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: pymelt.swmatthews.com.

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.

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.