Geochemistry, Mineralogy, Petrology & Volcanology

EGU Guest blogger

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Can limestone digestion by volcanoes contribute to higher atmospheric carbon dioxide levels?

Can limestone digestion by volcanoes contribute to higher atmospheric carbon dioxide levels?

By Frances Deegan and Ralf Halama

Cartoon showing carbon fluxes in subduction zones. Source: Deep Carbon Observatory.

Carbon – the element on everyone’s lips. Carbon is unquestionably one of the most important elements on Earth – terrestrial life is carbon-based and so are many of our energy sources. From the perspective of a human time-scale, biological and anthropogenic (caused by human activity) carbon fluxes are very important (e.g. through industrial activity and burning fossil-fuels). However, if we consider time-scales spanning millions of years (the geological time-scale), then Earth degassing becomes an important control on the climate evolution of our planet. Volcanoes are major contributors to long-term Earth degassing and this is what brings us to Indonesia – one of the most volcanically-dense regions on Earth and home to some of the most active and dangerous volcanoes.

Where does volcanic carbon dioxide come from? Indonesia is located above a subduction zone, which is where two tectonic plates meet and one sinks into the Earth’s mantle. This is where crust is destroyed or “recycled” back into the Earth and where arc volcanoes form on the upper plate. The carbon dioxide released from volcanoes in subduction zones comes from a mixture of sources – some comes from the sedimentary rocks that were pushed into the mantle during subduction and some comes from the Earth’s mantle. Scientists have also discovered another source of carbon at subduction zones – the crust sitting on top of the subduction zone, on which the volcanoes are built (the “upper arc crust”).

The limestone-gas connection. Volcanoes located in areas where the bedrock is made of limestone often have an unusually high outflux of carbon dioxide. Could there be a link? Limestone is made of calcium carbonate, CaCO3, and when hot magma reacts with CaCO3 it rapidly breaks down to release large amounts of CO2, contributing to this unusually high outflux of CO2. The exact process during magma-limestone interaction at depth and the timescales over which this can happen have for a long time remained speculative.

Thermally metamorphosed limestone (calc-silicate) fragment entrained in Merapi lava. Source: Frances Deegan.

New research results. At Merapi volcano, a team of scientists from the universities of Keele (UK), Uppsala (Sweden) and Swansea (UK) have found pieces of thermally-altered limestone (calc-silicate xenoliths) among the volcano’s erupted products. They argue that these altered limestone samples record the processes in the underlying magma reservoir and allow them to gain insights into the processes accompanying liberation of CO2 from limestone. New research led by PhD researcher Sean Whitley from Keele University, investigated isotopic ratios of carbon and oxygen in crystals of the mineral calcite in these altered limestone fragments at the Edinburgh Ion-Microprobe Facility. Using these data, scientists can chemically fingerprint the processes that led to formation of calcite in the limestone fragments. Most intriguingly, the team found an unusual carbon isotopic signature that demonstrates highly efficient remobilisation of limestone-derived CO2 into the magmatic system. They further found that this CO2 release occurs over geologically short timescales of hundreds to thousands of years and that during non-eruptive episodes up to half of the CO2 emissions at Merapi derive from the digestion of limestone in the magma storage region, rising to 95% during volcanic eruptions.

Although Merapi is currently considered a relatively minor emitter of CO2 on a global scale, the new results from Sean Whitley and co-workers show that volcanic CO2 liberation can potentially release large amounts of limestone-derived CO2 during eruptive episodes. This increasingly recognised contribution of limestone-derived CO2 to volcanic carbon budgets now requires reconsideration of global carbon cycling models throughout Earth history.

Link to article: Whitley, S., Gertisser, R., Halama, R., Preece, K., Troll, V.R., Deegan, F.M. 2019: Crustal CO2 contribution to subduction zone degassing recorded through  calc-silicate xenoliths in arc lavas. Scientific Reports 9:8803, DOI: 10.1038/s41598-019-44929-2.

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How do crystal aggregates form in magma chambers?

How do crystal aggregates form in magma chambers?

By Penny Wieser (PhD student at the University of Cambridge)

Clues into the inner workings of volcanoes can be gleamed from material which is erupted at the surface, or that which solidified at depth in the crust. Just before eruption, three main phases are present: a gas phase (containing water, carbon dioxide, sulphur, chlorine etc), a liquid melt phase (the magma), and a solid phase (consisting of lots of different kinds of crystals).

Olivine aggregate consisting of at least 17 separate crystals (taking with a normal camera). B-C) Backscatter electron images taken using an SEM showing attached olivine crystals (dark grey) surrounded by volcanic glass (solidified melt; lighter grey).

Petrologists spend their lives studying these crystals, and use them to determine how deep the magmas are stored in the crust, how long the magmas are stored, and how fast the magmas rise to the surface. However, many of the characteristics of erupted crystals are not fully understood, but may provide crucial insights into processes happening at depth within volcanoes. One example is individual crystals attaching together to form aggregates (also known as clusters, clots, or glomerocrysts). Despite these being really common in volcanoes, it’s still not exactly clear how they form. Previously it has been suggested that aggregates form during growth processes such as heterogeneous nucleation (where a new crystal nucleates on the surface of an existing crystal), dendritic growth (which occurs when crystals grow extremely fast, forming finger-like dendrites (this comes from the greek for ‘tree’)), or twinning (where two parts of a crystal grow together, with a specific orientation relative to one another). Or, crystals may “swim”  together as they settle through a liquid magma column (through a process known as synneusis), or land on top of one another after they settle to the solid base of the magma chamber. We wanted to have a look at these different possibilities in some more detail, we did this using a technique called electron backscatter diffraction (EBSD).

We took olivine crystals erupted at Kīlauea Volcano (Hawai’i), and chromite crystals from the Bushveld Complex of South Africa, and prepared flat, very well polished slices. These samples were then placed in the Scanning Electron Microscope (similar to a regular microscope, but with electrons instead of light). An electron beam was focused on the sample; electrons which enter the sample are diffracted by planes of atoms in the olivine and chromite crystals, and leave the sample as “backscattered electrons” (these are collected by an EBSD detector). Following a series of data processing steps, these measurements can be used to determine the orientation of the crystal lattice. This provides a quick, quantitative way to determine the relative orientations of neighbouring crystals within aggregates. These observations can then be compared to the orientations expected from the various different hypotheses.

EBSD-map, color coded according to the orientation of the separate crystals. The fact that all 6 attached crystals show similar colors indicates that their orientations are similar.

Crystals are lazy; they will grow in the way that uses the least energy. During crystal growth, any mismatches in the direction of the new part of the crystal lattice with respect to the existing lattice is energetically exhausting. So if a new crystal wishes to grow on the surface of an existing crystal, it should mimic the orientation of the existing crystal to conserve energy. Similarly, twinning only produces a finite number of possible geometries, called “twin laws” (1 in chromites, 3 in olivines). In contrast, physical aggregation processes can result in a much broader range of attachment geometries. The settling of crystals onto the floor of a magma chamber where they land on other crystals produces totally random aggregate geometries (analogous to a very bad tetris player). The swimming together of crystals in the liquid part of the magma chamber results in certain crystal faces sticking together (which can be predicted from observations of the way that crystals orientate themselves as they fall).

Olivine and chromite aggregates show too broad a range of attachment geometries to have formed during crystal growth. Instead, we observe that large numbers of olivine crystals are attached along the crystal faces predicted from settling. So, just before eruption, olivines in in the liquid part of the volcanic plumbing system at Kīlauea Volcano must “swim together” as they settle. Chromite aggregates from the Bushveld complex show completely random orientations. This is best explained by the settling of individual crystals onto a firm substrate at the base of the chamber, where they randomly land on other chromite crystals. The fact that crystals in liquid magma are present as aggregates in some volcanic systems (e.g. Kīlauea), but individual crystals in others (e.g. Bushveld) has implications for calculations of the rates of crystal settling. This, in turn, affects the viscosity of erupted lavas (as slower settling rates means more crystals, and “stickier” magmas), and is important to understand the formation of economic deposits of platinum group elements (which are concentrated within settled piles of crystals within the Bushveld Complex).

Want to know more? Read all about it here!

Penny Wieser is a PhD student in volcanology at the University of Cambridge, UK. She uses a variety of analytical techniques to interrogate erupted crystal cargoes and their melt inclusion records to gain insights into the pre-eruptive storage of magma beneath Kīlauea Volcano, Hawai’i.

What I learned from chairing my first EGU session

What I learned from chairing my first EGU session

By Emily Bamber (PhD Student, University of Manchester)

At this year’s EGU meeting I was invited to co-convene the GMPV 5.7 session ‘Magma ascent, degassing and eruptive dynamics: linking experiments, models and observations’. At first, I felt nervous, as a PhD student who has so far only attended and presented at a few conferences. Afterwards I felt happy to be part of a session which presents cutting-edge research in the field I love, and able to share this experience with the volcanology network. Although it can be daunting, to stand on a stage and present in front of a large and experienced audience, ultimately the audience is welcoming, and supportive, and just as excited as I was to learn about new and interesting ideas in volcanology.

Practically, this experience gave me invaluable knowledge in how sessions are organised and managed, and I was able to meet lots of new people in my field as a result. Later, I was able to continue the discussion during the presentation of my own research at the poster session. Convening is a great experience for a PhD student, and a valuable way to represent the early-career community. Here I share some tips which I learned along the way:

  • Record the deadlines for important milestones when organising the session, as these can arrive quickly!
  • Read all of the abstracts prior to the session
  • Have questions ready for speakers during the oral presentation session
  • Keep an eye on the time for oral presentations and communicate with EGU staff (it may be their first day too!)
  • Enjoy the experience!

Emily Bamber is a 2nd year PhD student at the University of Manchester in the UK. She studies eruption dynamics at basaltic systems, through petrological studies of natural products collected in the field.

Twitter: @EmilyCBamber