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Chapter 3. Decay Systems and Geochronology II: U-Th-Pb

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Chapter 3. Decay Systems and Geochronology II: U-Th-Pb Isotope Geochemistry Chapter 3 Geochronology II DECAY SYSTEMS AND GEOCHRONOLOGY II: U AND TH 3.1 INTRODUCTION The U-Th-Pb system is certainly the most powerful tool in the geochronologist's tool chest. While we can use the three decay systems independently, the real power comes in using them in combination, particularly the 235U-207Pb and 238U-238Pb systems, as it allows a check of the fidelity of the age calculated and in some circumstances, to obtain accurate ages despite disturbances to the system that violate the conditions we discussed in Chapter 2. We’ll begin by discussing U-Th-Pb dating, which is useful on a wide range of time scales, from hundreds of thousands to billions of years. Indeed, as we will see in this and subsequent chapters, U-Pb dating provides the definitive ages of the solar system and the oldest rocks on Earth. Without question, it is the “gold standard” of geochronology. Rather than decaying directly to lead, uranium and thorium decay through a chain of intermediate daughters, some of which have half-lives of tens or hundreds of thousands of years. Measuring the ra- tios of intermediate radioactive parents and daughters enables an entirely new set of geochronological tools for use on those time scales. We’ll devote the second half of this chapter to uranium decay series geochronology. 3.1.1 Chemistry of U, Th, and Pb U and Th are, strictly speaking, rare earth elements, although they belong to the actinide series instead of the lanthanide series. The other rare earths we have met so far, Nd and Sm, are lanthanides. As in the lanthanide rare earths, an inner electron shell is being filled as atomic number increases in the acti- nides. Both U and Th generally have a valence of +4, but under oxidizing conditions, such as at the sur- face of the Earth, U has a valence of +6. In six-fold coordination, U4+ has an ionic radius of 89 pm1 (100 pico meters = 1 Å); U6+ has an ionic radius of 73 pm in 6-fold and 86 pm in 8-fold coordination. Th4+ has an ionic radius of 94 pm. These radii are not particularly large, but the combination of somewhat large radius and high charge is not readily accommodated in crystal lattices of most common rock- forming minerals, so both U and Th are highly incompatible elements. Th is relatively immobile under most circumstances. In its reduced form, U4+ is insoluble and therefore fairly immobile, but in the U6+ form, which is stable under a wide range of conditions at the surface of the Earth, U forms the soluble oxyanion complex, UO 2– . As a result, U can be quite mobile. U and Th can form their own phases in 4 sedimentary rocks, uranite and thorite, but they are quite rare. In igneous and metamorphic rocks, U and Th are either dispersed as trace elements in major phases, or concentrated in accessory minerals (when they are present) such as zircon (ZrSiO4), which concentrates U more than Th, and monazite ([La,Ce,Th]PO4) which concentrates Th more than U. These elements may be also concentrated in other accessory phases such as apatite (Ca5(PO4)3(OH)), xenotime (YPO4) and titanite (or sphene, CaTiSiO5). However, zircon is by far and away the most important from a geochronological perspective. U and Th are refractory elements, and we can therefore expect the Th/U ratio of the Earth to be the same as chondrites or nearly so. There is, however, some debate about the exact terrestrial Th/U ratio, and we can be no more precise than to say it is 4±0.22. This ratio is 3.8 in the CI chondrite Orgueil, but may be low due to mobility of U in hydrous fluid in the CI parent body. 1In eight-fold coordination, the effective ionic radius of U4+ is 1.00Å. In zircon, a mineral which highly concentrations U, U is in 8-fold coordination. This is probably a pretty good indication that 8-fold coordination is the preferred con- figuration. The figure for 6-fold coordination is given for comparison to other radii, which have been for 6-fold coor- dination. Th has a radius of 1.05Å in 8-fold coordination. 2 The uncertainty results from the mobility of U. The CI carbonaceous chondrites experienced mild alteration in hy- drous conditions on the parent body. U was mobilized under these conditions and thus the U/Th ratio varies in these meteorites. For this reason, they cannot be used to precisely determine the U/Th ratio of the Solar System and the 73 December 4, 2013 Isotope Geochemistry Chapter 3 Geochronology II The geochemical behavior of Pb is more complex than that of the elements we have discussed so far and consequently, less well understood. It is a relatively volatile element, so its concentration in the Earth is certainly much lower than in chondrites. It is also a chalcophile element. If the core contains, as some believe, substantial amounts of S, it is possible that a significant fraction of the Earth's Pb is in the core (it is, however, difficult to distinguish loss of Pb from the Earth due to its volatility from loss of Pb from the silicate portion of the Earth due to extraction into the core). Pb can exist in two valence states, Pb2+ and Pb4+. Pb2+ is by far the most common state; the Pb4+ state is rare and restricted to highly alka- line or oxidizing solutions. The ionic radius of Pb2+ is 119 pm in 6-fold coordination and 129 pm in 8- fold coordination. As a result of its large ionic size, Pb is an incompatible element, though not as in- compatible as U and Th (incompatibility seems to be comparable to the light rare earths). The most common Pb mineral is galena (PbS). In silicates, Pb substitutes readily for K (ionic radius 133 pm) in po- tassium feldspar, but less so in other K minerals such as biotite. Most naturally occurring compounds of Pb are highly insoluble under most conditions. As a result, Pb is usually reasonably immobile. How- ever, under conditions of low pH and high temperature, Pb forms stable and somewhat soluble chlo- ride and sulfide complexes, so that Pb can sometimes be readily transported in hydrothermal solutions. Although Pb is clearly less incompatible that U and Th, these 3 elements have been extracted from the mantle and concentrated in the crust to approximately the same degree. The reason for this is not yet completely understood, and we will discuss this problem in more detail in Chapter 6. 3.1.2 The 238U/235U Ratio and Uranium Decay Constants Until the last few years, it had been assumed that the 238U/235U ratio was constant. In this case, the 207Pb*/206Pb* ratio is a function only of time (and the decay constants). The conventionally accepted value of this ratio was 138.88 (Jaffey et al., 1971; Steiger and Jäger, 1977). However, as precision in iso- topic measurements has improved, it became apparent (1) that this value varied somewhat, and (2) that mean value in terrestrial materials is actually a little lower (e.g., Sterling et al, 2007; Weyer et al., 2008; Amelin et al., 2010; Mattinson, 2010; Hiess et al., 2012). Sterling et al. (2007) found a range of about 4 per mil in this ratio in natural terrestrial materials, while Weyer et al. (2008) found a range of about 1.4 per mil. Hiess et al. (2012) demonstrated a variation of 5 per mil in uranium-bearing minerals commonly analyzed in geochronological work (zircon, apatite, monazite, xenotime, baddeleyite, titanite), but this range is defined by relatively few ‘outliers’ and almost all zircons fell within a much smaller range of 137.77 to 137.91, a 0.1 per mil variation. These variations result in slight differences in bond strength and diffusivity that result from the mass differences of the two U isotopes. We will postpone discussion of the causes of isotopic variations resulting from chemical effects such as these until Chapter 8, where we will discuss them at length. Somewhat greater variations occur in meteorites as a consequence of very slight chemical and isotopic heterogeneity in the solar nebula and the decay of the short-lived, and now extinct, radionuclide 247Cm (more on that in Chapter 5). Amelin estimated the mean terrestrial 238U/235U to be 137.821±0.014; Hiess et al., (2012) estimated it to be 137.818±0.045. There is excellent agreement between these two values, but both differ from the con- ventional value. Goldmann et al. (2013) have proposed a slightly lower value of 137.79±0.03 based on measurements of meteorites. In this text, we will adopt a value for the 238U/235U of 137.82 and we will assume the value to be constant. However, you should be aware that, at least as of this writing, a value of 137.88 remain the ‘official’ value (the one recommended by the IGC Subcommission on Geochronol- ogy) and that almost all the ages in the literature are based on that value. Furthermore, the highest pre- cision geochronology may require in the analysis of the 238U/235U as well as Pb isotope ratios. Finally, this value of 137.82 is the present-day value; it changes through time as a result of the two isotopes de- caying at different rates. If we need to know the ratio at some other time (for example, Problem 1), we need to calculate it based on equation 2.4.
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