After a long day of travelling, I’ve finally arrived in St Lucia!
Before I start showing off photos of the Caribbean and various volcanic outcrops, I figured it would be good to outline what I’m doing out here and why.
I’ll be starting with some fieldwork here in St Lucia, then moving on to La Soufriere volcano on St Vincent, with the aim of collecting as many cumulates (the rocks mentioned in my last post) as possible! We already have a large cumulate sample collection from the Lesser Antilles, but nearly all were picked up loose from river beds, canyons, etc, which means that we have no idea how old they are or which eruptions they came from. My plan is to collect cumulates and blocks of host lava from outcrops of pyroclastic flow deposits, of which we know the age.
By doing this, we can study the relationship between the cumulates and the lavas that erupted them. Many of the cumulate samples contain zones of interstitial melt between the crystals, like the one in the thin section below (from Martinique, another Lesser Antilles island in my study).
Comparing the chemical composition of the host lava and melt in the cumulates can allow us to understand how the cumulates and final erupted lavas are related. If the compositions are the same, it may suggest that the host lava was generated in a cumulate rich zone in the crust. If different, this may indicate that there are magmas existing in the crust that we don’t see at the surface, providing an insight into the complex nature of the magma plumbing system below the Antilles.
Another aspect of the trip will be to collect mafic (low silica, high magnesium content) lavas from northern St Lucia and St Vincent. These tend to be less affected by processes such as assimilation of crustal material, hence they can be used to determine the 87Sr/86Sr composition of less contaminated, more primitive magmas in the arc. If the cumulate plagioclase ends up showing much higher 87Sr/86Sr ratios than these “less contaminated” magmas this will provide strong evidence for the assimilation of arc crust hypothesis (see last post for details).
Once again, feel free to leave any questions or comments, and I look forward to posting some exciting photos from the Lesser Antilles in the next couple of days!