BIRTH OF SILICIC MAGMAS AT EYJAFJALLAJöKULL VOLCANO, ICELAND

Large volume of silicic magmas in Iceland has attracted the attention of earth scientists over the last 150 years. Their origin is still debated but recently measured isotope ratios put firm constraints on their petroenesis. During post-glacial time, volcanoes in the rift-zones of Iceland (high geothermal gradient) produce dacites and rhyolites with significantly lower ¹⁸O/¹⁶O than contemporaneously erupted basaltic magmas. These silicic magmas also have respectively lower and higher Th and B isotope ratios than the basalts that suggest partial melting of hydrothermally altered basaltic crust. In contrast, volcanoes in off-rift tectonic context (low geothermal gradient), produce silicic magmas with “mantle-like” O- and B-isotope ratios suggesting limited crustal contribution. The somewhat lower (²³⁰Th/²³²Th) in the off-rift silicic magma can result from either a prolonged differentiation time from the basaltic magma, or an AFC-process in which the fractional crystallization dominates.

The Eyjafjallajökull volcano is located in the S-Iceland off-rift zone that is characterized by a low geothermal gradient. During the first explosive phase of the 2010 eruption, mingled benmoreite tephra was produced. It is composed of basaltic, intermediate and silicic tephra grains having an alkali rhyolitic composition. The rhyolite has major- and trace-element compositions indistinguishable from the cryptotephra produced during the penultimate eruption (AD 1821-23). In-situ, laser ablation ICP-MS analyses reveal trace element characteristics that are fully compatible with an origin through fractional crystallization of the basaltic tephra. Isotope ratios measured in the mingled bulk 2010 tephra strongly suggest a silicic magma mixing end-member with identical isotope composition as the basaltic end-member. The small volume of silicic magma produced during post-glacial time at the off-rift Eyjafjallajökull volcano thus is readily explained by prolonged fractional crystallization alone.