The Composition and Evolution of the Earth's Mantle

The Earth's mantle is its largest geochemical reservoir, and the source of the vast majority of its volcanism. Hence, establishing the composition of the mantle and understanding its melting history is key to unravelling global geochemical cycles and magma generation. Constraints on mantle composition and melting come predominantly from its melting products: basalts.

However, these are the end product of an array of processes, and reconstructing mantle source properties from them is therefore non-unique. Furthermore, because basalts are aggregates of a wide range of partial melts, they average source properties and composition, hence do little to constrain the true range of compositions of the upper mantle or the spatial scale on which this variability occurs.

This project will reconstruct the composition and evolution of the oceanic upper mantle by analysing a comprehensive suite of peridotites recovered during IODP Expedition 399. The project combines a number of different approaches, and the student will have the flexibility to combine or focus on specific components according to their interests.

The first component is the mineralogy and mineral major element composition of the upper mantle. Backscattered electron images of the Expedition 399 samples will be used to reconstruct the original primary mineralogy of the peridotites (olivine, orthopyroxene, clinopyroxene, spinel). These images also provide context for the quantitative major element analyses of these primary minerals using combined EDS-WDS.

This dataset reflects the history of melting of the upper mantle as well as the subsequent migration of partial melts, and the student will quantify this history. A key aspect will be to determine the spatial scale of variations in compositions and histories.

The second component is the trace element record of melting, melt migration and cooling. Mineral trace elements will be measured by laser ablation ICP-MS, and will be used (in tandem with the mineral major elements) to inform melting and melt migration. They also enable a reconstruction of the cooling rate of the section (Dygert et al., 2017), which provides critical input to studies on thermal evolution and hydrothermal fluid flow undertaken by other members of the Expedition 399 team.

The third component is the radiogenic isotope composition of the upper mantle. Whole-rock measurements of Nd, Hf and Pb isotopes using both TIMS and multicollector ICP-MS will establish a baseline for the composition of the section as a whole and the magnitude and scale of variability within it. There is potential for a follow-up study of selected isotopes by micromilling-TIMS, which will test the presence and significance of potential sample-scale variability (Warren et al., 2009).

Together, these components will provide unprecedented constraints on the nature of the oceanic upper mantle.