The Origin of Rhyolitic Magmas at Krafla Central Volcano (Iceland)

Abstract

We present a detailed petrologic study of rhyolites from seven eruptions spanning the full (∼190 k.y.) history of rhyolitic volcanism at Krafla volcano, northeast Iceland. The eruptions vary widely in size and style, but all rhyolites are crystal-poor (<6 modal%: plagioclase + augite ± pigeonite ± orthopyroxene ± titanomagnetite ± fayalite) and have similar evolved compositions (73.7–75.8 wt% normalised whole-rock SiO2) and trace element patterns. Macrocryst rim compositions from each eruption cluster within a narrow range and are appropriate for equilibrium with their carrier melt. Crystal cores and interiors display complex growth patterns and commonly host resorption surfaces, but compositional variations are slight (e.g., typically <10 mol% An for plagioclase, Mg# <10 for pyroxene), and consistent with an overall trend of cooling and differentiation by crystal fractionation. Although most crystal core and interior compositions are broadly appropriate for equilibrium with melts similar to their host whole-rock, variable growth histories, juxtaposition of grains with distinct trace element compositions, and scatter in melt inclusion compositions imply mixing of antecrysts from compositionally similar evolved melts and/or assimilated felsic mush or intrusions before final rim growth. Evidence for mafic recharge (e.g., coupled increases in An and Fe in plagioclase) is absent in most crystals; rhyolite storage and fractionation thus occurred largely in isolation from the underlying mafic system. Comparison of observed matrix glass compositions with published experimental work on melting of altered (meta)basalts casts doubt on prior models favouring rhyolite generation by partial melting of hydrothermally altered basalts, instead supporting recent isotopic and modeling arguments for a crystallisation-driven process [Hampton, R. L. et al. (2021). Journal of Volcanology and Geothermal Research 414, 107229]. MELTS fractional crystallisation and assimilation-fractional crystallisation (AFC) models at 1 kbar predict liquid major and trace element compositions similar to Krafla rhyolites after ∼60–70 vol% crystallisation of a quartz tholeiite melt representative of the evolved crystal-poor basalts commonly erupted within Krafla caldera. We thus suggest that stalling and crystallisation of these evolved basalts at shallow depth forms crystal mushes from which evolved (broadly dacitic to rhyolitic) melts are extracted. These melts ascend and mix with other compositionally similar melt bodies and/or assimilate felsic intrusive material in the uppermost crust. The Daly gap between ∼57–71 wt% SiO2 at Krafla is consistent with preferential extraction of evolved melts from quartz tholeiite mushes in the ∼50–70% crystallinity window. Residual solid (cumulate) compositions predicted by MELTS are exclusively mafic, hence efficient silicic melt extraction from quartz tholeiite mushes may also explain the apparent compositional bimodality in some Icelandic plutonic suites.

Explosive Activity on Kīlauea's Lower East Rift Zone Fueled by a Volatile‐Rich, Dacitic Melt

“…Rhyolites comprise approximately 10% of erupted lavas in Iceland (Jónasson, 2007), and have been noted in a variety of settings among mid-oceanic ridges (Wanless et al, 2010). While the continuum of melt inclusion major element contents from basalt to dacite, and good fit of major, volatile and trace element contents to MELTS fractional crystallization models indicate that Kīlauea dacites result from extensive fractional crystallization, silicic magma occurrences in Iceland and MOR have been variably attributed to fractional crystallization, crystal-melt segregation, and partial melting of hydrothermally altered crust based on isotopic, volatile, major and trace element data (Elders et al, 2011;Geist et al, 2021;Masotta et al, 2018;Rooyakkers, Stix, Berlo, Petrelli, Hampton, et al, 2021;Wanless et al, 2010).…”

Geochemistry, Magmatic Processes and Timescales of Recent Rhyolitic Eruptives of the Ōkataina Volcanic Centre, Taupō Volcanic Zone, Aotearoa/New Zealand

“…The large margin of error on the equilibrium KD(Fe-Mg) opx-liq value of 0.21 ± 0.09 (Rooyakkers et al 2021; which, similarly to En, can be consistent for compositions which differ in other aspects.…”

“…The idea that most cores are antecrystic and derived from a heterogeneous mush is further supported by the equilibrium tests (Putirka 2008;Rooyakkers et al 2021;Table 4.1). While the large margin of error suggests nearly all core compositions are in equilibrium with the wholerock (approximate proto-melt) compositions of their eruptives (Figure 4.10), there is a much wider spread of compositions than in the rims (Figure 4.11).…”

“…This is independent of pressure or temperature, but does vary according to XSi (the cation fraction of SiO2 in the equilibrium liquid) with the relationship KD(Fe-Mg) opxliq = 0.4805-0.3733XSi liq (Putirka 2008). However, as noted by Rooyakkers et al (2021), this KD value is derived mainly from data from mafic sources. Rooyakkers et al ( 2021) compiled rhyolitic experimental data (Klimm et al 2003(Klimm et al , 2008Castro et al 2013;Huang et al 2019) to determine a rhyolite-appropriate equilibrium KD(Fe-Mg) opx-liq value of 0.21 ± 0.09.…”

“…However, it was carried out to assess which of the diverse core compositions were in equilibrium with the erupted whole-rock (i.e. the approximate proto-melt) composition (Figure (after Putirka 2008;Rooyakkers et al 2021) between rim compositions and the melt phases of each magma, showing all rims are in equilibrium with the final (erupted) melt compositions.…”