Klyuchevskoy Volcano, Kamchatka, Russia: The Role of High-Flux Recharged, Tapped, and Fractionated Magma Chamber(s) in the Genesis of High-Al2O3 from High-MgO Basalt |
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| Authors: | KERSTING ANNIE B; ARCULUS RICHARD J |
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| Institution: | 1Department of Geological Sciences, University of Michigan Ann Arbor, Michigan 48109-1063
2Department of Geology and Geophysics, University of New England Armidale, N.S.W., 2351 Australia |
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| Abstract: | Mineral Chemistry, and major and trace element variations ofthe basalts from Klyuchevskoy, the world's most active islandare volcano, are most consistently explained by the persistenceof a non-steady state, erupting, recharging, and fractionatingmagma chamber in which fractionation of a parental high-MgObasalt melt produces high-Al2O3 basalt. Although fractionalcrystallization is the dominant controlling mechanism, periodicrecharge with a more primitive high-MgO basalt is also an importantprocess contributing to the chemical evolution of the magmas.Hybrid basalts are the mixed product of high-Al2O3 basalt rechargedwith high-MgO basalt. The lavas range in composition from high-MgO, low-Al2O3 ( 12wt. % MgO, 14 wt. % Al2O3) to high-Al2O3, low-MgO ( 18 wt. %Al2O3, 4 wt. % MgO). The high-MgO lavas are characterized byphenocrysts of olivine (cores FO90–80 and rims FO85–75)with chromite inclusions Cr/(Cr + Al) 0.7], clinopyroxene (Wo46–42En48–42Fs15–7),and the rare occurrence of orthopyroxene (En72–70). Allthe phenocrysts are normally zoned and set in a groundmass ofplagioclase, pigeonite, clinopyroxene, magnetite, orthopyroxene.The high-Al2O3 basalts contain plagioclase (An85–55),olivine (Fo80–65), clinopyroxene (Wo45–30En50–38Fs23–11), orthopyroxene (En72–70) phenocrysts, that preserve bothnormal and reverse zoning in a groundmass of plagioclase, pigeonite,olivine, clinopyroxene, magnetite, orthopyroxene. Hybrid basaltshave characteristics of both high-MgO basalts and high-Al2O3basalts and preserve complicated normal-to-reverse, reverse-to-normal,and normally zoned phenocrysts. No hydrous minerals are presentin any of the lavas. The varied basaltic magmas erupted from Klyuchevskoy are derivedfrom a magma chamber(s) located near the base of the Kamchatkacrust (pressures 0.5–0.9 GPa) and characterized by relativelyhigh crystallization temperatures, some in excess of 1150C.Under these conditions, the fractionation of a parental high-MgOmagma, produced principally from the melting of a fluid-fluxed,peridotitic mantle wedge, results in the production of a chemicallydiverse spectrum of basalts ranging from high-MgO, low-Al2O3to high-Al2O3, low-MgO basalt, traversing the relatively primitiveend of both the calc-alkalic and tholeiitic differentiationtrends. |
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