GGRiP 2015 (Geochemistry group research in progress)

I am giving a keynote talk on the Monday of this conference that links our most recent XANES data (collected at Diamond Light Source) from basalts erupted south of Iceland to the cycling of oxygen through the solid Earth.  This work was inspired by a paper by Lecuyer and Ricard (1999), which builds up an inventory of ferric iron for terrestrial reservoirs and demonstrates that there must be a return flux of oxidised iron from Earth’s surface to the deep mantle.

An exciting implication of the Lecuyer and Ricard work is that we will potentially see a signal of ancient ferric iron cycling in the modern day composition of basalts that sample mantle containing recycled oceanic crust. This is what we have tested in this study.

 

 

AGU 2014

Come and find me or any of my collaborators at AGU this year to discuss our latest results.  Margaret Hartley and I have some great new XANES data collected at Diamond Light Source probing fO2 in enriched mantle domains and tracking its evolution during magmatic processes. I have an invited talk in V038: The Geochemical Diversity of the Mantle Inferred from Hotspots: Five Decades of Debate, where I will present evidence for the ubiquity of concurrent mixing and crystallisation in destroying the primary chemical diversity leaving the mantle at mid-ocean ridges. With Mark Hoggard’s fantastic record of dynamic support in the world’s ocean basins, we have begun to reconstruct spatio-temporal variability in mantle potential temperature over the last 100Ma.

Controls on OIB and MORB Geochemical Variabilty


Authors: Oliver Shorttle & John Maclennan

Concurrent mixing and crystallisation is visible on a local scale looking at melt inclusion and whole rock suites. Here we show that this basic magmatic process extends not only off of Iceland onto the adjacent Reykjanes Ridge, but by spatial statistical analysis can be seen to be present in global MORB datasets. Homogenisation of primary mantle chemical diversity is therefore a ubiquitous phenomenon occurring in magmatic systems. Understanding how this operates is going to be key for reconstructing mantle compositional diversity.


Authors: Oliver Shorttle & Yves Moussallam, Margaret E Hartley, Marie Edmonds, John Maclennan and Bramley J Murton

Recent evidence from Cottrell and Kelley (2013) has indicated that the mantle heterogeneity sampled by MORB and typically identified from studying radiogenic isotope tracers, may also be associated with redox heterogeneity in the mantle. This compelling observation has major implications for the flux of redox sensitive elements throughout the Earth system, for mantle dynamics, and for the melting process itself. In this work we have characterised the changes in mantle fO2 that occur towards the Iceland plume using a suite of basalt samples.


Authors: Margaret E Hartley, Oliver Shorttle, John Maclennan, Yves Moussallam and Marie Edmonds

Melt inclusions record the primary diversity of melts leaving the mantle in terms of their trace and isotopic compositions, and there is the potential for melt inclusions to also record redox heterogeneity of the source.  However, post entrapment processes such as diffusion and crystallisation may compromise the melt inclusion record, resetting melt inclusion fO2 during shallow level processes. To investigate the potential of the melt inclusion archive in terms of fO2 we have studied a suite of melt inclusions from the AD 1783 Laki eruption, Iceland.

A History of Global Mantle Potential Temperatures from Oceanic Crustal Thicknesses


Authors: Mark Hoggard, Nicholas J White and Oliver Shorttle

We know from geophysical observations of gravity anomalies and petrological measurements on primitive basalts that mantle potential temperature is likely to vary by several hundred degrees in the modern Earth.  A record of potential temperature variation in the past is preserved in the crustal thickness of old seafloor, which will be thicker if high potential temperatures during its formation increased melt production. Here, we use Mark’s extensive compilation of reflection and wide-angle seismic profiles to constrain crustal thicknesses throughout the oceanic realm. These observations when combined with a mantle melting model allow us to back out a unique record of spatio-temporal syn- and post-rift variations in mantle temperature.

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).

Plume-ridge interaction

Melt region traversal distance

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).

Ice in the Eocene

Ice rafted debris

A multiply striated dropstone from ODP site 913, found in > 30 Ma sediments. Figure from Tripati et al. (2008).

The onset of northern hemisphere glaciation is commonly placed at 2 to 15 Ma.  Here, by studying sediment records from an ODP core from the north Atlantic we demonstrate that ice must have been at least transiently present in the northern hemisphere back into the Eocene. We find evidence for ice rafted debris throughout the sedimentary section studied, most dramatically in the form of the dropstone photographed above.


Online [publisher]: http://dx.doi.org/10.1016/j.epsl.2007.09.045

Reference: Tripati, Aradhna K., Robert A. Eagle, Andrew Morton, Julian A. Dowdeswell, Katie L. Atkinson, Yannick Bahé, Caroline F. Dawber, Emma Khadun, Ruth M.H. Shaw, Oliver Shorttle, Lavaniya Thanabalasundaram. Evidence for glaciation in the Northern Hemisphere back to 44 Ma from ice-rafted debris in the Greenland Sea. Earth and Planetary Science Letters 265, no. 1 (2008): 112-122.

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