Elements, isotope variations

Elemental isotopic compositions (isotope ratios) can be used mass spectrometrically in a routine sense to monitor a substance for the presence of different kinds of elements, as with chlorine or platinum. It can also be used in a precise sense to examine tiny variations in these ratios, from which important deductions can be made in a wide variety of disciplines. 

In the process of SNMS analysis, sputtered atoms are ionized while passii through the ionizer and are accelerated into the mass spectrometer for mass analysis. The ion currents of the analyzed ions are measured and recorded as a function of mass while stepping the mass spectrometer through the desired mass or element sequence. If the purpose of the analysis is to develop a depth profile to characterize the surface and subsurface regions of the sample, the selected sequence is repeated a number of times to record the variation in ion current of a selected elemental isotope as the sample surfiice is sputtered away. 

Ellam RM, Hawkesworth CJ (1988) Elemental and isotopic variations in subduction related basalts evidenee for a three component model. Contrib Mineral Petrol 98 72-80 Elkins Tanton LT, Grove TL, Doimelly-Nolan J (2001) Hot, shallow mantle melting under the Caseades voleanoe are. Geology 29 631-634... 

The heavy elements carry clear excesses in a s-process component as can be seen from Figure 7. This has been demonstrated for Kr, Sr, Xe, Ba, Nd, Sm (Ott and Begemann 1990 Prombo et al. 1993 Lewis et al. 1994 Richter 1995 Hoppe and Ott 1997). The models can be made to fit very precisely the measured data (Lattanzio and Boothroyd 1997 Busso et al. 1999). Mo and Zr can occur as microcrystals of Mo-Zr-C within graphite grains. Typical s-process patterns are observed, with isotopic variations of about a factor of more than 5 (Nicolussi et al. 1997 Nicolussi et al. 1998a). 

Clayton RN, Hinton RW, Davis AM (1988) Isotopic variations in the rock-forming elements in meteorites. Phil Trans R Soc Lond A 325 483-501... 

Deines P (2003) A note on intra-elemental isotope effects and the interpretation of non-mass-dependent isotope variations. Chem Geol 199 179-182... 

A clear avenue of future research is to explore the S-Fe redox couple in biologic systems. Bacterial sulfate reduction and DIR may be spatially decoupled, dependent upon the distribution of poorly crystalline ferric hydroxides and sulfate (e.g., Canfield et al. 1993 Thamdrup and Canfield 1996), or may be closely associated in low-suUate environments. Production of FIjS from bacterial sulfate reduction may quickly react with Fefll) to form iron sulfides (e.g., Sorensen and Jeorgensen 1987 Thamdrup et al. 1994). In addition to these reactions, Fe(III) reduchon may be coupled to oxidation of reduced S (e.g., Thamdrup and Canfield 1996), where the net result is that S and Fe may be cycled extensively before they find themselves in the inventory of sedimentary rocks (e.g., Canfield et al. 1993). Investigation of both S and Fe isotope fractionations produced during biochemical cycling of these elements will be an important future avenue of research that will bear on our understanding of the isotopic variations of these elements in both modem and ancient environments. 

Until only a few years ago, I would never have imagined that a volume on the stable isotope geochemistry of elements like Mg, Fe or Cu would be written. In fact, a comic book of blank pages entitled The Stable Isotope Geochemistry of Fluorine would have been a more likely prospect. In volume 16 of this series, published in 1986, I wrote Isotopic variations have been looked for but not found for heavy elements like Cu, Sn, and Fe.. .. Natural variations in isotopic ratios of terrestrial materials have been reportedfor other light elements like Mg and K, but such variations usually turn out to be laboratory artifacts. I am about ready to eat those words. 

We have known for many years that large isotopic fractionations of heavy elements like Pb develop in the source regions of TIMS machines. Nonetheless, most of us held fast to the conventional wisdom that no significant mass-dependent isotopic fractionations were likely to occur in natural or laboratory systems for elements that are either heavy or engaged in bonds with a dominant ionic character. With the relatively recent appearance of new instrumentation like MC-ICP-MS and heroic methods development in TIMS analyses, it became possible to make very precise measurements of the isotopic ratios of some of these non-traditional elements, particularly if they comprise three or more isotopes. It was eminently reasonable to reexamine these systems in this new light. Perhaps atomic weights could be refined, or maybe there were some unexpected isotopic variations to discover. There were. 

Table 1.9 gives a summary of the respective heavy elements and the isotope variations observed so far. 

Lithium is a conservative element in the ocean with a residence time of abont one million year. Its isotope composition is maintained by inputs of dissolved Li from rivers (average 5 Li + 23%c, Huh et al. 1998) and high-temperature hydrothermal fluids at ocean ridges at one hand and low temperature removal of Li into oceanic basalts and marine sediments at the other. Any variance in these sources and sinks thus should cause secular variations in the isotope composition of oceanic Li. And indeed in a first attempt Hoefs and Sywall (1997) interpreted Li isotope variations in well preserved carbonate shells as indicating secular variations of the oceanic Li-cycle. 

Natural isotope variations in chlorine isotope ratios might be expected due to both the mass difference between Cl and Cl as well as to variations in coordination of chlorine in the vapor, aqueous and solid phases. Schauble et al. (2003) calculated equilibrium fractionation factors for some geochemically important species. They showed that the magnitude of fractionations systematically varies with the oxidation state of Cl, but also depends on the oxidation state of elements to which Cl is bound with greater fractionations for 2+ cations than for 1+ cations. Silicates are predicted to be enriched compared to coexisting brines and organic molecules are enriched to dissolved Cl. ... 

The heaviest elements with observed fractionations of about 3 to 4%c are mercury and thallium. This is surprising because isotope variations due to mass-dependent fractionations should be much smaller. Schauble (2007) demonstrated that isotope variations for the heaviest elements are controlled by nuclear volume, a fractionation effect being negligible for the light elements. Nuclear volume fractionations may... 

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