Oxygen Isotopes in the Solar System

Mass-independent O isotopic fractionations in CAIs have long been attributed to the mixing of nucleosynthetic components, but no other major elements have shown such nucleosynthetic contribution arguing against this hypothesis. Hence, Clayton (1) proposed that self-shielding of CO in the proto-solar nebula resulted in the preferential production of O isotopes, which were then accommodated in solid planetary objects such as Earth, Mars, and meteorite parent bodies. Yurimoto and Kuramoto (2), and Lyons and Young (3) also employed the same process (albeit in different astrophysical settings) to explain the mass-independent O isotope fractionation in CAIs. As a consequence of this process, these authors concluded that the solar O is close to the extreme O isotope composition observed in CAIs (DO ~ -50 permil), and differs from those of planetary bodies such as Earth or Mars (DO ~ 0). However, since the average values for DO for bulk chondrites and achondrites are close to zero, and their variance becomes smaller with increasing size of a planetary object (Figure 1), Ozima et al (4) argued that all the planetary objects could have been derived by random sampling of a whole planetesimal population which represents the DO value of the protosolar nebula or the Sun. In this preliminary statistical argument, Ozima et al.(4) assumed a specific relation between variance (standard deviation) and the size of planetary object that may not in general be valid. To circumvent this difficulty, here we used a more general statistical approach. The result supports the previous conclusion that terrestrial objects (meteorite parent bodies, Mars, Earth, Moon etc) should have the same O isotope as the Sun. We also discuss its implications on the evolution of the early solar system as well as the origin of the Oxygen isotopes in CAIs.