Trace element concentrations in plagioclase phenocrysts of Trace element concentrations in plagioclase phenocrysts of Karymsky andesite: Evidence for basalt replenishmentsKarymsky andesite: Evidence for basalt replenishments

Pavel IzbekovPavel Izbekov11, John Eichelberger, John Eichelberger11, , Thomas A. VogelThomas A. Vogel22 and Lina Patino and Lina Patino22

1 Alaska Volcano Observatory, Geophysical Institute University of Alaska Fairbanks

2 Department of Geological Sciences, Michigan State University

Purpose of presentationPurpose 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, July 1998

Location of Karymsky and Academy NaukLocation of Karymsky and Academy Nauk

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

Karymsky volcano, 54003'N 159027' E, is one of the most active volcanoes of the Eastern volca-nic zone of Kamchatka arc, which lies ~220 km inland and parallel to the Kamchatka trench. According to recent geophysical data this corresponds to approximately 120 km above the upper surface of the subducted slab.

Karymsky and Academy NaukKarymsky and Academy Nauk

Academy NaukAcademy NaukAcademy 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. 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, 6100-5400 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.

KarymskyKarymskyKarymsky

Karymsky and Karymsky Lake, September 1997

1996 eruption of Karymsky1996 eruption of Karymskyand Academy Naukand Academy Nauk

• Eruptions started simultaneously

• The erupted magmas were considerably different in chemical composition

• Significant extension occurred perpendicular to the fault connecting two eruptive vents

New ventNew vent

Karymsky, view from SWAugust 1999

Academy Nauk eruptive center,view from North, July 1998

Basalt of Academy Nauk and andesite of Karymsky,Basalt of Academy Nauk and andesite of Karymsky,Comparison of mineralogy and chemistryComparison of mineralogy and chemistry

Aca

dem

y N

au

k b

asal

tK

arym

sky

and

esi

te

Ol

Pl

Pl

Pl

CPx

Pl

Basalt AndesiteOlivine 4 NoneMagnetite <0.3 1Matrix glass 59 72Orthopyroxene <0.3 1Plagioclase 30 23Clinopyroxene 6 3Total 100 100

Modal Abandances, vol. %

AcademyNauk basalt

Karymskyandesites

AcademyNauk basalt

Karymskyandesites

SiO2 52.55 62.75 Eu 0.93 1.49 Al2O3 19.21 16.50 Gd 2.75 5.08 TiO2 0.74 0.91 Tb 0.46 0.86

FeO* 7.64 5.83 Dy 2.93 5.45 MnO 0.15 0.15 Ho 0.60 1.14 CaO 10.55 5.35 Er 1.64 3.15 MgO 5.51 1.90 Tm 0.24 0.47 K2O 0.60 1.62 Yb 1.49 2.99

Na2O 2.91 4.72 Lu 0.24 0.48 P2O5 0.14 0.26 Ba 170.96 398.12Total 100 100 Th 0.58 1.61

Ni 28.50 2.46 Nb 1.38 3.24 Cr 82.50 9.00 Y 15.70 30.68

V 206.00 127.08 Hf 1.70 3.93 Zr 73.50 141.92 Ta 0.11 0.25

Cu 63.50 31.54 U 0.31 0.87La 5.40 11.49 Pb 2.63 5.88Ce 12.27 26.25 Rb 7.56 21.41Pr 1.74 3.60 Cs 0.31 0.86Nd 8.37 16.91 Sr 454.09 357.18Sm 2.51 4.86 Sc 31.86 22.71

Chemical composition of basalt and andesite

Variations of melt composition in Karymsky andesiteVariations of melt composition in Karymsky andesite

50

55

60

65

70

75

80

1/1/96 7/19/96 2/4/97 8/23/97 3/11/98

wt.

%

65

67

69

71

73

75

1/1/96 4/10/96Time

SiO

2

Glass ofAN basalt

Composition of volcanic glass from Karymsky tephra produced during 1996-1998 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.

Types of Plagioclase PhenocrystsTypes of Plagioclase Phenocrysts

Academy Nauk basalt Karymsky andesite

Rimmed

Oscillatory

Analytical methodsAnalytical 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.

Results of analysesResults of analyses

Microprobe data ICP-MS data

0

20

40

60

80

100

120

140

160

43 49 55 61 67 73 79 85 91

An, mole %

Fre

quen

cy

KarymskyAN basalt

Examples of individual Plagioclase phenocrystsExamples of individual Plagioclase phenocrysts

Basalt of Academy Nauk

A

BC D

E

F

Sieved texture;

An80-An88;

Low Ba/Ca and Sr/Ca ratios.

Examples of individual Plagioclase phenocrystsExamples of individual Plagioclase phenocrysts

C D

Basalt of Academy Nauk

No melt inclusions in the core;

An77-An90;

Low Ba/Ca and Sr/Ca ratios.

Examples of individual Plagioclase phenocrystsExamples of individual Plagioclase phenocrysts

Karymsky andesite,oscillatory zoned phenocryst

A B

No melt inclusions;

An47-An60;

High Ba/Ca and Sr/Ca ratios.

Examples of individual Plagioclase phenocrystsExamples of individual Plagioclase phenocrysts Karymsky andesite,rimmed phenocryst

Sharp boundary between core and rim;

Rim has the same composition as oscillatory zoned plagioclases, while core has the same composition as plagioclases from Academy Nauk basalt

ConclusionConclusion

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.