06/10/2002 1

MAGMA MIXING AT KARYMSKY: MAGMA MIXING AT KARYMSKY: PETROLOGIC CONSTRAINTS AND PETROLOGIC CONSTRAINTS AND

MODELMODEL

Pavel IzbekovPavel Izbekov11, John Eichelberger, John Eichelberger11 and Boris Ivanov and Boris Ivanov22

11 Alaska Volcano Observatory, Alaska Volcano Observatory, Geophysical Institute, UAF, FairbanksGeophysical Institute, UAF, Fairbanks

22 Institute of Volcanic Geology and Institute of Volcanic Geology and Geochemistry, Petropavlovsk-Geochemistry, Petropavlovsk-Kamchatsky, RussiaKamchatsky, Russia

2

Introduction

• Mixing of compositionally distinct magmas is well Mixing of compositionally distinct magmas is well documented by the presence of enclaves and zoning of documented by the presence of enclaves and zoning of phenocrysts in igneous rocks phenocrysts in igneous rocks

• The question is how fast can the compositionally distinct The question is how fast can the compositionally distinct magmas mix?magmas mix?

• We present results of a detailed petrologic study of andesite We present results of a detailed petrologic study of andesite and basalt that erupted simultaneously at Karymsky volcanic and basalt that erupted simultaneously at Karymsky volcanic center beginning in January 1996.center beginning in January 1996.

• Continuous eruption of Karymsky offered an unique Continuous eruption of Karymsky offered an unique opportunity to determine compositional variations of its opportunity to determine compositional variations of its magma caused by basaltic recharge. These variations magma caused by basaltic recharge. These variations indicate that mixing can be surprisingly fast and thorough.indicate that mixing can be surprisingly fast and thorough.

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Location

160 E0 165 E0

60N

0

52N

0

56N

0

100 km

Area ofLandsatimage

KamchatkaPeninsula,Russia

BeringSea

Ray Sterner, Johns Hopkins University, Applied Physics Laboratory, 1998

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Geological background

• Karymsky volcano and Academy Nauk Karymsky volcano and Academy Nauk caldera belong to a chain of volcanoes, caldera belong to a chain of volcanoes, calderas, and maars, the location of which is calderas, and maars, the location of which is controlled by a local north-trending fault.controlled by a local north-trending fault.

• Karymsky is a ~5300-yr-old andesitic Karymsky is a ~5300-yr-old andesitic stratovolcano located in the center of a stratovolcano located in the center of a ~7900-yr-old caldera. During the past 500 yr, ~7900-yr-old caldera. During the past 500 yr, the volcano has been in a state of frequent, the volcano has been in a state of frequent, but intermittent eruptive activity.but intermittent eruptive activity.

• Academy Nauk caldera is centered 9 km Academy Nauk caldera is centered 9 km south of Karymsky on the same fault system. south of Karymsky on the same fault system. Since its caldera-forming event (ca. 40,000 yr Since its caldera-forming event (ca. 40,000 yr B.P.), the volcanic activity within the caldera B.P.), the volcanic activity within the caldera was confined to phreatomagmatic eruptions was confined to phreatomagmatic eruptions of basalt, which have occurred at least twice of basalt, which have occurred at least twice since 5000 yr B.P.since 5000 yr B.P.

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1996 eruption of Karymsky and Academy Nauk

• The magmas erupted simultaneously.The magmas erupted simultaneously.

• The erupted magmas had strongly contrasting bulk The erupted magmas had strongly contrasting bulk compositions. Academy Nauk vent produced basalt compositions. Academy Nauk vent produced basalt (52.2 wt% SiO(52.2 wt% SiO22), while Karymsky summit vent ), while Karymsky summit vent

erupted andesite (62.4 wt% SiOerupted andesite (62.4 wt% SiO22).).

• The eruptive vents are located along the same The eruptive vents are located along the same active fault.active fault.

• Significant ground deformation occurred between Significant ground deformation occurred between eruptive vents. Extension between eruptive vents eruptive vents. Extension between eruptive vents occurred gradually, rather than catastrophically.occurred gradually, rather than catastrophically.

The most recent episode of volcanic activity at The most recent episode of volcanic activity at Karymsky started on January 2, 1996, after 13 yr of Karymsky started on January 2, 1996, after 13 yr of dormancy. It began with simultaneous eruption of dormancy. It began with simultaneous eruption of andesite from the central vent of Karymsky andesite from the central vent of Karymsky volcano and basalt from a new vent, which formed volcano and basalt from a new vent, which formed in the northern part of Academy Nauk caldera.in the northern part of Academy Nauk caldera.

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 eruption of basalt coincident with the start of the most recent cycle of activity at Karymsky is the most recent cycle of activity at Karymsky is suggestive of eruptive triggering. suggestive of eruptive triggering.

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Questions to answer:

Karymsky, view from SWAugust 1999

Academy Nauk eruptive center,view from North, July 1998

• Was there a mixing of andesite Was there a mixing of andesite and basalt?and basalt?

• If yes, how can we explain the If yes, how can we explain the homogeneity of andesite? Was the homogeneity of andesite? Was the mixing that fast?mixing that fast?

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Samples and analytical techniques

• Electron microprobe (major elements)Electron microprobe (major elements)

Cameca SX-50 at University of Alaska Cameca SX-50 at University of Alaska Fairbanks;Fairbanks;

15 kV acceleration voltage;15 kV acceleration voltage;

10 nA beam current;10 nA beam current;

5 micron beam (10 microns for glass)5 micron beam (10 microns for glass)

• LA-ICP-MS (Ba and Sr)LA-ICP-MS (Ba and Sr)

Micromass Platform ICP- HEX-MS Micromass Platform ICP- HEX-MS at Michigan State University;at Michigan State University;

Cetac LSX 200 laser ablation Cetac LSX 200 laser ablation system equipped with UV laser;system equipped with UV laser;

30 micron beam.30 micron beam.

Samples of pyroclastics Samples of pyroclastics of Karymsky vs. time of Karymsky vs. time of their eruption. of their eruption. Length of hori-zontal Length of hori-zontal bars corresponds to bars corresponds to approximate time of approximate time of lava flows effusion.lava flows effusion.

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Petrography

Basalt Andesite Olivine 4 None Magnetite <0.3 1 Matrix glass 59 72 Orthopyroxene <0.3 1 Plagioclase 30 23 Clinopyroxene 6 3 Total 100 100

Modal Abundances, vol. %

Pl

Pl

CPx

Pl

Karymsky andesite

Ol

Pl

Academy Nauk basalt

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Karymsky: Glass composition vs. time

Compositions of volcanic ash glass plotted against the date of eruption. Melt of Karymsky 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 andesite, which erupted in February 1996, was the most mafic. Error bars correspond to 22 in electron microprobe analyses of ash samples. in electron microprobe analyses of ash samples.

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.

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Karymsky and Academy Nauk plagioclases

Academy Nauk Karymsky

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

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Karymsky: Calcic cores in plagioclases

Karymsky

0

5

10

15

20

25

30

35

40 46 52 58 64 70 76 82 88

Composition of plagioclase, An (mol %)

Fre

quen

cy, n

orm

aliz

ed to

100

AN basaltKarymsky andesite

N=650

N=35

• Approximately 25 vol.% of Approximately 25 vol.% of plagioclase phenocrysts in plagioclase phenocrysts in Karymsky andesite contain calcic Karymsky andesite contain calcic cores.cores.

• Composition and texture of cores Composition and texture of cores match those of plagioclases in match those of plagioclases in Academy Nauk basalts.Academy Nauk basalts.

• Widths of sodic rims are consistent Widths of sodic rims are consistent with introduction of calcic cores at with introduction of calcic cores at the onset of eruption (the onset of eruption (2.5 × 102.5 × 10–9–9 mm/smm/s plagioclase growth rate) plagioclase growth rate)

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Karymsky: Xenocrysts of olivine

Pl

OPx

Ol

Ca-core

a

b

0

5

10

15

20

25

30

35

40

66 68 70 72 74 76 78 More

Composition of olivine, Fo mol.%

Frequency

, n

orm

aliz

ed t

o 1

00 AN basalt

Karymsky andesite

N=79

N=104

c

Photomicrograph (a) and a simplified sketch Photomicrograph (a) and a simplified sketch (b) of Karymsky andesite from the lava flow, (b) of Karymsky andesite from the lava flow, which effusively erupted during April-August which effusively erupted during April-August 1996. Note olivine xenocryst attached to the 1996. Note olivine xenocryst attached to the calcic core of plagioclase – both likely calcic core of plagioclase – both likely introduced to andesite by basaltic introduced to andesite by basaltic replenishment in January, 1996. replenishment in January, 1996. Composition of olivines is shown in figure C. Composition of olivines is shown in figure C.

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Summary

• By late February 1996 Karymsky erupted texturally By late February 1996 Karymsky erupted texturally homogeneous andesites, no mafic enclaves were homogeneous andesites, no mafic enclaves were found.found.

• Within two months of the onset of eruption, the Within two months of the onset of eruption, the composition of melt of Karymsky andesite, as composition of melt of Karymsky andesite, as recorded by glass in tephra, shifted toward a more recorded by glass in tephra, shifted toward a more mafic composition and then gradually returned to its mafic composition and then gradually returned to its original state and remained constant for the original state and remained constant for the following 4 years. following 4 years.

• Andesite contains xenocrysts of basaltic origin, i.e Andesite contains xenocrysts of basaltic origin, i.e calcic plagioclase and olivine, at least part of which calcic plagioclase and olivine, at least part of which was most likely introduced by basaltic replenishment was most likely introduced by basaltic replenishment in January 1996, at the onset of the eruptive cycle.in January 1996, at the onset of the eruptive cycle.

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Viscosity constraints on mixing

AN basalt, dry

Karymsky andesite, dryAN basalt, 4 wt.% HO2

Karymsky ansdesite, 4 wt.% H02

Variations of viscosity vs. temperature for Academy Nauk basalt (2 wt. % H2O) and Karymsky andesite (1.5 wt.% H2O) at 200 MPa total pressure.

The plausibility of The plausibility of mixing can be roughly mixing can be roughly tested using estimates tested using estimates of viscosities for of viscosities for Academy Nauk basalt Academy Nauk basalt and Karymsky andesite. and Karymsky andesite.

• Isobaric crystallization of Isobaric crystallization of basalt was modeled using basalt was modeled using COMAGMAT algorithm COMAGMAT algorithm (Ariskin);(Ariskin);

• Viscosity was calculated using Viscosity was calculated using Shaw (1972) model;Shaw (1972) model;

• Volume of injected basalt was Volume of injected basalt was assumed negligible assumed negligible comparative to the volume of comparative to the volume of the stored andesite and did the stored andesite and did not affect the temperature of not affect the temperature of the produced hybrid the produced hybrid significantly.significantly.

Variations of viscosity vs. temperature for Academy Nauk basalt (2 Variations of viscosity vs. temperature for Academy Nauk basalt (2 wt. % Hwt. % H22O) and Karymsky andesite (1.5 wt.% HO) and Karymsky andesite (1.5 wt.% H22O) at 200 MPa total O) at 200 MPa total pressure.pressure.

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Comparative model

“Dispersion” “Ponding”

Turbulenthybrid mixture

Stagnant mush

Turbulenthybrid mixture

Basalticreplenishment Basaltic

replenishment

-100-400 years recharge period;- highly crystalline Hb-bearing dacites in storage;- high viscosity contrast of mixing magmas;- abundant mafic enclaves;- “dusty-zoned” plagioclase.

-10 years recharge period;- two-Px andesites residing inshallow-level magma reservoir;- low viscosity contrast of mixing magmas;- highly homogenous mixture.

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Conclusion

• Homogeneity of andesite erupted by late February 1996 Homogeneity of andesite erupted by late February 1996 suggests to us that thorough mixing of injected basalt and suggests to us that thorough mixing of injected basalt and andesite occurred in a period of time as short as two months.andesite occurred in a period of time as short as two months.

• Perhaps the rapid and effective blending was facilitated by an Perhaps the rapid and effective blending was facilitated by an only modest contrast in viscosities and temperatures only modest contrast in viscosities and temperatures between the magmas and by a vigorous fluid dynamic regime between the magmas and by a vigorous fluid dynamic regime in Karymsky reservoir.in Karymsky reservoir.

• Karymsky is a well-mixed end member case that reflects the Karymsky is a well-mixed end member case that reflects the short recurrence interval of recharges to the system. In short recurrence interval of recharges to the system. In contrast, Trident volcano in Alaska may represent an contrast, Trident volcano in Alaska may represent an intermediate case, where both “clotting” and direct mixing intermediate case, where both “clotting” and direct mixing occurs, and Kizimen volcano in Kamchatka – a poorly-mixed occurs, and Kizimen volcano in Kamchatka – a poorly-mixed end member case, where clotting along is dominant.end member case, where clotting along is dominant.