Hekla Revisited: Fractionation of a Magma Body at Historical Timescales

Hekla is an elongate volcano that lies at the intersection of the South Iceland Seismic Zone and the Eastern Volcanic Zone. We report major and trace element, oxygen isotopic, and H 2 O analyses on rocks, glass, melt inclusions, and minerals from almost all of the historical lavas and tephra deposits. This new dataset confirms the remarkable observation that not only are many eruptions compositionally zoned from felsic to mafic, but the extent of zoning relates directly to the length of repose since the previous eruption. Compositional data are consistent with the origin of the basaltic andesites and andesites by fractional crystallization, with no measurable crustal interaction once basaltic andesite has been produced. Although the 1104 CE Plinian rhyolite and 1158 CE dacite are also created by fractional crystallization, uranium–thorium isotopic disequilibria measured by others require that they evolved in a separate body, where magma is stored in a molten state for >10 ⁴  years. Consistent trace element trends and ratios, as well as oxygenisotopic data, preclude significant crustal input into the evolving magma. The phenocryst assemblages are dominated by crystals that formed from their host melt; an exception is the 1158 CE dacite, which contains abundant crystals that formed from the 1104 CE rhyolite melt. A suite of thermobarometers indicates that most crystals formed in the lower crust at temperatures ranging from ∼1010 to 850 °C. Hekla's unique and systematic petrological time series and geophysical activity are attributed to the unusual geometry of the magma body, which we propose to be a tabular, vertically elongate macrodike, extending from the lower to the upper crust. The vertical body is recharged with basaltic andesite magma at the end of each eruption, which then undergoes cooling and crystallization until the subsequent eruption. The entire system is supplied by a lower-crustal body of basaltic andesite, which is produced by fractional crystallization of basaltic magma in a reservoir that is thermochemically buffered to ∼1010 °C. Cooling and crystallization of recharged basaltic andesite magma in a background geothermal gradient from the lower to the shallow crust accounts for the systematic relationship between repose and composition.