The ultraviolet spectra of carbonaceous chondrites

Abstract

Optical spectroscopy of meteorites is an important laboratory tool for the study of the origin and evolution of solar and galactic interstellar minor bodies. In the region from the visible to the ultraviolet, the absorption spectra have hitherto been measured on the Bruderheim (Egan and Hilgeman, 1975), the Orgueil (Huffman, 1977), the Murchison (Sakata et al., 1977) and the Yamato (Yabushita et al., 1987) chondrites. These measurements were performed with pulverized specimens mixed with KBr powder or suspended in a transparent liquid. Due to these methods of sample preparation, the spectra observed were not very clear and the origin of the observed absorption features was not identified. In the present study, we have studied carbonaceous chondrites in the from of a thin film prepared by vacuum evaporation.A tiny fragment of the Belgica carbonaceous chondrite B-7904 (type CM2) was allotted to us by the National Institute of Polar Research, Tokyo. The vacuum evaporations were made by using an ordinary vacuum evaporator with a tantalum boat as a resistive heater. The vaporized chondrite inclusions were recondensed on a quartz substrate kept at room temperature. The evaporation temperature was carefully controlled at around 1050 ° C. Below this temperature, no appreciable condensation took place on the substrate, although we observed a significant pressure rise caused presumably by various gases released from the chondrite specimen, including the thermally degraded organic compounds (Shimoyama et al., 1991).The absorption spectrum obtained for an evaporated film with mass density 4.3 #g cm -2 is presented in Figure 1. The spectrum exhibits a sharp peak at 226 nm and a broad shoulder around 270 nm. These features are considerably clearer than those observed previously for the pulverized samples (Sakata et al., 1977;Yabushita et al., 1987). In order to identify the inclusion responsible for the spectral features observed here, we have performed a series of compositional analyses both on the raw meteorite specimen and the evaporated films. When we combined the results from these analyses all together, we came to the conclusion that the most plausible inclusion giving rise to the observed absorption features must