Using dissolved H2O in rhyolitic glasses to estimate palaeo-ice thickness during a subglacial eruption at Bláhnúkur (Torfajökull, Iceland)

Abstract

The last decade has seen the refinement of a technique for reconstructing palaeo-ice thicknesses based on using the retained H 2 O and CO 2 content in glassy eruptive deposits to infer quenching pressures and therefore ice thicknesses. The method is here applied to Bláhnúkur, a subglacially erupted rhyolitic edifice in Iceland. A decrease in water content from ∼0.7 wt.% at the base to ∼0.3 wt.% at the top of the edifice suggests that the ice was 400 m thick at the time of the eruption. As Bláhnúkur rises 350 m above the surrounding terrain, this implies that the eruption occurred entirely within ice, which corroborates evidence obtained from earlier lithofacies studies. This paper presents the largest data set (40 samples) so far obtained for the retained volatile contents of deposits from a subglacial eruption. An important consequence is that it enables subtle but significant variations in water content to become evident. In particular, there are anomalous samples which are either water-rich (up to 1 wt.%) or water-poor (∼0.2 wt.%), with the former being interpreted as forming intrusively within hyaloclastite and the latter representing batches of magma that were volatile-poor prior to eruption. The large data set also provides further insights into the strengths and weaknesses of using volatiles to infer palaeo-ice thicknesses and highlights many of the uncertainties involved. By using examples from Bláhnúkur, the quantitative use of this technique is evaluated. However, the relative pressure conditions which have shed light on Bláhnúkur's eruption mechanisms and syn-eruptive glacier response show that, despite uncertainties in absolute values, the volatile approach can provide useful insight into the mechanisms of subglacial rhyolitic eruptions, which have never been observed.

Pre-eruptive volatile content, degassing paths and depressurisation explaining the transition in style at the subglacial rhyolitic eruption of Dalakvísl, South Iceland

“…Dixon and Clague, 2001;Tuffen et al, 2010). The same methodology was undertaken as with samples from Bláhnúkur in Owen et al (2012) where a detailed description of the analytical procedure can be found.…”

“…Various palaeo-pressure conditions were modelled based on the water solubility-pressure relationships given by VolatileCalc (Newman and Lowenstern, 2002). See Tuffen et al (2010) and Owen et al (2012) for more information on this technique.…”

“…Bulk sample vesicularities were measured using the bead displacement method described in Owen et al (2012), which is a modified technique from Stevenson (2005). Five repeat measurements were performed per sample.…”

“…Water speciation reveals three samples (D4, D11c and D14c) with elevated molecular water (H 2 O m ) relative to total water (H 2 O t ) which is diagnostic of post-quenching hydration (Denton et al, 2009). Furthermore, their speciation ratios are similar to samples considered to be hydrated from Bláhnúkur in Owen et al (2012). These samples have been removed from the database.…”

Transport and modification of glaciovolcanic glass from source to sink on Mars

“…Fig. 2) as an effective tool for inferring palaeo-ice thicknesses (Tuffen et al, 2010;Owen et al, 2012). Analyses by the CheMin XRD instrument on the NASA Mars Curiosity rover indicated that 27±14% of the local regolith was composed of amorphous component, indicative of basaltic volcanic glass (Bish et al, 2013).…”