Evidence for two-stage mixing in magmatic inclusions and rhyolitic lava domes on Niijima Island, Japan

Journal of Volcanology and Geothermal Research, 29 (1986) 71--98

71

Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

EVIDENCE FOR TWO-STAGE MIXING IN MAGMATIC INCLUSIONS AND RHYOLITIC LAVA DOMES ON NIIJIMA ISLAND, JAPAN

TAKEHIRO KOYAGUCHI*

Geological Institute, Faculty of Science, University of Tokyo, Hongo, Tokyo 113, Japan (Received July 7, 1985; revised and accepted October 31, 1985)

ABSTRACT Koyaguchi, T., 1986. Evidence for two-stage mixing in magmatic inclusions and rhyolitic lava domes on Niijima Island, Japan. In: I. Kushiro (Editor), M. Sakuyama and H. F u k u y a m a Memorial Volume. J. Volcanol. Geotherm. Res., 29: 71--98. Fine-grained inclusions are found in the Achiyama and the Mukaiyama rhyolitic lava domes o n Niijima Island, Japan. The existence of fine-grained rims in the inclusions indicates that these inclusions are a quenched mafic magma incorporated into their rhyolite host. The chemical composition and the phenocryst assemblage of the inclusions in the Achiyama dome are identical with those of the effusive basaltic lava on Niijima Island (Wakago basalt). The inclusions of the Mukaiyama dome are dacitic and have a disequilibrium phenocryst assemblage (olivine, clinopyroxene, orthopyroxene, hornblende, biotite, plagioclase, quartz and opaque minerals); whole-rock compositions lie on a mixing line between the host rhyolite and Wakago basalt. The dacitic magma is, therefore, a product of mixing of the basaltic and rhyolitic magmas. The occurrence of two different inclusions (basaltic and dacitic) in two different domes can be explained by a two-stage magma mixing model. The dacite is formed in the first stage by mixing in a vertically stratified magma chamber caused by an interfacial instability between rhyolitic and basaltic layers. A small a m o u n t of rhyolitic magma is entrained into the lower basaltic magma layer, and two magmas can mix essentially in a liquid state. The mixed dacitic magma is formed in the lower layer. The second stage is mixing caused by forced convection during the ascent through a conduit just before eruption. The stably stratified silicic and mafic magmas in a magma chamber can turn over and efficiently mix because of the formation of an unstable flow in a conduit. Under this condition, the system reaches thermal equilibrium immediately after the mafic magma is disaggregated and dispersed in the silicic host. The mafic magma solidifies when the silicic endmember is predominant in the mixture; hence the equilibrium temperature is below the solidus of the mafic magma. Mafic inclusions are formed at this stage. Whether the inclusions are basaltic or dacitic depends on whether the final ascent through a conduit (eruption) occurred before or after the formation of the layer of a mixed dacitic magma in a magma chamber.

*Present address: Department of Earth Sciences, Faculty of Science, Ehime University, Matsuyama 790, Japan.

0377-0273/86/$03.50

© 1986 Elsevier Science Publishers B.V.

72 INTRODUCTION There is increasing evidence that intimate mixing of mafic and silicic magmas is a c o m m o n p h e n o m e n o n in many calc-alkalic volcanic fields (Eichelberger, 1975; Sakuyama, 1979; Luhr and Carmichael, 1980). If a basaltic magma intrudes a rhyolitic magma chamber, separate layers of rhyolite and basalt would form due to the density contrast between the two magmas. Furthermore, the stable stratification cannot be easily disrupted by any natural convection (e.g. Huppert and Sparks, 1980). H o w to disrupt this stratification is one of the fundamental problems concerning rhyolite/basalt magma mixing. Many investigations on this problem have been made especially on processes in a magma chamber (e.g. Eichelberger, 1980; Huppert et al., 1982, 1983). The role of dynamic mixing in a conduit also has been taken into account in recent studies of fluid dynamics (Kouchi and Sunagawa, 1985; Koyaguchi, 1985). Mechanisms obtained by experimental studies of simplified systems, however, do n o t seem to completely explain the petrographic features of natural hybrid lavas, probably because those lavas are the consequence of processes both in a magma chamber and in a conduit, including mixing and crystallization. The origin of basaltic and mixed magmatic inclusions in some rhyolitic lava domes in the Izu Islands is discussed based on petrographic and petrochemical data in this paper. Two possible mechanisms of magma mixing are investigated using experimental results of fluid dynamics. One is interfacial mixing between two convective layers in a magma chamber. The other is mixing and disruption of the stratification during magma ascent in a conduit. The boundary condition of heat and mass transfer for each mechanism is analyzed in order to clarify aspects of mafic magma crystallization during mixing. Finally, a two-stage magma mixing model beneath Niijima Island is demonstrated. In this paper I refer to homogeneous hybrid magmas as " m i x e d " and banded or inclusion-bearing magmas as "mingled".

Geologic setting Rhyolitic lava domes and basaltic base-surge deposits (Wakago basalt) occur on Niijima Island, Japan (Fig. 1). The geology of Niijima Island was described by Tsuya (1938) and Miyaji (1965). Niijima Island rises from a submarine ridge, 200 m deep trending from NE to SW, oblique to the trend of Izu--Mariana arc. Volcanic activity is divided into three stages. Hornblende rhyolitic and hornblende-biotite rhyolitic lava domes were formed at the first and second stages, respectively. Three biotite rhyolitic lava domes were formed at the third stage (Miyazukayama, Achiyama and Mukaiyama domes from oldest to youngest). They preserve their original volcanic topographies, and thus are distinct from the domes of the other stages. The eruption of Wakago basalt belongs to the third stage, after the activity of the

73

beach sand

Tok Y°~i ~

139" D,,q,-~

I

140OE

I

34°N

2 Km = i

~

pyroclastic deposits (und.f.)

B

Mukaiyamalava dome

~

Mukaiyama base-surge

W

Achiyamalava dome

m

Wakago basalt

~

Miyazukayama lava dome

I]~

Stage II

I~

Stage I

Stage III

N: Niijima Island, K: Kozushima Island Fig. 1. Geological map of Niijima Island after Tsuya (1938).

Miyazukayama d o m e and before that of the Achiyama dome (Miyaji, 1965). An old d o c u m e n t records an eruption which t o o k place at Mukaiyama in 886. A '4C-age of a carbonized tree in the base-surge deposits from the Mukaiyama volcano is consistent with this d o c u m e n t (Isshiki, 1973). Yokoy a m a and Tokunaga (1978) pointed o u t that the activity of the Mukaiyama volcano can be further divided into three substages; the base-surge, the pyroclastic cone, and the lava dome substages from oldest to youngest. This paper is mainly concerned with mafic inclusions in the Achiyama and the Mukaiyama rhyolitic lava domes, the host rhyolites, and Wakago basalt. In addition to these volcanic rocks, mafic inclusions in the Jinakayama lava dome in Niijima Island, and those in the Membo rhyolitic lava in Kozushima Island (about 30 Km SW of Niijima Island; Fig. 1 ) w e r e also investigated for comparison.

Analytical methods Whole-rock major-element compositions were determined by the Rigaku X R F at University of Tokyo. Details of the analytical m e t h o d have been reported by Matsumoto and Urabe (1980). Mineral compositions were determined by electron probe microanalysis, using the JEOL JXA-5 of the Geological Institute and the J E O L JCXA-733 of the Ocean Research Institute of University of Tokyo. The analytical procedure is similar to that given by Nakamura and Kushiro (1970), and the correction procedure of Bence and Albee (1968) was applied.

74 PETROGRAPHY

Major-element compositions and modal compositions are given in Tables 1 and 2, respectively. TABLE 1 Representative major-element analyses of inclusions, host rhyolites, and Wakago basalt No.

M-17

M-21

M-22

ON-1

H-1

H-2

ACH-3c

ACH-3r

SiO2 TiO2 A1203 FeO* MnO MgO CaO Na20 K20 P2 O s

77.6 0.11 12.8 0.82 0.07 0.07 0.92 4.67 2.90 tr

78.3 0.11 12.8 0.72 0.07 tr 0.88 4.21 2.85 tr

77.8 0.11 12.9 0.83 0.07 0.01 0.89 4.53 2.89 tr

77.8 0.12 13.0 0.86 0.07 0.06 0.95 4.20 2.89 tr

78.3 0.10 12.8 0.76 0.07 0.04 0.76 4.15 2.99 tr

77.5 0.11 13.1 0.84 0.07 0.10 0.94 4.39 2.97 tr

50.2 1.14 16.9 12.7 0.19 4.91 10.3 3.30 0.32 tr

52.3 1.12 16.9 12.3 0.19 4.76 9.62 2.32 0.50 tr

No.

ACH-10c

ACH-10r

M-5c

M-5r

M-6c

M-6r

M-10c

M-10r

SiO2 TiO2 A1203 FeO* MnO MgO CaO Na20 K20 P2Os

51.7 1.18 17.3 12.8 0.21 4.68 9.58 2.29 0.32 tr

56.1 1.00 17.7 9.84 0.22 3.72 8.40 2.56 0.46 0.04

63.0 0.65 15.6 7.09 0.15 3.12 6.15 3.16 1.14 tr

62.3 0.64 15.8 6.93 0.15 3.04 6.42 3.46 1.22 0.01

62.6 0.65 15.8 6.89 0.15 3.23 6.62 3.03 1.05 0.01

62.4 0.65 16.0 6.88 0.14 3.20 6.60 3.31 0.90 tr

64.1 0.58 15.8 6.14 0.15 2.84 6.00 3.23 1.19 tr

64.2 0.57 15.8 6.15 0.14 2.66 5.95 3.32 1.22 tr

No.

M-16c

M-16r

TNE-4c

TNE-4r

JIN-3

KZ-14c

KZ-14r

SiO2 TiO2 A1203 FeO* MnO MgO CaO Na20 K2 0 P2Os

62.4 0.64 15.7 7.07 0.15 3.14 6.53 3.26 1.12 tr

63.5 0.61 15.5 6.55 0.14 2.97 6.11 3.50 1.19 tr

67.1 0.48 14.9 4.87 0.13 2.04 4.79 4.25 1.50 tr

67.8 0.47 15.2 4.82 0.12 1.94 4.49 3.63 1.53 tr

63.3 0.73 17.0 6.01 0.16 2.29 6.05 3.51 0.90 0.08

52.4 0.91 18.4 8.89 0.15 5.71 10.5 2.43 0.62 tr

52.4 0.91 18.7 8.76 0.15 5.48 10.1 2.84 0.73 tr

75 XRF data are recalculated to give a total of 100%. M-17, M-21: Mukaiyama rhyolite (lava dome); M-22, ON-l: Mukaiyama rhyolite (essential block pyroclastic cone): H-l, H-2: Mukaiyama rhyolite (base-surge deposits); ACH-3, ACH-10: inclusions from Achiyama dome; M-5, M-6: inclusions from Mukaiyama dome; c: core; r: rim; tr: trace; FeO*: total Fe as FeO; M-10, M-16, TNE-4 : inclusions from Mukaiyama dome; JIN-3: inclusion from Jinakayama dome, Niijima Island (Stage I); KZ-14: inclusion from Membo lava, Kozushima Island. Wet analyses (by H. Haramura) ACH-26

WAK-10

ACH-4c

ACH-4r

SiO2 TiO2 A1203 Fe203 FeO MnO MgO CaO Na20 K20 P2Os H20(+)

76.12 0.07 13.31 0.74 0.24 0.06 0.21 1.14 4.62 2.78 0.04 0.39

49.53 1.14 16.97 3.26 9.40 0.21 5.14 10.22 2.39 0.42 0.16 1.21

49.30 1.17 17.12 8.20 4.98 0.21 5.03 10.43 2.35 0.22 0.11 1.19

52.03 1.05 16.01 6.81 5.59 0.20 4.56 9.28 2.66 0.70 0.07 0.85

total

99.72

100.05

100.31

99.81

ACH-26: Achiyama rhyolite (lava dome); WAK-10: Wakago basalt; ACH-4: inclusion form Achiyama dome.

Host rhyolites T h e r o c k s c o n t a i n e u h e d r a l p h e n o c r y s t s o f plagioclase, q u a r t z , b i o t i t e , a n d small a m o u n t s o f m a g n e t i t e and i l m e n i t e in a glassy o r c r y p t o c r y s t a l l i n e g r o u n d m a s s . O n l y o n e e u h e d r a l h o r n b l e n d e p h e n o c r y s t was f o u n d f r o m the M u k a i y a m a r h y o l i t e a m o n g 20 thin sections. T h e r e is s y s t e m a t i c d i f f e r e n c e in a p p a r e n t densities o f r o c k s a m o n g t h e M u k a i y a m a ejecta, b e t w e e n t h e base-surge d e p o s i t s a n d the lava d o m e or essential b l o c k s in t h e p y r o c l a s t i c c o n e . T h e b u l k densities o f p u m i c e grains in base-surge d e p o s i t s r a n g e f r o m 0 . 8 9 to 1.35 g / c m 3, w h e r e a s t h o s e o f the lava d o m e or essential b l o c k s in t h e p y r o c l a s t i c c o n e are 1.27 to 1.96 g / c m 3 . Since t h e densities o f p o w d e r e d s a m p l e s are n e a r l y u n i f o r m (2.38 + 0.03 g/ c m 3 ), t h e d e n s i t y d i f f e r e n c e s r e f l e c t p o r o s i t y .

Wakago basalt W a k a g o basalt c o n t a i n s o n l y a small a m o u n t o f plagioclase, olivine a n d rare augite p h e n o c r y s t s . T h e g r o u n d m a s s is c o m p o s e d o f d a r k b r o w n glass or c r y p t o c r y s t a l l i n e s u b s t a n c e , a n d small a m o u n t s o f plagioclase, olivine, clinop y r o x e n e , a n d o p a q u e minerals. T h e g r o u n d m a s s is p a r t l y l e u c o c r a t i c . S o m e

76 TABLE 2 Representative modal compositions of inclusions, host rhyolites, and Wakago basalt No.

WAK-10

M-17

M-21

pl-1 pl-2 pl-3 qz ol cpx opx hb bt o.m. gm por

0.4 0.0 0.0 0.0 0.2 0.0 0.0 0.0 0.0 0.0 99.4 n.d.

13.9 0.0 0.0 4.7 0.0 0.0 0.0 0.0 0.3 ~0.1 81.1 36.1

10.2 0.0 0.1 7.6 0.0 0.0 0.0 0.0 0.4 0.1 81.6 17.6

M-22 8.7 0.0 0.4 9.7 0.0 0.0 0.0 0.1 0.8 0.1 80.2 17.6

ON-1 16.1 0.0 0.0 7.0 0.0 0.0 0.0 0.0 0.9 ~0.1 76.0 29.8

H-1

H-2

9.0 0.0 0.0 3.3 0.0 0.0 0.0 0.0 0.4 ~0.1 87.3 49.2

14.3 0.0 0.5 3.6 0.0 0.0 0.0 0.0 0.7 ~0.1 80.9 58.0

ACH-26 7.3 0.0 0.0 1.2 0.0 0.0 0.0 0.0 0.3 ~0.1 91.2 31.9

Sample numbers as in Table 1. pl-l: plagioclase without glass inclusions or dusty zone; pl-2: plagioclase wtih concentric dusty zone; pl-3: plagioclase with many glass inclusions (for details see text); qz: quartz; ol: olivine; cpx: clinopyroxene; opx: orthopyroxene; hb: hornblende; bt: biotite; o.m.: opaque minerals; gm: groundmass; pot: porosity (vol.%).

b o u n d a r i e s b e t w e e n l e u c o c r a t i c and s u r r o u n d i n g basaltic m a t r i x are sharp, and o t h e r s are diffuse. This r o c k c o n t a i n s small a m o u n t s o f r e s o r b e d q u a r t z a n d sodic plagioclase. Glass a r o u n d these minerals is colorless, b u t grades into s u r r o u n d i n g dark b r o w n glass. These l e u c o c r a t i c parts o f the g r o u n d mass and q u a r t z and sodic plagioclase p h e n o c r y s t s m a y indicate t h a t Wakago basalt is c o n t a m i n a t e d b y a r h y o l i t i c m a g m a to s o m e e x t e n t . T h e y were carefully e x c l u d e d b e f o r e crushing f o r the c h e m i c a l analysis. Melting e x p e r i m e n t s at 1 a t m were p e r f o r m e d in a CO2/H2 gas a t m o sphere. T h e liquidus t e m p e r a t u r e o f Wakago basalt is b e t w e e n 1 2 2 0 ° and 1 2 3 0 ° C at fo~ = 10-9 a t m , and t h e liquidus phase is plagioclase.

Inclusions Fine-grained inclusions are f o u n d in the A c h i y a m a a n d M u k a i y a m a lava d o m e s . T h e inclusions are n o t o b s e r v e d in t h e base-surge deposits and the essential b l o c k s in the p y r o c l a s t i c c o n e o f the M u k a i y a m a v o l c a n o . The inclusions in t h e A c h i y a m a lava d o m e are basaltic (SiO2 = 4 9 - - 5 2 % ) , while t h o s e o f the M u k a i y a m a lava d o m e are dacitic (SiO~ = 6 1 - - 6 8 % ) (Fig. 2).

Inclusions in the A c h i y a m a dome. T h e inclusions are ellipsoidal, ranging f r o m several m m t o 70 cm in diameter. T h e y have a l m o s t t h e same chemical and m o d a l c o m p o s i t i o n s (olivine a n d calcic plagioclase) as Wakago basalt. R e s o r b e d q u a r t z a n d sodic plagioclase are f o u n d in s o m e inclusions. The g r o u n d m a s s is c o m p o s e d o f h o r n b l e n d e , plagioclase, c l i n o p y r o x e n e , o r t h o p y r o x e n e , o p a q u e minerals, a n d interstitial glass a n d q u a r t z . T h e g r o u n d m a s s

77

ACH-3

ACH-4

ACH-10

M-5

M-6

M-10

M-16

0.5 0.5 0.0 0.0 0.3 0.0 0.0 0.0 0.0 0.0 98.7 n.d

TNE-4

1.1 0.0 0.1 0.0 0.3 0.0 0.0 0.0 0.0

0.0 0.0 <0.1 0.0 <0.i 0.0 0.0 0.0 0.0

0.1 1.6 2.5 1.9 0.0 <0.1 0.0 1.5 0.0

0.1 0.7 5.2 0.7 0.0 0.2 0.6 <0.1 0.0

0.0 0.9 4.7 1.8 <0.I <0.1 0.2 0.2 <0.1

0.0 2.0 5.9 0.8 0.0 0.1 0.2 0.1 0.0

0.0 2.2 2.1 1.2 0.0 <0.1 0.1 0.3 0.0

0.0 98.5 n.d.

0.0 I00.0 n.d.

0.I 92.3 n.d.

0.4 92.1 n.d.

0.2 92.0 n.d.

0.2 90.7 n.d.

<0.i 94.1 n.d.

Z x Z

[]

Achiyamalava

x ×

_/ Z

•

Jinakayama lava

__

Z

[]

Membolava (Kozushima)

/

f ! J

N

50

55

• 610

65

710

Si02wt% Fig. 2. SiO2 content of inclusions in the Achiyama, Mukaiyama, and Jinakayama rhyolite domes on Niijima Island, and in the Membo rhyolite lava on Kozushima Island. All the data used for the fi~ares are normalized to give a total of 100%. g r a i n s i z e o f a n i n c l u s i o n is p o s i t i v e l y c o r r e l a t e d w i t h t h e s i z e o f t h e inc l u s i o n . S o m e i n c l u s i o n s h a v e c r e n u l a t e m a r g i n s , a n d f i n e - g r a i n e d r i m s (less t h a n l m m t o 1 c m in w i d t h ) a r e c o m m o n ( F i g s . 3 a n d 4). G r o u n d m a s s crystals are acicular and show a spherulitic texture (Fig. 4), probably caused b y r a p i d c o o l i n g (e.g. L o f g r e n , 1 9 8 0 ) . T h e s e p e t r o g r a p h i c f e a t u r e s i n d i c a t e

78

A a C

°'I

-

4

L1

B

Fig. 3. Basaltic inclusions f r o m the A c h i y a m a lava d o m e . a and b: basalt-rhyolite c o n t a c t characterized by a fine-grained rim (Sample No. ACH-3). canct d: a diffuse c o n t a c t witho u t fine-grained rim (Sample No. ACH-10). b.i. = basaltic inclusion, r ----rhyolitic host. D I F ~ diffuse contact. Chemical traverses along solid lines ( A - B a n d C - D ) are s h o w n in Fig. 5. It is also notable that vesicles are u n e v e n l y distributed in b o t h samples.

t h a t the groundmass of the inclusions (more than 98 vol.% of each inclusion) was in a liquid state when the basaltic magma was incorporated into the rhyolite host, and that it crystallized after the disruption into droplets, while the marginal parts were chilled against the low-temperature host rhyolitic magma. The groundmass of the inclusions is porous and contains m a n y vesicles ranging from less than I mm to several cm in diameter. The vesicles are heterogeneously distributed within an inclusion (Fig. 3). The vesicle-rich part could be the original upper side of the inclusion at the time when it was in a liquid state, because vesicles would have migrated upwards in each basaltic droplet.

79

Fig. 4. A p h o t o m i c r o g r a p h ( o p e n - n i c o l ) s h o w i n g the fine-grained rim o f the inclusion. r ---- r h y o l i t e host; i = inclusion.

56 F

55}-

A

B

ACH-3

54 ~- rim

52 51 f 50

rim

• •

o"

v 57 E

~

ACH-IO

rim

56 f -~ 55 .-e-_q~_ -4154 C -053 l- rim _e__e_~-e--e52~- ~-~---e-¢ ,,¢ 51

so~

0

5

10

15

(cm)

Fig. 5. Chemical zoning of basaltic inclusions in the Achiyama dome. In most of inclusions, the chemical composition is uniform from rim to core. Some inclusions (e.g. sample nos. ACH-3 and A C H - 1 0 ) have a slightly siUcic rim. Especially some boundaries between embayed rhyolite and basalt are diffuse (see sample no. A C H - 1 0 ) and a slightly silicic part is formed in the inclusions. Bars indicate the size of the analyzed chips.

80

The inclusions are nearly aphyric and no evidence of crystal accumulation is observed. Therefore, their bulk compositions would represent those of incorporated magmas. The compositional variation within an inclusion was examined by analyzing incremental samples (Fig. 5). The chemical composition within a single inclusion is nearly uniform except for the following examples. The rims of some inclusions (e.g. ACH-3 and -10) are slightly wt.% 2L..,

,

,

,

[

. . . .

i

. . . .

FeO* 6 4 2 6

MgO

4 2 0 10

CaO

8 6 4 2 0 4 3

;._

2

K20

~

-

0

50

60

70

80

SiO 2 w t % Fig. 6. SiO2 variation diagrams of inclusions, host rhyolite, and Wakago basalt on Niijima Island. Star = Wakago b a s a l t ; solid circle = inclusions in the A c h i y a m a lava d o m e ; o p e n circle ----i n c l u s i o n s in t h e M u k a i y a m a laval d o m e . F e O * = t o t a l i r o n as F e O . All analyses o f h o s t r h y o l i t e , c o l l e c t e d f r o m 13 localities all over the M u k a i y a m a lava d o m e , p l o t w i t h i n t h e range i n d i c a t e d b y rectangles.

81 silicic (Fig. 5), suggesting a mixing with host magma by molecular diffusion. Especially when the rhyolite magma is e m b a y e d in a large basaltic inclusion, the contact between the rhyolite and the basalt is diffuse (Fig. 3 c and d). For example, in a large {more than 30 cm in diameter) inclusion (ACH-10), some leucocratic diapirs are observed in the side opposite to the vesicle-rich part. This texture indicates that the rhyolitic diapir invaded from the b o t t o m side into the basaltic droplets, when the basalt was still in a liquid state. The contact between the basalt and the leucocratic diapirs are diffuse and the chemical composition of the inclusion is partly silicic (Fig. 5). Inclusions in the Mukaiyama dome. The dacitic inclusions in the Mukaiyama dome have a complicated phenocryst assemblage: olivine, quartz, plagioclase, o r t h o p y r o x e n e , clinopyroxene, hornblende, biotite, and magnetite. The total a m o u n t of phenocrysts is a b o u t 10vol.% (Table 2). The groundmass is c o m p o s e d of plagioclase, hornblende, clinopyroxene, orthopyroxene, opaque minerals, and interstitial quartz, alkali feldspar, and colorless glass. Groundmass plagioclase, hornblende, and pyroxenes are acicular. A fine-grained rim is f o u n d on some inclusions, b u t is less c o m m o n than in the basaltic inclusions of the Achiyama dome. Groundmass grain size and the size of the inclusion are correlated. The bulk chemical compositions of the dacitic inclusions plot on a mixing line between the host rhyolite and Wakago basalt, although A1203 and K 2 0 contents are slightly deviated from the mixing line (Fig. 6). Within a single inclusion the chemical composition is nearly uniform (Fig. 7); it varies at most by 1% in SiO2. This observation indicates that reaction between inclusions and host is limited. The composition of the core is independent of the size of the inclusion (Fig. 8). rim

core M-6

_o_-e-e_-e-. M-g

M-tO

°4 63 f 62

M-16 63 62

~. -e'--e---e--

. . . .

5~ . . . .

:NE-4

1"0 ' (c'm)

Fig. 7. Chemical zoning of dacitic inclusions in the Mukaiyama lava dome. The chemical composition is uniform within each inclusion, but varies among different inclusions (sample nos. M-5, 6, 10, 16, and TNE-4). Bars indicate the size of the analyzed chips.

82 I

I

!ooo

1

I

I

I

I

0 O

O

O

O

Mukai-yama

J

Jinaka-yama 6O

55

50 I 10

0

1 20

I 30

Diameter of

I 40

I 50

I 60

h" 70

Basic Inclusion (cm)

Fig. 8. R e l a t i o n s h i p b e t w e e n SiO2 c o n t e n t a n d size o f incusions. O p e n circle = M u k a i y a m a d o m e ; o p e n s q u a r e = A c h i y a m a d o m e ; solid circle = J i n a k a y a m a d o m e ; solid square = M e m b o lava.

Mineral chemistry Representative chemical compositions of phenocrystic phases are listed in Table 3. TABLE 3 Representative chemical compositions of phenocrysts Olivine No.

WAK-10

ACH-8

core SiO 2 TiO2 A1203 FeO MnO MgO CaO Cr203 NiO

37.9 0.04 0.11 23.7 0.35 38.0 0.17 . --

rim

.

37.6 -0.10 25.4 0.40 36.6 0.25 . --

core 38.0 0.01 0.03 24.2 0.41 36.8 0.20 .

M-14 rim 38.2 0.02 0.03 24.5 0.39 36.7 0.17 --

core 38.9 0.04 0.23 20.7 0.35 40.2 0.21 0.01 0.07

M-10 rim 38.6 0.01 0.23 22.6 0.45 38.6 0.18 0.01 0.08

core 39.5 0.03 0.25 14.1 0.28 46.4 0.10 0.02 0.18

total

100.3

100.4

99.7

100.0

100.7

100.8

100.9

Mg#

74.1

72.0

73.0

72.8

77.6

75.3

85.4

83 TABLE

3 (continued) Pyroxene

No.

M-4 core

M-10 core

rim

SiO2 TiO2 A1203 FeO MnO MgO CaO Na20 Cr203 V203 NiO

53.2 0.14 0.73 22.1 1.77 20.9 0.90 0.04 0.03 . --

50.3 0.31 2.77 27.5 1.35 16.4 0.81 0.12 -. . 0.03

53.7 0.14 0.86 20.0 1.17 22.5 1.19 0.05 -. --

total

99.7

99.6

99.6

Wo En Fs Mg#

1.92 61.6 36.5 62.8

1.80 50.6 47.6 51.5

2.47 65.1 32.5 66.7

r.r.

52.4 0.01 7.35 11.5 0.28 28.1 0.62 0.04 0.07 . 0.08 100.5 1.27 80.3 18.5 81.3

core

rim

51.7 0.35 2.34 10.5 0.64 14.2 20.3 0.37 --

WAK-10

ACH-8

core

core

51.9 0.42 3.47 10.1 0.54 14.1 21.1 0.42 ----

--

50.4 0.72 3.14 12.5 0.35 14.9 18.1 0.25 -0.02 --

50.2 0.56 3.74 8.83 0.22 15.0 20.4 0.26 --0.03

100.5

101.0

100.3

99.3

42.0 41.0 17.0 70.7

43.4 40.5 16.2 71.5

37.2 42.7 20.1 68.0

42.4 43.3 14.3 75.2

Mg#: Mg/(Mg + Fe) × 100 mole ratio. WAK-10: Wakago basalt. ACH-8: inclusion from Achiyama dome. M-4, -10, and -14: inclusions from Mukalyama dome. r.r: reaction rim of olivine phenocrysts.

Biotite No.

Hornblende

M-22

M-10 core

M-22

core

rim

SiO2 TiO2 A12 0 3 FeO MnO MgO CaO Na: O K20

36.6 3.87 14.1 19.1 0.45 12.4 0.02 0.70 7.95

36.7 4.03 13.9 19.2 0.41 12.3 0.02 0.73 8.43

37.2 3.97 13.5 18.7 0.52 12.3 0.01 0.74 8.24

SiO2 TiO2 A12 03 FeO MnO MgO CaO Na2 O K20

total

95.1

95.6

95.2

Mg#

53.5

53.3

53.9

Cr2 03 V203 NiO

M-22 : M u k a i y a m a r h y o l i t e .

M-4

core

rim

core

core

rim

48.0 1.52 6.73 13.3 0.48 14.9 11.0 1.51 0.12

49.0 1.38 6.24 13.5 0.67 14.9 10.4 1.38 0.10

45.3 2.04 10.25 13.3 0.34 14.2 11.0 2.22 0.14

44.5 2.01 9.65 12.6 0.49 14.2 11.0 2.38 0.17

48.5 1.10 5.99 18.5 0.53 11.6 9.82 1.29 0.19

0.06 0.03 0.04

0.05 -0.02

-0.03 0.02

----

----

total

97.6

97.6

98.9

97.0

97.5

Mg#

66.7

66.2

65.5

66.8

52.8

84 TABLE 3 (continued) Plagioclase No.

M-22 core

SiO2 TiO2 A1203 FeO MgO CaO Na20 K20

63.1 0.02 22.6 0.09 0.05 4.57 8.95 0.51

total

99.9

An Ab Or

21.4 75.8 2.86

No.

ACH-8* core

WAK-10 rim 65.1 0.03 21.4 0.12 0.04 3.39 9.83 0.68 100.6 15.4 80.9 3.70

M-10* core

core 46.2 0.03 33.4 0.62 0.11 18.2 1.33 0.01 99.9 88.2 11.7 0.06

TNE-6** core

ACH-8 rim 48.4 0.06 32.0 0.72 0.14 16.5 2.31 0.02 100.1 79.7 20.2 0.11

rim

SiO2 TiO2 A1203 FeO MgO CaO Na20 K20

63.3 0.01 22.3 0.05 0.01 4.22 9.19 0.64

63.8 0.02 21.6 0.11 -3.11 9.63 0.74

53.5 0.02 28.4 0.41 0.04 12.0 5.01 0.07

51.8 0.02 29.0 0.59 0.25 12.9 4.34 0.07

total

99.7

99.1

99.5

99.1

An Ab Or

19.5 77.0 3.52

14.5 81.4 4.12

56.7 42.9 0.39

61.9 37.8 0.38

core 45.3 -34.4 0.57 0.10 18.3 1.00 0.03 99.7 90.8 9.00 0.18

M-16"** core 48.4 0.03 32.2 0.78 0.16 16.7 2.07 0.01 100.3 81.6 18.4 0.06

T N E - 6 , M-16: i n c l u s i o n s f r o m M u k a i y a m a d o m e . * : plagioclase w i t h c o n c e n t r i c d u s t y z o n e . * * : plagioclase w i t h m a n y glass inclusions. * * * : plagioclase f o r m i n g a n aggregate t e x t u r e w i t h olivine p h e n o c r y s t .

rim 46.4 0.01 33.8 0.65 0.06 17.7 1.54 0.04 100.2 86.2 13.6 0.22

rim 48.5 0.05 31.9 0.75 0.15 16.7 2.25 0.03 100.4 80.3 19.6 0.15

85 TABLE 3 (continued) Magnetite No.

Ilmenite H-1

M-22

H-1

core core SiO2 TiO2 A1203 Fe203 FeO MnO MgO CaO Cr203 V203 total usp

0.67 7.02 1.73 52.2 36.5 1.25 0.58 0.08 0.02 0.02

M-22

core 0.79 7.10 1.82 51.9 37.0 1.29 0.55 0.03 0.00 0.03

rim

core

0.75 7.67 1.79 50.1 37.1 1.31 0.51 0.06 0.06 0.03

SiO2 TiO2 A1203 Fe203 FeO MnO MgO CaO Cr203

V203

0.56 46.5 0.39 11.0 37.4 2.63 1.34 0.03 0.00 0.00

100.1

100.5

99.5

total

99.9

22.4

22.9

24.6

hem

11.0

0.56 47.6 0.42 9.3 38.3 2.70 1.34 0.04 0.00 0.00 100.2 9.35

rim 0.59 47.0 0.45 10.2 37.7 2.85 1.29 0.04 0.00 0.00 100.1 10.3

H-l: Mukaiyama rhyolite, recalculated with the method of Buddington and Lindsley (1964). usp: ulvospinel component (tool.%); hem: hematite component (mol.%).

Plagioclase. P h e n o c r y s t s o f plagioclase o c c u r in all samples. Plagioclase in Wakago basalt is Ans0-90 and t h a t in the h o s t r h y o l i t e s is An10_30 (Fig. 9). T h e s e plagioclase p h e n o c r y s t s are u n z o n e d or n o r m a l l y z o n e d . Plagioclase in t h e dacitic inclusion is variable in c o m p o s i t i o n ranging f r o m An 10 to Ang0. Sodic plagioclase shows a c o n c e n t r i c t e x t u r e ; it is u n z o n e d in the core and is s u r r o u n d e d b y a dark z o n e o f sieved calcic plagioclase with n u m e r o u s , n a r r o w (less t h a n 10 ~ m wide) glass inclusions, resulting f r o m partial melting ( T s u c h i y a m a , 1 9 8 5 ) . The u n z o n e d core has the same chemical variation as plagioclase in t h e h o s t r h y o l i t e . Several crystals o f calcic (Ans0) plagioclase f o r m an aggregate with olivine or c l i n o p y r o x e n e crystals. I n t e r m e d i a t e plagioclase shows irregular zoning, and c o n t a i n s m a y large ( m o r e t h a n 100 p m in d i a m e t e r ) glass and opaque-minerals inclusions. S o m e plagioclase p h e n o c r y s t s o f this t y p e f o r m an aggregate with o r t h o p y r o x e n e , h o r n b l e n d e , m a g n e t i t e and c l i n o p y r o x e n e . C o m p o s i t i o n a l variation o f plagioclase in the basalitic inclusions in the A c h i y a m a d o m e is bimodal: Anls_2s and Anso-90 (Fig. 9). T h e sodic plagioclase shows a c o n c e n t r i c t e x t u r e with d u s t y z o n e as described above.

Quartz. B i p y r a m i d a l p s e u d o m o r p h s o f high q u a r t z up to several m m in d i a m e t e r are c o m m o n in t h e h o s t r h y o l i t e . In dacitic inclusions, b i p y r a m i d a l q u a r t z p h e n o c r y s t s are fringed b y fine-grained h o r n b l e n d e a n d / o r p y r o x e n e s ,

86

[ ] w i OIt h[ ]

w i t h hb

with Cpx [ ~ k r e g u i a r zoning Dconcentric r ~ with opx decomposition []

MUKAIYAMA INCLUSION i M

i 0

, MM

i 2O

, 17

B-.L

•

I

I 40

I

=,

610

I

= = .

I 80

I

ACHIYAMA INCLUSION q-~:lRn

'

,

2'0

,

4'0

ff

~o

'

.o

6IO

i

80

6i 0

I

m,

(~ I 0

d 16o

HOST RHYOLITE mmm i

0

~

m_=

•

2'0

410

]

i

i

I00

WAKAGO BASALT • I

0

i

2'0

'

410

I

-I

80

mmmm I

i

100

An c o n t e n t

Fig. 9. An/(An + Ab) ratio of plagioclase phenocrysts in inclusions, host rhyolite, and Wakago basalt. Plagioclase crystals which show different textures are shown by different symbols. some of which are in turn surrounded by quartz overgrowth. Quartz is also found in Wakago basalt and basaltic inclusions as corroded bipyramidal pseudomorphs with pyroxene corona, as described by Sato (1975).

Olivine. Olivine occurs in Wakago basalt and in basaltic inclusions as euhedral to subhedral crystals up to 0.5 mm in diameter. Small amounts of olivine occur in dacitic inclusions, enclosed by thick reaction rims of o r t h o p y r o x e n e or forming an aggregate with calcic plagioclase and clinopyroxene. Olivine is Fo70-73 in Wakago basalt and basaltic inclusions, and is slightly more magnesian (FoTs-80) in dacitic inclusions (Fig. 10).

Clinopyroxene. Clinopyroxene phenocrysts are rarely f o u n d in Wakago basalt and both types of inclusions as euhedral to subhedral crystals up to 1 mm long. Mg# of clinopyroxene is similar to or slightly less than that of olivine within each sample (Fig. 10). In dacitic inclusions some clinopyroxene crystals form aggregates with hornblende, orthopyroxene, magnetite, and plagioclase, and sometimes with olivine. Orthopyroxene. This mineral occurs in dacitic inclusions as euhedral grains up to 1 mm long, or as reaction rims on olivine phenocrysts. Orthopyroxene reaction rims have a Mg# similar to that of olivine (75--80) (Fig. 10). Euhedral o r t h o p y r o x e n e is generally less magnesian (Mg# = 60--65) {Fig. 10), and shows a reverse or irregular zoning. Euhedral orthopyroxene comm o n l y forms aggregates with plagioclase, magnetite, and hornblende.

87 Inclusion

Mukeiyarne

or thopyroxene

•

510 ~ clinopyroxene

610

I

reaction rim

,'o FI ~

610

50 olivine F

I

50

60

17

80

90

81o '

Jo

10

910

Inclusion

Achiyema

olivine I

~

50

60

I

'°L

81o

9'o

I

710

810

910

Basalt

Wakego

olivine 510

6O

M g l M g + Fe

xlO0

Fig. 10. Mg/(Mg + Fe) ratio of mafic minerals (olivine, orthopyroxene, and clinopyroxene) in inclusions and Wakago basalt.

Hornblende. Euhedral to subhedral oxyhornblende phenocrysts up to several mm long occur in dacitic inclusions. Euhedral green hornblende rarely occurs in the Mukaiyama rhyolite. The two types of hornblende are chemically distinct. Oxyhornblende is more enriched in A1203 and Na20 and depleted in SiO2 than green hornblende (Table 3). Biotite. Euhedral crystals of biotite less than several mm long are c o m m o n in the host rhyolites. Decomposed biotite crystals with or w i t h o u t relict cores are rare in dacitic inclusions. The relict biotite has chemical composition similar to phenocrysts in the host rhyolites {Table 3 ). Opaque minerals. Spinel inclusions (several tens of/~m in diameter) are rare in olivine phenocrysts in Wakago basalt and in basaltic and dacitic inclusions. Euhedral to subhedral magnetite and ilmenite phenocrysts up to 0.1 mm in diameter occur in the host rhyolite and the dacitic inclusions. Analyses of magnetite and ilmenite are generally variable within a sample, because of fine exsolution lamellae. Lamellae are absent in magnetite and ilmenite in the base-surge deposits and the essential blocks in the pyroclastic cone of the Mukaiyama volcano. In those samples, the chemical compositions of magnet-

88 ite and ilmenite are nearly uniform; ulvospinel component in magnetite is about 22 mol.%, and hematite component in ilmenite is about 10 mol.%. The above magnetite-ilmenite pair can coexist in equilibrium at about 700°C and fo~ = 10-1~ to 10 -16 atm (Buddington and Lindsley, 1964). The base-surge deposits have higher porosity than the ejecta of other modes of emplacement, suggesting the higher H20 content in the magma at the time of eruption. Therefore, the above value of temperature does not necessarily represent that of the whole rhyolitic magma of the Mukaiyama volcano, but may represent that of the volatile-rich upper zone of the rhyolitic magma chamber. DISCUSSION

Origin o f dacitic inclusions in the Mukaiyama volcano Petrographic features suggest that the dacitic inclusions are samples of a magma formed by mixing of basaltic and rhyolitic magmas. The dacitic inclusions have a complicated phenocryst assemblage, including the disequilibrium pair of magnesian olivine and quartz. Plagioclase has a very wide chemical variation (Fig. 9), sodic plagioclase has been partly melted, and some calcic plagioclase forms aggregates with olivine. Relict biotite in the inclusions has the same chemical composition as biotite in the host rhyolite (Table 3). Furthermore, the bulk chemical compositions of the dacitic inclusions roughly plot on a mixing line between the host rhyolite and Wakago basalt (Fig. 6). The most probable mafic and silicic endmembers are Wakago basalt and the host rhyolite. The other phenocrysts (orthopyroxene, oxyhornblende, intermediate plagioclase, and magnetite), are characterized by an irregular chemical zoning. They may have crystallized from the heterogeneous intermediate liquids during the mixing. The dacitic composition of the inclusion cannot be explained by a reaction between a basaltic droplet and the rhyolite host. If the dacitic inclusion resulted from the reaction between the droplet and host, each inclusion should be chemically zoned. The rim should be more silicic than the core, and the degree of the reaction might be correlated with the size of the inclusion. However, within a single inclusion, the bulk chemical composition is nearly uniform from rim to core (Fig. 7), and the composition of the core is independent of the size of inclusions (Fig. 8). It is suggested that the hybridization (formation of slightly zoned dacitic magma} occurred prior to disaggregation and quenching to form inclusions. In order to explain the occurrence of dacitic inclusions, a two-stage magma mixing model is proposed. One stage is mixing caused by the instability of the interface between two layers in a vertically stratified magma chamber. The dacitic mixed magma was formed at this stage. The other stage is mixing caused by forced convection during ascent in a conduit just before eruption. Inclusions were incorporated into their hosts at this stage.

89

Mixing mechanism When basaltic magma intrudes a rhyolitic magma chamber, separate layers of basalt and rhyolite are thought to be formed (e.g. Eichelberger, 1980; Huppert et al., 1982). The m a x i m u m penetration height of basaltic magma injected upward into silicic magma can be estimated from simple energy conservation laws (Rice, 1981; Sakuyama and Koyaguchi, 1984): (pV2)/2 > Apgh

(1)

where p and V are the density and velocity of basalt, respectively, Ap is the density difference between the t w o magmas, g is the gravitational acceleration and h is the penetration height. For the rhyolite/basalt system, the height is estimated to be several tens of cm at the maximum, assuming that the injection velocity is 1 m/s, and p and Ap are 2.6 and 0.4 g/cm 3 , respectively. Consequently, the basaltic magma would pond beneath the rhyolite. The compositional stratification and thermal gradient of the rhyolite/basalt layers are suitable for double diffusive convective layers. In the natural rhyolite/basalt magma system, where the Q = {3AC/~AT (where AC and AT are the differences in composition and temperature between the t o p and the b o t t o m of the chamber, respectively, and /3 and c~ are the corresponding coefficients of expansion) is a b o u t 10, two or more convective layers are stably stratified (e.g. Turner, 1979). Convective disruption of the stratification ("rollover") only happens in a magma chamber when the densities of the magma layers become equal (Q = 1). At present, there seems to be only one mechanism to accomplish "rollover" in a magma chamber. If the basaltic magma in the lower layer crystallizes and exsolves vapour bubbles as it loses heat across the interface, the exsolution may cause a density inversion in the magma chamber and the stratification may be broken (Eichelberger, 1980; H u p p e r t et al., 1982). This process, however, can play a significant role only in a very shallow magma chamber at a pressure less than 1 kbar, and requires a very high water content for the basaltic magma. Assuming a pressure greater than 1 kbar (for example 1.5 kbar) and an initial water content of 3 wt.% in the basaltic magma, a b o u t 50% crystallization of the basaltic magma is required before the exsolution of vapour bubbles begins. Further crystallization is required in order that the bulk density of basaltic magma (melt + crystals + vapour) becomes equal to that of overlying magma. As mentioned above, more than 90 vol.% of the inclusion crystallized after the disaggregation into droplets. If the disaggregation occurred during disruption of the stratification, density inversion caused by volatile exsolution cannot account for the breaking of the stratification. Consequently, we must consider mixing mechanisms which can occur even though the mafic magma remains denser than the silicic magma (i.e. Q ~> 1) in order to explain the Niijima mingled lavas.

Entrainment between convective layers. When Q ~ 1, t w o magmas of the lower and upper convective layers can mix in a magma chamber only locally

90 around the interface by intermittent penetrative convection (entrainment). Griffiths (1979) described the following instabilities of the interface between two convective layers in the experiments using MgCl2, CaCl2, NaC1, and KC1 solutions. At Q ( 3 , penetrative convection produces interfacial waves. These waves become large enough when Q ~ 2 to break and cause local mixing of two layers around the interface. In the experiments on double diffusive convective layers, upward migration of the interface has been c o m m o n l y observed (e.g. Marmorino and Caldwell, 1976; Griffiths, 1979; McDougall, 1981). This p h e n o m e n o n is interpreted as follows. Convective velocities are greater in the lower layer than in the upper because of significantly greater coefficient of the thermal expansion in the lower layer in these experiments. The difference in convective velocities causes an imbalance in entrainment of fluid from the interface, and the migration occurs. A difference in convective velocities between the rhyolitic and basaltic magmas can be reasonably presumed, judging from the viscosity contrast, and it may cause entrainments of rhyolitic magma into the lower basaltic layer. If so, magma of intermediate compositions is inevitably produced in the lower layer. In all these experiments using aqueous solutions the interfacial instabilities (e.g. influences of entrainment) are observed at lower Q (usually less than 7) than the value which is expected in the natural silicic/mafic magma system (about 10). Intensity of entrainment is a function of the Froude number (Fr) of the interface, which is expressed as: Fr = u /( g Ap l/ P o) 1/2

(2)

where u and l are typical velocity and length scales of the motion near the interface, Ap is density difference, and P0 is density of the lower fluid. For fixed density ratio Q, Fr is variable depending on convective velocities. Furthermore, a large difference in convective velocities between the rhyolitic and basaltic layers due to a large viscosity contrast may induce another kind of interfacial instability even at the higher Q. Much more experimental and theoretical investigations are required for the quantitative estimation of entrainment. Mixing in a conduit. Koyaguchi (1985) pointed o u t based on fluid dynamic

experiments that the rhyolite/basalt stratification in a magma chamber can be easily disrupted while the magmas ascend through a conduit. The results of the experiments will be reviewed briefly. Two types of experiments (down-flow and up-flow experiments) were performed using dyed water (density is 1 . 0 0 g / c m 3 and viscosity is a b o u t 10 -2 poise), commercially available glue (1.03 g/cm 3 , 60 poise), and dilute glycerin (1.12 g/cm 3 , less than 10 -I poise). In the d o w n - f l o w experiment, water initially overlies glue in an irrigator. When the t w o liquids flow down through a vinyl tube connected at the b o t t o m of the irrigator, less viscous water flow penetrates downward into

91

the glue along the central axis of the tube and emerges from the tube before the glue. During the overturning, the boundary of the two liquids is unstable with boudin-shaped waves. In the up-flow experiment, glue and dyed dilute glycerin, which are initially stably stratified in a beaker, are forced to flow upward by an aspirator through the inverted irrigator and vinyl tube. The results of the up-flow experiment are shown in Fig. 11. The denser and less viscous glycerin flows through the glue, and when the flow rate is small (several mm/s), overturning of the two liquids occurs with the boudin-shaped flow. As the flow rate is increased (more than several cm/s and Re is 10 -2 to 10 -1 ), overturning with turbulent flow occurs. The results of these experiments demonstrate that the stable stratification of the rhyolitic and basaltic magmas in a magma chamber can be disrupted during the ascent in a conduit in spite of their density contrast. Flow during

=ii ¸

= i~ ~ ~iiii i ~!

!il if!if!!!i~i!!!i~!i~i~i::~i!ii~iii i~ ~i ii

Fig. 11. Photographs of the up-flow experiments using glue (colorless) and dilute glycerin (dark-colored). Glycerin is being sucked up the tube. a When the flow rate of the glue is small, overturning occurred with "boudin-shaped" flow. b As the flow rate of the glue is increased, the turbulent overturning occurs, and the two liquids are efficiently mixed.

92 overturning is unstable, so that considerable mixing of two magmas is expected. If a fracture forms from a magma chamber to the surface before the magma begins to ascend, the magma can flow up through the fracture with large pressure gradients (Szekely and Reitan, 1971). In such a case, the rhyolitic and basaltic magmas can turbulently overturn and efficiently mix in a conduit.

Boundary conditions o f heat and mass transfer during mixing When magmas are juxtaposed by mixing under given boundary conditions, compositional change (homogenization) by molecular diffusion is limited because diffusivity of heat is much greater than that of material in silicate melts (Murase and McBirney, 1973; Hofmann, 1980; Sparks and Marshall, 1986). The difference in diffusivities of heat and material is independent of any physical processes of mixing, whereas a boundary condition can be much influenced by mixing mechanisms. Accordingly the nature of the resultant mixture would be much affected by the mixing mechanisms. There are two types of boundaries between basaltic inclusions and host rhyolite; one is sharp and characterized by a fine-grained rim, and the other is diffuse. The diffuse contacts are characteristically found where rhyolitic magma occupies e m b a y m e n t s in large basaltic inclusions (Figs. 3 and 5). The local proportion of basaltic magma was large so that the temperature was locally and temporarily high around the rhyolite-filled embayment. Thus the rhyolitic and basaltic magmas remained in a liquid state and could undergo diffuse exchange before the large inclusion reached thermal equilibrium with the rhyolitic host and solidified. This observation strongly suggests the importance of the influence of boundary conditions even within a single mixing process. In this section, heat and mass transfer under two different types of boundary conditions will be discussed. Spherical basaltic droplets in rhyolitic host. The boundary condition first is that of spherical basaltic droplets homogeneously distributed in the rhyolitic host (Fig. 12A). This boundary condition would represent that of mass and heat transfer during the formation of the inclusions. Thermal history of the droplets can be estimated by a simple diffusion model (e.g. Carslaw and Jaeger, 1959; Sparks et al., 1977). Assuming that the initial temperatures of basaltic and rhyolitic magmas are 1200°C and 800°C, and the thermal diffusivity of magmas is a b o u t 10 -a cm2/s, each droplet of several to several tens of cm in diameter is chilled to an equilibrium temperature within several hours. The equilibrium temperature mainly depends on the mixing ratio of basaltic and rhyolitic magmas. The m o d e of mixing after the thermal equilibrium is much affected by the equilibrium temperature. Mixing in a liquid state occurs only when the equilibrium temperature is higher than the

93

T°,[ -_B

Tb ¢D

-t fk

Tr

E

I--

---~~_'~Rh ...... "~...... ~

R ~liquidus ~solidus

quldus ~oli'dus

Cb

Or

Cb C~ C2

C~_~C~ C,

Composition

rhyolite basalt

•

• O

° .: .,:

(A) Overturn of stratification in a conduit

(B) ~Entrainment" in a magma chamber

Fig. 12. S c h e m a t i c i l l u s t r a t i o n o f m e c h a n i s m o f mass a n d h e a t t r a n s f e r o f m i x i n g in a c o n d u i t (A), a n d m i x i n g b y e n t r a i n m e n t in a m a g m a c h a m b e r (B). B = b a s a l t ; R = r h y olite. F o r e x a p l a n a t i o n see t e x t .

solidus of basaltic magma (or the temperature at which basaltic magma extensively crystallizes and behaves as a virtual solid). When the rhyolitic magma is predominant in the mixture, on the other hand, the equilibrium temperature would be below the solidus of the basaltic magma and the basaltic droplets would solidify to inclusions (star in Fig. 12A). The cooling rate of a droplet of several tens of cm in diameter is estimated to be more than several degrees centigrade per hour under this condition. This value is comparable to the experimentally obtained cooling rate for the growth of acicular crystals (e.g. Lofgren, 1980). The acicular crystals in the groundmass of the inclusion would have formed under this condition. Koyaguchi (1986) pointed o u t that textures of mixed and mingled lavas from the Abu volcano group, SW Japan, systematically change as a function of the mixing ratio of basaltic and dacitic magmas. The basaltic to basaltic andesitic lavas have relatively homogeneous matrices with disequilibrium phenocryst assemblages, while the andesitic to dacitic lavas are characterized by basaltic inclusions. He explained this systematic change by the difference in the equilibrium temperature depending on the mixing ratio as is mentioned above.

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The equilibrium temperature is also a function of heat capacities of magmas, latent heat, and heat of mixing. Furthermore, mixing after thermal equilibration would be much affected by the volume fraction of crystals, even if the equilibrium temperature is above the solidus (Sparks and Marshall, 1986). These factors are, however, ignored in the qualitative discussions hereafter.

Rhyolite~basalt layers. Thermal and compositional evolution of the lower layer of basaltic magma in a stratified magma chamber will be discussed next. Application of the two-layer model (Huppert and Sparks, 1980) indicates that the cooling rate of the basaltic layer is inversely proportional to the cube root of the viscosity. Therefore, the cooling rate of the basaltic layer overlain by a rhyolitic layer is much smaller than that overlain by another basaltic layer as has been calculated by Huppert and Sparks (1980). When the basaltic layer is I km thick, the cooling rate is initially less than 1 degree centigrade per hour and further decreases with time. Viscosities of the upper and lower layers are tentatively assumed to be 104 poise here, since application for two-layer system with a viscosity contrast has not been established yet. The small value of cooling rate cannot explain the morphology of crystals in the inclusions. If the rhyolitic magma of the upper layer is entrained into the lower basaltic layer by the interfacial instability as is suggested in the previous section, a small amount of rhyolite is successively supplied into the lower layer and immediately mixed into the basalt by covection in the lower layer. The mechanism of mass and heat transfer under this boundary condition is schematically shown in Fig. 12B. As a small amount of rhyolitic magma (temperature ( T ) = Tr, composition ( C ) = Cr) is entrained and mixed in the lower basaltic layer (T = Tb, C = Cb ), the lower layer reaches thermal equilibrium (T = T~, C = Cb + Cr). If there are sufficient time and vigorous convection, the lower layer can be completely homogenized (T = TI, C = C1 ). As rhyolitic magma is additionally entrained into the lower layer of mixed magma, temperature and composition reach (T2, C2 ) through (T2, C1 + C~). The points (Tn, Cn -1) can approach to the line joining (Tb, Cb) and (Tr, Cr), if the rate of the entrainment is sufficiently small compared with the rate of homogenization within the lower layer. Under this boundary condition, the basaltic and rhyolitic magmas may mix essentially in a liquid state in a wide range of mixing ratios (Fig. 12B). The dacitic magma with 64% SiO 2 can be produced by the mixing of basaltic and rhyolitic magmas with SiO2 contents of 50 and 78%, respectively, in the proportion of 1:1. Assuming that the initial temperatures of basaltic and rhyolitic magmas are 1200°C and 800°C, respectively, the temperature of the basaltic magma would immediately fail by more than one hundred degrees under the boundary condition of Fig. 12A, even if latent heat is taken into account. According to experimental studies (Marsh, 1981; Yamaguchi, 1983), nearly 50vo1.% of basaltic magma would crystal-

95

lize, hence the dacitic mixture would be porphyritic (about 25% phenocrysts) under this condition. The dacitic inclusions of the Mukalyama dome, however, contain only a small a m o u n t of phenocrysts (less than 10vol.%). Furthermore, most of the phenocrysts (plagioclase of a b o u t Ans0, hypersthene, oxyhornblende) are considered to have crystallized from the melt of intermediate compositions. These observations would suggest that the formation of the mixed dacitic magma occurred under the boundary condition of Fig. 12B, where two magmas can mix in a liquid state w i t h o u t an extensive crystallization of the basaltic magma.

Two-stage magma mixing model in Niijima domes The occurrence of the basaltic and dacitic inclusions can be successfully explained by a two-stage magma mixing model as is shown in Fig. 13. Injection of the basaltic magma into the rhyolitic magma chamber results in separate layers of basaltic and rhyolitic magmas. In a magma chamber, mixed magma was formed by an entrainment of rhyolitic magma into the lower basaltic magma. The t w o magmas can mix essentially in a liquid state during the hybridization of this stage. If the homogehization within the lower layer

~ Basaltic inclusion

t

0

conduit

GbCa=-r21

Gr(an-r 2)

4/J~

4~ut

•

•

inclusion Da¢itl¢=

tottover

magma chnmbnr

(P,T3

t

CP.T}

(P,T)

|

~,C ~L~T

>7

7--t

1

Fig. 13. Schematic m o d e l for mixing in a magma chamber and in a conduit. In a magma chamber, mixing proceeds from left to right; injection, stratification, formation o f the layer o f m i x e d magma and rollover. In the conduit, the stratification is disturbed and the disaggregation of lower magma occurs. The composition o f inclusions depends on when the eruption (or final ascent o f the stratified magma) occurs.

96

is n o t perfect, a stepwise-zoned magma chamber with basaltic, intermediate and rhyolitic layers upwards may be formed. There is no information a b o u t the b o u n d a r y between the intermediate and basaltic layer, because no basaltic inclusions have been found in the Mukaiyama dome. Independent of the degree of mixing in a magma chamber, if stratified magmas ascend through a conduit, the stratification can be easily disrupted. The mafic magma is disaggregated and dispersed in the rhyolitic host magma, and temperatures of these magmas immediately converge to an equilibrium temperature. If the rhyolitic magma is predominant, the equilibrium temperature is below the solidus of the mafic magma and the mafic magma solidifies. Whether the inclusions are basaltic or mixed in Niijima rhyolitic lava domes depends on whether the final ascent occurred before or after the formation of an intermediate layer in a magma chamber. Mafic mixed inclusions are often found in calk-alkalic or silicic lavas, such as the Coso volcanic field, California (Bacon and Metz, 1984; Bacon, 1986), or net-vein complexes, such as the complexes of St. Kilda and Ardnamurchan, Scotland (Vogel, 1982; Marshall and Sparks, 1984). Although the foregoing discussions only concern the Niijima mingled lavas, the two-stage magma mixing model may be applied to the magma mixing of these provinces. ACKNOWLEDGEMENTS

During the course of this study I have benefitted from discussions with Profs. I. Kushiro, S. Aramaki, M. Toriumi, and T. Fujii of the University of T o k y o , Prof. C.M. Scarfe of the University of Alberta, Dr. K. Ozawa of the University of T o k y o , and Dr. Y. Tatsumi of K y o t o University. Review by Dr. C.R. Bacon helped to improve and clarify the manuscript. I thank all of these persons. REFERENCES Bacon, C.R., 1986. Magmatic inclusions in silicic and intermediate volcanic rocks. J. Geophys. Res., in press. Bacon, C.R. and Metz, J., 1984. Magmatic inclusions in rhyolites, contaminated basalts, and compositional zonation beneath the Coso volcanic field, California. Contrib. Mineral. Petrol., 85: 346--365. Bence, A.E. and Albee, A.L., 1968. Empirical correction factors for the electron microanalysis of silicates and oxides. J. Geol., 76: 382--403. Buddington, A.F. and Lindsley, D.H., 1964. Iron-titanium oxide minerals and synthetic equivalents. J. Petrol., 5(Part 2): 310--357. Carslaw, H.S. and Jaeger, J.C., 1959. Conduction of Heat in Solids. Oxford Press, Oxford, 510 pp. Eichelberger, J.C., 1975. Origin of andesite and dacite, evidence of magma mixing at Glass Mountain in California and the other Circum-Pacific volcanoes. Geol. Soc. Am. Bull., 86: 1381--1391. Eichelberger, J.C., 1980. Vesiculation of mafic magma during replenishment of silicic magma reservoirs. Nature, 288: 446--450.

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Griffiths, R.W., 1979. The transport of multiple components through thermohaline diffusive interface. Deep-Sea Res., 26A: 383--397. Hofmann, A.W., 1980. Diffusion in natural silicate melts: a critical review. In: R.B. Hargraves (Editor), Physics of Magmatic Processes. Princeton University Press, Princeton, N.J., pp 385--417. Huppert, H.E. and Sparks, R.S.J., 1980. The fluid dynamics of a basaltic magma chamber replenished by influx of hot, dense, ultrabasic magma. Contrib. Mineral. Petrol., 75: 279--289. Huppert, H.E., Sparks, R.S.J. and Turner, J.S., 1982. Effect of volatiles on mixing in calcalkaline magma systems. Nature, 297: 554--557. Huppert, H.E., Sparks, R.S.J. and Turner, J.S., 1983. Laboratory investigations of viscous effects in replenished magma chambers. Earth Planet. Sci. Left., 65: 377--381. Isshiki, N., 1973. C-14 age of Mukal-yama Volcano of Nii-jima Island, Izu Island. Bull. Volcanol. Soc. Jpn., Ser. II, 1 8 : 1 6 9 - - 1 7 0 (in Japanese). Kouchi, A. and Sunagawa, I., 1985. A model for mixing basaltic and dacitic magmas as deduced from experimental data. Contrib. Mineral. Petrol., 89: 17--23. Koyaguchi, T., 1985. Magma mixing in a conduit. J. Volcanol. Geotherm. Res., 25: 365--369. Koyaguchi, T., 1986. Textural and compositional evidence for magma mixing and its mechanism, Abu volcano group, Southwestern Japan. Contrib. Mineral. Petrol., 93: 33--45. Lofgren, G.E., 1980. Experimental studies on the dynamic crystallization of silicate melts. In: R.B. Hargraves (Editor), Physics of Magmatic Processes. Princeton University Press, Princeton, N.J., pp. 487--551. Luhr, J.F. and Carmichael, I.S.E., 1980. The Colima volcanic complex, Mexico, I. Postcaldera andesites from Volcan Colima. Contrib. Mineral. Petrol., 7 1 : 3 4 3 - - 3 7 2 . Marmorino, G.O. and Caldwell, D.R., 1976. Heat and salt transport through a diffusive thermohaline interface. Deep-Sea Res., 23: 59--67. Marsh, B.D., 1981. On the crystallinity, probability of the occurrence, and rheology of lava and magma. Contrib. Mineral. Petrol., 78: 85--98. Marshall, L.A. and Sparks, R.S.J., 1984. Origin of some mixed-magma and net-veined ring intrusions. J. Geol. Soc. London, 141: 171--182. Matsumoto, R. and Urabe, T., 1980. An automatic analysis of major elements in silicate rocks with X-ray flurescence spectrometer using fused disc samples. J. Jpn. Assoc. Miner. Petrol. Econ. Geol., 7 5 : 2 7 2 - - 2 7 8 (in Japanese). McDougall, T.J., 1981. Double-diffusive convection with a non-linear equation state. Part II. Laboratory experiments and their interpretation. Prog. Oceanogr., 10: 71--89. Miyaji, Y , 1965. The geomorphology of the volcanoes of Niijima Island, Izu archipelago. Geograph. Rev. Jpn., 3 8 : 6 4 3 - - 6 5 7 (in Japanese). Murase, T. and McBirney, A.R., 1973. Properties of some c o m m o n igneous rocks and their melts at high temperatures. Geol. Soc. Amer. Bull., 84: 3563--3592. Nakamura, Y. and Kushiro, I., 1970. Compositional relations of coexisting orthopyroxene, pigeonite and augite in a tholeiitic andesite from Hakone volcano. Contrib. Mineral. Petrol., 26: 265--275. Rice, A., 1981. Convective fractionation: A mechanism to provide cryptic zoning (macrosegregation), layering crescumulates, banded tufts and explosive volcanism in igneous processes. J. Geophys. Res., 86: 405--417. Sakuyama, M., 1979. Evidence of magma mixing; petrological study of Shirouma Oike calc-alkaline andesite volcano, Japan. J. Volcanol. Geotherm. Res., 5: 179--208. Sakuyama, M. and Koyaguchi, T., 1984. Magma mixing in mantle xenollth-bearing calc alkaline ejecta, Ichinomegata volcano, NE Japan. J. Volcanol. Geotherrn. Res., 22: 199--224. Sato, H., 1975. Diffusion coronas around quartz xenocrysts in andesite and basalt from Tertiary volcanic region in northeastern Shikoku, Japan. Contrib. Mineral. Petrol., 50: 49--64.

98 Sparks, R.S.J. and Marshall, L.A., 1986. Thermal and mechanical constraints on mixing between mafic and silicic magmas. J. Volcanol. Geotherm. Res., 29: Sparks, R.S.J., Sigurdsson, H. and Wilson, L., 1977. Magma mixing: a mechanism for triggering acid explosive eruptions. Nature, 267 : 315--318. Szekely, J. and Reitan, P.H., 1971. Dike filling by magma intrusion and by explosive entrainment of fragments. J. Geophys. Res., 76: 2602--2609. Tsuchiyama, A., 1985. Dissolution kinetics of plagioclase in the melt of the system diopside-albite-anorthite, and origin of dusty plagioclase in andesite. Contrib. Mineral. Petrol., 89: 1--16. Tsuya, H., 1938. Volcanoes of Nii-zima, one of the seven Izu Islands. Bull. Earthquake Res. Inst. Tokyo. Univ., 1 6 : 1 7 1 - - 2 0 0 (in Japanese). Turner, J.S., 1979. Buoyancy Effects in Fluids. Cambridge Univ. Press, Cambridge 368 pp. Vogel, T.A., 1982. Magma mixing in the acid-basic complex of Ardnamurchan: implications on the evolution of shallow magma chambers. Contrib. Mineral. Petrol., 79: 411--423. Yamaguchi, T., 1983. Roles of H20 on fractional crystallization of the magmas beneath Hotaka and Akagi Volcanoes, and their bearing on the petrography of Quaternary volcanic rocks of NE Japan. PhD. thesis, Univ. Tokyo. Yokoyama, S. and Tokunaga, T., 1978. Base-surge deposits of Mukaiyama Volcano, Niijima, Izu Islands. Bull. Volcanol. Soc. Jpn., 2 3 : 2 4 9 - - 2 6 2 (in Japanese).