Day 6 – conquering La Soufriere

Day 6 started earlier than normal, as we were required at the foot of the trail up La Soufriere, St Vincent’s active volcano, at 8 in the morning. Our guide Brace (from the Soufriere Monitoring Unit) rocked up slightly late, with only trainers for footwear! (Which later proved to have much better grip than my boots….). After a short drive through the banana plantations to the foot of the volcano, we began our slow and sticky climb.

The path proved to be surprisingly well maintained, and the cloud cover sheltered us from the worst of the sun. We made rapid progress, pausing for a break just over halfway up at the valley shown below.

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River valley halfway up La Soufriere, with exposed older lava flows

Brace was eager to point out that we would be surprised by the size of the crater and lava dome that awaited us at the top, and he was definitely right! The lava dome that takes up most of the 1.6 km diameter crater is over 100m high, and on one side shows yellow sulphur deposits from recent degassing. Once a year, the Soufriere Monitoring Unit staff descend into the crater to make gas measurements, though the tricky and long path down meant that we weren’t able to access the crater floor on the day.

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The majestic crater and lava dome at the summit of La Soufriere

On our way down, we stopped to collect even more samples from the 1902 eruption, from well exposed deposits in ravines next to the trail. These yielded even more cumulates with attached host lava, and another monster cumulate almost equalling “King Vincy” in size! We were also able to get in situ samples from what we believe to be the 1580 eruption (from stratigraphic logs), adding another unit from the volcano’s history to the study.

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Looking windswept at the summit

Heading back, we stopped by a tempting looking roadcut through the Yellow Tephra formation, and within seconds found several useful samples in an extremely cumulate rich layer. This took us to a grand total of 100 samples for the trip! This means that we now have cumulate and juvenile magma samples from 4 different units from La Soufriere, as well as a range of mafic magmas erupted on the island. After a long evening session of more describing, packing and taping up boxes of rock, we were ready to send our new cumulate collection back to Durham.

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Another sunset, this time viewed from our villa balcony.

Day 5 – how do we get the rocks home?

After our ridiculously successful day of sampling, we decided to spend Monday trying to find out the best and cheapest way to post the rock samples back to the UK. We started by visiting our local contact Kemron, part of the Soufriere Monitoring Unit who are responsible for the monitoring at La Soufriere volcano. He quickly arranged us a guide for our climb up the volcano the following day, and pointed us in the direction of a local shipping agent.

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An enormous pile of cumulates ready for inspection, and posting

A quick stop at the post office on the way confirmed that the cost of air mail would be ridiculous, so we headed for the shipping company, only to be informed that the process was rather complicated and required us to get a customs broker on top of filling out several forms. Kemron came to the rescue, using his talent for having the phone number of literally anyone in St Vincent you might need, and sorted things for us with a small local shipping company.

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Sunset from Indian Bay, St Vincent

After a wander round Kingstown, St Vincents capital, for lunch, we returned to the villa for a mammoth session of describing the many samples collected the day before. It was at this point I realised just how variable and interesting the various cumulates were – far too much to work on in one PhD! We rewarded ourselves for our efforts with a warm end of day swim while viewing the spectacular sunset at Indian Bay.

Day 4 – too many rocks!

Our first full day in St Vincent began in a dried up river valley known as the Dry Rabacca, thanks to a tip off from Dr Paul Cole at VMSG in January. Little did we know that we would spend the next 3 and a half hours in this cumulate hunting paradise! A simple look at the floor within a few metres of the entrance led to a pile of cumulate finds, and exposures from the 1902 eruption of La Soufriere volcano at the edges of the valley immediately provided in situ cumulate samples.

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Contemplating the vast deposits of the 1902 eruption (photo credit George Cooper)

The finds kept on coming, including incredibly useful cumulates with their host lava attached and unusual green lithics interpreted as mafic hornfels (which could represent crustal material assimilated below the arc!) By far the best find was a very large and heavy, approximately 20cm across in situ cumulate, “King Vincy” which contains varying crystal textures. I’ll add more info on this once I return from the field and have time to study it in detail. After 3 hours, we had found so many potential samples that we had to be selective and whittle them down to those with host lava attached or unusual textures/compositions.

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A happy looking George holding up “King Vincy”, by far the largest cumulate we’ve sampled

After a long and successful morning, we stopped for lunch and a relaxing swim at Owia salt pond in the north of the island. A glance at the geological map revealed that the mafic looking rocks around the salt pond were in fact lava flows from flank eruptions at La Soufriere. A casual look around whilst swimming and eating led to us discovering cumulates in the mafic lava flows! Of course we took the opportunity to sample these and their host lavas, which don’t appear to have been geochemically analysed before. Their age is currently unclear, though some amateur local geologists we met on the beach were convinced that they were formed during the main 1902 event.

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Owia salt pond
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A cumulate within the mafic lavas at Owia salt pond

We ended the day by collecting one last mafic lava sample, from the basalts exposed at Big Sand Bay, before heading back to the villa to enjoy the sunset and catalogue the days finds – 35 samples in total (more than we collected in 2 days in St Lucia)!

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The sunset view from the balcony, with “King Vincy” proudly displayed on the table

 

 

 

 

 

 

Day 3 – St Vincent time!

With our work on St Lucia done, we made our way bright and early to the airport for our early flight to the next target island, St Vincent. After a quick stop at the airport beach we boarded our mere 20 minute flight and arrived at our new accommodation overlooking the sea on the south coast of St Vincent.

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Vigie beach next to St Lucia airport

We decided to spend the afternoon at Black Point, the site of the most mafic basalt in the Lesser Antilles arc. A trip through the tunnel on the headland, previously used to transport sugar, led to a spectacular exposure of the basaltic lava flows and deposits from older explosive eruptions. The lowermost unit in the sequence consists of multiple pyroclastic flow deposits separated by thin ashy horizons, followed by the Yellow Tephra formation (believed to have been erupted between 3600 and 4500 years bp) and finally the mafic basalt lava flow.

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Labelled are the formations exposed at Black Point, St Vincent, produced by the earlier stages of the island’s volcanic activity.

As well as sampling the basalts, which may represent magmas from near-mantle depths below St Vincent, we were able to collect several in situ cumulates and blocks of juvenile magma from the pyroclastic deposits. Majority of the cumulates found were of the “salt and pepper” type, consisting of white plagioclase and black amphibole crystals of similar sizes. Most of these cumulates also contained a small amount of olivine. These samples could potentially represent “non cumulate gabbros” – frozen equivalents of the final erupted magmas. Comparing the chemistry of the plagioclase in the cumulates and juvenile magmas from the same deposits will allow me to test this.

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Taking field notes at Black Point

After another successful day of sampling (and avoiding being forced into buying coconuts from enthusiastic local salesmen) we retreated to the villa for a well deserved glass of local rum.

 

St Lucia day 2 – into the Piton jungle

Day 2 saw us head to the south-west side of St Lucia, towards the town of Soufriere and the towering Pitons, two ancient volcanic plugs that dominate the landscape.

The area around Soufriere and the Pitons is mostly jungle, making finding outcrops (especially cumulate bearing ones!) rather difficult. We began our search in some road cuts en route to the town of Soufriere, through some massive pyroclastic density current deposits from the Soufriere Volcanic Centre (the most recent phase of activity on the island). Unfortunately our morning of searching yielded no cumulates from any of these outcrops – at this point we realised how lucky we were to have found so many on day 1!

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Gros Piton, viewed from our roadside lunch stop

We stopped for lunch by the pitons, before heading on to the Belfond quarry, the site of a Soufriere Volcanic Centre dacite lava dome. Here we were much more successful, as the quarry had recently excavated more of the dome, leaving rock piles full of cumulates and cumulate like mushy material. Our samples here included “mafic crystal clots” (zones of interlocking crystals dominated by mafic minerals such as amphibole and pyroxene) included in the dacite magma, plagioclase and amphibole dominated cumulates with aligned minerals, and enclaves of mafic magma within the felsic host dacite. The presence of such enclaves and mafic crystal clots may indicate that mixing of mafic and felsic magmas occurred before the eruption of the dacitic done-forming magma. It wasn’t all plain sailing however, as we were forced to retreat to the car several times due to torrential rain!

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George looking unhappy after getting drenched for the second time by a tropical downpour!

The remainder of the afternoon was spent driving around the jungle below the Pitons on steep winding roads hunting for small roadcut outcrops. “Rachel” (our hired Kia Rio) did well to make it up the steep slopes to get us to the localities, but the heavily weathered outcrops meant that we were unable to find any more useful samples. We ended the day with a seafood dinner in Marigot Bay, before reflecting on a successful couple of days sampling in St Lucia.

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Sunset over the Pitons

The next few days will be spent on St Vincent, where we plan to collect even more cumulates, the most mafic lavas in the Lesser Antilles, and climb to the crater of La Soufriere volcano.

 

 

 

St Lucia day 1 – smuggling cumulates

I’m glad to report that the first day of sampling on St Lucia was a great success – in fact, far more successful than we expected!

After enjoying our breakfast on the terrace, George (my co-supervisor and companion for the trip) and I struggled through the noisy Caribbean traffic towards the northernmost part of the island. Our aim was to collect samples of mafic lavas from the first stage of the island’s formation, estimated to be around 15-18 million years ago, and to search these outcrops for any cumulates they may contain.

Our first port of call was the well exposed sections below Fort Rodney (locality shown below). Here we collected two mafic lava samples, from the lower pyroxene containing flows and the uppermost amphibole and plagioclase rich flows. A walk further west then yielded the first cumulate samples of the trip! These were found in unusual reworked volcaniclastic deposits that appear to be interspersed with the mafic lavas. These deposits may represent reworked material from explosive eruptions occurring at the same time as the effusive magmatism.

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After rewarding ourselves with lunch by the sea, we took a detour to look at the outcrops on either side of the small bay known as Smugglers Cove (see below, photo looking towards the northern side of the bay). The rocks here are mostly volcaniclastic and conglomerate deposits, from the same initial phase of the island’s formation.

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On closer inspection, these were found to contain an amazing amount of cumulates, leading us to collect the hoard in the following photo.

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The cumulates found were typical of the Lesser Antilles, dominated by amphibole and plagioclase crystals, though a couple also contained olivine or pyroxene as an additional phase. One of the lower volcaniclastic layers on the northern side of the bay contained so many cumulates it was impossible to sample them all – you can see three in the photo below! We are hoping that these samples will provide useful information about the processes occurring in the crust prior to eruptions that contributed to the island’s formation, and how these differ to later events on St Lucia.

LOC 3 NORTH CUMULATES

The day was rounded off successfully by finding cumulates within the lava flow units exposed at the southern end of Rodney Bay, which will allow us to study the relationship between them and their host magmas on St Lucia.

That’s all for day 1, here’s hoping that tomorrows locales down near the scenic Pitons can live up to today’s finds!

 

Hello from St Lucia!

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).

 

thin section with melt zone

 

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!

 

A bit about me and the PhD….

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).

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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.

sub zone

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.

plag thin section zoned

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…..)!