Magma decompression rates during explosive eruptions of Kīlauea volcano, Hawaii, recorded by melt embayments

Abstract

The decompression rate of magma as it ascends during volcanic eruptions is an important but poorly constrained parameter that controls many of the processes that influence eruptive behaviour. In this study we quantify decompression rates for basaltic magmas using volatile diffusion in olivine-hosted melt tubes (embayments) for three contrasting eruptions of Kīlauea volcano, Hawai'i. Incomplete exsolution of H 2 O, CO 2 and S from the embayment melts during eruptive ascent creates diffusion profiles that can be measured using microanalytical techniques, and then modelled to infer the average decompression rate. We obtain average rates of ~0.05-0.45 MPa s -1 for eruptions ranging from Hawaiian style fountains to basaltic subplinian, with the more intense eruptions having higher rates. The ascent timescales for these magmas vary from around ~5 to ~36 minutes from depths of ~2 to ~4 km respectively. Decompression-exsolution models based on the embayment data also allow for an estimate of the mass fraction of pre-existing exsolved volatiles within the magma body. In the eruptions studied this varies from 0.1-3.2 wt%, but does not appear to be the key control on eruptive intensity. Our results do not support a direct link between the concentration of pre-eruptive volatiles and eruptive intensity; rather they suggest that for these eruptions decompression rates are proportional to independent estimates of mass discharge rate. Although the intensity of eruptions is defined by the discharge rate, based on the currently available dataset of embayment analyses it does not appear to scale linearly with average decompression rate. This study demonstrates the utility of the embayment method for providing quantitative constraints on magma ascent during explosive basaltic eruptions.

Melt inclusion constraints on petrogenesis of the 2014–2015 Holuhraun eruption, Iceland

“…However, the rate of 0.75 m s −1 is based on the outgassing of volatile radionuclides upon early CO 2 loss at depth within the magmatic system, and thus represents a different stage of magma ascent than has been considered in our calculation. Our calculated ascent rates are slower than the 0.05-0.45 MPa s −1 (~ 1.7-15 m s −1 ) obtained for various eruptions at Kilauea, Hawaii (Ferguson et al 2016), but our higher estimates are comparable with ascent rates of ~ 0.2-2.0 m s −1 modelled for Stromboli (e.g., Pichavant et al 2013). …”

“…Closed-system degassing has been argued to be significant in basaltic magmatic systems (e.g., Allard et al 2005;Lloyd et al 2014;Ferguson et al 2016). Volatile saturation models (Newman and Lowenstern 2002;Witham et al 2012) predict that the Holuhraun carrier liquid becomes water-saturated at pressures between ~ 1.0 and 0.4 kbar during closed-system degassing, with the variability arising from the choice of initial magmatic CO 2 content and hence the mass percent of pre-existing exsolved vapour present.…”

Evidences of Plug Pressurization Enhancing Magma Fragmentation During the September 2016 Basaltic Eruption at Piton de la Fournaise (La Réunion Island, France)

“…Using a N V of 9.0 × 10 3 mm -3 , we found an estimated decompression rate of 0.14 MPa s -1 , equivalent to a magmatic ascent rate of around 4 m s -1 . This value is consistent with typical decompression rates of basaltic eruptions (Ferguson et al, 2016;Shea, 2017) and similar to those calculated for the initial stages of the June 2014 eruption (0.15 MPa s -1 ), but it is higher than the initial stage of the July 2015 eruption (0.08 MPa s -1 ), which did not produce significant golden pumice and pre-eruptive nucleation (Thivet et al submitted). Considering the estimated minimum depth of the magma reservoir and the decompression rate associated with the golden pumice material, the duration of the magma propagation from the reservoir to the surface should be at least 60 s.…”

Diffusive exchange of trace elements between alkaline melts: Implications for element fractionation and timescale estimations during magma mixing

“…Demouchy et al, 2006;Chakraborty, 2008;Costa et al, 2008, Faak et al, 2014 and melt inclusions (e.g. Ferguson et al, 2016), as well as the use of rocks showing evidence of magma mixing as volcanic chronometers (e.g. Perugini et al, 2015, Rossi et al, 2017.…”