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E447: Novel constraints on the volatile contents of mantle melts feeding Large Igneous Provinces (Lead Supervisor: Sally Gibson, Earth Sciences)

Supervisors: Sally Gibson (Earth Sciences) and Helen Williams (Earth Sciences)

Importance of the area of research:

Volcanic eruptions in Large igeneous Provinces (LIPs) represent the largest manifestations of melting in our planet's mantle. Outgassing of volatiles during these eruptions has often been associated with major mass extinction events. While LIPs are primarily a consequence of adiabatic decompression melting in upwelling mantle plumes, lithological heterogeneity has widely been invoked as the cause of geochemical variability in the compositions of their primitive melts. This heterogeneity relates to the presence of readily-fusible pyroxenite (in addition to peridotite) and eclogite in both the lithospheric and convecting mantle. For the convecting mantle, this has been inferred from analysis of Ni in olivine phenocrysts (e.g. Sobolev et al., 2007). 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 source regions of LIPs is important because of the implications for our understanding of melting in the Earth’s mantle and the volatile budgets of volcanoes associated with LIPs.

Project summary:

The extent to which variations in lithological heterogeneity controls the compositions and volatile budgets of magmas erupted from volcanoes in 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 pyroxenites and peridotite others have suggested that it is controlled by crystallisation temperatures. This project will use combinations of novel trace elements in olivine and non-traditional (Fe) isotopes (Williams & Bizimis, 2014) to better determine the contribution of ancient subducted oceanic lithosphere and/or metasomatised sub-continental lithospheric mantle to LIP magmas and hence constrain the nature of the lithological heterogeneity of contributing mantle source regions and quantify volatile contents of the corresponding primary mantle melts.

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 a diverse range of volatile-rich and poor melts erupted in Large Igneous Provinces in order to constrain the lithological composition of their source regions. The sample set will include ferropicrites, which represent the most likely candidates of pyroxenite melts in the mantle, as well as picrites and lamprophyres (Gibson et al., 2006). Volatile contents of mantle source regions will be established using SAG's new analytical data for mantle peridotites and pyroxenites and input into numerical models of melting and crystal fractionation. These will use existing partition coefficient data and allow the student to quantify the mantle contribution of C, S, F, Cl etc to LIP magmas. Code to model this is currently being developed at the University of Cambridge.

Please contact the lead supervisor directly for further information relating to what the successful applicant will be expected to do, training to be provided, and any specific educational background requirements.

References:

Gibson, S.A., Thompson, R. N., Day, J.A., 2006. Timescales and mechanisms of plume-lithosphere interactions: 40Ar/39Ar geochronology and geochemistry of alkaline igneous rocks from the Paraná-Etendeka large igneous province. Earth and Planetary Science Letters 1-17.

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)

Sobolev A. V. & 7 others (2011) and references therein. Linking mantle plumes, large igneous provinces and environmental catastrophes. Nature, 477, 312-316.

Follow this link to find out about applying for this project.

Other projects available from the Lead Supervisor can be viewed here.

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