Planetary mass-dependent fractionations

Hydrogen isotopes have also been fractionated during planetary geologic processes. Compared to the Earth, Venus and Mars have significantly elevated D/H ratios (5D = 125 000 permil and 4000 permil, respectively). These fractionations are thought to result from preferential loss of H relative to D from the atmospheres of these planets (Robert el al., 2000) atmospheric escape of hydrogen from the Earth was apparently not important. 

Ion microprobe analyses of hydrous minerals in Martian meteorites reveal two different sources of hydrogen. One is interpreted as magmatic water, with 5D = 900 permil, and thought to reflect the mantle composition the other is thought to reflect the atmospheric composition, with 5D =4000 permil (Leshin, 2000). The incorporation of atmospheric water into these meteorites suggests some kind of cycling of water between the atmosphere and lithosphere on Mars. 

Once differences in oxygen isotope composition for different bodies were established, the compositions of individual samples were smeared along slope +1/2 lines by mass-dependent 

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 

Oxygen isotopic compositions of minerals in CAIs on an oxygen three-isotope diagram. Axes are given in delta notation 81S0 =[((180/160)sampie/(180/160)s,andard)-1] x 1000, and similarly for S170. After Clayton et al. (1977). 

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Besides the problem of accounting for the chemical abundances of planetary noble gases, there are characteristic differences in isotopic composition between planetary noble gases in meteorites and the solar gases that presumably represent the nebula from which meteorites formed. For Ar and Kr the differences are modest or perhaps nonexistent or can ultimately be explained in terms of a reasonable degree of mass-dependent isotopic fractionation. For Ne (Figure 3.3) and Xe (Figure 7.6), the... 

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