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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