Klyuchevskoy volcano, kamchatka, Russia

Abstract

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. Show more