Solar oxygen mass fraction

When differences in the oxygen isotopic composition of CAIs were first measured in 1973, Robert Clayton and his coworkers attributed these mass-independent variations to mixing of normal solar system gas (plotting on or above the terrestrial mass-fractionation... 

Figure 3 The distribution of neon isotopes in mantle-derived rocks, indicating the presence of an atmospheric component, a radiogenic component adding Ne (produced by neutrons from uranium fission acting on oxygen and magnesium), and a solar component. It is this latter that indicates that gases in the mantle were derived from the capture of solar material in the early history of the Earth. M = MORB (midocean ridge basalts) P = plume or ocean island basalts (OIB) A = atmosphere. Solar neon is represented by the horizontal line at Ne/ Ne = 12.5 MFL is the mass fractionation line. The presence of solar neon in ocean basalts was first identified by Craig and Lupton (Craig H and Lupton JE (1976) Earth and Planetary Science Letters 31 369-385). (Reprinted with permission from Farley and Poreda (1993).

Stable isotope analysis of Earth, Moon, and meteorite samples provides important information concerning the origin of the solar system. 8lsO values of terrestrial and lunar materials support the old idea that earth and moon are closely related. On the other hand three isotope plots for oxygen fractionation in certain meteoric inclusions are anomalous. They show unexpected isotope fractionations which are approximately mass independent. This observation, difficult to understand and initially thought to have important cosmological implications, has been resolved in a series of careful experimental and theoretical studies of isotope fractionation in unimolecular kinetic processes. This important geochemical problem is treated in some detail in Chapter 14. 

Mass-independent isotopic fractionations are widespread in the earth s atmosphere and have been observed in O3, CO2, N2O, and CO, which are all linked to reactions involving stratospheric ozone (Thiemens 1999). For oxygen, this is a characteristic marker in the atmosphere (see Sect. 3.9). These processes probably also play a role in the atmosphere of Mars and in the pre-solar nebula (Thiemens 1999). Oxygen isotope measurements in meteorites demonstrate that the effect is of significant importance in the formation of the solar system (Clayton et al. 1973a) (Sect. 3.1). 

By its great mass, the Sun constitutes the major part of the Solar System. In this sense, it is more representative than the planets, which have been the scene of intensive chemical fractionation. The composition of the solar photosphere can thus be compared with the contents of meteorites, stones that fall from the sky, a second source of information on the composition of the protosolar cloud, provided that volatile elements such as hydrogen, helium, carbon, nitrogen, oxygen and neon are excluded. Indeed, the latter cannot be gravitationally bound to such small masses as meteorites and tend to escape into space over the long period since their formation. 

In cosmochemistry, we use stable-isotope fractionations to study evaporation and condensation in the solar nebula, aqueous processes on asteroids, and even ion-molecule reactions to form organic molecules in interstellar clouds. The oxygen isotopes also show large mass-independent shifts that may be related either to chemical or physical processes or to incomplete mixing of the products of nucleosynthesis. These topics will be covered in detail in later chapters. 

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