PhD positions for 2017 start

I am involved in three exciting projects at the University of Cambridge, each of which is part of the Cambridge Earth Systems Science DTP and will base the student in the Department of Earth Sciences.


Chemical geodynamics: Combining geochemical and geophysical constraints on mantle structure and processes

 

Degenerate mantle chemical structures obtained from limited surface sampling.

Degenerate mantle chemical structures obtained from limited sampling at Earth’s surface.  The development and application of novel statistical techniques will enable us to quantify the sptial information content of ocean island and MORB geochemistry.

Large geochemical datasets have been used to infer hemispherical compositional structure in Earth’s mantle, the presence of chemically isolated domains at the core mantle

boundary, and define an isotope ‘zoology‘ of domains tracing processes from lithospheric delamination to continental crustal recycling.  However, these observations have not been rigourously combined with complementary geohpysical datasets, which probe the mantle’s seismic, density, and phase structure.  In this project the student will combine field, analytical, and statistical work to bridge the gap between the geochemical and geophysical pictures of mantle structure and dynamics.

Lead supervisor: Oliver Shorttle
Co-supervisor: John Maclennan


Ice and fire: feedbacks between glaciation, volcanism and climate

Deglaciation has triggered dramatic increases in volcanism on Iceland and been linked to globally increased volcanic fluxes at arcs.  However, still relatively few datasets provide local geological and geochemical evidence for a causal link between deglaciation and volcanism.  This project will collect new geochemical data on an exceptional suite of samples from Mount Haddington, James Ross Island, on the northern Antarctic Peninsula. These data will probe the volcano’s response to multiple deglaciations over 6 Myr.

Lead supervisor: Marie Edmonds
Co-supervisors: Joanne Johnson (BAS), Oliver Shorttle


The redox structure of the Icelandic plume: new constraints from iron stable isotopes

The oxygen content of the mantle, as captured in its oxidation state (fO2), determines how the mantle melts, how magmas transport volatile elements, and ultimately buffers the redox evolution of Earth’s surface environment.  At present our understanding of mantle oxidation state is limited by the available geochemical proxies, none of which uniquely capture fO2 variability.  To overcome these challenges this project will make Fe isotope analyses on a suite of geochemically well characterised samples (e.g. Murton et al. 2002; Shorttle et al. 2015) from the Reykajnes Ridge.  Fe isotopes will be sensitive to the oxidation state of the mantle, as well as its lithology.  With independent constraints on mantle lithology, and fO2, the component of redox heterogeneity captured by Fe isotopes will be identified, establishing the utility of this proxy in magmatic rocks and opening the potential to map the redox structure of the Iceland plume.

Lead supervisor: Helen Williams
Co-supervisors: John Maclennen, Oliver Shorttle

Spatial geochemical structure in Icelandic basalts

Pb isotope binary mixing arrays

Binary mixing arrays in Pb isotope space shift systematically across Iceland, revealing a length scale on which either mixing of melts in the crust or mantle operates. The observations are similar to the recent work indicating the presence of ‘double volcanic chains’ in ocean islands such as Hawaii and the Galapagos. Figure modified from Shorttle et al. (2013).

The mantle is compositionally heterogeneous on a fine scale, this can be observed in exhumed mantle sections (e.g. Allegre & Turcotte, 1986) and in melt inclusion suites from single eruptions (e.g. Maclennan, 2008). However, this compositional variability may also show long rage structure, with basalt compositions sampling the mantle exhibiting systematic changes as a function of their eruption location. This has been demonstrated most strikingly with basalts from Hawaii (Abouchami et al. 2005), which depending on their origin north or south on the island chain exhibit distinct Pb isotopic compositions. Here we show that similar spatial patterns to those found on Hawaii are also present on Iceland, with Icelandic basalts showing systematic shifts in composition that are only recorded by Pb isotopes (see figure above).

Basalts are a probe of mantle compositional structure, and seeing such systematic spatial patterns in their compositions it is tempting to infer that there are stepped changes in the chemistry of the underlying mantle (e.g. Weis et al. 2011). However, on Iceland we observe that the composition of erupted basalts changes systematically north to south across the island. We can make this observation because in contrast to many ocean islands, such as Hawaii, volcanism on Iceland is distributed across en-echelon fissure systems affording greater spatial resolution of isotopic shifts.

Our observations from Iceland raise the question of how the geochemical asymmetry seen in double-chain volcanism truly represents underlying mantle chemical structure, if, when we have greater spatial resolution, we see more gradational shifts. To really project observations made at the surface back down into the mantle we need a lot more information on melt transport out of the mantle, to know how spatial patterns in mantle heterogeneity are being mapped into basalt chemistry.


Online [publisher, open access]: http://dx.doi.org/10.1016/j.gca.2013.08.032

ReferenceShorttle, Oliver, John Maclennan, and Alexander M. Piotrowski. Geochemical provincialism in the Iceland plume. Geochimica et Cosmochimica Acta 122 (2013): 363-397

Thesis: Characteristics of a heterogeneous mantle

A copy of my PhD thesis, carried out at the University of Cambridge between Oct. 2009 and Oct. 2012 can be found here. I was supervised by John Maclennan and Alex Piotrowski.

A more digestible read of Chapters 4, 5 and 6 can be found in the associated papers, which are respectively: