Constraining mantle carbon: CO2-trace element systematics in basalts and the roles of magma mixing and degassing

Mixing and degassing systematics

The systematics of mixing and degassing magmas, shown schematically (top) and as seen in the melt inclusion record (bottom). Primary melts are generated with correlated C-trace element systematics (left). Degassing only affects carbon, reducing its concentration in saturated (high C) melts (middle). Subsequent mixing of this variably degassed suite of inclusions can generate the appearance of no degassing having occurred.

The mantle is an important, yet poorly understood, part of Earth’s carbon cycle, which interacts with Earth’s surface through volcanism and subduction. The CO2 flux in to and out of the mantle regulates the mass of CO2 in Earth’s crust and hydrosphere, exerting control over the evolution of Earth’s climate and carbon availability for life. However, carbon’s volatility, and therefore tendency to degas from magmas and emanate at Earth’s surface diffusely, has made identifying the present-day mantle carbon distribution difficult.

Droplets of magma trapped within crystals as they grow deep in the crust offer a chance of observing CO2 concentrations in magmas prior to degassing. The behaviour of CO2 during magma evolution is encoded in the covariation of CO2 and trace element concentrations. In a small number of datasets, a correlation between CO2 and either Ba or Nb has been reported; consequently identical behaviour, in particular a lack of degassing, has been inferred. These, apparently undegassed, datasets underpin our understanding of carbon distribution in the mantle.

In this paper, we argue that many of the melts supplied from the mantle should be oversaturated in CO2 vapour at the pressure of magma storage, whilst others will be sufficiently depleted in CO2 that they should be strongly undersaturated. Such a population of melts will tend to partially degas at the earliest stages of melt evolution, before subsequent mixing and fractionation. We show that positive correlations between CO2 and both Ba and Nb are a natural consequence of this process. Furthermore, our new model makes specific predictions about the covariance of CO2 with a gamut of trace elements, if partial degassing and mixing has taken place.

Since we demonstrate that positive correlations between CO2 and trace element concentrations are arise from partial degassing and mixing, we cannot use this metric alone as a criterion for identifying whether a dataset has been affected by degassing. Mantle carbon contents, derived by assuming such melts preserve primary CO2 concentrations, are likely to be underestimates. We find the maximum CO2/Ba ratio in a dataset is the best proxy for mantle carbon content.


Online [publisher, open access]: https://doi.org/10.1016/j.epsl.2017.09.047

Reference: Simon Matthews, Oliver Shorttle, John F Rudge, John Maclennan. Constraining mantle carbon: CO2-trace element systematics in basalts and the roles of magma mixing and degassing. Earth and Planetary Science Letters (2017).

Olivine-hosted melt inclusions as an archive of redox heterogeneity in magmatic systems

Olivine hosted melt inclusions from the AD 1783 Laki eruption. These inclusions were trapped early in the magma’s life, preserving many chemical signals from the subsequent reprocessing, which occurred as the magma cooled and crystallised in the crust. However, our new study shows the susceptibility of tracers of magma oxidation state in the melt inclusions to resetting, even following eruption.

The amount of oxygen in magmas affects their physical and chemical properties, and ultimately their impact on chemical cycles linking planetary oceans and atmospheres to their deepest interiors.  A key archive of information on oxygen in magmas is the abundance of Fe2+, reduced iron, compared with Fe3+, oxidised iron. The abundance of these two forms of Fe evolves as magmas are stored in the Earth’s crust, meaning that the primary Fe2+/Fe3+ that magmas have when they enter the crust from the mantle will not be preserved at the point a magma comes to erupt.

An important information source for getting back at the chemical state of primitive magmas, before their crustal evolution, is in melt inclusions — small pockets of melt trapped as crystals grow.  However, in this study we show that even these archives of early magma history are susceptible to chemical resetting.  Firstly, in magma chambers at high temperature diffusion can occur, resetting all the melt inclusions to record the same activity of oxygen in the magma (oxygen fugacity).   Secondly, even after eruption, as the magma is flowing along the surface of the Earth, changes in the oxygen fugacity of the surrounding magma can propagate through to the melt inclusions changing theirFe2+/Fe3+ ratio.

These results mean that to reconstruct the oxygen fugacity of primitive magma we have to 1) select our samples very carefully, and 2) characterise the crustal and eruptive processes that could have reset the melt inclusions.


Online [publisher]: https://doi.org/10.1016/j.epsl.2017.09.029

Reference: Margaret Hartley, Oliver Shorttle, John Maclennan, Yves Moussallam, Marie Edmonds. Olivine-hosted melt inclusions as an archive of redox heterogeneity in magmatic systems. Earth and Planetary Science Letters (2017).

Geology beyond the solar system: Easter 2018

Over the last two terms the Earth Science course has introduced you to the basic physical and chemical processes that occur on the Earth. In these three lectures we will now move beyond considering the Earth specifically, and apply this fundamental geological knowledge in the wider context of rocky planets.

A transformation in our understanding of planet formation and evolution has occurred since 19951, fuelled by two key observational campaigns: the Kepler mission, which has detected thousands of planets outside our solar system; and the ALMA observatory, which provides unparalleled images of planetary systems being born. One of the most profound results from this flurry of discovery is that the most abundant type of planet in the universe may be rocky and roughly Earth-sized. This realisation begins a new era of geological-uniformitarianism, where we must now apply the principles of geology that have been founded on the study of Earth in our exploration of these new worlds.

The course will reflect this philosophy; although we now leave the Earth, the aim is to use some of the same core geophysical, geochemical, and petrological concepts you have learnt in previous terms. So, whilst we will need to introduce some new nomenclature, this course is also a chance to reinforce your understanding of concepts you are already covered in lectures and practicals.

Location: Physiology lecture theatre
When: Easter term, 2018

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

Posts below comprise summaries of some of the 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.

Michaelmas: week 8 (the Christmas vacation edition)

Map Exercise: Complete map exercise 4 from the examples book.

Thin Section: Describe the rock and thin section Me3.

— Due for the first supervision of Lent Term, before lectures start —

Go over your notes and read around the subject a bit!
A structured way of doing this is by identifying the 1A tripos questions that relate to the Michaelmas term of the course (which you can access through Moodle), and use these essay titles to construct essay plans. Where you can’t think what you would say, head back to the notes and then onto relevant textbooks to find the information you need and to furnish your answers with more quantitative detail and real-world examples of the processes you are describing.

A great book for John’s part of the course is Fowler’s Solid Earth, which contains much more than you need to know, but has good clear explanations of key physical processes. The minerals part of the course is nicely expanded upon in Putnis’s Introduction to mineral sciences. For Marian’s part of the course there are a couple of books you might look at, Klein’s Earth Materials and Philpotts’s Principles of Igneous and Metamorphic Petrology are both good; each of which will also contain some examples of real-world occurrences of metamorphic terranes you can use in your essays and photomicrographs to help you relate geological processes to the textures you can see under the microscope. All of these should be in college libraries/UL so you shouldn’t need to buy them.

As a general read, Langmuir’s update of How to Build a Habitable Planet couldn’t be better, but this is a big book so expect reading it to take a while.

After all of this reading you should have plenty of questions for the first supervision next term, so come back with a list of things you want to discuss.

Michaelmas: Week 7

Essay: How can metamorphic rocks exposed at the surface of the Earth give us information about the pressure and temperature regimes in which they formed?

Map exercise: Complete map exercise 3 from the examples booklet.

— Due for the supervision in the week starting Monday 28th November —

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 significantly hotter mantle beneath Iceland

BSE image of Borgarhraun olivine

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.

Read the full article on Eos: A significantly hotter mantle beneath Iceland

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

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