Origin and dynamics of rhyolitic magmatism at Krafla Central Volcano (Iceland)

Abstract:

We studied rhyolites from seven eruptions spanning the full 190 k.y. history of rhyolitic volcanism at the basalt-dominated Krafla caldera. Macrocrysts in these rhyolites principally record growth from evolved melts and commonly host rhyolitic melt inclusions, but some cores are antecrystic. Crystal zoning patterns reflect an overall trend of cooling and fractional crystallisation and are not consistent rhyolite generation in by partial melting of altered basalts. Further, MELTS trace element models for this process provide a poor fit to observed compositions, instead implying a dominant role for AFC from basaltic parental melts. We thus propose that the Daly gap at Krafla reflects the presence of uneruptible intermediate mush bodies, from which silicic melts are extracted. Parallel REE patterns among all but the oldest rhyolite imply that the process of rhyolite genesis has been similar across much of the system’s lifespan. Distinct trace element characteristics for the oldest rhyolite imply a different source and possibly deeper fractionation prior to development of a long-lived shallow magmatic system. Three rhyolite ridges (each ~0.15 km³) erupted around the caldera margins at ca. 65 ka derive from a common magma body and reflect a distinct phase of post-caldera rhyolitic volcanism, possibly resulting from recharge after caldera collapse at ca. 110 ka. Rhyolitic eruptions since ca. 24 ka have been focused in the central-eastern region of the caldera and were small (<0.02 km³). Products from this period are either aphyric or show textual evidence for basalt-rhyolite mixing. Basalt-rhyolite interactions in the C.E. 1724 Víti eruption were syn-eruptive. Compositional overlap between Víti pumices and quenched rhyolite magma sampled during drilling suggests that the currently active magma body was previously tapped in the Víti event. Rapid basalt triggering of the Víti eruption raises the possibility of similar explosive eruptions triggered by ascending basalt.