One might have expected that the composition of the Earth and the other inner planets would mirror that of the original rock component (represented by the CI chondrites) of the solar nebula. However the moderately volatile elements (that have condensation temperatures in the range 400-1100 K) are strongly depleted both in the Earth, Venus and Mars as well as in many classes of meteorites (Fig. 1.2).
1 The planets: their formation and differentiation 100000
Ordinary chondrites (H, L, LL)
Volatile element depletion
Fig. 1.2 The depletion of potassium, a volatile element, relative to uranium, a refractory element, both relative to CI, in the inner Solar System; CM and CV are classes of carbonaceous chondrite
This depletion in the volatile elements is most readily shown by measurements of the abundances of the gamma-ray emitters, potassium (moderately volatile) and uranium (refractory). The initial solar nebula value of the K/U ratio, as given by the CI meteorites, is near 60 000, but the ratio for the Earth is 10 000 (or 12 500) while that for Venus is similar within rather wide limits. The martian K/U ratio is about 20 000 .
It has been well established that this depletion is a bulk planetary and not a surficial effect. Potassium and uranium, although distinct in ionic radius and valency, are both incompatible (see Section 1.6), being concentrated in residual melts (and crusts) and so remain together during planetary differentiation. Alternative suggestions to account for the depletion of potassium: that it is buried in planetary cores, or is evaporated during planetary accretion, are untenable. As well as potassium, many other elements in the periodic table are also depleted, independently of the size of the planet.
Another useful volatile/refractory pair in this context are rubidium and strontium . The Rb/Sr ratio of the Earth is 0.03, an order of magnitude lower than the primordial solar nebula ratio as given by the CI meteorites. This, coupled with primitive 87Sr/86Sr values in lunar samples and meteorites tells us that this volatile-element depletion occurred close to the formation of the Solar System and was not due to later planetary processes such as evaporative loss during accretion.
This depletion is well illustrated by the composition of the Earth, plotted relative to CI (Fig. 1.3). The striking feature of this plot, that resembles the abundances of the elements in the other terrestrial planets as well as in many classes of meteorites other than CI, is that the depletion of the elements correlates with volatility, not with any other chemical parameters. Thus the diagram includes elements of diverse geochemical affinities that all plot on the depletion trend. These include those that enter metal phases (siderophile elements), sulfides (chalcophile elements) or silicates (lithophile elements) during crystallization of a molten planetary body.
In summary, the inner nebula was depleted in volatile elements whose condensation temperatures are less than about 1100 K. This event was probably due to early intense solar activity, so that the Sun was already formed, but occurred before the formation of chondrules. The consequence is that the inner planets do not contain the primordial solar nebular abundances (CI) of the chemical elements.
There are three reasons to raise this topic of volatile-element depletion in planetary compositions in a book about planetary crusts. In the first instance, many of these elements are incompatible and so finish up being highly concentrated in crusts. Secondly, the abundance of the moderately volatile element potassium, whose 40K isotope decays to 40Ar, is a major heat source and so is a driving force in planetary tectonics. Finally, many of the volatile elements (notably carbon, but including nitrogen, phosphorus, sulfur, potassium, sodium and copper) that are essential to life as it occurs on the Earth, are low in abundance in the inner nebula. There is thus a certain irony in the fact that, at least in our Solar System, these elements that are essential for life, are depleted in the habitable zone .
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