Geochemical evolution of historical lavas from Askja Volcano, Iceland: Implications for mechanisms and timescales of magmatic differentiation

The mechanisms and the timescales of magmatic evolution were investigated for historical lavas from the Askja central volcano in the Dyngjufjoll volcanic massif, Iceland, using major and trace element and Sr, Nd, and Pb isotopic data, as well as U-238-Th-230-Ra-226 systematics. Lavas from the volcano show marked compositional variation from magnesian basalt through ferrobasalt to rhyolite. In the magnesian basalt-ferrobasalt suite (5-10 wt% MgO), consisting of lavas older than 1875 A.D., Sr-87/Sr-86 increases systematically with increasing SiO2 content; this suite is suggested to have evolved in a magma chamber located at similar to 600 MPa through assimilation and fractional crystallization. On the other hand, in the ferrobasalt-rhyolite suite (1-5 wt% MgO), including 1875 A.D. basalt and rhyolite and 20th century lavas, Sr-87/Sr-86 tends to decrease slightly with increasing SiO2 content. It is suggested that a relatively large magma chamber occupied by ferrobasalt magma was present at similar to 100 MPa beneath the Oskjuvatn caldera, and that icelandite and rhyolite magmas were produced by extraction of the less and more evolved interstitial melt, respectively, from the mushy boundary layer along the margin of the ferrobasalt magma chamber, followed by accumulation of the melt to form separate magma bodies. Ferrobasalt and icelandite lavas in the ferrobasalt-rhyolite suite have a significant radioactive disequilibrium in terms of (Ra-226/Th-230), and its systematic decrease with magmatic evolution is considered to reflect aging, along with assimilation and fractional crystallization processes. Using a mass-balance model in which simultaneous fractional crystallization, crustal assimilation, and radioactive decay are taken into account, the timescale for the generation of icelandite magma from ferrobasalt was constrained to be <similar to 3 kyr which is largely dependent on Ra crystal-melt partition coefficients we used. (C) 2010 Elsevier Ltd. All rights reserved.