MAGMA MIXING AT KARYMSKY: PETROLOGIC CONSTRAINTS AND MODEL

1. MAGMA MIXING AT KARYMSKY: PETROLOGIC CONSTRAINTS AND MODEL Pavel Izbekov1, John Eichelberger1 and Boris Ivanov2 1 Alaska Volcano Observatory, Geophysical Institute, UAF, Fairbanks 2 Institute of Volcanic Geology and Geochemistry, Petropavlovsk-Kamchatsky, Russia

2. Introduction • Mixing of compositionally distinct magmas is well documented by the presence of enclaves and zoning of phenocrysts in igneous rocks • The question is how fast can the compositionally distinct magmas mix? • We present results of a detailed petrologic study of andesite and basalt that erupted simultaneously at Karymsky volcanic center beginning in January 1996. • Continuous eruption of Karymsky offered an unique opportunity to determine compositional variations of its magma caused by basaltic recharge. These variations indicate that mixing can be surprisingly fast and thorough.

3.  Ray Sterner, Johns Hopkins University, Applied Physics Laboratory, 1998 Location

4. Geological background • Karymsky volcano and Academy Nauk caldera belong to a chain of volcanoes, calderas, and maars, the location of which is controlled by a local north-trending fault. • Karymsky is a ~5300-yr-old andesitic stratovolcano located in the center of a ~7900-yr-old caldera. During the past 500 yr, the volcano has been in a state of frequent, but intermittent eruptive activity. • Academy Nauk caldera is centered 9 km south of Karymsky on the same fault system. Since its caldera-forming event (ca. 40,000 yr B.P.), the volcanic activity within the caldera was confined to phreatomagmatic eruptions of basalt, which have occurred at least twice since 5000 yr B.P.

5. 1996 eruption of Karymsky and Academy Nauk The most recent episode of volcanic activity at Karymsky started on January 2, 1996, after 13 yr of dormancy. It began with simultaneous eruption of andesite from the central vent of Karymsky volcano and basalt from a new vent, which formed in the northern part of Academy Nauk caldera. • The magmas erupted simultaneously. • The erupted magmas had strongly contrasting bulk compositions. Academy Nauk vent produced basalt (52.2 wt% SiO2), while Karymsky summit vent erupted andesite (62.4 wt% SiO2). • The eruptive vents are located along the same active fault. • Significant ground deformation occurred between eruptive vents. Extension between eruptive vents occurred gradually, rather than catastrophically. The eruption of basalt coincident with the start of the most recent cycle of activity at Karymsky is suggestive of eruptive triggering. The eruption of basalt coincident with the start of the most recent cycle of activity at Karymsky is suggestive of eruptive triggering.

6. Questions to answer: Karymsky, view from SW August 1999 Academy Nauk eruptive center, view from North, July 1998 • Was there a mixing of andesite and basalt? • If yes, how can we explain the homogeneity of andesite? Was the mixing that fast?

7. Samples and analytical techniques • Electron microprobe (major elements) • Cameca SX-50 at University of Alaska Fairbanks; • 15 kV acceleration voltage; • 10 nA beam current; • 5 micron beam (10 microns for glass) • LA-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. Samples of pyroclastics of Karymsky vs. time of their eruption. Length of hori-zontal bars corresponds to approximate time of lava flows effusion.

8. Petrography Karymsky andesite Academy Nauk basalt Pl CPx Ol Pl Pl Pl Modal Abundances, vol. %

9. Samples of pyroclastics of Karymsky vs. time of their eruption. Length of hori-zontal bars corresponds to approximate time of lava flows effusion. Samples of pyroclastics of Karymsky vs. time of their eruption. Length of hori-zontal bars corresponds to approximate time of lava flows effusion. Karymsky: Glass composition vs. time Compositions of volcanic ash glass plotted against the date of eruption. Melt of Karymsky andesite, which erupted in February 1996, was the most mafic. Error bars correspond to 2s in electron microprobe analyses of ash samples.

10. Karymsky Academy Nauk Karymsky and Academy Nauk plagioclases BSE images of Academy Nauk (a) and Karymsky (b) plagioclases and their corresponding compositional profiles. Note contrasting textures and compositions.

11. Karymsky Karymsky: Calcic cores in plagioclases • Approximately 25 vol.% of plagioclase phenocrysts in Karymsky andesite contain calcic cores. • Composition and texture of cores match those of plagioclases in Academy Nauk basalts. • Widths of sodic rims are consistent with introduction of calcic cores at the onset of eruption (2.5 × 10–9 mm/s plagioclase growth rate)

12. a 40 c N=79 AN basalt 35 b Karymsky andesite 30 Pl N=104 25 Frequency, normalized to 100 20 15 Ca-core OPx 10 Ol 5 0 66 68 70 72 74 76 78 More Composition of olivine, Fo mol.% Karymsky: Xenocrysts of olivine Photomicrograph (a) and a simplified sketch (b) of Karymsky andesite from the lava flow, which effusively erupted during April-August 1996. Note olivine xenocryst attached to the calcic core of plagioclase – both likely introduced to andesite by basaltic replenishment in January, 1996. Composition of olivines is shown in figure C.

13. Summary • By late February 1996 Karymsky erupted texturally homogeneous andesites, no mafic enclaves were found. • Within two months of the onset of eruption, the composition of melt of Karymsky andesite, as recorded by glass in tephra, shifted toward a more mafic composition and then gradually returned to its original state and remained constant for the following 4 years. • Andesite contains xenocrysts of basaltic origin, i.e calcic plagioclase and olivine, at least part of which was most likely introduced by basaltic replenishment in January 1996, at the onset of the eruptive cycle.

14. Viscosity constraints on mixing The plausibility of mixing can be roughly tested using estimates of viscosities for Academy Nauk basalt and Karymsky andesite. • Isobaric crystallization of basalt was modeled using COMAGMAT algorithm (Ariskin); • Viscosity was calculated using Shaw (1972) model; • Volume of injected basalt was assumed negligible comparative to the volume of the stored andesite and did not affect the temperature of the produced hybrid significantly. Variations of viscosity vs. temperature for Academy Nauk basalt (2 wt. % H2O) and Karymsky andesite (1.5 wt.% H2O) at 200 MPa total pressure. Variations of viscosity vs. temperature for Academy Nauk basalt (2 wt. % H2O) and Karymsky andesite (1.5 wt.% H2O) at 200 MPa total pressure.

15. Comparative model

16. Conclusion • Homogeneity of andesite erupted by late February 1996 suggests to us that thorough mixing of injected basalt and andesite occurred in a period of time as short as two months. • Perhaps the rapid and effective blending was facilitated by an only modest contrast in viscosities and temperatures between the magmas and by a vigorous fluid dynamic regime in Karymsky reservoir. • Karymsky is a well-mixed end member case that reflects the short recurrence interval of recharges to the system. In contrast, Trident volcano in Alaska may represent an intermediate case, where both “clotting” and direct mixing occurs, and Kizimen volcano in Kamchatka – a poorly-mixed end member case, where clotting along is dominant.