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Chapter 12. Noble Gas Isotope Geochemistry

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Chapter 12. Noble Gas Isotope Geochemistry Isotope Geochemistry Chapter 12 Noble Gas Geochemistry Noble Gas Isotope Geochemistry 12.1 INTRODUCTION The noble gases are those elements on the extreme right of the periodic table. The name is derived from their unreactive character, a consequence of having outer electron orbitals filled. Isotopes of all 6 noble gases are produced to some degree by nuclear processes. 4He is, of course, produced by alpha de- cay while 40Ar is produced by electron capture decay of K. Isotopes of Kr and Xe are produced by U fis- sion, while Rn consists of only short-lived isotopes of the U and Th decay series. In addition, 129Xe is the decay product of the extinct radionuclide 129I (half life: 15.8 Ma) and the heavy Xe isotopes were pro- duced by fission of the extinct nuclide 244Pu (half life 82 Ma). While neon isotopes are not produced di- rectly by radioactive decay, significant variations occur in Ne isotopes as a consequence of reaction be- tween ‘fissogenic’ neutrons and magnesium as well as between alpha particles and 18O. Finally, cosmic ray interactions can also produce isotopic variations in the noble gases, as we have already seen. Cos- mic ray interactions produce a great many nuclides (Chapter 4), but because noble gases are so rare the effects are more significant than for other elements. In addition to nuclear processes noble gas isotope ratios also vary as a consequence of mass-dependent fractionations. The processes that produced these fractionations mainly occurred in the young solar system and Earth and provide important insights into that very early episode of history, as was recognized more than half a century ago (Damon and Kulp, 1958). In the following sections, we will consider the data on He, Ne, Ar, Kr, and Xe separately (radon nu- clides are too short-lived to be of interest here) and examine how noble gases and their isotope ratios vary in solar system materials as this provides an essential background for understanding noble gas isotope variations on this planet. In later sections we will see how all the noble gases can be used to- gether to inform broad theories of Earth’s formation, structure and evolution. Much more detail on no- ble gas geochemistry can be found in books by Ozima and Podosek (2002) and Porcelli et al. (2002). 12.1.1 Noble Gas Chemistry Although none of the noble gases are known to form chemical bonds in nature (they can be induced to do so in laboratory experiments and Xe may be reactive in nature at high pressure), they nonetheless differ in their solubility in liquids (including water, oil, and magma), ionization potential, polarizabil- ity, diffusivity, and the extent to which they absorb onto solid surfaces. Noble gas solubility is a com- plex function of a variety of factors including temperature, pressure, and salinity, but under typical sur- face conditions, water solubility increases with increasing atomic number. Varying temperature de- pendence of noble gas solubility in water enables dissolved noble gas concentrations in groundwater to be used as a paleothermometer (e.g., Kipfer et al., 2002). In magma, solubility decreases with atomic weight (i.e., He is the most soluble, Xe the least). Table 12.1. Abundances of Noble Gases In equilibration between water and petroleum, the heavier noble gases appear to partition into in the Atmosphere and Sun the petroleum phase more extensively than the Element Atmosphere Sun -6 9 lighter noble gases. As one might expect, diffu- He 5.24 × 10 2.288 × 10 diffusivity increases with decreasing atomic Ne 1.818 × 10-5 2.146 × 106 4 weight. Being a particularly small and Ar 0.0934 1.025× 105 8 electrically neutral atom, He diffuses Kr 1.14 × 10-6 55.15 particularly rapidly. Even at modest tem- Xe 8.7 × 10-8 5.391 peratures of the upper crust (150˚C), radiogenic Rn ~6 × 10-20 4He will diffuse too rapidly out of minerals to be Atmospheric concentrations are mixing ratio, i.e., mo- retained. Nevertheless, on the scale of the whole lar concentrations; solar concentrations are molar con- crust, significant transport of even He occurs centrations relative to Si ≡ 1 × 106. 427 12/4/13 Isotope Geochemistry Chapter 12 Noble Gas Geochemistry only through advection. The heavier noble gases, Xe in particular, can be strongly absorbed on to min- eral surfaces. For example, Yang and Anders (1982) found mineral/gas distribution coefficients for chromite and carbon decreased from 0.1 for Xe to 0.6 x 10-3 for Ne. Helium has the highest first ioniza- tion potential of all elements (2373 kJ/mol) and ionization potential drops with increasing atomic num- ber such that the ionization potential of krypton (1351 kJ/mol) is only a little higher than that of hydro- gen (1312 kJ/mol) and the ionization potential of xenon (1170 kJ/mol) is well below that of hydrogen. The atmosphere is the largest terrestrial reservoir of most noble gases (He is the exception) and is well mixed and consequently isotopically homogeneous (concentration of the noble gases are listed in Table 12.1). Concentrations of noble gases in rocks and minerals are extremely low and recovering them in sufficient quantities to determine their isotopic composition is a trick rivaling Moses’ producing wa- ter from rock. The first part of the trick is to choose the right rock. Because they readily escapes to the atmosphere when lavas erupt subareally, data on isotopic composition of noble gases in the mantle are restricted to samples from deep wells and hot springs, submarine-erupted basalts, fluid inclusions in minerals that crystallized at depth (including xenoliths and in some cases phenocrysts), and, in the case of Iceland, subglacially erupted basalts. When basalts are erupted under several kilometers of water or ice, the solubility of noble gases is such that at least some of the gases remain in the melt and are trapped in the quenched glassy rims and vesicles of pillow basalts. Several approaches can then used to extract the noble gases from rocks and minerals. First, the rock or mineral can be fused in vacuum, al- lowing noble gases to be collected, cryogenically separated, and analyzed in a mass spectrometer. Second, the sample can be step-heated, terminating with complete fusion, with each temperature-release fraction analyzed separately, much as is done in 40Ar/39Ar dating (Chapter 3). Third, the sample may be crushed in vacuum, which releases only those gases trapped in vesicles and fluid inclusions. The remaining material can then be step-heated to release gases trapped in the crystalline structure or glass. Finally, samples may be attacked or etched with acids, an approach largely restricted to meteorites. In some cases, distinct compositions result from different release strategies for the same sample, that in turn reflects multiple origins of the gas (but we don’t have space to go into that detail of the craft). 12.1.2 Noble Gases in the Solar System Figure 12.1. Abundance patterns of noble gases in the solar system. Planetary patterns, which include the atmospheres of Figure 12.1 shows the relative the terrestrial planets as well as gases in chondrites, show abundances of the noble gases in depletions in the light noble gases relative to the Solar pat- various solar system materials. Noble terns of the Sun and solar wind. Based on data in Hunten et gas geochemistry divides these into al., (1988), Busemann et al., (2000); Pepin and Porcelli (2002), two basic patterns: solar and planetary, Ozima and Podesek (2006), and Weiler (2002). terms we will use throughout this 428 12/4/13 Isotope Geochemistry Chapter 12 Noble Gas Geochemistry chapter. The former include the spectroscopically measured “solar” abundances in the Sun itself and the solar wind (as determined from spacecraft measurements), even though the latter shows small de- pletions in the light noble gases. This reflects some elemental fractionation in the acceleration of ele- ments into the solar wind as well as fractionation in the retention of these gases in various target mate- rials such as foils deployed in spacecrafts and lunar soils. Solar abundance patterns also occur as a component of the noble gas inventory of meteorites and in interplanetary dust particles. Solar gases in these materials originate through solar wind implantation and are released by heating at relatively low temperatures. The remaining “planetary” patterns show much stronger depletions in the light relative to the heavy noble gases, particularly He and Ne. Planetary patterns are observed in both terrestrial planet atmospheres and as the major component of the noble gas inventory of meteorites. Step heating and successive leaching reveals carbonaceous chondrites contain a variety of noble gases components, including solar gases (implanted by the solar wind) and cosmogenic nuclides (Figure 5.16), but the most abundant components have planetary type patterns. One of most ubiquitous of these planetary components is released by leaching with oxidizing acids such as HNO3 and HClO4. The carrier of these noble gases, called “Q”, appears to be carbon-rich component present in the matrix and in coatings on chondrules, but its exact nature is unclear (the so-called “P1” component in meteorites appears to be es- sentially the same as “Q”). In contrast to ordinary chondrites, which are gas-poor, at least some ensta- tite chondrites also contain a significant inventory of noble gases, and their abundance patterns are ba- sically similar to that of the Q component of carbonaceous chondrites. These planetary abundance pat- terns suggest a fractionation process operating in the early solar system in which noble gases were lost from planets and planetary embryos in proportion to atomic mass.
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