Groundmass pargasite in the 1991–1995 dacite of Unzen volcano: phase stability experiments and volcanological implications

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Abstract

Pargasite commonly occurs in the dacitic groundmass of the 1991–1995 eruption products of Unzen volcano. We described the occurrence and chemical compositions of amphibole in the dacite, and also carried out melting experiments to determine the low-pressure stability limit of amphibole in the dacite. The 1991–1995 ejecta of the Unzen volcano show petrographic evidence of magma mixing, such as reverse compositional zoning of plagioclase and amphibole phenocrysts, and we used a groundmass separate as a starting material for the experiments. Reversed experiments show that the maximum temperature for the crystallization of amphibole is 930°C at 196 MPa, 900°C at 98 MPa, and 820°C at 49 MPa. Compared with the experimental results on the Mount St. Helens dacite, present experiments on the Unzen dacitic groundmass show that amphibole is stable to pressures ca. 50 MPa lower at 850°C. Available Fe–Ti oxide thermometry indicates the crystallization temperature of the groundmass of the Unzen dacite to be 880±30°C, suggesting that the groundmass pargasite crystallized at >70 MPa, corresponding to a depth of more than 3 km in the conduit. The chlorine content of the groundmass pargasite is much lower than that of phenocrystic magnesiohornblende in the 1991–1995 dacite of Unzen volcano, indicating that vesiculation/degassing of magma took place before the crystallization of the groundmass pargasite. The present study shows that the magma was water oversaturated and that the degassing of magma along with magma mixing caused crystallization of the groundmass amphibole at depths of more than 3 km in the conduit.

Introduction

Amphibole is a common phenocryst component of calc-alkalic andesites and dacites (Jakes and White, 1972; Sakuyama, 1983). The 1991 dacite of Unzen volcano, however, contains pargasite in the groundmass along with phenocrystic magnesiohornblende. The presence of amphibole in the groundmass is unusual, because the crystallization of amphibole generally requires high water pressures (e.g., Helz, 1982), whereas groundmass minerals are commonly considered to crystallize at shallow depths either after effusion or in the conduit. One of the purposes of the present study is to determine whether groundmass amphibole crystallized deep in the conduit or at shallow depth. The secondary purpose of this study arose from a volcanological view point. The 1991–1995 eruption of Unzen volcano was mostly effusive, although the dome lava showed a variable degree of self explosivity, generating pyroclastic flows and two Vulcanian explosions (Sato et al., 1992; Nakada and Fujii, 1993). The self explosive nature of the dacite lava was caused by high excess pore gas pressure (Sato et al., 1992; Sparks, 1997), and the presence of the groundmass amphibole in the ejecta may be related to the high excess pore gas pressure in the extruded viscous lavas or in the shallow conduit. These ideas led us to conduct experimental work on the low-pressure stability limit of amphibole in the Unzen 1991–1995 dacite.

The stability of amphibole has been investigated for more than 3 decades, though few detailed works have been done in the low-pressure range (e.g., review by Helz, 1982). Pure pargasite stability was investigated by Boyd (1959). It was demonstrated that at 900°C, pargasite is stable to pressures of ca. 30 MPa. In volcanic rock systems, many experimental works have been concerned with the role of amphibole in the generation of calc-alkalic magmas and experiments have been conducted mainly in the pressure range of 200 MPa to 3 GPa (Green and Ringwood, 1968; Kushiro et al., 1968; Holloway and Burnham, 1972; Helz, 1973; Cawthorn, 1976; Allen and Boettcher, 1983; Sisson and Grove, 1993). The low-pressure stability limit of amphibole in calc-alkalic volcanic rocks has been experimentally examined only in the Mount St. Helens dacites. Rutherford et al. (1985)and Rutherford and Hill (1993)demonstrated that amphibole is unstable at less than 100 MPa in a Mount St. Helens dacite by reversed experiments, although the experiments of Mertzbacher and Eggler (1984)showed the presence of amphibole at 800°C and 100 MPa in another Mount St. Helens dacite. Therefore, the low-pressure stability of amphibole in calc-alkalic systems has not been well constrained experimentally.

In this paper, we first describe the composition and texture of the groundmass amphiboles in the 1991–1995 dome dacite of Unzen volcano, and then present the results of experiments on the groundmass composition of the Unzen dacite designed to constrain amphibole stability. Finally, we discuss the crystallization conditions of amphibole in the 1991–1995 Unzen dacite and its volcanological implications.

Section snippets

Groundmass amphibole in the 1991–1995 dacite of Unzen volcano

The 1991–1995 eruption products of the Unzen volcano show limited variations in petrography and bulk rock chemistry (Nakada and Motomura, 1999). The SiO2 contents range from 64–66 wt.%, and total phenocryst contents vary from 20 to 30 vol.%. Phenocryst phases are plagioclase, magnesiohornblende, biotite, quartz, magnetite, ilmenite, apatite, and rare orthopyroxene and clinopyroxene. Magnesiohornblende phenocrysts are 0.3–5 mm in length, sometimes with a thin rim of pargasite. The boundary

Starting material

The 1991–1995 dacite of Unzen volcano showed petrographic evidence of magma mixing, and the bulk rock sample is not appropriate as a starting material for the study of amphibole stability in the groundmass. Therefore, we used a groundmass separate of the 1992 dacite of Unzen volcano. Part of a hot block, ca. 2 m across, in the mud flow deposit of May 28, 1992 was sampled, crushed and sieved to 0.15–0.25 mm in diameter. The hot block is >500°C 20 cm interior from the surface at the time of

Amphibole stability and chemistry

Here we compare our experimental data on the stability and composition of amphiboles in the groundmass of the Unzen 1991 lava with those of previously studied systems. Pargasite stability has been studied by Boyd (1959)and Westrich and Holloway (1981). The former study determined the low pressure stability of pargasite to be ca. 1000°C at 100 MPa and 910°C at 50 MPa. Although incorporation of iron in the system may decrease the high-temperature stability limit of amphibole (Gilbert et al., 1982

Conclusions

(A) Pargasite constitutes 1–2 vol.% of the groundmass of the 1991–1995 dacite of Unzen volcano. It is long prismatic and often shows rounded outline. Al2O3 content of the pargasite is ca. 10–13 wt.% with Mg/(Mg+Fe) of ca. 0.66–0.72, whereas those of phenocrystic magnesiohornblende is ca. 7–9 wt.% and 0.60–0.67. Chlorine content of amphibole is 0.05–0.08 wt.% in the core of phenocryst and is 0.01–0.03 wt.% in the rim of phenocryst and the groundmass.

(B) Experimental study showed that pargasite

Acknowledgements

This study was defrayed from the Grant-in-Aid from the Ministry of Education, Science, Sports and Culture of Japan. Kind guidance of Prof. Eiichi Takahashi at the onset of experimental work is gratefully acknowledged. Prof. Jun-ichi Hirabayashi helpfully informed us about F contents of Unzen dacites. Dr. Atsushi Yasuda helped us during the EPMA analyses at ERI, the University of Tokyo. Comments by Prof. Malcolm Rutherford, Dr. Jenni Barclay, and Dr. Atsushi Goto are of much help to improve the

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