1. Pavel Izbekov1, John Eichelberger1, Thomas A. Vogel2 and Lina Patino2
Trace element concentrations in plagioclase phenocrysts of Karymsky andesite: Evidence for basalt replenishments
Pavel Izbekov1, John Eichelberger1, Thomas A. Vogel2 and Lina Patino2
1 Alaska Volcano Observatory, Geophysical Institute
University of Alaska Fairbanks
2 Department of Geological Sciences, Michigan State University

2. 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, August 1999

3. Location of Karymsky and Academy Nauk
 Ray Sterner, Johns Hopkins University,
Applied Physics Laboratory, 1998
Karymsky volcano, 54003'N ' E, is one of the most active volcanoes of the Eastern volcanic zone of Kamchatka arc, which lies ~220 km inland and parallel to the Kamchatka trench. Karymsky showed eruptive activity in 20th century during , , , , , , 1996 – now

4. Karymsky and 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 eruption. 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% SiO2). However at least twice in its history, BP and 2800 BP, Karymsky produced andesitic basalt (52-56% SiO2)
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 1996.
Karymsky
Academy Nauk
Karymsky and Karymsky Lake, September 1997

5. 1996 eruption of Karymsky and Academy Nauk New vent
Eruptions started simultaneously
The erupted magmas were considerably different in chemical composition
Significant extension occurred perpendicular to the fault connecting two eruptive vents
Karymsky, view from SW
August 1999
Academy Nauk eruptive center,
view from North, July 1998
New vent

6. Basalt of Academy Nauk and andesite of Karymsky, comparison
Modal abundances, vol. % Ol
Academy Nauk basalt Pl
Chemical composition of basalt and andesite Pl
CPx
Karymsky andesite Pl Pl

7. Hypothesis
Basaltic dike penetrated the crustal portion of the Karymsky magma system,
triggering release of andesitic magma from a high-level reservoir
Test I. Variation of melt composition of Karymsky andesite in time
Test II. Xenoliths and xenocrysts?

8. Test I. Variations of melt composition in Karymsky andesite
SiO2
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.

9. Test II. Study of Plagioclase Phenocrysts
Academy Nauk basalt
Karymsky andesite
Oscillatory
Rimmed

10. 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.

11. Plagioclase phenocrysts of Academy Nauk basalt
Basalt of Academy Nauk C D F B E A
An80-An88;
Low Ba/Ca and Sr/Ca ratios.

12. Oscillatory zoned plagioclases of Karymsky andesiteB
No melt inclusions;
An47-An60;
High Ba/Ca and Sr/Ca ratios.

13. Rimmed plagioclases of Karymsky andesite
Rim has the same composition as oscillatory zoned plagioclases, while core has the same composition as plagioclases from Academy Nauk basalt

14. Results of analyses Microprobe data ICP-MS data
Bimodal distribution of microprobe analyses;
Separate clusters of points with low and high ratios of Ba/Ca and Sr/Ca.

15. Conclusion
ICP-MS data
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;
This may serve as an additional evidence that in 1996 the eruption of Karymsky was triggered by basalt dike penetrated the crustal portion of the Karymsky magma system.