Supervisors: (Earth Sciences) and Helen Williams (Earth Sciences)
Importance of the area of research:
Lithological heterogeneity of Earth’s convecting mantle has widely been invoked as the cause of geochemical variability in the compositions of primitive melts generated by adiabatic decompression melting in upwelling mantle plumes, i.e. basalts and picrites erupted at ocean islands and Large Igneous Provinces. The presence of readily-fusible pyroxenite in the convecting mantle is thought to result from stirring of subducted oceanic crust with peridotite, and has commonly been inferred from analysis of Ni in olivine phenocrysts1. Nevertheless, such interpretations are not wholly consistent with evidence from Sr, Nd, Pb and Os isotopic ratios and imply that Ni may be controlled by factors other than bulk composition of the source. Establishing the extent of lithological heterogeneity in the convecting mantle is important because of the implications for geodynamic models of mantle convection, as well as our understanding of melting in the Earth’s mantle2 and where volcanoes erupt.
The extent to which variations lithological heterogeneity controls the compositions of primitive magmas erupted from volcanoes at ocean islands and Large Igneous Provinces remains highly controversial. While some workers believe that the observed variability in Ni contents of olivine phenocrysts relates to the relative proportions of pyroxenite and peridotite others have suggested that it is controlled by crystallisation temperatures. This project seeks to understand how combinations of novel trace elements in olivine and non-traditional (Fe) isotopes might be combined to better determine the contribution of ancient subducted oceanic lithosphere and/or delaminated sub-continental lithospheric mantle to mantle plume derived melts and hence constrain the nature of the lithological heterogeneity of the deep convecting mantle.
What the student will do:
The project is novel because it focuses on the high-precision analysis of non-traditional (i) trace elements in olivine phenocrysts (Zn, Co, Ga, Sc, Mn and V) and (ii) Fe isotopic ratios of the host basalts and picrites. All of the samples will be from Large Igneous Provinces or ocean islands. The sample set will include ferropicrites, which represent the most likely candidates of pyroxenite melts in the mantle, as well as picrites and basalts. Key insights into the relative controls of source composition and crystal fractionation will be derived from numerical models involving partition coefficients calculated from existing run products of experiments at 1 atmosphere to 5 GPa3 together with the trace-element analyses of olivines and Fe isotope data for corresponding picrites and basalts.
1.Sobolev & 19 others, A. V. The amount of recycled crust in sources of mantle-derived melts. Science 316, 412–417 (2007).
2.Williams, H. M. & Bizimis, M. Iron isotope tracing of mantle heterogeneity within the source regions of oceanic basalts. Earth Planet. Sci. Lett. 404, 396–407 (2014).
3.Tuff, J., Takahashi, E. & Gibson, S. A. Experimental constraints on the role of garnet pyroxenite in the genesis of high-Fe mantle plume derived melts. J. Petrol. 46, 2023–2058 (2005).
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