐type asteroids are (1) albedo at 0.56 μm, (2) continuum slope, and (3) depth of the electronic absorption band due to octahedrally coordinated Fe2+ in olivine and pyroxene. These quantities were numerically extracted from the spectra of 23 OCs representing all metamorphic grades and 39 S‐type asteroids to be plotted in a three‐dimensional coordinate system. The spectral characteristics of three OCs which were comminuted, melted, recrystallized, and recomminuted are also presented in the same format. The results show that although laboratory simulation of melt alteration in an asteroidal regolith does alter OC spectra, the spectral parameters of these altered meteorites do not change enough to leave the parametric region defined by “unaltered” OC spectra. When the region containing the 39 S‐asteroid spectra is compared with that of the altered and unaltered OCs, it is found that not one of the OCs falls within the S‐asteroid region. The differences may be largely attributed to spectral differences between the respective metallic components. The range of S‐asteroid parameters is then compared with potential pure “end‐member” components most likely to result from magmatic differentiation of a chondritic protoasteroid: olivine, orthopyroxene, clinopyroxene, and Fe,Ni meteorite metal (alternatively represented by the M‐asteroids). It is found that the S‐asteroid array is consistent with random mixtures of the differentiated components except for a notable dominance of the spectral characteristics of the opaque (metallic) component. These results suggest that the M‐asteroids may form a composition continuum with the S‐asteroids. We discuss a scenario consistent with this analysis and with the Bell et al. theory of the geological structure of the asteroid belt. Ordinary chondritic protoasteroids of all sizes were probably the dominant primary condensates in the inner portion of the main asteroid belt. These were later heated by electromagnetic induction or by 26Al nuclide decay. As a result, the smaller ones were subjected to various degrees of metamorphism, while the larger ones were subjected to large‐scale magmatic differentiation. Petrological domains of the S‐asteroids (beneath a mixed regolith) may be large and supply achondrites, irons, and stony irons to Earth rather than well‐mixed breccias of these components. The (smaller) OC protoasteroids may still be abundant in the asteroid belt but, if small enough to escape differentiation, may also be small enough to escape Earth‐based identification.