The use of TGA-MS in the Analysis of Perlites and Subglacially Erupted
Volcanic Glass From Iceland
Jo Denton (Department of Environmental Science, Lancaster University, Lancaster
LA1 4YQ; ph +44 (0)1524 593975; e-mail j.denton1@lancaster.ac.uk); [*Hugh
Tuffen*](Department of Environmental Science, Lancaster University, Lancaster LA1
4YQ; ph +44 (0)1524 593571; e-mail h.tuffen@lancaster.ac.uk); Jennie S Gilbert
(Department of Environmental Science, Lancaster Universtiy, Lancaster LA1 4YQ;
ph +44 (0)1524 593022; e-mail J.S.Gilbert@lancaster.ac.uk)
Perlitised lavas are commonly found in subglacially erupted rhyolitic deposits. Perlite
is silicic glass that contains abundant, intersecting, arcuate and gently curved cracks
surrounding cores of intact glass, generally less than a few millimetres across
[McPhie et al., 1993]. Knowledge of how perlite forms is required for us to better
understand how meltwater and lava interact during and after subglacial eruptions.
Perlite formation is currently poorly understood and there are two contrasting models
detailed in the literature. Friedman et al. [1966] state that diffusion of water into the
glass modifies/increases the volume of the material. The resultant stresses generate
the cracks. Marshall [1961] argues that the cracks appear first and are a mechanical
effect of the rapid cooling of the material. These cracks act as conduits through which
water can enter from the environment and hydrate the glass. The formation of perlite
is of interest to many branches of subglacial volcanism, for example, the interaction
of meltwater with magma as well as the potential use of degassing patterns as a
palaeo-climatic indicator. In this project, analysis of perlitic samples using TGA-MS
will be combined with a study of perlitic textures to shed new light on how perlite is
generated.
A thermogravimetric analyser (TGA) allows the weight of a sample to be measured as
a function of temperature while being subjected to a controlled heating program. The
addition of a mass spectrometer (MS) allows the user to identify the volatiles being
exsolved at specific temperatures/temperature ranges. TGA analyses at Lancaster
University can be carried out on samples that weigh up to 100 mg over a temperature
range of 0-1500 ºC. The balance sensitivity is 0.1 µg.
The TGA-MS has been used previously by the authors to successfully establish a
suitable heating program through which all the volatiles from a sample are driven off.
Preliminary results are presented of analyses of perlitised rhyolitic glass from
Bláhnúkur, Iceland. The temperature at which the volatile species are exsolved and
the degassing intensity will reflect the original thermal history of the material and the
relative timing of cooling and fracturing of the lava.
Results are also presented of analyses of a lava lobe, from Bláhnúkur, which contains
discrete textural zones. These zones range from central microcrystalline rhyolite to an
obsidian carapace and reflect different cooling rates, degassing patterns and/or
varying degrees of meteoric water ingress. The lobe has been described in detail in
Tuffen et al. [2001; 2002]. The degassing patterns (e.g. temperature of water
exsolution) produced by TGA-MS analysis reveal information on the structure of the
glass which, in turn, reflects the temperature and overlying pressure of the eruptive
environment. This information will address issues surrounding the degassing and
emplacement of subglacial rhyolite lavas.
References
Friedman, I., Smith, R.L., Long, W.D., 1966. Hydration of natural glass and
formation of perlite. Geol Soc Am Bull 77:323-327.
Marshall, R.R., 1961. Devitrification of natural glass. Geol Soc Am Bull 72:14931520.
McPhie, J., Doyle, M., Allen, R.L., 1993. Volcanic Textures: A guide to the
interpretation of textures in volcanic rocks. University of Tasmania. Centre for Ore
Deposit and Exploration Studies.
Tuffen, H., Gilbert, J., McGarvie, D., 2001. Products of an effusive subglacial rhyolite
eruption: Bláhnúkur, Torfajökull, Iceland. Bull Volcanol 63:179-190.
Tuffen, H., Pinkerton, H., McGarvie, D.W., Gilbert, J.S., 2002. Melting of the glacier
base during a small-volume subglacial rhyolite eruption: evidence from Bláhnúkur,
Iceland. Sed Geol 149:183-198.
POSTER
CORRESPONDING AUTHOR: H TUFFEN, FIRST AUTHOR IS A STUDENT