The History of Planetary Degassing as Recorded by Noble Gases

Details The History of Planetary Degassing as Recorded by Noble Gases The History of Planetary Degassing as Recorded by Noble Gases

Dec 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003trgeo...4..281p&link_type=abstract

Treatise on Geochemistry, Volume 4. Editor: Ralph F. Keeling. Executive Editors: Heinrich D. Holland and Karl K. Turekian. pp. 3

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Noble gases provide unique clues to the structure of the Earth and the degassing of volatiles into the atmosphere. Since the noble gases are highly depleted in the Earth, their isotopic compositions are prone to substantial changes due to radiogenic additions, even from scarce parent elements and low-yield nuclear processes. Therefore, noble gas isotopic signatures of major reservoirs reflect planetary differentiation processes that generate fractionations between these volatiles and parent elements. These signatures can be used to construct planetary degassing histories that have relevance to the degassing of a variety of chemical species as well.It has long been recognized that the atmosphere is not simply a remnant of the volatiles that surrounded the forming Earth with the composition of the early solar nebula. It was also commonly thought that the atmosphere and oceans were derived from degassing of the solid Earth over time (Brown, 1949; Suess, 1949; Rubey, 1951). Subsequent improved understanding of the processes of planet formation, however, suggests that substantial volatile inventories could also have been added directly to the atmosphere. The characteristics of the atmosphere therefore reflect the acquisition of volatiles by the solid Earth during formation (see Pepin and Porcelli, 2002; Chapter 4.12), as well as the history of degassing from the mantle. The precise connection between volatiles now emanating from the Earth and the long-term evolution of the atmosphere are key subjects of modeling efforts, and are discussed below.Major advances in understanding the behavior of terrestrial volatiles have been made based upon observations on the characteristics of noble gases that remain within the Earth. Various models have been constructed that define different components and reservoirs in the planetary interior, how materials are exchanged between them, and how the noble gases are progressively transferred to the atmosphere (see Chapter 2.06). While there remain many uncertainties, an overall process of planetary degassing can be discerned. The present chapter discusses the constraints provided by the noble gases and how these relate to the degassing of the volatile molecules formed from nitrogen, carbon, and hydrogen (see also Chapter 3.04). The evolution of particular atmospheric molecular species, such as CO2, that are controlled by interaction with other crustal reservoirs and which reflect surface chemical conditions, are primarily discussed elsewhere (Chapter 8.09).Noble gases provide the most detailed constraints on planetary degassing. A description of the available noble gas data that must be incorporated into any Earth degassing history is provided first in Section 4.11.2, and the constraints on the total extent of degassing of the terrestrial interior are provided in Section 4.11.3. Noble gas degassing models that have been used to describe and calculate degassing histories of both the mantle ( Section 4.11.4) and the crust ( Section 4.11.5) are then presented. These discussions then provide the context for an evaluation of major volatile cycles in the Earth ( Section 4.11.6), and speculations about the degassing of the other terrestrial planets ( Section 4.11.7), Mars and Venus, that are obviously based on much more limited data. The processes controlling mantle degassing are clearly related to the structure of the mantle, as discussed in Section 4.11.4. Further descriptions of mantle noble gas reservoirs and transport processes based upon multi-tracer variations in mantle-derived materials are provided in Chapter 2.06. An important aspect is the origin of planetary volatiles and whether initial incorporation was into the solid Earth or directly to the atmosphere; these issues are discussed in detail in Chapter 4.12. Basic noble gas elemental and isotopic characteristics are given in Ozima and Podosek (2001) and Porcelli et al. (2002). The major nuclear processes that produce noble gases within the solid Earth, and the half-lives of the major parental nuclides, are given in Table 1. Table 1. Major nuclear processes producing noble-gas isotopes in the solid earth.a DaughterNuclear processParent half-lifeYield (atoms/decay)Comments 3He6Li(n, α)3H(β-)3He3He/4He=1×10-8b 4Heα-decay of 238U decay series nuclides4.468 Ga8c 4Heα-decay of 235U decay series nuclides0.7038 Ga7c238U/235U=137.88 4Heα-decay of 232Th decay series nuclides14.01 Ga6cTh/U=3.8 in bulk Earth 21Ne18O(α, n)21Ne21Ne/4He=4.5×10-8b 21Ne24Mg(n, α)21Ne21Ne/4He=1×10-10b 40Ar40K β- decay1.251 Ga0.1048b40K=0.01167% total K 129Xe129I β- decay15.7 Ma1129I/127I=1.1×10-4 at 4.56 Gad 136Xe238U spontaneous fission4×10-8e 136Xe244Pu spontaneous fission80.0 Ma7.00×10-5244Pu/238U=6.8×10-3 at 4.56 Gaf a From data compilations of Blum (1995), Ozima and Podosek (2001), and Pfennig et al. (1998).b Production ratio for upper crust (Ballentine and Burnard, 2002).c Per decay of series parent, assuming secular equilibrium for entire decay series.d Hohenberg et al. (1967).e Eikenberg et al. (1993) and Ragettli et al. (1994).f Hudson et al.(1989).

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