Details
| Original language | English |
|---|---|
| Pages (from-to) | 168-180 |
| Number of pages | 13 |
| Journal | Journal of Volcanology and Geothermal Research |
| Volume | 175 |
| Issue number | 1-2 |
| Publication status | Published - 12 Apr 2008 |
Abstract
The compositions of homogenized melt inclusions trapped in plagioclase (Pl) microphenocrysts (40-200 μm length) from mafic enclaves within dacitic rocks erupted at Unzen volcano in 1991-95 were investigated. The SiO2 contents of the melt inclusions vary from 58 to 70 wt.% and Pl anorthite content is An50-70. The stability fields (in terms of temperature and water activity) of natural Pl and coexisting melts from the melt inclusions were estimated from data of phase equilibria experiments performed with a synthetic andesite composition at T = 900-1140 °C, P = 200 MPa, log fO2 = NNO-2-NNO, and water activity of about 0-1. This composition is representative of the average composition of mafic enclaves from the 1991-95 eruption and nearly identical to the composition of andesitic lavas from 1663 Unzen eruption. The temperatures and H2O melt concentrations, calculated using the compositions of coexisting Pl and melt inclusions, provide an estimation of the conditions of andesitic magma evolution within the mafic enclaves prior to eruption. The formation of melt inclusions in plagioclase microphenocrysts occurred at a maximum temperature of ~ 1010 ± 35 °C for a melt containing 2 wt.% H2O and a minimum temperature of ∼ 945 °C ± 30 °C for a melt with ∼ 4 wt.% dissolved H2O. The compositional range of the melt inclusions indicates that the composition of the mafic enclave was not significantly contaminated by the host magma when inclusions were formed. The difference between concentrations of dissolved S (up to 0.06 wt.%) and Cl (up to 0.05 wt.%) in melt inclusions in Pl of mafic enclaves and concentrations of S (< 0.005 wt.%) and Cl (0.05-0.11 wt.%) in melt inclusions in phenocrysts of the dacitic magma clearly implies that two distinct sources for S and Cl in the 1991-95 eruption of Unzen volcano need to be considered. Sulfur degassing was generated by a release of fluids from the high-temperature andesitic magma whereas Cl was degassed from the low-temperature silicic magma. The combination of data from melt inclusions and phase equilibria experiments indicates that the mafic end-member magma at Unzen was already partially crystallized and contained significant proportions (20 to 40 wt.%) of Pl and orthopyroxene (Opx) when melt inclusions started to form. Since clinopyroxene (Cpx) and magnetite (Mt) crystallize after Pl and Opx, the temperatures derived for the mafic end-member magma from coexisting Opx-Cpx and Ilm-Mt pairs do not represent temperatures near to the liquidus. Assuming that the injected mafic magma was nearly aphyric, its initial temperature might have been higher than estimated in previous studies.
Keywords
- andesite, experiment, magma mixing, melt inclusions, phase equilibria, storage conditions, Unzen volcano, volatiles
ASJC Scopus subject areas
- Earth and Planetary Sciences(all)
- Geophysics
- Earth and Planetary Sciences(all)
- Geochemistry and Petrology
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In: Journal of Volcanology and Geothermal Research, Vol. 175, No. 1-2, 12.04.2008, p. 168-180.
Research output: Contribution to journal › Article › Research › peer review
}
TY - JOUR
T1 - Storage conditions and evolution of andesitic magma prior to the 1991-95 eruption of Unzen volcano
T2 - Constraints from natural samples and phase equilibria experiments
AU - Botcharnikov, Roman E.
AU - Holtz, Francois
AU - Almeev, Renat R.
AU - Sato, Hiroaki
AU - Behrens, Harald
N1 - Funding Information: We thank S. Nakada, J. Eichelberger and K. Uto for the organization of this special issue of JVGR. We thank Otto Diedrich for the preparation of samples for analysis and O. Beermann for the help with experiments. J.-I. Kimura and C. Mandeville are greatly acknowledged for the critical and constructive comments that improved the quality of the data presentation and interpretation. This study was funded by the DFG (ICDP project Ho1337/11). Copyright: Copyright 2008 Elsevier B.V., All rights reserved.
PY - 2008/4/12
Y1 - 2008/4/12
N2 - The compositions of homogenized melt inclusions trapped in plagioclase (Pl) microphenocrysts (40-200 μm length) from mafic enclaves within dacitic rocks erupted at Unzen volcano in 1991-95 were investigated. The SiO2 contents of the melt inclusions vary from 58 to 70 wt.% and Pl anorthite content is An50-70. The stability fields (in terms of temperature and water activity) of natural Pl and coexisting melts from the melt inclusions were estimated from data of phase equilibria experiments performed with a synthetic andesite composition at T = 900-1140 °C, P = 200 MPa, log fO2 = NNO-2-NNO, and water activity of about 0-1. This composition is representative of the average composition of mafic enclaves from the 1991-95 eruption and nearly identical to the composition of andesitic lavas from 1663 Unzen eruption. The temperatures and H2O melt concentrations, calculated using the compositions of coexisting Pl and melt inclusions, provide an estimation of the conditions of andesitic magma evolution within the mafic enclaves prior to eruption. The formation of melt inclusions in plagioclase microphenocrysts occurred at a maximum temperature of ~ 1010 ± 35 °C for a melt containing 2 wt.% H2O and a minimum temperature of ∼ 945 °C ± 30 °C for a melt with ∼ 4 wt.% dissolved H2O. The compositional range of the melt inclusions indicates that the composition of the mafic enclave was not significantly contaminated by the host magma when inclusions were formed. The difference between concentrations of dissolved S (up to 0.06 wt.%) and Cl (up to 0.05 wt.%) in melt inclusions in Pl of mafic enclaves and concentrations of S (< 0.005 wt.%) and Cl (0.05-0.11 wt.%) in melt inclusions in phenocrysts of the dacitic magma clearly implies that two distinct sources for S and Cl in the 1991-95 eruption of Unzen volcano need to be considered. Sulfur degassing was generated by a release of fluids from the high-temperature andesitic magma whereas Cl was degassed from the low-temperature silicic magma. The combination of data from melt inclusions and phase equilibria experiments indicates that the mafic end-member magma at Unzen was already partially crystallized and contained significant proportions (20 to 40 wt.%) of Pl and orthopyroxene (Opx) when melt inclusions started to form. Since clinopyroxene (Cpx) and magnetite (Mt) crystallize after Pl and Opx, the temperatures derived for the mafic end-member magma from coexisting Opx-Cpx and Ilm-Mt pairs do not represent temperatures near to the liquidus. Assuming that the injected mafic magma was nearly aphyric, its initial temperature might have been higher than estimated in previous studies.
AB - The compositions of homogenized melt inclusions trapped in plagioclase (Pl) microphenocrysts (40-200 μm length) from mafic enclaves within dacitic rocks erupted at Unzen volcano in 1991-95 were investigated. The SiO2 contents of the melt inclusions vary from 58 to 70 wt.% and Pl anorthite content is An50-70. The stability fields (in terms of temperature and water activity) of natural Pl and coexisting melts from the melt inclusions were estimated from data of phase equilibria experiments performed with a synthetic andesite composition at T = 900-1140 °C, P = 200 MPa, log fO2 = NNO-2-NNO, and water activity of about 0-1. This composition is representative of the average composition of mafic enclaves from the 1991-95 eruption and nearly identical to the composition of andesitic lavas from 1663 Unzen eruption. The temperatures and H2O melt concentrations, calculated using the compositions of coexisting Pl and melt inclusions, provide an estimation of the conditions of andesitic magma evolution within the mafic enclaves prior to eruption. The formation of melt inclusions in plagioclase microphenocrysts occurred at a maximum temperature of ~ 1010 ± 35 °C for a melt containing 2 wt.% H2O and a minimum temperature of ∼ 945 °C ± 30 °C for a melt with ∼ 4 wt.% dissolved H2O. The compositional range of the melt inclusions indicates that the composition of the mafic enclave was not significantly contaminated by the host magma when inclusions were formed. The difference between concentrations of dissolved S (up to 0.06 wt.%) and Cl (up to 0.05 wt.%) in melt inclusions in Pl of mafic enclaves and concentrations of S (< 0.005 wt.%) and Cl (0.05-0.11 wt.%) in melt inclusions in phenocrysts of the dacitic magma clearly implies that two distinct sources for S and Cl in the 1991-95 eruption of Unzen volcano need to be considered. Sulfur degassing was generated by a release of fluids from the high-temperature andesitic magma whereas Cl was degassed from the low-temperature silicic magma. The combination of data from melt inclusions and phase equilibria experiments indicates that the mafic end-member magma at Unzen was already partially crystallized and contained significant proportions (20 to 40 wt.%) of Pl and orthopyroxene (Opx) when melt inclusions started to form. Since clinopyroxene (Cpx) and magnetite (Mt) crystallize after Pl and Opx, the temperatures derived for the mafic end-member magma from coexisting Opx-Cpx and Ilm-Mt pairs do not represent temperatures near to the liquidus. Assuming that the injected mafic magma was nearly aphyric, its initial temperature might have been higher than estimated in previous studies.
KW - andesite
KW - experiment
KW - magma mixing
KW - melt inclusions
KW - phase equilibria
KW - storage conditions
KW - Unzen volcano
KW - volatiles
UR - http://www.scopus.com/inward/record.url?scp=42549100320&partnerID=8YFLogxK
U2 - 10.1016/j.jvolgeores.2008.03.026
DO - 10.1016/j.jvolgeores.2008.03.026
M3 - Article
AN - SCOPUS:42549100320
VL - 175
SP - 168
EP - 180
JO - Journal of Volcanology and Geothermal Research
JF - Journal of Volcanology and Geothermal Research
SN - 0377-0273
IS - 1-2
ER -