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, 47254752, doi:10.1002/2016GC006497.