Iron isotopes trace primordial magma ocean cumulates melting in Earth’s upper mantle

Williams, Matthews, Rizo, Shorttle, Science Advances 7(11), eabc7394 (2021). doi: 10.1126/sciadv.abc7394

Evidence for an ancient magma ocean on Earth is preserved in the geochemistry of 3.7 billion year old metabasalts from Isua, Greenland. Previously work suggested these rocks are derived from melting a mantle source formed by Bridgmanite crystallisation and accumulation in the lower mantle. Bridgmanite crystallisation has previously been proposed to result in oxidation of the mantle, as it incorporates Fe3+ into its crystal structure, even when the magma it crystallises from contains only Fe2+. To balance the reaction Fe-metal is produced, which could be extracted efficiently to the core.

This reaction is thought to be associated with a fractionation in Fe-isotopes, such that the remaining bridgmanite enriched mantle should have an excess of 57Fe and 56Fe over 54Fe. In this study we demonstrated that such a fractionation is present within the Isua rocks, providing further confirmation for these magma ocean processes having taken place on the early Earth. However, the concentration of trace elements in the lavas suggest a more complex process, involving remelting and recrystallisation in the lower mantle.

To explore the consequences of these processes further, we performed THERMOCALC phase-equilibria modelling to determine how these magma-ocean derived mantle lithologies will melt in the upper mantle. We found that our hypothesised source for the Isua metabasalts melts at an anomalously low temperature. This would likely mean that such heterogeneities were rapidly melted out of the mantle, their evidence being largely erased.

The image is Figure 5 in the manuscript. It shows one such iteration of the phase-equilibria calculations. See the manuscript for further explanation.

Do olivine crystallization temperatures faithfully record mantle temperature variability?

Matthews, Wong, Shorttle, Edmonds, Maclennan, G3 (2021). doi: 10.1029/2020GC009157

Mantle temperatures are thought to vary substantially in the present day throughout the Earth, as a consequence of the vigorous convective cycling within our planet’s interior. Additionally, mantle temperatures are thought to vary through time. Estimating mantle temperatures in the ancient Earth can be more complex than for the present day Earth. In many cases the constraints we would like to use have to be indirectly inferred for the Earth’s past.

However, petrological techniques can be applied equally to both modern and ancient volcanic rocks. The temperature at which a magma starts crystallising is determined in large part by the temperature of the mantle whence it derived. Mantle composition also exerts a control (which we explored for Iceland in Matthews et al., 2016). In this new contribution we assess to what extent crystallisation temperatures can be relied on as a proxy for mantle temperature in the absence of information about mantle composition.

Central to our methods is an open source python module that we have developed for calculating mantle melting behaviour- pyMelt. To use the model without installing the python module, see our web-based app:

The image is from the Supplementary Information for the manuscript. It shows how we can use an electron probe to map the aluminium contents of olivine crystals. By measuring the aluminium content of coexisting olivine and spinel we can estimate the temperature they crystallised from magma.

Heavy 𝛿57Fe in ocean island basalts: A non-unique signature of processes and source lithologies in the mantle

Soderman, Matthews, Shorttle, Jackson, Ruttor, Nebel, Turner, Beier, Millet, Widom, Humayun, Williams, GCA 292, 309-332 (2021). doi: 10.1016/j.gca.2020.09.033

Despite being many tens of kilometres beneath our feet, Earth’s mantle plays an important role in the development of our planet. It acts as a vast chemical reservoir, exchanging with the surface through volcanism and subduction of tectonic plates. The mantle may also act as an archive of the chemical and tectonic changes that have occurred during our planet’s life.

An important tracer of past tectonics is the presence of ancient recycled crust, returned to the mantle by subduction. The presence of recycled crust has been inferred beneath volcanic islands such as Hawaii and Iceland, thought to have been transported in hot upwellings from the base of the mantle. However, most of the geochemical tools available to us only imply the presence of recycled crust indirectly. A more direct observation must relate to the mineralogical makeup of the mantle component, also known as its lithology.

Fe makes up a significant proportion of mantle rocks, and (in-part) determines their lithology. Subtle fractionations in its isotopes exist between different minerals due to variations in the way Fe atoms are bonded in their crystal structures. ‘Heavy’ Fe-isotope signatures have been linked to melting of recycled crust, but lithology is not a unique control on Fe-isotope fractionations. Here we combined new data with some novel models to assess the role of recycled crust in generating the Fe-isotope variability we observe in erupted lavas.

The image is Figure 6 from the manuscript. This shows the results of THERMOCALC phase-equilibra calculations for two mantle lithologies: KLB-1 (more ‘normal’ mantle) and G2 (‘recycled’ mantle material). Software we developed allowed us to calculate the degree of Fe-isotope fractionation that would be generated as these mantle components produce magma.

Reconstructing Magma Storage Depths for the 2018 KÄ«lauean Eruption from Melt Inclusion CO2 Contents: The Importance of Vapor Bubbles

Wieser, Lamadrid, Maclennan, Edmonds, Matthews, Iacovino, Jenner, Gansecki, Trusdell, Lee, Ilyinskaya, G3 22(2), e2020GC009364 (2020). doi: 10.1029/2020GC009364

The CO2 content of magmas is controlled primarily by pressure. Since pressure increases with depth in the crust, the CO2 contents of magmas can be useful tool for estimating how deep magma chambers sit beneath volcanoes. However, by the time magma reaches the surface it has lost most of its CO2. A widely used to tool to get around this is to measure the CO2 content in tiny droplets of magma trapped within crystals (melt inclusions) as they grew within the magma chamber.

However, crystals never behave as perfect pressure vessels. In many cases they have been shown to leak, removing the signal of magma chamber depth they once preserved. In other cases, they don’t leak, but the CO2 from the magma exsolves to form a bubble within the melt inclusion. This study analyses this behaviour in detail, and derives estimates of magma chamber depths beneath KÄ«lauea.

This study provided an ideal testing ground for the new VESIcal software I am involved in developing. In particular it allowed comparison of different CO2 solubility models and how they take into account the secondary control of magma composition on CO2 solubility.

The image is Figure 10 from the manuscript. It demonstrates the disparity in pressure estimates derived from different CO2 solubility models. Whilst none of the models are incorrect, they all work best in different situations. Something important to consider when using melt inclusions as barometers.

The global melt inclusion C/Ba array: mantle variability, melting process, or degassing?

Matthews, Shorttle, Maclennan, Rudge, GCA 293, 525-543 (2021). doi: 10.1016/j.gca.2020.09.030

In this paper we present new melt inclusion data from four Icelandic eruptions: Háleyjabunga, Stapafell, Berserkjahraun, and Heilagsdalsfjall. We used Secondary Ion Mass Spectrometry, Electron Microprobe Analysis, and Raman Analysis, to fully characterise the inclusions’ major, trace, and volatile element compositions. We then compiled this new data with previously published data from Iceland and the rest of the world.

The ratio of C/Ba in melt inclusions and submarine glasses has been used to estimate the magnitude and heterogeneity in carbon content of the mantle. Though mantle carbon contents are thought to be low (generally), the mantle is a significant reservoir of carbon on a planetary scale, and is likely to help regulate the surface carbon cycle on planetary timescales. It also has first order control on where magmas can form in the mantle, and ultimately make their way to the surface.

We find there’s a strong covariation of C/Ba ratio with indexes of geochemical enrichment, often thought to track with the contribution of recycled components to magma genesis. However, we show that this is likely a consequence of crustal processing rather than being a property of the mantle. This study lays the groundwork for future work in quantifying the small-scale carbon heterogeneity we think is very likely to be present in Earth’s mantle.

The image is Figure 5 from the manuscript, and shows off our new data alongside data presented in many other studies.

Decoupling of zircon U-Pb and trace-element systematics driven by U diffusion in eclogite-facies zircon (Monviso meta-ophiolite, W. Alps)

Garber, Smye, Feineman, Kylander-Clark, Matthews, CMP 175, 1-25 (2020). doi: 10.1007/s00410-020-01692-2

The mineral zircon is used extensively for dating metamorphic processes. The U-Pb system is particularly useful; however, understanding U-Pb dates requires a knowledge of their mobility (or lack thereof) during crustal residence and metamorphism.

In this manuscript, data are presented that suggest U diffusion into zircon can be significantly faster than suggested by experiments. This has important implications for the types of process that can be recorded by zircon dating. My contribution to this study was providing an estimate of the redox state of U in the fluids likely to be reacting with the zircons studied here.

The image is Figure 12 from the manuscript. It shows my predictions for the dominant speciation of U in aqueous fluids at elevated temperature and pressure.

The Hebridean Igneous Province plumbing system: A phase equilibria perspective

Nicoli & Matthews, Lithos 348 105194 (2019), doi: 10.1016/j.lithos.2019.105194

The Little Minch Sill Complex on the Isle of Skye, Scotland, is part of the British Tertiary Igneous Province and represents part of an extensive magmatic plumbing system. Decades of work has constrained the petrogenetic histories of the sills extremely well, and has allowed us to form a picture of how the crystal cargoes and their melts are related.

In this article we use this excellent petrological foundation to explore how the crystal and magma chemistry betrays the pressures and temperatures at which the magmas derived, and the conditions in which the sills were in intruded. Examining fossilised magmatic plumbing systems is important for developing an understanding of present day magmatic systems. In particular, we can begin to understand the paths magmas take on their way to the surface, and what makes some magmas more likely to erupt than others.

Image is Figure 8 from the manuscript. Bar 4 are the estimates derived in this study, all others are comparisons from the literature. See manuscript for citations.

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

The mantle is an important, yet poorly understood, part of Earth’s carbon cycle; interacting with Earth’s surface through volcanism and subduction. The CO2 flux balance 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 tendancy 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 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.

Matthews, S., O. Shorttle, J. F. Rudge and J. Maclennan (2017), Constraining mantle carbon: CO2-trace element systematics in basalts and the roles of magma mixing and degassing, Earth and Planetary Science Letters, 480, 1-14. doi:10.1016/j.epsl.2017.09.047

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

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.

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.

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 °C for Iceland, and °C for MORB.

Matthews, S., O. Shorttle, and J. Maclennan (2016), The temperature of the Icelandic mantle from olivine-spinel aluminum exchange thermometry, Geochem. Geophys. Geosyst., 17, 4725–4752, doi:10.1002/2016GC006497.