Volatile elements noble gases

One of the salient characteristics of the composition of the Earth is the depletion in volatiles compared to the parental solar nebula relative abundances. This is most pronounced in the noble gases (Fig. 1). However, the acquisition by the planet of these unreactive elements at even these levels pose considerable problems. A comparison between noble gases on the terrestrial planets and other solar system objects reveals significant differences in both elemental ratios and isotopic compositions and indicate that complex processes were involved in sequestering planetary volatiles from the nebula, as well as providing important indications of the sources and evolutionary history of planetary volatiles. Also, noble gas isotope inventories that are produced by nuclear... 

For the group 6 and 7 elements, also the oxide/hydroxide molecules have been synthesized. For elements of group 8, the tetroxide is the species of choice, since this molecule is very volatile. For future studies with the p-elements around atomic number 114 the elements are expected to be volatile in their atomic state and should behave like noble metals or even like a noble gas. 

Due to the expected high volatility of elements with atomic numbers 112 to 118 in the elemental state [104], see also Chapters 2 and 6, gas phase chemical studies will play an important role in investigating the chemical properties of the newly discovered superheavy elements. An interesting question is, if e.g. elements 112 and 114 are indeed relatively inert gases (similar to a noble gas) [105] due to closed s2 and p /22 shells, respectively, or if they retain some metallic character and are thus adsorbed quite well on certain metal surfaces, see Chapter 6, Part II, Section 3.2. Extrapolations by B. Eichler et al. [106] point to Pd or Cu as ideal surfaces for the adsorption of superheavy elements. 

Comets are rich in volatile elements, but they probably delivered no more than 10% of Earth s volatile inventory. There are several reasons for this. Comets have a very low impact probability with Earth over their dynamical lifetime ( 10 Levison et al., 2000), limiting the amount of cometary material that Earth could have accreted. In addition, if most of Earth s water was acquired from comets, it seems likely that Earth s noble gas abundances would be higher than observed by several orders of magnitude (Zahnle, 1998). Einally, water measured spectroscopically in comets differs isotopically from that of seawater on Earth, with the cometary D/H ratio being greater by a factor of 2 (Lunine et al., 2000). 

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. 

This chapter provides an overview of available noble gas data for solar system bodies apart from the Earth, Mars, and asteroids. Besides the Sun, the Moon, and the giant planets, we will also discuss data for the tenuous atmospheres of Mercury and the Moon, comets, interplanetary dust particles and elementary particles in the interplanetary medium and beyond. In addition, we summarize the scarce data base for the Venusian atmosphere. The extensive meteorite data from Mars and asteroidal sources are discussed in chapters in this volume by Ott (2002), Swindle (2002a,b) and Wieler (2002). Data from the Venusian and Martian atmospheres are discussed in more detail in chapters by Pepin and Porcelli (2002) and Swindle (2002b). Where appropriate, we will also present some data for other highly volatile elements such as H or N. 

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. 

Figure 2. Noble gas M/ Kr abundance ratios in terrestrial planet atmospheres and volatile-rich meteorites, plotted with respect to solar relative abundances and compared to the range of elemental fractionations (with respect to ambient gas-phase abundance ratios) determined from laboratory adsorptive experiments and from analyses of natural sedimentary materials (Pepin 1991). These fractionations generally fall in the darker shaded area of the figure, except for a number of measurements on carbon black (Wacker 1989) displaying the smaller or reversed patterns within the lighter shading. Data from references cited in the text and in Pepin (1991).

Comet accretion models. Noble gases, as well as water, carbon, and nitrogen, could have been supplied to the inner planets by accretion of volatile-rich icy comets scattered inward from the outer solar system. Although noble gas isotopic distributions in comets are unknown, solar isotopic compositions would be expected in cometary gases acquired from the nebula. There is experimental evidence that the relative elemental abundances of heavier species (Xe, Kr, and Ar) trapped in water ice at plausible comet formation temperatures ( 30 K) approximately reflect those of the ambient gas phase, and trapped noble gas abundances per gram of water are substantial (Bar-Nun et al. 1985 Owen et al. 

Porcelli D, O Nions RK, Galer SJG, Cohen AS, Mattey DP (1992) Isotopic relationships of volatile and lithophile trace elements in continental ultramafic xenoliths. Contrib Mineral Petrol 110 528-538 Porcelli D, Ballentine CJ, Wieler R (2002) An introduction to noble gas geochemistry and cosmochemistiy. Rev Mineral Geochem 47 1-18... 

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