Evolution of Deeper Basaltic and Shallower Andesitic Magmas during the AD 1469-1983 Eruptions of Miyake-Jima Volcano, Izu-Mariana Arc: ...
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JOURNAL OF PETROLOGY VOLUME 44 NUMBER 12 PAGES 2113±2138 2003 DOI: 10.1093/petrology/egg072
Evolution of Deeper Basaltic and Shallower
Andesitic Magmas during the AD 1469---1983
Eruptions of Miyake-Jima Volcano, Izu---
Mariana Arc: Inferences from Temporal
Variations of Mineral Compositions in
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Crystal-Clots
MIZUHO AMMA-MIYASAKA1* AND MITSUHIRO NAKAGAWA2
1
HOKKAIDO BRANCH, GEOLOGICAL SURVEY OF JAPAN, AIST, N8W2, KITA-KU, SAPPORO 060-0808, JAPAN
2
DEPARTMENT OF EARTH AND PLANETARY SCIENCES, GRADUATE SCHOOL OF SCIENCE, HOKKAIDO
UNIVERSITY, SAPPORO 060-0810, JAPAN
RECEIVED SEPTEMBER 2, 2002; ACCEPTED MAY 29, 2003
Miyake-jima volcano has erupted at least 13 times during the end-member magmas suggest that the basaltic magma has
period 1469---1983. To understand the historic magmatic pro- differentiated gradually since 1469, and that its magmatic
cesses, we focus on the mineral assemblage and chemical com- temperature has fallen from 1220 to 1180 C. Conversely, the
positions of crystal-clots in single samples from each of the andesitic magma has changed in a complex fashion to become
eruptions. Most of the historic lavas consist of nearly aphyric more mafic (the magmatic temperature rose from 1050 to
to weakly porphyritic basalt to andesite, but there also exist 1100 C). As a result of this study, it is estimated that the
megacryst-bearing rocks. The megacrysts are considered to be basaltic magma after the 1983 eruption was the least mafic, and
xenocrysts from a deep-seated plutonic body. Many samples of the andesitic magma the most mafic, of the historic eruptions.
each eruption contain two types of clots beside megacrysts,
termed here B-type and A-type. The B-type clots are composed
of olivine, clinopyroxene and plagioclase, whereas the A-type KEY WORDS: andesite; basalt; crystal-clots; evolution of magma;
clots additionally contain magnetite and orthopyroxene. Compo- Miyake-jima volcano; magma mixing
sitional relationships between these mafic minerals suggest that
the minerals in the same type of clots are in equilibrium.
Comparing the chemical compositions of the minerals in the INTRODUCTION
two types of clots in each sample, they are derived from distinct The present state of the magma plumbing system
magmas: the B-type clots from basaltic magma and the A-type beneath an active volcano provides important clues
clots from andesitic magma. During the historic activity, the about the nature of the volcanic activity and the poten-
magma plumbing system appears to have included two magma tial for future eruptions. Temporal changes in the
storage systems: a deep-seated basaltic and a shallower andesitic magma plumbing system can be evaluated through
one. In many cases, basaltic magma has injected into shallower detailed petrological studies of stratigraphically well-
andesitic magma to form mixed magma; however, andesitic constrained eruption sequences. Many studies have
magma has sometimes erupted alone without extensive injections focused on the magma plumbing system and eruption
of basaltic magma. Temporal variations of mineral composi- processes based on the eruptive sequence of a single
tions in the clots and estimated whole-rock compositions of the eruption (e.g. Reagan et al., 1987; Wolfe et al., 1987;
*Corresponding author. Telephone: 81-11-709-1813. Fax: Journal of Petrology 44(12) # Oxford University Press 2003; all rights
81-11-709-1817. E-mail: m.miyasaka@aist.go.jp reservedJOURNAL OF PETROLOGY VOLUME 44 NUMBER 12 DECEMBER 2003
Garcia et al., 1989, 1992, 1996, 2000; Wolf & focused on the historic 1940 and 1962 eruptions and
Eichelberger, 1997; Marianelli et al., 1999; Nakagawa established that deeper basaltic magma and shallower
et al., 1999, 2002; Streck et al., 2002). A series of papers andesitic magma existed beneath the volcano during
on the Pu'u O'o eruption of Kilauea (Garcia et al., this period. It is not clear, however, how long the
1989, 1992, 1996, 2000) has revealed changing mag- magma plumbing system has existed and how it has
matic processes over a period of 17 years; Reagan et al. evolved through time.
(1987) and Streck et al. (2002) discussed the role of In this paper, we describe the evolution of the whole-
basalt replenishment in the long-lived (about 30 years) rock and mineral chemistry of the historic lavas from
eruption of Arenal volcano, Costa Rica. Clearly, to 1469 to 1983. We focus on the mineral assemblage and
study the evolution of magma plumbing systems, it is chemical compositions of crystal-clots in single samples
important to investigate as long a period of activity to clarify whether minerals in each sample are in equi-
as possible [e.g. Wright & Fiske (1971) for Kilauea, librium or not. We demonstrate the existence of two
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Hawaii; Borgia et al. (1988) for Arenal, Costa Rica; magma storage systems since 1469, one basaltic and
Fichaut et al. (1989) for Mt. Pelee, Martinique; Belkin one andesitic. We carefully investigate the evolution
et al. (1993), Villemant et al. (1993) and Tedesco et al. of both magma types for the past 500 years and the
(1998) for Vesuvius, Italy; D'Antonio et al. (1999) for interaction between these magmas during each erup-
Campi Flegrei caldera, Italy; Nakano & Yamamoto tion episode. This provides important new insights into
(1991) for Izu---Oshima, Japan]. In most of these stu- the evolution of the historic magma plumbing system,
dies, however, the evolution of the magma plumbing and allows us to estimate the possible state of the system
systems was discussed mainly in terms of the temporal just before the 2000 eruption.
change in whole-rock compositions. Although detailed
analysis of phenocryst minerals is useful to understand
magmatic processes (e.g. Nakamura, 1995; Umino & GEOLOGY AND 1469---1983
Horio, 1998; Nakagawa et al., 1999, 2002; Streck et al.,
2002), a systematic mineralogical study of the eruptive
ERUPTIONS OF MIYAKE-JIMA
activity of an active volcano for a considerable period VOLCANO
(for hundreds to thousands years), based on the evolu- Miyake-jima volcano is a composite volcano with two
tion of both whole-rock and mineral chemistry, has not nested calderas (Fig. 1), and is composed of tholeiitic
been carried out, except for that by Borgia et al. (1988). basalt and andesite. Based on the eruption style and
Miyake-jima volcano has erupted at least 13 times petrological characteristics of lavas and scoria, Tsukui
since AD 1469. During the period 1469---1983, magma et al. (2001) divided the volcanic activity of the last
effused mainly from flank fissures. The most recent 10 000 years into four stages: 10 000---7000, 4000---2500,
eruption, which began in June 2000 (Nakada et al., 2500 years BP to AD 1154, and since AD 1469. During
2001), however, is distinctive compared with other the first stage, the main cone was constructed, and the
historic eruptions. Earthquake swarms occurred begin- outer caldera filled. Erupted materials are porphyritic
ning the night of June 26, and a submarine eruption basalts. The second stage began after a 3000 year
took place on the morning of June 27. A summit erup- repose period, consisting of andesitic lavas and scoria
tion (ash plume) followed on July 8, and the summit erupted from lateral and central vents. The third stage
area suddenly subsided. The collapse continued began with a large-scale eruption that formed the inner
`silently' until mid-August, resulting in the formation caldera, which has subsequently been filled with nearly
of a new caldera with a volume of about 06 km3 . The aphyric to weakly porphyritic basalts.
volcano is now (April, 2003) discharging a large quan- The latest stage, since1469, began after a 300 year
tity of volcanic gases (SO2, etc.) and inhabitants of repose period. The eruptions were characterized by
Miyake-jima have been evacuated since September the effusion of lavas and scoria mainly from NE---SW-
2000. Several petrological studies have been carried trending flank fissures (Fig. 1). Eruptions occurred
out on recent historic eruptions, except the 2000 erup- every 50---70 years before 1811, and have become
tion, of Miyake-jima volcano (e.g. Iwasaki et al., 1982; more frequent (every 20---70 years) since then
Fujii et al., 1984; Soya et al., 1984; Sato et al., 1996; (Table 1). Most of the eruptions continued for a short
Amma-Miyasaka & Nakagawa, 1998, 2002). Sato et al. duration (typically a day to a month) whereas the
(1996) dealt with most of the historic lavas since 1643. 1763 eruption was prolonged, lasting for 6 years.
They showed that whole-rock compositions became The volume of magma erupted is estimated to be
more silicic from 1643 to 1874, and then more mafic 50066 km3 for each eruption. Although these values
from 1940 until 1983. They proposed that mafic may be underestimated, especially for the older
magma was recently injected into the magma plumb- eruptives, there seems to be little correlation between
ing system. Amma-Miyasaka & Nakagawa (2002) the eruptive mass and the duration of each eruption.
2114AMMA-MIYASAKA AND NAKAGAWA MAGMA EVOLUTION, MIYAKE-JIMA
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Fig. 1. Location of Miyake-jima volcano and distribution of lavas and pyroclastic deposits erupted during the period 1469---1983. These
eruptions were characterized by the effusion of lavas and scoria mainly from NE---SW-trending flank fissures. The 1643 lava flows are
composed of three lobes: main-N, main-S and central. Modified from Isshiki (1984).
Table 1: Summary of eruptive history of Miyake-jima volcano from 1469 to 1983
Age (date AD) Vent location Duration Volume* (km3 DRE) Rocks Total phenocryst (vol. %) SiO2 (wt %)
1983 SW flank 15 h 0.007 nearly aphyric---weakly porphyritic 1.0---8.5 52.0---54.4
1962 NE flank 30 h 0.006 nearly aphyric 0.2---1.5 53.1---55.5
1940 summit 25 days 0.015 nearly aphyric---weakly porphyritic 2.8---8.0 54.2---54.7
NE flank 23 h nearly aphyric 0.7---2.6 54.1---56.4
megacryst bearing 1.4---27.3 51.4---55.5
1874 N flank 4---5 days 0.01 nearly aphyric 1.6---3.0 54.5---55.3
megacryst bearing 3.7---15.7 52.8---55.2
1835 W flank 10 days 50.001 nearly aphyric---weakly porphyritic 1.3---7.9 52.1---55.5
1811? NE flank 7 days 50.001 megacryst bearing 3.1---10.7 54.6---55.2
1763---1769 SW flank 6 years 0.066 nearly aphyric 0.6---1.4 53.0---54.5
1712 SW flank 14 days 0.001 nearly aphyric 1.3---3.2 50.6---52.1
1643 SW flank 21 days 0.012 nearly aphyric 0.3---1.3 50.8---53.7
1595 SE flank? ? 50.001 nearly aphyric 1.2---1.7 51.8---52.6
1535 SE flank? ? 0.003 nearly aphyric 0.9---1.7 51.8---52.6
1469 W flank? ? 0.002 nearly aphyric 1.4---2.7 53.2---54.9
*Quoted from Tsukui & Suzuki (1998) except for the 1811 lavas.
2115JOURNAL OF PETROLOGY VOLUME 44 NUMBER 12 DECEMBER 2003
ANALYTICAL METHODS 1835 lavas, however, do not contain olivine pheno-
crysts, whereas the 1643 and 1712 lavas lack ortho-
For each of the 1469---1983 eruptions, we collected
pyroxene. In addition, in some of the 1940 and 1962
samples to represent all source vents as well as the
lavas both olivine and orthopyroxene phenocrysts are
complete eruptive sequence. Mineral core composi-
absent (Amma-Miyasaka & Nakagawa, 1998).
tions were determined by a single analysis from the
Plagioclase is the dominant phenocryst phase and its
centre of the minerals for representative samples that
maximum size is 46 mm in length. The An content of
reflect the whole-rock variations of each eruption. The
the plagioclase phenocrysts is in the range 53---96 mol %
JEOL 733 and 8800 electron probe microanalysers
(Fig. 2). Histograms for the 1835, 1962 and 1983
at Hokkaido University were used for the mineral
samples show unimodal distributions with a peak of
analyses. Operating conditions were 15 kV accelerat-
An 80---90, whereas the other samples show bimodal
ing voltage and 20 nA beam current with a minimum
distributions. Although the peaks of the An contents
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spot size of 1 mm. Each element was counted for 30 s on
are variable in each eruption, most of them are in the
the peak and 20 s on the background. Corrections were
range of An 80---90 and An 60---75.
made according to the ZAF method. Whole-rock com-
Clinopyroxene less than 24 mm in length is the
positions were determined by X-ray fluorescence
dominant mafic phenocryst. Most of the phenocrysts
(XRF), using a Philips PW-1404 system with a Rh
are augite, although pigeonite sometimes occurs in
tube at Hokkaido University. Glass beads were used
samples of the 1712 and 1940 lavas (Fig. 3). The Mg-
for major element analysis, and pressed pellets for trace
number of most of the clinopyroxene phenocrysts is in
element analysis. Major element compositions were
the range 60---79; however, one phenocryst in the 1962
determined for 230 samples, and trace element compo-
lava is more mafic (Mg-number 480). There is little
sitions for 198 samples from the 1469---1983 eruptive
difference in the compositions of the phenocrysts from
products. Whole-rock compositions of all the samples
samples of different eruption ages, although composi-
for which we have analysed mineral compositions are
tional variations in the 1835 and 1962 lavas seem to be
listed in Table 2 and representative mineral composi-
smaller compared with other eruptives (Fig. 3).
tions are given in the Appendix. The complete whole-
Olivine phenocrysts are usually smaller than 20 mm
rock and mineral composition dataset is included in
in length. The Fo content of the olivine phenocrysts is
Electronic Appendices 1---5, which may be downloaded
in the range 52---78 mol % (Fig. 3). The variations in
from the Journal of Petrology web site at http://www.
the phenocryst composition in the samples older than
petrology.oupjournals.org/.
the 1763 eruption are greater than those in the samples
younger than the 1874 eruption.
PETROGRAPHY AND MINERAL Orthopyroxene and magnetite phenocrysts are rare
and smaller than 16 mm and 04 mm in length, respect-
CHEMISTRY ively. The Mg-number of orthopyroxene phenocrysts
Although most of the 1469---1983 lavas are nearly is in the range 52---75 (Fig. 3). Although most of them
aphyric to weakly porphyritic, with less than 9 vol. % are in the range Mg-number 60---75, some of the
of phenocrysts, there also exist rocks characterized by orthopyroxene phenocrysts in the 1962 and 1983
the presence of anorthite (up to 3 cm in length) and lavas have Mg-number 560.
olivine megacrysts (Table 1). Phenocryst minerals are
plagioclase, olivine, clinopyroxene, orthopyroxene and
magnetite. The groundmass is composed of plagioclase, Megacryst-bearing rocks
clinopyroxene, magnetite and brown glass, and ranges Megacryst-bearing rocks occur only in the 1811, 1874
in texture from intersertal to hyalo-ophitic. and 1940 flank eruption sequences (Table 1), and are
orthopyroxene-bearing clinopyroxene---olivine basalt
to andesite (Figs 2 and 3). The total volume of pheno-
Megacryst-free rocks (nearly aphyric to crysts varies from 1 to 27 vol. %, and gradually
weakly porphyritic rocks) increases from the 1811 (511 vol. %), to 1874
All of the dated lavas except for the 1811 lavas include (516 vol. %) and to 1940 (527 vol. %) eruptions.
nearly aphyric rocks (Table 1), and their modal Petrographic characteristics and mineral compositions
volumes of phenocrysts are less than 3 vol. %. Some of of the megacrysts in the 1940 samples were described
the 1835 and 1940 summit and 1983 lavas are weakly by Amma-Miyasaka & Nakagawa (2002). Although
porphyritic, containing 3---9 vol. % phenocrysts. We the mineral assemblage and chemical compositions of
generally call these `megacryst-free rocks'. Mafic pheno- the megacrysts in the 1874 samples are similar to those
crysts in the megacryst-free rocks usually consist of clino- of the 1940 eruption, clinopyroxene megacrysts are also
pyroxene, orthopyroxene and olivine (Figs 2 and 3). The found in the 1811 megacryst-bearing rocks in addition
2116Table 2: Representative whole-rock compositions during the period 1469---1983
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AMMA-MIYASAKA
Age: 1983 1983 1962 1962 1962 1962 1962 1962 1940 1940 1940 1940 1940 1940
Sample no.: 1522 2203 1802 1810 122508 2233 122005 1901 2003 2008 1803 1820* 122002 1815
from https://academic.oup.com/petrology/article/44/12/2113/1460172
wt %
SiO2 52.69 52.87 53.24 52.77 54.62 52.78 53.23 51.67 53.39 53.99 55.67 50.46 53.46 55.60
TiO2 1.31 1.39 1.36 1.38 1.37 1.40 1.39 1.40 1.32 1.32 1.31 0.97 1.39 1.27
Al2O3 14.80 15.39 14.46 14.46 14.53 14.65 14.60 14.49 14.78 14.90 14.87 16.26 14.56 14.72
AND NAKAGAWA
Fe2O3 14.21 14.44 14.19 14.55 13.67 14.74 14.25 14.92 13.90 13.71 13.16 12.49 14.21 12.93
MnO 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.19 0.20 0.20
MgO 4.36 4.00 3.51 3.77 3.52 3.98 3.98 3.97 3.86 3.91 3.54 6.30 3.91 3.31
CaO 9.13 9.23 8.01 8.34 8.04 8.64 8.64 8.65 8.52 8.47 7.99 10.10 8.46 7.70
Na2O 2.79 2.77 3.11 3.01 3.06 2.84 2.80 2.88 2.91 2.93 2.97 2.18 2.85 3.25
K2O 0.58 0.53 0.66 0.62 0.64 0.56 0.55 0.57 0.60 0.59 0.62 0.41 0.61 0.71
P2O5 0.16 0.15 0.17 0.17 0.17 0.15 0.16 0.16 0.16 0.15 0.15 0.11 0.15 0.18
2117
Total 100.23 100.97 98.91 99.27 99.80 99.94 99.80 98.91 99.64 100.17 100.48 99.47 99.80 99.87
MAGMA EVOLUTION, by
FeO*/MgO 2.93 3.25 3.64 3.47 3.49 3.33 3.22 3.38 3.24 3.15 3.34 1.78 3.27 3.51
ppm
Sc 49 47 48 49 46 52 51 52 46 47 45 36 51 41
V 386 437 357 389 301 439 418 434 365 360 313 312 411 289
Cr 21 10 7 7 9 9 9 7 11 12 10 33 9 11
Ni 12 10 8 8 7 10 9 10 10 8 8 48 9 7
Cu 119 147 134 142 98 142 142 138 132 116 82 109 139 98
Rb 8 7 8 8 8 8 8 7 8 8 8 6 8 9
Zn 117 120 122 123 116 124 119 121 120 117 117 98 125 120
MIYAKE-JIMA
Sr 235 236 235 235 234 234 233 234 236 234 235 235 233 234
guest on 14 January 2021
Y 34 35 37 36 38 35 35 35 37 35 37 25 35 40
Zr 72 71 81 77 83 71 70 71 74 74 82 50 74 87
Ba 190 187 213 203 231 191 186 196 203 204 224 136 201 237
Pb 4 4 3 4 5 4 5 4 4 3 4 5 8 5
Ga 16 17 16 16 16 16 16 16 16 16 15 14 15 16Table 2: continued
Age: 1940 1874 1874 1874 1874 1874 1874 1835 1835 1811? 1811? 1763 1763 1763
Sample no.: 1818* 1828* 1841 1842 11008* 11207* 31910* 1918 1919 1806* 1808* 1603 1609 2021
wt %
SiO2 50.79 52.14 53.11 54.93 52.72 52.81 52.97 52.38 53.26 54.10 53.20 50.12 51.50 52.97
TiO2 0.96 1.18 1.35 1.36 1.21 1.24 1.22 1.25 1.29 1.13 1.17 1.29 1.35 1.36
Al2O3 16.55 16.13 14.56 14.89 15.72 15.62 15.58 14.95 14.92 15.27 14.88 14.90 14.68 14.82
Fe2O3 12.31 13.08 14.03 13.62 13.22 13.35 13.08 13.85 13.48 12.90 13.38 15.00 14.82 14.35
MnO 0.19 0.20 0.20 0.20 0.19 0.20 0.23 0.20 0.20 0.19 0.19 0.20 0.20 0.20
MgO 6.24 4.47 3.60 3.67 4.27 3.96 4.10 4.47 3.45 3.92 3.94 4.88 3.97 3.99
JOURNAL OF PETROLOGY
CaO 10.19 9.57 8.18 8.25 9.35 9.15 9.17 9.21 8.15 8.39 8.26 9.96 8.74 8.53
Na2O 2.20 2.59 3.06 3.01 2.60 2.76 2.74 2.61 3.06 2.92 2.99 2.45 2.86 2.93
K2O 0.39 0.50 0.64 0.64 0.53 0.55 0.57 0.57 0.66 0.64 0.65 0.48 0.59 0.59
P2O5 0.11 0.14 0.17 0.18 0.14 0.15 0.15 0.14 0.18 0.14 0.15 0.12 0.15 0.15
2118
Total 99.93 100.00 98.90 100.75 99.96 99.78 99.80 99.63 98.65 99.60 98.81 99.40 98.86 99.89
VOLUME 44
FeO*/MgO 1.77 2.63 3.50 3.34 2.78 3.03 2.87 2.79 3.51 2.96 3.05 2.76 3.36 3.23
ppm
Sc 41 43 46 48 45 42 41 48 42 42 40 53 49 47
V 321 332 340 334 349 338 347 368 320 304 309 452 417 393
Cr 36 20 9 10 20 17 12 28 10 17 15 32 10 7
NUMBER 12
Ni 44 18 7 10 17 11 12 13 8 15 13 15 10 8
Cu 115 124 130 129 132 130 137 95 97 140 140 134 137 124
Rb 6 7 8 8 7 8 8 8 9 8 8 6 8 8
Zn 101 105 122 123 110 110 106 112 116 113 114 112 119 123
Sr 233 241 238 237 237 244 243 231 242 228 227 230 234 237
Y 25 31 38 38 33 33 34 34 37 35 34 31 35 36
Zr 50 63 81 79 65 68 73 72 83 76 77 60 72 74
DECEMBER 2003
Ba 145 182 218 219 179 191 208 186 224 207 212 165 189 213
Pb 5 5 4 4 1 11 3 5 5 4 4 4 5 6
Ga 14 17 16 16 15 16 18 15 16 16 16 16 17 17
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Sample no.: 2023 1504 1936 1620 1627 2222 2110 2113 2501 2116 2117 2502 1921 2227 2315
wt %
SiO2 51.75 49.36 51.24 49.64 51.08 51.81 51.24 50.68 51.08 50.64 51.38 51.63 51.78 53.46 54.17
TiO2 1.36 1.34 1.31 1.31 1.38 1.19 1.14 1.15 1.15 1.14 1.15 1.16 1.17 1.18 1.18
Al2O3 14.44 14.55 14.93 14.71 14.45 15.12 15.34 15.33 15.39 15.23 15.35 15.47 14.95 15.04 15.09
Fe2O3 14.73 15.54 14.73 15.30 15.33 13.76 13.68 13.88 13.74 13.86 13.68 13.70 13.61 13.33 13.01
MnO 0.20 0.21 0.21 0.21 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.19
MgO 3.85 4.94 4.98 4.89 4.31 4.80 5.13 5.05 5.09 5.03 5.08 5.14 4.47 4.40 4.18
CaO 8.48 10.12 10.06 10.05 9.18 9.61 10.02 10.03 9.99 10.06 10.02 10.06 9.25 9.09 8.72
Na2O 2.93 2.48 2.40 2.51 2.67 2.52 2.40 2.41 2.37 2.43 2.37 2.35 2.59 2.59 2.68
K2O 0.60 0.49 0.45 0.46 0.53 0.52 0.44 0.45 0.45 0.46 0.45 0.44 0.59 0.59 0.62
P2O5 0.16 0.13 0.13 0.12 0.14 0.13 0.12 0.12 0.12 0.12 0.12 0.12 0.14 0.14 0.15
Total 98.50 99.16 100.44 99.20 99.27 99.66 99.71 99.30 99.58 99.17 99.80 100.27 98.75 100.02 99.99
AMMA-MIYASAKA AND NAKAGAWA
FeO*/MgO 3.44 2.83 2.66 2.81 3.20 2.58 2.40 2.47 2.43 2.48 2.42 2.40 2.74 2.72 2.80
2119
ppm
Sc 47 54 52 53 53 44 46 48 47 45 50 45 45 45 43
V 404 462 455 446 452 362 386 380 383 388 388 373 357 352 340
Cr 7 31 32 32 8 37 42 42 43 42 45 42 34 30 30
Ni 9 15 15 14 11 16 15 17 16 16 17 17 14 12 13
Cu 147 157 163 160 141 118 140 122 126 127 133 128 123 117 103
Rb 8 6 7 6 7 7 7 6 6 6 6 6 8 8 8
Zn 121 114 114 112 122 111 106 108 106 109 110 104 109 111 110
Sr 236 229 229 230 233 229 232 230 227 230 228 229 227 227 226
Y 35 30 30 31 33 32 29 29 29 30 29 29 34 34 36
Zr 74 60 59 59 67 65 59 58 58 57 58 58 74 75 80
Ba 200 164 160 168 178 177 170 161 161 164 160 168 198 203 214
Pb 4 4 4 4 3 3 5 3 4 3 2 3 5 3 4
Ga 17 16 16 16 16 16 16 15 15 15 16 16 16 16 15
MAGMA EVOLUTION, MIYAKE-JIMA
*Megacryst-bearing rocks.
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Fig. 2. Histograms of An contents of plagioclase phenocryst cores in megacryst-free and megacryst-bearing rocks erupted during the period
1469---1983. The megacryst-bearing rocks have more Ca-rich plagioclase compared with the megacryst-free rocks. The 1811 lavas consist only
of the megacryst-bearing rocks.
to olivine and plagioclase. The chemical compositions of butions of plagioclase with An 490 and An 80---90
these megacrysts are more Mg rich or Ca rich compared are recognized in all the samples, phenocrysts with An
with other phenocrysts in the megacryst-free rocks. 590 are rare in the 1811 samples.
Plagioclase up to 30 mm in length forms the domi- Olivine is the dominant mafic phenocryst. The
nant phenocryst phase. The compositions of plagio- compositions in the 1811 and 1940 samples range from
clase phenocrysts are in the range An 53---97, Fo 70 to Fo 86 (Fig. 3). The 1874 lavas, however,
nearly the same as those in the megacryst-free rocks also include Fo-poor phenocrysts (Fo 570). Olivine
(Fig. 2). Plagioclase megacrysts (longer than 5 mm) in megacrysts (longer than 2 mm) have similar composi-
all the rocks have An 490. Although bimodal distri- tions (Fo 480) among the three eruptions.
2120AMMA-MIYASAKA AND NAKAGAWA MAGMA EVOLUTION, MIYAKE-JIMA
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Fig. 3. Core compositions of olivine and pyroxene phenocrysts in megacryst-free and megacryst-bearing rocks erupted during the period
1469---1983. Continuous lines with numerals are isotherms of Lindsley (1983). All data are plotted in terms of quadrilateral pyroxene
components diopside---enstatite---hedenburgite---ferrosilite.
Clinopyroxene less than 40 mm in length is also an to andesite, whereas some of the megacryst-bearing
abundant mafic phenocryst. All of the phenocrysts rocks plot in the low-K field. Although lavas from the
are augite (Fig. 3), and their Mg-numbers are in 1469 to the 1983 eruptions show variations in
the range 60---83. The compositional range in the SiO2 (506---564 wt %) and K2O (035---082 wt %),
rocks is nearly the same as that for the megacryst- there are no systematic changes in their whole-rock
free rocks. The 1811 megacryst-bearing rocks show a composition with time (Fig. 5). Comparing the differ-
compositionally bimodal distribution of the clino- entiated megacryst-free rocks in each eruption,
pyroxene phenocrysts (Fig. 3). The phenocrysts with SiO2 and K2O contents seem to increase from 1469
high Mg-number often coexist with plagioclase and to 1712, then decrease to 1940 and increase again to
olivine megacrysts. 1983. On the other hand, the chemical compositions of
Orthopyroxene and magnetite phenocrysts are rare, the differentiated megacryst-bearing rocks in each
and smaller than 12 mm and 03 mm in length, respec- eruption are nearly constant, whereas the entire com-
tively. The compositional range of orthopyroxene phe- positional ranges of SiO2 and K2O have gradually
nocrysts is narrow (Mg-number 62---73) compared become wider from 1811 to 1940. The compositional
with the megacryst-free rocks (Fig. 3). variations of both megacryst-free and megacryst-
bearing rocks are shown in representative Harker
diagrams in Fig. 6.
WHOLE-ROCK GEOCHEMISTRY The SiO2 contents of the megacryst-free rocks range
All of the rocks during the period 1469---1983 are clas- from 506 to 564 wt % (Figs 4---6). Among these rocks,
sified as tholeiites in a SiO2---FeO*/MgO diagram at least three distinct trends (I, II and III) can be
(Fig. 4). Most of the lavas consist of medium-K basalt recognized, especially in SiO2---TiO2, SiO2---FeO*/
2121JOURNAL OF PETROLOGY VOLUME 44 NUMBER 12 DECEMBER 2003
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Fig. 5. Temporal variations of whole-rock SiO2 and K2O contents
for megacryst-free and megacryst-bearing rocks erupted during the
Fig. 4. (a) SiO2---FeO*/MgO and (b) SiO2---K2O diagrams for period 1469---1983.
megacryst-free and megacryst-bearing rocks erupted during the per-
iod 1469---1983. Dividing line in the SiO2---FeO*/MgO diagram is
from Miyashiro (1974) and that in the SiO2---K2O diagram from Gill
(1981). All analyses are normalized to 100 wt % volatile-free with FeO*/MgO and higher Cr (e.g. No. 1522 of 1983 and
total iron (FeO*) calculated as FeO. No. 1918 of 1835 in Table 2), compared with other
1763---1983 lavas.
The SiO2 content of the megacryst-bearing rocks
MgO and SiO2---Cr diagrams, and these seem to corres- ranges from 514 to 555 wt % (Figs 4---6). With an
pond to eruption age (Fig. 7). Trend I consists mainly increase of SiO2, the Al2O3, Cr and Ni contents
of the 1469, 1535 and 1595 lavas. Two samples (e.g. decrease, whereas the TiO2 contents and the FeO*/
No. 2222 in Table 2) from one of the lava lobes of the MgO ratio increase. The compositional variation of
1643 eruption (main-N, Fig. 1) are also classified as the megacryst-bearing rocks in each eruption forms
part of this trend. The SiO2 content of the Trend I distinct trends in most of the variation diagrams
rocks ranges from 518 to 549 wt %. This trend is the (Fig. 6). Although the 1811 lavas consist of SiO2
most mafic (poor in SiO2, TiO2, FeO*/MgO and rich 546---552 wt % andesites, the 1874 and 1940 lavas
in Cr) of the three trends. Trend II consists mainly of have a wider compositional range of SiO2 528---
lavas from the 1643 main-S lava lobe (Fig. 1; No. 1620 552 wt % and SiO2 514---555 wt %, respectively.
in Table 2) and 1712 lavas. Compared with the rocks The 1811 lavas are rich in TiO2 and have higher
of Trend I, those of Trend II are in the range of SiO2 FeO*/MgO, whereas the 1874 lavas are poor in TiO2
506---532 wt %, and are characterized by lower and FeO*/MgO among the megacryst-bearing rocks.
SiO2 and Cr, and by higher TiO2 and FeO*/MgO. Comparing the whole-rock compositions of the
Trend III is composed mainly of the 1763---1983 lavas, megacryst-bearing rocks with the megacryst-free sam-
and also contains the 1643 lavas from the central lava ples, the megacryst-bearing rocks are rich in Al2O3 and
lobe (Fig. 1; No. 1627 in Table 2). The rocks of this Ni, and poor in TiO2, FeO*/MgO and V. Further-
trend have SiO2 520---564 wt %, and are the most more, the felsic ends of the trends of variations of
differentiated (lowest Cr, and highest SiO2, TiO2 and the megacryst-bearing rocks intersect with those of the
FeO*/MgO). There also exist some samples with lower megacryst-free samples at around SiO2 55---56 wt %.
2122AMMA-MIYASAKA AND NAKAGAWA MAGMA EVOLUTION, MIYAKE-JIMA
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Fig. 6. Selected SiO2 variation diagrams for major elements (TiO2, Al2O3), FeO*/MgO and trace elements (V, Cr and Ni) for megacryst-free
and megacryst-bearing rocks erupted during the period 1469---1983.
MINERAL ASSEMBLAGE AND are, however, nearly aphyric and have less than 3 vol. %
phenocrysts. This suggests that plagioclase accumula-
CHEMICAL COMPOSITIONS OF
tion is a minor source of compositional variation in
CRYSTAL-CLOTS these rocks. In addition, SiO2 and K2O do not system-
It is widely accepted that two types of magmas atically change during the period 1469---1983 (Fig. 5).
have erupted from the tholeiitic volcanoes of the If differentiation had proceeded in a closed magma
Izu---Mariana arc: `plagioclase-controlled' (plagioclase- storage system, these values should have increased
accumulated) and `differentiated' magmas (Nakano & with time. These observations suggest that simple dif-
Yamamoto, 1991; Nakano et al., 1991; Tsukui & ferentiation does not play an important role during the
Hoshino, 2002). This is consistent with the evidence period. On the other hand, phenocryst minerals show
that there exist no disequilibrium phenocryst assem- wide and polymodal compositional distributions (Figs 2
blages, such as olivine and quartz, or olivine and horn- and 3), suggesting that all of the phenocrysts could
blende, in all of the rocks formed during the 1469---1983 not crystallize simultaneously from a single magma.
eruptions of Miyake-jima volcano. Most of these rocks To investigate whether these rocks are equilibrium
2123JOURNAL OF PETROLOGY VOLUME 44 NUMBER 12 DECEMBER 2003
crystallization products or not, we examined the
mineral assemblage and chemical compositions of
crystal-clots found in each sample, as described by
Amma-Miyasaka & Nakagawa (2002), because it can be
considered that minerals in the same crystal-clot have
crystallized simultaneously from the same magma.
Types of crystal-clots
On the basis of the mineral assemblage and the chemi-
cal compositions of the minerals within the crystal-clots
in the 1469---1983 rocks, crystal-clots can be divided
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into four types (Figs 8 and 9): megacryst-type (M-type);
basaltic-type (B-type); andesitic-type (A-type); basal-
tic andesitic-type (AB-type). The M-type crystal-clots
are recognized in megacryst-bearing rocks, and mostly
consist of Ca-rich plagioclase and Mg-rich olivine with
occasional Mg-rich clinopyroxene (e.g. in the 1811
lavas). They are the least differentiated of the four
types of clots: An 88---97, Fo 77---85 and Mg-
number 77---84. The B-type crystal-clots consist of
plagioclase, clinopyroxene and olivine, characterized
by the absence of orthopyroxene and magnetite.
Mineral core compositions are An 72---94, Mg-
number 68---79 and Fo 62---78, respectively, more
evolved than those of the M-type clots. In contrast, the
A-type crystal-clots are characterized by the presence
of magnetite (Usp 24---46) and orthopyroxene (Mg-
number 59---73), as well as plagioclase, clino-
pyroxene and olivine. Core compositions of plagio-
clase, clinopyroxene and olivine in these clots are An
55---91, Mg-number 60---77 and Fo 60---73,
slightly more evolved compared with those in the B-
type clots. In addition to these clots, a fourth type of
crystal-clot (AB-type)exists in the 1535 and 1595
lavas. The crystal-clots contain Mg-rich orthopyrox-
ene (Mg-number 70---76; Fig 3 and 9) without mag-
netite. The core compositions of coexisting plagioclase
and clinopyroxene are An 73---85 and Mg-number
70---75, respectively. The mineral assemblage and
chemical compositions are intermediate between the
A-type and B-type clots.
Compositional relationship of minerals
within and among the clots
The compositional relationship between olivine and
pyroxenes can be investigated on the basis of the
Fe---Mg distribution in these minerals. If these minerals
Fig. 7. Variation of TiO2, FeO*/MgO, and Cr vs SiO2 for coexist in equilibrium, the Mg-number of olivine
megacryst-free rocks ( symbol in Fig. 6) erupted during the period should be nearly the same as or slightly lower than
1469---1983. The megacryst-free rocks can be divided into three
trends. The 1643 lavas flowed down separately into three lobes:
that of clinopyroxene and orthopyroxene (Obata et al.,
main N, main S and central lobes, which respectively correspond to 1974; Brey & Kohler, 1990). We have determined
Trend I, II and III. the average compositions and compositional ranges of
2124AMMA-MIYASAKA AND NAKAGAWA MAGMA EVOLUTION, MIYAKE-JIMA
equilibrium from the same magma, and that different
types of crystal-clots have crystallized from distinct
magma compositions.
The Fo content of olivine phenocrysts is strongly
dependent on the FeO/MgO of the magma and
KD(olivine/liquid) is constant over a wide range of P, T,
fO2and H2O (Roeder & Emslie, 1970). Based on the Fo
content of olivine in each type of clot, M-type clots are
formed in the most mafic magma, and the A-type clots
in the most differentiated magma. The presence or
absence of magnetite and orthopyroxene in these clots
suggest that the magma producing the M-type and B-
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type clots is basaltic, and that producing the AB-type
and A-type clots is probably andesitic (e.g. Gill, 1981).
DISCUSSION
Origin of M-type clots and megacrysts
Amma-Miyasaka & Nakagawa (2002) investigated the
origin of M-type clots and megacrysts in the 1940 lava.
We defined the M-type clots and megacrysts by their
size [plagioclase of L 403 mm (where L is the radius of
the equivalent circle) and olivine of L 402 mm] and
the least differentiated composition of minerals (plagio-
clase of An 88---97, olivine of Fo 77---85 and
clinopyroxene of Mg-number 77---84). Examining
these crystals in detail, we pointed out the following
petrographical features: (1) spherical olivine in plagio-
clase; (2) wide, homogeneous cores of plagioclase;
(3) kink-banding of olivine megacrysts. These features
cannot be explained by normal crystallization
processes. The spherical olivine and wide, homoge-
neous cores of plagioclase can be formed by long-term
diffusion, and the kink bands can be formed under
conditions that can transmit strain. Such a condition
could be achieved within a plutonic body. This is
supported by the evidence that the mineral assem-
blage, chemical compositions and crystal size distribu-
tions of both the megacrysts and M-type clots are
identical to those of plutonic xenoliths (smaller than
5 cm in diameter, consisting only of An 93---97
anorthite and Fo 83---85 olivine crystals) found in
Fig. 8. Photomicrographs illustrating the various types of crystal- some of the 1940 lavas.
clots (M, B, A and AB) during the period 1469---1983. The M-type M-type crystal-clots have also been recognized in the
photograph is through crossed nicols and the other photographs are megacryst-bearing rocks of the 1811 and 1874 erup-
in plane-polarized light.
tions. The clots in the rocks of the 1874 eruption consist
only of plagioclase and olivine, as in the case of the
olivine and pyroxenes in each type of clot of each 1940 megacryst-bearing lavas. On the other hand, the
eruption age (Fig. 10). The Mg-number of clinopyrox- clots in the 1811 lavas also contain clinopyroxene.
ene is slightly higher compared with that of olivine There is no significant difference in the petrographic
in each type of clot. Moreover, the Mg-number of characteristics and chemical composition of plagioclase
orthopyroxene is nearly the same as that of olivine in and olivine megacrysts among the three lavas of known
the A-type clots. This suggests that the olivine and age. Thus, we consider that the megacrysts in the 1811
pyroxene in each type of clot have crystallized in and 1874 lavas have the same origin as those in the
2125JOURNAL OF PETROLOGY VOLUME 44 NUMBER 12 DECEMBER 2003
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Fig. 9. Mineral assemblage and mineral core compositions (plagioclase, clinopyroxene and olivine) in four types of crystal-clots (M, B, A and
AB) in the rocks erupted during the period 1469---1983 (see text).
from N---NE flank fissures. Therefore, it may be sug-
gested that these lavas have captured both the mega-
crysts and M-type clots as xenocrysts from the same
body located beneath the N---NE sector of the volcano.
Evidence for magma mixing
Both A-type and B-type crystal-clots coexist in each
sample of the 1469---1983 lavas, except for 1835, 1962
and 1983. Histograms of minerals occurring in A-, B-
and AB-type crystal-clots in each eruption are shown
in Fig. 11. Plagioclase in these lavas shows composi-
tionally bimodal distribution. The peaks of the An
contents are variable among lavas of different eruption
ages. Plagioclase phenocrysts in the B-type clots are,
however, always more Ca rich than those in the A-type
clots in each eruption. Histograms of olivine and clino-
pyroxene do not clearly show compositionally bimodal
distributions; however, these minerals are always more
Mg rich in the B-type clots. This coexistence of two
types of crystal-clots with distinct chemical composi-
tions is considered to be a disequilibrium feature.
The relationship between the crystal-clots and their
host rocks can be investigated on the basis of Fe---Mg
Fig. 10. Mg-number of (a) clinopyroxene core and (b) orthopyrox-
partitioning (Fig. 12). If minerals crystallize in equili-
ene core vs Mg-number of olivine core in the A-, B- and M-type clots brium with the host magma, Fe---Mg distribution
in the rocks erupted during the period 1469---1983. The AB-type clots between the mineral-cores and the magma must be
lack olivine phenocrysts. Dashed line in (a) is after Obata et al. constrained by plausible partition coefficients, shown
(1974).
by the lines in Fig. 12. In the case of the rocks with both
A-type and B-type clots, most of the mafic minerals in
1940 lavas. Although the whole-rock chemistry of the the clots do not plot near equilibrium. Most olivine and
megacryst-bearing rocks of each eruption seems to clinopyroxene in the B-type clots are more Mg rich
define distinct trends, this may reflect the difference than possible equilibrium, whereas those in the A-
in the ratio of component minerals in the plutonic type clots are more Fe rich. Orthopyroxene, contained
rocks. All of the megacryst-bearing rocks erupted only in the A-type clots, is also more Fe rich than that
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Fig. 11. Histograms of core compositions of plagioclase, clinopyroxene, orthopyroxene and olivine in A-, B- and AB-type crystal-clots in the
rocks erupted during the period 1469---1983. The M-type clots are considered to be xenocrysts and are excluded. It should be noted that the
rocks containing only B-type clots have never erupted during the period 1469---1983.
required to be in equilibrium with the whole-rock (An 60---90). The compositional variation of clino-
FeO*/MgO ratio (Fig. 12). This suggests that the pyroxene in the 1983 lavas is also relatively extensive,
mafic minerals in the B-type clots should have crystal- compared with that in the 1962 and 1835 lavas. The
lized from a more mafic (lower FeO*/MgO) magma compositional relationships between minerals and
than the host magma, and those of the A-type clots from whole rocks also show that most of the mafic minerals
a more differentiated (higher FeO*/MgO) magma. in these rocks are in equilibrium with their host magma;
Our data suggest that most of the 1469---1983 lavas however, Fe-rich clinopyroxene in the 1983 sample
contain two types of phenocryst that originated from (No. 1522 in Table 2) and olivine in the 1962 sample
distinct magmas, and indicate that these lavas could be (No. 122508 in Table 2) might not be in equilibrium
produced by magma mixing of two end-member mag- (Fig. 12). The lack of B-type clots indicates that these
mas, a differentiated A-type (andesitic) and a mafic lavas may have been produced without mixing with the
B-type (basaltic) magma. The 1535 and 1595 lavas mafic B-type magma. Although it is also possible that
that have another type of crystal-clot (AB-type, nearly aphyric basaltic magma mixed with the A-type
Fig. 11), in addition to the A-type and B-type, might magma, the proportion of the basaltic magma would be
be the results of mixing between three magmas. In minor, because the mineral compositions are nearly in
summary, at least two distinct magmas have existed equilibrium with the host magma (Fig. 12). Consider-
beneath the Miyake-jima volcano since 1469, and have ing the wide compositional variations of the phenocryst
usually mixed during eruption. minerals (plagioclase and clinopyroxene) especially
in the 1983 lavas, however, we could not exclude
the possibility of mixing between distinct andesitic
Rocks without obvious evidence magmas. In conclusion, we suggest that lavas without
for magma mixing the B-type clots are not mixing products between the
The 1983, 1962 and 1835 lavas contain only the A-type A-type and B-type magmas, and that only pure A-type
crystal-clots (Fig. 11). Although histograms of phe- magma (or magmas) has erupted during the 1835,
nocryst compositions in these lavas do not show 1962 and 1983 eruptions. Furthermore, it is noteworthy
clear bimodal distributions, the compositional range that rocks containing only the B-type clots have never
of the plagioclase phenocrysts is relatively wide erupted during the period 1469---1983.
2127JOURNAL OF PETROLOGY VOLUME 44 NUMBER 12 DECEMBER 2003
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Fig. 12. Fe---Mg partitioning between whole rock and cores of A-type and B-type olivine and pyroxenes from 1469---1983 megacryst-free rocks.
The megacryst-bearing rocks are excluded, because the FeO*/MgO of these rocks may be affected by the presence of xenocryst minerals. Lines
represent partition coefficients of KD [(XFeO/XMgO)mineral/(XFeO/XMgO)liquid] of Beattie (1993) for olivine and orthopyroxene, and Baker &
Eggler (1987) for clinopyroxene. FeO contents of whole rocks are calculated by assuming Fe2 /(Fe2 Fe3 ) 09. We have also tried Fe2 /
(Fe2 Fe3 ) 081 by wet chemical analysis (Isshiki, 1960; Iwasaki et al., 1982), but 09 better explains the equilibration relationships in
non-mixing rocks.
Except for the 1835, 1962 and 1983 lavas, the FeO*/
End-member magmas and their MgO ratios of the whole rocks are intermediate
relationships between those of the A-type and B-type magmas in
Estimation of whole-rock compositions of each eruption. This is consistent with our conclusion
end-member magmas that these rocks are mixing products between these
Using the Fe---Mg mineral---melt distribution coeffi- magmas. Whole-rock FeO*/MgO ratios of the 1962
cients for olivine/liquid and orthopyroxene/liquid and most of the 1835 and 1983 lavas are nearly the
(Beattie, 1993), we can estimate the FeO*/MgO ratios same as the estimated FeO*/MgO ratios of the A-type
of both the A-type and B-type magmas (Fig. 13). magmas, whereas the whole-rock ratios of the 1535,
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Fig. 13. Estimated FeO*/MgO ratios of A-type and B-type magmas erupted during the period 1469---1983. We used KD(olivine/liquid) 0303,
KD(orthopyroxene/liquid) 0284 of Beattie (1993) and the representative core compositions of mafic minerals in each type of clot. Calculated
FeO*/MgO values of the A-type liquid are plotted at the SiO2-rich end and those of the B-type liquid at the SiO2-poor end.
1595, 1712 and most of the 1643 lavas are similar to 27 km in radius and 100 m thick for the 1940 eruption,
those of the B-type magmas. FeO*/MgO ratios might assuming the depth of the magma to be 3 km. Hypo-
reflect the mixing ratio between the A-type and B-type centres of long-period earthquakes (Ueki et al., 1984)
magmas. and geomagnetic change (Nakagawa et al., 1984) asso-
ciated with the 1983 eruption suggested that magma
was located at a depth of 2---3 km. In addition to these
Relationship of the end-member magmas shallower `sources', the inflation and deflation accom-
Plots of incompatible-element ratios (K2O/Ba, Y/Ba panying the 1983 eruption suggested a magma reser-
and Y/Zr) in the 1469---1983 megacryst-free rocks voir about 1---2 km SW of the summit crater, at a depth
define linear trends that pass through the origin with estimated to be about 8 km (Tada & Nakamura,
R2 values of 089---093 (Fig. 14). This strongly suggests 1988). Sasai et al. (2001) also suggested that the pres-
that both the A-type and B-type magmas may have sure source during 1995---1999 lies beneath the south-
been derived from a single primary magma. This con- ern flank of the volcano at a depth of 8 km below sea
clusion is consistent with Sr isotopic data. Notsu et al. level. Considering that the depths of short-period
(1983) and Notsu & Aramaki (1984) measured 87 Sr/ earthquakes are 1---10 km below sea level (Minakami
86 et al., 1963; Miyazaki & Sawada, 1984; Ueki et al.,
Sr ratios of the rocks from Miyake-jima volcano, and
confirmed that the isotopic ratio is concentrated in a 1984), the magma storage systems appear to have
narrow range (070350---070369), suggesting that the existed at least at two depths (c. 2---3 km and 8 km).
source region of the magmas beneath Miyake-jima On the other hand, geobarometry did not successfully
volcano is isotopically uniform. reveal the crystallization depth of the A-type and
B-type magmas of the volcano. We used the pseudo-
ternary normative diagram (cpx---ol---SiO2) of Walker
Depths of the end-member magmas et al. (1979) and Baker & Eggler (1983) for lavas with
The depths of the magma storage systems have been only the A-type clots and those with both the A-type
estimated based on geophysical observations carried and B-type clots, and the clinopyroxene geobarometer
out since the 1940 eruption. Takahashi & Hirano (Nimis, 1995) for both types of clots. These calculations
(1941) estimated that the size of the chamber was show, however, no difference between the A-type and
2129JOURNAL OF PETROLOGY VOLUME 44 NUMBER 12 DECEMBER 2003
(A-type) and a deeper basaltic (B-type), have existed
during the period 1469---1983. The relative position of
these magma storage systems may be consistent with
the fact that the A-type magma (or magmas) has
erupted alone during the 1835, 1962 and 1983 erup-
tions, whereas rocks containing only the B-type clots
have never erupted during the period 1469---1983. The
absolute pressure of both magmas should be less than
3 kbar; the shallower andesitic magma storage systems
might be located at a depth of 2---3 km, and the deeper
basaltic magma storage systems might be at 8 km.
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Magma storage systems and
their evolution
Our petrological analysis of the 1469---1983 lavas
reveals that the magma plumbing system during this
period has involved two distinct magma storage
systems, filled with the andesitic (A-type) and basaltic
(B-type) magmas. In many of the eruptions, the resul-
tant lavas are the mixing products between these two
magmas, although shallower andesitic magmas have
sometimes erupted alone. Because we can identify
phenocryst minerals that are derived from each end-
member magma during mixing events, the evolution
of both end-member magmas can be studied by using
these phenocrysts. We can demonstrate the evolution
of the chemical composition of these phenocrysts and of
the deduced magmatic temperature during the past
500 years.
Evolution of the end-member magmas and their
magmatic temperature
The core composition of olivine in the B-type crystal-
clots has gradually become Fe rich with time (Fig. 15).
The average composition has decreased systematically
from Fo 75 in the 1469 lavas to Fo 69 in the 1940
lavas. Although the compositional range of clinopyr-
Fig. 14. Incompatible-element variation diagrams for the mega- oxene in the B-type magma is wide, the average pyr-
cryst-free rocks erupted during the period 1469---1983. R2 values are oxene composition in each eruption is nearly the same
for all points. in the range of Mg-number 72---74 throughout the
period 1469---1983. According to Brey & Kohler
(1990), the ratio of the Mg-number of clinopyroxene
B-type magmas. Whole-rock compositions are pro- to the Fo content of olivine increased from 1 to more
jected around 1 atm for both lavas on the normative than 1, with falling equilibrium temperature. There-
diagram, and equilibration pressures calculated based fore, a temporal change in both Mg-number of
on clinopyroxene compositions are 0---3 kbar for both clinopyroxene and Fo content of olivine in the B-type
the A-type and B-type clots. clots would indicate that the temperature of the basal-
The B-type magma appears to have entrained plu- tic magma has fallen systematically with time. The
tonic xenoliths before mixing with the A-type magma temperature of the basaltic magma calculated by
during the 1940 eruption (Amma-Miyasaka & Loucks (1996) actually fell from 1220 C in the 1469
Nakagawa, 2002). Thus, we suggest that basaltic eruption to 1180 C in 1940. Using the temporal
magma should exist at a deeper level beneath the variation of phenocryst minerals in the B-type clots,
volcano than andesitic magma. We propose that two the evolution of the whole-rock chemistry of the basal-
distinct magma storage systems, a shallower andesitic tic magma can also be evaluated. The FeO*/MgO
2130AMMA-MIYASAKA AND NAKAGAWA MAGMA EVOLUTION, MIYAKE-JIMA
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Fig. 15. Temporal variations of core compositions of B-type clinopyroxene and olivine and magmatic temperature for the rocks erupted
during the period 1469---1983. Numbers in parentheses represent the eruption intervals of B-type magma. The B-type magma erupted every
48---69 years during the period 1469---1983. Large filled triangles are average core compositions of the minerals.
ratios of the basaltic end-member magmas during a complex fashion from 1070 C in the 1469 eruption
1469---1595 were the lowest (Fig. 13). Since then, the to 1100 C in 1983 (Fig. 16). As in the case of the
ratio has increased, accompanied by falling magmatic basaltic magma, temporal changes in the whole-rock
temperature. chemistry of the andesitic magma are also evident: the
In contrast, the core composition of clinopyroxene FeO*/MgO ratio has decreased from 44 in the 1469
and orthopyroxene in the A-type clots has not changed eruption to 33 in 1983 (Fig. 13).
systematically as it has for the B-type clots (Fig. 16). Thus, the two magmas, which have formed end-
The average core composition of clinopyroxene is Mg- member components for magma mixing during the
number 67 in the 1469 lavas, which is the most Fe past 500 years, have evolved following separate paths
rich during the past 500 years. Although the composi- (Fig. 17a). The basaltic magma has become more
tion has fluctuated since then, it has gradually become differentiated with time while cooling, whereas the
more Mg rich with time to Mg-number 72 in the andesitic magma has become more mafic while heat-
1983 lavas. Except for the 1763 eruption, the average ing. A temporal fall in the magmatic temperature of
Mg-number of orthopyroxene in the 1469 A-type clots the basaltic magma could be explained by fractional
reflects the most differentiated magma during the per- crystallization of a primary basaltic magma, and it
iod 1469---1983, and has gradually become Mg rich up suggests that the basaltic end-member magma has
to 1983. Although orthopyroxene in the A-type clots in been continuously in existence at least for 500 years.
the 1763 lava is the most Fe rich among the 1469---1983 The complicated and reverse evolution of the andesitic
samples, it is rare in 1763. Using the average composi- magma during the past 500 years cannot be explained
tions of both clinopyroxene and orthopyroxene in the by a closed system as in the case of the basaltic magma.
A-type clots, magmatic temperatures were calculated In each eruption, the deeper basaltic magma was
according to the Wells (1977) geothermometer for each injected into the shallower andesitic magma and
eruption. The temperature appears to have risen in erupted a mixed magma. It suggests that the shallower
2131JOURNAL OF PETROLOGY VOLUME 44 NUMBER 12 DECEMBER 2003
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Fig. 16. Temporal variations of core compositions of A-type pyroxenes and magmatic temperature for the rocks erupted during the period
1469---1983. Numbers in parentheses represent the eruption intervals of A-type magma. The A-type magma erupted every 21---69 years during
the period 1469---1983, and about every 20 years since 1940. Large filled circles are average core compositions of the minerals.
andesitic magma chamber was an open magma by fractional crystallization of the basaltic magma. We
storage system for injection of the basaltic magma. In used the composition of the 1535 basaltic lava (No.
this case, if only a small quantity of the basaltic magma 2321 in Table 3) as the starting material because the
was injected, the shallower andesitic magma would FeO*/MgO values of the 1535 and 1595 lavas
become more differentiated mainly by the effect of (240---248) are the lowest among the 1469---1983
fractional crystallization. In contrast, when basaltic megacryst-free rocks, and nearly the same as the
magma was injected on a large scale, the andesitic calculated FeO*/MgO of the B-type liquid (24---25,
magma could have become more mafic. Moreover, Fig. 13). According to MELTS calculations, the
the andesitic magma might be nearly replaced by the FeO*/MgO continuously increases as temperature
basaltic magma. In conclusion, the temporal fluctua- falls, in the range of SiO2 553 wt % (Fig. 18a). Liquid
tion of magmatic temperature and the changing lines of descent are partially dependent on pressure and
composition of the andesitic magma during the past water content. Liquid compositions jump to SiO2 455
500 years depend on the quantity of basaltic magma wt % when magnetite begins to crystallize. The FeO*/
injected. MgO continuously increases as temperature falls, and
liquid lines of descent for SiO2 455 wt % have a wide
range. The compositional range of the 1469---1983
Evaluation of our model by MELTS calculation megacryst-free rocks lies between the liquid lines of
Our explanation for the evolution of the basaltic descent for SiO2 553 wt % and SiO2 455 wt %.
magma can be tested using the MELTS calculation Comparing the FeO*/MgO ratios of the end-
(Ghiorso & Sack, 1995). The chemical evolution of the member magmas calculated from phenocryst compo-
andesitic magma might reflect mixing with injected sitions (44 for the 1469 andesitic magma and 24---32
basaltic magma. The andesitic magma existing before for the basaltic magma) with the results of MELTS
the 1469 eruption, however, might have been formed calculations, the FeO*/MgO of the basaltic magma
2132AMMA-MIYASAKA AND NAKAGAWA MAGMA EVOLUTION, MIYAKE-JIMA
Table 3: Whole-rock magma during the past 500 years, the composition of
compositions of end-member basaltic magma after the 1983 eruption must be SiO2
452 wt %, FeO*/MgO 432 and Fo content of the
magmas of the 1469 eruption B-type olivine 570 (Fig. 17b). On the other hand, it is
difficult to estimate the composition of the shallower
Basaltic Andesitic andesitic magma because it shows a more complicated
Sample no.: 2321 estimated temporal change compared with the basaltic magma.
It has, however, become more mafic from 1874 to 1983,
wt % probably because of the injections of the basaltic
SiO2 5150 6277 magma. After the 1983 eruption, the andesitic
TiO2 117 092 magma may have had SiO2 553 wt %, FeO*/MgO
533 with A-type clots (pyroxenes; Mg-number 470).
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Al2O3 1553 1296
FeO* 1265 833
In other words, the compositions of andesitic and
MnO 020 052
basaltic magmas are indistinguishable after the 1983
eruption. Before the 2000 eruption, we proposed that
MgO 525 189
the erupted magma would be a mixing product of these
CaO 1027 632
basaltic and andesitic magmas, or would be andesitic
Na2O 235 356
alone.
K2O 045 110
Geshi et al. (2002) have already discussed the mag-
P2O5 012 031
matic system during the 2000 eruptive activity mainly
H2O 050 130
based on the whole-rock chemistry of the eruptive
Total 10000 10000 rocks. However, we consider that detailed analysis of
FeO*/MgO 241 440 phenocryst minerals, as investigated in this paper, are
essential to reveal the magmatic system. We will
develop detailed petrological analysis of the 2000
eruption in another paper (Amma-Miyasaka et al.,
submitted).
can be explained by both 1 and 3 kbar pressures
(Fig. 18b). Geophysical constraints suggest that the
magmas have been stored at a depth of 8 km; there-
fore, we consider that the basaltic magma may have Implications for studies of the magma
stagnated at around 3 kbar pressure. On the other plumbing system and its evolution in
hand, the FeO*/MgO ratio of the andesitic magma active volcanoes
can be explained only by the 1 kbar calculated results Several studies have already focused on the structures
(Fig. 18b). This result is consistent with the geophy- of magma plumbing systems and their evolution in
sical observations (the shallower andesitic magma active volcanoes over hundreds to thousands of years:
storage system might be located at a depth of Fichaut et al. (1989), Belkin et al. (1993), Villemant et al.
2---3 km). It is suggested that the 1469 andesitic (1993) and Sato et al. (1996). These workers argued
magma (Table 3) could have been formed by frac- that the increase of SiO2 content or FeO*/MgO ratio
tional crystallization of basaltic magma at a shallow with time is due to fractionation or to the change of the
level (about 1 kbar). It is evident that the 1469 ande- mixing ratio. On the other hand, they also speculated
sitic magma has changed its composition over 500 that a sudden drop of SiO2 or FeO*/MgO may indi-
years in a complex fashion (Fig. 18b), and it is also cate the injection of a new batch of mafic magma. If the
consistent with our model that the shallower andesitic end-member magmas had not changed for consider-
magma chamber was an open magma storage system able durations, these explanations might be correct.
for injection of the basaltic magma. Temporal change of the end-member magmas, how-
ever, may be common as in the case of Miyake-jima
volcano, in which case temporal variations of the
Possible state of the magma plumbing whole-rock chemistry cannot simply correspond to the
system just before the 2000 eruption degree of fractionation and/or to mixing ratio of
Our model for the evolution of the magma plumbing the end-member magmas. It should be noted that
system and eruption processes should be able to esti- the detailed analysis of eruptive rocks from each
mate the state of the system just before the 2000 erup- eruption using the mineral assemblage and chemical
tion of Miyake-jima volcano. According to simple compositions in crystal-clots can clarify the evolution of
temporal differentiation of the deep-seated basaltic the magma system of active volcanoes.
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