Understanding the Behavior of Lead Isotopes in Geochronology and Isotope Geochemistry, Study notes of Geology

Download Understanding the Behavior of Lead Isotopes in Geochronology and Isotope Geochemistry and more Study notes Geology in PDF only on Docsity! Geol. 656 Isotope Geochemistry 112 RADIOGENIC ISOTOPE GEOCHEMISTRY: THE MANTLE II ISOTOPE GEOCHEMISTRY OF THE MANTLE: THE PB PICTURE Pb is by far the most powerful of the isotopic tools available to us because three parents decay to three isotopes of Pb. We have seen that the two U decay systems make Pb isotopes particularly use- ful in geochronology. The same is true in isotope geochemistry. Our next step, therefore, is to see if Pb isotopes are consistent with the picture provided by Sr, Nd, and Hf. First we need to consider the spe- cial features of the Pb isotope system. We noted earlier that the slope on a plot of 207Pb/204Pb–206Pb/204Pb is proportional to time. Since Pb is a volatile element, we cannot assume the U/Pb ratio of the Earth is the same as the chondritic one. Hence the Pb isotope ratios of the bulk Earth are not known precisely, as is the Nd or Hf isotope ratio. Pb isotope ratios are, however, constrained by the assumptions that (1) the solar nebula has a uniform Pb isotopic composition when it formed (which we take to be equal to the composition of Pb in troilite in the Canyon Diablo iron meteorite) and (2) the Earth formed from this nebula 4.55 Ga ago. Thus the 207Pb/204Pb and 206Pb/204Pb ratios of the Earth today must lie on a unique isochron, called the Geochron, whose slope corresponds to 4.55 Ga and which passes through Canyon Diablo initial Pb (Figure 17.1; Table 17.1). Indeed, all planetary bodies that formed from the solar nebula at that time (4.55 Ga ago), and have remained closed system since then must plot on this isochron. While there are no good grounds to question assumption 1, assumption 2 might be ques- tioned in detail. The solar system certainly formed 4.55 Ga ago, but the accretion of the inner planets may have required a significant amount of time. Indeed, computer models of planetary accretion suggest the process may take as much as 100 Ma. In this case, the Earth might be as young as 4.45 Ga, and would have begun with slightly different initial Pb isotope ratios, because of growth of radiogenic Pb over this 100 Ma period. On the other hand, several lines of evidence suggest the Earth could be no younger than about 4.45 Ga. This evidence includes terrestrial Xe isotope data, which we will discuss subsequently, and the presence of 4.45 Ga rocks on the Moon. It is clear from isotope data among other evidence tha t the Earth and Moon are closely . 10 12 14 16 18 20 228 10 12 14 16 18 3.0 2.0 1.0 Geoc hron µ = 10 µ = 9 µ = 8 20 7 P b/ 20 4 P b 206Pb/204Pb Figure 17.1. Evolution of Pb isotope ratios. The curve lines represent the evolutionary paths for systems having µ values of 8, 9, and 10. The hash marks on the evolution curves mark Pb isotope composi- tions 1.0, 2.0, and 3.0 Ga ago. TABLE 17.1. PB ISOTOPE RATIOS IN CANYON DIABLO TROILITE 206Pb/204Pb 9.307 207Pb/204Pb 10.294 208Pb/204Pb 29.476 Docsity.com Geol. 656 Isotope Geochemistry 113 related planetary bodies, and it is unlikely the Moon is substantially older than the Earth. The point is that we cannot be quite certain that bulk Earth Pb isotope ratios must lie on the geochron shown in Figure 17.1. But they must lie between this line and a 4.45 Ga isochron, which on Figure 17.1 would be essentially parallel to the 4.55 Ga geochron shown but shifted to higher 206Pb/204Pb by about 0.6. When the Earth formed its Pb isotope ratios were (roughly) the same as that of the Canyon Diablo iron. As time passed the 207Pb/204Pb and 206Pb/204Pb ratios increased. At first, the 207Pb/204Pb ratio in- creased rapidly because there was about as much 235U as 238U around and 235U was decaying to Pb more rapidly than 238U. But as the 235U was consumed, the rate of increase of 207Pb/204Pb slowed until the . Marquesas St. Helena Mangaia & Tubuai Gough Tristan Atlantic & Pacific MORB Marion/P.E. Christmas Heard Comores Kerguelen Crozet Reunion St. Paul N. Amsterdam Bouvet Azores Guadalupe Easter Rarotonga S. Felix & S. Ambrosio Societies Galapagos Juan Fernandez Samoa Hawaii 36.5 37 37.5 38 38.5 39 39.5 40 40.5 41 17 17.5 18 18.5 19 19.5 20 20.5 21 21.5 22 15.4 15.5 15.6 15.7 15.8 206Pb/204Pb Comores Kerguelen Marion/P.E. Crozet Reunion St. Paul Heard Atlantic & Pacific MORB Indian MORB Tristan St. Helena Mangaia & Tubuai Gough Bouvet Guadalupe S. Felix & S. Ambrosio Marquesas Azores Easter Societies Samoa Hawaii Rarotonga Galapagos 17 17.5 18 18.5 19 19.5 20 20.5 21 21.5 22 Juan Fernandez N. Amsterdam Christmas 4.55 Ga Geochron 4.45 Ga Geochron Indian MORB 20 7 P b/ 20 4 P b 20 8 P b/ 20 4 P b Figure 17.2. Pb isotope systematics of oceanic basalts on the 207Pb/204Pb –206Pb/204Pb and 208Pb/204Pb–206Pb/204Pb diagrams. Docsity.com