High temperature silicon isotope geochemistry

Silicon (Si) is the defining element of silicate reservoirs yet, despite its dominance in major Earth processes, there is still no clear understanding of how much is hosted in Earth's core, how the enriched continental crust forms or even if the crust is isotopically different from the mantle because of a long history of weathering, erosion and subduction. With the advent of multiple collector inductively coupled plasma mass spectrometry it has become relatively straightforward to explore small (100. ppm level) mass dependent variations in Si isotopic composition resulting from high temperature fractionation and to develop new isotopic fingerprints of magmatic processes and source regions. This paper reviews the technique developments, the new data and the veracity of current interpretations.Only a small Si isotopic effect is associated with basalt formation via mantle melting. However, there now is compelling evidence, based on a considerable number of samples (> 100), that the silicate Earth is isotopically fractionated by 100-200. ppm per amu to a heavy composition relative to that of chondrites and also all differentiated stony meteorites. This could reflect variability in the circumstellar disc, but this is not well supported by data for enstatite chondrites which are isotopically light. The most plausible current explanation is that Si is a light element in Earth's core and that differences in the bond stiffness between silicate- and metal-hosted Si resulted in substantial fractionation. Interestingly, the Moon has the same Si isotope composition as Earth's mantle, which is hard to explain unless the Moon's atoms were mainly derived from Earth.Differentiated magmatic sequences such as those of Hekla, Iceland display a systematic relationship between isotopic composition and Si content. More complex magmatic suites, such as I- and S-type granites, reveal a range of isotopic compositions not well correlated with chemical composition. Similar effects are found in lower crustal granulite facies xenoliths. Nonetheless the overall composition of the continental crust is only slightly heavy relative to the mantle; in fact the effect is barely resolvable. Therefore, the amounts of surface dissolved heavy Si removed to the mantle via weathering over time have been small or these losses have been balanced by subduction of isotopically light clay.