Journal of Petrology | Volume 44 | Number 12 | Pages 2113-2138 | 2003
© Oxford University Press 2003; all rights reserved
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 Crystal-Clots
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
* Corresponding author. Telephone: +81-11-709-1813. Fax: +81-11-709-1817. E-mail: m.miyasaka{at}aist.go.jp
RECEIVED SEPTEMBER 2, 2002; ACCEPTED MAY 29, 2003
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONGEOLOGY AND 1469-1983 ERUPTIONS...ANALYTICAL METHODSPETROGRAPHY AND MINERAL...WHOLE-ROCK GEOCHEMISTRYMINERAL ASSEMBLAGE AND CHEMICAL...DISCUSSIONCONCLUSIONSUPPLEMENTARY DATAAPPENDIX: REPRESENTATIVE MINERAL...REFERENCES |
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Miyake-jima volcano has erupted at least 13 times during the period 1469–1983. To understand the historic magmatic processes, we focus on the mineral assemblage and chemical compositions of crystal-clots in single samples from each of the eruptions. Most of the historic lavas consist of nearly aphyric to weakly porphyritic basalt to andesite, but there also exist megacryst-bearing rocks. The megacrysts are considered to be xenocrysts from a deep-seated plutonic body. Many samples of 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 clots additionally contain magnetite and orthopyroxene. Compositional 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 two types of clots in each sample, they are derived from distinct magmas: the B-type clots from basaltic magma and the A-type clots from andesitic magma. During the historic activity, the magma plumbing system appears to have included two magma storage systems: a deep-seated basaltic and a shallower andesitic one. In many cases, basaltic magma has injected into shallower andesitic magma to form mixed magma; however, andesitic magma has sometimes erupted alone without extensive injections of basaltic magma. Temporal variations of mineral compositions in the clots and estimated whole-rock compositions of the end-member magmas suggest that the basaltic magma has differentiated gradually since 1469, and that its magmatic temperature has fallen from 1220 to 1180°C. Conversely, the andesitic magma has changed in a complex fashion to become more mafic (the magmatic temperature rose from 1050 to 1100°C). As a result of this study, it is estimated that the basaltic magma after the 1983 eruption was the least mafic, and the andesitic magma the most mafic, of the historic eruptions.
KEY WORDS: andesite; basalt; crystal-clots; evolution of magma; Miyake-jima volcano; magma mixing
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONGEOLOGY AND 1469-1983 ERUPTIONS...ANALYTICAL METHODSPETROGRAPHY AND MINERAL...WHOLE-ROCK GEOCHEMISTRYMINERAL ASSEMBLAGE AND CHEMICAL...DISCUSSIONCONCLUSIONSUPPLEMENTARY DATAAPPENDIX: REPRESENTATIVE MINERAL...REFERENCES |
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The present state of the magma plumbing system beneath an active volcano provides important clues about the nature of the volcanic activity and the potential for future eruptions. Temporal changes in the magma plumbing system can be evaluated through detailed petrological studies of stratigraphically well-constrained eruption sequences. Many studies have focused on the magma plumbing system and eruption processes based on the eruptive sequence of a single eruption (e.g. Reagan et al., 1987
; Wolfe et al., 1987; Garcia et al., 1989, 1992, 1996, 2000; Wolf & Eichelberger, 1997; Marianelli et al., 1999; Nakagawa et al., 1999, 2002; Streck et al., 2002). A series of papers on the Pu'u O'o eruption of Kilauea (Garcia et al., 1989, 1992, 1996, 2000) has revealed changing magmatic processes over a period of 17 years; Reagan et al. (1987) and Streck et al. (2002) discussed the role of basalt replenishment in the long-lived (about 30 years) eruption of Arenal volcano, Costa Rica. Clearly, to study the evolution of magma plumbing systems, it is important to investigate as long a period of activity as possible [e.g. Wright & Fiske (1971) for Kilauea, Hawaii; Borgia et al. (1988) for Arenal, Costa Rica; Fichaut et al. (1989) for Mt. Pelee, Martinique; Belkin et al. (1993), Villemant et al. (1993) and Tedesco et al. (1998) for Vesuvius, Italy; D'Antonio et al. (1999) for Campi Flegrei caldera, Italy; Nakano & Yamamoto (1991) for Izu–Oshima, Japan]. In most of these studies, however, the evolution of the magma plumbing systems was discussed mainly in terms of the temporal change in whole-rock compositions. Although detailed analysis of phenocryst minerals is useful to understand magmatic processes (e.g. Nakamura, 1995; Umino & Horio, 1998; Nakagawa et al., 1999, 2002; Streck et al., 2002), a systematic mineralogical study of the eruptive activity of an active volcano for a considerable period (for hundreds to thousands years), based on the evolution of both whole-rock and mineral chemistry, has not been carried out, except for that by Borgia et al. (1988).
Miyake-jima volcano has erupted at least 13 times since AD 1469. During the period 1469–1983, magma effused mainly from flank fissures. The most recent eruption, which began in June 2000 (Nakada et al., 2001), however, is distinctive compared with other historic eruptions. Earthquake swarms occurred beginning the night of June 26, and a submarine eruption took place on the morning of June 27. A summit eruption (ash plume) followed on July 8, and the summit area suddenly subsided. The collapse continued ‘silently’ until mid-August, resulting in the formation of a new caldera with a volume of about 0·6 km3. The volcano is now (April, 2003) discharging a large quantity of volcanic gases (SO2, etc.) and inhabitants of Miyake-jima have been evacuated since September 2000. Several petrological studies have been carried out on recent historic eruptions, except the 2000 eruption, of Miyake-jima volcano (e.g. Iwasaki et al., 1982; Fujii et al., 1984; Soya et al., 1984; Sato et al., 1996; Amma-Miyasaka & Nakagawa, 1998, 2002). Sato et al. (1996) dealt with most of the historic lavas since 1643. They showed that whole-rock compositions became more silicic from 1643 to 1874, and then more mafic from 1940 until 1983. They proposed that mafic magma was recently injected into the magma plumbing system. Amma-Miyasaka & Nakagawa (2002) focused on the historic 1940 and 1962 eruptions and established that deeper basaltic magma and shallower andesitic magma existed beneath the volcano during this period. It is not clear, however, how long the magma plumbing system has existed and how it has evolved through time.
In this paper, we describe the evolution of the whole-rock and mineral chemistry of the historic lavas from 1469 to 1983. We focus on the mineral assemblage and chemical compositions of crystal-clots in single samples to clarify whether minerals in each sample are in equilibrium or not. We demonstrate the existence of two magma storage systems since 1469, one basaltic and one andesitic. We carefully investigate the evolution of both magma types for the past 500 years and the interaction between these magmas during each eruption episode. This provides important new insights into the evolution of the historic magma plumbing system, and allows us to estimate the possible state of the system just before the 2000 eruption.
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GEOLOGY AND 1469–1983 ERUPTIONS OF MIYAKE-JIMA VOLCANO |
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TOPABSTRACTINTRODUCTIONGEOLOGY AND 1469-1983 ERUPTIONS...ANALYTICAL METHODSPETROGRAPHY AND MINERAL...WHOLE-ROCK GEOCHEMISTRYMINERAL ASSEMBLAGE AND CHEMICAL...DISCUSSIONCONCLUSIONSUPPLEMENTARY DATAAPPENDIX: REPRESENTATIVE MINERAL...REFERENCES |
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Miyake-jima volcano is a composite volcano with two nested calderas (Fig. 1), and is composed of tholeiitic basalt and andesite. Based on the eruption style and petrological characteristics of lavas and scoria, Tsukui et al. (2001)
divided the volcanic activity of the last 10 000 years into four stages: 10 000–7000, 4000–2500, 2500 years BP to AD 1154, and since AD 1469. During the first stage, the main cone was constructed, and the outer caldera filled. Erupted materials are porphyritic basalts. The second stage began after a 3000 year repose period, consisting of andesitic lavas and scoria erupted from lateral and central vents. The third stage began with a large-scale eruption that formed the inner caldera, which has subsequently been filled with nearly aphyric to weakly porphyritic basalts.
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The latest stage, since1469, began after a 300 year repose period. The eruptions were characterized by the effusion of lavas and scoria mainly from NE–SW-trending flank fissures (Fig. 1). Eruptions occurred every 50–70 years before 1811, and have become more frequent (every 20–70 years) since then (Table 1). Most of the eruptions continued for a short duration (typically a day to a month) whereas the 1763 eruption was prolonged, lasting for 6 years. The volume of magma erupted is estimated to be <0·066 km3 for each eruption. Although these values may be underestimated, especially for the older eruptives, there seems to be little correlation between the eruptive mass and the duration of each eruption.
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ANALYTICAL METHODS |
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TOPABSTRACTINTRODUCTIONGEOLOGY AND 1469-1983 ERUPTIONS...ANALYTICAL METHODSPETROGRAPHY AND MINERAL...WHOLE-ROCK GEOCHEMISTRYMINERAL ASSEMBLAGE AND CHEMICAL...DISCUSSIONCONCLUSIONSUPPLEMENTARY DATAAPPENDIX: REPRESENTATIVE MINERAL...REFERENCES |
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For each of the 1469–1983 eruptions, we collected samples to represent all source vents as well as the complete eruptive sequence. Mineral core compositions were determined by a single analysis from the centre of the minerals for representative samples that reflect the whole-rock variations of each eruption. The JEOL 733 and 8800 electron probe microanalysers at Hokkaido University were used for the mineral analyses. Operating conditions were 15 kV accelerating voltage and 20 nA beam current with a minimum spot size of 1 µm. Each element was counted for 30 s on the peak and 20 s on the background. Corrections were made according to the ZAF method. Whole-rock compositions were determined by X-ray fluorescence (XRF), using a Philips PW-1404 system with a Rh tube at Hokkaido University. Glass beads were used for major element analysis, and pressed pellets for trace element analysis. Major element compositions were determined for 230 samples, and trace element compositions for 198 samples from the 1469–1983 eruptive products. Whole-rock compositions of all the samples for which we have analysed mineral compositions are listed in Table 2 and representative mineral compositions are given in the Appendix. The complete whole-rock and mineral composition dataset is included in Electronic Appendices 1–5, which may be downloaded from the Journal of Petrology web site at http://www.petrology.oupjournals.org/.
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PETROGRAPHY AND MINERAL CHEMISTRY |
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TOPABSTRACTINTRODUCTIONGEOLOGY AND 1469-1983 ERUPTIONS...ANALYTICAL METHODSPETROGRAPHY AND MINERAL...WHOLE-ROCK GEOCHEMISTRYMINERAL ASSEMBLAGE AND CHEMICAL...DISCUSSIONCONCLUSIONSUPPLEMENTARY DATAAPPENDIX: REPRESENTATIVE MINERAL...REFERENCES |
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Although most of the 1469–1983 lavas are nearly aphyric to weakly porphyritic, with less than 9 vol. % of phenocrysts, there also exist rocks characterized by the presence of anorthite (up to 3 cm in length) and olivine megacrysts (Table 1). Phenocryst minerals are plagioclase, olivine, clinopyroxene, orthopyroxene and magnetite. The groundmass is composed of plagioclase, clinopyroxene, magnetite and brown glass, and ranges in texture from intersertal to hyalo-ophitic.
Megacryst-free rocks (nearly aphyric to weakly porphyritic rocks)
All of the dated lavas except for the 1811 lavas include nearly aphyric rocks (Table 1), and their modal volumes of phenocrysts are less than 3 vol. %. Some of the 1835 and 1940 summit and 1983 lavas are weakly porphyritic, containing 3–9 vol. % phenocrysts. We generally call these ‘megacryst-free rocks’. Mafic phenocrysts in the megacryst-free rocks usually consist of clinopyroxene, orthopyroxene and olivine (Figs 2 and 3). The 1835 lavas, however, do not contain olivine phenocrysts, whereas the 1643 and 1712 lavas lack orthopyroxene. In addition, in some of the 1940 and 1962 lavas both olivine and orthopyroxene phenocrysts are absent (Amma-Miyasaka & Nakagawa, 1998).
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Plagioclase is the dominant phenocryst phase and its maximum size is 4·6 mm in length. The An content of the plagioclase phenocrysts is in the range 53–96 mol % (Fig. 2). Histograms for the 1835, 1962 and 1983 samples show unimodal distributions with a peak of An = 80–90, whereas the other samples show bimodal distributions. Although the peaks of the An contents are variable in each eruption, most of them are in the range of An = 80–90 and An = 60–75.
Clinopyroxene less than 2·4 mm in length is the dominant mafic phenocryst. Most of the phenocrysts are augite, although pigeonite sometimes occurs in samples of the 1712 and 1940 lavas (Fig. 3). The Mg-number of most of the clinopyroxene phenocrysts is in the range 60–79; however, one phenocryst in the 1962 lava is more mafic (Mg-number >80). There is little difference in the compositions of the phenocrysts from samples of different eruption ages, although compositional variations in the 1835 and 1962 lavas seem to be smaller compared with other eruptives (Fig. 3).
Olivine phenocrysts are usually smaller than 2·0 mm in length. The Fo content of the olivine phenocrysts is in the range 52–78 mol % (Fig. 3). The variations in the phenocryst composition in the samples older than the 1763 eruption are greater than those in the samples younger than the 1874 eruption.
Orthopyroxene and magnetite phenocrysts are rare and smaller than 1·6 mm and 0·4 mm in length, respectively. The Mg-number of orthopyroxene phenocrysts is in the range 52–75 (Fig. 3). Although most of them are in the range Mg-number = 60–75, some of the orthopyroxene phenocrysts in the 1962 and 1983 lavas have Mg-number <60.
Megacryst-bearing rocks
Megacryst-bearing rocks occur only in the 1811, 1874 and 1940 flank eruption sequences (Table 1), and are orthopyroxene-bearing clinopyroxene–olivine basalt to andesite (Figs 2 and 3). The total volume of phenocrysts varies from 1 to 27 vol. %, and gradually increases from the 1811 (<11 vol. %), to 1874 (<16 vol. %) and to 1940 (<27 vol. %) eruptions. Petrographic characteristics and mineral compositions of the megacrysts in the 1940 samples were described by Amma-Miyasaka & Nakagawa (2002). Although the mineral assemblage and chemical compositions of the megacrysts in the 1874 samples are similar to those of the 1940 eruption, clinopyroxene megacrysts are also found in the 1811 megacryst-bearing rocks in addition to olivine and plagioclase. The chemical compositions of these megacrysts are more Mg rich or Ca rich compared with other phenocrysts in the megacryst-free rocks.
Plagioclase up to 30 mm in length forms the dominant phenocryst phase. The compositions of plagioclase phenocrysts are in the range An = 53–97, nearly the same as those in the megacryst-free rocks (Fig. 2). Plagioclase megacrysts (longer than 5 mm) in all the rocks have An >90. Although bimodal distributions of plagioclase with An >90 and An = 80–90 are recognized in all the samples, phenocrysts with An <90 are rare in the 1811 samples.
Olivine is the dominant mafic phenocryst. The compositions in the 1811 and 1940 samples range from Fo = 70 to Fo = 86 (Fig. 3). The 1874 lavas, however, also include Fo-poor phenocrysts (Fo <70). Olivine megacrysts (longer than 2 mm) have similar compositions (Fo >80) among the three eruptions.
Clinopyroxene less than 40 mm in length is also an abundant mafic phenocryst. All of the phenocrysts are augite (Fig. 3), and their Mg-numbers are in the range 60–83. The compositional range in the rocks is nearly the same as that for the megacryst-free rocks. The 1811 megacryst-bearing rocks show a compositionally bimodal distribution of the clinopyroxene phenocrysts (Fig. 3). The phenocrysts with high Mg-number often coexist with plagioclase and olivine megacrysts.
Orthopyroxene and magnetite phenocrysts are rare, and smaller than 1·2 mm and 0·3 mm in length, respectively. The compositional range of orthopyroxene phenocrysts is narrow (Mg-number = 62–73) compared with the megacryst-free rocks (Fig. 3).
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WHOLE-ROCK GEOCHEMISTRY |
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TOPABSTRACTINTRODUCTIONGEOLOGY AND 1469-1983 ERUPTIONS...ANALYTICAL METHODSPETROGRAPHY AND MINERAL...WHOLE-ROCK GEOCHEMISTRYMINERAL ASSEMBLAGE AND CHEMICAL...DISCUSSIONCONCLUSIONSUPPLEMENTARY DATAAPPENDIX: REPRESENTATIVE MINERAL...REFERENCES |
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All of the rocks during the period 1469–1983 are classified as tholeiites in a SiO2–FeO*/MgO diagram (Fig. 4). Most of the lavas consist of medium-K basalt to andesite, whereas some of the megacryst-bearing rocks plot in the low-K field. Although lavas from the 1469 to the 1983 eruptions show variations in SiO2 (50·6–56·4 wt %) and K2O (0·35–0·82 wt %), there are no systematic changes in their whole-rock composition with time (Fig. 5). Comparing the differentiated megacryst-free rocks in each eruption, SiO2 and K2O contents seem to increase from 1469 to 1712, then decrease to 1940 and increase again to 1983. On the other hand, the chemical compositions of the differentiated megacryst-bearing rocks in each eruption are nearly constant, whereas the entire compositional ranges of SiO2 and K2O have gradually become wider from 1811 to 1940. The compositional variations of both megacryst-free and megacryst-bearing rocks are shown in representative Harker diagrams in Fig. 6.
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The SiO2 contents of the megacryst-free rocks range from 50·6 to 56·4 wt % (Figs 4–6). Among these rocks, at least three distinct trends (I, II and III) can be recognized, especially in SiO2–TiO2, SiO2–FeO*/MgO and SiO2–Cr diagrams, and these seem to correspond to eruption age (Fig. 7). Trend I consists mainly of the 1469, 1535 and 1595 lavas. Two samples (e.g. No. 2222 in Table 2) from one of the lava lobes of the 1643 eruption (main-N, Fig. 1) are also classified as part of this trend. The SiO2 content of the Trend I rocks ranges from 51·8 to 54·9 wt %. This trend is the most mafic (poor in SiO2, TiO2, FeO*/MgO and rich in Cr) of the three trends. Trend II consists mainly of lavas from the 1643 main-S lava lobe (Fig. 1; No. 1620 in Table 2) and 1712 lavas. Compared with the rocks of Trend I, those of Trend II are in the range of SiO2 50·6–53·2 wt %, and are characterized by lower SiO2 and Cr, and by higher TiO2 and FeO*/MgO. Trend III is composed mainly of the 1763–1983 lavas, and also contains the 1643 lavas from the central lava lobe (Fig. 1; No. 1627 in Table 2). The rocks of this trend have SiO2 = 52·0–56·4 wt %, and are the most differentiated (lowest Cr, and highest SiO2, TiO2 and FeO*/MgO). There also exist some samples with lower FeO*/MgO and higher Cr (e.g. No. 1522 of 1983 and No. 1918 of 1835 in Table 2), compared with other 1763–1983 lavas.
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The SiO2 content of the megacryst-bearing rocks ranges from 51·4 to 55·5 wt % (Figs 4–6). With an increase of SiO2, the Al2O3, Cr and Ni contents decrease, whereas the TiO2 contents and the FeO*/MgO ratio increase. The compositional variation of the megacryst-bearing rocks in each eruption forms distinct trends in most of the variation diagrams (Fig. 6). Although the 1811 lavas consist of SiO2 = 54·6–55·2 wt % andesites, the 1874 and 1940 lavas have a wider compositional range of SiO2 = 52·8– 55·2 wt % and SiO2 = 51·4–55·5 wt %, respectively. The 1811 lavas are rich in TiO2 and have higher FeO*/MgO, whereas the 1874 lavas are poor in TiO2 and FeO*/MgO among the megacryst-bearing rocks. Comparing the whole-rock compositions of the megacryst-bearing rocks with the megacryst-free samples, the megacryst-bearing rocks are rich in Al2O3 and Ni, and poor in TiO2, FeO*/MgO and V. Furthermore, the felsic ends of the trends of variations of the megacryst-bearing rocks intersect with those of the megacryst-free samples at around SiO2 = 55–56 wt %.
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MINERAL ASSEMBLAGE AND CHEMICAL COMPOSITIONS OF CRYSTAL-CLOTS |
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TOPABSTRACTINTRODUCTIONGEOLOGY AND 1469-1983 ERUPTIONS...ANALYTICAL METHODSPETROGRAPHY AND MINERAL...WHOLE-ROCK GEOCHEMISTRYMINERAL ASSEMBLAGE AND CHEMICAL...DISCUSSIONCONCLUSIONSUPPLEMENTARY DATAAPPENDIX: REPRESENTATIVE MINERAL...REFERENCES |
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It is widely accepted that two types of magmas have erupted from the tholeiitic volcanoes of the Izu–Mariana arc: ‘plagioclase-controlled’ (plagioclase-accumulated) and ‘differentiated’ magmas (Nakano & Yamamoto, 1991
; Nakano et al., 1991; Tsukui & Hoshino, 2002). This is consistent with the evidence that there exist no disequilibrium phenocryst assemblages, such as olivine and quartz, or olivine and hornblende, in all of the rocks formed during the 1469–1983 eruptions of Miyake-jima volcano. Most of these rocks are, however, nearly aphyric and have less than 3 vol. % phenocrysts. This suggests that plagioclase accumulation is a minor source of compositional variation in these rocks. In addition, SiO2 and K2O do not systematically change during the period 1469–1983 (Fig. 5). If differentiation had proceeded in a closed magma storage system, these values should have increased with time. These observations suggest that simple differentiation does not play an important role during the period. On the other hand, phenocryst minerals show wide and polymodal compositional distributions (Figs 2 and 3), suggesting that all of the phenocrysts could not crystallize simultaneously from a single magma. To investigate whether these rocks are equilibrium 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 chemical compositions of the minerals within the crystal-clots in the 1469–1983 rocks, crystal-clots can be divided into four types (Figs 8 and 9): megacryst-type (M-type); basaltic-type (B-type); andesitic-type (A-type); basaltic 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, clinopyroxene and olivine. Core compositions of plagioclase, 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 orthopyroxene (Mg-number = 70–76; Fig 3 and 9) without magnetite. 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.
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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 coexist in equilibrium, the Mg-number of olivine should be nearly the same as or slightly lower than that of clinopyroxene and orthopyroxene (Obata et al., 1974
; Brey & Kohler, 1990). We have determined the average compositions and compositional ranges of olivine and pyroxenes in each type of clot of each eruption age (Fig. 10). The Mg-number of clinopyroxene is slightly higher compared with that of olivine in each type of clot. Moreover, the Mg-number of orthopyroxene is nearly the same as that of olivine in the A-type clots. This suggests that the olivine and pyroxene in each type of clot have crystallized in equilibrium from the same magma, and that different types of crystal-clots have crystallized from distinct magma compositions.
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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-type clots is basaltic, and that producing the AB-type and A-type clots is probably andesitic (e.g. Gill, 1981). |
DISCUSSION |
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TOPABSTRACTINTRODUCTIONGEOLOGY AND 1469-1983 ERUPTIONS...ANALYTICAL METHODSPETROGRAPHY AND MINERAL...WHOLE-ROCK GEOCHEMISTRYMINERAL ASSEMBLAGE AND CHEMICAL...DISCUSSIONCONCLUSIONSUPPLEMENTARY DATAAPPENDIX: REPRESENTATIVE MINERAL...REFERENCES |
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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 >0·3 mm (where L is the radius of the equivalent circle) and olivine of L >0·2 mm] and the least differentiated composition of minerals (plagioclase 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 plagioclase; (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, homogeneous 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 assemblage, chemical compositions and crystal size distributions 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 some of the 1940 lavas. M-type crystal-clots have also been recognized in the megacryst-bearing rocks of the 1811 and 1874 eruptions. The clots in the rocks of the 1874 eruption consist only of plagioclase and olivine, as in the case of the 1940 megacryst-bearing lavas. On the other hand, the clots in the 1811 lavas also contain clinopyroxene. There is no significant difference in the petrographic characteristics and chemical composition of plagioclase and olivine megacrysts among the three lavas of known age. Thus, we consider that the megacrysts in the 1811 and 1874 lavas have the same origin as those in the 1940 lavas. Although the whole-rock chemistry of the megacryst-bearing rocks of each eruption seems to define distinct trends, this may reflect the difference in the ratio of component minerals in the plutonic rocks. All of the megacryst-bearing rocks erupted from N–NE flank fissures. Therefore, it may be suggested that these lavas have captured both the megacrysts 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 compositionally 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 clinopyroxene 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 compositions is considered to be a disequilibrium feature.
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The relationship between the crystal-clots and their host rocks can be investigated on the basis of Fe–Mg partitioning (Fig. 12). If minerals crystallize in equilibrium with the host magma, Fe–Mg distribution between the mineral-cores and the magma must be constrained by plausible partition coefficients, shown 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 the clots do not plot near equilibrium. Most olivine and clinopyroxene in the B-type clots are more Mg rich than possible equilibrium, whereas those in the A-type clots are more Fe rich. Orthopyroxene, contained only in the A-type clots, is also more Fe rich than that required to be in equilibrium with the whole-rock FeO*/MgO ratio (Fig. 12). This suggests that the mafic minerals in the B-type clots should have crystallized from a more mafic (lower FeO*/MgO) magma than the host magma, and those of the A-type clots from a more differentiated (higher FeO*/MgO) magma.
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Our data suggest that most of the 1469–1983 lavas contain two types of phenocryst that originated from distinct magmas, and indicate that these lavas could be produced by magma mixing of two end-member magmas, a differentiated A-type (andesitic) and a mafic B-type (basaltic) magma. The 1535 and 1595 lavas that have another type of crystal-clot (AB-type, Fig. 11), in addition to the A-type and B-type, might be the results of mixing between three magmas. In summary, at least two distinct magmas have existed beneath the Miyake-jima volcano since 1469, and have usually mixed during eruption.
Rocks without obvious evidence for magma mixing
The 1983, 1962 and 1835 lavas contain only the A-type crystal-clots (Fig. 11). Although histograms of phenocryst compositions in these lavas do not show clear bimodal distributions, the compositional range of the plagioclase phenocrysts is relatively wide (An = 60–90). The compositional variation of clinopyroxene in the 1983 lavas is also relatively extensive, compared with that in the 1962 and 1835 lavas. The compositional relationships between minerals and whole rocks also show that most of the mafic minerals in these rocks are in equilibrium with their host magma; however, Fe-rich clinopyroxene in the 1983 sample (No. 1522 in Table 2) and olivine in the 1962 sample (No. 122508 in Table 2) might not be in equilibrium (Fig. 12). The lack of B-type clots indicates that these lavas may have been produced without mixing with the mafic B-type magma. Although it is also possible that nearly aphyric basaltic magma mixed with the A-type magma, the proportion of the basaltic magma would be minor, because the mineral compositions are nearly in equilibrium with the host magma (Fig. 12). Considering the wide compositional variations of the phenocryst minerals (plagioclase and clinopyroxene) especially in the 1983 lavas, however, we could not exclude the possibility of mixing between distinct andesitic magmas. In conclusion, we suggest that lavas without the B-type clots are not mixing products between the A-type and B-type magmas, and that only pure A-type magma (or magmas) has erupted during the 1835, 1962 and 1983 eruptions. Furthermore, it is noteworthy that rocks containing only the B-type clots have never erupted during the period 1469–1983.
End-member magmas and their relationships
Estimation of whole-rock compositions of end-member magmas
Using the Fe–Mg mineral–melt distribution coefficients for olivine/liquid and orthopyroxene/liquid (Beattie, 1993), we can estimate the FeO*/MgO ratios of both the A-type and B-type magmas (Fig. 13). Except for the 1835, 1962 and 1983 lavas, the FeO*/MgO ratios of the whole rocks are intermediate between those of the A-type and B-type magmas in each eruption. This is consistent with our conclusion that these rocks are mixing products between these magmas. Whole-rock FeO*/MgO ratios of the 1962 and most of the 1835 and 1983 lavas are nearly the same as the estimated FeO*/MgO ratios of the A-type magmas, whereas the whole-rock ratios of the 1535, 1595, 1712 and most of the 1643 lavas are similar to those of the B-type magmas. FeO*/MgO ratios might reflect the mixing ratio between the A-type and B-type magmas.
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Relationship of the end-member magmas
Plots of incompatible-element ratios (K2O/Ba, Y/Ba and Y/Zr) in the 1469–1983 megacryst-free rocks define linear trends that pass through the origin with R2 values of 0·89–0·93 (Fig. 14). This strongly suggests that both the A-type and B-type magmas may have been derived from a single primary magma. This conclusion is consistent with Sr isotopic data. Notsu et al. (1983)
and Notsu & Aramaki (1984) measured 87Sr/86Sr ratios of the rocks from Miyake-jima volcano, and confirmed that the isotopic ratio is concentrated in a narrow range (0·70350–0·70369), suggesting that the source region of the magmas beneath Miyake-jima volcano is isotopically uniform.
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Depths of the end-member magmas
The depths of the magma storage systems have been estimated based on geophysical observations carried out since the 1940 eruption. Takahashi & Hirano (1941)
estimated that the size of the chamber was 2·7 km in radius and 100 m thick for the 1940 eruption, assuming the depth of the magma to be 3 km. Hypocentres of long-period earthquakes (Ueki et al., 1984) and geomagnetic change (Nakagawa et al., 1984) associated with the 1983 eruption suggested that magma was located at a depth of 2–3 km. In addition to these shallower ‘sources’, the inflation and deflation accompanying the 1983 eruption suggested a magma reservoir about 1–2 km SW of the summit crater, at a depth estimated to be about 8 km (Tada & Nakamura, 1988). Sasai et al. (2001) also suggested that the pressure source during 1995–1999 lies beneath the southern flank of the volcano at a depth of 8 km below sea level. Considering that the depths of short-period earthquakes are 1–10 km below sea level (Minakami et al., 1963; Miyazaki & Sawada, 1984; Ueki et al., 1984), the magma storage systems appear to have existed at least at two depths (c. 2–3 km and 8 km). On the other hand, geobarometry did not successfully reveal the crystallization depth of the A-type and B-type magmas of the volcano. We used the pseudoternary normative diagram (cpx–ol–SiO2) of Walker et al. (1979) and Baker & Eggler (1983) for lavas with only the A-type clots and those with both the A-type and B-type clots, and the clinopyroxene geobarometer (Nimis, 1995) for both types of clots. These calculations show, however, no difference between the A-type and B-type magmas. Whole-rock compositions are projected around 1 atm for both lavas on the normative diagram, and equilibration pressures calculated based on clinopyroxene compositions are 0–3 kbar for both the A-type and B-type clots.
The B-type magma appears to have entrained plutonic xenoliths before mixing with the A-type magma during the 1940 eruption (Amma-Miyasaka & Nakagawa, 2002). Thus, we suggest that basaltic magma should exist at a deeper level beneath the volcano than andesitic magma. We propose that two distinct magma storage systems, a shallower andesitic (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 eruptions, 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.
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 resultant 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 clinopyroxene in the B-type magma is wide, the average pyroxene composition in each eruption is nearly the same 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 to the Fo content of olivine increased from 1 to more than 1, with falling equilibrium temperature. Therefore, a temporal change in both Mg-number of clinopyroxene and Fo content of olivine in the B-type clots would indicate that the temperature of the basaltic magma has fallen systematically with time. The temperature of the basaltic magma calculated by Loucks (1996) actually fell from 1220°C in the 1469 eruption to 1180°C in 1940. Using the temporal variation of phenocryst minerals in the B-type clots, the evolution of the whole-rock chemistry of the basaltic magma can also be evaluated. The FeO*/MgO ratios of the basaltic end-member magmas during 1469–1595 were the lowest (Fig. 13). Since then, the ratio has increased, accompanied by falling magmatic temperature.
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In contrast, the core composition of clinopyroxene and orthopyroxene in the A-type clots has not changed systematically as it has for the B-type clots (Fig. 16). The average core composition of clinopyroxene is Mg-number = 67 in the 1469 lavas, which is the most Fe rich during the past 500 years. Although the composition has fluctuated since then, it has gradually become more Mg rich with time to Mg-number = 72 in the 1983 lavas. Except for the 1763 eruption, the average Mg-number of orthopyroxene in the 1469 A-type clots reflects the most differentiated magma during the period 1469–1983, and has gradually become Mg rich up to 1983. Although orthopyroxene in the A-type clots in the 1763 lava is the most Fe rich among the 1469–1983 samples, it is rare in 1763. Using the average compositions of both clinopyroxene and orthopyroxene in the A-type clots, magmatic temperatures were calculated according to the Wells (1977)
geothermometer for each eruption. The temperature appears to have risen in a complex fashion from 1070°C in the 1469 eruption to 1100°C in 1983 (Fig. 16). As in the case of the basaltic magma, temporal changes in the whole-rock chemistry of the andesitic magma are also evident: the FeO*/MgO ratio has decreased from 4·4 in the 1469 eruption to 3·3 in 1983 (Fig. 13).
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Thus, the two magmas, which have formed end-member components for magma mixing during the past 500 years, have evolved following separate paths (Fig. 17a). The basaltic magma has become more differentiated with time while cooling, whereas the andesitic magma has become more mafic while heating. A temporal fall in the magmatic temperature of the basaltic magma could be explained by fractional crystallization of a primary basaltic magma, and it suggests that the basaltic end-member magma has been continuously in existence at least for 500 years. The complicated and reverse evolution of the andesitic magma during the past 500 years cannot be explained by a closed system as in the case of the basaltic magma. In each eruption, the deeper basaltic magma was injected into the shallower andesitic magma and erupted a mixed magma. It suggests that the shallower andesitic magma chamber was an open magma storage system for injection of the basaltic magma. In this case, if only a small quantity of the basaltic magma was injected, the shallower andesitic magma would become more differentiated mainly by the effect of fractional crystallization. In contrast, when basaltic magma was injected on a large scale, the andesitic magma could have become more mafic. Moreover, the andesitic magma might be nearly replaced by the basaltic magma. In conclusion, the temporal fluctuation of magmatic temperature and the changing composition of the andesitic magma during the past 500 years depend on the quantity of basaltic magma injected.
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Evaluation of our model by MELTS calculation
Our explanation for the evolution of the basaltic magma can be tested using the MELTS calculation (Ghiorso & Sack, 1995
). The chemical evolution of the andesitic magma might reflect mixing with injected basaltic magma. The andesitic magma existing before the 1469 eruption, however, might have been formed by fractional crystallization of the basaltic magma. We used the composition of the 1535 basaltic lava (No. 2321 in Table 3) as the starting material because the FeO*/MgO values of the 1535 and 1595 lavas (2·40–2·48) are the lowest among the 1469–1983 megacryst-free rocks, and nearly the same as the calculated FeO*/MgO of the B-type liquid (2·4–2·5, Fig. 13). According to MELTS calculations, the FeO*/MgO continuously increases as temperature falls, in the range of SiO2 <53 wt % (Fig. 18a). Liquid lines of descent are partially dependent on pressure and water content. Liquid compositions jump to SiO2 >55 wt % when magnetite begins to crystallize. The FeO*/MgO continuously increases as temperature falls, and liquid lines of descent for SiO2 >55 wt % have a wide range. The compositional range of the 1469–1983 megacryst-free rocks lies between the liquid lines of descent for SiO2 <53 wt % and SiO2 >55 wt %.
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Comparing the FeO*/MgO ratios of the end-member magmas calculated from phenocryst compositions (4·4 for the 1469 andesitic magma and 2·4–3·2 for the basaltic magma) with the results of MELTS calculations, the FeO*/MgO of the basaltic magma 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; therefore, we consider that the basaltic magma may have stagnated at around 3 kbar pressure. On the other hand, the FeO*/MgO ratio of the andesitic magma can be explained only by the 1 kbar calculated results (Fig. 18b). This result is consistent with the geophysical observations (the shallower andesitic magma storage system might be located at a depth of 2–3 km). It is suggested that the 1469 andesitic magma (Table 3) could have been formed by fractional crystallization of basaltic magma at a shallow level (about 1 kbar). It is evident that the 1469 andesitic magma has changed its composition over 500 years in a complex fashion (Fig. 18b), and it is also consistent with our model that the shallower andesitic magma chamber was an open magma storage system for injection of the basaltic magma.
Possible state of the magma plumbing system just before the 2000 eruption
Our model for the evolution of the magma plumbing system and eruption processes should be able to estimate the state of the system just before the 2000 eruption of Miyake-jima volcano. According to simple temporal differentiation of the deep-seated basaltic magma during the past 500 years, the composition of basaltic magma after the 1983 eruption must be SiO2 >52 wt %, FeO*/MgO >3·2 and Fo content of the B-type olivine <70 (Fig. 17b). On the other hand, it is difficult to estimate the composition of the shallower andesitic magma because it shows a more complicated temporal change compared with the basaltic magma. It has, however, become more mafic from 1874 to 1983, probably because of the injections of the basaltic magma. After the 1983 eruption, the andesitic magma may have had SiO2 <53 wt %, FeO*/MgO <3·3 with A-type clots (pyroxenes; Mg-number >70). In other words, the compositions of andesitic and basaltic magmas are indistinguishable after the 1983 eruption. Before the 2000 eruption, we proposed that the erupted magma would be a mixing product of these basaltic and andesitic magmas, or would be andesitic alone.
Geshi et al. (2002) have already discussed the magmatic system during the 2000 eruptive activity mainly based on the whole-rock chemistry of the eruptive rocks. However, we consider that detailed analysis of 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).
Implications for studies of the magma plumbing system and its evolution in active volcanoes
Several studies have already focused on the structures of magma plumbing systems and their evolution in active volcanoes over hundreds to thousands of years: Fichaut et al. (1989), Belkin et al. (1993), Villemant et al. (1993) and Sato et al. (1996). These workers argued that the increase of SiO2 content or FeO*/MgO ratio with time is due to fractionation or to the change of the mixing ratio. On the other hand, they also speculated that a sudden drop of SiO2 or FeO*/MgO may indicate the injection of a new batch of mafic magma. If the end-member magmas had not changed for considerable durations, these explanations might be correct. Temporal change of the end-member magmas, however, may be common as in the case of Miyake-jima volcano, in which case temporal variations of the whole-rock chemistry cannot simply correspond to the degree of fractionation and/or to mixing ratio of the end-member magmas. It should be noted that the detailed analysis of eruptive rocks from each eruption using the mineral assemblage and chemical compositions in crystal-clots can clarify the evolution of the magma system of active volcanoes.
Coexistence of deeper basaltic (less differentiated) and shallower basaltic–andesitic (differentiated) magma storage systems might be common for island arc and oceanic island basalt to basaltic andesite volcanoes, for example, Kilauea and Arenal volcanoes (e.g. Wright & Fiske, 1971; Reagan et al., 1987; Garcia et al., 1989, 1992, 1996, 2000; Marianelli et al., 1999; Streck et al., 2002). In these cases, new batches of basaltic magma from distinct mantle sources were supplied to the shallower magma systems during the recent eruptions. Magma storage systems at Miyake-jima volcano have, however, survived at least for 500 years without the injection of a new batch of basaltic magma. We consider that this might depend on the production rate of magma. The erupted volume of the Pu'u O'o eruption of Kilauea (started in 1983 and continuing in 2000) is <2 km3 (Garcia et al., 2000), and that of the 1968–1996 eruption of Arenal volcano is >0·35 km3 (Reagan et al., 1987). These values are much larger than the total erupted volume of Miyake-jima volcano during 1469–1983 (<0·15 km3; Tsukui et al., 2001). In the case of higher production rate, the evolution of the geochemical characteristics of the erupted magma would reflect a frequent supply of the newly arriving basaltic magmas. We would emphasize that magma storage systems could be maintained over at least 500 years without supply of basaltic magma from mantle source, as in the case of Miyake-jima volcano.
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CONCLUSION |
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TOPABSTRACTINTRODUCTIONGEOLOGY AND 1469-1983 ERUPTIONS...ANALYTICAL METHODSPETROGRAPHY AND MINERAL...WHOLE-ROCK GEOCHEMISTRYMINERAL ASSEMBLAGE AND CHEMICAL...DISCUSSIONCONCLUSIONSUPPLEMENTARY DATAAPPENDIX: REPRESENTATIVE MINERAL...REFERENCES |
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We have constrained the magma plumbing system beneath Miyake-jima volcano for the past 500 years based on the mineral assemblage and chemical compositions of crystal-clots in single samples, as follows:
- the magma plumbing system during historic activity of Miyake-jima volcano has been fed by two magma storage systems; a deep-seated basaltic (about 3 kbar) and a shallower andesitic (1 kbar) system.
- In many cases, the deeper basaltic magma injected into the shallower andesitic magma to form mixed magmas; moreover, the shallower andesitic magma rarely erupted alone (1983, 1962 and 1835) without extensive injections of the basaltic magma.
- The basaltic magma has differentiated with time and its temperature has also fallen. The andesitic magma has become more mafic, as a result of the magma being periodically refilled by the deeper basaltic magma.
- According to the temporal variations of these end-member magmas, we estimate that the compositions of the two magmas had become similar after the 1983 eruption.
- Analysis of crystal-clots in single samples is a useful method to clarify magmatic processes.
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SUPPLEMENTARY DATA |
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TOPABSTRACTINTRODUCTIONGEOLOGY AND 1469-1983 ERUPTIONS...ANALYTICAL METHODSPETROGRAPHY AND MINERAL...WHOLE-ROCK GEOCHEMISTRYMINERAL ASSEMBLAGE AND CHEMICAL...DISCUSSIONCONCLUSIONSUPPLEMENTARY DATAAPPENDIX: REPRESENTATIVE MINERAL...REFERENCES |
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Supplementary data for this paper are available on Journal of Petrology online.
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APPENDIX: REPRESENTATIVE MINERAL COMPOSITIONS IN EACH TYPE OF CRYSTAL-CLOTS |
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TOPABSTRACTINTRODUCTIONGEOLOGY AND 1469-1983 ERUPTIONS...ANALYTICAL METHODSPETROGRAPHY AND MINERAL...WHOLE-ROCK GEOCHEMISTRYMINERAL ASSEMBLAGE AND CHEMICAL...DISCUSSIONCONCLUSIONSUPPLEMENTARY DATAAPPENDIX: REPRESENTATIVE MINERAL...REFERENCES |
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ACKNOWLEDGEMENTS |
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We wish to express our gratitude to S. Nakada, M. Takahashi, S. Togashi and H. Sato for valuable advice and constructive criticisms. We are indebted to S. Terada and M. Ikeda for supporting electron probe microanalyser and XRF analysis, and to T. Kuwashima and H. Nomura for preparing thin sections. We gratefully acknowledge K. Kobayashi of Miyake-jima for supporting our fieldwork. The paper was greatly improved by constructive comments from J. Gill, G. Morris, J. Davidson and M. Wilson. We wish to express our gratitude to K. Das, J. Riehle and N. Riggs for reading our manuscript. This study is partly supported by the Ministry of Education of Japan (No. 14080201) and the Earthquake Research Center, University of Tokyo.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONGEOLOGY AND 1469-1983 ERUPTIONS...ANALYTICAL METHODSPETROGRAPHY AND MINERAL...WHOLE-ROCK GEOCHEMISTRYMINERAL ASSEMBLAGE AND CHEMICAL...DISCUSSIONCONCLUSIONSUPPLEMENTARY DATAAPPENDIX: REPRESENTATIVE MINERAL...REFERENCES |
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