I’m Josh, a first year IAPETUS DTP funded PhD student at Durham University, studying Caribbean volcanoes!
I’ll be using this blog to document the progress of my PhD, starting with my exotic fieldwork in St Lucia and St Vincent, followed by lab work, conference visits, future fieldtrips, etc, all with the aim of showing just how interesting and important it is to study volcanoes.
The project aims to understand the processes occurring deep below volcanoes in the Lesser Antilles arc in the Caribbean. It is being run in collaboration with the NERC funded VOiLA project (www.voila.ac.uk).
More specifically, I plan to study strontium isotope compositions in the magmas from these volcanoes. Strontium atoms can be produced by radioactive decay of rubidium atoms, forming strontium atoms with a mass number of 87 (87Sr). However, strontium atoms with a mass number of 86 (86Sr) are not a radioactive decay product, hence the amount of 86Sr on Earth remains constant over time. The ratio of these different isotopes of strontium (87Sr/86Sr) can be measured in volcanic rocks and is affected by the processes that generate magmas at subduction zones like the Lesser Antilles.
Volcanic rocks that come directly from the mantle tend to have low 87Sr/86Sr ratios, while sedimentary and crustal rocks have high 87Sr/86Sr ratios (shown on the subduction zone diagram below). Many of the lavas erupted in the Lesser Antilles have high 87Sr/86Sr, suggesting that crustal or sedimentary material is added to the magmas that form by melting of the mantle below the arc. The question my project aims to answer is whether these high 87Sr/86Sr signatures are caused by addition of subducting sediment to the mantle wedge, or by assimilation of arc crust as the magmas ascend.
To do this I’ll be looking at cumulate rocks – these form in the crust by accumulation of crystals that settle out of cooling magma bodies. Sometimes, these cumulates are entrained in rising magmas and erupted, allowing us to sample them. By studying the crystals in these samples, we can understand what is happening in the crust, or “magma plumbing system” below the volcano that erupted them.
The image below is from a thin section (a 30 micron thick slice of cumulate rock) which shows that some plagioclase crystals have zoning (see below). These zones exist due to differences in the chemical composition of different parts of the crystal, which records the composition of the magmas that it grew from.
The darker central zones, or “cores” were likely formed earlier and deeper in the crust, with the paler outer zones (“rims”) added by growth from a later magma with a different composition. The aim is to measure the 87Sr/86Sr ratio in the different zones of the plagioclase. If the “cores” have high 87Sr/86Sr, this suggests that subducting sediments are the main contributor to the high ratio in the erupted magmas. However, if the “rims” have higher 87Sr/86Sr than the cores, it is likely that the rising magma has assimilated arc crust, producing the higher 87Sr/86Sr ratio magma from which the rims formed.
So why attempt to understand these processes? Eruptions in the Lesser Antilles tend to be very explosive and fuelled by volatiles in the rising arc magmas. The arc crust can be a source for these volatiles, hence it is important to know how the magma interacts with the crust in order to understand where and how these explosive magmas are generated.
That’s a brief (and hopefully not too complicated!) overview of the aims of the project. Feel free to send questions or comments, either on here or on social media (links at the side).
I’ll be following this up with a post about the aims of the fieldwork, then updating regularly from the field in the Caribbean over the next couple of weeks (good internet connection permitting…..)!