Stable isotope variations of heavy elements

Advances in thermal ionization mass spectrometry (TIMS) and the introduction of multi-receiver inductively coupled plasma mass spectrometry (MC-ICP-MS) have made it possible to study the isotopic compositions of large migrating natural variability and heavy metal systems that could not be accurately measured at the present time.

The emergence of MC-ICP-MS has allowed isotopic measurements of elements such as Zn, Cu, Fe, Cr, Mo, and Tl to be performed to an accuracy of 40 × 10- 6 , Fe, Cr, Mo, and Tl. This technique combines the advantages of the inductively coupled plasma (ICP) technique (high ionization efficiency for almost all elements) with the high accuracy of a hot ion source mass spectrometer equipped with a Faraday collector. Elements are introduced from solution and plasma ionization ions, and the instrumental mass fractionation is then corrected by adding an external tracer under the same operating conditions or by comparing the sample to a standard sample. As with conventional inductively coupled plasma mass spectrometers (ICP-MS), all MC-ICP-MS instruments require argon as the plasma working gas. As a result, mass interference is an inherent property of the technique, which can be overcome using a desolvation nebulizer.

The technique for the determination of Cu-Zn isotope ratios was first described by Maréchal et al. ( 1999) and Zhu et al. ( 2000a). The variation observed at low temperatures is in the order of a few thousandths of a degree, which is much larger than originally expected for isotopes of heavier elements with small mass differences. Such fractionation differences depend on, for example, redox and biologically relevant reactions. Whether the fractionation mechanism is related to the observed changes is not known so far, but it should be the same as for the lighter elements. Among other things, speciation and absorption phenomena seem to be extremely important influences. Since most metals can be coordinated to multiple coordinating groups, isotopic effects between dissolved aqueous solutions are particularly important, especially those in different redox states (Anbar & Rouxel, 2007). Further, the absorption of dissolved species on the particle surface is another important fractionation mechanism. A few studies have shown that small amounts of <1‰ fractionation, such as metal ions, can move from solution to the oxidized surface. In general, the preferential adsorption of heavier isotopes to the oxidized surface of the metal is related to the shorter metal-oxygen bonding and the fact that heavy isotopes have a lower absorption coordination number relative to aqueous solutions ( Balistrieri et al., 2008). The largest fractionation observed so far ( 1. 18‰) occurs between dissolved and absorbed Mo elements ( Barling & Anbar, 2004).

Schauble ( 2004) applied the theory of stable isotope fractionation to unconventional isotope systems. He showed that differences in coordination numbers between *** storage phases control the isotopic fractionation of cations. Lighter isotopes preferentially occupy higher coordination positions. Thus, differences in the isotopic compositions of lithophile elements such as Mg, Ca, and Li reflect changes in coordination number.

Equilibrium fractionation has been documented for some transition metals (e.g., Fe), but this fractionation should be very small and may be masked by kinetic fractionation at low temperatures and by biological systems (Schauble, 2004). For any of the transition metals, evidence will also have to be found that biological effects are an important factor controlling the variation of natural isotopes.

Table 1. 9 gives the various heavy elements and the isotopic variations observed so far.

Table 1. 9 Natural isotopic variations of heavy elements and some of their important geochemical properties