Compositional evolution isotopic ratios

Now consider the simultaneous evolution of and c)D. Suppose an air mass having initial compositions 3, = 0 cools and precipitates, always as an equilibrium process. According to the preceding analysis, measurements of both isotopic ratios in precipitation will plot on a curve with a slope approximately given by... 

Fig. 6-5. The evolution of the lagoon s waters in response to oscillations in biological productivity. The results show the adjustment of the system from an initial composition equal to that of seawater. This figure shows isotope ratios, calcium concentration, the saturation index, and productivity.

Taylor SR, McLennan SM (1985) The Continental Crust Its Composition and Evolution. Blackwell, Boston Tuit CB, Ravizza G (2003) The marine distribution of molybdenum. Geochim Cosmochim Acta 67 A4950 Tumlund JR, Keyes WR, Peiffer GL (1993) Isotope ratios of molybdenum determined by thermal ionization mass spectrometry for stable isotope studies of molybdenum metabolism in humans. Anal Chem 65 1717-1722... 

If the model outlined above is valid, sihcic magmatism in the LFB involved net crustal growth, as juvenile mantle-derived liquids, or their differentiates were instrumental in the formation and compositional evolution of hornblende granites, and, to a lesser extent, the cordierite granites. The amount of new crust generated is estimated by determining the overall mantle component present within both granitic types, and this is best done isotopically, since the trace-element ratios are poorly constrained for the potential basaltic end-members. 

Matsubaya O., Sakai H., Torii T., Burton H., and Kerry K. (1979) Antarctic saline lakes-stable isotope ratios, chemical compositions and evolution. Geochim. Cosmochim. Acta 43, 7-25. 

Figure 1 Systematics of Nd- and Hf-isotopic evolution in the bulk Earth, continental crust, and mantle. Daughter elements Nd and Hf are more incompatible during mantle melting (more likely to go into a partial melt of mantle rock) than Sm and Lu, respectively. As a result, the continental crust has a lower Sm/Nd and Lu/Hf ratio than the mantle, and lower Nd- and Hf-isotope ratios. Young continental crust has isotope ratios similar to the mantle, and the older the continental terrain, the lower the Nd- and Hf-isotope ratios. Rb-Sr behaves in the opposite sense, such that the parent element Rb is more incompatible than the daughter element Sr. (a) Schematic example of the evolution of Nd- and Hf-isotope ratios of a melt and the melt residue from a melting event around the middle of Earth history from a source with the composition of the bulk Earth, (b) The same scenario as in (a), but with the isotope ratios plotted as e d and snf. The bulk Earth value throughout geological time is defined as e d and SHf = 0> and e-value of a sample is the parts per 10 deviation from the bulk Earth value.

Transport balance or box models have been used by many workers in the past in efforts to understand the trace element and isotope characteristics of the Earth s major silicate reservoirs, i.e. continental crust, and upper and lower mantle (e.g. Jacobsen Wasserburg 1979 Zartman Haines 1988). Although simple mass balance calculations can be applied to present-day trace element concentrations and Pb, Nd and Hf isotope compositions of major reservoirs, e.g. continental crust and depleted mantle, to test the hypothesis that these reservoirs are complementary, transport balance models are needed to test ideas on their evolution in time. The reason is that the isotope ratio variations are the result of time-integrated trace element variations in the reservoir, modified by fluxes between them. Below, recent transport balance models in which the evolution of the continental crust is examined using Th-U-Pb (Kramers T olstikhin 1997) and... 

The initial Xe/ °Xe or Xe/ Xe ratio, as well, potentially contains information about the evolution of the system before decay began (Swindle 1998 Swindle et al. 1991b), just as the initial isotopic compositions of Sr or Nd do. However, it is not commonly used, and Gilmour et al. (2001) suggest that it should not be used, in part because some of the initial Xe isotopic ratios derived (such as the one for Renazzo in Fig. 3a) are less than 1, less than any suspected trapped Xe component (Ott 2002). They argue that plots such as Figure 3 are more complicated than described above, and that the trapped Xe must be accompanied by some trapped iodine as well. 

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