U: isotopic composition

Summary. U isotope compositions potentially can provide an important tool for tracing groundwaters from different aquifer conditions. However, clear interpretations of (234u/238y) quantification of the responsible processes, remain difficult. 

Figure 10 High-precision U isotopic compositions of NIST SRM 960, as measured by thermal ionization mass spectrometry (TIMS) and multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). (From Halliday et al., 1998b.)...

Isotope Z Element A Relative At. Mass Std. At. Weight (u) Isotopic Composition (%)... 

Element has no stable nuclides the value given in parentheses is the atomic mass number of the isotope of longest known half-life. However, three such elements (Th, Pa and U) do have a characteristic terrestrial isotopic composition, and for these an atomic weight is tabulated. 

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In these situations, addition of a tracer of unique isotopic composition is required, and the nature of the tracers added depends on the measurement technique. For example, short-lived and Th (with respective half-lives of 70 and 1.9 years) are commonly used as a tracer for alpha spectrometric analysis of U and Th, whereas longer-lived... 

For precise measurement of isotopic composition by mass spectrometry, it is also common to use either a natural, known isotopic ratio to correct for instrumental mass fractionation (e g., internal normalization) or to add a tracer for this purpose. For example for natural uranium samples, one can use the natural U/ U of 137.88 to correct for fractionation. Alternatively, one can use an added double spike of ratio -unity... 

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Figure 8. ° Pb7 Pb vs. Th/Us (derived using Eqn. 5 in the text) diagram for mid-ocean ridge and ocean island basalt based on a recent data set with mostly mass spectrometry measurements (Turner et al. 1997 Bourdon et al. 1996 Dosso et al. 1999 Claude-lvanaj et al. 1998, 2001 Sims et al. 2002). The data show a relatively well defined array that intersect a closed-system hne for the bulk Earth starting with an initial lead isotope composition equal to Canyon Diablo (T = 4.55 Ga). This intersect was used by Allegre et al. (1986) to define the Th/U ratio of the Earth.

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