## 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.

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.

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.

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?

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.

Co-supervisors: Heye FreymuthOliver Shorttle [ESC-homepage, IoA-homepage]

## Hot mantle rising

A crossed polarised light image of the 2.7 billion-year-old Belingwe komatiite from Zimbabwe. Needle-like crystals of olivine give komatiites their characteristic texture. Komatiites form from magmas with temperatures greater than 1,500°C and were abundant during the Archaean, more than 2.5 billion years ago. As Earth’s mantle cooled over time, fewer komatiites fomed. Trela and colleagues have identified lavas formed just 89 million years ago from a mantle source with a similarly high temperature. Field of view is 3mm across. Specimen courtesy of Mike Bickle.

The Earth formed from a molten ball of rock, cooling rapidly at first, as it quenched to the rocky mantle we sit upon today.  Over the ensuing 4.5 billion years, Earth’s mantle underwent a slow cooling.  Although the precise temperature change of the Earth’s mantle over this period is debated, it has likely fallen by ~200K since the Archean Eon.

This change in mantle temperature has had an important impact on the way in which the planet’s interior interacts with the oceans and atmosphere.  Higher mantle temperatures in the Archean would have meant more voluminous magmatism, and possibly more frequent large igneous provinces, which have been linked to environmental catastrophism and mass extinction.

The types of lava produced during melting of the mantle have also changed.  Whilst the basalts erupting at mid-ocean ridges today typically have MgO contents of around 8wt%, Archean komatiites had MgO > 25wt%.  As MgO in a primitive lava is a proxy for the temperature of the mantle that melted to form it, the composition of ancient lavas can be used to track changes in mantle temperature.  It is using this type of observation, in addition to Al-in-olivine thermometry (e.g., Matthews et al., 2016) that Trela et al. used to identify young, 89 Ma, komatiites in Costa Rica.

These Costa Rican komatiites of the Tortugal suite have been inferred to record crystallisation temperatures of > 1550°C (e.g., Alvarado et al. 1997, Trela et al. 2017) and mantle potential temperatures of up to 1700°C.  These temperatures are at the limits of the calibration range of current petrological tools and are a challenge for any thermodynamic description of mantle melting to model accurately.  So work needs to be done extending igneous thermometers up to these temperatures.  Equally, the geodynamic implications of such high mantle potential temperatures being present in the upper mantle has yet to be fully understood.  If these temperature estimates are substantiated by further observations then it is perhaps most likely we are seeing an unusually efficient dredge up of a thermal boundary layer, possibly that at the core-mantle boundary.

Online [publisher]: https://dx.doi.org/10.1002/2016GC006497

Discussing the article by Trela et al. https://dx.doi.org/10.1038/NGEO2954

Reference: Oliver Shorttle. Hot mantle rising. Nature Geosciences (2017)

## A significantly hotter mantle beneath Iceland

A false color image of an olivine crystal (centre) found in the Borgarhraun eruption of north Iceland. The color picks out variations in the crystal’s composition. We estimated the temperature at which the crystal grew by comparing the composition of the olivine to that of the spinel crystal which has been trapped inside it (small red circle inside crystal) . Horizontal scale = 1.5mm.

We have shown that the Icelandic mantle is unusually hot, a result which has been featured in AGU’s Eos magazine.  We measured the chemistry of olivine and spinel crystals that grew from magmas sourced directly from the Icelandic mantle. These crystals recorded crystallisation at almost 1400°C, indicating that the underlying mantle must be at least this hot. By developing a model to account for how the temperature of the mantle changes during melting, we were able to show that this crystallisation temperature is consistent with a mantle temperature prior to melting of closer to 1500°C.  This is more than 160°C hotter than the mantle underlying most regions on Earth.

And head here for a more detailed summary of the science.

## AGU 2015

I will be at AGU for the whole week: on Thursday speaking about work we have been doing combining geochemical and geophysical indicators of mantle potential temperature to understand what drives melting anomalies on Earth; followed by a talk on Friday presenting some early results investigating the thermodynamics of melt transport and the chemical signals this produces in basalts. On Wednesday Simon Matthews has a poster with new Al-olivine thermometry data from Iceland and neat modelling results showing how petrological estimates of crystallisation temperature can be used to estimate mantle potential temperature – the essence: mantle lithology matters!

### Lithology and temperature: How key mantle variables control rift volcanism

Authors: Oliver Shorttle, Mark Hoggard, Simon Matthews, John Maclennan

Session: T44C: Tectonic, Magmatic, and Geodynamic Studies of Extensional Processes: Applications in Iceland and the Nubia-Somalia-Arabia Plate System II

When/where: Thursday 17th December, 16:15 in Moscone South 304

Here we pick apart the various roles of mantle potential temperature and source composition in generating melting anomalies on Earth.  Taking Iceland as a case study, we show how crustal production rates (a proxy for melt flux) and estimates of the enriched-lithology’s melt supply to the crust can be used to constrain the source composition.  Knowing the source composition we can then make more accurate estimates of the thermal structure of the melting region, and so invert petrological estimates of crystallisation temperature into mantle potential temperature. Irrespective of Iceland’s source composition, the mantle must represent a thermal anomaly of at least 100ºC.

We extend our analysis to rifting globally by using a compilation of continental margin crustal thickness estimates. By making reasonable assumptions about mantle source composition, these crustal thickness estimates track the post break-up thermal evolution of the mantle. These observations allow us to evaluate the hypothesis that even away from plumes continental insulation drives up mantle potential temperature prior to rifting. However, the crustal thickness records provide little evidence for a long term increase of mantle temperature due to continental insulation: either it decays rapidly following break-up, or was not generated during the pre-break-up lifetime of the continent.

### Geochemical constraints on magma formation and transport processes

Authors: Oliver Shorttle, Paula Antoshechkina, Paul Asimow, Rajdeep Dasgupta, John Rudge

Session: DI51C: Melt and Liquids in Earth and Planetary Interiors II

When/where: Friday 18th December, 08:30 in Moscone South 303

The key question motivating this work is what proportion of geochemical diversity in basalts can be attributed to the melt transport history a given melt has experienced? The implications of this question are broad, as it leads to us questioning the origin of geochemical differences observed between ocean islands and mid-ocean ridges, or as a function of spreading rate and mantle potential temperature: Are these various tectonic regimes driving different styles and rates of melt transport that map into geochemical differences?

To answer these questions we performed some simple calculations of focussed melt flow to quantify the geochemical diversity generated just from varying the amount of melt focusing.  We observe a significant response in terms of the major and trace element chemistry of basalts, suggesting that some portion of local geochemical variability could be mapping in the diverse transport history melts have experienced through the mantle.

### The Temperature of the Icelandic Mantle Plume from Aluminium-in-Olivine Thermometry

Authors: Simon Matthews, Oliver Shorttle, John Maclennan

Session: DI31B: Melt and Liquids in Earth and Planetary Interiors

When/where: Wednesday 16th December, 08:00 in Moscone South – Poster Hall

Petrological estimates of mantle potential temperature are a key observation underpinning our models of mantle geodynamics. However, the process of going from the direct observable, a crystallisation temperature, back into a mantle potential temperature is not straight forward. A crystallisation temperature at the very list gives a minimum bound on the mantle temperature, but depending on the thermal history of the magma that crystal grew from, and the magma’s origin within the melting region, that temperature could be 100ºC less than the temperature of initial mantle melting.  To recover the mantle temperature before melting requires a model for the thermal evolution of the mantle during decompression and partial melting.

Here we combine a multi-lithology model of mantle melting with new Al-olivine thermometer estimates for the crystallisation temperature of forsteritic Icelandic olivines. By using geochemical and geophysical constraints on melt production we are able to arrive at a valid range of potential temperatures for the Icelandic mantle that are consistent with available observations.  Combining constraints in this way enables us to propagate uncertainty through relevant model parameters and analytical uncertainty on the crystallisation temperature, to obtain rigorously defined uncertainties. All viable model solutions show the Icelandic mantle to be significantly hotter than typical mid-ocean ridge mantle.

## Fe-XANES analyses of Reykjanes Ridge basalts: Implications for oceanic crust’s role in the solid Earth oxygen cycle

The ferric iron content (presented as  $\rm{Fe^{3+}/\sum{Fe}}$) and oxygen fugacity ($\rm{\Delta{QFM}_{(10)} (2 kbar)}$) in Reykjanes Ridge basalts approaching the Iceland plume. These samples form a 700km transect of mantle chemical structure as the chemical and thermal anomaly of the Iceland mantle plume is approached. Towards Iceland basalts become more oxidised and increasingly enriched in incompatible trace elements. These signals are consistent with ancient recycled oceanic crust present in the Icelandic mantle.

There is abundant evidence for extreme chemical heterogeneity in the Earth’s mantle, resulting from billions of years of differentiation during magma production, and the subsequent recycling of these crustal rocks back into the mantle. One way we can access a record of these processes is by studying the chemistry of recent volcanic eruptions in locations such as Iceland: where the mantle melts, its chemical character is mapped into the magmas produced, which can erupt as basalts to form an accessible archive of mantle composition.

One way we might expect the Earth’s history of subduction recycling to manifest in the composition of basalts is in their oxygen fugacity, as constrained by the proportion of $\rm{Fe^{3+}/\sum{Fe}}$, oxidised to reduced iron, in the basalt. Material that spends time at Earth’s surface has the potential to become oxidised by interaction with the atmosphere and hydrosphere. By compiling a large database of ocean floor basalt $\rm{Fe^{3+}/\sum{Fe}}$ compositions and the results of scientific drilling studies, Lecuyer and Ricard (1999) showed that igneous ocean crust often becomes significantly oxidised by hydrothermal alteration, shifting an initial composition of $\rm{Fe^{3+}/\sum{Fe}\sim{}0.1}$ to a mean crustal value of $\rm{Fe^{3+}/\sum{Fe}=0.22\pm{0.08}}$. A recent study by Cottrell and Kelley (2013) found that enriched mantle material, possibly produced by recycling, actually appears reduced compared with ambient mantle. However, the Cottrell and Kelley (2013) sample set specifically avoided mantle plume influenced sections of ridge, such as the Reykjanes Ridge near Iceland. This study therefore aimed to probe the oxidation state of a mantle plume, which we also have good independent evidence for containing recycled oceanic crust.

Performing Fe-XANES analyses on 64 Reykjanes Ridge basalts on beamline I18 at Diamond Light Source we found that as basalts become more enriched closer to Iceland, they also become more oxidised (Figure above). Neither degassing, nor simple fractional melting processes can account for this trend, which we instead attribute to the presence of recycled oxidised material in the Iceland plume. By performing simple fractional melting calculations, assuming reasonable ferric iron partition coefficients (Mallmann and O’Neill, 2009), we find that the oxidised signature of enriched Icelandic basalts is consistent with altered recycled oceanic crust present in the plume source in similar proportions as found by Shorttle et al. (2014).

Although more work needs to be done on the petrological modelling of ferric iron during crustal and mantle processing, our results are an indication of the role the solid Earth may have the global oxygen cycle. During the last 500 million years of Earth history oxygenation of the oceans may have enabled a flux of oxygen back into the mantle through oxidation of igneous crust at the ridge axis. In this way oxygen levels at Earth’s surface are coupled to the redox evolution of the mantle, as oxidised material is returned into it at subduction zones for long term storage. Occasionally, in locations such as Iceland, we may sample the return flux of this oxidised material to the shallow mantle, where it is involved in melting.

Online [publisher, open access]: http://dx.doi.org/10.1016/j.epsl.2015.07.017

Reference: Oliver Shorttle, Yves Moussallam, Margaret Hartley, John Maclennan, Marie Edmonds, Bramley Murton. Earth and Planetary Science Letters 427 (2015): 272-285.

Data: The published version of the ferric iron data file is space separated rather than comma separated.  Download a comma separated version here.

Publicity: From fiery giants

## Spatial geochemical structure in Icelandic basalts

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:

## Identifying lithological heterogeneity

Concurrent mixing and crystallisation destroys the primary geochemical variability leaving the mantle. MgO tracks the progressive crystallisation of basalts, whilst Nb/Zr, an incompatible trace element ratio records the collapse in primary chemical diversity that occurs in response to magma mixing in the crust. Primitive basalts, with MgO > 9.5 wt%, show the greatest geochemical diversity, which correlates with major element composition, implying lithological heterogeneity in the source. Figure modified from Shorttle & Maclennan (2011).

Our understanding of the mantle, long thought to be largely homogeneous in its major element composition, has undergone significant revision in the last decade as increasing evidence points towards it containing lithological heterogeneity. In this paper we present new evidence from Iceland that demonstrates the presence of pyroxenitic components in the plume source. Using a novel data projection (shown above) we account for fractional crystallisation processes and concurrent mixing and crystallisation to see through to the major element – trace element relationships characteristic of enriched and depleted mantle domains.

We also consider the consequences of lithological heterogeneity for melt production. Iceland is characterised by high crustal thicknesses, perhaps up to 40 km at the island’s centre (Darbyshire et al., 2000), and the eruption of enriched basalts. This combination has led some to claim that an enriched, fusible, source lithology is alone enough to explain the high crustal thickness observed (Foulger & Anderson, 2005). However, the thermodynamic modelling we present in this paper of a bi-lithological peridotite-pyroxenite mantle shows that the latent heat cost of melting requires significant excess temperature (> 100°C) even in the case of a pure pyroxenite mantle.

Online [Department repository, full text]: http://eprints.esc.cam.ac.uk/2228/7/ggge2058.pdf

Online [Publisher]: http://dx.doi.org/10.1029/2011GC003748

ReferenceShorttle, Oliver, and John Maclennan. Compositional trends of Icelandic basalts: Implications for short–length scale lithological heterogeneity in mantle plumes. Geochemistry, Geophysics, Geosystems 12, no. 11 (2011).

## Plume-ridge interaction

Map of melt region traversal distances for mantle flow paths extending away from the Iceland plume centre. The flow of plume material beneath spreading centres, with the concomitant decompression and partial melting progressively extracts the enriched components giving the plume mantle its distinctive trace element and isotopic character. Melt regions are marked out as regions in black, 120 km wide. Figure modified from Shorttle et al. (2013), based on the model presented in Shorttle et al. (2010).

The dispersal of mantle plumes in the shallow mantle causes excess volcanism and surface uplift over thousands of kilometres. These phenomena result primarily from a mantle plume’s excess temperature with respect to ambient mantle, which will be the main reason a plume is buoyantly ascending through the mantle in the first place. However, mantle plumes also tend to produce basalts that have distinct trace element and isotopic compositions, that are unlikely to be due solely to changing mantle potential temperature. Like surface uplift, these geochemical characteristics of a mantle plume can be used to trace its dispersal in the shallow mantle if it intersects passive melting features like mid-ocean ridges.

In the simplest model of plume dispersal, the geophysical and geochemical observables recording the presence of a mantle plume will track each other, so that when a mid-ocean ridge is shallow because of underlying hot mantle, it will also erupt trace element enriched basalts. However, in two classic cases of plume-ridge interaction, the Galapagos and Iceland, geophysical and geochemical tracers of plume dispersal in the shallow mantle are decoupled and apparently asymmetric about the plume axis. These observations have led to models of asymmetric plume flow in response to prevailing mantle convection, fracture zone blocking of plume outflow, or tilted mantle plumes.

In this paper we show that by allowing a realistically located centre of plume symmetry to be found for Iceland and the Galapagos, geophysical indicators of plume dispersal can be shown to be radially symmetric. However, geochemical enrichment along the ridges either side of the Iceland and Galapagos plumes remain highly asymmetric. These observations can be reconciled by considering that the enriched plume component, carrying much of the trace element load that gives the plume its distinctive geochemical character, is more fusible (i.e. is a pyroxenitic heterogeneity) than ambient mantle. The implication of this is that partial melting during outflow of the plume material will preferentially deplete it in the enriched component, leaving any basalts that the source goes on to produce relatively depleted. It is then the asymmetry in the distribution of spreading centres about the Iceland and Galapagos plumes that ingrows asymmetry in the chemistry of the plume material (see figure above for Iceland), as spreading centres cause the laterally flowing plume material to decompress and undergo small degrees of melting.

Our model therefore explains decoupling between geophysical and geochemical indicators of plume dispersal without requiring complex dynamics, just the observation that enriched mantle domains will be more readily extracted from a source than depleted mantle.

Online [Department repository, full text]: http://eprints.esc.cam.ac.uk/1400/2/G3_Shorttle_Maclennan.pdf

Online [publisher]: http://dx.doi.org/10.1029/2009GC002986

ReferenceShorttle, Oliver, J. Maclennan, and S. M. Jones. Control of the symmetry of plume‐ridge interaction by spreading ridge geometry. Geochemistry, Geophysics, Geosystems 11, no. 7 (2010).