PhD positions for 2018 start

I am involved in several exciting projects at the University of Cambridge, each of which is part of the Cambridge Earth Systems Science DTP (projects I am involved in) and will base the student in the Department of Earth Sciences.  Several of the projects below are cross-disciplinary and link areas of the geosciences and astronomy.  My research life is based both in the Institute of Astronomy and Department of Earth Sciences.

If you have any questions about these projects, please feel free to contact me or one of the other listed supervisors.


Tracing mantle carbon

The project will ask the question ‘How reliable are magmatic archives as recorders of mantle carbon content?’. There are significant limitations to the most ubiquitous approaches of estimating the carbon content of Earth’s mantle: Using seawater noble gas contents provides a bulk upper mantle estimate, not easily resolving the constituent mantle sources; whilst C/trace-element ratios in melt inclusions are frequently lowered by C degassing. This project will provide new constraints on the distribution and cycling of C within the mantle by employing C isotopes as an independent tracer of degassing, and by targeting for analysis eruptions with geochemical signatures from: the lower mantle (Iceland); crustal recycling (The Canaries); and the archetypal depleted upper mantle.

Lead supervisor: Oliver Shorttle [ESC-homepage, IoA-homepage]
Co-supervisors: John Maclennan


Cooking the crust: 4.4 billion years of weathering-driven continental maturation?

The crustal composition has likely evolved over time because of weathering. This project will quantify this over Earth history. This has previously been neglected, but could be a major control on crustal composition with implications for the evolution of the surficial environment and for the igneous processes which generate crust. The student will measure stable isotope ratios of Mg and Li on river sediments to characterise average crustal compositions and variability, whilst associated “model” ages will be determined from radiogenic isotope systems such as neodymium. The Li and Mg stable isotope systems are highly-sensitive to water rock interaction, such as weathering, and their fractionations are most prominent at low temperature. Therefore, when combined, these three isotope systems will enable the weathering-driven maturation of the crust to be constrained.

Lead supervisor:  Ed Tipper
Co-supervisor: Oliver Shorttle  [ESC-homepage, IoA-homepage]


Carbon and sulfur cycling in the Earth’s mantle over the last 4 Ga: new clues from novel stable isotopes

This project will explore carbon and sulfur cycling and mantle source region heterogeneity using a combination of novel stable isotope and geochemical tracers. Through this it will be possible to explore the interplay between partial melting processes, mantle chemical and mineralogical heterogeneity and volatile element cycling. Key ocean island localities will be targeted, which possess abundant evidence for recycled components in their source regions. The study will also extend its reach back over Earth history by considering komatiites, with this project’s analyses contributing important new results to the debate about the origin (thermal vs. compositional) of these enigmatic high-degree melts.

Lead supervisor: Helen Williams
Co-supervisors: Oliver Shorttle [ESC-homepage, IoA-homepage]


The alternative chemistry of deep planetary interiors (Lead Supervisor: Simon Redfern

Ab initio structure prediction methods have now reached a maturity that allow them to be used to model the enthalpic stabilities of phases across pseudo-binary composition sections, The project uses this development to search for new structures in key silicates, oxides and carbonates at high pressures, where “unusual” chemical configurations may be stabilised. Such computational predictions demand experimental verification using high-pressure structural techniques such as vibrational spectroscopy and X-ray diffraction of samples pressurised in the diamond anvil cell. This project seeks to identify such unexpected phases by first adopting particle swarm structure prediction methods based on quantum mechanical computational results, combined with experimental studies of these structures for key candidate silicate, oxide and carbonate chemistries.

Lead supervisor: Simon Redfern
Co-supervisors: Oliver Shorttle [ESC-homepage, IoA-homepage]


Beyond Imaging: Optical Spectroscopy for Mineral Characterization

During this project, we wish to answer the following question: (1) how does the optical absorption signature of minerals such as olivine vary at the micron scale, and what are the key components leading to this variation? (2) Are defects such as dislocations in olivine (a sign of compression and deformation) perceptible with optical tools, and to what extent? (3) How does optical absorption and cathodoluminescence of minerals be used as complementary techniques?

Lead supervisor: Emilie Ringe
Co-supervisors: Oliver Shorttle [ESC-homepage, IoA-homepage]


Stable isotope tracers for the evolution of the crust-mantle system

This project will explore the changing composition of the Earth’s mantle and crust from the perspective of novel isotopic tracers. Recently, differences in the 238U/235U in OIB and MORB have been related to recycling of surface material and the different ages of the OIB and MORB sources (Andersen et al. 2015). Similar effects might be expected for stable isotope systems such as the transition metals (e.g. δ98Mo) and δ138Ba (e.g.Freymuth et al. 2015, Nielsen et al. 2018). Mantle-derived samples representing a time-series of Earth’s history will be used to characterise the isotopic evolution of the mantle in these systems and relate them to the history of plate tectonics and the formation of continental crust via geochemical modeling.

Lead supervisor: Helen Williams
Co-supervisors: Heye FreymuthOliver Shorttle [ESC-homepage, IoA-homepage]

Concurrent mixing and crystallisation in MORB

Concurrent mixing and crystallisation in MORB over a range of length scales.

As MgO drops during crystallisation basalts become more mixed. This concurrent mixing and crystallisation is tracked by a measure of chemical variability at a given MgO (σ*), shown here for La/Yb in a global compilation of mid-ocean ridge basalts (Gale et al., 2013). In the plot to the left the amount of chemical variability present at a given MgO is represented by points coloured red for high variability and blue for low. Each horizontal band of colour indicates how variability changes during differentiation, for a suite of MORB geographically normalised over a particular length scale. Taken together, these σ*-MgO profiles record how coherent mixing patterns emerge from global datasets as the length scale of sample grouping becomes progressively more local. Plots to the right indicate the normalised data distribution of La/Yb at 10,000 km and 50 km. Figure modified from Shorttle (2015).

The basalts sampled at mid-ocean ridges and ocean islands provide critical insight into the chemical evolution of the Earth: with their isotope ratio and trace element abundances capturing processes as diverse as planetary accretion and the recent subduction of organic sediments.  Basalts are used in this way with the understanding that their composition reflects that of the solid mantle source from which they were derived. However, we can only use the chemical inventory of basalts to probe mantle geochemistry if we are confident that we can ‘see through’ the chemical signals that have been imparted to them in the tens to thousands of years preceding their eruption. During this interval basalts are produced by partial melting of the mantle, extracted by porous and focussed flow, aggregated in mantle and crustal magma chambers and cooled, leading to their crystallisation. At each of theses stages there is the opportunity for basalts to mix, leading to a reduction in the amount of information they carry about their source, and therefore limiting our ability to reconstruct mantle composition and history.

This paper focusses on the mixing basalts experience during crustal (and upper mantle) storage in magma chambers. I use Iceland as a case study, which nicely illustrates the chemical signature of mixing in whole rock and melt inclusion records, to then explore mixing systematics along the more sparsely sampled submarine portions of the mid-ocean ridge system. The significance of magma mixing in mid-ocean ridge basalts (MORB) has previously been recognised from global compilations of MORB (e.g. Rubin et al., 2001) and from micro-analytical studies of melt inclusions (e.g. Sobolev & Shimizu (1993) and Maclennan (2008)), amongst other observations. In this paper I focus on investigating the length scale at which coherent mixing relationships become visible in suites of basalts, and the spatial statistical methods we can use to understand the meaning of this length scale.

Applying geographic normalisation to the global dataset of Gale et al., (2013), I find that coherent mixing relationships are visible at length scales < 300 km – approximately the scale of first order ridge segmentation. Statistical tests repeatedly randomly repositioning samples demonstrate that this result is only apparent in the real dataset – i.e. spatial structure exists in the samples of natural MORB that statistically relates them to one another over ~100 km length scales. This observation is another hint at the geochemical provincialism present in basalts (e.g. Shorttle et al. 2013). Yet, it remains challenge to identify at which point in the system, from mantle to crust, that basalts erupting > 100 km apart gain similarity to one another. One possibility is that this relates to a decorrelation length scale in mantle compositional structure.

The challenge posed by magma mixing from a mantle geochemical perspective is even knowing that the information loss has occurred. If primitive basalts are erupted, containing early crystallising phases, then perhaps concurrent mixing and crystallisation (CMC) will be caught in the act and basalts will be only partially mixed. However, extensive mixing may occur before a crystal record is even produced, for example during transport out of the mantle (Rudge et al., 2013). In this case it is important to appreciate how mixing may bias the accessible geochemical record of basalts. In particular, developing an understanding of magma mixing is going to be important for understanding the relationship of ocean island basalt chemistry to that of MORB.  As their very different geodynamic settings will likely translate into different magma storage and transport histories, of which we are unable to fully predict the geochemical consequences.


Online [publisher]: http://dx.doi.org/10.1016/j.epsl.2015.04.035

Online [preprint]: http://www.shorttle.com/paper_store/os_mix2015.pdf

Reference: Oliver Shorttle. Geochemical variability in MORB controlled by concurrent mixing and crystallisation. Earth and Planetary Science Letters 424 (2015): 1-14.

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: