Nikita Mironov1, Maxim Portnyagin2
1 Vernadsky Institute of Geochemistry and Analytical Chemistry RAS, Moscow, Russia
2 IFM-GEOMAR / SFB 574, Wischhofstr. 1-3, 24148 Kiel, Germany
Klyuchevskoy volcano (Kamchatka) - from source to surface
Klyuchevskoy is a magnificent ~5 km-high stratovolcano famous for its frequent eruptions and vigorous (>60*10^6 t/year) magma output. The volcano is located in the Central Kamchatka Depression (CKD), adjacent to the Kamchatka-Aleutian arc junction where the edge of the Pacific plate is subducting beneath Kamchatka. This study is aimed at elucidating evolution of Klyuchevskoy magmas from their origin in the mantle wedge to the moment of the eruption. We have studied >400 melt inclusions in olivine Fo92-67 , which cover the entire compositional range of olivine phenocrysts known for this volcano. The olivines were separated from rock varieties ranging from high-MgO basalts (HMB) to moderate-MgO high-alumina basaltic andesites (HAB). The age of the samples ranged from 6.9 cal ka to 1966 AD. Melt inclusions were analyzed for major, volatile (S, Cl, H2O, F) and trace elements using electron and ion microprobes. To quantify conditions of magma origin, the data on melt inclusions were combined with those on composition of crystal and fluid inclusions in minerals and with the results of numerical modeling.
Klyuchevskoy parental magmas have high-magnesian (#Mg~72 mol.%, MgO~12-13 wt. %) basaltic (SiO2~48 wt.%), nepheline-normative (Ne~3-4 wt%), moderately-K (K2O~0.6 wt.%) and high-H2O (up to 3.5 wt.%) composition. Primary magmas originate in the upper mantle at 12 to 21 kbar pressure (40-70 km depths) and temperature 1300-1320 °C through fluid-fluxed melting of lithologically heterogeneous peridotite source consisting of moderately depleted lherzolite and amphibole wehrlite. The presence of amphibole wehrlite in the source region is a distinctive feature of primitive magmas of Klyuchevskoy volcano and also of a number of other volcanoes from island-arc setting. The amphibole wehrlite could be formed either due to H2O-CO2 metasomatism of the mantle wedge or represent delaminated lower crustal cumulates. Fluid component, which triggers melting in the mantle, is estimated to be rich in H2O, K2O, Ba, Cl, B and Pb and likely to to originate by dehydration of altered oceanic crust and sediments subducting beneath CKD. On the way from the source region to the crust primary magmas interact with previously metasomatized lithospheric mantle. This interaction leads to enrichment of primary magmas in most of incompatible trace elements, such as K, Ba, Th, U, Sr, REE, Zr and Hf, to increasing δ18O, 87Sr/86Sr and ratios of more to less incompatible elements (e.g. K/Ti, La/Yb, Ba/Zr). The extent to which primary Klyuchevskoy melts assimilate metasomatized mantle varied through time and decreased during periods of high magma production rate in the deep mantle and likely faster magma passage to the crust.
Klyuchevskoy parental magmas start to crystallize olivine, high-Ca pyroxene and Cr-spinel at 1250-1300 °C temperature, oxygen fugacity ΔNNO~0 and 10-12 kbar pressure corresponding to the Moho depth beneath CKD (35-40 km) (Balesta et al., 1981). The following fractional crystallization occurred at magma decompression. At pressures more than 6 kbar (>20 km depth in the lower crust) parental melts of all rock types crystallize at a constant geothermal gradient dT/dP=25 °C/kbar which results in crystallization rate of dF/dP=3%/kbar. At a depth less than 20 km, the rates of magma cooling and crystallization of parental melts of HAB increase to ~35-50 °C/kbar and ~10-20 %/kbar, respectively. This suggests magma stalling in the upper crust beneath Klyuchevskoy volcano. High-Al basaltic melts originate at 10-17 km depth, 1000-1060 °C temperature, oxygen fugacity ΔNNO=0-1 and water content in melt up to 5 wt.% as a result of 40 % fractionation of parental Klyuchevskoy magmas. Crystallization at the lower to upper crustal levels was accompanied by release of sulphate- and carbonate -rich fluids. Final stages of magma evolution are characterized by exsolution of predominantly H2O-rich fluid and accompanied by massive degassing-driven crystallization of plagioclase, olivine, pyroxenes and Fe-oxides at nearly constant or even slightly increasing temperature of magma just before and during eruption.
Our data show that parental Klyuchevskoy magmas are very rich in volatile components (H2O, CO2, S, Cl) and substantial amount of the volatiles initially dissolved in primitive melts is released to fluid phase at subsequent magma evolution. We estimated that fluxes of volatiles to the exosphere due to activity of Klyuchevskoy volcano can account for up to 1.5 % of the average annual flux from all island-arc volcanoes. Therefore larger Klyuchevskoy eruptions may have significant climatic effect. Particularly large effect is anticipated during periods of enhanced Klyuchevskoy activity, for example ~7 and 3 ka BP.
The estimated conditions of magma evolution beneath Klyuchevskoy volcano agree well with independent data based on seismic tomography studies and distribution of epicenters of the earthquakes in the crust (Fedotov et al, 1988; Gorel’chik et al, 2001; Lees et al., 2007). This has allowed us to put forward a self-consistent general model of the structure and evolution of magma plumbing system beneath Klyuchevskoy volcano utilizing data from petrology, geochemistry and geophysics.
This research was supported by the German-Russian KALMAR project and the Russian Foundation for Basic Research (grants № 07-05-00807 and 03-05-64629).
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