Posts below comprise summaries of articles I have worked on.  For a complete list of the papers I have been involved in see my cv (including links to online versions of the articles).  For a more general summary of some of the work we have done, including press coverage, head to the research page.

Hot mantle rising

 

Belingwe Komatiite

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)

Publicity: Interview on Radio 4

The temperature of the Icelandic mantle from olivine-spinel aluminum exchange thermometry

Potential temperature estimates at Iceland

A comparison of previous estimates of mantle potential temperature at Iceland (red horizontal bars) to our new estimate based on combining crystallisation temperatures with crustal thickness and geochemical constraints (red/yellow histograms).  Estimates of mantle potential temperature for MORB are shown for reference in blue.

Variations in mantle temperature are a primary control on the melting behaviour of the mantle. Despite its importance for understanding present day volcanism and the thermal evolution of the Earth, mantle temperature has remained difficult to quantify. Proxies, such as crustal thickness, seismic velocity, and melt chemistry must be used; however, each suffers from its own uncertainties and trade-offs with other equally uncertain parameters. Melting anomalies, such as Iceland, have been variously linked to raised mantle temperature, unusually fusible mantle, or enhanced mantle flow.
Several studies have recently used olivine crystallisation temperatures, derived from olivine-spinel aluminium-exchange thermometry, as a proxy for mantle temperature. When offsets in olivine crystallisation temperatures are used to infer mantle temperature variation directly, it is implicitly assumed the method does not suffer from trade-offs arising from greater mantle fusibility or enhanced mantle flow.

Summary of new crystallisation temperatures from Iceland

Summary of the new crystallisation temperature estimates we made in this study.  Crystallisation temperatures were calculated from the composition of olivine-spinel pairs using the Wan et al. (2008) and Coogan et al. (2014) Al-exchange thermometer.

Using a new set of crystallisation temperatures determined for four eruptions from the Northern Volcanic Zone of Iceland, we demonstrate crustal processes, rather than mantle processes, are responsible for the crystallisation temperature variation within our dataset. However, the difference between Icelandic crystallisation temperatures and those from MORB, are most easily accounted for by substantial mantle temperature variations between the two locations.

The thermal structure of the mantle melting region will determine the chemical and thermal properties of the melts entering the crust. As lithological heterogeneity can exert a large effect on the thermal structure of the melting region, we assess its effect on crystallisation temperature using a forward thermal model of multi-lithology melting. Using crystallisation temperature estimates from Iceland and MORB as examples, we demonstrate that in the absence of further constraints on the thermal structure of the melting region (e.g. crustal thickness), crystallisation temperature provides only a weak constraint on mantle temperature.

By inversion of our thermal model, fitting for crystallisation temperature, crustal thickness, and fraction of bulk crust derived from pyroxenite melting, we demonstrate that a mantle temperature excess over ambient mantle is required for Iceland. We estimate a mantle temperature of \mathsf{1480^{+37}_{-30}}°C for Iceland, and \mathsf{1318^{+44}_{-32}}°C for MORB.


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

Reference: Simon Matthews, Oliver Shorttle, John Maclennan. The temperature of the Icelandic mantle from olivine-spinel aluminum exchange thermometry. Geochemistry, Geophysics, Geosystems (2016)

PublicityA significantly hotter mantle beneath Iceland

A statistical description of concurrent mixing and crystallisation during MORB differentiation: implications for trace element enrichment

Melt formation, mixing, and transport schematic

A cartoon of the processes operating from mantle to crust, which give rise to the appearance of trace element over-enrichment in mid-ocean ridge basalts. Diverse melts are produced in the mantle, potentially from chemically heterogeneous sources (left). These melts are transported out of the mantle by channelised flow, which preserves some primary heterogeneity of the melts supplied to the crust (centre). In the crust melts fractionate crystal phases and mix (centre and right).  The supply of heterogenous melts to crustal magma chambers, and their subsequent mixing, gives rise to apparent trace element over-enrichment in the final erupted basalt.

Basalts are our window into Earth’s chemical structure, and being a product of mantle melting and transport, constrain the processes by which it continues to differentiate.  However, to interpret the basalt record in terms of mantle source and melting conditions requires our being able to see through the processes that affect basalt chemistry following melting, during their transport through the mantle, and their storage in the crust.  The extent to which basalts reflect mantle processes vs. crustal processes has been a topic of long-running debate, with workers such as Klein and Langmuir (1987) and Gale et al. (2014) arguing for the importance of mantle processes, whereas more recently O’Neill and Jenner (2012) and Coogan and O’Hara (2015) have emphasised the role of crustal magma chambers in modifying the trace and minor element chemistry of basalts.  A key observation in this debate has been the presence of trace element over-enrichment in mid-ocean ridge basalts, whereby differentiated basalts (those with low MgO) appear to have higher concentrations of incompatible trace elements than can be accounted for by simple fractional crystallisation (see figure below).

Trace element enrichment gradients.

The rate of enrichment of trace elements during magmatic differentiation (expressed as the gradient in trace element – MgO space). More negative numbers indicate stepper gradients, which when they fall below the orange line, cannot be accounted for by simple fractional crystallisation.

In this article we demonstrate how apparent trace element over-enrichment during differentiation can result simply from the chemically heterogeneous melts being supplied to the crust.  The key aspect of the model is that once resident in crustal magma chambers, the probability of melts mixing is proportional to their degree of differentiation (a proxy for their residence time).  Therefore, as melts differentiate, they progressively interact with other melts, until at low MgO (~5  wt%), magmas have a composition close to that of the mean melts being supplied from the mantle.  A consequence of this process, which we call concurrent mixing and crystallisation (CMC; Maclennan, 2008), is that the overall trend of trace element enrichment during differentiation is steeper than what it would be predicted in the absence of mixing.  What’s more, the most incompatible elements have the most variable abundance in melts supplied to the crust, and therefore exhibit the greatest degree of over-enrichment.  Thus, this model reproduces the observations presented in the figure above, whereby highly incompatible elements such as Th and U have the steepest enrichment gradients.

The importance of these results is that in our model, although mixing destroys much mantle-derived chemical variability and therefore information on the melting and melt transport process, the mean composition of the magmas supplied from the mantle is not affected.  As such, magmas retain bulk information on their sources and conditions of formation.  This means that global correlations like those established by Klein and Langmuir (1987) and Gale et al. (2014) will be valid, and the elemental ratios forming the basis for isotope evolution models of the Earth’s mantle (e.g., Sm/Nd for the <sup>143</sup>Nd isotope system) will not have been perturbed.


Online [publisher]: https://dx.doi.org/10.1093/petrology/egw056

Reference: Oliver Shorttle, John F. Rudge, John Maclennan, and Ken Rubin. Journal of Petrology (2016): 1-35, doi:10.1093/petrology/egw056.

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

Ferric iron proportions and oxygen fugacity of Reykjanes Ridge basalts.

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

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.

Quantifying lithological heterogeneity

Melt production as a function of lithology

Combining constraints on melt production (crustal thickness, tc) and the fraction of melts supplied by pyroxenite melting (from geochemistry, Fpx) unique source lithologies can be identified beneath Iceland. In these triangular diagrams position denotes the proportion of each lithology in the source, whilst colour indicates the relative proportion of enriched and depleted melts produced. Figure modified from Shorttle et al. (2014).

The presence of lithological diversity in the mantle has major implications for solid Earth dynamics, the mantle melting process and potentially the environmental impact of eruptions. In this paper we consider the processes affecting the representation of lithological diversity in erupted basalts, such as the biasing effect of the melting process, in order to construct quantitative estimates of the abundance of different lithologies in the mantle. Being able to form such estimates is the first step for understanding how lithological heterogeneity is affecting melt production and ultimately influencing the solid Earth’s interaction with the surface environment.

We focus our study on Iceland, and first use high MgO basalts from Iceland’s neovolcanic zones to characterise the trace element enriched and depleted endmember melts entering the Icelandic crust. These compositions can then be used in a mass balance with the average Icelandic crustal composition (as represented by evolved, mixed, basalts) to calculate the proportion of melt on Iceland being supplied from enriched vs. depleted mantle domains. With this estimate of enriched and depleted melt proportions, and knowing that the lithologies contributing to melting in the Icelandic mantle (pyroxenite and lherzolite, Shorttle et al. 2011), it is possible to construct a melting model to determine how abundant each lithology must be in the Icelandic source. However, as can be seen from the figure above, using only the constraint on proportion of enriched and depleted endmembers does not uniquely constrain a valid source lithology. To achieve a unique result we add in the constraint that the total melt production must be consistent with the volume of melt production on Iceland (the crustal thickness).

Even using enriched-depleted melt proportions and crustal thickness, source lithology remains non-unique unless mantle potential temperature is also known. Specifically, there is a strong trade off in harzburgite abundance against lherzolite proportion in the source – all valid solutions have ~10% pyroxenite component. To obtain added constraints on source we consider the buoyancy of the possible Icelandic source lithologies, and whether they would be consistent with recent volume flux estimates (Jones et al. 2014). Considering this additional dynamical constraint, the Icelandic source is required to have significant (>20%) harzburgite component. Although there is this uncertainty on the maximum temperature of the plume source, our model nonetheless constrains its minimum temperature to be significantly above ambient mantle, by ~130ºC (see figure above).


Online [publisher, open access]: http://dx.doi.org/10.1016/j.epsl.2014.03.040

ReferenceShorttle, Oliver, John Maclennan, and Sarah Lambart. Quantifying lithological variability in the mantle. Earth and Planetary Science Letters 395 (2014): 24-40.

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:

 

Identifying lithological heterogeneity

Concurrent mixing and crystallisation seen through to reveal 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).

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