Pavel Izbekov 1 , James Gardner 2 , Ivan Melekestsev 3 , and John Eichelberger 1

1. RECURRENT CALDERA-FORMING ERUPTIONS: KSUDACH CASE STUDYPreliminary results of the ongoing petrological and experimental study Pavel Izbekov1, James Gardner2, Ivan Melekestsev3, and John Eichelberger1 1 Alaska Volcano Observatory, Geophysical Institute, University of Alaska Fairbanks, Alaska 2 University of Texas at Austin, Texas 3 Institute of Volcanology and Seismology, Petropavlovsk-Kamchatsky Petropavlovsk-Kamchatsky, August 2004

2. Landsat 7 ETM & SRTM DEM Introduction • The development of volcanic centers is often cyclic: cone – caldera – cone – caldera ... • Ksudach caldera complex in Kamchatka appears to represent an exemplary case where at least three such cycles have been completed since middle Pleistocene. Petropavlovsk-Kamchatsky, August 2004

3. Introduction cont. We use petrological and experimental approaches to determine the relationships of magmas erupted at Ksudach and to infer their pre-eruptive conditions, which potentially may illuminate the mechanism of such cyclicity, as well as the mechanism of the recurrent caldera-forming eruptions. Petropavlovsk-Kamchatsky, August 2004

4. Russia Alaska Japan Location Southern part of the Eastern Volcanic Front of Kamchatka Ca. 150 km to the SW from PK Petropavlovsk-Kamchatsky, August 2004

5. from Volynets et al. (1999) Background • 5 caldera-forming eruptions at one single volcanic center: • 2 larger Pleistocene calderas and • 3 Holocene calderas (8800 BP, 6300-6000 BP, and 1800 BP) • Cycles of a cone-building effusive activity and caldera-forming events: (1) The first caldera has destroyed an older Pleistocene shield volcano. (2) Then a new cone has been constructed inside the caldera and was subsequently destroyed by a collapse of the second Pleistocene caldera. (3) The intra-caldera cone has been constructed again and was destroyed by the first Holocene caldera-forming eruption KS-4 (8800 BP). Then three nested calderas were formed sequentially with only a subtle intra-caldera effusive activity between the caldera-forming eruptions. (4) After KS-1 eruption (1800 BP) the cone-building activity resumed again and culminated in the explosive eruption of 1907 AD. Petropavlovsk-Kamchatsky, August 2004

6. Questions We focused first on the composition of the most evolved magmas erupted during a sequence of three caldera-forming eruptions occurred at Ksudach during the Holocene: the dacite of the initial fall deposit of KS-4 (8800 yr. BP, 67.4 wt % SiO2), the KS-3 rhyolite (~6000 yr. BP, 70.3 wt % SiO2), and the KS-1 rhyolites (1800 yr. BP, 71.5-72.1 wt % SiO2). 1. Do the products of the Holocene caldera-forming eruptions at Ksudach represent snapshots of a single magma reservoir? 2. What are the pre-eruptive conditions for the last Holocene caldera-forming eruption (1800 BP)? Petropavlovsk-Kamchatsky, August 2004

7. Samples Petropavlovsk-Kamchatsky, August 2004

8. Methods • Mineral and glass compositions were determined by electron microprobe (3-10 micron, 10 nA, 15 kV beam). • Pre-eruptive conditions for KS-1 were reproduced experimentally in Rene and TZM pressure vessels (NNO, water-saturated, 100 MPa). • Whole rock compositions were determined by XRF and ICP-MS Petropavlovsk-Kamchatsky, August 2004

9. WR chemistry Petropavlovsk-Kamchatsky, August 2004

10. WR chemistry cont. Petropavlovsk-Kamchatsky, August 2004

11. Mineral compositions Magnetite-ilmenite thermometry indicates that the dacite of KS-4 was last equilibrated at 914-924C and fO2 of NNO+0.4, KS-3 rhyolite at 894-927C and fO2 of NNO+0.1, KS-1 rhyolite at 870-907C and NNO+0.6. It appears that there is a weak overall cooling trend from older to younger magmas of similar silicic composition. Petropavlovsk-Kamchatsky, August 2004

12. Mineral compositions Plagioclase is a dominant phenocryst phase in the studied KS-1, KS-3, and KS-4 products. Most show oscillatory-zoning. Their compositions range from An52.33 in KS-4 to An436 in KS-1, with KS-3 plagioclase compositions being intermediate, which is consistent with the view that these magmas have been derived from a single, slowly cooling source. However, the presence of “dusty-zoned” plagioclases, in which resorbed An46-49 cores are mantled by An72-75 inclusion-rich rims, as well as rare anorthite xenocrysts (An93-96) indicate that the composition of silicic magmas was likely modified by mixing with basalt. Petropavlovsk-Kamchatsky, August 2004

13. Experimental results: KS-1 P-T phase diagram for KS-1 rhyolite based on the phase diagram for Karymsky dacite, which has a similar bulk composition. Red symbols indicate the conditions of experiments, which used the natural KS-1 rhyolite pumice (sample 02IPE45). The equilibrium mineral phases observed in KS-1 experiments match those in Karymsky experiments with the exception of one run at 900ºC, where plagioclase is stable. Petropavlovsk-Kamchatsky, August 2004

14. 100 MPa, NNO buffer Experimental results cont. Shaded areas correspond to the average composition (±1 sigma) of natural plagioclase rims and the average temperature estimated using Fe-Ti oxides (±1 sigma). Petropavlovsk-Kamchatsky, August 2004

15. Variations of experimental melt composition as a function of temperature 100 MPa, NNO buffer Experimental results cont. Shaded areas correspond to the average composition (±1 sigma) of matrix glass and the average temperature estimated using Fe-Ti oxides (±1 sigma) for the KS-1 natural sample. Petropavlovsk-Kamchatsky, August 2004

16. Summary of experiments • Compositions of natural plagioclase, orthopyroxene, clinopyroxene, and matrix glass of KS-1 has been reproduced experimentally at 892±9°C, 100 MPa, and oxygen fugacity near NNO buffer. • Water in glass inclusions averages 3.8±1 wt.%, which matches saturation at ~100 MPa (Andrews et al., AGU-2003, V31G-01). The pre-eruptive pressure corresponds to approximately 3-4 km depth. Petropavlovsk-Kamchatsky, August 2004

17. Possible scenarios: Model 1 • A single, long-lived, fractionating magma chamber continually generates silicic magmas, which periodically ascend and erupt. Eruptions may be triggered by basaltic replenishments, which do not however, prevent the magma system from a gradual cooling. • Pro’s: The volume of silicic material appears to increase in time, as well as the concentration of SiO2 in magmas and apparent period of quiescence between eruptions. Petropavlovsk-Kamchatsky, August 2004

18. Possible scenarios: Model 2 • Silicic inputs originate from a partially molten basement. They segregate and ascend periodically, flushing an evolved, shallow-level andesitic reservoir. Basaltic magmas from even deeper source transport heat and matter, and may trigger volcanic eruptions • Pro’s: Magmas of contrasting composition are involved in each eruption. The younger KS-1 rhyolite has lower REE concentrations as compared to the slightly less evoloved KS-3 and KS-4 rhyodacites. Also, the oxidation state of the magmas appears to vary with time. Petropavlovsk-Kamchatsky, August 2004

19. Conclusions • The rhyolite of KS-1, the most recent caldera forming eruption at Ksudach, was last equilibrated at 892±9°C, 100 MPa, at water saturated conditions. The pre-eruptive pressure corresponds to approximately 3-4 km depth, which coincides with the regional stratigraphic boundary between Cretaceous metamorphic basement and an overlying volcano-sedimentary layer. • The silicic magmas of the Holocene caldera-forming eruptions at Ksudach most likely has originated from the same reservoir and then their temperatures may reflect its cooling from ca. 920°C at 8800 BP to ca. 890°C at 1800 BP. This reservoir, however, may not be at the depth, where the erupted magmas has last equilibrated. Petropavlovsk-Kamchatsky, August 2004