A microanalytical oxygen isotopic and U-Th geochronologic investigation and modeling of rhyolite petrogenesis at the Krafla Central Volcano, Iceland

Conduit formation and crustal microxenolith entrainment in a basaltic fissure eruption: Observations from Thríhnúkagígur Volcano, Iceland

“…Thríhnúkagígur is exceptional in that a significant quantity of microxenolithic material (~10 % by volume) was entrained into the ascending magma, and one-of-a-kind in that the source of this scoured material is accessible for direct observation in a cave system directly underlying the vent. Elsewhere in Iceland, a similar proportion of assimilated hydrothermally altered crust has been invoked to explain O, B, and Sr isotope ratios in pristine basalts and melt inclusions [Gee et al 1998;Bindeman et al 2008;Brounce et al 2012;Hampton et al 2021]. Thríhnúkagígur highlights the challenges to identifying cryptic crustal contamination of basaltic magmas as the tephra and dike end-member compositions are insufficiently distinct from one another to produce clear mixing trends in the erupted lava compositions, even though entrained tephra is observed in thin section.…”

“…In contrast to these shallow magma-country rock interactions which primarily facilitate rapid mechanical incorporation of crustal material during magma transport, crustal assimilation occurs during magma storage over longer timescales. In longer-lived magmatic systems with persistent crustal reservoirs in Iceland, assimilated crust may account for as much as 10-30 % of erupted volumes of basalt, such as in the Grímsvötn-Laki system [Bindeman et al 2008;Brounce et al 2012] and at Krafla [Nicholson et al 1991;Hampton et al 2021]. Low-density and mechanically weak lithologies are common within the Icelandic crust on multiple scales from individual cinder cones and tephra layers to hyaloclastite units, including on the Reykjanes Peninsula [Marks et al 2010;.…”

In situ probing of the present-day zircon-bearing magma chamber at Krafla, Northeastern Iceland

“…The low-δ 18 O character of Icelandic felsic magmas suggests generation by open-system processes of partial melting of the hydrothermally-altered basaltic crust and melt-fluid-rock interactions (Jónasson, 1994;Gunnarsson et al, 1998;Bindeman et al, 2012;Pope et al, 2013). Other petrogenetic processes involved in the formation of felsic magmas include crustal assimilation coupled with fractional crystallization, and near-solidus differentiation (Nicholson et al, 1991;Jónasson, 2007;Elders et al, 2011;Pope et al, 2013;Hampton et al, 2021). Furthermore, magma mixing and the following hybridization have been documented and acted as a potential trigger for volcanic eruptions (e.g., Borisova et al, 2012 and references therein).…”

“…Natural data and experiments on crystallization of Icelandic glassy rhyolite and finely-crystallized (≤1 mm) granophyre partial melting demonstrate efficient partial melting of the felsic crust (e.g., Masotta et al, 2018), and efficient assimilation of hydrothermally altered crust (Hampton et al, 2021). Masotta et al (2018) suggest that the IDDP-1 (Figure 1) rhyolite magma could originate from a high-degree partial melting of felsic rocks rich in quartz and feldspars by a basaltic intrusive event.…”