Volatile elements reservoirs

The potential components that might have delivered volatile elements to the Earth are the PSN (the major reservoir in the Solar System) and solid matter bodies such as meteorites and comets. The composition of meteoritic volatiles is thought to have been derived from the PSN through elemental and isotopic fractionation. Contributions from sources outside the Solar System such as pre-solar grains or species affected by interstellar chemistry are attested by the discovery of pre-solar grains in primitive meteorites on one hand, and by the large variation of the D/H ratio in the Solar System on another hand, but their extent is a matter of debate. A comparison of the abundances of... 

Of the major volatile elements described above, water, a variety of carbon compounds, nitrogen and sulfur are the volatile compounds which dominate in the modern Earth. In this section we review the modern-Earth geochemical cycles for water, carbon, nitrogen, and sulfur and look in some detail at volatile mass balances between the Earth s surface reservoirs and the deep Earth. Then, having established how the modern Earth works we seek to determine how these geochemical cycles might have operated in the early Earth. 

Kramers (2003) calculated major and minor (noble gas) volatile element abundance patterns in the Outer Earth Reservoir (the atmosphere, hydrosphere, oceanic and continental crust, and recycled components in MORB-source mantle). These are presented, normalized to solar abundances, together with data for chondrites in Fig. 5.6. The following observations can be made ... 

FIGURE 5.6 Major volatile element and noble gas abundances in the outer Earth reservoir of Kramers (2003) and in carbonaceous chondrites relative to Al and solar abundances. The data show that apart from xenon the Earth and chondritic meteorites have similar element distribution patterns and that both are strongly depleted in the noble gases and in H, C, and N relative to solar abundances. 

FIGURE 5.8 The Al-normalized concentrations of the major volatile elements (H, C, N, Cl, Br and I) and the noble gases (Ne, Ar, Kr, Xe) in the outer Earth reservoirs relative to Al-normalized concentrations in carbonaceous chondrites (after Kramers, 2003). 

Given that more than 99% of the mass in the solar system is contained in the Sun and that the Sun, in contrast to the rocky material that constitutes the meteorites, has retained its full complement of volatile elements, it is clear that the Sun is the primary reservoir of... 

Some short-lived radionuclides were sufficiently abundant at the start of the solar system to produce variations in the abundance of their daughter isotopes in early-formed objects (Table 10.2). The half-lives of these nuclides are between about 0.1 and 100 Ma (Table 10.2). Hence, the parent isotopes are no longer present today, but they were synthesized in stars shortly before solar system formation and therefore they were present in the early solar nebula. The isotopic record of these nuclides provides information about stellar nucleosynthetic sites active shortly before the birth of the solar system and the time scales over which the early solar system formed and first differentiated. Depending on half-life and chemical affinities of parent and daughter isotopes, extinct radionuclide systems can be used to date processes as diverse as the formation of CAIs and chondrules, volatile element depletion and planetary difierentiation (e.g., core segregation and differentiation of early silicate reservoirs). In particular, they are powerful tools to study the Earth s accretion and core formation [90-92],... 

In most cases, the activator impurity must be incorporated during crystal growth. An appropriate amount of impurity element is dissolved in the molten Ge and, as crystal growth proceeds, enters the crystal at a concentration that depends on the magnitude of the distribution coefficient. For volatile impurities, eg, Zn, Cd, and Hg, special precautions must be taken to maintain a constant impurity concentration in the melt. Growth occurs either in a sealed tube to prevent escape of the impurity vapor or in a flow system in which loss caused by vaporization from the melt is replenished from an upstream reservoir. 

Volcanic activity has a significant effect on the mobilization of metals, particularly the more volatile ones, e.g., Pb, Cd, As, and FFg. Effects of volcanism are qualitatively different from those of the weathering and other near-surface mobilization processes mentioned above, in that volcanism transports materials from much deeper in the crust and may inject elements into the atmospheric reservoir. 

Developing a model for the composition of the Earth and its major reservoirs can be established in a four-step process. The first involves estimating the composition of the silicate Earth (or primitive mantle, which includes the crust plus mantle after core formation). The second step involves defining a volatility curve for the planet, based on the abundances of the moderately volatile and highly volatile lithophile elements in the silicate Earth, assuming that none have been sequestered into the core (i.e., they are truly lithophile). The third step entails calculating a bulk Earth composition using the planetary volatility curve established in step two, chemical data for chondrites, and... 

After the accretionary event in which the Earth acquired its volatiles, other processes took place which caused it to lose them. There are two lines of evidence which tell us about the early Earth s loss of volatiles. The first comes from a comparison between the volatile concentrations in the outer Earth and those of carbonaceous chondrite meteorites (the most primitive and most volatile-rich of all the meteorite groups). It is clear from Fig. 5.6 that the Outer Earth Reservoir has two to three orders of magnitude less volatiles than carbonaceous chondrites. In addition it is evident that the lighter major elements are more depleted than the heavy ones. 

The solar system formed from a molecular cloud fragment—traditionally called the solar nebula—that was rather well mixed. Therefore, isotopic abundances in almost all available solar system materials are very similar to each other, and elemental abundances in primitive meteorites are also similar to the values in the Sun. The major exceptions to this rule are the noble gases. Because they are chemically inert and volatile, they are very strongly depleted in solid matter. As a consequence, numerous noble gas components can be recognized throughout the solar system which are not necessarily related to the composition of the bulk nebula. Still, one major question in cosmochemistry is to what extent planetary bodies contain reservoirs that reflect the noble gas composition in the nebula or the presolar cloud. 

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