- Start date: 1 February 2013
- End date: 31 January 2018
- Funder: Natural Environment Research Council (NERC)
- Value: £732,579
- Primary investigator: Dr Jason Harvey
It is a fundamental tenet in geochemistry that isotopes of individual elements are not disturbed from their natural ratios during high temperature processes. This has led to the assumption that the production of basalt, erupted at the surface, faithfully records the isotopic signature of the mantle from which it was generated. However, as more actual samples of mantle material are analysed, with increasingly sensitive instrumentation, it is clear that for some isotope systems this is not a valid assumption. For example the Pb and Os isotope composition of basalts does not reflect the range of compositions of the mantle from which they came, e.g. the apparent “Pb paradox” (Allègre, 1969) and the “Os isotopic gap” (eg Shirey and Walker, 1998). These long-standing problems challenge the current understanding of how chemical signatures are transferred from the mantle to the crust.
In contrast to previous studies, which use basalts to infer mantle composition, this project aims to take advantage of recent improvements in analytical capability to resolve the Pb paradox and Os isotopic gap through the analysis of mantle material. Preliminary work that I have performed, and that of my collaborators (Professor Shirey, Carnegie Institution, Washington DC & Prof. Warren, Stanford University, CA), suggests that the abundance and distribution of siderophile and chalcophile elements, such as Pb and Os, is controlled by volumetrically minor, yet disproportionately influential, sulphides. Moreover, these sulphides can account for the discrepancies between long-held assumptions and the emerging evidence for isotopic disequilibrium in the mantle that is not recorded in erupted basalts.