HR: 1340h
AN: V33C-1527    [Abstracts]
TI: Incremental formation, differentiation, and explosion of an arc magma chamber: Isotopic and physical study of multi-caldera Ksudach volcano, Kamchatka, Russia
AU: * Bindeman, I
EM: bindeman@uoregon.edu
AF: Geological Sciences, 1272 University of Oregon, Eugene, OR 97403, United States
AU: Izbekov, P
EM: pavel@gi.alaska.edu
AF: Geophysical Institute, University of Alaska, Fairbanks, AK 99775, United States
AU: Chen, C
EM: china@earth.sinica.edu.tw
AF: Institute of Earth Sciences, Academia Sinica, P.O. Box 1-55 Nankang, Taipei, 11529, Taiwan
AU: Ponomareva, V
EM: ponomareva@kscnet.ru
AF: Institute of Volcanology, Piip Blvd 6, Petropavlovsk, 106999, Russian Federation
AU: Melekestsev, I
EM: melekes@kscnet.ru
AF: Institute of Volcanology, Piip Blvd 6, Petropavlovsk, 106999, Russian Federation
AB: Ksudach volcano is the most explosive Holocene volcano in Kamchatka that had four episodes of caldera formation in Holocene: 8800 BP (2km3), 6300 BP (1km3), 6000 BP (7-8km3), and 1800 BP (18-19 km3, as well as two episodes in Pleisocene (one at ca 160ky and the other younger). While mineral compositional trends suggest fractional crystallization is sufficient to explain magma diversity, oxygen isotopic trends in conjunction with Sr and Nd isotopes suggest progressive formation of a large magma chamber by assimilation of shallow crust prior to the 1800 BP eruption. Lavas and smaller volume pyroclastic units belonging to the pre-1800BP sequence form a temporal array of decreasing d18O values of phenocrysts and increasing Sr isotopic values. Oxygen isotopes in Holocene Ksudach melts decrease from 5.4 to 4.7 permil, while Sr isotopes increase from 0.70331 to 0.70338, and two isotopes anticorrelate. Piston cylinder phase equilibria experiments suggest shallow, 1.0 kbar preeruptive storage conditions for the products of the 1800 BP KS1 caldera-forming eruption. Low-d18O values, shallow pressures of storage, and temporal isotopic trends allows us to suggest that at least 18-19km3 of siliceous magma was stored in the very shallow crust prior to the 1800 BP caldera formation, and that an entire magma reservoir was emptied in the course of that eruption. Older and smaller episodes of caldera-formations did not emptied the magma chamber completely: trend of d18O depletion persisted while Sr and Nd isotopes exhibited quickly recuperating wiggles toward pre-caldera values.Temporal isotopic trends suggest that the dominant petrogenetic process for thousands of years before the 1800 BP eruption was assimilation and melting of low-d18O hydrothermally altered carapace around this magma body. Following the 1800 BP caldera-forming eruption, largely basaltic Shtyubel" stratocone started to form that had produced ~1km3 dacitic differentiate in the course of 1907 AD eruption. These youngest products of the intracaldera Shtyubel cone have more normal, yet diverse, in d18O and 87/86 Sr values, signifying a fundamentally new cycle of magmatic activity. This study demonstrates that tens of cubic kilometers of siliceous magma can be produced, stored for thousands of years, and erupted effusively and explosively from a magma chamber only 3km below the surface. At the same time, roof melting and assimilation proceeds rapidly at such shallow depths.
DE: 8419 Volcano monitoring (7280)
DE: 8440 Calderas
DE: 8455 Tephrochronology (1145)
DE: 8488 Volcanic hazards and risks
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting