immiscibility synthesis as an indication of cooling rates of basalts

Journal of Volcanology and Geothermal Research, 45 ( 1991 ) 289-310 289 Elsevier Science Publishers B.V., Amsterdam

Immiscibility synthesis as an indication of cooling rates of basalts

B6atrice Martin a and Ikuo Kushiro b

a Laboratoire de P~trologie Min&alogique, Universit~ Pierre et Marie Curie, 4, Place Jussieu, 75230 Paris Cedex 05, France b Geological Institute, University of Tokyo, Tokyo 113, Japan

(Received July 18, 1989; accepted in revised form May 18, 1990)

ABSTRACT

Martin, B.,and Kushiro, I., 1991. Immiscibility synthesis as an indication of cooling rates of basalts. J. H)lcanol. Geotherm. Res., 45: 289-310.

Silicate liquid immiscibility of iron-rich and silica-rich compositions have been experimentally reproduced at various cooling rates to evaluate the possible effect of cooling rate on the size of the globules. The experiments were performed at constant cooling rates from 1 to 32°C/hr under atmospheric pressure and controlled oxygen fugacity. A tholeiitic basalt which occurs as a 36 cm thick dike in the lzu Peninsula (Japan) was used as starting material. The sample was melted at the liquidus temperature (1190 ± 5°C) and cooled to 1000°C.

Two immiscible liquids occur in the charges, as do in the dike. In the experiments, the size of the globules is strongly related to the cooling rate; the diameter of the globules decreases exponentially with increasing cooling rate and the number of globules decreases with decreasing cooling rate. These facts together with the typical coalescence shape suggests a growth of globules by coalescence rather than a decrease of the nucleation rate. Compositional variations of the liquid just prior to splitting within each charge do not affect those of the two immiscible liquids. However, the compositional differences between the two liquids slightly increase with decreasing cooling rate.

Using the cooling rate vs size relationships, the size of globules in basalts allows a cooling rate estimate. In the lzu dike, the 10 largest silica-rich globules range from 2.3 iLm to 0.8 #m in the center and the margin, suggesting cooling rates of about 15°C/hr and greater than 32°C/hr respectively, at temperatures near 1000°C. Other estimates from natural samples show a good agreement between deduced cooling rates and observed textures.

I n t r o d u c t i o n

Since the discovery of natural liquid immiscibility by Roedder and Weiblen (1970) in lunar rocks, other occurrences have been reported in many terrestrial rocks (e.g., Roed- der and Weiblen, 1971; De, 1974; Philpotts, 1979; Fujii et al., 1980). Immiscibility has been found mainly in tholeiitic and high- alumina basalts, but also in alkalic and calc- alkalic rocks (e.g., Philpotts, 1971; Fujii et al., 1980). Following the pioneering experiments on immiscibility by Greig (1927) and Roedder (1951) regarding the compositional fields for two-liquid region in synthetic systems, several

(t377-(1273/91/$03.50 © 1991 - Elsevier Science Publishers B.V.

experimental studies have been performed (e.g., Roedder and Weiblen, 1970; McBirney and Nakamura, 1974; Naslund, 1976; Watson, 1976; Visser and Koster Van Groos, 1976). Roedder and Weiblen (1970) found that under equilibrium conditions, immiscibility occurs at late stage of crystallization in lunar rocks at temperatures between 1135 and 1045°C. Immiscibility was also the subject of many experimental investigations with a view to understand the lack of intermediate compositions between basic and acidic rocks in intrusive suites (e.g., McBirney and Nakamura, 1974; Philpotts, 1979; Dixon and Rutherford, 1979). Effects of fo2, pressure and chemi-

290

cal composition on liquid immiscibility have also been examined. For example, Naslund (1976) found that the immiscible region expands with increasing fo2 in the KzO-NazO- AI203-SiO2-FeO-Fe203 system. Watson and Naslund (1977) found that the immiscible region expands with increasing pressure to 5 kbar in the K20-FeO-AI203-SiO2 under CO2- saturated conditions. Details of these studies as well as others are given in a comprehensive review by Roedder (1979).

These previous experiments focused on the conditions required for immiscibility to occur and the composition of two immiscible liquids. Most of these experiments were performed under near-equilibrium conditions. However, in extrusive volcanic rocks and shal- low intrusive bodies, rapid cooling involves non-equilibrium conditions. We report here the results of liquid immiscibility synthesis performed at various cooling rates showing a strong dependence of globule size upon cooling rate. We show that this result is in good agreement with the observations of a 36-cm-wide tholeiitic basalt dike, wherein the globule size increases from the margin to the center. The variolitic textures in dikes and pillows described in the literature as products of liquid immiscibility support these conditions.

Observation in natural samples

Previous observations of globular textures in dikes and pillows

Globular textures are commonly inter- preted as resulting from liquid immiscibility. Size variation of globules in lamprophyre dikes was early described by Zimmerle (1958), Philpotts (1971), Velde (1971) and more recently by Carstens (1982). In pillow lavas similar phenomena appear. Their textural zonations are now commonly explained by a cooling rate effect (Lofgren et al., 1974). Mdvel (1975) observed variolitic texture in spilitic pillow lava, and attributed the size

B. M A R T I N A N D I. K U S H I R O

140"

35"20

1 ~0"

130" 140"

Fig. 1. Location of the Ajiro dike, Izu Peninsula, Japan.

variation of the globules to the cooling rate. Globules in the lamprophyre dikes and pillow lavas are crystallized. Chlorite substitutes for glass (M6vel, 1975) and suggests the former existence of a liquid. Consequently, the size variation of immiscible globules expected by Roedder (1979) in dikes, pillows or flows has indeed been found in nature.

The Ajiro basalt dike and liquid immiscibility

A fresh tholeiitic 36-cm-wide basalt dike (1-2 Ma; Nakano et al., 1980) has been sam- pled in Izu Peninsula, Japan (Fig. 1). The bulk-rock composition is shown in Table 1. A high alumina content (16.01% A1203 and a low alkali content (0.30% K20 + 2.11% Na20) characterize this particular tholeiite which belongs to the Ajiro group originally described by Kuno (1950). Note also the high FeO*/MgO ratio (2.7) and the slight oxida- tion shown by the Fe203/FeO* ratio. This ox- idation induces the presence of quartz in the norm although this tholeiite is olivine-bearing (0.4% in volume). This aphyric basalt shows a

IMMIS('IBII.I 1Y SYNTIIESIS AS AN INDICATION OF CO()I.ING RATES OF BASAl I'S 291

Fig. 2. SEM photomicrographs of natural immiscible liquids: photos 1-4 illustrate the iron-rich globules in the Ajiro dike: note the size variation between the margin (left) and the center (right).

I-ram-wide chilled margin, a brownish interstitial glass (11% modal percent) inward, mi- crocrysts and rounded xenocrysts of olivine, as well as augite, subcalcic augite, pigeonite and Ti-magnetite dendrites and phenocrysts.

The Ajiro basalt dike contains evidence of two immiscible liquids, except in the chilled margin (Fig. 2). The globules are glassy and crystal free. Iron-rich and silica-rich globules coexist in the dike, but the iron-rich globules prevail over the silica-rich ones. We observed under the microscope several thin sections in order to find out and measure the largest globules which are present in the mar-

gin and in the center of the dike. The silica- rich globules were directly measured under transmitted light using 400 × magnification. The iron-rich globules were measured with a SEM using a magnification of 2000. SEM photographs were taken in all parts of the thin sections, and used to measure the largest size of the iron-rich globules in the margin and the center. The uncertainty to indeed find the largest globule in each part of the dike led us to consider the 10 largest globules, a number arbitrarily chosen. The average of the 10 largest silica-rich globules is 2.3 itm in the center, and 0.8 Itm in the margin, the max-

292

TABLE 1

Bulk rock composition of basalts

1 2 3 4

Whole-rock composition Si02 50.45 52.29 50.30 48.76 TiO2 1.31 1.17 1.06 1.39 AI2 03 16.01 14.75 15.80 18.53 Fe203 5.72 1.88 nd 4.12 FeO 7.76 10.56 8.75 8.07 MnO 0.23 (I.22 0.14 0.19 MgO 4.74 5.30 9.17 5.47 CaO 10.02 9.89 11.86 10.01 Na20 2.11 2.60 2.36 2.69 K20 0.30 I).33 0.06 0.72 H 2 0 - 0.47 0.54 nd - H2 O+ 0.69 (/.64 nd 0.40 P205 0.11 0.16 0.28 0.28 Total 99.92 100.33 99.86 100.63

CIPW norm: Q 8.63 4.25 - - Or 1.77 1.95 0.33 4.28 Ab 17.85 21.97 19.96 22.74 An 33.32 27.63 32.30 36.31 Mt 8.29 2.73 2.18 * 5.99 Di 12.78 17.00 19.58 9.45 Hy 13.34 21.00 18.85 17.38 OI - - 3.94 0.83 II 2.49 2.22 2.02 2.64 Ap 0.26 [).37 0.84 0.67

1. Starting material: tholeiitic basalt dike, Ajiro. 2. Rattlesnake Hill basalt, Connecticut (Philpotts, 1979). 3. MORB (Dixon and Rutherford, 1979); * norm calculated. with 1.5% Fe203 (Kay et al., (1970). 4. Olivine basalt, Fuji volcano (Kuno, 1960).

imum diameter being 8.0 #m and 1.2 Izm, respectively. The size variation is also veri- fied for the iron-rich globules. The maximal diameter of iron-rich globules is 5.2 #m and 2.0 #m in the center and the margin, respectively. The bulk composition of rock was determined across the dike from one margin to the other (Fig. 3) taking samples every 3 cm. Small compositional variations of K20 and Fe203 are insufficient to explain the size variation of globules in this dike. Variations of the number of globules were not determined.

Figure 4 illustrates the two immiscibility fields discovered by Greig (1927) and Roed- der (1951) in the (TiO2 + FeO + MnO

B. MARTIN AND I. KUSHIRO

52 o~

o 51

1.5 0 1.3

12

o 0.19

0.18 o

~ 10.2 0 10.0 0 ~ 2.2

~ 2.1

~ 0.3

~ 0.1 0 0.1

a. 0-0

°oOOOOOOOoO °

OoOOOOoOOOo o°

o o o o o o o o O O o ° °

° O o o o O o O o O o O °

o o o o o o o o o o o o o

O o o ° o o o O ° ° o o o

OOoOoOOOOO ° °

° ° O o o o ° o o o o o o

o o O ° ° ° ° O o O O o o

O o o o o o o o o o o o o

3 9 15 21 27 33 39

DISTANCE (era) Fig. 3. Bulk composition of the Ajiro dike, every 3 cm from one margin to the other (36 cm wide).

TiO'2* FeO ÷ MnO* MgO

• CaO,-P205

50

4O

3O

20

10

so 40 50 .o ~o 80 90 ( S )

N a L K ~ K 2 0 . A I 2 0 3 S i O 2

Fig. 4. Glass composition in the chilled margin (open triangles) and the center (stars) of the Ajiro dike. Solid triangles: pair of immiscible liquids in lunar basalts analyzed by Roed- der and Weiblen (1971). Solid squares: immiscible liquids of the Fuji basalt analyzed by Fujii et al. (19801.

I M M I S C I B I L I T Y S Y N T H E S I S A S A N I N D I C A T I O N O F ( ' 0 O L I N ( ; R A T E S O F B A S A U I ' S 293

+ MgO + CaO + P2Os)-(Na20 + K20 + AleO3)-SiO2 diagram (M-A-S diagram, here- after) and the projected glass compositions determined at various distances from the contact. Analyses of immiscible liquids found in the lunar basalt by Roedder and Weiblen (1971) and the Fuji basalts by Fujii et al. (1980) are plotted for comparison. The compositions of the chilled margin (triangles) scatter in the middle of the immiscible field, from a point close to that representing the starting material to more siliceous compositions. The composition of the interstitial glass 3 cm from the contact (stars) also has a wide compositional range (Fig. 4). Such compositions probably represent a mixture of the two immiscible liquids in various proportions. Further inward, from 3 cm to the center, the glass is more silica-rich. This composition suggests either a differentiated liquid or the silica-rich pole of immiscibility where large in- terstices permit better analyses. The globule size is too small to judge whether the effect of quenching increases the compositional difference between globules and matrix, as suggested by Gelinas et al. (1976).

The fine-grained texture of the dike attests to the rapidity of the cooling as expected by the chilled margin and the thinness of the dike (36 cm). The globule size in the Ajiro basalt dike is small compared to those described in other basalts by Roedder (1979). Texture and size of globules thus appear to be related.

It is interesting to note that, to the best of our knowledge, liquid immiscibility has never been reported from alkali basalts. A study of a 28-cm-wide dyke of alkali basalt (Mar- tin, 1987), in many respects similar to the Ajiro dyke, has failed to show any hetero- geneity in the glass, an indirect confirmation of the probable absence of this phenomenon in alkali basalts, unlike some alkaline lamprophyre where such immiscibility is established.

Experimental method

The starting material for the present experiments is a sample from the margin of the dike, larger than the chilled margin (Ta- ble 1). The experiments were conducted at constant cooling rates from I to 32°C/hr at atmospheric pressure. The oxygen fugacity was controlled with a CO2/H2 gas mixing method. We adopted a constant ratio of CO2/ H2 rather than constant fo2 in the cooling experiments, because it reproduces more closely the f% of cooling lavas. Two series of experiments were carried out with CO2/H2 ratios of 10.88 and 14.00. The oxygen fugacity of the experiments with the former ratio is close to that determined by Philpotts (1979) for a basalt from Rattlesnake Hill in the temperature range between 1150 and 1070°C (Fig. 5; Table 1). The experiments with the higher ratio were designed to investigate the effect of slightly higher fo2, assuming that only a small variation of oxygen fugacity might occur in a thin dike.

o

==

¢0

E I-

1200

1150

1100

1050

1000

950

900 15

, , , , , , ,

( I )

.,o.~;9o e" i i i i i i i

1 4 1 3 1 2 1 1 1 0 9 8

-log f02 Fig. 5. Results of equilibrium experiments made with the Ajiro basalt showing the stable phases at foz = 10-~ atm (solid squares) as well as the results obtained by Philpotts (1979) on a basalt from Rattlesnake Hill (thin lines). The two thick curves represent constant CO2/H2 ratios, 14.00 and 10.88, used for the present cooling experiments.

294 I~ M A R ' I I N A N D I. K U , g I I I R O

The samples were fine powder, less than 2 l~m in diameter. About 0.1 g of powder was pressed into pellets 5 mm in diameter or sintered in a Pt foil at room temperature. Each sample was hung with a 0.1-mm-thick Pt wire in a vertical furnace using the wire- loop method (Donaldson et al., 1975). This method reduces the loss of FeO from charge to Pt, which is estimated at less than 1%. The charges were melted at temperature just above the liquidus (1190 ~: 5°C) to produce homogeneous liquid. Temperature was measured and controlled with a Pt/Pts7 Rh~3 ther- mocouple. Almost all samples were cooled to 1000°C which is below the solidus (1050 + 10°C). The total duration of the run ranges from 3 to 190 hours. Finally, the charge was quenched into cold water when the final temperature was reached. The run products were cut in half with a thin diamond saw and pol- ished for microscopic observations under reflected light. The glassy globules, matrix and minerals were analyzed using a JEOL JSM- 840 Scanning Electron Microscope equipped with an energy-dispersive spectrometer. An- alytical conditions were 15 kV acceleration voltage, 1 n A current, a minimum size electron beam (2 #m) and a 100-s counting time. The minerals and the glass of the natural dike were analyzed with a JEOL JCMA 733 MK-II electron microprobe using the following conditions: 15 or 20 kV acceleration voltage, 20 or 40 n A current depending on minerals, a 20-s counting time for major elements and 50 for minor elements.

Experimental results

The liquidus and solidus of the starting material were determined in equilibrium experiments using an oxygen fugacity of 10 ~ atmosphere, a value which is close to the wfistite-magnetite buffer at temperatures near 1200°C. The liquidus temperature of plagioclase was found at 1190 4- 5°C, and the solidus temperature was determined as 1050 + 10°C

(Fig. 5). The results of equilibrium and cooling experiments are given in Table 2A-B, respectively. Note that under non-equilibrium conditions glass is preserved at temperature of 1000°C, which is far below the stable solidus.

Immiscible liquids in the cooling experiments

In the cooling experiments, the charges quenched at a temperature of 1000°C contain two different glasses located in the interstitial areas between crystals of plagioclase, pyroxene and oxide. The immiscibility is evident under reflected light owing to the sharp contrast of reflectivity between the dark silica-rich glass and the clear iron-rich one (Fig. 6). Liq- uid immiscibility may take two aspects. One is a silica-rich liquid which forms numerous globules surrounded by an iron-rich glassy matrix, and vice versa. In the experimental charges, the common occurrence is that of silica-rich globules in iron-rich glass. In the dike, however, the iron-rich globules prevail over the silica-rich globules. Note that a calcic pyroxene is found within the dike, whereas it is absent in the charges. The silica-rich globules are similar in size and shape, and distributed throughout the charge. The glasses form areas between crystals. Similar globule size appear within each area, but the size varies from one area to another. The silica-rich globules are largest near the rim of the charge than in the center. The globules are frequently attached to plagioclase crystals, as observed by McBirney and Nakamura (1974), and are semi-spherical if they are attached to plagioclase laths. Glob- ules are not observed close to pyroxene crystals: the same observation was made by Roed- der (1984) and the proposed explanation is that if a pyroxene crystallizes in an iron-rich liquid which contains 80% normative pyroxene, the evidence of the liquid former existence is eliminated. Iron-rich globules are heterogeneously distributed at the rim of the charge, from the surface to a distance of about

IMM[SCIBII.ITY SYNTHESIS AS AN INDICAq ION OF COO[.INO RATES OF BASALTS

T A B L E 2

Experimental results

A. Results o f the equi l ibr ium experiments

295

Run no. Temperature fo2 = 10-9 Duration Products (°C) (atm) (hr)

CO2/H2

I 1210 10.5 3 GI 3 1190 15.3 8 G1 + Pl + Px 8 1310--,11911 14.6 1~20 GI + Pl 5 1311---,1188 15.3 1--,20 GI + PI

II 1187 14.0 48 GI + PI 13 1185 14.6 96 GI + PI + Px 2 1163 22.7 15 GI + PI + Px 6 1320---,1163 22.7 1430 GI + PI + Px 4 1137 34.4 40 G1 + P1 + Px 7 1308--,1133 34.4 1---,48 GI + PI + Px

28 1066 169 24 melted 27 1066 147 24 melted 25 1060 134 24 melted 16 1059 139 48 melted 23 1042 198 24 not melted 18 1038 198 48 melted 21 1027 268 24 not melted 20 1010 268 24 not melted

B. Results of the cooling experiments

Run no. Cooling rate Temperature (°C) CO2/H: Products (°C/hr)

initial quench

33 16 1208 1025 14.00 PI 34 8 1210 1025 14.1)0 PI 32 32 1210 1020 14.01t PI 35 6 1212 1020 14.00 P1 36 1 1200 1014 14.00 PI 68 32 1200 1010 14.00 PI 37 4 1190 1010 14.00 PI 39 2 1190 1010 14.00 PI

56 2 1190 1000 14.00 P1 42 6 1192 1000 14.00 PI 41 8 1192 1000 14.00 PI 55 12 1190 1000 14.00 PI 54 32 1190 1000 14.00 P1 69 64 1200 1000 14./10 PI

38 12 1192 1010 11).88 PI

311 1 1205 1000 10.88 PI 24 8 1187 1000 10.88 PI 411 12 1194 986 10.88 PI 57 12 1190 1000 10.88 PI 29 32 1195 1000 10.88 PI

Px GI Px GI Px GI Gx G1 P x T m G I L1 L2 PxTmG1 LI L2 PxTm G I L l L2 PxTm G I L l L2

Px Tm LI L2 Px Tm LI L2 Px Tm L1 L2 Px Tm LI L2 Px Tm LI L2 Px Tm LI L2

Px Tm GI

Px Tm L I L2 Px Tm L1 L2 Px Tm L1 L2 Px Tm LI L2 Px - LI L2

10-50 pm inwards. The distance of 10-50 Izm from the rim of the charge seems to be the region where each type takes over from the

other. Coalescence and irregular shapes de- velop commonly in this outer zone, and are seldom observed elsewhere.

296 B. MARTIN AND 1. KUSHIRO

Fig. 6. Experimental liquid immiscibility: photomicrographs 1-4 show silica-rich globules in the charges at I, 8, 12 and 32°C/hr.

In the experiments immiscibility does not occur when the charge is quenched above 1014 ° . The residual liquid (27.1% in volume) becomes immiscible at a temperature of 1010°C after the crystallization of plagioclase (47.1%), clinopyroxene (25.1%) and ti- tanomagnetite (< 1%). This temperature is in good agreement with those determined by Dixon and Rutherford (1979) for MORB basalts (Table 1, analysis 3) cooled under 1 and 2°C/hr rate. Dixon and Rutherford found that the immiscible liquids appear after crystallization of olivine, plagioclase, pyroxene, and ilmenite. Above 1010°C, according to these authors, the residual liquid is homogeneous. In the present experiments, the glass is

still homogeneous in the center of the charge at 1010°C. Iron-rich globules occur first in the rim without reaching the center of the charge. At 1000°C, immiscibility with silica-rich globules extends throughout the charge. Oxide minerals continue to crystallize below 1000°C. Consequently, in the present cooling experiments the upper limit of the two-liquid region (1010°C) is lower than that (1035°C) determined by Philpotts (1979) under equilibrium conditions (Table 1, analysis 2). Even though the SiO2 content is 2% lower and A1203 is 1.25% higher than in the Rattlesnake basalt, the phase diagram should not be significantly different and Philpott's diagram may serve as a basis for the present discussion. The tern-

IMMIS('IBILI FY SYNTHESIS AS AN INDICATION OF COO[_ING RATES OF BASALTS 297

perature difference simply reflects the non- equilibrium conditions of the present experiments.

Size distribution of globules with cooling rate

In each charge cut in the middle, the prevalent silica-rich and peripheral iron-rich globules were counted and measured within a given area chosen as comprising a small number of crystals. Three SEM photographs were taken using a magnification of 2000 in three different domains of each charge (2 mm in diameter), for the silica-rich and the iron-rich globules, respectively. The total area wherein the globules were counted and measured cor- responds to a surface of 7200 itm 2. Several hundreds up to one thousand of globules are present in the areas thus defined, depending on the cooling rate.

The main result of this study is that the globule size increases with decreasing cooling rate. Figure 6 presents micrographs showing charges cooled at 32, 12, 8 and l°C/hr. The diameter of the silica-rich globules clearly decreases with increasing cooling rate. Tiny globules are difficult to measure when the charge is cooled at 64°C/hr. This difficulty could explain why no size variation between 32°C/hr and 64°C/hr has been detected. Fig- ure 7 illustrates the fact that the average diameter of silica-rich globules is related to the cooling rate by an exponential law. This relationship also holds if a selection of the 10 largest silica-rich globules from each sample is used (Fig. 8). The curve only shifts to larger diameters for a given cooling rate, suggesting that a rapid estimation of the cooling rate might be effectively made by counting only the 10 largest globules. The variation in globule size is inversely correlated with their number: the slower the cooling rate, the less abundant the globules are (Figs. 9, 10). The iron- rich globules were not measured in run 29 (32°C/hr), which contains an unusually small number of iron-rich globules.

Figure 7A, B shows that oxygen fugacity has an effect on the size of the silica-rich globules: the size decreases with increasing fo_,. For example, the average diameter of the 10 largest globules diminishes from 3.2 to 2.1 ILm as CO2/H2 increases from 10.88 to 14.00, at 8°C/hr. This effect becomes smaller with increasing cooling rate, and for cooling rates higher than 20°C/hr there is no detectable in- fluence. The size of iron-rich globules are not sensitive to the variation of oxygen fugacity within the range studied: their size remains identical for both fugacities at 1 and 8°C/hr (Fig. 7C, D).

The precision of the curves depends on the cooling rate. The standard deviation increases with decreasing cooling rate (Table 3). How- ever, for a given diameter of globules the de-

TABLE 3

Average diameter of globules and their standard deviation in the charges

Run Cooling Aver. diam. s.d. Number rate (#m) (°C/hr)

Silica-rich globules (CO2/H 2 = 1(I.881 29 32 0.37 0.26 327 40 12 0.80 0.41 183

7 12 0.56 0.37 444 24 8 0.93 1.64 254 3(1 1 1.63 1.63 175

Silica-rich globules (CO2/H2 = 14.011) 69 64 0.52 0.21 285 54 32 0.41 0.23 5114 55 16 0.44 0.35 874 41 8 0.45 0.37 327 42 6 0.54 0.411 256 56 2 1.25 0.91 276

Iron-rich globules (CO2/H 2 = 111.88) 40 12 0.68 0.41 922 24 8 0.53 0.34 851 311 1 1.00 0.58 769

Iron-rich globules (CO2/H 2 = 14.00) 54 32 0.34 0.24 277 55 16 0.38 0.26 639 41 8 0.69 0.48 696 42 6 0.64 0.49 894 56 2 0.84 0.91 689

298 B. MARTIN A N D 1. K U S H I R O

40

O

.~ ao

_== g 20 O

A

60

50

10

0.2

CO2/H2=14

i i ,

0.4 0.6 0.8 1.0

i i l i

1.2 1.4 1.6 1.8

Average diameter of g lobu les (Urn)

O

._== i o

B

40

30

20

10

0

0.2

CO2/H2=10.88

- ° ' 1

0.4 0.6 0.8 1.0 1.2 1,4 1.6 1.8

Average d iamete r of g lobules (~Jm)

40

30

2o

g

0 t 0

0.2

- - - i

0.4 0.6 0.8 1.0 1.2

CO2/H2=t 4

40

30 J= O

20

g

o 10

0 i i i

1.4 1.6 1.8 0.2

Average diameter of g lobules 0Jm)

CO2/H2=t 0 .88

©

i i -- i i i i

0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

Average d iameter of g lobu les (urn)

40

3O

o G e

20

[ 0 10

I E ,

' CO2/H2=4.78

Lc-Fa-Qz system

i i { ~ - - - 1 2 3

Average diameter of globules (}Jm)

Fig. 7. Relationship between globule size and cooling rate, using the average diameter. (A) Silica-rich globules in Ajiro experiments with CO2/H2-14.00; (B) as A with CO2/H2 = 10.88. (C) Iron-rich globules with CO2/H2 = 14.00; (D) as C with CO2 H2 = 10.88. (E) Silica-rich globules with CO2/H2 =4.78 in the synthetic system Lc26-Fa40-Qz34 (wt.%).

IMMIS('IBILITY SYNTHESIS AS AN INDICATION 0 | ' COOI.ING RATES OF BASAI3"S 299

.c

o~

==

0

60

50

40

30

20

10

A ,a C02/1.'12=14

i i i i i i i

0 1 2 3 4 5 6 7

A v e r a g e d i a m e t e r of the 10 l a r g e s t globules (pm)

(3 o

4)

=- := o o (3

40

30

20

10

C02/H2=10.88

1 2 3 4 5 6 7

A v e r a g e d i a m e t e r of t h e 10 l a r g e s t globules (pro)

0

®

=_=

0

C

40

30

20

10

0

C021H2=14

©

i i i i

1 2 3 4 5 6

A v e r a g e d i a m e t e r o f t h e 1 0 l a r g e s t g l o b u l e s OJm)

4 0

30

20

10

C02/H2=14

Fuj i

0

0

0

0 0 0 ,

1 2 3 4 5 6 7

A v e r a g e d i a m e t e r of t h e 1 0 l a r g e s t globules (urn)

Fig. 8. Relationship between globule size and cooling rate, using the average of the 10 largest globules. (A) Silica-rich globules with CO2/H2 = 14.00; (B) as A with CO2/H 2 = 10.88. (C) Iron-rich globules with CO2/H2 = 14.00.

termination of the cooling rate is more impre- cise at high cooling rate than at low cooling rate, because of the shape of the curve.

Composition of homogeneous glass, globules and their matrix

The composition of several homogeneous glasses quenched at temperatures above 1010°C with C O 2 / H 2 = 14 are given in Ta- ble 4A. Glass analyses from runs 39, 37, 36,

35, 32 and 33 provide the composition of the liquid just prior to splitting, since only a few globules are found in these glasses. To test the compositional uniformity of the charge, the glass was analyzed at different locations within run 39. Minor variations are found between glasses surrounded by plagioclase and those by iron-bearing minerals. The composition of the homogeneous glass surrounded by plagioclase is similar in runs 39, 37, and 38. An average of 3 analyses from 36 is plot-

3{)0 B. MARl'IN AND I. KUSHIRO

TABLE 4

Composition of homogeneous glasses

A. Average composition and standard deviation 1020, 1025, and 1036°C

(sd) of homogeneous glasses prior to splitting for charges quenched at 1010, 1014,

Quench T: 1010°C 1010°C 1014°C 1020°C 2°C/hr 4°C/hr l°C/hr 6°C/hr #39 #37 #36 #35 (3pts.) sd (6pts.) sd (3pts.) sd (3pts.) sd

SiO2 TiO2 A1203 FeO" MnO MgO CaO Na20 K20 P2 05 Total

55.43 (0.92) 54.49 (1.00) 55.42 (0.47) 52.96 3.39 (0.33) 4.79 (0.09) 4.74 (0.10) 4.13 8.89 (0.47) 8.92 (0.42) 8.98 (0.23) 9.36

18.15 (0.55) 19.70 (0.74) 18.48 (0.38) 21.01 0.31 (0.11 ) 0.27 (0.09) 0.34 (0.14) 0.28 0.79 (0.11) 0.78 (0.18) 0.84 (0.13) 0.95 7.18 (0.171 6.66 (0.10) 6.73 (0.111 7.15 1.88 (0.07) 1.97 (0.12) 1.71 (0.14) 1.85 0.94 (0.02) 1.09 (0.07) 1.08 (0.05) 1.01 0.47 (0.25) 0.30 (0.21) 0.39 (0.29) 0.35

97.43 98.97 98.71 99.05

(0.56) (0.16) (0.29) (0.08) (0.17) ({/.081 (0.11) (0.15) (0.06) (0.20)

1020°C 1025°C 1025°C 1036°C 32°C/hr 8°C/hr 16°C/hr l°C/hr #32 #34 #33 #17 (10pts.) sd (3pts.) sd (Spts.) sd (7pts.) sd

SiO2 52.46 (0.72) 52.43 (0.75) 53.41 (0.49) 54.72 (0.82) TiO2 4.17 (0.19) 4.05 (0.19) 4.19 (0.08) 4.00 (0.12) A1203 9.24 (0.21) 9.29 (0.25) 9.57 (0.24) 9.72 (0.36) FeO* 21.14 (0.69) 21.02 (0.49) 19.84 (0.37) 20.59 (0.44) MnO 0.24 (0.09) 0.25 (0.08) 0.24 (0.06) 0.31 (0.10) MgO 0.91 (0.16) 1.05 (0.19) 1.18 (0.15) 1.26 (0.14) CaO 7.21 (0.23) 7.21 (0.03) 7.22 (0.08) 7.33 (0.23) Na20 1.98 (0.141 2.01 (0.14) 1.99 (0.17) 2.13 (0.20) K20 1.08 (0.06) 0.92 (0.05) 1.07 (0.04) 0.91 (0.05) P205 0.34 (0.18) 0.21 (0.02) 0.31 (0.17) 0.27 (0.13) Total 98.77 98.44 99.02 101.24

B. Representative analyses of homogeneous liquid for charges quenched at 1010°C with CO2/H2 of 14 and 10.88, respectively

(CO2/H z = 14.00) (CO2/H2 = 10.88) Run 37 Run 38

SiO2 52.66 53.08 53.33 52.78 53.73 TiO2 4.44 5.00 4.69 4.41 4.24 A1203 8.73 8.47 8.62 8.79 9.35 FeO * 20.97 21.31 21.28 21.72 20.89 MnO 0.15 0.18 0.30 0.08 0.09 MgO 0.81 0.46 0.71 0.80 1.10 CaO 7.17 7.05 6.78 7.24 7.17 Na20 2.47 1.79 1.95 1.67 1.92 Ke O 1.03 0.96 1.12 1.05 1.02 P205 0.60 0.28 0.31 0.25 0.37 Total 99.03 98.58 99.09 98.79 99.88

IMMISCIBILITY SYNTHESIS AS AN INDICATION OF COOLING RATES OF BASALTS 3111

TABLE 5

Composition of the most silica-rich globules and iron-rich matrix at various cooling rates. For a CO2/H 2 ratio of 10.88: l, 8, 12

and 32°C/hr; and for a CO2/H2 ratio of 14.00:2 and 8°C/hr

Sifica-rich globules analyses

CO2/H 2 = 10.88 CO2/H 2 = 14.00

l°C/hr 8°C/hr 12°C/hr 2°C/hr 8°C/hr #30 #24 #57 #56 #41

SiO2 68.11 64.71 66.14 69.36 65.73 TiO2 2.11 2.70 2.51 1.69 2.90 AI2 03 11/.07 1 (/.32 10.21 10.36 9.30 FeO * 9.27 11.14 10.70 8.52 10.80 MnO 0.19 0.08 0.05 0.34 0.23 MgO 0.11 0.41 0.32 0.09 0.26 CaO 3.86 4.60 4.15 3.69 4.58 Na20 1.90 2.13 1.98 2.09 2.04 K20 2.11 1.94 1.73 1.93 1.76 1'205 - 0.21 0.24 0.25 0.21 Total 97.73 98.24 98.03 98.32 97.81

Iron-rich liquid analyses

CO2/H2 = 10.88 CO2/H2 = 14./)0

l°C/hr 8°C/hr 12°C/hr 32°C/hr 2°C/hr 8°C/hr #30 #24 #57 #29 #56 #41

SiO2 47.36 45.90 48.56 46.58 44.93 47.14 TiO2 7.07 7.18 5.87 6.83 6.35 6.36 AI2 03 7.26 7.04 7.52 7.75 6.73 7.21 FcO 26.73 28.94 25.62 26.28 25.76 25.64 MnO 0.43 0.19 0.33 0.21 0.39 0.40 MgO 0.51 0.53 0.96 1.12 0.84 0.74 CaO 8.81 8.74 8.35 8.03 9.20 8.56 Na20 1.05 1.30 1.65 1.77 1.62 1.30 K 20 0.50 0.49 0.58 0.73 0.54 0.61 1,205 0.52 0.55 0.70 11.59 1.11 0.71

Total 100.24 100.86 100.14 9.89 97.47 98.67

ted in Figure 11. It should be emphasized that, although the composition of the liquid varies slightly within the charge, once the two liquids split their composition remains constant for a given temperature and only their relative proportion varies. The variation of oxygen fugacity does not change the composition of the homogeneous glass in the studied range (Table 4B).

Glass compositions of globules and matrix are listed in Table 5. The iron-rich globules observed in the peripheral area of the charges have the same composition as the iron-rich matrix found in the center of the

charge. The same liquid this takes two forms in the peripheral and central zones of the charge. Similar observations can be made for the silica-rich liquid. Both the silica-rich and iron-rich glasses lie within the immiscible field mapped by Roedder (1951) in the A-M-S diagram. Tie lines between the silica- rich and iron-rich compositions defined in the present experiments are roughly paral- lel to those found for the immiscible liquids in Apollo 11 basalts (Fig. 11). The compositional gap between the two immiscible liquids is however wider in lunar basalts than in the Ajiro basalt. The widest compositional gap is

31)2

run 29

1OOOO /mw~ ]

o - - ] 3 2 e C l h r

run 57 I I h 1 2 " C / h r

i l i I-.--,

run 2 4

run 30 0 0 .8 1.6 2 .4

8 e C l h r

m

3.2 4.0

lOC/hr

4.8 5 .6

G l o b u l e d i a m e t e r ( ~ m )

Fig. 9. Distribution of globule diameters for charges cooled at 32, 12, 8 and l°C/hr with CO2/H 2 ratio of 10.88. The counting is based on 327, 444, 264 and 175 globules in the run products 29, 57, 24 and 30. The histograms are drawn with values calculated for a l-ram 2 surface. With decreasing cooling rates, the number of globules decreases and their size increases.

shown in the Fuji basalt (Fujii et al., 1980) (Fig. 12): the silica-rich liquids plot close to those in the lunar basalt in the A-M-S diagram whereas the iron-rich liquids contain more iron and PzO5 (up to 5.16%) than the lunar basalts. We found that the composi-

Fig. 10, Relationship between the number of silica-rich globules and cooling rate. Number of globules per surface (includ- ing crystals) in Ajiro experiments: (A) with CO2/H 2 = 14.00; (B) with CO2/H 2 = 10.88. Note a slight curvature at low cooling rates, (C) Number of globules per surface (free of crystal), in the synthetic system Lc26-Fa40-Qz34. The number of globules decreases with increasing cooling rate.

3O

c J= o 20 ®

L1

111

J: (9 & 20

o

B. MARTIN AND I. KUSHIRO

A ' '

/ ~ C02/H2=14"00

0 i i i 0 1000 2000 3000

Number of globules (XlO2/mm 2)

3O

10

C02/H2=10 88

0 t i A 0 1000 2000 30oo

Number of globules (X102/mm)

30

10

0 0 1000 2000 3000

0 "~ 2 0 ®

Number of globules (XlO21mm2)

IMMIS('IBII.ITY SYNTHESIS AS AN INDICATION OF COOLING RATES OF BASAIXS 303

TiO2,FeO+MnO*MgO

• c a ~ o s

...w ~" 0

' ~ ' 0

/ . . . .7 .... .:., ;.. ...;. ,,.,o. (A) (s)

40 50 60 70 8 0 90 80 90

Na20+ K20 *AI203 SiO 2

Fig. I 1. Composition of starting material, homogeneous liquid, and immiscible liquids of Ajiro experiments in the A-M-S diagram. The compositional gap between the pairs of immiscible liquids increases with decreasing cooling for various oxygen fugacities. Left: CO2/H 2 = 14.00. Right: CO2/H2 = 10.88. Solid star: Ajiro starting material. Solid squares: composition of homogeneous glass prior to splitting (average of 3 analyses of the run at 1014°C; run 37 at 1010°C). Solid dots and white triangles: silica-rich globules and iron-rich matrix analyzed in the Ajiro experiments. Asterisks: analyses of natural immiscible liquids of lunar basalts from Roeddcr and Weiblen (1970).

TIO'2*FeO*MnO* MgO b~(M)

+C'0÷P205 / ~

70

60

50

40

3O

2O

(A) -. (s) 30 40 50 60 70 80 90

N~O.K20.̂ V~03 si%

Fig. 12. Composition of starting material, and immiscible liquids of Fuji experiments with CO_,/H2=14.00 in the A-M-S diagram. The compositional gap increases with decreasing cooling rate. Asterisks: analyses of natural immiscible liquids of the Fuji volcano by Fujii et al. (1980). Solid star: Fuji starting material. Open circles and solid triangles: iron-rich globules and silica-rich matrix analyzed in the Fuji experiments.

tional gap between the iron-rich and silica- rich glasses increases slightly with decreasing cooling rate. This gap increases more significantly with increasing oxygen fugacity (Fig. 11), a result which agrees with results reported by Naslund (1976). Analyses published by Dixon and Rutherford (1979) (runs TR-18 and TR-21 at 1 and 2°C/hr) are consistent with an increasing compositional gap with decreasing cooling rate. The compositional variations observed with varying cooling rates show that SiO2 and K20 increase and P205 decreases slightly in the silica-rich globules, whereas the A1203, MgO, Na20 and K20 contents decrease slightly and CaO increases in the iron-rich matrix with lower cooling.

Textures and phase composition in the charges

The texture of the charges is micro- subophitic and changes slightly with cooling rate in keeping with observations made by

304 B. MARTIN A N D I. K U S H I R O

Lofgren et al. (1974). This minor textural variation might explain that the liquid line of descent is not modified, the liquid prior to splitting having the same composition at I°C/ hr and 32°C/hr. Nevertheless the crystal size changes in harmony with the globule size.

The compositions of the phases in the charges are as follows: plagioclase (from An~79 Ab4t.30r0~ to An77.4 Ab21.60r~.o, subcalcic pyroxene (from W0302 En21.3 Fs4~5 to Wos.3Enss.sFs35.9), Ti-magnetite and ilmenite. Augite pyroxene is absent from the charges though present in the dike. Plagio- clase is the first phase to crystallize, followed by clinopyroxene and then Ti-magnetite. Pla- gioclase never contains glass inclusions in the experimental charges.

For a given SiO2 content, the iron-rich glasses of the charges have higher Na20, K20, TiO2 and A1203 contents and a lower CaO content than those in the Apollo 11 lunar basalts analyses published by Roedder and Weiblen (1971). The silica-rich glasses of the present experiments are less silicic than those of the lunar basalts. In Apollo samples, the temperature at which the two liquids were quenched was probably different. Consequently, the compositional discrepancy between the present experiments and the natural samples should reflect either a different quench temperature or another liquid line of descent.

Results on an olivine-basalt from the Fuji volcano

Analogous experiments were performed on a Fuji olivine-basalt (Mishima lava flow) belonging to the high-alumina basalt series (Kuno, 1960, p.140 analysis la) which contains more A1203 (18.53%) and less Si02 (48.76%) than the Ajiro basalt (Table 1, analysis 4). In contrast with the experiments reported above, iron-rich globules were found throughout the charges. This result is, however, in good agreement with the result ob-

tained on the natural Fuji basalt in which the iron-rich type dominates. The size of globules in the charges is larger at 2°C/hr cooling rate than at 32°C/hr. However, the correlation with cooling rate does not follow the sim- ple law found in the Ajiro experiments for intermediate values. Silica-rich globules were observed in two runs only, and always in very small number. The main mineralogical difference between the Fuji and the Ajiro experimental products is the presence of olivine in the former. The compositional gap between the iron-rich and silica-rich liquids increases with decreasing cooling rate as it does in the Ajiro basalt (Fig. 12). However, important compositional variations of iron-rich glasses occur between 16°C/hr and l°C/hr, larger than recorded for the silica-rich liquid. The iron-rich glass contains more FeO and P20~ in the experiments, a fact that is observed in the analyses reported by Fujii et al. (1980) for immiscible globules in the Fuji basalt lava.

Results on the Lc-Fa-Qz system

Complementary experiments were performed in the synthetic system Lc-Fa-Qz to observe the behaviour of crystal-free immiscible liquids. The composition of the starting material, Lc26 Fa40 Qz34, plots in the center of the immiscibility region in the system Lc-Fa-Qz (Fig. 13). The samples were melted at temperatures near 1180°C and cooled to 1150°C with CO2/H2 = 4.78. The run dura- tions were short, between 1 and 30 hours. The run products contain two different glasses without significant amounts of crystals. The globules are iron-rich in a silica-rich matrix as in the Fuji experiments, but contrary to the Ajiro results. However, iron-rich globules may contain numerous tiny blebs of silica-rich composition in their rim (probably the composition of the matrix). The iron-rich globules are distributed homogeneously in the charge, but show a slight tendency to be concentrated in small zones. The nature of the relationship

IMMISCIBII_ITY SYNTHESIS AS AN INDICATION OF COOl.IN(; RATES OF BASALTS 305

Fe28iO 4

KAISi206 50 SiO 2

Fig. 13. Composition of the synthetic starting material (Lcz<,- Fa40-Qz34) in the Lc-Fa-Qz diagram and immiscible liquids at various cooling rates. Star: starting material. Open dots: I°C/ hr. Solid dots: 2°C/hr. Solid squares: 4°C/hr. Open triangles: 16°C/hr.

between globule size and cooling rate con- firms our previous results on the Ajiro basalt (Fig. 7E). The average of the iron-rich globules diameter varies between 3.16 and 0.24 pm for 1 and 32°C/hr respectively, a range significantly larger than that found for Ajiro and Fuji basalts. The major discrepancy with the results obtained on Ajiro and Fuji basalts is that the compositional gap does not increase with decreasing cooling rates for the Lc26 Fa40 Qz34 compositions (Fig. 13).

Discussion

The mechanisms of nucleation and growth of globules must be discussed before the relationship obtained in the present experiments is applied to natural basalts. The thermo- dynamic processes controlling the nucleation and growth of globules are similar to those of crystals (Roedder, 1979). The main difference between liquid globules and crystals is that the growth of globules may involve coalescence. Nucleation and growth are explained in term of free energy and interracial tension

by Philpotts (1976). The supersaturation nec- essary for globules to form can be produced by crystallization or supercooling, whereas the surface tension provides the energy to coalesce.

Growth mechanisms of globules

Nucleation. The globules nucleate sponta- neously as suggested by the homogeneous distribution of silica-rich globules within iron- rich liquid in the charges. The run products obtained from the synthetic Lc26Fa40 Qz34 composition show that the nucleation of immiscible globules is operative in the absence of crystals and for a short run duration (from 1 to 30 hours). A common result in all experiments is that the number of globules is high for high cooling rates and decreases with decreasing cooling rate. This correlation suggests a decreasing rate of nucleation with decreasing cooling rate. However, coalescence shapes and a large size distribution suggest a coalescence process rather than a variation of the nucleation rate.

The reason why iron-rich globules nucleate first in the peripheral zone of the charge (Ajiro experiments), and silica-rich globules appear in the centre of the charge is a possible consequence of the liquid composition. The composition of the liquid before splitting determines the proportion of the two immiscible liquids. The less abundant liquid fraction nucleates as globules rather than form- ing the matrix. When more pyroxene crystals form at the rim of the charges, iron is de- pleted in the residual liquid, and when the resulting liquid splits, the iron-rich liquid forms globules.

Growth by diffusion. In Ajiro experiments the compositional gap between the pair of liquids increases symmetrically with decreasing cooling rates. If our dynamic experiments can be related to a stable solvus, such a relationship suggests that the solvus of the

3 0 0 B. MARl ' IN AND I. KUSHI RO

two-liquid region may be symmetrical (Fig. 11). The results on the synthetic composition Lc26 Fa40 Qz34 do not show a similar trend, a fact which may be related to its composition. The analysis of silica-rich globules at 32°C/hr (run 29) are not plotted because the small globule size makes measurements unreliable. A variable compositional gap suggests that, either the cooling affects slightly the course of crystallization, or the composition of the immiscible liquids being fixed at given temperature, diffusion continues for low cooling rates. Whatever the cooling rate the homogeneous liquid before splitting is similar enough in the various experiments to allow for the rejection of the first hypothesis (Table 4A). We conclude that the liquid line of descent is not significantly changed between 1 and 32°C/hr following Dixon and Rutherford (1979) for values between 1 and 2°C/hr. The second hypothesis suggests that diffusion between the two liquids significantly helps the growth of globule for low cooling rates. A variable compositional gap is also found for high-alumina basalt composition of the Fuji experiments. Its higher viscosity necessitates more time to equili- brate. The viscosity of high-alumina basalt may so affect the growth of globules that the exponential relationship between cooling rate and globule size cannot be demon- strated. Moreover, iron-rich globules grow in a more viscous matrix in Fuji experiments, whereas silica-rich globules grow in a less viscous matrix in Ajiro experiments. The viscosity thus appears as a controlling factor for the growth of globules. In Fuji and lunar basalts, the larger compositional gap may reflect the more complete crystallization (closer to equilibrium), the samples being continuously cooled down to ambient temperature. This circumstance might explain why the silica-rich type is prevalent in our experimental charges, and the iron-rich one in the Ajiro dike.

Growth by coalescence. Evidence for coalescence is shown by photomicrographs and by size distribution. With coalescence, the number of globules decreases and the diameter distribution shows more scatter. The irregular forms of globules, which depend on their size, do not persist because of the surface tension (Philpotts, 1976). Coalescence is possible when two globules come into contact. The probability of collision depends on the initial size distribution, time duration, viscosity or mechanical constraints by crystallization. Then the coalescence uses the energy provided by the interfacial tension of the globules.

(1) Initial size distribution: in the Ajiro experiments, the silica-rich globules prevail over the iron-rich ones, increasing their probability of coalescing.

(2) Time duration: results of an additional run at 64°C/hr show that the size of globules at 32°C/hr and 64°C/hr are similar. A first possibility is that there is a cooling rate limit above which the size of globules does not vary. Above a cooling rate of 32°C/hr, the globules being homogeneous and constant in size, growth by diffusion might prevail over the coalescence, and the coalescence may become operative below cooling rates of 32°C/hr. A second possibility is that the time difference between 32 and 64°C/hr is not large enough to produce a different result. The growth duration depends on the temperature at which the liquid splits and the cooling rate. The experiments show that globules nucleate at 1010°C under disequilibrium conditions. The solidification of the chilled margin occurs before the liquids splits, and the margin of the dike does not contain globules. At a 1000°C quenching temperature, the growth duration varies between 10 hours and 20 minutes with 32°C/hr, respectively. At 64°C/hr, the duration is reduced to 10 minutes. The difference between 20 and 10 minutes is probably too short to result in a visible variation in the globules size. Nevertheless, the idea that

IMMISCIBILITY SYNTHESIS AS AN INDICATION OF COOLING RATES OF BASALTS 307

collision of globules is stopped by quench just after their nucleation suggests the existence of a cooling rate value below which coalescence becomes operative.

(3) Viscosity: the opportunity for globules to come into contact depends on the viscosity of their enclosing matrix which may be the conjugate liquid or crystals. The viscosity increases with increasing silica, decreasing temperature and abundance of crystals. The higher viscosity of the silica-rich liquid diminishes the probability for the iron-rich globules to come into collision and might explain that the iron-rich globules are smaller on the average than the silica-rich ones. The large size of globules in the Lc26 Fa40 Qz34 composition is probably due to the lower viscosity or/and the absence of crystals which do not hamper the probability for globules to come into contact,

(4) Interfacial tension: once globules are in contact the energy to coalesce is provided by interfacial tension which produces high pres- sures within globules (Philpotts, 1976). Co- alescence efficiency is raised by small radii, pressure being higher in small globules. The coalescence of small globules should be easier than those of large radii. However, if we as- sume that coalescence is a rapid phenomenon once globules are into contact, a variation of coalescence efficiency due to the size of globules may be negligible.

Effect of falling temperature. Most of the prop- erties of the two liquids such as interfacial tension, viscosity, composition and diffusion change with falling temperature. However, the effect of falling temperature during the growth of globules should be relatively small because the temperature range where immiscible spheres grow is small (< 30°C), and the composition of the two liquids is nearly constant over this temperature. For example, the Ajiro basalt viscosity estimated using the method by Shaw (1972) indicates an increase viscosity of only 3% when the temperature decreases from 1030 to 1000°C.

Effect of plagioclase. McBirney and Naka- mura (1974) suggested an effect of the plagioclase crystallization on immiscible liquids. The fact that plagioclase often contains immiscible liquids inclusions might support this hypothesis. In basic andesites Luais (1987) suggests that inclusions of immiscible liquids within plagioclase are formed by the compositional gradient near their surface. In the experimental charges, however, plagioclase does not contain inclusions, and the globule size in the mesostasis is not related to plagioclase crystallization. Moreover, rate processes are faster in liquids than in crystals (Roed- der, 1979), so that crystallization of plagioclase should not help the growth of globules.

Limitation in the growth of globules

So long as the two liquids are not crystallized, the globules are able to coalesce. The presence of crystals also limits the growth of globules. Our observation is that globular structures in lamprophyre dikes have larger sizes than those of the tholeiitic basalt. In these rocks the globular structure contains crystals which may be magmatic. The lower viscosity of the lamprophyre liquid probably allows a rapid growth of globules. In intrusive rocks the immiscible liquids segregate by density difference, and might crystallize elsewhere. This case was discussed for the Skaer- gaard layered intrusion to explain the compositional gap between acidic and basic rocks (McBirney and Nakamura, 1974).

Application of the results

Several applications on various samples are given in Table 6. The experimental results provide a mean of estimating the cooling rate for tholeiite basalts by measuring the diameter of silica-rich or iron-rich globules. The measurements of the 10 largest silica-rich globules gives an average diameter of 2.3 #m in the center of the dike. This value suggests

308 B. MARq IN AND I. KUStlIRO

TABLE ¢~

Estimation and comparison of cooling rates in various samples by plotting the average diameter of the 10 largest globules on curves established from our experiments. Results are slightly different when using the silica-rich or iron-rich globules diameter, and for various oxygen fugacities

Av. diameter Cool. rate Av. diameter Cool. rate (#m) (°C/hr) (l~m) (°C/hr) Si-globule CO2/H2 Fe-globulc CO2/H2 ( 10 largest) 14.00/10.88 ( 10 largest) 14.00

Ajiro dike margin Ajiro dike centre Ajiro flow Oshima (Okada) Fuji (Saruhashi)

0.8 >32 >32 1.5 29 2.3 I 1 15 2.4 16 3.3 4 8 2.3 17 3.7 4 6 3.3 9 4.7 2 3 6.3 1

a cooling rate of about l l °C/hr supposing a CO2/H2 ratio of 14.00 (Fig. 8A). The estimate of the cooling rate is slightly higher (15°C/hr) if we choose to consider a CO2/H2 ratio of 10.88 (Fig. 8B). In the dike margin the average measured diameter is smaller than the lower limit obtained in experiments, and the cooling rate proposed for the zone adjacent to the chilled margin is higher than 32°C/hr or 64°C/hr. Another sample was chosen for an Ajiro flow with greater crystal sizes than the dike. Bigger globules were indeed observed in the flow. The average diameter of 3.3 ll, m for the 10 largest silica-rich globules suggests cooling rates of 4 and 8°C/hr with CO2/H2 ratio of 14.00 and 10.88, respectively. The cooling rates thus estimated show a good agreement between globule and crystal sizes. It appears also from these results that an un- derestimate of oxygen fugacity leads to an overestimate of the cooling rate. Using the 10 largest iron-rich globules and the curve in Figure 8C, the cooling rates determined are slightly higher than with silica-rich globules even if the difference is not important. The estimates deduced from the iron-rich globules size might be less representative than the estimates from the silica-rich ones for two reasons. First, the iron-rich globules used to determine the curve are in an anomalous position in the charges. Second, the number of the iron-rich globules is insufficient in the

charges 29, 57 and 69, so that the presence of the iron-rich globules seems more random. However, the most important point is that the relative estimates found by the two methods are similar.

Caution must, however, be exerted when attempting to apply our experimental results to natural rocks. One of the aspects deserv- ing consideration is the fact that the relationship between globule size and cooling rate has been established for temperatures between 1190 and 1000°C when globules form between 1010 and 1000°C. The estimates given may be meaningful only for this temperature inter- val. It is furthermore evident that in natural conditions the cooling rate will not remain constant, and is likely to be high at high temperature and to decrease with lowering temperature.

Another aspect that should be considered is the fact that the proposed values are probably not absolute rates. A charge of 5 mm in diameter cannot produce large globules and crystals, and could limit the size of globules or even the shape of the curves. Neverthe- less, immiscibility appears at low temperature in basalts with the crystallization of a fine- grained groundmass. Consequently, for cooling rates that are high enough, small crystals produced in small experimental charges are equivalent to a groundmass. But, for low cooling rates (<< l°C/hr) larger charges should

I M M I S ( ' I B I I . I T Y SYN'I |tf~SIS AS AN I N D I C A T I O N OF COOI. IN( I RATES OF BASAI~I'S 3 0 9

be used. The curves proposed in this paper should be applied only for cooling rates of about l°C/hr or more.

Conclusion

The experiments reported in this paper allow estimates of cooling rates for tholeiitic basalt compositions. The method consists of measuring the diameter of immiscible globules of iron-rich or silica-rich composition. The cooling rate is easily deduced by using the established relationships between cooling rates and globule size. These estimates, however, are valid for the cooling history of magmas for temperatures between 1010 and 1000°C.

The globules grow mainly by coalescence, which is responsible of the exponential shape of the established relationship. The viscosity varies with the composition of magmas, cooling rate, and the presence of crystals, and seems the main factor controlling the coalescence of globules. A decrease of the oxygen fugacity, however, facilitates the growth of globules. Iron-rich globules coalesce with more difficulty in a silica-rich matrix than silica-rich ones in an iron-rich matrix, and measurements of silica-rich globules may provide more consistent estimates. The absence of an exponentional relationship between globule size and cooling rates in a Fuji high alumina-basalt may be ascribed to the high viscosity of surrounding liquid.

The composition of the liquid prior to splitting is not modified by crystallization within 1 and 32°C/hr because the composition of the residual liquid is constant for various cooling rates. Compositional variations of the residual liquid after crystallization of plagioclase, pyroxene, and iron oxides within each charge do not affect those of the two immiscible liquids. The variable size of the compositional gap between the two immiscible liquids with varying cooling rates is a possible consequence of diffusion. Because of the non-

equilibrium condition of these experiments diffusion between the two immiscible liquids continues till equilibrium is reached. A consequence is that the compositional gap between the two liquids increases at low cooling rates.

The composition of the liquid prior to splitting determines the proportions of each immiscible liquid, as well as the form of the globules, and nature of the matrix that devel- ops in each liquid. The less abundant liquid fraction forms globules.

To extend our results further, additional experiments should be made for rates of cooling lower than l°C/hr.

Acknowledgements

The authors are indebted to The Japan So- ciety For The Promotion of Sciences which supported this work. Thanks are also due to CNRS who made B.M. stay in Tokyo possible through their exchange program with Japan. The first author is also very grateful to Pr. D. Velde and Pr. J. Fabri6s for guidance and en- couragement. Dr. A.R. McBirney is thanked for critically reviewing the manuscript.

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immiscibility synthesis as an indication of cooling rates of basalts

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