Blog

Global trends in novel stable isotopes in basalts: Theory and observations

Soderman, Shorttle, Matthews, Williams, Geochimica et Cosmochimica Acta, 318, 388-414 (2021). doi: 10.1016/j.gca.2021.12.008

In this paper we assessed the utility of novel stable isotopes (Mg, Ca, Fe, V, and Cr) in lavas for tracing mantle lithological heterogeneity and melting processes, and in particular the prospects for combining multiple stable isotope proxies to uniquely identify these processes. Major element isotope systems may better respond to lithological heterogeneity because, unlike trace elements, their concentrations do not vary by orders of magnitude between different mantle components.

This work (led by PhD student C. Soderman) significantly expanded the capabilities of the stable-isotope fractionation code I originally developed (and published a proof-of-concept).

VESIcal Part II: A critical approach to volatile solubility modelling using an open-source Python3 engine

Wieser, Iacovino, Matthews, Moore, Allison, Earth and Space Science, Accepted Manuscript (2022). doi: 10.1029/2021EA001932

In the second VESIcal manuscript we use the new ability to compare volatile solubility models to assess and review the CO2-H2O solubility calculations. In particular, we compare the calibration ranges of each model (for composition, pressure, and temperature), explore the sensitivities of each model, highlight the limitations of often-used methods, and suggest best practices for the presentation and archiving of data and calculations.

This work is particularly important for researchers using dissolved H<sub>2<\sub>O and CO<sub>2</sub> in magmas and melt inclusions for assessing magma storage pressures and igneous processes.

VESIcal Part I: An Open-Source Thermodynamic Model Engine for Mixed Volatile (H2O-CO2) Solubility in Silicate Melts

Iacovino, Matthews, Wieser, Moore, Bégué, Earth and Space Science 8 (11), e2020EA001584 (2021) doi: 10.1029/2020EA001584

An interactive version of this manuscript is available on myBinder:

We introduce our open-source solubility modelling engine, VESIcal, in this manuscript. Many of the most commonly used volatile solubility models are built into VESIcal and calculations can be performed using a common interface. This is the first time multiple solubility models have been so easily accessible at the same time, allowing their full potential to be exploited. Furthermore, calculations on large datasets are automated and can be performed in minutes or less.

I was one of the principle developers of the code and oversaw the construction of the generic model engine, and the implementation of several of the empirical models. The code is hosted in a GitHub repository (github.com/kaylai/VESIcal), and the documentation is available at vesical.readthedocs.io.

VESIcal Workshop at vVMSG2022

VESIcal is a new tool for calculating solubilities of CO2-H2O fluids/vapours in magmatic systems. It provides a powerful interface for many of the most frequently used solubility models, enabling the calculation of saturation pressures, degassing paths, and fluid/melt compositions. The tool leverages the power and flexibility of the python programming language, but most calculations require very little coding experience. In this short workshop we will introduce VESIcal, some of the solubility models that are built into it, and we will demonstrate how fast and easy it is to obtain results, even when using large datasets. 

Workshop Instructors: Simon Matthews (University of Iceland) & Penny Wieser (Oregon State University)

The workshop will take place on Wednesday 12th January 10:00-12:00 GMT on Zoom, as part of the virtual-VMSG meeting programme. Attendees will receive an email from the Geological Society of London with a link to the Zoom meeting. Following the meeting, all the workshop resources will be available at github.com/simonwmatthews/VESIcal_Workshop_VMSG22 including links to the lectures and practical demonstrations.

To participate in the hands-on exercises, you do not need to install anything on your own computer. Instead, all the calculations are performed on a cloud computing server. The only thing you need to access the server is a modern web browser (I find Chrome offers the best performance, but other browsers should work just fine).

Workshop Prerequisites

None! Other than an interest in volatile solubility modelling. You do not need any prior experience with coding (in fact you don’t need this to use VESIcal at all), using computation servers, or anything technical beyond using a web browser. We will show you everything else, and we will be on hang to help you get started.

However, please follow the instructions below before the workshop, and ideally as possible.

Preparation Instructions

  1. Go to the server web address provided in the VMSG email from the Geological Society of London (coming on Friday). [Please, please, please do not share this address publicly, otherwise I will have to start manually approving access for each user. To use a long-lived and public version of the server see https://server.enki-portal.org/, but if you do get access to this public server please do not use it for the workshop.]
  2. You should see a screen with a large orange button in the middle that says “Sign in with GitLab”. Click this button.
  3. If you have a GitLab account already then you can proceed to sign in. If not, please follow the directions to create one, it is free.
  4. You should see a loading screen with a progress bar, and then eventually a screen titled “ENKI Server”. You may also get some warning messages appear- it is safe to dismiss these. On the welcome screen click “Close this screen”.
  5. You should now see a window with two panes, the larger (right-most) pane having “Launcher” written in the tab. It should look very similar to this screenshot:
If you do not see this, or you get stuck at any other step, please email me (simonm@hi.is) as soon as possible and we will solve the problem. If you do see this screen (or one very similar- workshop materials may appear on the left-hand side by the time you login) then congratulations! You may now log out (click File in the top left, then Logout from the menu that appears).

NOTE: We are providing access to the server in advance of the workshop so that you can check you can get access to it and solve any problems before the workshop starts. Please do not run calculations on here before the workshop. All computation time costs credits, and we have only a limited quantity available for the workshop. If you want to explore the ENKI-server before the workshop, see http://www.enki-portal.org for more information. Any misuse of the server will result in you being banned from it

For more information about VESIcal…

For more information about VESIcal, see https://vesical.readthedocs.io, or read the VESIcal papers here:

https://doi.org/10.1029/2020EA001584

https://doi.org/10.31223/X5K03T

Geochemical Constraints on the Structure of the Earth’s Deep Mantle and the Origin of the LLSVPs

Gleeson, Soderman, Matthews, Cottaar, Gibson, G3, 22, e2021GC009932 (2021). doi: 10.1029/2021GC009932

The Galápagos islands preserve significant geochemical variability in their lavas, which varies consistently with spatial position. It is thought that much of this heterogeneity derives from melting of recycled and primordial mantle components. At the base of the mantle from which the Galápagos plume rises is a large low shear velocity province (LLSVP), however its origin remains enigmatic. LLSVPS have variously been interpreted as primordial mantle heterogeneities or piles of recycled oceanic crust.

If recycled oceanic crust is contributing to the magmatism, we should expect to see evidence for pyroxenite melting, This study demonstrated that the strongest pyroxenite signatures are found in a narrow band offset from the centre of the mantle plume. The lack of evidence for pyroxenite in the centre of the plume, the part most likely to be sampling the LLSVP, could indicate that the LLSVP material is not recycled oceanic crust, but this recycled material might be present on the margins of the LLSVP.

This study made use of the pyMelt mantle melting package (github.com/simonwmatthews/pyMelt) and the THERMOCALC results we published in Soderman et al. (2021).

Conditions and dynamics of magma storage in the snæfellsnes volcanic zone, Western Iceland: insights from the Búðahraun and Berserkjahraun eruptions

Kahl, Bali, Guðfinnsson, Neave, Ubide, Van Der Meer, Matthews, Journal of Petrology 62(9) (2021). doi: 10.1093/petrology/egab054

The Snæfellsness volcanic zone in Iceland is an example of off-rift magmatism. Melt eruption rates are much lower than in Iceland’s neo-volcanic zones, and the eruptive products are more alkalic and show greater geochemical enrichment. It might, therefore, be expected that magma storage and transport processes work differently beneath Snæfellsness. In this manuscript the crystal cargoes of two eruptions Búðahraun and Berserkjahraun are analysed.

The chemically diverse crystals record complex petrogenetic histories most likely occurring during magma storage in both the lower- and mid-crust. The Berserkjahraun crystals I analysed for melt inclusions (published in Matthews et al., 2021) contributed to this study.

The photo shows the panoramic view across the mountains of Snæfellsness, with the Buðahraun lava flow in the foreground.

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.