TY - JOUR
T1 - Geochemical evolution of historical lavas from Askja Volcano, Iceland
T2 - Implications for mechanisms and timescales of magmatic differentiation
AU - Kuritani, Takeshi
AU - Yokoyama, Tetsuya
AU - Kitagawa, Hiroshi
AU - Kobayashi, Katsura
AU - Nakamura, Eizo
N1 - Funding Information:
We thank Ryoji Tanaka, Tatsue Nogi, and all other members of the Pheasant Memorial Laboratory (Okayama University) for technical help and useful discussion. We are grateful to T. Moriguti, E. Rose-Koga, J.Ö. Fridsteinsson, Á. Höskuldsson, and K. Grönvold for helping us collect rock samples in Iceland. Editorial handling by F.J. Ryerson and constructive reviews and comments by G. Zellmer, J.G. Brophy, and B. Jicha are acknowledged. In particular, critical improvement of the manuscript and encouragements by G. Zellmer are greatly appreciated. We are also indebted to J. Maclennan for fruitful suggestions in the course of this study. This work was supported by the Ministry of Education, Culture, Sports, Science, and Technology of the Japanese Government to T.K., and also by the program for the “Center of Excellence for the 21st Century in Japan” to ISEI, Okayama University.
PY - 2011/1/15
Y1 - 2011/1/15
N2 - The mechanisms and the timescales of magmatic evolution were investigated for historical lavas from the Askja central volcano in the Dyngjufjöll volcanic massif, Iceland, using major and trace element and Sr, Nd, and Pb isotopic data, as well as 238U-230Th-226Ra systematics. Lavas from the volcano show marked compositional variation from magnesian basalt through ferrobasalt to rhyolite. In the magnesian basalt-ferrobasalt suite (5-10wt% MgO), consisting of lavas older than 1875 A.D., 87Sr/86Sr increases systematically with increasing SiO2 content; this suite is suggested to have evolved in a magma chamber located at ∼600MPa through assimilation and fractional crystallization. On the other hand, in the ferrobasalt-rhyolite suite (1-5wt% MgO), including 1875 A.D. basalt and rhyolite and 20th century lavas, 87Sr/86Sr tends to decrease slightly with increasing SiO2 content. It is suggested that a relatively large magma chamber occupied by ferrobasalt magma was present at ∼100MPa beneath the Öskjuvatn 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 (226Ra/230Th), 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 <∼3kyr which is largely dependent on Ra crystal-melt partition coefficients we used.
AB - The mechanisms and the timescales of magmatic evolution were investigated for historical lavas from the Askja central volcano in the Dyngjufjöll volcanic massif, Iceland, using major and trace element and Sr, Nd, and Pb isotopic data, as well as 238U-230Th-226Ra systematics. Lavas from the volcano show marked compositional variation from magnesian basalt through ferrobasalt to rhyolite. In the magnesian basalt-ferrobasalt suite (5-10wt% MgO), consisting of lavas older than 1875 A.D., 87Sr/86Sr increases systematically with increasing SiO2 content; this suite is suggested to have evolved in a magma chamber located at ∼600MPa through assimilation and fractional crystallization. On the other hand, in the ferrobasalt-rhyolite suite (1-5wt% MgO), including 1875 A.D. basalt and rhyolite and 20th century lavas, 87Sr/86Sr tends to decrease slightly with increasing SiO2 content. It is suggested that a relatively large magma chamber occupied by ferrobasalt magma was present at ∼100MPa beneath the Öskjuvatn 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 (226Ra/230Th), 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 <∼3kyr which is largely dependent on Ra crystal-melt partition coefficients we used.
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U2 - 10.1016/j.gca.2010.10.009
DO - 10.1016/j.gca.2010.10.009
M3 - Article
AN - SCOPUS:78650191868
VL - 75
SP - 570
EP - 587
JO - Geochmica et Cosmochimica Acta
JF - Geochmica et Cosmochimica Acta
SN - 0016-7037
IS - 2
ER -