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

Mineral Chemistry, and major and trace element variations of the basalts from Klyuchevskoy, the world's most active island are volcano, are most consistently explained by the persistence of a non-steady state, erupting, recharging, and fractionating magma chamber in which fractionation of a parental high-MgO basalt melt produces high-Al2O3 basalt. Although fractional crystallization is the dominant controlling mechanism, periodic recharge with a more primitive high-MgO basalt is also an important process contributing to the chemical evolution of the magmas. Hybrid basalts are the mixed product of high-Al2O3 basalt recharged with high-MgO basalt. The lavas range in composition from high-MgO, low-Al2O3 (∼ 12 wt. % MgO, 14 wt. % Al2O3) to high-Al2O3, low-MgO (∼ 18 wt. % Al2O3, 4 wt. % MgO). The high-MgO lavas are characterized by phenocrysts 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). All the phenocrysts are normally zoned and set in a groundmass of plagioclase, 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 both normal and reverse zoning in a groundmass of plagioclase, pigeonite, olivine, clinopyroxene, magnetite, ± orthopyroxene. Hybrid basalts have characteristics of both high-MgO basalts and high-Al2O3 basalts and preserve complicated normal-to-reverse, reverse-to-normal, and normally zoned phenocrysts. No hydrous minerals are present in any of the lavas. The varied basaltic magmas erupted from Klyuchevskoy are derived from a magma chamber(s) located near the base of the Kamchatka crust (pressures ∼ 0.5–0.9 GPa) and characterized by relatively high crystallization temperatures, some in excess of ∼1150°C. Under these conditions, the fractionation of a parental high-MgO magma, produced principally from the melting of a fluid-fluxed, peridotitic mantle wedge, results in the production of a chemically diverse spectrum of basalts ranging from high-MgO, low-Al2O3 to high-Al2O3, low-MgO basalt, traversing the relatively primitive end of both the calc-alkalic and tholeiitic differentiation trends.