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).

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.

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

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.