Volume 82: Non-Traditional Stable Isotopes

Fang-Zhen Teng, James Watkins, and Nicolas Dauphas, editors

2017, i-xvi + 885 pages. ISBN 978-0-939950-98-0

Description

Traditional stable isotope geochemistry involves isotopes of light elements such as H, C, N, O, and S, which are measured predominantly by gas-source mass spectrometry. Even though Li isotope geochemistry was developed in 1980’s based on thermal ionization mass spectrometry (TIMS), the real flourish of so-called non-traditional stable isotope geochemistry was made possible by the development of multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS). Since then, isotopes of both light (e.g., Li, Mg) and heavy (e.g., Tl, U) elements have been routinely measured at a precision that is high enough to resolve natural variations. The publication of RIMG volume 55 (Geochemistry of Non-Traditional Stable Isotopes) in 2004 was the first extensive review of Non-Traditional Stable Isotopes summarizing the advances in the field up to 2003. When compared to traditional stable isotopes, the non-traditional stable isotopes have several distinctive geochemical features:

1. as many are trace elements, their concentrations vary widely in different geological reservoirs;
2. these elements range from highly volatile (e.g., Zn and K) to refractory (e.g., Ca and Ti);
3. many of these elements are redox-sensitive;
4. many of them are biologically active;
5. bonding environments, especially for the metal elements, are different from those of H, C, N, O and S;
6. many of these elements have high atomic numbers and more than two stable isotopes.

These features make the different elements susceptible to different fractionation mechanisms, and by extension, make them unique tracers of different cosmochemical, geological and biological processes, as highlighted throughout this volume.

Large variations have been documented in both natural samples and laboratory experiments for non-traditional stable isotopes. These studies suggest that the following factors control the degree of isotope fractionation in non-traditional stable isotopes during various processes: the relative mass difference between isotopes, change of the oxidation state, biological sensitivity, and volatility. Among these elements, Li displays the largest isotopic variation in terrestrial samples, and since Li is not volatile during geological processes and is not sensitive to redox reactions and biological processes, the large isotope fractionation is controlled mainly by the large relative mass difference. For many of the other elements, other factors may be equally, if not more, important. For example, Chlorine exhibits the second largest isotopic variation, which is due to kinetic isotope fractionation during volcanic degassing. Selenium isotopes also show large isotopic variations, but this reflects redox-controlled Se isotope fractionation, while the large mercury isotopic variations are mainly associated with biological processes.

From Chapter 1.

Fang-Zhen Teng, Isotope Laboratory, Department of Earth and Space Sciences University of Washington, Seattle WA, USA
Nicolas Dauphas, Origins Lab, Department of the Geophysical Sciences and Enrico Fermi Institute, The University of Chicago, Chicago IL, USA
James M. Watkins, Department of Earth Sciences, University of Oregon, Eugene OR, USA

January 2017

Table of Contents