K/U ratios

A lower K/U ratio for the Earth and the basaltic achondrites as compared to the chondritic (solar) ratio was emphasized more than ten years ago by Gast115 116 and Wanke117). Since the first lunar samples became available it has been suggested, especially by Gast30,118 that the Moon has a higher concentration of the refractory elements Ca, Al, REE, etc. 

Uranium shows a reasonable correlation with lanthanum (Figure 6), which yields an upper cmstal uranium content of 2.7 + 0.6 ppm. This value is within error of the averages derived from surface exposures, except for the value of Gao et al. (1998a) and Eade and Eahrig (1973), which are distinctly lower. The loess-derived K/U ratio of 7,400 is lower than that assumed for the upper cmst of 10,000 (Taylor and McLennan, 1985), and may reflect some potassium loss due to weathering, as discussed above. 

The measured " °Ar/ Ar ratio is 1.11 0.02, substantially less radiogenic than the terrestrial atmosphere. For a mixing ratio of 21-48 ppm (Donahue and Pollack, 1983), the atmosphere contains (1.8-4.4) X 10" atoms " °Ar. Divided by the mass of the planet, this corresponds to (3.6-9.0) X 10 atoms " °Ar g This is 0.2-0.5 times the value for the Earth. However, Venus appears to be deficient in potassium. Data for the K/U ratio of the surface indicate that K/U = 7,220 1,220 (Kaula, 1999), or 0.57 0.10 times that of the value of 1.27 X 10" commonly taken for the Earth. Assuming that Venus has the same uranium concentration as the total Earth (including the core) of 14 ppb, then 12-28% of the " °Ar produced in Venus is now in the atmosphere (see Kaula, 1999). This indicates that a substantial inventory of " °Ar remains within the planet, possibly also accompanied by up to an equivalent... 

TABLE 3.5 Argon isotopic compositions and K/U ratios in MORB, OIB and the atmosphere. 

By using elemental ratios that either are constant across a wide range of igneous compositions (e.g., K/U) or vary systematically with bulk composition (e.g., K/Rb), it is possible to extrapolate to obtain the upper crustal abundances of a number of other elements (Table IV). In this manner the abundance of Rb can be obtained from K/Rb (250) Sr from Rb/Sr (0.3), while U can be obtained from the upper crustal Th/U ratios (3.8) and K from K/U ratios (10,000). The rapidly increasing database for sedimentary rocks has recently permitted upper crustal estimates for a variety of other elements (e.g., Nb, Ta, Zr, Hf, Cr, Ni) from the sedimentary data. 

Fig. 14. Uranium is refractory like the rare earths, so that K/U ratios are an analogue for K/La ratios (fig. 11). K and U data are available for a wide variety of solar system material, since both elements are readily determined by gamma-ray spectroscopy. Both are incompatible in igneous processes and so tend to preserve their bulk planetary ratios during differentiation. This diagram illustrates that substantial volatile element depletion was widespread in the inner solar nebula, so that similar variations to K/U in volatile element/rare earth ratios are lo be expected. (From Taylor 1987a.)...

The surface of Venus has K/U ratios of the same order as those of terrestrial surface rocks (Surkov 1981), so that it also is depleted in volatile elements relative to primitive nebula values. This relative depletion of the more volatile elements thus appears to have been a widespread feature of the inner solar nebula rather than being a feature unique to the earth. Accordingly, a widespread loss of volatile elements occurred in the irmer solar system prior to the accretion of the terrestrial planets (Taylor 1987a). No information is yet available for the rare earth elements on Venus, but on the basis of the discussion above, they are probably similar to the terrestrial abundance levels. 

The gamma-ray observations relevant to planetary studies are spectroscopic in nature, aimed at identifying specific key elements present in planetary surfaces via their characteristic emission energies. The abundances of elements with different condensation temperatures and geochemical behavior relate directly to the origin and evolution of planetary bodies. For example, the K/U ratio provides a measure of the remelting of primordial condensates, while the K/Th ratio indicates the relative abundance of volatile to refractory elements. 

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