Rifting and recharge as triggers of the mixed basalt-rhyolite Halarauður ignimbrite eruption (Krafla, Iceland)
Résumé
We present a petrologic study of the ca. 110 ka Halarauður eruption (7 ± 6 km3 magma), associated with collapse of Krafla caldera in northeast Iceland. Whole-rock compositions of juvenile Halarauður products span a continuous range between quartz tholeiite basalt (50.0 wt% SiO2, 5.0 wt% MgO; Mg# 42) and rhyolite (74.6 wt% SiO2). Linear correlations between all major elements are consistent with two-component mixing of sub-equal volumes of these end-member magmas, whereas correlations between trace elements are influenced by diffusive fractionation during chaotic mixing. Evolved compositions (andesite to rhyolite) and compositional heterogeneity are typical of early-erupted units, reflecting tapping of the upper, more silicic regions of a compositionally heterogeneous reservoir undergoing chaotic mixing. Later-erupted deposits are more compositionally homogeneous and grade smoothly upward from andesite to basalt, reflecting tapping of denser hybrid magma and uncontaminated basalt from lower in the chamber. All erupted products host < 1-2 modal% macro-crysts, implying storage at near-liquidus temperatures. Geobarometry and MELTS modeling suggest shallow storage pressures of ~ 200 MPa (~ 8 km depth) for the quartz tholeiite. Plagioclase (An60-76) and augite (Mg# 68-75) macro-crysts crystallized from this basalt during shallow storage, while sparse glomerocrysts (plagioclase ± augite ± olivine ± orthopyroxene) in late-erupted basaltic material are derived from disaggregated cumulate mush and include more primitive compositions. Occasional narrow sodic rims on plagioclase crystals from the quartz tholeiite record short periods of re-equilibration with hybrid magmas during mixing, constrained by experimental growth rates as at most two months and possibly as short as tens of hours. A second population of calcic plagioclase (cores An83-91) with adhering primitive basaltic glass selvages (Mg# 53-59) occurs sparsely in deposits of the first eruptive phase and is scarce or absent in later-erupted units, providing evidence for eruption of a second, more primitive basalt that was of insufficient volume to skew whole-rock mixing trends. Nucleation delay models suggest that the absence of overgrowth rims or quench crystals in these glassy basaltic selvages reflect residence times of a few hours at rhyolitic temperatures before eruption. Short basalt-rhyolite mixing timescales reflect rapid destabilization of the magmatic system and triggering of the eruption by mafic recharge. The ascent of both primitive and evolved basaltic magmas from depth mirrors events in recent volcano-tectonic episodes in the north of Iceland, suggesting that mafic recharge was driven by a plate boundary rifting event.