Journal of Petrology

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Journal of Petrology | Volume 41 | Number 3 | Pages 431-448 | 2000
© Oxford University Press 2000

Primary Arc Basalts from Daisen Volcano, Japan: Equilibrium Crystal Fractionation versus Disequilibrium Fractionation during Supercooling

Y. TAMURA^1,*, M. YUHARA² and T. ISHII³

¹DEPARTMENT OF EARTH SCIENCES, KANAZAWA UNIVERSITY, KANAZAWA 920-1192, JAPAN
²RESEARCH INSTITUTE FOR HAZARDS, NIIGATA UNIVERSITY, NIIGATA 950-2181, JAPAN
³OCEAN RESEARCH INSTITUTE, UNIVERSITY OF TOKYO, TOKYO 164-8639, JAPAN

Received February 2, 1999; Revised typescript accepted September 2, 1999

	ABSTRACT

TOP ABSTRACT INTRODUCTION GENERAL GEOLOGY OF DAISEN... ANALYTICAL METHODS GEOCHEMISTRY OF DAISEN BASALTS... OLIVINE BASALTS COMPOSITIONS OF CALCULATED... PRIMARY MAGMA COMPOSITION SUPERCOOLING CONCLUSION REFERENCES

 
Daisen volcano, southwest Japan, has been thought to be an exclusively dacitic volcano, lavas having trace element patterns with a garnet signature. We studied the basalts at the western foot of the volcano and made two unexpected findings. (1) The homogeneity of ⁸⁷Sr/⁸⁶Sr in Daisen basalts (Sr isotopic variability 0·0003) contrasts with great variations in basalts from nearby monogenetic fields. Moreover, the basalts and dacites from Daisen volcano have the same ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd, indicating that these rocks have a close genetic relationship. The calculated primary basaltic magma of Daisen volcano suggests a segregation depth of 60 km (18 kbar), and higher Sm/Yb ratios in the Daisen basalts relative to NE Japan arc basalts suggest that the segregation took place between the garnet and spinel peridotite stability fields. (2) The effects of fractional crystallization were distinguished from those produced by supercooling on the basis of olivine morphology and chemical relationships between olivines and host basalts. Supercooled magmas become magnesian through fractionation of iron-rich olivines and are seemingly anomalous because they contain iron-rich olivines despite their magnesian bulk-rock chemistry.

KEY WORDS: arc volcanism; Daisen volcano; olivine fractionation; primary magma; supercooling

	INTRODUCTION

TOP ABSTRACT INTRODUCTION GENERAL GEOLOGY OF DAISEN... ANALYTICAL METHODS GEOCHEMISTRY OF DAISEN BASALTS... OLIVINE BASALTS COMPOSITIONS OF CALCULATED... PRIMARY MAGMA COMPOSITION SUPERCOOLING CONCLUSION REFERENCES

 
Mantle-derived primary magma is of fundamental importance, both for shedding light on the nature of mantle source rocks and for understanding the ways these rocks melt to yield primary magmas. Some of these processes have been investigated by studying the experimental melting of peridotite using the recently employed diamond-aggregate method (e.g. Hirose & Kushiro, 1993; Baker & Stolper, 1994; Kushiro, 1996), leading to a vigorous controversy on the interpretation of the results (e.g. Falloon et al., 1997). Thus further studies, including those of natural examples of mantle-derived primary magma, are needed. Most arc basalts, however, do not represent primary magmas, because their compositions (high FeO^*/ MgO and low Ni contents) cannot have been in equilibrium with mantle olivines (e.g. Sato, 1977; Tatsumi et al., 1983). We must apply additional methods to gain insight into the nature and origin of arc magmas.

The original goal of this research was to study the olivine basalts at the western foot of Daisen volcano so as to determine the primary basaltic magmas involved. While pursuing this goal, two previously unrecognized relationships emerged: (1) these basalts have similar ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios and the same general age as the older dacites and andesites of Daisen volcano itself. Although Daisen volcano has previously been considered to consist of only andesites and dacites, these basalts must now be interpreted to be members of the Daisen volcanic suite. (2) Some of these basalts show mineralogical and chemical evidence of supercooling.

Evidence of supercooling of basaltic magmas is well developed in layered intrusions (e.g. Ballhaus & Glikson, 1989; Tegner et al., 1993). Although derived supercooled magmas in arc volcanoes could have been tapped from magma chambers similar to those represented by layered intrusions, little attention has been paid to the evidence of such supercooling. The basalts from Daisen volcano are primitive olivine basalts, containing phenocrysts of olivine only. Our study of olivine mineralogy and basalt chemistry revealed two kinds of olivine basalts; one resulting from fractional crystallization of olivine from primary basalt magma and the other lying outside this fractionation path. Although these latter basalts tend to be more magnesian, they contain olivines that are distinctly enriched in FeO relative to those expected from a normal fractionation trend. Interestingly, normally zoned skeletal olivine phenocrysts are characteristic of these basalts. Their chemical compositions and skeletal shapes are thought to have resulted from supercooling of basaltic magmas, and their distinctive shapes suggest supercooling of about 10–30°C (Donaldson, 1976).

We discriminate between fractional crystallization and supercooled differentiation of Daisen basaltic magmas and, using these relationships, we estimate the primary magma composition of Daisen volcano.

	GENERAL GEOLOGY OF DAISEN VOLCANO

TOP ABSTRACT INTRODUCTION GENERAL GEOLOGY OF DAISEN... ANALYTICAL METHODS GEOCHEMISTRY OF DAISEN BASALTS... OLIVINE BASALTS COMPOSITIONS OF CALCULATED... PRIMARY MAGMA COMPOSITION SUPERCOOLING CONCLUSION REFERENCES

 
Daisen volcano is one of 245 Quaternary arc volcanoes in Japan (Ono et al., 1981), and it is associated with the subduction of the Philippine Sea Plate beneath the Eurasia Plate (Nakanishi, 1980; Fig. 1). Although subcrustal earthquakes, which normally provide evidence of a descending slab, are absent in southwest Japan, Nakanishi (1980) found evidence for ScS- to P-wave conversions at a dipping interface in the upper mantle and interpreted this to be the top of the aseismic Philippine Sea Plate.

View larger version (42K): [in this window] [in a new window] [Download PowerPoint slide]   Fig. 1. Location map and simplified topography showing height (m) of major peaks. Daisen volcano is associated with the subduction of the Philippine Sea Plate beneath the Eurasia Plate (Nakanishi, 1980). The volcano is actually a volcanic complex, consisting of clustered and overlapping lava domes and associated lava flows.

 

Daisen volcano is actually a volcanic complex, consisting of clustered and overlapping lava domes and associated lava flows and pyroclastic flows (Figs 1 and 2). The volcano rests unconformably on 185 Ma granites and gneisses (Ishiga et al., 1989). Most peaks, including the highest Misen lava dome, are dacitic in composition. Morris (1995) showed Y and heavy rare earth element (HREE) depletion in dacites from Daisen volcano and suggested they originated by slab melting, leaving an eclogite residue.

View larger version (41K): [in this window] [in a new window] [Download PowerPoint slide]   Fig. 2. Geologic map of Daisen volcano, simplified from Tsukui (1984, 1985), Tsukui et al. (1985) and Ota (1962). Eruption ages (Ma) are from Tsukui et al. (1985) and Uto (1989).

 

Figure 2 shows a simplified geologic map of Daisen volcano after Tsukui (1984, 1985), Tsukui et al. (1985) and Ota (1962); the lava eruption ages shown (Ma) are from Tsukui et al. (1985) and Uto (1989). The basalt lavas studied overlie basement granites in Mizoguchi area, just west of Daisen volcano, and consist at least 16 individual lava flows. The source vents for the lava flows were not recognized, but they are assumed to have been erupted in the immediate area. The basalts are magnesian olivine tholeiitic lavas. They contain 5–11 vol. % olivine phenocrysts, with rare Cr-spinel inclusions, ± augite phenocrysts. No plagioclase phenocrysts were observed. Uto (1989) reported a K–Ar age of 1·21 ± 0·16 Ma for these basalts, indicating that the basalts have the same general age as the older dacites and andesites (0·91–1·02 Ma) of Daisen volcano (Tsukui et al., 1985; Uto, 1989).

	ANALYTICAL METHODS

TOP ABSTRACT INTRODUCTION GENERAL GEOLOGY OF DAISEN... ANALYTICAL METHODS GEOCHEMISTRY OF DAISEN BASALTS... OLIVINE BASALTS COMPOSITIONS OF CALCULATED... PRIMARY MAGMA COMPOSITION SUPERCOOLING CONCLUSION REFERENCES

 
Major and trace elements
After initial splitting and jaw crushing, all samples were pulverized in an agate ball mill. H₂O^– and loss on ignition were determined at 110°C and 950°C (5 h), respectively. Major and trace elements were determined by X-ray fluorescence (XRF) at the Ocean Research Institute, University of Tokyo. Trace elements were analysed on pressed powder discs, and major elements were determined on fused glass discs. A mixture was used of 0·5 g of each powdered sample and 5 g of anhydrous lithium tetraborate (Li₂B₄O₇), with no matrix correction because of the high dilution used. Instrumental neutron activation analysis (INAA) was carried out with the Kyoto University reactor (KUR) and a gamma-ray spectrometer having a Ge (Li) detector at the RadioIsotope Center, Kanazawa University, using the procedures of Ishiwatari & Ohama (1997). Microprobe analysis was carried out on two JEOL JXA-8800 Superprobes. The probe at the Institute of Study of the Earth’s Interior, Okayama University, was equipped with four wavelength-dispersive spectrometers (WDS); the instrument at Kanazawa University had five WDSs. Olivine analyses were made with a counting time of 100 s, using a focused beam, an accelerating voltage of 25 kV, and a beam current of 20 nA to ensure reliable nickel values (SD count < 2%).

Isotopic compositions
⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios were determined with a thermal ionization mass spectrometer (Finnigan MAT 261; modified from MAT 260) at Niigata University. Sr isotopes were normalized to ⁸⁶Sr/⁸⁸Sr = 0·1194 and adjusted to 0·710241 for the ⁸⁷Sr/⁸⁶Sr value of the NBS-987 standard. Nd isotopic ratios were corrected by normalization to ¹⁴⁶Nd/¹⁴⁴Nd = 0·7219, and values of ¹⁴³Nd/¹⁴⁴Nd are given relative to a value of 0·512784 for the JB-1a standard. This Nd isotopic ratio of JB-1a corresponds to ¹⁴³Nd/¹⁴⁴Nd = 0·512638 of BCR-1 (Kagami et al., 1989). The blanks for the whole procedure were Rb < 0·25 ng, Sr < 0·52 ng, Sm < 0·025 ng and Nd < 0·22 ng.

	GEOCHEMISTRY OF DAISEN BASALTS AND DACITES

TOP ABSTRACT INTRODUCTION GENERAL GEOLOGY OF DAISEN... ANALYTICAL METHODS GEOCHEMISTRY OF DAISEN BASALTS... OLIVINE BASALTS COMPOSITIONS OF CALCULATED... PRIMARY MAGMA COMPOSITION SUPERCOOLING CONCLUSION REFERENCES

 
Plots of selected major and trace elements and Sr/Y vs SiO₂ are shown in Figs 3 and 4, and analyses are presented in Table 1. The 16 basalt lava flows studied (SiO₂ = 50 wt %, MgO = 7·1–9·6 wt %) have variation of TiO₂, Y and Nb as wide as, or wider than, that of the andesites and dacites from the main mass of Daisen volcano. On the other hand, the latter have a wider compositional variation of elements such as Al₂O₃, Na₂O, K₂O, Rb, Zr, Sr and Ba, and they have less variable and lower contents of MgO and Ni.

View larger version (23K): [in this window] [in a new window] [Download PowerPoint slide]   Fig. 3. Major elements (TiO2, Al2O3, Fe2O3, MgO, CaO, K2O, Na2O and P2O5) vs SiO2 in lavas of Daisen volcano. The volcanic association is clearly bimodal.

 

View larger version (23K): [in this window] [in a new window] [Download PowerPoint slide]   Fig. 4. Trace elements (Ni, Rb, Zr, Sr, Y, Ba and Nb) and Sr/Y vs SiO2 in lavas of Daisen volcano.

 

View this table: [in this window] [in a new window]   Table 1: Major element, modal analyses, trace element and isotopic data for selected basalt lavas and dacites from Daisen volcano

 

Subduction-zone affinities of the Daisen basalts can be shown in a normalized trace element plot (Fig. 5). Relative to normal mid-ocean ridge basalts (N-MORB), Daisen basalts show: enrichment in strongly incompatible large ion lithophile elements (LILE) such as Rb, Ba and K; strong depletion in Nb relative to K and Th; and enrichment in Pb over Ce. These patterns are considered to be distinctive of subduction-related magmas elsewhere (e.g. Pearce, 1982; McCulloch & Gamble, 1991; Price et al., 1999).

View larger version (33K): [in this window] [in a new window] [Download PowerPoint slide]   Fig. 5. MORB-normalized plot of trace element data for Daisen basalt lavas: normalizing values are from Sun & McDonough (1989). These basalts have typical arc trace element signatures.

 

Figure 6a shows an ⁸⁷Sr/⁸⁶Sr–SiO₂ plot for the Daisen basalts and dacites, and provides a comparison with basalts from nearby fields, Oki-Dogo, 100 km NNW and Yokota, 50 km west of Daisen volcano, respectively (Fujibayashi et al., 1988; Fujimaki et al., 1991). The Oki-Dogo alkali basalts (<1·0 Ma) and Yokota basalts (1·1–2·1 Ma) are associated with monogenetic volcano groups and are not associated with dacites and andesites. The homogeneity of ⁸⁷Sr/⁸⁶Sr in Daisen basalts (Sr isotopic variability 0·0003) contrasts with great variations in those from Yokota and Oki-Dogo, thus making the Daisen basalts geochemically distinct from nearby monogenetic volcanoes. Moreover, the basalts and dacites from Daisen volcano have the same ⁸⁷Sr/⁸⁶Sr values. Tamura & Nakamura (1996) reviewed ⁸⁷Sr/⁸⁶Sr isotopic data from 38 arc volcanoes and found that mantle-derived magmas can be isotopically homogeneous within an individual volcano. Thus, although the Daisen basalts and dacites are adjacent rather than juxtaposed, and the volcano has previously been referred to as dacitic, we consider the basalts and dacites to be genetically related and the Daisen magmatic system to be bimodal.

Figure 6b shows a conventional ¹⁴³Nd/¹⁴⁴Nd vs ⁸⁷Sr/⁸⁶Sr variation diagram for the Daisen volcano. Although the four dacites measured have slightly higher values of ¹⁴³Nd/¹⁴⁴Nd than the basalts, the Daisen Sr and Nd isotopic data cluster within the ocean island basalt (OIB) field of Zindler & Hart (1986). They also have higher ⁸⁷Sr/⁸⁶Sr and lower ¹⁴³Nd/¹⁴⁴Nd than MORB (Zindler & Hart, 1986) and NE Japan arc basalts (Shibata & Nakamura, 1997). Tamura & Nakamura (1996) showed that large isotopic variability from the same volcano could suggest some crustal involvement. Although ⁸⁷Sr/⁸⁶Sr ratios for all analysed Daisen samples are higher than those of NE Japan arc basalts (Shibata & Nakamura, 1997), the Sr isotopic variability from the volcano is <0·0003 (Fig. 6a). Moreover, all Daisen data cluster within the mantle array (Fig. 6b), suggesting that crustal involvement is minimal.

View larger version (44K): [in this window] [in a new window] [Download PowerPoint slide]   Fig. 6. (a) 87Sr/ 86Sr vs SiO2 for Daisen basalts and dacites compared with Oki-Dogo alkali basalts (<1·0 Ma, Fujimaki et al., 1991) and Yokota basalts (1·1–2·1 Ma, Fujibayashi et al., 1988), situated 100 km NNW and 50 km west of Daisen, respectively. The homogeneity of 87Sr/86Sr in Daisen basalts contrasts with great variations in basalts from these nearby monogenetic fields. (b) 143Nd/ 144Nd vs 87Sr/ 86Sr for basalts () and dacites () of Daisen volcano. The Sr and Nd isotopic data cluster within the mantle array. The MORB and OIB fields are from Zindler & Hart (1986). The NE Japan arc basalt field is from Shibata & Nakamura (1997).

 
Isotopically, it is clear that the Daisen basalts and dacites have a close genetic relationship, which is a common characteristic of arc volcanoes (e.g. Gill, 1981; Tamura & Nakamura, 1996).

Figure 7 shows an La/Sm–Sm/Yb plot for the Daisen basalts, NE Japan arc basalt field after Shibata & Nakamura (1997), and the average E-MORB, N-MORB and OIB compositions of Sun & McDonough (1989). The Daisen basalts have higher Sm/Yb ratios than arc basalts in NE Japan, and are intermediate between E-MORB and OIB magmas. In the same diagram, Ewart et al. (1998) showed that geochemical characteristics of Etendeka LTZ.H magmas extend from E-MORB to OIB magmas. These magmas were thought to suggest transitional signatures from melting in the garnet stability field (OIB-like) to the spinel peridotite field (MORB-like) (Ewart et al., 1998).

View larger version (27K): [in this window] [in a new window] [Download PowerPoint slide]   Fig. 7. La/Sm–Sm/Yb plot of the Daisen basalts. Comparative field from NE Japan arc basalts (Shibata & Nakamura, 1997). The average E-MORB, N-MORB and ocean island basalt (OIB) compositions are from Sun & McDonough (1989). The Daisen basalts have higher Sm/Yb ratios than arc basalts in NE Japan, and are intermediate between E-MORB and OIB magmas.

 

	OLIVINE BASALTS

TOP ABSTRACT INTRODUCTION GENERAL GEOLOGY OF DAISEN... ANALYTICAL METHODS GEOCHEMISTRY OF DAISEN BASALTS... OLIVINE BASALTS COMPOSITIONS OF CALCULATED... PRIMARY MAGMA COMPOSITION SUPERCOOLING CONCLUSION REFERENCES

 
Basalts from Daisen volcano contain 7–10 wt % MgO and ubiquitously bear olivine phenocrysts (5–11 vol %), with rare Cr-spinel inclusions. Augite phenocrysts occur in low modal abundance (0·1%), only in the least magnesian lavas (7·1–7·2 wt % MgO). All lavas are free of plagioclase phenocrysts (Table 1). They are isotopically homogeneous (Fig. 6), and variations of major and trace element compositions are relatively small, suggesting that olivine fractionation (the dominant phenocryst) was responsible for the limited variation now observed (Figs 3–5). Unexpectedly, however, the olivines from these basalts have a wide range of composition (Fo_66–89), and the relationships between host rocks and olivines cannot be explained by simple fractional crystallization of liquidus olivines.

Olivine morphology
The olivine phenocrysts are relatively small (up to 2 mm in diameter), but their shapes vary from granular to hopper or skeletal, indicating accentuated edge growth and/or incomplete facial growth (Donaldson, 1976). All of the 16 studied lava flows contain more or less skeletal olivine phenocrysts, together with granular olivines. Eight of the flows, however, are characterized by the great abundance of the skeletal olivines having highly irregular outlines (Fig. 8). It is not uncommon for olivine grains not visibly connected in thin section to be in optical continuity (Fig. 8e and f), suggesting that these are parts of a single skeletal grain. Most of the skeletal olivines have weak normal zoning and their compositions are iron rich (Fo₇₀). Donaldson (1976) showed that skeletal shape is the result of growth and not of resorption. Skeletal olivines from Daisen volcano are relatively subequant, not having acicular or bladed habits (Fig. 8). Olivine phenocrysts show subequant shapes when the degree of supercooling is relatively small (10–30°C) (Donaldson, 1976).

View larger version (119K): [in this window] [in a new window] [Download PowerPoint slide]   Fig. 8. Skeletal olivines in basalts from Daisen volcano. Groundmass phases are plagioclase, clinopyroxene and iron–titanium oxides. All photographs are taken under crossed polars, at diagonal positions (a, c, e and g) or extinction positions (b, d, f and h). (a, b) A skeletal olivine from d-53. The higher birefringence on its rim indicates a higher iron content (Fo70) than for its core (Fo83). (c, d) A skeletal olivine from d-57 having little zoning from core (Fo71.4) to rim (Fo70.8). Its gross outline approaches euhedralism, although its surface is rough and indented. (e, f) A single skeletal olivine from d-75. (Note that the grains, not visibly connected in the section, are in optical continuity.) (g, h) An externally euhedral hopper from d-84 showing little zonation from its interior (Fo72) to its rim (Fo70).

 

Measured olivine compositions
Cores and rims of about 20–30 olivine phenocrysts from each of 11 basalt lava flows were measured by electron microprobe (Table 2). Forsterite contents [Fo: values of 100Mg/(Mg + Fe) in olivine] range from 66 to 89, and most magnesian olivines (Fo₈₉) contain 0·4 wt % NiO (Fig. 9). Values of NiO are, on the whole, positively correlated with Fo values, but ranges of NiO abundance at the same values of Fo converge on both the most and least magnesian ends of the plot (Fig. 9). Sato (1977) pointed out that olivines crystallizing first from primary magmas should have NiO contents of 0·4 wt %, because olivines of upper-mantle lherzolites have uniform NiO contents of 0·4 wt %. Thus, the most magnesian olivines (Fo₈₉ and 0·4 wt % NiO) are assumed to represent those crystallized from the primary basalts of Daisen volcano.

View this table: [in this window] [in a new window]   Table 2: Representative analyses of olivine in Daisen basalts

 

View larger version (31K): [in this window] [in a new window] [Download PowerPoint slide]   Fig. 9. NiO vs Fo [100Mg/(Mg + Fe)] in olivine phenocrysts from 11 lavas. Twenty to 30 olivines were measured from each lava and both core and rim compositions are plotted. All phenocrysts are normally zoned. (Note that ranges of NiO abundance at the same Fo values are narrower toward both the magnesian and iron-rich ends of the plot.)

 

	COMPOSITIONS OF CALCULATED OLIVINES

TOP ABSTRACT INTRODUCTION GENERAL GEOLOGY OF DAISEN... ANALYTICAL METHODS GEOCHEMISTRY OF DAISEN BASALTS... OLIVINE BASALTS COMPOSITIONS OF CALCULATED... PRIMARY MAGMA COMPOSITION SUPERCOOLING CONCLUSION REFERENCES

 
Method of calculation
Basalts from Daisen volcano contain olivine phenocrysts only, suggesting that these basalts represent primitive magmas derived from a mantle-derived primary magma only through olivine fractionation. Equilibrium olivine compositions for the basalts, which would have been fractionated as they progressed from the primary magma, have been calculated using olivine fractionation models (e.g. Tatsumi et al., 1983; Yamashita & Tatsumi, 1994), estimated K_D(Fe/Mg)^ol/liq values, and D_Ni^ol/liq from Kinzler et al. (1990).

The compilation of Takahashi (1986) showed that pressure and temperature dependence of K_D(Fe/Mg)^ol/liq is compensated for by the liquid compositional variation. He used mol % (MgO + 0·33FeO) of the liquid as a compositional parameter; this parameter is somewhat arbitrary but is useful to show compositional dependence of K_D values. Figure 10 shows the relationship between K_D(Fe/Mg)^ol/liq and mol % (MgO + 0·33FeO) of liquid from the more recently acquired data of Hirose & Kushiro (1993), Baker & Stolper (1994) and Kushiro (1996); data from these studies are thought to be free from quench problems. Values of K_D(Fe/Mg)^ol/liq and the compositional parameter of liquid (MgO + 0·33FeO) correlate positively, which can be approximated by the line

View larger version (23K): [in this window] [in a new window] [Download PowerPoint slide]   Fig. 10. KD(Fe/Mg)ol/liq vs mol % (MgO + 0·33FeO) of liquids from diamond-aggregate experimental data. Values of KD(Fe/Mg)ol/liq increase gradually as liquids become more magnesian.

 
Estimated K_D values obtained by this equation and used in our calculations range from 0·293 to 0·310, and mostly from 0·30 to 0·31. This range overlaps the constant K_D value of 0·3 determined by Roeder & Emslie (1970). Falloon et al. (1997) suggested that the low K_D values obtained in some of these diamond-aggregate experiments represent non-equilibrium; we therefore do not use these low K_D values in this study.

Partition coefficients of Ni between olivine and liquid were obtained by using the equation (4) of Kinzler et al. (1990):

A series of olivine and basalt compositions were calculated from an original basalt as follows: (1) the composition of the equilibrium olivine was obtained by using the K_D(Fe/Mg)^ol/liq and D_Ni^ol/liq, assuming that Fe³⁺/(Fe²⁺ + Fe³⁺) in the melt is constant at 0·1. (2) A more primitive basalt composition was calculated as a mixture of the basalt and the equilibrium olivine in a weight ratio of 99:1. (3) Steps (1) and (2) were repeated using the calculated, more primitive basalt to obtain another, more primitive basalt. In Daisen volcano, the most magnesian olivines have values of Fo₈₉ and 0·4 wt % NiO. Therefore, the calculations of olivine and basalt compositions were repeated until the calculated equilibrium olivines had a value of Fo₈₉.

Real vs calculated olivines
Figure 11 shows real and calculated olivines from four representative lava flows of Daisen volcano (d-56l, d-73, d-57 and d-84). Interestingly, the comparison of real olivine compositions with those calculated using the models reveals there are two contrasting types of lavas in Daisen volcano. One type contains olivine phenocrysts whose compositions overlap with those of the calculated, equilibrium olivines, and continuously extend toward iron-rich compositions (d-56l and d-73, Fig. 11a and b). The other type of lava contains iron-rich olivines (Fo_70–80), but the calculated equilibrium olivines are too magnesian at given NiO contents and these compositions are far removed from those of the real olivines (d-57 and d-84, Fig. 11c and d). Moreover, the calculated compositions fall outside the entire distribution of observed Daisen olivine compositions. In other words, the host lavas are too magnesian at a given NiO content to be in equilibrium with not only their olivine phenocrysts but also any other olivine phenocrysts found in Daisen volcano.

View larger version (46K): [in this window] [in a new window] [Download PowerPoint slide]   Fig. 11. Plots of NiO vs Fo [100Mg/(Mg + Fe)] for real olivines () and calculated equilibrium olivines () from four basalt lavas. Calculated equilibrium olivines () were determined from the bulk composition of each of the lavas, as described in the text. Grey symbols show all measured olivine compositions from Daisen basalts. Measured olivine compositions partly overlap those of calculated equilibrium olivines in examples d-56l and d-73 (Fig. 10a and b) but are much more iron rich than calculated equilibrium olivines in examples d-57 and d-84 (Fig. 10c and d). In the latter pair, it should be noted that the calculated olivine compositions fall outside the distribution of all measured olivines from Daisen volcano.

 

	PRIMARY MAGMA COMPOSITION

TOP ABSTRACT INTRODUCTION GENERAL GEOLOGY OF DAISEN... ANALYTICAL METHODS GEOCHEMISTRY OF DAISEN BASALTS... OLIVINE BASALTS COMPOSITIONS OF CALCULATED... PRIMARY MAGMA COMPOSITION SUPERCOOLING CONCLUSION REFERENCES

 
We assume that the primary basalt magma was in equilibrium with Fo₈₉ olivine, on the basis of compositions of olivines observed in Daisen volcano (Table 2, Fig. 9). The comparison of calculated equilibrium olivines with real olivines gives us another constraint for determining primary magma composition. Using the fractional crystallization modelling, any basalt can be used to calculate an apparent primary magma in equilibrium with Fo₈₉. This is not true, however, when the composition of the calculated equilibrium olivine is far from the actual phenocrystic olivine in the original basalt. Table 3 shows compositions calculated for magmas in equilibrium with Fo₈₉ olivine (apparent primary magmas of Daisen volcano) calculated from actual Daisen basalts. Primary magmas of Daisen volcano, however, should be those that are in equilibrium with Fo₈₉ olivines containing 0·4 wt % NiO (Table 3). Thus, the primary basaltic magmas must have been similar to those calculated from d-55, d-56l, d-56m, d-56n, d-59, d-73 and d-85. Table 4 shows major and trace elements of these estimated primary basaltic magmas. We interpret the mean of these to represent the primary arc basalt of the Daisen volcano. These primary and apparent primary magma compositions and the actual Daisen basalt compositions were plotted in the Ol–Pl–Qz diagram of Walker et al. (1979), together with the isobaric liquid compositional trends of garnet lherzolite determined by Kushiro (1996) (Fig. 12). In this diagram, the basalts with equilibrium olivine phenocrysts cluster tightly, as do the primary magmas calculated from these basalts. The location of these primary basalts in Fig. 12 suggests that the depth of generation of the basalts, or the final depth of equilibrium with mantle lherzolite, was about 60 km (18 kbar). This seems to be consistent with higher Sm/Yb values of Daisen basalts relative to NE Japan arc basalts, which suggest a segregation depth between the garnet and spinel peridotite stability field (Fig. 7). Mantle diapir models and concurrent production of basalts and magnesian andesites from a single diapir (Tamura, 1994, 1995; Tamura & Nakamura, 1996) seem to apply at Daisen volcano. The slab-melting model of Morris (1995) seems inadequate to explain the concurrent generation of both basalts and dacites at this volcano.

View this table: [in this window] [in a new window]   Table 3: Apparent primary magmas of Daisen volcano (compositions of magmas in equilibrium with Fo89 olivines) calculated from actual Daisen basalts

 

View this table: [in this window] [in a new window]   Table 4: Estimated primary arc basalts of Daisen volcano

 

View larger version (21K): [in this window] [in a new window] [Download PowerPoint slide]   Fig. 12. Ol (olivine)–Pl (plagioclase)–Qz (SiO2) diagram [the projection of Walker et al. (1979)] showing basalt compositions from Daisen volcano and assumed primary magmas with isobaric liquid compositional trends for a relatively fertile garnet lherzolite (PHN1611) as determined by Kushiro (1996). Basalts for which olivine phenocrysts are in equilibrium, and primary magma compositions calculated from these, cluster tightly. Calculated primary magma compositions suggest a pressure of 18 kbar (60 km).

 

	SUPERCOOLING

TOP ABSTRACT INTRODUCTION GENERAL GEOLOGY OF DAISEN... ANALYTICAL METHODS GEOCHEMISTRY OF DAISEN BASALTS... OLIVINE BASALTS COMPOSITIONS OF CALCULATED... PRIMARY MAGMA COMPOSITION SUPERCOOLING CONCLUSION REFERENCES

 
We have shown that basalts from Daisen volcano contain only olivine phenocrysts (Table 1), show little variation in major and trace elements (Figs 3–5), and have homogeneous isotopic compositions (⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd) (Fig. 6). In spite of this apparent homogeneity, however, there exists a wide variation in morphology and composition of olivine phenocrysts. Two types of basalts exist: one in which olivine phenocrysts were in equilibrium with host magmas and one in which they were not (Fig. 11).

In Fig. 13 relationships between real and calculated olivines are schematically interpreted using phase diagrams for olivine.

View larger version (37K): [in this window] [in a new window] [Download PowerPoint slide]   Fig. 13. Schematic phase diagrams for olivine, showing: (a) the effect of fractional crystallization; (b) the relationship between bulk composition of a basalt and calculated olivines; (c) the effect of supercooling. (See text for details.)

 

Fractional crystallization (Fig. 13a)
Let us consider a primary magma cooling from initial temperature (T₁) above the liquidus. On reaching temperature T₂, it precipitates olivine of composition m. As the temperature falls slowly from T₂ to T₃, the composition of both the liquid and the olivine in equilibrium with it change gradually, and finally, olivine of composition o is precipitated.

Real Daisen basalts consist of groundmasses (liquids) and 6–11 vol. % olivine phenocrysts; liquids are more evolved than bulk compositions of the basalts. Moreover, olivines have normal zoning; cores and rims of olivines could have more primitive (Mg-rich) and more evolved (Fe-rich) compositions, respectively, than those in equilibrium with the basalt whole rocks. Thus, rather than assuming extreme fractional crystallization, it is realistic to consider that a basalt having a bulk composition b would contain olivine phenocrysts ranging from m to o.

In Daisen basalts, olivine phenocrysts from 11 lavas have a continuous composition ranging from Fo₈₉ to Fo₆₆ (Fig. 9), and the Fo₈₉ is thought to represent primary olivine composition m based on mantle-like high NiO values of 0·4 wt % (Sato, 1977).

Calculated olivine compositions assuming fractional crystallization (Fig. 13b)
Let us consider a basalt, such as b in Fig. 13b, and for theoretical convenience assume it to be a liquid. The composition of olivine in equilibrium with this liquid is shown as n. In the model shown in Fig. 13b, the compositions of both the liquid and the olivine in equilibrium with it have been changed to follow only the reverse of fractional crystallization, until the olivine in equilibrium with the liquid has the composition m (Fo₈₉ in Daisen volcano). Calculated olivines have composition ranging from m to n. If this model is applicable, it is expected that, in terms of 100Mg/(Mg + Fe²⁺) and wt % NiO of basalts and olivines, relatively primitive compositions among real olivine phenocrysts should overlap with the calculated olivines from the basalt (m–n). The ranges of real and calculated olivines from some basalts (e.g. d-56l and d-73, Fig. 11a and b) are in accord with this expectation, suggesting in this case that fractional crystallization plays a role in producing Daisen basalts from a primary magma.

Olivines from supercooling (Figs 13c and 14)
Let us consider a primary magma above the liquidus, having a temperature T₁ (Fig. 13c). If the temperature of the primary magma suddenly drops from T₁ to T₃, an equilibrium olivine composition at T₃ would be o, which is much more evolved (iron rich) than m and n. Donaldson (1976) has shown that skeletal olivines, of the type seen at Daisen, are characteristic of supercooled basaltic magmas.

View larger version (39K): [in this window] [in a new window] [Download PowerPoint slide]   Fig. 14. NiO vs Fo [100Mg/(Mg + Fe)] in olivine phenocrysts from Daisen volcano. Projected onto the diagram are: m, an olivine composition (Fo89 and 0·4% NiO) in equilibrium with the assumed primary magma; o, an assumed iron-rich olivine (Fo80 and 0·3 wt % NiO) crystallized during supercooling; c, olivine calculated to be in equilibrium with the composition produced by 7% subtraction of iron-rich olivine (o) from the primary magma. Vector E indicates the calculated olivine trend produced by equilibrium crystal fractionation and vector D indicates disequilibrium fractionation during supercooling from a primary magma. In the case of equilibrium crystal fractionation, actual compositions of olivine phenocrysts and the calculated E trend overlap. In the case of disequilibrium fractionation, olivine phenocrysts are iron rich (o), and such differentiated magmas, which originate from supercooling and subtraction of iron-rich olivine phenocrysts, become more magnesian than compositions produced by equilibrium crystal fractionation. Calculated magnesian olivines in equilibrium with these magmas (vector D) and actual iron-rich olivine phenocrysts crystallized during supercooling (as shown in Fig. 11c and d) never overlap.

 
Another interesting aspect of supercooling is that fractionation of such iron-rich olivines from primary magmas produces magnesian magma that cannot be generated by normal fractional crystallization. The trend of the magma compositions can be plotted on an olivine composition diagram (Fig. 14) as the trend line for olivines calculated to be in equilibrium with the magma compositions. In Fig. 14, vectors E and D indicate: (1) calculated olivine trends produced by equilibrium crystal fractionation (E), and (2) disequilibrium fractionation during supercooling (D) from a primary magma. The latter shows the effect of simple subtraction of iron-rich olivines (Fo₈₀ and 0·3 wt % NiO) from a primary magma, and the consequent production of magnesian melts, which should be in equilibrium with magnesian olivines (Fo₈₈). In the case of disequilibrium fractionation during supercooling, actual iron-rich olivine phenocrysts (o) and calculated magnesian olivines (c) of the differentiated magma never overlap. Thus, supercooling yields disequilibrium iron-rich olivines, which can exist in magnesian basalt (Fig. 11).

It must be stressed again that if only bulk composition data are available, one tends to use the most magnesian rocks to estimate primary magma. But the above reasoning suggests this is not always appropriate.

	CONCLUSION

TOP ABSTRACT INTRODUCTION GENERAL GEOLOGY OF DAISEN... ANALYTICAL METHODS GEOCHEMISTRY OF DAISEN BASALTS... OLIVINE BASALTS COMPOSITIONS OF CALCULATED... PRIMARY MAGMA COMPOSITION SUPERCOOLING CONCLUSION REFERENCES

 

1. On the basis of the isotopic data (⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd), the 1·2 Ma basalts at Daisen volcano, which have an arc trace element signature, should be considered to be part of the volcano. The volcanism of Daisen volcano is strictly bimodal. Basaltic primary magmas of Daisen volcano segregated at a pressure of 18 kbar (60 km). Concurrent generation of basalt and magnesian andesite (e.g. Tamura, 1994, 1995; Tamura & Nakamura, 1996) and the likelihood of garnet in the residual mantle would explain the garnet or the transitional signature in the trace element compositions of the Daisen basalts and dacites.
   
2. Differentiation by fractional crystallization produces different effects from those produced by supercooling, and these can be discriminated using the morphology of olivines and chemical relationships between olivines and host rocks. Supercooled magmas become magnesian through fractionation of iron-rich olivines, and such lavas are seemingly anomalous because they contain iron-rich olivines in spite of their magnesian bulk-rock chemistry.
   

	ACKNOWLEDGEMENTS

 
All field studies and some of the analytical work by Y. Tamura were carried out under the guidance of I. Kushiro, Okayama University, whose help is much appreciated. Comments on an earlier version of the manuscript were provided by H.Kagami and K.Shuto of Niigata University and S. Yamashita of Okayama University. We particularly thank R. S. Fiske of the Smithsonian Institution for help and comments. A. Ishiwatari, H. Ishida, H. Shukuno and M. Kondo assisted with INAA analysis at Kyoto and Kanazawa Universities. H. Asada of Okayama University helped with thin-section preparation. We benefited from the insightful and constructive reviews of T. H. Green and R. C. Price. Part of this work was supported by a grant from the Ministry of Education, Science, Sports and Culture (Y.T.).

	FOOTNOTES

 
^*Corresponding author. Telephone: +81-76-264-5979. Fax: +81-76-264-5746. e-mail: ytamura{at}kenroku.kanazawa-u.ac.jp

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TOP ABSTRACT INTRODUCTION GENERAL GEOLOGY OF DAISEN... ANALYTICAL METHODS GEOCHEMISTRY OF DAISEN BASALTS... OLIVINE BASALTS COMPOSITIONS OF CALCULATED... PRIMARY MAGMA COMPOSITION SUPERCOOLING CONCLUSION REFERENCES

 
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