Petrology and geochemistry of Klyuchevskoy Volcano, Kamchatka, U.S.S.R.: Implications for the chemical and physical evolution of island arcs.

• Main Author: Kersting, Annie Bernadette
• Other Authors: Arculus, Richard J., Essene, Eric J.
• Format: Thesis
• Language: unknown
• Published: 1991
• Subjects: Geology; Mineralogy; Geochemistry; Kamchatka
• Online Access: https://hdl.handle.net/2027.42/105389; http://gateway.proquest.com/openurl?url_ver=Z39.88-2004&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&res_dat=xri:pqm&rft_dat=xri:pqdiss:9124033

Klyuchevskoy Volcano, Kamchatka, U.S.S.R., is the most rapidly erupting island arc volcano in the world producing lavas with a narrow range in SiO$\sb2$ (51-54 wt%) but a comparatively large range in MgO and Al$\sb2$O$\sb3$. The basalts range from high-MgO, low-Al$\sb2$O$\sb3$ ($\approx$12 wt% MgO, 14 wt% Al$\sb2$O$\sb3$) to high-Al$\sb2$O$\sb3$, low-MgO ($\approx$18 wt% Al$\sb2$O$\sb3$, 4 wt% MgO). Klyuchevskoy is characterized by non steady-state erupting, recharging and fractionating magma chamber(s) in which fractionation of parental high-MgO basalt melts produce high-Al$\sb2$O$\sb3$ basalts. Parental high-MgO magmas are generated above the subducting slab through melting of a fluid-fluxed mantle wedge. Although fractionation of olivine and clinopyroxene is the dominant mechanism, periodic recharge with primitive, higher-MgO basalts is also an important process contributing to the chemical evolution of the magmas. The lavas differentiate at high temperatures in excess of 1150$\sp\circ$C and moderate pressure between 0.5 and 0.9 GPa at the base of the crust. The $\delta\sp{18}$O (SMOW) of whole rock basalts from Klyuchevskoy range from +7.2 to +7.8$\perthous$. These values are significantly higher (1-2$\perthous$) than other island arc basalts or MORB with comparable Sr and Nd isotopic characteristics. Mineral separates of plagioclase (+7.7 to +8.6$\perthous$), olivine (+6.9 to +7.9$\perthous$) and clinopyroxene (+6.9$\perthous$) are in isotopic equilibrium with the melt. Assimilation of lithospheric crust is the most likely process responsible for heavy $\delta\sp{18}$O values of the Klyuchevskoy basalts. This process appears decoupled from processes contributing to the Nd and Sr isotopic, petrographic or major and trace element characteristics of the Klyuchevskoy basalts. Aqueous, two-phase, fluid inclusions (5-10 $\mu$m) in plagioclase phenocrysts have primary textures, yet low homogenization temperatures (72 to 187$\sp\circ$C) and $\delta$D values ($-$99 to $-$150$\perthous$). The $\delta$D values are ...