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

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

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.

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).