Silicon Isotope Geochemistry

Abstract

In contrast to many other stable isotopes of the elements discussed in this book, those of silicon are not strictly speaking “Non-Traditional Stable Isotopes” because they have been studied for more than 60 years. After the pioneering works of Reynolds and Verhoogen (1953) and Allenby (1954), a steady increase in silicon isotope studies of geological materials has led to a substantial corpus of data. These data were compiled by Ding et al. (1996) alongside new measurements that, collectively, included over a thousand samples of rocks, minerals, waters and biological materials. Most of these data were produced using the well established method of gas source mass spectrometry after sample decomposition and silicon purification via fluorination techniques. As for many non-traditional stable isotopes, silicon isotope research has flourished with the advent of second generation of multicollector plasma source mass spectrometers (MC–ICP–MS). These instruments eliminated the requirement of hazardous gaseous fluorine sample preparation methods while permitting improved analytical precision in both wet plasma (De La Rocha 2002) and in dry plasma (Cardinal et al. 2003). Subsequent analytical developments involving high mass resolution MC–ICP–MS combined with improved silicon purification methods (Georg et al. 2006) made this analytical technique more robust and precise enough to study even the subtle silicon isotope variations produced during high temperature geological processes (Savage et al. 2014). Silicon is the fourteenth element of the Periodic Table. Its atomic mass was precisely determined to be 28.08553 ± 0.00039 in atomic mass units (a.m.u.) on a pure silicon reference material (NIST SRM–990, Barnes et al. 1975). This 95% confidence limit error includes the overall natural isotopic variation range for 30Si/28Si known by the time, estimated to be about 5‰ from the analysis of biological, meteoritic and terrestrial materials (Tilles 1961). As detailed below, the current database suggests …