Hot mantle rising

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)

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

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 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)

A significantly hotter mantle beneath Iceland

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.

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

AGU 2016

I will be at AGU for the whole week.  On Monday afternoon I will presenting work we have been doing combining new Fe-XANES observations and thermodynamic models of mantle melting to understand solid Earth redox.  On Wednesday afternoon Paula Antoshechkina will be presenting our preliminary model incorporating carbonate melting into the pMELTS thermodynamic framework.  See you in San Francisco!

The solid Earth’s involvement in oxygen cycling: Observations and theory

Authors: Oliver Shorttle, Edward Stolper, Paula Antoshechkina, Paul Asimow, Eleanor Jennings, Glenn Gaetani, David Graham, Margaret Hartley, Helen Williams, Maryjo Brounce, Saemundur Halldorsson

Session: V13B Magmatic and Tectonic Influences on Elemental Cycling and Earth’s Climate and Oxidation State Posters

When/where: Monday 12th December, 13:40 – 18:00 in Moscone South – poster hall

We have undertaken a targeted study of basalts erupted along the South East Indian Ridge to test the relative controls of mantle temperature and chemical heterogeneity on Fe3+/ΣFe.  Among this suite of basalts there is short length scale heterogeneity and a long wavelength transition to cooler mantle.  Despite these factors, the Fe3+/ΣFe and the oxidation state of erupted basalts is remarkably uniform.  This result suggests that basalt  fO2 is being buffered during mantle melt extraction.

Silicate and Carbonatite Melts in the Mantle: Adding CO2to the pMELTS Thermodynamic Model of Silicate Phase Equilibria

Authors: Paula Antoshechkina, Oliver Shorttle

Session: V33C Deep Carbon: From the Mantle to the Surface and Back Again III Posters

When/where: Wednesday 14th December, 13:40 – 18:00 in Moscone South – poster hall

The transport of carbon in the mantle via carbonated melting of peridotite is critical for the solid Earth volatile cycle, yet most models of mantle melting only consider the thermodynamics of silicate melting and treat carbon as a trace species.  To address this issue and form a self-consistent thermodynamic description of carbonated peridotite melting we have expanded and updated the CO2-fluid database constructed by Ghiorso and Gualda (2012, 2015) to include more recent high pressure experiments.  In the initial stages of calibrating the model a key question we will answer is whether a Na2CO3 liquid component is required in addition to CaCO3.

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.

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.

How the Ferric Iron Proportion in Basalts Changes Towards the Iceland Plume

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.

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

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.