Trace element concentrations in plagioclase phenocrysts of Karymsky andesite: Evidence for basalt replenishments Pavel Izbekov 1, John Eichelberger 1, Thomas A. Vogel 2 and Lina Patino 2 1 Alaska Volcano Observatory, Geophysical Institute University of Alaska Fairbanks 2 Department of Geological Sciences, Michigan State University

Purpose of presentation To discuss the results of comparative study of plagioclase phenocrysts from Karymsky andesites and Academy Nauk basalt, which erupted simultaneously in January 1996 Karymsky, July 1998

Location of Karymsky and Academy Nauk  Ray Sterner, Johns Hopkins University, Applied Physics Laboratory, 1998 Karymsky volcano, 'N ' E, is one of the most active volcanoes of the Eastern volca-nic zone of Kamchatka arc, which lies ~220 km inland and parallel to the Kamchatka trench. According to recent geophysical data this corresponds to approximately 120 km above the upper surface of the subducted slab.

Karymsky and Academy Nauk Academy Nauk Karymsky is a stratovolcano located in the center of a 5-km- diameter caldera, which formed ~6600 years ago as a result of caldera-forming. The active cone of Karymsky was constructed throughout the last 5100 years with a repose period between 2800 and 500 BP. It is mostly composed by andesite of relatively constant composition (59-62% SiO 2 ). However at least twice in its history, BP and 2800 BP, Karymsky produced andesitic basalt (52-56% SiO 2 ) Academy Nauk is located ~9 km on the South from Karymsky on the same longitudinally oriented fault. It is a 4-km-diameter caldera formed ~28,000-48,000 years ago. AN produced small volumes of basalt from eruptive centers located within caldera 5500, 4100, 950 BP and in January KarymskyKarymsky Karymsky and Karymsky Lake, September 1997

1996 eruption of Karymsky and Academy Nauk Eruptions started simultaneously The erupted magmas were considerably different in chemical composition Significant extension occurred perpendicular to the fault connecting two eruptive vents New vent Karymsky, view from SW August 1999 Academy Nauk eruptive center, view from North, July 1998

Basalt of Academy Nauk and andesite of Karymsky, Comparison of mineralogy and chemistry Academy Nauk basalt Karymsky andesite Ol Pl CPx Pl Modal Abandances, vol. % Chemical composition of basalt and andesite

Variations of melt composition in Karymsky andesite SiO 2 Glass of AN basalt Composition of volcanic glass from Karymsky tephra produced during was studied using Cameca SX-50 electron microprobe at University of Alaska, Fairbanks  Analytical conditions: »15 kV accelerating voltage; »10 nA beam current; »10 micron electron beam. The microprobe study of volcanic glass from Karymsky volcano tephra supported our hypothesis that the current eruption have been triggered by a basaltic dike penetrated the crustal portion of the Karymsky magma system.

Types of Plagioclase Phenocrysts Academy Nauk basalt Karymsky andesite Rimmed Oscillatory

Analytical methods Composition of plagioclase phenocrysts of Academy Nauk basalts and Karymsky andesites was studied by Electron microprobe (major elements) Cameca SX-50 at University of Alaska Fairbanks; 15 kV acceleration voltage; 10 nA beam current; 5 micron beam. ICP-MS (Ba and Sr) Micromass Platform ICP- HEX-MS at Michigan State University; Cetac LSX 200 laser ablation system equipped with UV laser; 30 micron beam.

Results of analyses Microprobe dataICP-MS data

Examples of individual Plagioclase phenocrysts Basalt of Academy Nauk A B C D E F Sieved texture; An 80 -An 88 ; Low Ba/Ca and Sr/Ca ratios.

Examples of individual Plagioclase phenocrysts C D Basalt of Academy Nauk No melt inclusions in the core; An 77 -An 90 ; Low Ba/Ca and Sr/Ca ratios.

Examples of individual Plagioclase phenocrysts Karymsky andesite, oscillatory zoned phenocryst A B No melt inclusions; An 47 -An 60 ; High Ba/Ca and Sr/Ca ratios.

Examples of individual Plagioclase phenocrysts Karymsky andesite, rimmed phenocryst Sharp boundary between core and rim; Rim has the same composition as oscillatory zoned plagioclases, while core has the same composition as plagioclases from Academy Nauk basalt

Conclusion Compositional similarity of cores of plagioclase phenocrysts of Karymsky andesites to plagioclase phenocrysts of Academy Nauk basalts suggests that the cores came from the Academy Nauk basaltic source and were introduced to andesite by basalt replenishments of 1996 and/or earlier events.