Kuritani, T. (1999) Phenocryst crystallization during ascent of alkali basalt magma at Rishiri Volcano, northern Japan, Journal of Volcanology and Geothermal Research, 88, 77-97, doi: 10.1016/S0377-0273(98)00105-X. < 20150918134015-868101 > pub

Map on Earth or stone. Places and analyses are shown.

  • abstract:

    Phenocrysts in volcanic rocks are commonly used to deduce crystallization processes in magma chambers. A fundamental assumption is that the phenocrysts crystallized in the magma chambers at isobaric and nearly equilibrium conditions, on the basis of their large sizes. However, this assumption is not always true as demonstrated here for a porphyritic alkali basalt (Kutsugata lava) from Rishiri Volcano, northern Japan. All phenocryst phases in the Kutsugata lava, plagioclase, olivine, and augite, have macroscopically homogeneous distribution of textures showing features characteristic of rapid growth throughout the crystals. Rarely, a core region with distinct composition is present in all phenocryst phases. Phenocrysts, excluding this core, are occasionally in direct contact with each other, forming crystal aggregates. The equilibrium liquidus temperature of plagioclase, the dominant phase (~35 vol%) in the Kutsugata lava, can never exceed the estimated magmatic temperature, unless the liquidus temperature increases significantly due to vesiculation of the magma during ascent. This suggests that most phenocrysts in the Kutsugata lava were formed by decompression of the magma during ascent in a conduit, rather than by cooling during residence in a magma reservoir. In the magma chamber before eruption, probably located at depth of more than 7 km, only cores of the phenocrysts were present and the magma was nearly aphyric (< 5 vol% crystals), though the observed rock is highly porphyritic with up to 40 vol% crystals. The Kutsugata magma is inferred to have been rich in dissolved H2O (> 4 wt.%) in the magma chamber, and liquidus temperatures of phenocryst phases were significantly suppressed. Large undercooling caused by decompression and degassing of the magma was the driving force for significant crystallization during ascent because of the increase in liquidus temperature due to vapor exsolution. Low ascent rate of the Kutsugata magma, which is suggested by pahoehoe lava morphology and no association of pyroclastics, gave sufficient time for crystallization. Furthermore, the large degree of superheating of plagioclase in the magma chamber caused plagioclase crystallization with low population density and large crystal size, which characterizes the porphyritic nature of the Kutsugata lava. Alkali basalt is likely to satisfy these conditions and similar phenomena are suggested to occur in other volcanic systems.

  • doi: 10.1016/S0377-0273(98)00105-X
  • modified at 2019-07-18
ISEI/main/127/D/4/Kuritani 17 < 20091014092142923.hkitagawa >
20151109091821-112596 Tixhr 269462790 thumb Km 11 < 20151109091821-112596 >pub
  • IGSN: IEDRM0088
  • classification: volcanic:basaltic
  • physical-form: hand specimen
  • quantity (kg): 2.285
  • description: Kutsugata Lava, South Lava
  • modified at 2019-07-18
20151109090906-874432 Tixhr 1796037154 thumb Km 3a < 20151109090906-874432 >pub
  • IGSN: IEDRM0087
  • classification: volcanic:basaltic
  • physical-form: hand specimen
  • quantity (g): 523.41
  • description: Kutsugata Lava, South Lava 1 (augite phenocryst-poor lava)
  • modified at 2019-07-18
ISEI/main/127/D/6/Kuritani 29 < 20091014092122569.hkitagawa >
20151125083843-709246 Tixhr1023603247 thumb Fm-17a < 20151125083843-709246 >pub
  • IGSN: IEDRM008C
  • classification: volcanic:basaltic
  • physical-form: hand specimen
  • quantity (g): 1184.68
  • description: Kutsugata Lava, North Lava
  • modified at 2019-07-18
20151125083641-768130 Tixhr1522928762 thumb Fm-15a < 20151125083641-768130 >pub
  • IGSN: IEDRM008B
  • classification: volcanic:basaltic
  • physical-form: hand specimen
  • quantity (g): 1012.69
  • description: Kutsugata Lava, North Lava
  • modified at 2019-07-18
ISEI/main/127/D/3/Kuritani 20 < 20091014092137832.hkitagawa >
20151124084009-856544 Tixhr1753106849 thumb Ku 20 < 20151124084009-856544 >pub
  • IGSN: IEDRM008A
  • classification: volcanic:basaltic
  • physical-form: hand specimen
  • quantity (g): 163.53
  • description: Kutsugata Lava, South Lava
  • modified at 2019-07-18
ISEI/main/127/D/5/Kuritani 27 < 20091014092125951.hkitagawa >
20151113080017-796937 Tixhr572971475 thumb KR 9 < 20151113080017-796937 >pub
  • IGSN: IEDRM0089
  • classification: volcanic:basaltic
  • physical-form: hand specimen
  • quantity (g): 673.57
  • description: Kutsugata Lava, South Lava 2 (augite phenocryst-rich lava)
  • modified at 2019-07-18
ISEI/main/127/D/6/Kuritani 21 < 20091014092136165.hkitagawa >
20151130084246-069488 Tixhr1286665639 thumb Ta-26 < 20151130084246-069488 >pub
  • IGSN: IEDRM008E
  • classification: volcanic:basaltic
  • physical-form: hand specimen
  • quantity (g): 108.49
  • description: Kutsugata Lava, North Lava
  • modified at 2019-07-18
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name spots global-id
caption assembly stone
Table 1. Major and trace element compositions and modal compositions of representative samples pub major-trace (Kuritani et al., 1999) 14
manual
  • name
  • Kuritani, T.