Oxygen Isotopes in Asteroidal Materials

Abstract

Measurement of the variation of the 17O/16O and 18O/16O ratios of extraterrestrial materials has proven to be a key tool in understanding the formation and evolution of the Solar System. This chapter attempts to collate the huge data set of oxygen isotopic measurements of asteroidal material, as sampled by meteorites, in order to understand some of the processes involved in the formation of these primitive bodies and how such events have affected the oxygen isotopic ratios, ultimately offering a window back to the very origin of Solar System and its primordialoxygen isotopic heterogeneity. Oxygen is generally a major element in the gas, liquid and solid phases that have interacted throughout the evolution of the Solar System, its chemical properties ensuring that there is abundant opportunity for reactions and exchange, the details of which can be recorded in the isotopic ratios of the products. Such processes can be crudely subdivided into the following six categories: • Early solar nebular isotopic heterogeneity • High-temperature modifi cation of nebular components • Metamorphism and melting in asteroids • Aqueous alteration in asteroids • Mixing of components/brecciation • Terrestrial weathering It is clear from the isotopic variation in the earliest-formed nebular components, such as refractory inclusions and chondrules, that there was considerable oxygen isotopic heterogeneity in the early solar nebula, spanning over 50‰, and perhaps much greater, in both δ17O and δ18O. The origin of this variation is discussed elsewhere in this volume. However, isotopic exchange between the 16O-rich solids and 17O- and 18O-rich gas phase(s) resulted in a wide range of isotopic variation in these early-formed components, offering insight into the conditions of these processes and the evolution of the reservoirs with time and/or space. Once accreted onto the early planetisemals, much of the primitive nebular components experienced considerable modification. The metamorphism and melting experienced by some of these bodies, most probably the result of heating from the decay of short-lived radionuclides (e.g., 26Al), resulted in progressive homogenization, within individual bodies, of any original isotopic heterogeneity. Complete homogenization appears to have been achieved in the HED parent body, indicating very high levels of melting and the development of a large magma ocean. The carbonaceous and ordinary chondrite parent bodies experienced more modest heating, sufficient to mobilize water ice that accreted along with the more refractory materials. The subsequent reactions between the liquid and solid phases at low temperature imparted large isotopic fractionations which record some of the conditions of this alteration, such as water:rock ratio, temperature, and isotopic signature, and thus potentially the origin of the water ice. The evolution of the asteroidal bodies has been far from simple, and an important use of oxygen isotope measurements has been in the identifi cation of the components present in some meteorites, the result of breakup, reassembly and mixing of different bodies during impact processing. The final oxygen isotopic signatures imparted into meteorite samples are those associated with terrestrial weathering. The isotopic compositions of the weathering products depend on location (primarily latitude) while the magnitude of the weathering is dependent on many factors, with some meteorites (e.g., highly reduced enstatite chondrites and the matrix-rich CO3s) particularly susceptible to such effects.