High and low pressure phase equilibria of a mildly alkalic lava from the 1965 Surtsey eruption: Experimental results

Lithos, 26 ( 1991 ) 223-243 Elsevier Science Publishers B.V., Amsterdam

223

High and low pressure phase equilibria of a mildly alkalic lava from the 1965 Surtsey eruption: Experimental results P. T h y * NASA, Johnson Space Center, SN2, Houston, TX 77058, U.S.A. (Received January 2, 1990; accepted July 9, 1990 )

LITHOS

%

ABSTRACT Thy, P., 1991. High and low pressure phase equilibria of a mildly alkalic lava from the 1965 Surlsey eruption: Experimental results. Lithos, 26: 223-243. Melting experiments have been performed on a primitive, mildly alkalic glassy lava ( 10 wt.% MgO) from the 1965 eruption of the Surtsey volcano located at the tip of the south-eastern propagating rift zone of Iceland. At atmospheric pressure, approximately on the FMQ oxygen buffer, olivine (Fosj) crystallizes from 1240°C, followed by plagioclase (An7o) from 1180°C and augite from 1140°C. The experimental glasses coexisting with olivine, plagioclase and augite arc ferrobasaltic enriched in FeO ( 13.6-14.2 wt.% ) and TiO2 (4.0-4.4 wt.%). In high pressure, piston-cylinder, graphite-controlled runs, olivine occurs as the liquidus phase until 14 kbar, above which augite is the liquidus phase. Low-Ca pyroxene is not a liquidus phase at any pressure. The high pressure liquids are, relative to the one atmosphere liquids, significantly enriched in A1203 and Na20 and depleted in CaO as a result of changes in the crystallizing assemblages. Furthermore, liquidus augite is dominantly subcalcic and shows significant enrichment in AI and depletion in Ti. Subliquidus plagioclase is enriched in sodium relative to low pressure phase compositions. Evaluated in normative projections, contrasting liquid lines of descent are revealed as a function of pressure. At one atmosphere, the multisaturated liquids are located close to the thermal divide defined by the plane olivine-plagioclase-augite,but appear, with advanced degrees of crystallization, to be moving away from the thermal divide toward normative quartz. The augites crystallizing in the one atmosphere experiments are calcic and slightly nepheline normative. In the 10 and 12.5 kbar experiments, the augites become subcalcic and dominantly hypersthene normative. Because of this shift in augite compositions, transitional basaltic liquids may at high pressure evolve from the tholeiitic side of the olivineplagioclase-diopsidenormative divide onto the alkalic side. With increasing pressure above 15 kbar, the liquidus augite compositions move back toward the olivine-plagioclase-diopsidenormative divide.

Introduction M i l d l y alkalic a n d t r a n s i t i o n a l basalts v a r y c o n t i n uously in c o m p o s i t i o n f r o m slightly n e p h e l i n e to h y p e r s t h e n e n o r m a t i v e a n d fall in b o t h the alkalic basalt a n d the o l i v i n e t h o l e i i t e v o l u m e s o f the exp a n 4 e d basalt t e t r a h e d r o n o f Y o d e r a n d Tilley ( 1 9 6 2 ) . Because these t y p e s o f b a s a l t s t r a d d l e the n o r m a t i v e d i v i d e b e t w e e n alkalic a n d t h o l e i i t i c ba*Present address: Department of Geology, University of Botswana, Private Bag 0022, Gaborone, Botswana.

salts, u n d e r s t a n d i n g t h e i r d e t a i l e d c r y s t a l l i z a t i o n b e h a v i o r is crucial for p r e d i c t i n g e v o l u t i o n p a t h s ( Y o d e r a n d Tilley, 1962; M i y a s h i r o , 1978). T h e i m p o r t a n t low pressure, phase e q u i l i b r i u m cons t r a i n t is a t h e r m a l m a x i m u m on the olivine, plagioclase a n d augite s a t u r a t e d cotectic ( P r e s n a l l et al., 1978, 1979) that a p p e a r s in n a t u r a l systems to be l o c a t e d relatively close to, if not on, the n o r m a tive d i v i d e ( M a h o o d a n d Baker, 1986; Sack et al., 1987). U s i n g p s e u d o - q u a t e r n a r y projections, Sack et al. ( 1 9 8 7 ) suggested that the c o m p o s i t i o n s o f multisaturated liquids form a narrow band between

224 a nepheline saturated, eutectic-like point on one side of the divide and a low-Ca pyroxene saturated, pseudo-invariant point on the other side. These projections, together with the location of the thermal maximum, suggest that transitional basalts will evolve toward the low-Ca pyroxene saturated reaction point, whereas mildly alkalic basalts will evolve toward nepheline saturation. Consequently, otherwise geochemically continuous lava suites may exhibit highly diverse evolution paths. It is a further consequence that olivine normative, transitional basalts will evolve toward the same pseudo-invariant point as determined for quartz normative and depleted tholeiites (Grove et al., 1982; Grove and Bryan, 1983).

Mildly alkalic lavas and the rift zones of Iceland

Recent volcanic activity in Iceland occurs along two types of axial rift zones. The main rift zone, which is the landward extension of the mid-Atlantic ridge, produces tholeiitic lavas. Flank zones, offthe main rift zone, produce transitional and mildly alkalic olivine basalts (Jakobsson, 1972). The most prominent of these is the south-eastern propagating rift zone, which shows a progressive evolution from tholeiitic lavas in central Iceland, transitional lavas in southern Iceland, to mildly alkalic lavas in the Vestmannaeyjar (Jakobsson, 1979). The Surtsey volcano is the southernmost island of the Vestmannaeyjar and was formed by submarine eruptions during 1963 to 1987 (Thorarinsson, 1964). The erupted lavas are mildly alkalic and nepheline normative olivine basalts (Jakobsson, 1979). In order to understand better the origin and evolution of the lavas in the south-eastern propagating rift, the February 1965 lava flowing from the eastern vent of the Surtsey volcano was selected for one atmosphere and high pressure melting experiments. The sample used (SU106; Table 1) contains olivine and plagioclase microphenocrysts and microlties in a glassy matrix. Chromian spinel is present as inclusions in olivine and in the groundmass. Compositional!y it is relatively primitive with a MgO of 10 wt.% (Table 1 ) and appears to have been only slightly modified by crystal fractionation (Thy, 1991 ). The April 1964 lava that was investigated by Tilley et al. (1967) contains 9 wt.% MgO (Table 1).

P. THY TABLE 1 Chemical composition of starting material. The Surtsey lava of February 27, 1965, Iceland (SU 106 ) 1

2

3

Fe203 FeO FeO *+ MnO MgO CaO Na20 K20 P205 H 2 0 ++ Total

46.58 1.85 15.16 1.58 10.35 I 1.77 0.17 10.09 10.22 2.94 0.42 0.14 0.30 99.80

46.82 1.85 15.07 1.48 10.42 11.75 0.19 10.02 10.29 2.92 0.49 0.22 0.12 99.89

46.56 2.02 15.93 1.61 10.32 11.77 0.20 9.00 10.51 3.2 I 0.51 0.26 0.02 100. [ 5

FeO*/MgO Mg ~'

1.17 60

1.31 58

or ab an ne di ol il ap

2.48 18.50 27.10 3.55 18.88 25.64 3.53 0.33

1.17 60 C I P W weight n o r m ++ * 2.90 18.81 26.68 3.24 18.96 25.37 3.53 0.52

SiO2 TiO 2

A1203

3.01 17.73 27.55 5.1 I 18.83 23.32 3.84 0.62

+Iron calculated as FeO*. M g # = M g / ( M g + Fe), atomic. % with all iron as Fe. + + H 2 0 calculated from loss of ignition (anal. I ) or gravimetrically determined (anal. 2 a n d 3 ). + + + C I P W weight n o r m calculated with all iron as FeO. 1. X R F analysis by S. G r u n d v i g (Aarhus). N a , O by INAA. Same powder used as starting material. 2. X R F analysis by I. Sorensen ( C o p e n h a g e n ) of another part o f same sample. Analysis supplied by S.P. Jakobsson (Reykjavik). 3. Starting composition used by Tilley et al. ( 1967 ). First lava o f April, 1964.

This paper reports the results of the experimental study. These results have implications for the petrogenetic constraints on the origin and evolution of mildly alkalic and transitional tholeiitic lavas. A subsequent paper (Thy, 1,991 ) attempts, in the light of the experimental results, to explain important differences between lavas erupted at the tip of the propagating rift and behind it. Experimental methods

Sample preparation The sample was crushed in a steel jaw crusher and pulverized in an agate shatter box for approximately 10 min to pass a 20 mesh sieve. A few grams of the powder were ground to

PHASE EQUILIBRIAOF LAVAFROM THE 1965 SURTSEY ERUPTION: RESULTS an estimated grain-size of 10 # m in an agate m o r t a r u n d e r acetone (referred to as rock powder). Another few grams were fused in a graphite crucible at 1300°C and an oxygen fugacity of 10 - s for 30 min, quenched to a glass and ground as described above (referred to as glass p o w d e r ) . Both powders were dried at 110°C for 24 h and stored in a desiccator.

One atmosphere runs The one atmosphere runs were performed in a vertical, platinum-wound, quenching furnace. The oxygen fugacity was controlled approximately at the f a y a l i t e - m a g n e t i t e - q u a r t z ( F M Q ) oxygen buffer by a flow o f CO2 and H2 mixed in constant proportions (CO2/H2 = 24 ) by a m a n o m e t e r system (cf., Nafziger et al., 1971; Deines et al., 1974). The temperature was monitored and controlled by P t / P t l 0 R h thermocouples calibrated against a standard thermocouple certified by the U.S. National Bureau o f Standards. The rock p o w d e r was sin-

225

tered to 0.002" Pt wire loops. A few charges were prepared from the glass powder (runs marked G in Table 2 ). The beads were inserted into the furnace either directly at the chosen run temperature ( u p - t e m p e r a t u r e or melting runs; R and G in Table 2) or inserted at a temperature slightly above liquidus ( 1250°C( and subsequently dropped, as fast as permitted by furnace cooling, to the run temperature (down-temperature or crystallization runs; D in Table 2 ). All runs were of durations more than 20 hours and were quenched by dropping into water. The temperature determinations are believed to be internally precise to within 2°C.

High pressure runs

The high pressure experiments were carried out with a solidmedia, piston-cylinder apparatus o f the type developed by

TABLE 2 Run data at 1 atm Run no.

Method"

Temperature (~C)

Run time (h)

Run products + +

Phase proportions+ + + gl

4 14 11 12 10 15 9 8 23 6 24 16 13 32 5 21 17 33 27 18 26 3l 19 20* 28 30 29 25

R D R R R D R D D R D D R G R D D G R D R G D D R G R D

1242 1238 1235 1235 1225 1218 1208 1201 199 193 184 180 176 174 169 162 153 131 130 124 118 117 110 1090 1086 1085 1074 1067

24 23 23 24 22 22 24 21 23 21 46 25 22 24 23 26 24 46 30 39 24 58 25 43 41 44 45 63

gl, sp, ol gl, sp, gl, sp, ol gl, sp, ol gl, sp, ol gl, sp, ol gl, sp, ol gl, sp, ol gl, sp, ol gl, sp, ol gl, sp, ol gl, sp, ol, pl gl, sp, ol, pl gl,-, ol, pl gl, sp, ol, pl gl, sp, ol, pl gl, sp, ol, pl gl, -, ol, pl, cpx gl, sp, ol, pl, cpx gl, sp, ol, pl, cpx gl, sp, ol, pl, cpx gl, sp, ol, pl, cpx gl, sp, ol, pl, cpx gl, sp, ol, pl, cpx gl, sp, ol, pl, cpx gl, -, ol, pl, epx gl, sp, ol, pl, cpx Solid (no glass)

ol

pl

cpx

100

0

100 100 99 97 98 96 95 95 93 91 91 83 79 78

0 0 1 3 2 4 5 5 7 7 7 9 10 11

2 2 8 11 11

42 41 49 34

17 17 17 18

32 32 30 34

9 l0 4 14

32

19

37

12

÷ R-Charge prepared from rock powder and brought directly to the run temperature. D-Charge prepared from rock powder and brought from above liquidus temperature ( 1250 ° C) to the run temperature as fast as possible. Run time given as the total time at the chosen temperature. G-As R, except that a glass was used prepared in a graphite crucible at 1300 ° C in a controlled atmosphere (fo2 = 10 -a). * ÷gl-glass; sp-spinel; ol-olivine; pl-plagioclase; epx-clinopyroxene; -phase not present or not observed, but expected. * + ÷ Phase proportions (wt.%) estimated from least-squares linear regressions using the analytical data in Table 5. All oxides were weighted by 1.00, except SiO2 and A1203 which were weighted 0.40 and 0.50, respectively. *The spinel occurring in this run is a magnetite.

P. THY

226 TABLE 3 Run data at high pressure Run no.

Pressure kbar

Temperature (°C)

Run time (h)

Run products +

Phase proportions + + gl

27 31 32 26 15 24 19 25 21 40 41 48 55 54 39 34 28 37 35 36 38 46 43 45 47 14 16 12 20 13 3 6 17 42

30 20 20 20 20 20 20 20 20 17.5 17.5 17.5 17.5 17.5 15 15 15 15 15 15 15 12.5 12.5 12.5 12.5 10 10 10 10 10 l0 10 10 7

1390 1380 1360 1340 1320 1300 1280 1240 1230 1330 1310 1290 1270 1250 1330 1320 1310 1300 1270 1250 1230 1300 1280 1260 1240 1320 1300 1280 1270 1240 1220 1180 1160 1230

23 27 25 22 26 20 17 21 23 32 t8 28 35 24 35 21 23 23 27 25 21 21 20 22 27 23 24 23 24 23 22 19 19 17

gl, cpx gl gl gl, cpx, sp gl, cpx gl, cpx gl, cpx solidus solidus gl, cpx gl, cpx, sp gl, cpx, sp gl, cpx, sp gl, cpx, sp gl gl, cpx gl, cpx gl, cpx gt, cpx gl, cpx, sp gl, cpx, sp, pl gl, ol gl, ol gl, ol, cpx gl, ol, cpx, pl gl gl gl, ol gl, ol gl, ol, pl gl, ol, pl, cpx gl, ol, pl, cpx solidus +++ gl, ol

ol

pl

cpx

sp

58 59

42 41

67 57

33 43

0 0

37

62

I

92 85

8 15

70

30 55

39 100 97 80 46

0 3 2 6

10

18 38

96 94 95 83 35

4 6 5 6 13

0 3 27

8 25

97

3

2

+gl-glass; ol-olivine; cpx-clinopyroxene; pl-plagioclase; sp-spinel. + +Phase proportions (wt.%) estimated from least-squares linear regressions using the analytical data in Table 6. All oxides were weighted by 1.00, except SiO2 and AI203 which were weighted 0.40 and 0.50, respectively. + + +Solidus phases not identified.

Boyd and England (1960). The glass p o w d e r was used as starting material. The powder was loaded in graphite containers and sealed in platinum capsules. Before loading into the furnace assemblage, the experimental capsules were dried for 30 min in an oxygen-free a t m o s p h e r e at 1050°C. All furnace parts, except the Pyrex glass and talc sleeves, were also dried at 1050°C in an oxygen-free atmosphere. The glass sleeves were kept in an oven at about 110°C before use. All experiments were of the piston-out type with an over-pressure of about 5 kbar and without application of pressure corrections to thermocouple emf. The W 3 R e / W 2 5 R e t h e r m o couples were adjusted to the 1968 International Practical Temperature Scale ( A n o n y m o u s , 1969). The experimental techniques are, except for the use o f graphite capsules, essen-

tially similar to those used by Presnall et al. ( 1979 ) and Sen and Presnall (1984) in the same laboratory. Pressures are believed to be accurate to within 0.5 kbar and temperatures to within 10°C (Presnall et al., 1979). All high pressure runs were of durations longer than 17 h (Table 3 ). The graphite container is expected to maintain a relatively low fo2. This condition has been substantiated by T h o m p s o n and Kushiro (1972) who observed wiistite in h e m a t i t e graphite runs at high pressures. Furthermore, Takahashi and Kushiro (1983) noted that Fe 3+ concentrations in high pressure, graphite encapsuled glasses were below detection limits with Mtissbauer spectroscopy. A lowfo2 is also supported for the present study by the precipitation of Ni in a few 10 kbar runs containing NiO powder.

PHASE EQUILIBRIAOF LAVAFROM THE 1965 SURTSEYERUPTION: RESULTS

227

Analytical techniques

IRON LOSS

401 [] I0 All phases were identified with reflected light and a scanning electron microscope with back-scattered electron images. Phases were verified and selected runs analyzed with the electron microprobe without back-scattered analyzer. The analyses were performed with an ARL-SEMQ, fully automatic microprobe with an accelerating voltage of 15 kV and a beam current of approximately 25 nA. Because of the possibility for loss of sodium, an electron beam with a diameter of 5-10 #m was moved over pockets of glass during analyses. The small areas of interstitial glass in low temperature runs, howcver, proved difficult to analyze due to interference from adjacent mineral phases. Standards were natural minerals and glasses. All analyses are corrected for matrix effects by a standard alpha-factor method (Bence and Albee, 1968; Albee and Ray, 1970). An evaluation of the accuracy and precision of glass analyses is given in Table 4.

Iron and sodium loss The use of the wire-loop technique greatly minimizes platinum/sample volume ratios and consequently iron loss to the Pt suspension wire (Presnall and Brenner, 1974). Therefore, only 3-4% of the original sample FeO was lost and no attempt was made to use FePt alloy wires (e.g., Grove, 1981 ). The amount of Na20 lost in the one atmosphere runs reach 6-7% for low temperature runs, but average 3-4% (excluding some near liquidus runs). The elemental losses were calculated by mass balance (see note to Table 2 ) showing good fits with sums of the residuals averaging 0.20 (range 0.03-0.30), mainly due to sodium loss. The iron loss in the platinumgraphite-contained, high pressure runs is significant, in particular for runs close to or above liquidus temperatures. The Fe loss decreases with decreasing temperature and increasing pressure (Fig. 1 ). Runs more than 30°C above liquidus may TABLE 4 Precision of microprobe analyses VG-A99

SiO2 TiO2 AIzO3 FeO* MnO MgO CaO Na20 K20

P205 Total

WRAB-4

1

2

3

1

2

3

50.90 4.06 12.97 13.18 0.19 5.18 9.38 2.73 0.80 0.41 99.80

50.93 3.96 12.29 13.43 0.20 5.01 9.66 2.46 0.81 0.46 99.21

0.38 0.17 0.09 0.16 0.05 0.05 0.14 0.18 0.04 0.05

47.94 0.69 '.7.75 8.58 0.12 9.95 11.52 2.44 0.11 0.06 99.16

47.46 0.68 17.65 8.48 0.15 10.10 11.74 2.53 0.09 0.06 98.94

0.44 0.07 0.12 0.14 0.03 0.10 0.20 0.05 0.04 0.03

1. Acceptedanalysesof VG-A99 (Jarosewich et al., 1979) and WRAB4 (B. Evans). 2. Averageof 24 analyses. 3. One standard deviation.

°l,tl?

20/"

[]

+----oA2/ -,o 1 • o¢ o

-201 -2

__:A_

l 0

= 2

I 4

I 6

"

, 8

, I0

12

/IF(K) Fig. l. Iron loss versus liquidus temperature for high pressure runs. Iron loss calculated as original FeO minus run FeO (wt.%). Iron loss for subliquidus runs have been estimated by least-squares, linear regression of the run products. Temperature is plotted as run temperature minus liquidus temperature. have lost close to 100% iron. In subliquidus runs Fe loss varies between 30% near the liquidus to 5-7% more than 30°C below it.

Experimental results One atmosphere The details of the one atmosphere runs are given in Table 2 and summarized in Figs. 2-4. Olivine crystallizes from 1240 ° C and is present until the solidus. Plagioclase starts crystallizing from 1180 ° C and augite from about 1140°C. In addition, chromian spinel appears in most runs, but low-Ca pyroxenes were not detected in any runs. The spinel in the run at 1090°C is a magnetite. The crystallization sequence and liquidus temperatures are consistent with those obtained by Tilley et al. (1967) on a slightly more iron rich sample from the same eruption (olivine 1220°C, plagioclase 1180°C, augite 1155°C). Although the experiments were brought to run temperatures by different paths, no significant compositional differences could be detected between the two types of runs. Texturally, skeletal textures appear frequently in the crystallization runs, while euhedral forms dominate in the melting experiments. Chromian spinel failed to nucleate in most of the melting experiments in which the relatively reduced glass was used as starting composi-

228

P. THY

/

1400 --

SU 106

0

/

kkq~k~

T °C

t,'~/

0 / " ~~ ~ ~ (~/(~sp I, cpx +-sp

.-///f

~sp

1300-

•

1200--

1100-

-JU. o~-*~/

/

,. /

.pi.~px.-~p i÷oL ~L) I + pl

~ , 6 ~

e~"-

I000

(~ I+cpx • subsolidus

I 5

I

I 15

I0

I 20

I 25

I 30

P kbar Fig. 2. Melting relations of the Surtsey lava (SU 106 ) as a function of temperature ( T ° C) and pressure (P kbar). Liquid ( / ) glass, ol-olivine, p/-plagioclase, cpx-augite, sp-Al-spinel. The one atmosphere results (Table 2) are summarized. The spinel in

the one atmosphere experiments is a chromian spinel, except for a subsolidus run which contains magnetite. The experimental details are summarized in Tables 2 and 3.

sum06- latm

1250

SUI06- l arm

1250

Toc

OI " - ~ i

,il

T°C

pl\f

1200-

1200-

tl50--

llSO-

cpx~ /

lP1"dr

gs 1100--

1050 2

~

....

I 4

I 6

~

I0

MgO Fig. 3. Temperature (T°C) versus wt.% MgO in glasses for the one atmosphere experiments. Ol, pl and cpx mark the in-

come of olivine, plagioclase and augite, respectively. Solid is the temperature at which no liquid can be detected (Table 2). tion. This result may be due to insufficient run time for the fo2 of the charge to equilibrate with the furnace gas (cf., Fudali, 1965). The standard deviations of replicate analyses on the experimental charges (Table 5A) are, for high temperature runs, reasonably within the analytical precision of the microprobe analyses (Table 4). Due to the fine grain-size, however, glasses and minerals

I Ioo-j, old,.

*

1050

I 40

I 20

I

60

I

80 I00 %liquid Fig. 4. Temperature (T°C) versus percentage liquid remaining (% liquid) for the one atmosphere experiments (Table 2 ). Same data as shown on Fig. 3. 0

in the low temperature runs proved difficult to analyze. These problems are reflected in the large variation in, and the relatively few, replicate analyses made (Table 5). These limitations in the experimental and analytical procedures are largely similar to those experienced by Mahood and Baker (1986), Grove et al. (1982) and Grove and Bryan ( 1983 ), among many others, showing that experimental charges with less than 30% liquid remaining rarely can be analyzed with the microprobe.

PHASE EQUILIBRIA OF LAVA FROM THE 1965 SURTSEY ERUPTION: RESULTS

229

TABLE 5A Chemical composition of experimental glasses at 1 a t m Run4 (6) + SiO2 TiO2 A1203 FeO* MnO MgO CaO Na20 K20 P205 Total FeO*/MgO

47.34 1.83 14.99 11.20 0.19 9.37 10.85 2.49 0.37 0.17 98.80 1.20

0.24 0.06 0.09 0.19 0.03 0.09 0.18 0.02 0.04 0.03

Run24 (5)

Run 14 (5)

Run 11 (20)

Run 12 (11)

Run 10 (7)

Run 15 (5)

Run9 (12)

47.20 1.90 15.09 11.10 0.17 9.27 10.86 2.54 0.35 0.21 98.69 1.20

47.53 1.78 15.25 11.05 0.18 9.38 10.84 2.57 0.35 0.20 99.14 1.18

47.24 1.81 15.11 11.24 0.21 9.42 10.76 2.54 0.37 0.21 98.91 1.19

47.97 1.84 15.49 11.32 0.19 8.86 10.75 2.78 0.37 0.21 99.78 1.28

48.03 1.90 15.60 11.15 0.20 8.15 11.06 3.00 0.39 0.19 99.67 1.37

47.70 1.83 15.82 11.13 0.17 8.66 ll.00 2.88 0.38 0.21 99.78 1.29

0.35 0.06 0.10 0.21 0.04 0.18 0.06 0.04 0.03 0.06

Runl6 (5)

SiO2 48.04 0.71 47.98 TiO2 2.01 0.06 2.19 AI20~ 16.21 0.06 14.98 FeO* 10.81 0.18 11.44 MnO 0.21 0.04 0.18 MgO 7.05 0.28 6.55 CaO 11.66 0.09 11.93 Na20 3.13 0.12 3.02 K20 0.37 0.05 0.53 P205 0.24 0.03 0.21 Total 99.73 99.00 FeO*/MgO 1.53 1.75

0.65 0.08 0.27 0.27 0.04 0.24 0.20 0.15 0.03 0.04

Runl3 (16)

0.25 0.11 0.29 0.20 0.03 0.18 0.13 0.12 0.03 0.03

Run32 (10)

0.25 47.59 0.60 0.08 2.01 0.11 0.08 16.01 0.57 0.24 10.92 0.28 0.02 0.21 0.04 0.06 7.15 0.33 0.13 11.41 0.21 0.02 3.18 0.13 0.05 0.44 0.03 0.04 0.23 0.03 99.15 1.53

47.48 2.27 14.85 11.19 0.22 6.73 11.44 3.34 0.48 0.26 98.26 1.66

0.80 0.06 0.28 0.17 0.04 0.17 0.24 0.18 0.03 0.02

Run5 (12) 0.45 0.12 0.14 0.17 0.04 0.12 0.14 0.05 0.04 0.04

47.52 2.28 14.74 11.87 0.21 6.65 11.42 3.20 0.51 0.26 98.66 1.78

Run21 (5) 0.46 0.09 0.32 0.19 0.04 0.17 0.24 0.23 0.03 0.04

47.52 2.35 15.11 11.80 0.22 6.25 11.64 3.15 0.50 0.27 98.81 1.89

0.60 0.10 0.25 0.45 0.01 0.09 0.17 0.05 0.01 0.05

Run33 (2)

Run 8 (5) 0.39 0.06 0.23 0.22 0.03 0.20 0.22 0.17 0.04 0.03

47.34 2.01 15.69 11.31 0.21 7.77 11.38 2.90 0.37 0.24 99.22 1.46

Run27 (4)

0.33 47.62 0.10 4.30 0.11 12.40 0.12 14.07 0.03 0.26 0.07 4.97 0.24 10.63 0.09 3.36 0.05 0.82 0.04 0.55 98.98 2.83

47.44 4.02 13.07 13.98 0.29 5.55 11.01 3.58 0.72 0.50 100.16 2.52

Run23 (5) 0.34 0.05 0.16 0.08 0.04 0.14 0.24 0.07 0.04 0.03

47.37 1.95 15.83 11.07 0.20 7.78 11.43 2.89 0.38 0.22 99.12 1.42

Runl8 (16)

Run6 (13) 0.24 I).04 0.10 0.10 0.06 0.07 0.19 0.03 0.02 0.02

Run26 (I)

0.14 47.17 0.80 47.54 0.14 3.42 0.14 4.57 0.13 12.77 0.17 13.20 0.36 13.59 0.20 13.59 0.14 0.25 0.04 0.23 0.04 5.32 0.12 4.80 0.29 11.71 0.23 9.16 0.18 3.25 0.11 3.63 0.06 0.71 0.04 1.16 0.03 0.43 0.06 0.61 98.62 98.49 2.55 2.83

47.79 1.85 15.95 11.10 0.19 7.50 11.33 2.97 0.39 0.22 99.29 1.48

0.37 0.15 0.36 0.22 0.02 0.27 0.28 0.17 0.04 0.04

Runl9 (7) 47.57 4.37 12.22 14.21 0.27 4.14 10.16 3.37 1.08 0.63 98.02 3.43

0.50 0.24 0.17 0.20 0.05 0.09 0.14 0.13 0.05 0.06

+ N u m b e r in parenthesis refers to total n u m b e r o f analyses used for calculating each average (first c o l u m n ) a n d one s t a n d a r d deviation (second c o l u m n ) listed under each run number. FeO*-all iron given as FeO. TABLE 5B Chemical composition of experimental olivines at 1 arm

SiO2 FeO MnO MgO CaO NiO Total Fo mol.% range ++

SiO2 FeO MnO MgO CaO NiO Total Fo mol.% range

Run4 (5) +

Runll (8)

Runl2 (8)

Runl0 (9)

40.43 0.57 14.34 0.45 0.19 0.04 43.90 0.17 0.20 0.06 0.14 0.04 99.20 84.5 83.8-85.0

38.87 0.83 13.97 0.34 0.21 0.03 46.37 0.87 0.48 0.15 0.18 0.06 100.08 85.5 84.9-86.2

38.65 0.69 14.24 0.33 0.22 0.04 46.16 0.59 0.37 0.12 0.18 0.04 99.82 85.2 84.8-85.8

Run 16 (7)

Run 13 (14)

Run32 (7)

38.48 0.26 18.59 0.48 0.25 0.07 41.61 0.64 0.25 0.05 0.16 0.06 99.34 80.0 79.2-80.7

38.97 0.97 17.65 0.62 0.25 0.03 42.38 0.93 0.40 0.13 0.19 0.12 99.84 81.1 80.3-82.5

39.45 17.92 0.28 40.49 0.31

0.53 0.40 0.03 1.30 0.12

98.45 80. t 79.2-81.4

Runl5 (7)

38.65 1.14 39.29 0.26 14.61 0.98 14.99 0.28 0.21 0.04 0.22 0.04 46.03 2.04 43.75 0.40 0.27 0.08 0.24 0.06 0.23 0.05 0.21 0.06 99.99 98.7 t 84.9 83.9 83.6-84.3 81.7-85.7

Run9 (13)

Run8 (2)

Run23 (5)

Run6 (21)

Run24 (5)

38.93 0.47 14.93 0.73 0.23 0.04 44.91 0.92 0.29 0.09 0.27 0.06 99.56 84.3 82.9-85.5

38.50 15.50 0.24 44.57 0.91 0.24 99.96 83.7 83.4-84.0

38.85 0.24 15.69 0.19 0.21 0.05 43.77 0.24 0.22 0.06 0.26 0.08 99.00 83.3 83.1-83.4

38.81 0.92 15.83 0.84 0.24 0.04 44.95 0.74 0.31 0.12 0.22 0.12 100.36 83.5 82.2-85.9

38.33 0.29 16.57 0.80 0.22 0.04 43.30 0.58 0.20 0.04 0.17 0.02 98.79 82.3 81.0-83.4

Run33 (4)

Run27 (7)

Run 18 (4)

Run26 (1)

Run 19 (3)

38.27 22.96 0.40 37.79 0.23

37.51 27.20 0.48 32.92 0.31

37.09 26.51 0.45 34.49 0.18

98.42 68.3

98.72 69.9 69.0-71.3

Run5 (9)

Run21 (7)

38.13 l . l l 18.79 0.48 0.27 0.04 42.89 0.88 0.25 0.05 0.14 0.07 100.47 80.3 79.4-81.2

38.04 0.43 39.45 0.41 17.82 1.69 23.76 0.92 0.25 0.04 0.38 0.02 42.53 1.40 35.32 1.45 0.31 0.19 0.11 0.00 0.16 0.07 99.11 99.02 81.0 72.6 79.2-85.0 70.7-74.1

38.10 25.94 0.43 34.42 0.37

0.30 0.41 0.09 0.58 0.06

99.26 70.3 69.8-71.3

0.35 0.65 0.04 0.63 0.04

99.65 74.6 74.0-75.6

0.53 1.25 0.02 0.45 0.06

+ N u m b e r in parenthesis refers to total n u m b e r o f analyses used for calculating each average (first c o l u m n ) a n d one standard deviation (second column ) listed u n d e r each run number. + + Forsterite mole % a n d total range in analyzed grains.

P. THY

230 TABLE 5C Chemical composition of experimental plagioclases at 1 atm

SiO2 TiO2 AI203 FeO ° MgO CaO Na20 K20 Total An mol.% range +* Ab mol.% Or mol.%

Run 16 (4) +

Run 13 (2)

Run32 (4)

Run5 (6)

50.20 0.61 0.12 0.04 29.87 0.96 0.64 0.11 0.19 0.02 14.60 0.58 3.43 0.26 0.12 0.05 99.17 69.7 66.2-71.9 29.6 0.7

50.25 0.09 31.31 0.54 0.22 15.05 3.34 0.10 100.90 71.0 70.7-71.2 28.5 0.6

50.53 0.47 49.81 0.66 0.10 0.03 0.10 0.02 29.53 0.07 30.18 0.57 0.82 0.07 0.85 0.30 0.22 0.03 0.20 0.03 14.47 0.37 14.85 0.24 3.47 0.15 3.32 0.05 0.09 0.05 0.12 0.07 99.23 99.43 69.4 70.7 67.8-71.1 69.9-71.5 30.1 28.6 0.5 0.7

Run21 (2)

Run33 (3)

Run27 (5)

Run 18 (12)

51.51 0.12 29.38 0.77 0.19 14.11 3.48 0.11 99.67 68.7 67.2-70.2 30.7 0.6

52.92 0.17 52.32 0.55 50.65 0.98 0.09 0.04 0.15 0.03 0.18 0.08 28.12 0.46 28.20 0.33 28.92 0.73 1.01 0.16 1.29 0.08 1.07 0.34 0.27 0.19 0.26 0.02 0.25 0.10 13.15 0.32 12.95 0.28 14.06 0.42 3.94 0.08 3.83 0.10 3.64 0.14 0.22 0.01 0.18 0.06 0.15 0.03 99.72 99.18 98.92 64.0 64.4 67.5 63.4-65.1 63.6-65.3 65.2-70.2 34.7 34.5 31.6 1.3 1.1 0.9

Run26 (1)

Run 19 (9)

51.51 0.24 29.15 1.27 0.26 13.70 4.16 0.17 100.46 63.9

52.12 0.58 0.29 0.09 27.71 0.61 1.37 0.21 0.36 0.11 13.42 0.44 3.99 0.22 0.24 0.03 99.50 64. I 60.5-66.2 34.5 1.4

35.1 0.9

÷Number in parenthesis refers to total number of analyses used for calculating each average (first column) and one standard deviation (second column) listed under each run number. + +An, Ab. and Or mole % and total range in analyzed grains. FeO*-all iron given as FeO. TABLE 5D Chemical composition of experimental clinopyroxenes at 1 atm

SiOz

Ti02 M_,O3 FeO* MnO MgO CaO Na20 Total En mol.% range + + Fs mol.% range Wo tool.% rangc

Run 33 (3) +

Run 27 (5)

Run 18 (3)

Run 19 (9)

49.60 0.32 1.60 0.07 4.01 0.63 8.12 0.60 0.26 0.10 13.20 0.35 21.65 0.66 0.45 0.07 98.89 39.6 38.7-40.2 13.7 I2.9-14.8 46.7 45.0-48.3

48.10 0.86 1.82 0.28 4.86 0.93 8.24 0.50 0.20 0.03 13.63 0.46 21.01 0.9I 0.42 0.05 98.28 40.9 39.9-42.2 13.9 12.9-14.8 45.3 43.2-46.8

46.45 0.34 2.89 0.28 6.30 0.15 8.39 0.08 0.17 0.03 12.69 0.08 22.39 0.21 0.39 0.01 99.67 37.9 37.8-38.0 14.1 14.0-14.2 48.1 47.8-48.3

47.36 1.45 3.16 0.53 5.68 0.91 9.13 0.32 0.20 0.04 11.73 0.42 22.11 0.32 0.55 0.11 99.92 35.8 34.5-37.1 15.6 14.9-16.0 48.5 47.6-49.8

+Number in parenthesis refers to total number of analyses used for calculating each average (first column) and one standard deviation (second column ) listed under each run number. + + En, Fs and Wo mole % and total range for analyzed grains. FeO*-all iron calculated as FeO. TABLE 5E Chemical composition of representative experimental spinels at 1 arm

Cr2Os TiO2 AI20~ FeO + Fe203 + MnO MgO Total M g / ( M g + F e 2+ ) Cr/(AI+Cr)

Run 4

Run 9

Run 16

Run 5

Run 20

23.10 2.57 30.06 17.17 12.40 0.22 13.98 99.51 0.592 0.340

25.33 2.92 33.84 17.45 4.49 0.15 14.31 98.49 0.594 0.334

23.10 2.15 35.21 19.28 6.75 0.31 12.93 99.73 0.544 0.306

22.78 4.24 23.60 22.98 16.33 0.31 10.71 100.95 0.454 0.393

0.12 1.52 19.92 6.93 53.38 0.77 18.60 101.24

+ Iron distributed according to spinel stoichiometry.

231

PHASE EQUILIBRIA O F LAVA FROM THE 1965 SURTSEY E R U P T I O N : RESULTS

often coexists with the augite (Fig. 2 ). The analytical results on the high pressure runs are subject to the same limitations as the 1 atm runs.

The phase proportions and the amounts of liquid remaining, were estimated by least-squares linear regression with the analytical data presented in Table 5. The proportions of crystallizing phases (Table 2 ) are approximately 1 : 1 for coexisting olivine and plagioclase and 2:3:1 for coexisting olivine, plagioclase and augite. Both the amount of crystallization (liquid remaining) and the composition of the glass show correlation with the equilibrium temperature (Figs. 3 and 4). The former with a significant change in the slope when a new phase appears (Fig. 4).

Phase compositions and liquid-mineral equilibria

Glasses The one atmosphere glasses (Table 5A) on simple variation diagrams show systematic trends (Fig. 5 ) that can be related to the crystallizing phases. The variation of the magnesium rich glasses is controlled by olivine crystallization. The appearance of plagioclase in glasses with about 7 wt.% MgO causes a marked change in FeO and A I 2 0 3 . The subsequent appearance of augite in glasses with 6-5 wt.% MgO results in a bend in CaO. A sharp rise in the TiO2 and P205 concentrations seems to coincide with the onset ofaugite crystallization and high degree of crystallization (Fig. 5 ). The compositions of the high pressure glasses (Table 6A), in part, reflect experimental and analytical limitations. In particular the loss of iron in some of the near liquidus glasses is evident (Fig. 6). This loss may, for the 10 kbar glasses, cause an

High pressure The high pressure runs are summarized in Fig. 2 and details are given in Table 3. Liquidus temperatures show a significant rise with increasing pressure of 6°C/kbar (Fig. 2). At l0 kbar, olivine appears at about 1290°C, followed by plagioclase at 1250°C and augite from 1230°C. The crystallization temperature of augite, however, sharply increases with pressure and at 12.5 kbar augite precedes plagioclase and from 14 kbar augite consistently appears as the liquidus phase. With the breakdown of plagioclase at 15 kbar, an Al-spinel SU 106-1 arm

I0-

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w t % MgO

Fig. 5. Compositional variation of the one atmosphere glasses. Data are from Table 5A, calculated to 100% and with all iron as FeO.

232

P. THY

TABLE 6A Chemical composition of experimental glassesat high pressures

SiO2 TiO2 AI203 FeO* MnO MgO CaO Na20 K20

P205 Total FeO*/MgO

SiO2 TiO2 A1203 FeO* MnO MgO CaO Na20 K20 PzO5 Total FeO*/MgO

7 kbar Run 42 (4) +

l0 kbar Run 12 (6)

l0 kbar Run 20 (9)

10 kbar R u n 13 (7)

10 kbar Run 3 (2)

10 kbar Run 6 (6)

46.77 2.02 16.00 9.22 0.15 8.77 11.53 3.30 0.42 0.23 98.41 1.05

48.47 1.91 15.87 8.91 0.20 8.29 11.26 3.40 0.45 0.24 99.00 1.07

47.67 1.89 16.07 10.30 0.20 7.05 11.12 3.51 0.44 0.23 98.48 1.46

47.79 2.30 16.03 9.03 0.19 7.72 11.03 3.52 0.46 0.26 98.33 1.17

46.63 3,88 15.41 9.95 0.24 6.95 10.23 3.58 0.69 0.54 98.10 1.43

45.59 4.01 14.97 14.86 0.26 5.17 9.68 4.15 0.85 0.50 100.04 2.87

0.82 0.09 0.07 0.24 0.04 0.11 0.09 0.06 0.03 0.05

0.23 0.07 0.15 0.11 0.05 0.09 0.09 0.06 0.03 0.03

0.50 0.12 0.14 0.29 0.03 0.17 0.10 0.07 0.05 0.05

0.69 0.13 0.24 0.29 0.03 0.09 0.23 0.12 0.11 0.06

0.36 0.24 0.23 0.44 0.05 0.20 0.39 0.33 0.04 0.03

12,5 kbar Run 46 (6)

12.5 kbar Run 43 (11)

12.5 kbar Run 45 (9)

12.5 kbar Run 47 (4)

48.89 1.96 15.08 9.63 0.18 9.63 10.96 3.20 0.40 0.21 100.14 1.00

47.14 2.08 15.30 10.97 0.22 8.23 10.82 3.40 0.41 0.22 98.79 1.33

47.85 2.16 16.61 11.75 0.18 7.30 9.60 3.76 0.47 0.29 99.97 1.61

45.85 2.85 16.88 13.85 0.24 5.24 8.28 4.88 0.77 0.46 99.30 1.67

0.65 0.05 0.18 0.19 0.02 0.09 0.16 0.08 0.04 0.05

0.53 0.23 0.15 0.33 0.05 0.16 0.15 0.19 0.04 0.08

0.39 0.10 0.20 0.15 0.03 0.23 0.22 0.08 0.05 0.03

15 k b a r Run 34 (4)

15 kbar Run 28 (8)

15 kbar R u n 35 (6)

15 kbar Run 38 (6)

17.5 kbar R u n 41 (10)

17.5 kbar Run 48 (4)

17.5 kbar R u n 54 (5)

20 kbar Run 26 (7)

20 kbar Run 15 (7)

48.04 2.25 17.26 8.42 0.16 8.43 10.20 3.58 0.56 0.28 99.18 1.00

48.38 2.32 17,72 7.72 0.16 8.13 10.13 3.98 0.60 0.28 99.42 0.95

48.02 2.25 18.05 10.26 0.19 7.29 9.19 3.72 0.61 0.31 99.89 1.41

46.18 2.34 17.75 13.39 0.18 4.98 7.92 5.37 0.62 0.34 99.07 2.69

45.87 2.27 17.22 13.42 0.19 6.70 8.80 4.10 0.57 0.35 99.49 2.00

45.95 2.43 17.92 11.90 0.20 5.95 8,12 4,28 0,70 0,36 97,81 2,00

47.69 2.43 17.60 13.40 0.17 4.41 6.59 5.65 0.85 0.41 99.20 3.04

45.53 2.18 16.99 12.61 0.20 7.05 8.31 4.40 0.66 0.30 98.23 1.79

46.51 2.38 18.82 11,55 0,17 5.84 7.94 4,70 0.67 0.40 98.98 1.98

0.74 0.10 0.15 0.18 0.02 0.03 0.04 0.03 0.04 0.06

0.24 0.10 0.30 0.47 0.02 0.47 0.15 0.14 0.19 0.04

0.50 0.29 0.49 0.37 0.04 0.75 0.29 0.17 0.02 0.06

0.55 0.17 0.25 0.77 0.01 0.47 0.84 0.31 0.34 0.03

0.55 0.06 0.51 0.30 0.03 0.11 0.09 0.23 0.04 0.04

0.70 0.08 0.14 0.15 0.02 0.05 0.08 0,09 0.03 0.04

0.60 0.09 0.33 0.43 0.02 0.16 0.10 0.23 0.04 0.06

0.38 0.19 0.42 0.32 0.03 0.43 0.38 0.12 0.10 0,04

0.61 0.56 0.69 1.63 0.06 0.64 0.17 0,22 0,05 0,17

0.39 0.16 0.36 0.16 0.04 0.19 0.19 0.18 0.06 0.04

+ N u m b e r in parenthesis refers to total n u m b e r of analyses used for calculating each average (first c o l u m n ) a n d one standard deviation (second column ) listed u n d e r each run number. FeO*-all iron given as FeO.

TABLE 6B Chemical composition of experimental olivines at high pressures

SiO2 FeO MnO MgO CaO Total Fo range ++

7 kbar ÷ Run 42 (2)

10 kbar R u n 12 (4)

10 kbar R u n 20 (6)

10 kbar R u n 13 (3)

10 kbar Run 3 (2)

10 kbar Run 6 (3)

12.5 kbar Run 46 (3)

12.5 kbar Run 43 (2)

12.5 kbar Run 45 (2)

12.5 kbar Run 47 (4)

39.82 16.41 0.27 42.51 0.12 99.13 82.2 82.2

39.81 0.42 14.63 0.24 0.19 0.02 43.90 0.05 0.14 0.04 98.67 84.3 84.0-84.5

39.39 0.84 18.06 0.54 0.24 0.02 40.92 0.39 0.20 0.05 98.81 80.2 79.7-81.0

38.91 0.22 15.86 0.37 0.21 0.06 43.15 0.66 0.25 0.02 98.38 82.9 82.5-83.5

39.20 18.46 0.29 41.28 0.26 99.49 79,9 78.9-80.0

36.86 0.94 25.04 0.17 0.36 0.02 36.35 0.14 0.28 0.01 98.89 72.1 72.0-72.2

40.73 0.22 13.71 0.04 0.19 0.03 44.90 0.98 0.28 0.05 99.81 85.4 85.1-85.7

39.06 17.31 0,25 42.31 0.24 99.17 81.3 81.1-81.6

39.92 20.35 0.28 40.48 0.14 101.17 78.0 77.6-78.4

37.97 0.67 22.35 0.27 0.27 0.01 36.66 0.84 0.22 0.06 97.47 74.5 74.0-74.9

+ N u m b e r in parenthesis refers to total n u m b e r of analyses used for calculating each average (first column ) a n d one s t a n d a r d deviation (second c o l u m n ) listed u n d e r each run number. + + Forsterite mole % a n d total range in analyzed grains.

233

PHASE EQUILIBRIA OF LAVA FROM THE 1965 SURTSEY ERUPTION: RESULTS TABLE 6C Chemical composition o f experimental plagioclases at high pressures 10 kbar Run 13 (3) +

Si02

51.50 0.80 0.51 0.08 29.07 0.49 0.45 0.06 0.52 0.33 12.96 1.03 4.39 0.26 0.19 0.02 99.59 61.3 57.7-64.3 ' 37.6 1.1

TiO2 A1203 FeO* MgO CaO Na:O K20 Total An range + + Ab Or

10 kbar R u n 3 (3)

10 kbar Run 6 (3)

12.5 kbar Run 47 (2)

15 kbar Run 38 (4)

53.10 0.36 0.11 0.09 29.02 0.15 0.55 0.24 0.16 0.02 12.77 0.36 4.91 0.16 0.19 0.04 100.81 58.4 57.1-59.8 40.6 1.0

53.41 0.07 0.25 0.03 28.01 0.89 0.99 0.27 0.55 0.36 12.20 0.38 5.26 0.04 0.28 0.07 t00.95 55.3 54.4-56.5 43.2 1.5

53.42 0.53 28.57 0.80 0.35 12.09 5.76 0.32 101.84 52.8 51.9-53.7 45.5 1.7

54.16 0.75 0.14 0.04 27.39 0.25 0.56 0.03 0.17 0.05 10.88 0.14 5.48 0.09 0.37 0.04 99.15 51.2 50.8-52.1 46.7 2.1

÷ N u m b e r in parenthesis refers to total n u m b e r o f analyses used for calculating each average (first c o l u m n ) a n d one standard deviation (second column ) listed u n d e r each run number. + +An, Ab, a n d O r mole % a n d total range in An for analyzed grains. FeO*-all iron calculated as FeO.

TABLE 6D Chemical composition of experimental clinopyroxenes at high pressures

SiO2

Ti02 A1203 FeO* MnO MgO CaO Na20 Total En ÷ + Fs Wo

SiO2 TiO2 AI203 FeO* MnO MgO CaO NazO Total En ÷ + Fs Wo

10 kbar Run 3 (4) +

10 kbar R u n 6 (2)

12.5 kbar R u n 45 (5)

12.5 k b a r R u n 47 ( 1 0 )

15 kbar Run 34 (7)

15 kbar R u n 28 (4)

15 kbar Run 35 (6)

50.09 1.15 6.36 7.08 0.18 16.40 18.08 0.55 99.89 49.1 11.9 39.0

47.16 2.62 9.34 9.51 0.24 11.66 16.64 1.30 98.47 40.3 18.4 41.3

50.60 1.04 7.64 7.23 0.19 16.20 16.96 0.65 100.51 49.9 12.5 37.6

47.95 1.74 10.67 8.73 0.20 12.83 15.49 1.37 98.98 44.5 17.0 38.6

48.27 1.04 9.50 5.76 0.23 17.90 14.85 0.86 98.41 56.3 10.2 33.6

50.32 0.96 8.65 6.66 0.25 18.21 13.32 0.84 98.37 57.8 11.9 30.4

48.75 1.39 11.39 7.79 0.17 14.03 13.46 1.36 98,52 49,8 15.9 34,3

15 kbar Run 38 (8)

17.5 kbar R u n 4 1 (8)

17.5 kbar R u n 48 (2)

17.5 kbar Run 54 (5)

20 kbar R u n 26 (14)

20 kbar Run 15 (8)

48.01 1.59 11.04 10.15 0.20 13.21 13.14 1.44 98.78 46.6 20.1 33.3

49.16 1.14 9.28 7.66 0.21 16.22 14.64 0.83 99.14 52.3 13.9 33.9

48.64 1.11 11.38 9.06 0.20 14.46 12.70 1.29 98.84 50.4 17.7 31.8

47.78 1.18 11.23 10.54 0.21 13.17 12.83 1.63 98.57 46.5 20.9 32.6

47.69 1.30 12.03 8.93 0.22 13.61 13.39 1.65 98.84 48.2 17.7 34.1

48.24 0.94 11.75 7.58 0.19 14.70 14.39 1.58 99.37 50.2 14.5 35.3

1.67 0.33 1.31 0.92 0.06 1.46 0.77 0.12

0.32 0.34 1.56 1.04 0.02 1.75 1.23 0.29

1.01 0.18 1.02 0.65 0.03 1.10 0.64 0.11

1.54 0.34 2.01 0.78 0.04 1.79 1.69 0.08

0.64 0.29 1.16 1.03 0.06 1.43 1.36 0.48

0.94 0.13 1.33 0.73 0.04 1.28 1.20 0.29

1.47 0.29 1.90 0.14 0.05 2.07 1.95 0.14

0.60 0.31 0.80 1.77 0.04 0.72 1.27 0.26

0.76 0.25 1.63 0.24 0.06 0.90 0.99 0.18

0.92 0.47 1.72 0.38 0.09 1.66 1.05 0.22

1.56 0.27 2.06 0.36 0.06 1.56 0.75 0.30

÷ N u m b e r in parenthesis refers to total n u m b e r o f analyses used for calculating each average (first c o l u m n ) a n d one s t a n d a r d deviation (second c o l u m n ) listed u n d e r each run number. En, Fs and Wo mole % of average composition. FeO*-all iron calculated as FeO.

234

P. THY

TABLE 6E Chemical composition of representative experimental spinels at high pressures

SiO2 TiO2

AI203 FeO* MnO MgO CaO Total

15 kbar Run 36

15 kbar Run 38

17.5 kbar Run 41

17.5 kbar Run 54

20 kbar Run 26

2.60 62.60 16.51 0.09 16.24 0.72 98.76

1.06 0.58 62.99 16.67 0.13 16.63 1.96 100.02

0.13 0.39 64.80 16.41 0.15 17.23 0.10 99.21

0.30 0.51 63.16 20.02 0.11 15.16 99.26

0.14 0.75 62.43 21.43 0.09 14.21 99.05

FeO*-all iron calculated as FeO. Fe203 calculated by charge balance is low or not present. -below limit of detection. Cr203 is below limit of detection for all analyses.

early olivine control to be masked by Fe loss. Some of the 17.5 and 20 kbar runs also show erratic compositional variations. Despite these problems, most of the high pressure runs appear to show systematic increase in FeO and possibly a slight decrease in SiO2 with advanced degree of crystallization. The TiO2 increase observed for the one atmosphere glasses is not seen in the high pressure glasses. The A1203, CaO and Na20 variations are dependent on pressure (Fig. 6). Relative to the one atmosphere glasses, CaO decreases and A1203 and Na20 increase as a function of pressure (10 and 15 kbar). Compared to the one atmosphere glasses, these differences can be related to enhanced augite and suppressed plagioclase crystallization. Olivine

The one atmosphere olivines range from Fo86 to (Table 5B) and the Fo contents show a positive correlation with temperature. The grain-to-grain variation is mostly below 2.5 mol.% Fo, but may reach much higher values (Table 5B). These values are too high to be attributed to analytical uncertainties and must in part record disequilibrium. The Fe 2+ and Mg partitioning between coexisting olivine and glass, nevertheless, supports an approach to equilibrium. The KD values for the Fe and Mg distribution between high temperature ( > 1140 °C) olivines and liquids [K Fe/ug ( o l / l i q ) = X ve (ol) X Mg ( l i q ) / X Mg (ol) X ve (liq)] average 0.25 with all iron calculated as Fe 2+ (Fig. 7). If these calculations are corrected for Fe 3+ in the glass, KD atFO68

tains an average of 0.29 (range 0.26-0.31 ) with a liquid F e 2 + / ( F e 2+ + F e 3+ ) =0.86 as suggested by Presnall et al. (1979) to be typical for oceanic basalts. This KD is within the 0.30_+ 0.03 obtained by Roeder and Emslie (1970). The low temperature olivine ( < 1140°C; F e / M g > 0 . 3 ) and glass pairs, however, reflect relatively disequilibrium (Fig. 7) with KD ranging between 0.23 and 0.35, but, nevertheless, with an average of 0.29. The high pressure olivines have compositions FO85_74 (Table 6B) and show a decrease in Fo with falling temperature. The Fe 2+ and Mg partitioning gives distribution coefficients that average at KD= 0.31 (range 0.30-0.31 ), slightly higher than for the one atmosphere runs. The low temperature runs reflect disequilibrium with a KD of 0.22. Except for the 7 kbar run (KD=0.37), there is no indication for disequilibrium due to the loss of Fe. Most high pressure runs crystallizing olivine have suffered some Fe loss, but appear from considerations Of KD'S tO have equilibrated to the changing bulk system. This observation is contrary to that of Jaques and Green (1979), who observed significant disequilibrium due to Fe loss. Plagioclase

One atmosphere plagioclases vary from An71 tO An64 (Table 5C) and show a slight positive correlation with normative anorthite of the coexisting glass (Fig. 8) and crystallization temperature. The high pressure plagioclases are less calcic, ranging between An61 and An51 (Table 6C; Fig. 8 ). This difference between the low and high pressure plagioclases exceeds the effect of relative sodium losses. Such a "pressure effect" has also been observed by Bender et al. (1978) and Green et al. (1979) in experiments on tholeiitic mid-ocean ridge basalts and is consistent with the elevated Na20 in the high pressure liquids (Fig. 6). Both the low and high pressure grain-to-grain variation ( < 6-7 tool.% An) often exceeds the variation expected as an effect of the analytical uncertainty. This variation points to an approach to equilibrium only, a common problem for most experimental work involving plagioclase (e.g., Johannes, 1978). A ugite

The augite compositions show large variation (Tables 5D and 6D) in part due to lack ofequilib-

235

PHASE EQUILIBRIA OF LAVA FROM THE 1965 SURTSEY ERUPTION: RESULTS

SU 106- 7-20kbar

-0.4 0 E -0.2~

c~l O-i

. [] • i~ '~'--~*~ - ~ -

&'-~'b5-

• 2~ -,~-•~ :sz&o-o-

o-

014-

-0

"~"z9"~*-~'- u45~_o-

012-

0

- 1.5:~

(D

-0 10--

O z

8--

-2 6--

-0 4--

t3~

~18--

~48

14--

- 4 ~ H-

[]

*A° '~"*¢' ~ o - o-

°o

--2 -0

12--

la_

D

10-

~ 52 --

--15

© 7

•

o 12.5 ,:~ 15

48-%z2~/.

46 I

I

2

0

I

4

6

1

I I0

8

0

-I0

D.~Q 0 z~

@ 17.5

•V

•

44

l arm

20 Kbar I

I

I

2

4

6

I 8

-5

I I0

12

wt% MgO Fig. 6. Compositional variation of the high pressure glasses. Data are from Table 6A, calculated to 100 oY0and with all iron as FeO. For comparison is shown the one atmosphere liquid line of descent from Fig. 5, together with tentative trends for 10 and 15 kbar as stippled lines.

OI-glass

0.5

80

/ b t:}-

o/ / "

-6 ~0.4 -

PI-glass

.~ 7 0 -

h

oT.OY

0.3-

[]

60-

/

D/

/

@

0.2-

~1

~ /

//~

I atm

•

7kb

0

04

' o'8'

50-

lo [] 125

0.1

/D

&'

O

d6'

o

Fe/Mg glass

Fig. 7. FeZ+-Mg partitioning between olivine and glass, calculating all iron as Fe 2+. Correcting the one atmosphere glasses for an estimated Fe3+ content gives a KD=0.29, slightly lower than 0.31 for the high pressure glasses. rium. Furthermore, the fine-grained augite in low temperature runs is difficult to analyze. The KD values for the distribution o f Fe and Mg between the one atmosphere pyroxenes and coexisting glasses (calculated with all iron as Fe z+ ) range between 0.22 and 0.26 (Fig. 9). The KD values for the high

40

•

Iatm

[]

IOkbar

0

125

20

I .BO

0

I I 40 50 60 Normative An glass

Fig. 8. Mol.% anorthite of experimental plagioclase versus normative anorthite of the coexisting glass.

pressure pyroxenes show large scatter, but appear to be systematically higher than for low pressure (Fig. 9), O f the analyzed 13 high pressure pyroxene-glass pairs, 9 give KD values between 0.24 and 0.32. These KD's are consistent with the 0.23 observed by G r o v e and Bryan ( 1 9 8 3 ) in one atmosphere experiments on mid-ocean ridge basalts and the 0.30 obtained

236

P. THY

~px- glass

0.5

Cpx

0.125

•

Ti

¢D

0.100

a,0.4 I.i_

A

~/

latm • lOkb []

r~ o

0.2 1

125

O

15

Z~

20

0.4

///

[]

0.050--

0.025--

17.5 •

1

-

I arm • I0 kb [] q2 5 0

/

0.075--

0.3--

0. I

5,

I 08

I

I 1.2

I

I 1.6 Fe/Mg

•

I

o.ooo

2.0 glass

0.I

0!2

01.3

I 0.4

o15:

I 0.6

0.7 AI

Fig. 9. The Fe and Mg distribution between coexisting experimental pyroxenes and glasses, calculating all iron as Fe 2+. KD= 0.23 is the one atmosphere distribution coefficient determined by Grove and Bryan (1984) for mid-ocean ridge tholeiitic basalts. CaFe

Cpx

0.125

• •

O.lOO -

o.o

a

17.5 kbar 20

/

-

/ o . 0.025

15

.5

0.000 Fe

_I

0.1

j_

0.2

l

0.3

l

0.4

i

0.5

I

l

0.6

0.7

AI Fig. 10. Mg-Ca-Fe variation of the experimental pyroxenes. Shaded field is the one atmosphere augites.

by Thompson (1974a) and Green et al. (1979) in high pressure experiments. The one atmosphere augites are calcic ( W o > 4 3 ; Fig. 10) and have Mg/ ( M g + F e ) in the range 0.75-0.68 (Table 5D) reflecting the relatively iron rich coexisting glasses. The high pressure pyroxenes are subcalcic (30 < Wo < 41; Fig. 10) and have systematically higher M g / ( M g + F e ) ratios of 0.85-0.69 (Table 6D). Except for the 20 kbar pyroxenes, there is a positive correlation between M g / ( M g + F e ) of the pyroxenes and the crystallization temperatures. The A1 and Ti substitution show extensive variation (Tables 5D and 6D). In particular, the high pressure augites, compared to the one atmosphere augites, contain significant higher amount of A1 in the pyroxene solid solutions (Fig. 11 ). Total A1,

Fig. 11. Ti versus AI (calculated to 6 oxygens) for the experimental pyroxenes. Symbols as for Fig. 10. (A) 1 atm, 10 kbar, 12.5 kbar and 15 kbar. (B) 17.5 kbar and 20 kbar.

furthermore, decreases as crystallization temperature increases for all pressures (except 20 kbar). The O A1203 (A1203cpx/A1203 liq, as w t . % oxides) for the low pressure pyroxenes vary from 0.31 to 0.49 and the D Ti°2 from 0.37 to 0.84. Such high values are far in excess of those determined by Grove and Bryan ( 1984; D AlzO3= 0.22 and D vi°2 = 0.27 ) which may be a compositional effect caused by a relatively lower activity of silica in the mildly alkalic lava investigated in this study compared to the tholeiitic lavas of Grove and Bryan (1984). The high D values are supported by values obtained by Mahood and Baker (1986) and Sack et al. (1987) in one atmosphere experiments on alkalic and mildly alkalic lavas. The D A1203 for the high pressure pyroxenes

P H A S E E Q U I L I B R I A O F LAVA F R O M T H E 1965 S U R T S E Y E R U P T I O N :

are, as expected, significant higher with D-values from 0.41 to 0.71; the D Ti°2 shows corresponding lower values of 0.29-0.65. The ferric iron content of the pyroxenes can be estimated from balancing charge deficiencies versus charge excesses (Fig. 12). Substitution of Na into M2 sites or AI into tetrahedral sites can be charge balanced by substituting A1, Cr, Ti and Fe 3+ into octahedral sites (Papike et al., 1974). Because of the relatively low fo2 imposed by the graphite container, the high pressure runs cluster around the Fe 3+ = 0 line (Fig. 12 ) and show no systematic evidence for the presence of ferric iron. On the other

237

RESULTS

C linopyroxenes 2 0 kbar

o

~

12.5

I;"A

17/7~

~

VlIIIIIA

ff/l

[7;l

IO

V777?1

I atm

Cpx

0.6

•

I arm

0

12.5 kb

÷

one anal. [

[] I0

0.025 0.050

~

~o.4-

[]

0.20

/ 0

I 0.1

I 0.2

I 03

I I 0.4 0.5 AIVI+ 2Ti

A 0.6

Cpx

0,6

L~ 15 kbar

@

t-r5

•

20

/

~ 0.4 -

zxz~ 0.2

0.0

m

/k

/ 0

zx~x

I 0.2

/

0./00

I

I

0.125 0,150 NO (0=6)

hand, the low pressure pyroxenes show a systematic departure from the Fe 3+ = 0 line indicating ferric iron in the solid solutions which probably occur as a Fe 3+vi-A1 iv substitution (Fig. 12). A strong positive correlation between A1~v and AI v~ suggests a dominantly Ca-tschermak substitution (AlVi-A1i') for the high pressure augites. For the low pressure pyroxenes, a combination of AlVLA1TM,Fe3+V'-Al~V and Ti-2A1 ~v substitutions (Sack and Carmichael, 1984) can account for the covariance of Ti and A1. Sodium substitution into augite shows a systematic rise with pressure (Fig. 13), probably reflecting a dominating high pressure jadeite (Na-AW) substitution (Thompson, 1974a) and a dominating aegirine component at one atmosphere ( N a - F e 3+ ).

Spinels

B I 0.1

I

0.075

Fig. 13. Sodium content of the experimental pyroxenes as a function of pressure.

0

~c~ °

0.0

I

I

I

I

0.3

0.4

0.5

0.6

Aft=÷ 2Ti Fig. 12. Charge balance of the experimental pyroxenes in the N a + A W versus A W + 2 T i diagram (Papike et al., 1974). Symbols as for Fig. 10. 1:1 marks the F e ~ + = 0 line. (A) 1 atm, 12.5 kbar and 10 kbar. (B) 15 kbar, 17.5 kbar and 20 kbar.

The spinels crystallizing in the one atmosphere runs are chromian spinels (Table 5E) with Cr/ (Cr+A1) below 0.50, M g / ( M g + F e 2+) between 0.65 and 0.40 and relatively low estimated Fe 3+. There is no systematic variation with the composition of the coexisting olivine or crystallization temperatures. A Fe-AI-Mg spinel appears at 1090°C (Table 5E). The high pressure spinels (Table 6E) show little compositional variations and are A1-Fe-

238 Mg spinels containing Cr and Fe 3+ below detection or calculation limits.

P. ~fHY

ne

pl A /

• ":a

":':." Liquid lines of descent Phase equilibrium constraints on the liquid lines of descent for the Surtsey basalt can be evaluated in normative projections of the expanded basalt tetrahedron of Yoder and Tilley ( 1962 ). To construct these projections, all glass and mineral compositions have been calculated into the C I P W molecular normative components nepheline (ne), olivine (ol), diopside (di), hypersthene (by), plagioclase (pl) and quartz (q), disregarding minor components (magnetite, ilmenite, apatite and orthoclase). Hypersthene is broken down into normative olivine and quartz and the amount of these two components are added to the original values. The ferric/ferrous iron ratio is assumed to be constant for both glasses [ F e Z + / ( F e 2+ + F e 3+ ) = 0 . 8 6 ] and minerals (0.0). This projection procedure follows that suggested by Presnall et al. ( 1979 ) and reduces the total n u m b e r of components to five which graphically can be shown as projections of two pseudo-quaternary systems with a c o m m o n p l - d i ol base.

One atmosphere The olivine, augite and plagioclase pseudounivariant' saturated, one atmosphere Surtsey liquids in part straddle the plane pl-di-ol. They plot, when projected from either diopside (Fig. 14) or plagioclase (Fig. 15), as an extension of, and in agreement with, comparable liquids determined for diverse alkalic lavas by Sack et al. (1987) and Mahood and Baker (1986). In the proximity of the p l di-ol divide, the pseudo-univariant liquids determined for tholeiitic basalts (Walker et al., 1979; Grove et al., 1982; Grove and Bryan, 1983; Tormey et al., 1987 ) appear to have comparable normative compositions to the alkalic liquid. This led Sack et al. ( 1987 ) to suggest that pseudo-univariant liquids are bridging a nepheline saturated, eutectic-type, ~Pseudo-univariant liquids refer, throughout this paper, to liquids saturated with olivine, plagioclase, and augite. Some liquids taken from Sack et al. (1987) are, in addition, saturated with magnetite.

,~

•

t

-

~o"

o Surtsey " Pantelleria

\ \

•

Aikalic

\ ~A99

v

py~ ol

\ q

oi- pl- aug soturated liquids

Fig. 14. One atmosphere, pseudo-univarianl liquids coexisting with olivine, plagioclase and augite projected from diopside onto the triangular diagrams ne-pl-ol and ol-pl-q. Normative, molecular projection and calculations are after Presnall et al. ( 1979 ). Shown are the two sigma precision ellipses for the microprobe standards of Table 4. SU 106 is the

starting composition from Table 1, showing the effect of maximum observed Na20 loss. The data points shown are alkalic lavas (Sack et al., 1987), Pantelleria (Mahood and Baker, 1986) and Surtsey (this study). Some of Sack et al.'s (1987) liquids are in addition saturated in Fe-Ti oxide minerals. All other data shown are not saturated in Fe-Ti oxides. Full drawn line is Walker et al.'s ( 1979 ) liquid line of descent for mid-ocean ridge type of basalts. The shaded area reflects either the uncertainty or a shift in the location of the low-Ca pyroxene saturated, pseudo-invariant according to Walker et al. ( 1979 ) and Grove and Bryan ( 1983 ). The location of the four-phase cotectic close to the pl-di-ol divide is uncertain. Liquids produced in diopside and olivine encapsuled experiments on an Oceanographer fracture zone basalt extend the multisaturated tholeiitic cotectic to higher temperature, relatively diopside-rich liquids and account for the significant curvature on the cotectic. A quantitatively similar curvature is present in the simplified CaO-MgO-AI203-SiO2 system (Presnall et al., 1979). pseudo-invariant point on the alkalic side of the p l di-ol divide and a low-Ca pyroxene saturated, pseudo-invariant, reaction point to the tholeiitic side of the divide. The thermal m a x i m u m on the olivine, plagioclase, and augite cotectic is generally assumed, based on an analogy with the simplified system, to be located relatively close to the plane pl-di-ol. Walker et al. (1979) located the thermal divide for midocean ridge types ofbasalts slightly to the alkalic side of the p i - d i - o l divide. This location is a shift relative to the position within the tholeiitic volume determined by Presnall et al. (1978) for the simplified system. The location of the thermal m a x i m u m is controlled by the composition of the crystallizing

239

PHASE EQUILIBRIA OF LAVA FROM THE 1965 SURTSEY ERUPTION: RESULTS

ne

ne

di

A ..

•

.

":;',,

o/'

"Z.°otO

suJo

ol

/

ol-pl-aug

\

di

o Surtsey ,, Pantelleria

\.Al,o,i~

~-"S~"/~,: '

Aug ire

\ ORB

saturated liquids

Glasses ~ ~\ \

q

Fig. 15. One atmosphere, pseudo-univariant liquids coexisting with olivine, plagioclase and augite projected from plagioclase onto the molecular, normative, triangular diagrams ne-di-ol and ol-di-q. Same projection method and data as for Fig. 14. mineral phases. The temperature m a x i m u m is reached when the liquid composition lies exactly on the plane defined by the crystallizing mineral phases. This means that the triangle plagioclase, augite and olivine becomes essentially ternary and the piercing cotectic an invariant point. As both the crystallizing olivine and plagioclase, for the present purpose, essentially plot on the same location as the ideal normative compositions, the composition of augite, which strongly departs from the ideal normative diopside, becomes crucial for understanding the liquid lines of descent and crystallization behavior in the proximity of the normative divide between alkalic and transitional and tholeiitic basalts. The one atmosphere Surtsey augites, coexisting with plagioclase and olivine, are relatively calcic and slightly nepheline normative (Fig. 16). Tie-lines between coexisting glass and augite for the Surtsey experiments vary from nearly parallel to the relevant pl-augite ( a u g ) - o l plane to slightly inclined toward silica (Fig. 16). Two of the liquids are on the projections located close to, if not on, the thermal m a x i m u m and their evolution, therefore, may be trapped on the thermal divide defined by olivine, augite and plagioclase. However, the trend observed for three of the liquids, away from the divide toward normative quartz (Figs. 14, 15), appears to be real and cannot be attributed to the experimental and analytical uncertainties. The total loss of sodium is relatively constant and amounts to 6-7%, which is insufficient to move the starting composi-

FZ Pigeomte " - . . / \ OI

~

o i - pl - aug s a t u r a t e d

liquids

q

Fig. 16. Pseudo-univariant and pseudo-invariant liquid-pyroxene relations projected from plagioclase on the molecular, normative ne-di-ol and ol-di-q diagrams. Projection method as for Fig. 14. Full drawn line is Walker et al's (1979) liquid line of descent for mid-ocean ridge type of basalts. The shaded area reflects either the uncertainty or a shift in the location of the low-Ca pyroxene saturated, pseudo-invariant according to Walker et al. and Grove and Bryan ( 1983 ). The data shown are alkalic lavas (AL; Sack et al., 1987 ), Pantelleria (PA; Mahood and Baker, 1986), Surtsey (SU; this study), PI2 (Oceanographer fracture zone basalt; Walker et al., 1979 ) and FAMOUS (FA; Grove and Bryan, 1983). Some of Sack et al.'s (1987) liquids are in addition saturated in either magnetite or ilmenite or both. All other data shown are not saturated in Fe-Ti oxides. The augites equilibrated with the alkalic and Pantelleria liquids fall within the general field for the Surtsey augites. Generalized tie-lines between coexisting augites and liquids are shown.

tion (SU106; Figs. 14, 15) to the saturated side of the thermal divide. This conflict in the detailed interpretation of the liquid line of descent may be related to shortcomings in the projection scheme and the relative positions of the thermal divide and the experimental liquids and starting compositions. Additional information on coexisting glasses and pyroxenes in relation to the p l - d i - o l divide is summarized on Fig. 16. The alkalic lavas experimentally examined by Mahood and Baker (1986) and Sack et al. (1987) crystallized augites very similar to those of the Surtsey experiments. Their tie-lines, therefore, are strongly inclined toward nepheline and away from the p l - a u g - o l plane and the whole sequence of these alkalic glasses obviously fall to the alkalic side of the thermal divide. The coexisting liquid and pyroxene data on tholeiites, melted by Grove et al. (1982), Grove and Bryan (1983) and

P. THY

240

Walker et al. ( 1979; sample P22 ), all fall within t h e same general field, on Fig. 16 represented by the FAMOUS experiments (Grove and Bryan, 1983). Most augites equilibrated with tholeiitic liquids are subcalcic. The projected tie-lines between coexisting glass and augites, therefore, are inclined toward quartz and these liquids clearly move away from the actual pl-aug-ol plane and the thermal maximum on that plane. The coexisting glasses and augites in Walker et al.'s (1979) experiments on their sample P 12 plot in an intermediate position between those of the relatively silicic tholeiites and the alkalic Surtsey lava (Fig. 16). It is a consequence of the variation in the augite compositions that the position of the thermal maximum on the pseudo-univariant cotectic appears to change from slightly within the alkalic volume to well within the transitional basalt volume. The cotectic mineral proportions can be estimated from the experiments on Pantelleria lavas (Mahood and Baker, 1986) which crystallized average proportions of 19% olivine, 53% plagioclase and 28% augite. The phase proportions of the alkalic lavas examined by Sack et al. (1987 ) appear, based on calculations of a few of their run products, to be similar to those obtained by Mahood and Baker (1986) ( 15% ol, 53% pl, 32% aug). The calculated Surtsey cotectic proportions (29% ol, 53% pl, 18% aug; Table 2) are very similar to those calculated for typical tholeiites (27% ol,. 57% pl, 16% aug; FAMOUS; Grove and Bryan, 1983 ). The last detectable liquid for the Surtsey basalt is located at approximately 1070°C (Table 2). The composition of this liquid can be estimated from graphic extrapolations like that shown in Fig. 3. This result, which linearly extrapolates the observed chemical variation 40-50°C down temperature, suggests that the final liquid contains 7.2 wt.% TiO2, 17.7% FeO and 3.0% MgO. Such estimates, however, are highly hypothetical as magnetite crystallization at a much earlier stage ( ~ 1090°C; Table 2; Fig. 5E ) will deplete TiO2 and FeO and strongly enrich SiO2 in the liquid. The onset of magnetite crystallization will shift the crystallizing assemblage away from the pl-di-ol divide well within the alkalic volume. In effect, the crystallization of magnetite a n d / o r ilmenite in alkalic lavas may cause a thermal divide to break down and the liquids to evolve toward quartz saturation (Osborn, 1979; Lapin et al., 1985; Mahood and Baker, 1986).

High pressures Due to the suppression of plagioclase crystallization with increasing pressure, liquids saturated with olivine, plagioclase and augite were only determined for three high pressure runs: two at 10 kbar and one at 12.5 kbar (Table 6A). These liquids are nepheline normative, plotting well within the alkalic volume (Fig. 17). In particular, the diopside component of the liquids appears to be strongly pressure sensitive and decreases with increasing pressure. This result is consistent with the general observation in many experimental studies that increasing pressure causes an expansion of the augite volume (e.g., Thompson, 1974b; Bender et al., 1978; Takahashi, 1980) at the expense of both the olivine and plagioclase volumes (Presnall et al., 1979 ). The high pressure effect on the liquids is paralleled by that on the pyroxene compositions that coexist with plagioclase and olivine. These pyroxenes become subcalcic at high pressure (Fig. 10). Such an effect is well established for simplified systems (cf., Lindsley, 1983 ) and natural systems (Green et al., 1979; Jaques and Green, 1980; Biggar, 1984). It is clear from the normative projections (Figs. 17 and 18B), and the mineral chemistry discussion, that the pyroxenes show wide normative variations primarily caused by extensive Ca-tschermak and hypersthene substitutions (cf., Biggar, 1984). The reason for this extensive variation may be due to metastable nucleation and growth of Ca-tschermakitic ne

di

ol

q o l - p l - a u g saturated liquids Fig. 17. High pressure, pseudo-univariant, liquid-augite relations for the Surtsey experiments projected from plagioclase. Same diagram and calculations as for Fig. 15. Extreme examples of tie-lines and suggested orientations of the liquid lines of descent are shown.

PHASE EQUILIBRIA OF LAVA FROM THE 1965 SURTSEY ERUPTION: RESULTS

ne

di

20

./~ ~ ',, ",~\17.5 kbar

kbar

ol

k.iquJdus augites

q

di

:ff'ob,,I0 kbor

B

ires

pl

ol

Fig. 18. (A) The liquidus augites from the 12.5 kbar runs (coexisting only with olivine) and 15, 17.5 and 20 kbar projected from plagioclase. Same projection as for Figs. 15 and 16. ( B) All experimentally determined augites projected from either nepheline or quartz on the pl-di-ol pseudo-ternary. clinopyroxene in place of the stable augite. Such a metastable component will tend to spread the augites toward nepheline normative compositions. Aluminous subcalcic augites similar to the experimental augites occur in nature and are frequently reported as megacrysts in alkalic lavas (Binns et al., 1970; Irving, 1974). The experiments presented here suggest that these megacrysts are cognate and formed at pressures approximating that at the crustmantle boundary. Although the equilibrium compositions of the augites cannot be assessed in detail, the main point to make here is that independent of the choice of tie-lines between coexisting pyroxene and glass, the pseudo-univariant liquids move away from the plaug-ol plane toward normative nepheline during progressive crystallization at 10 and 12.5 kbar (Fig.

241

17 ). The high pressure shift in augite compositions away from normative diopside is the main reason for the comparative shift in the cotectics. Because of the shift in augite compositions, the thermal maximum on the pseudo-univariant cotectic shifts from slightly within the alkalic volume, at one atmosphere pressure, into the olivine tholeiite volume at high pressure. Because of such a shift, transitional basaltic magma may evolve from the tholeiitic side of the ol-di-pl divide into the alkalic volume. The same direction of crystallization has been suggested by Stolper (1980) and Biggar (1984). Between 14 and 15 kbar pressures, plagioclase breaks down and is replaced by an Al-spinel (Fig. 1 ). Many of the 15-20 kbar runs contain augite and spinel as the only crystallizing phases above the solidus and, therefore, do not constrain univariant relations. These liquidus augites (Fig. 18B) show little pressure effect on normative diopside, but reveal a progression toward nepheline normative compositions with increasing pressure (Fig. 18A). Presnail et al. (1979) showed that in the C a O - M g O A1203-SIO2 system, the univariant forsterite, anorthite and diopside saturated cotectic breaks down at pressures above 7 kbar due to the expansion of the spinel field. Instead, two univariant spinel saturated cotectics are developed piercing the anorthite-diopside-forsterite join from within the alkalic into the tholeiitic side of the divide and terminating in enstatite saturated invariant points. Because the natural 15-20 kbar Surtsey augites are not constrained by univariant relations, the position of possible univariant relations and related invariant points at pressures above 12.5 kbar cannot directly be evaluated. Because the position of the cotectics are dependent on the crystallizing augites, the positions of the spinel saturated invariant points determined by Presnall et al. ( 1979 ) may be shifted into the alkalic volume for the basaltic system. At pressures above 15 kbar, crystallization ofaugite and spinel (and subsequent olivine) may initially drive a transitional basalt into the alkalic volume but finally reverse this trend away from normative nepheline along a spinel-augite-olivine cotectic. Conclusions

The olivine, plagioclase and augite saturated, one atmosphere Surtsey liquids straddle the divide be-

242 tween alkalic a n d tholeiitic basalts. Because the augites crystallizing from these liquids are slightly n e p h e l i n e n o r m a t i v e , the liquid line of descent is toward increasing hypersthene. This effect places the one a t m o s p h e r e t h e r m a l d i v i d e slightly w i t h i n the alkalic basalt volume. Pressures o f 10-12.5 kbar causes the augite c o m p o s i t i o n s to shift toward subcalcic a n d hypersthene n o r m a t i v e c o m p o s i t i o n s well within the t r a n s i t i o n a l basalt v o l u m e of the basalt tetrahedron. Such a shift causes the t h e r m a l maxim u m to m o v e w i t h i n the h y p e r s t h e n e n o r m a t i v e v o l u m e a n d p o i n t s to the possibility that fractionalion oftholeiitic magmas at high pressure may cause these to evolve toward n o r m a t i v e alkalic compositions.

Acknowledgments The a u t h o r is in debt to D.C. Presnall, U n i v e r s i t y of Texas at Dallas for access to the laboratory a n d for help a n d i n t r o d u c t i o n to the e x p e r i m e n t a l techniques. The e x p e r i m e n t a l work was in part d o n e d u r i n g a fellowship from the N o r d i c Volcanological Institute, Iceland. The samples were kindly supplied by S.P. J a k o b s s o n from the collections at the M u s e u m of N a t u r a l History, Reykjavik, Iceland. The m i c r o p r o b e work was d o n e d u r i n g a visit to the Nordic Volcanological Institute, Iceland. Discussions with D.C. Presnall, J.D. Hoover, S.P. Jakobsson a n d N. Oskarsson d u r i n g v a r i o u s stages o f this study are gratefully acknowledged. The m a n u s c r i p t was written d u r i n g a N a t i o n a l Research CouncilNASA Research Associateship a n d b e n e f i t t e d from c o m m e n t s from D.R. Baker, J.S. Beard, J. Longhi. T h e study has been s u p p o r t e d by D a n i s h N a t u r a l Science Research Council, Nordic C o u n c i l a n d U n i v e r s i t y of Aarhus.

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