Volcanic Glass: Frozen Mantle Geochemistry in Iceland

Summer time in research groups usually means two things: conferences and field work.  For me this summer it was field work and I had my first postdoctoral research trip to Iceland. Along with myself was Margaret Hartley a lecturer in Earth Sciences at University of Manchester (my boss) and Emma Waters our new PhD student starting in September of this year.

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Fig 1: Myself stood in front of the subglacial volcano Herðubreið in the NE of Iceland

Last year I finished my PhD project at Manchester trying to determine the eruption ages of subglacial volcanoes in Iceland and have now started a new Icelandic based research project as a postdoc.  Instead of being focussed on helium a light noble gas to determine eruption ages, I am using krypton and xenon, the heaviest noble gases.  These noble gases are used to determine the amount of volatiles (water, carbon dioxide, nitrogen, sulphur, halogens and noble gases) present in the mantle. Specifically, those that have existed since the Earth formed and those that have been recycled by tectonic processes (subduction zones, spreading ridges etc).  Iceland is a great place to study mantle processes as Iceland is literally a living laboratory.  Iceland has not only a hot spot of upwelling magma from the lower mantle but a spreading ridge that taps into the melting of the upper mantle.

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Fig. 2: Large block of basaltic glass found at Kalfstindar in the SW of Iceland

The best type of sample needed to analyse heavy noble gases and halogens is basaltic glass (shiny dark extrusive volcanic rock – see Fig.2).  This is what is produced when a basaltic eruption meets ice and cools extremely fast, hence its glassy appearance.  The best place to find basaltic glass is near the base of a volcanic edifice on the rim of pillows (see Fig. 4).  Pillows are formed when an eruption is under high pressure, in this case, caused by the overlying ice.  A lot of the time the volcanic edifice you want to sample is covered in scree from erosion and you have to climb up to wherever a decent outcrop may be.  This is where you must channel your inner mountain goat!

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Fig. 3: Outcrop on the side of Maelisfell in the SW of Iceland just south of Þingvallavatn

Another great place to find glass fragments is throughout hyaloclastite.  This is where the volcanic eruption has penetrated through the ice and meets melt-water.  The overlying melt-water is significantly lower in pressure than when erupting under ice and so the eruption changes from forming pillow lavas to a very fine-grained, cemented, and often sandy coloured layer above the pillows (see slightly yellow-ish rock protruding behind me in Fig.1).

Fig. 4: Some glassy pillow rims at an outcrop South of Hitarvatn on the Snaefellsnes peninsula

Picking glassy fragments out of the hyaloclastite can be a lot easier than hammering the rims off of pillows and so can speed up the sampling process.  Glass from pillows compared to glass from the hyaloclastite layer may have varying geochemistry and volatile signatures and so this is something I plan on testing in the lab. Now that I am back in Manchester the next step is to take the samples of glass I have collected in Iceland and clean, sieve and pick out as much glass as possible for noble gas and halogen analysis.  Keep an eye out on a future blog based on the specifics of using the Helix-MC!

Iceland is one of the best places to do field work (I’m not biased… much) as not only is it an amazingly beautiful location, especially if you enjoy geology, you have to be aware that there still are active volcanoes threatening to erupt. As well as dodging sheep on the roads, having your truck break down and looking out for movies and TV shows being filmed, you have to keep your wits about you in Iceland.

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Fig. 5: Pahoehoe sheet flow inflation cracks in the NE of Iceland

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One Response to Volcanic Glass: Frozen Mantle Geochemistry in Iceland

  1. Leslie Street says:

    Katie, thanks for sharing this exciting update. It is great to learn what you do on your holidays, sorry I mean field trips.

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