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

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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

introduction
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.
geological background
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.
1996 eruption of karymsky and academy nauk
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.

questions to answer
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?
samples and analytical techniques
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.

petrography
Petrography

Karymsky andesite

Academy Nauk basalt

Pl

CPx

Ol

Pl

Pl

Pl

Modal Abundances, vol. %

karymsky glass composition vs time

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.

karymsky and academy nauk plagioclases

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.

slide11

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)
karymsky xenocrysts of olivine

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.

summary
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.
viscosity constraints on mixing
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.

conclusion
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.
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