Contrib Mineral Petrol (1991) 107 : 8-20 Contributions to Mineralogy and Petrology �9 Springer-Verlag 1991

Reequilibration of chromite within Kilauea lki lava lake, Hawaii

P.A.H. Scowen 1 *, P.L. Roeder 1, and R.T. Helz 2

1 Department of Geological Sciences, Queen's University Kingston Ontario Canada K7L 3N6 2 US Geological Survey, Reston VA 22092 USA

Received April 10, 1990/Accepted August 8, 1990

Abstract. Chromite mainly occurs as tiny inclusions

within or at the edges of olivine phenocrysts in the 1959

Kilauea Iki lava lake. Liquidus chromite compositions

are only preserved in scoria that was rapidly quenched

from eruption temperatures. Analyses of drill core taken

from the lava lake in 1960, 1961, 1975, 1979, and 1981 show that chromite becomes richer in Fe +2, Fe +3, Ti

and poorer in Mg, AI, Cr than the liquidus chromite.

The amount of compositional change depends on the

time elapsed since eruption, the cooling history of the

sample, the extent of differentiation of the interstitial

melt, and the position of the chromite inclusion within

the olivine phenocryst. Compositional changes of the

chromite inclusions are thought to be a result of reequili-

bration with the residual melt by cationic diffusion (Mg, A1, Cr outwards and Fe +2, Fe +3, Ti inwards) through

olivine. The changing chemical potential gradients pro-

duced as the residual melt cools, crystallizes and differ-

entiates drives the reequilibration process. Major and

minor element zoning profiles in olivine phenocrysts

suggest that volume diffusion through olivine may have

been the major mechanism of cationic transport through

olivine. The dramatic compositional changes observed

in chromite over the 22 years between eruption and 1981

has major implications for other molten bodies.

Introduction

The internal processes in magma chambers generally

cannot be observed directly as they happen. Usually they

are inferred by studying rocks, which may not be

pristine, and by analogy with experimental models. The unique lava lakes of Kilauea Volcano are "natural labo-

* Now at Research School of Earth Sciences, Australian National University, GPO Box 4, Canberra ACT 2601, Australia Offprint requests to: P.A.H. Scowen

ratories" of semi-enclosed tholeiitic lava that do allow

the direct study of some of the processes thought to

be important in magma chambers. Detailed documenta- tion of the 1959 eruption of Kilauea Volcano in conjunc-

tion with extensive sampling of both the eruption scoria

and the lava lake itself, in the form of drill core, makes

Kilauea Iki lava lake particularly conducive to study.

The quenched samples obtained through repeated drill-

ing of Kilauea Iki are instantaneous records of the pro-

cesses that were active during the cooling and crystalliza- tion of the lava lake.

The objective of the present work is to document

the chemical variability of chromite in Kilauea Iki lava

lake and to outline the major factors controlling the

observed chemical trends. This study is an elaboration

and continuation of work by Evans and Wright (1972)

on the composition of liquidus chromite from the 1959

Kilauea Iki and 1965 Makaopuhi lava lakes. Evans and

Wright used only four samples from the Kilauea Iki

eruption. Three of these were scoria and the other was an air-quenched lake surface sample. A much more com-

prehensive suite of samples is now available from

Kilauea Iki lava lake and the work presented here is based on the petrography and electron microprobe anal-

ysis of 44 samples which include eruption scoria and

drill core from 1960, 1961, 1975, 1979, and 1981.

Evans and Wright (1972) pointed out that unaltered

liquidus compositions of chromite are only preserved in rapidly air-quenched scoria and that, even where en-

closed by olivine, "early liquidus chromites are highly sensitive to subsequent events". New electron micro-

probe analysis of unaltered liquidus chromites from rap-

idly quenched eruption scoria are compared below with chromites in progressively older lava lake samples

(drilled 1, 2, 16, 19, and 22 years after eruption) to illus-

trate the extent of compositional change that has oc- curred. Particular emphasis is placed on the relationship and correlation between the composition and depth of inclusion of a chromite crystal within its host olivine

phenocryst.

Background

The 1959 summit eruption of Kilauea Volcano filled Kilauea Iki

pit crater to a depth of approximately 110 m with 38 x ] 0 6 m 3

of tholeiitic basalt creating the lava lake. Pyroclastic debris accu-

mulated to form an adjacent scoria cinder cone. A detailed account

by Richter et al. (1970) describes the eruptive sequence that lasted

36 days and consisted of 17 separate eruptive phases.

Between 1960 and 1962 four holes were drilled into the upper

crust of the lava lake. Three more were drilled in 1967, followed

by three in 1975, two in 1976, six in 1979, and seven in 1981.

Drilling was able to proceed to progressively deeper levels as the

lava cooled and crystallized over time. Helz (1980) summarized

drilling results from 1967 to 1979. Additional information on the

drill core is in Helz and Wright (1983) and all the work done

on Kilauea Iki to 1987 has been reviewed by Helz (1987a).

Petrography and chemistry of the I959 eruption

Most samples from Kilauea Iki lava lake are porphyritic olivine

basalt consisting of olivine phenocrysts in a fine-grained ground-

mass of glass, clinopyroxene, plagioclase, and opaque minerals in

varying amounts (Richter and Moore 1966). Unlike the majority

of Kilauea summit eruptions, the 1959 eruption is very rich in

olivine phenocrysts. Wright (1973) calculated an average bulk MgO

content of 15.4 wt % corresponding to about 20 wt % olivine. The

original Richter and Murata (1966) tripartite classification of oliv-

ine phenocrysts (i.e., euhedral, rounded, and skeletal) has been

modified and enlarged by Helz (1987b). Using eruption samples

and lava lake drill core, Helz concentrated on the morphology

and internal features of the phenocrysts in thin section to divide

the large and varied olivine population into five classes which are:

(1) irregular, block crystals, 1-12 mm long with multiple planar

extinction discontinuities; (2) euhedral or skeletal crystals, 0.5-

5 mm long; (3) round or strongly resorbed grains; (4) angular

or conchoidal fragments; (5) subhedral crystals containing sul-

phide-bearing inclusions. These five classes of olivine phenocrysts are determined on

petrographic criteria and they cannot be distinguished on the basis

of their composition or style of zoning. Helz (1987b) found that

all classes of olivine phenocrysts in the eruption scoria have core

compositions between Fo84 and Fo89, have essentially the same

NiO, MnO, and CaO content, and are only moderately zoned (de-

crease/increase < 3% Fo) in their outer 10-30 gm rim with normal

and reverse zoning equally abundant. In contrast to this, olivine

from the lava lake is only zoned normally and often shows consid-

erable zoning depending on when the sample was drilled and the

cooling history of the sample.

Kilauean basalts contain chromite as an ubiquitous accessory

mineral that occurs usually within or at the edges of olivine pheno-

crysts (Fig. I a). Chromite embedded in olivine tends to occur as

tiny euhedral crystals generally < 100 gin in diameter and com-

monly 20~40 gm across. Independent crystals of chromite in the

groundmass are rare but they do occur in both the eruption samples

and lava lake drill core. In lake samples these groundmass chro-

mites are often very large (with diameters up to 500 gm) because

their growth was not restricted by enclosure in olivine or by rapid

quenching on eruption. Groundmass chromites tend to be anhedral and isolated from the interstitial glass by clinopyroxene (Fig. 1 b).

Kilauea Iki chromite exhibits a large range of solid solution

extending from titanian-chromite (50 wt % Cr203, 1.2 wt % TiO2)

to chromian-titanomagnetite (11 wt % Cr203, 13 wt % TiOz). This wide chemical substitution into the spinel structure makes it a par-

ticularly sensitive indicator of the prevailing physical and chemical

environment at the time of its formation. The usefulness of chro-

mite as a petrogenetic indicator has long been recognized (Irvine

1965, 1967; Thayer 1970; Hill and Roeder 1974; Dick and Bullen 1984).

Fig. 1 a, b. Photomicrographs of Kilauea Iki samples: a chromite

crystals in a class I olivine phenocryst that is surrounded by pris-

matic plagioclase crystals and high relief clinopyroxene crystals,

plane-polarized light, width of field = 12 ram; b clinopyroxene crys-

tals separating opaque chromite from interstitial glass, plane-polar-

ized light, sample K175-1-141.5, width of field = 6 mm

Primary compositional trends of liquidus chromite in the 1959

Kilauea Iki and the 1965 Makaopuhi lava lakes were described

by Evans and Wright (1972). They found variations between indi-

vidually homogeneous chromite crystals but no systematic compo-

sitional differences between chromite wholly enclosed in olivine

and chromite surrounded by glass. The observed trends between

chromites (decreases in Cr2Oa, MgO, AIzO3 accompanied by in-

creases in FeO, FezO3, TiOz, V~O3) they attributed to decreasing

temperature and oxygen fugacity fro) and to increasing crystalliza- tion of chromite and olivine.

Sampling

Forty-four samples were used in this study. Five are tephra collect-

ed during different eruptive phases and the remaining 39 are lava

lake drill core. Relevant information on the samples is in the Ap-

pendix. All core samples, except two from hole KI79-5, are from

holes drilled near the centre of the lava lake (Fig. 2). Drill core

samples were chosen from the bottom and generally hottest section

of each hole. Most of the lake samples consisted of a liquid-crystal

"mush" that was quenched by the drilling process. However, some

of the samples were entirely crystalline when drilled (Appendix).

The samples give good coverage of the lava lake in time and quenching temperature.

Regardless of the fact that the five classes of olivine phenocrysts appear to be chemically indistinguishable, only class 1 olivine phe-

10

0 100 200 meters

b

3 5 0 0 -

> 3400 _r

0 3 3 0 0

o

>

r 3200

3 1 0 0

�9 N10E

"~ O "T" 0J

~ C C ~ C C C C C ~ C C C C C C C C C C 0 -

-io5o ~ g

lOO -

.&n,su,,.oe 300 �9

1,48 '.1 20~0 ~ " ~ / / ~'~pre 1959 surface (1:2000) "~ " ~ j J at 3 1 3 ft = 95.4 m

. . . . ~ ~ 400 �9 1955 (1:5000)

l l l l l l l l , l l l l l l l l l l l l l l l I

"0

w

~>

=o

~176 i

100 "O

Fig. 2. a Plan view of the post-1959

surface of Kilauea lava lake with the

network of levelling stations (small dots). Only those drill holes that were sampled

for this study are shown. See Helz

(1987a) for locations of all the 1961~

1981 drill holes, b Cross-section of

Kilauea Iki lava lake along the principal

N-S levelling stations. The drill holes are

projected onto this section. Vertical

exaggeration is 4:1. The two pre-

eruption profiles of the lava lake,

determined from 1948 and 1955 air

photos, do not coincide with the actual

location of the bottom of the take below

hole KI79-5. Helz (1980) discusses this

discrepancy

nocrysts and their chromite inclusions were analyzed in this study

for two main reasons. Firstly, the large size of the class 1 pheno-

crysts allows the greatest possible range in depths of inclusion of

chromite crystals. By virtue of their large size and 'kink-bands',

class 1 olivine phenocrysts tend to be the most conspicuous feature

in any thin section and this makes misidentification unlikely. The

second reason is that these phenocrysts have not grown in the

lava lake. Helz (1987b) points out that class 1 olivine phenocrysts

in the 1981 drill core are indistinguishable from class 1 phenocrysts

in 1959 eruption scoria and that, despite up to 22 years of slow

cooling in the lava lake, the internal features and size of these

olivine crystals remain unchanged, (compare Figs. 25.4 and 25.5,

pp. 695-696, in Helz 1987 b). Other more euhedral classes of olivine

phenocrysts, such as class 2, have increased in size since eruption

by overgrowth within the lava lake.

Chromites with mean diameters less than 20 gm were generally

not analyzed in order to minimize X-ray loss. Evans and Wright

(1972) found that for chromite crystals 20 pm in diameter the X-ray

intensity loss was about 1% relative to the amount present and

1.5% for 10 pm crystals. Chromite crystals in glass inclusions with-

in olivine were not analyzed if the area of glass present was greater

than, or equal to, the area of the chromite. Total Fe in chromite

was recalculated to give FeO and F%O3 assuming spinel stoichiom-

etry. Primary and secondary standards were run to check the accura-

cy and precision of the data. Values obtained for all major oxides

of the chromite secondary standard were within 3 % of the accepted

analysis. Reproducibility of the results was good and the precision

does not vary significantly among samples of differing ages or

composition (Scowen 1986).

Analytical method

Chromite, olivine, and glass were analyzed with an ARL-SEMQ

electron microprobe using energy dispersive spectrometry (EDS) at 15 kV. K-ratios were corrected for matrix effects using the proce- dure of Bence and Albee (1968) with the alpha correction factors

of Albee and Ray (1970). In addition, zoning profiles in olivine,

and chromium and titanium contents in glass were obtained using

wavelength dispe[sive spectrometry (WDS) at 25 kV.

R e s u l t s

Chromite and olivine zoning

T w e l v e c h r o m i t e s in s a m p l e s f r o m v a r i o u s yea r s a n d

d e p t h s in the l a v a l ake w e r e e x a m i n e d fo r z o n i n g , T h e

c h r o m i t e s r a n g e in size w i t h m e a n d i a m e t e r s 3 5 - 5 0 0 g m

a n d d e p t h o f i nc lus ion in o l iv ine 0 - 1 9 0 0 gin. A d i s t a n c e

0.8

11

0 . 7 - -

0.6 -

0.5

0 . 4 -

0 . 2 -

0 0.3

C r / ( C r + A I )

[] []

~ ~ E] rn EIN

[]

�9 C =�9 �9

�9 m = ==~= �9

�9 . . . . ; . . . , ;

�9 . ; ; ' " ~ . ' ~ . f . r ~ + ." . " . ' . . o - . 1 = " �9 _~. �9 ++.

�9 . . . a . . . = �9 ~..+._r .~ �9 �9 � 9 1 4 9 =1 .~ m = . , , -

, I

F e 3 / ' ( F e 3 + C r , A I )

+ =.

+"

+

DE]

I , I , I

�9 #

~- + �9 + +

�9 �9 � 9 1 6 2 . ; r �9 . . ~. �9 .~ �9 ~ . ~ , ' . ~ ' . � 9 , , � 9 - , ~ . . �9 �9

�9 # 1 �9 e l = �9 �9 I

�9 . . .~= ~ . . . " ' . ' . ~ . . . - . . - . .~- � 9

i I ,

0 . 4

+

++

Fig. 3. Composition of selected chromites projected onto two faces

of Stevens' (1944) spinel composition prism. Symbols are as fol-

lows: open squares=eruption sample chromites; small filled squares=1979+t981 drill hole chromites embedded in olivine;

Fe2/ ' (Fe 2 + Mg ) 0.7 0.8

crosses= 1979+1981 chromites in contact with the groundmass.

The heavy line shows the compositional variation from core (C)

to rim (R) of one large, zoned chromite crystal from sample KI81-

1-230.4 which was quenched from ~ 1140 ~ C

of 0 gm signifies that chromite is on the edge of an oliv-

ine phenocryst and in contact with the groundmass.

Chromites that are wholly surrounded by olivine are

homogeneous. The compositional variation, described

below, of chromite crystals embedded in olivine pheno-

crysts involves variations between individual chromites rather than within single crystals. On the other hand,

large chromite crystals that are partially or entirely in

contact with interstitial glass exhibit significant zoning.

Eighteen analyses across a large, 500 ]am diameter,

groundmass chromite (from sample K181-1-230.4, glass-

quenching T=l140 ~ C) show a progressive change in

composition from core to rim as indicated by the heavy lines in Fig. 3.

The compositional zoning of two olivine crystals from eruption scoria is compared to the zoning of two olivine crystals from the lava lake in Fig. 4. All four

profiles are taken from rim to core to rim and are located

well away from any included chromite crystals. The two olivines from the scoria show no detectable zoning

whereas the two lava lake olivines are zoned with respect

to Fe and Mg and with respect to minor elements such

as Cr and Ti. Lava lake olivine also has a higher total

Fe content.

Melt composition

Evolution of the melt within Kilauea Iki lava lake has

been documented by Helz (1987a). Figure 5 shows the

MgO, SiO2, and Cr203 content of glass for samples

examined in the present study. There is a good correla-

tion between decreasing MgO and Cr203 and increasing

SiO2. The CrzO3 content of the glass is low (< 0.1 wt %)

and it was possible to analyze only those samples that

contained sufficient glass well away from high-chromi-

um phases (e.g., chromite and clinopyroxene).

Chromite compositional trends

Figure 3 shows that chromite in the 1959 eruption scoria

is compositionally distinct from chromite in the 1979- ~981 lava lake drill core. Chromite from the eruption

12

4O

o~

20

a) scoria olivine (Iki 22)

MgO

FeO

Or203 0.08

t ,,o, 0.00 "1 I I

0 5OO 1000

b) scoria olivine (Iki 33) c) 1979 lava lake olivine

MgO

FeO

Cr20 ~

d) 1981 lava lake olivine

TIC)=

i i ! i s J i

Cr~O 3 Cr=O~

1000 2000 0 1000 3000 0 2000 4000

Distance (p, m)

Fig. 4a-d. Zoning profiles rim-core-rim for four olivine phenocrysts; a, b from eruption scoria; e, d from the lava lake. Analyses by WDS at 25 kV. Effective detection limit for Cr203 is about 0.01 wt%

T'C 1100 1200

56

52

48

.08

.06

.04

.02

+ +

+ +

Wt.% +

i s,o2 ++ )*

Wt,%

Cr203

[] [5

s

+ +4~ +

+ +

, + + + ) ~ + +~ 4 6 Wt.% MgO 8 10

Fig. 5. Composition of glass from eruption scoria (open squares) and from lava lake samples (crosses). Cr203 was determined using WDS at 25 ku The effective detection limit for Cr203 is approxi- mately 0.01 wt %. The temperature scale at the top of the figure is calculated from the wt % MgO of the melt (Helz and Thornber 1987)

scoria has lower Fe+2/ (pe+2+Mg) ratios and a nar-

row range of low Fe + 3/(Fe + 3 + A1 + Cr) values. The data

for the lava lake chromites show that inclusions in oliv-

ine overlap with chromites found in direct contact with

the glass or groundmass, and both these overlap the

range in compositional zoning found in a large chromite

crystal within the glass. Chromites are divided into four groups according

to the year the sample was collected and shown in Fig. 6.

For each group of eruption scoria, 1960+1961, 1975,

and 1979+1981, distance to groundmass is plotted

against composition for samples quenched from 1000 ~ C

or higher. The following trends can be identified:

1. Chromite inclusions in eruption scoria vary but there

is no systematic correlation between composition and

depth within the host olivine phenocryst.

2. For the 1960+1961, 1975, and 1979+1981 samples,

chromite inclusions close to the edges of olivine pheno-

crysts have more Ti, Fe +3, Fe +2 and less Mg, A1, Cr

than those more deeply embedded in olivine.

3. As the time-elapse since eruption increases, chromite

crystals more deeply embedded within olivine increase

in Ti, Fe + 3, Fe + 2 and decrease in Mg, AI, Cr.

4. By 1979 + 1981 even the most deeply embedded chro-

mite crystals have an Mg/Fe + 2 ratio well below the aver-

age for inclusions in the eruption scoria.

The depths to which changes are detected varies for each

cation. From 1959 (eruption) to 1981 the most dramatic

change is in Mg and Fe +2 where even the deepest inclu-

sion ( ~ 1900 gm) has been affected. Detectable changes

in Fe +3 and A1 extend to ~900 ~tm by 1979-1981,

whereas changes in Cr and Ti are confined to chromites

embedded within ~ 200 btm from the edge of the olivine

phenocryst.

Figure 7 shows the effect on composition of slow

cooling before quenching. Chromites from 1979+ 1981

samples quenched above 1000~ C show little variation

of Mg with depth of inclusion. Samples that cooled to

below 1000 ~ C show a greater change in Mg, with lower

Mg contents near the olivine phenocryst rim. The Fe +2

profiles mirror Mg except that the trend is reversed.

All temperature subgroups of Cr of the 1979+1981

chromites show the same trend regardless of the amount

of cooling before quenching. Trends for Ti are similar

to Cr but in the opposite direction. Profiles for A1 and

Fe +3 in the 1979+1981 samples are intermediate be-

tween the Mg (Fe + 2) trend and the Cr (Ti) trend. Repre-

sentative analyses of chromite inclusions embedded at

various depths within olivine, of olivine, and of

quenched-liquid composition for two 1979 samples and

one 1981 sample are in Table 1.

13

Z uJ

X 0

t~

Z o

0

00 Z uJ

0

o~ z 0

Ti

C r

AI

Fe+3

F e + 2

Mg

E R U P T I O N S C O R I A 3 -

2 -

1

~,,113 0o~0 �9 �9

t i t i

1 0 �9 ~,e e l e ~ -

4 I a a

6

e 5

L.:" ...: 4 � 9 1 4 9 �9

3

2 I n i - t

5

~,~:..,::,..,.~. .

1

n 0 i i

'! - ~ 1 7 6 �9

o0 o0:0 0o00

1 9 6 0 + 1 9 6 1

.

. - "t ""

i i n n i

�9 t " ' ' ' / . ' ? p. !. i 1 i i

~p

t~ t,

t:. ~�9 @

~ l � 9

i ' / , . . . t . . . . "

i J t I

2":"" ~

}~ ~ o ' ' ' 4 0 0 6 0 0 8 0 0 1 0 0 0

1 9 7 5

~ . .

I ;:..~,-., .... i n i

!...:ii"" "" !'.

I n n n

i

"i:~'~: " I �9

t

J.

i:!?:.., . .

�9 ~ �9

~

�9 �9 o

"."%: .~ . .

�9 �9

�9 . I ' ; ' - : " "

n n ~ ~ 1 0 n 0 0 i 2 0 0 4 0 0 8 0 0 8 0 0 1 2 0 0

1 9 7 9 + 1 9 8 1

! i -

�9 ~;4i",',':~,.., , :'~-,~', , "..- ",-'-" .

! " . J i

�9 �9 , ": �9 �9 ~.

!,,,."o ! : "

� 9

i i

i I n i i ~ u

�9 �9 �9 � 9 �9

�9 ~ ' e e 6 e �9 -~

o � 9 �9

m

i r . � 9 o e , , �9 l � 9

- o . o j ~ , e eoe~.=o p j � 9 �9 �9 �9 �9 �9 �9

i !i i

I I I I I I I 1 ~

2 0 0 4 0 0 6 0 0 8 0 0 1 0 0 0 1 2 0 0 1 4 0 0 1 6 0 0 1 8 0 0 2 0 0 0

DISTANCE TO GROUNDMASS (gm)

Fig. 6. Composit ion of chromite inclusions at varying depths within age of two 200 s EDS analyses on one chromite inclusion. Errors

olivine phenocrysts. Only those samples quenched from 1000 ~ C are covered by the dimensions of the dot

or higher are included in these plots. Each dot represents the aver-

All the samples used in this study, except two, come

from holes drilled near the middle of the lava lake where

the drill core samples the upper crust and liquid-crystal

" m u s h " at the base of the upper crust. The two excep-

tions come from hole K179-5 which was collared in the

north end of the lava lake (Fig. 1) and went through

the lake completely into pre-1959 rock. Figure 8 shows

the relationship between chromite composition and

depth of inclusion in olivine for K179-5-309.4, the chilled

lower crust at the base of the lake, and for K179-5-~ 63.0,

14

taken approximately halfway through the lava lake.

Sample KI79-5-309.5 cooled rapidly from ~ 1 2 0 0 ~

but, unlike the eruption scoria, it remained at tempera-

tures between 400 and 700~ until drilled. The Fe +3,

A1, Cr, and Ti content and flat profiles in K179-5-309.4

are comparable to the eruption scoria trends. Only the

z lU

x 0

t~

z O N

(9

11-

I0-

9

8 Cr

7

6

5

4

6

/ "

I I l I I I I I I I

5 - - - - - ) 1 0 7 9 . 1 9 8 1 T > I O 0 0 ~ C e r u p t i o n s c o r i a / ~ ,

Mg 4

�9 :. r . - ~ " .

# ,_'.,~Z'�9 ~ �9

~ , ~ * � 9 �9 1 9 7 9 . 1 9 8 1 T < ~ O 0 0 ~

2 I I t I i I I I [ I

0 200 400 600 800 1000 1200 1400 1600 1800 2000

D I S T A N C E T O G R O U N D M A S S (~m)

Fig. 7. Mg and Cr contents of chromite inclusions at varying depths in olivine. Original liquidus chromite compositions are shown by the eruption scoria field (dashed lines). The continuous line encloses the compositional field for 1979+1981 samples quenched from temperatures above 1000 ~ C. Analyses of chromite in 1979+ 1981 samples quenched from below 1000~ C are shown by individual dots

Mg and Fe + z content of the chromites show any detect-

able change with position in olivine and all chromites

have lower Mg/Fe +2 ratios than the eruption scoria

chromites. Rapid eruptive quenching does not preclude

changes in Fe +2 and Mg if the temperature of the rock

is subsequently maintained between 400-700 ~ C. Sample

KI79-5-J63.0 cooled to 990-995 ~ C before drilling (Ap-

pendix). It cooled more slowly than KI79-5-309.4 but

its position near the north edge of the lake probably

allowed faster cooling than samples at an equivalent

depth in other more centrally located drill holes. Except

for one chromite, 20 gm from the edge of the olivine

phenocryst, the KI79-5-163.0 chromites have the same

Cr and Ti content as those in the eruption scoria. The

chromite inclusions are slightly enriched in Fe § and

depleted in A1 as compared to the eruption scoria but

only the chromite close to the edge of the olivine pheno-

cryst shows any dramatic change in A1 and Fe +3. All

the KI79-5-J63.0 chromites have lower Mg/Fe § ratios

than the eruption scoria chromites.

Correlation o f chromite and olivine compositions

There is a positive linear correlation between the Mg

(Mg x 100/(Mg + Fe + 2)) of olivine and chromite (Fig. 9).

Eruption scoria contains the most Mg-rich olivines

which coexist with Mg-rich chromites. Four of 14 coex-

isting olivine-chromite pairs from the 1960+ 1961 drill

core have compositions comparable to the lowest Mg

of olivine-chromite pairs in the eruption scoria. The

other 1960+1961 samples and all the 1975 and 1979+

1981 drill core have coexisting olivine-chromite composi-

tions that are more Fe-rich than those in the eruption-

scoria. These later drill core samples show a wide range

in the olivine-chromite compositions. For a given Mg =~

Table 1. Representative analyses of glass, chromite, and olivine for three samples with different quenching temperatures

KI79-3-166,1 T(MgO)= 1070 ~ C KI79-3-172.9 T(MgO)= 1110 ~ C KISJ-1-209.8 T(MgO)= 1135 ~ C

GL CHR OL CHR OL GL CHR OL CHR OL GL CHR OL CHR OL

SiO~ 58.46 0.05 38.19 0.05 38.85 51.87 0.00 38.87 0.00 39.46 51.25 0.05 38.84 0.14 38.87 TiO2 2.63 7.44 0.00 1.95 0.00 5.27 5.91 0.00 1.20 0.00 3.79 8.54 0.04 1.97 0.00 A1203 16.02 8.87 0.00 t2.40 0.00 12.68 10.16 0.00 12.09 0.00 13.89 9.53 0.00 13.35 0.00 Fe203 ND 18.16 ND 11.84 ND ND 16.37 ND 6.76 ND ND 17.95 ND 14.47 ND CrzOa 0.15 29.40 0.06 42.29 0.10 0.00 32.72 0.26 50.49 0.20 0.13 26.09 0.J0 38.39 0.09 FeO a 6.32 28.41 21.46 21.95 17.94 11.59 25.88 18.17 17 .73 12.72 10.56 25.84 17.84 20.31 16.99 MnO 0.08 0.31 0.36 0.15 0.29 0.22 0.00 0.36 0.00 0.20 0.12 0.33 0.34 0.24 0.33 MgO 2.34 7.28 40,02 8.67 43.01 4.65 8.27 41.63 11 .14 45.67 5.84 9.15 41.89 9.57 42.68 CaO 5.87 0.11 0.23 0.09 0.26 8.42 0.00 0.28 0.08 0.18 9,98 0_06 033 0.12 0.26 Na20 4.79 0.00 0.00 0.00 0.00 3.16 0.00 0,00 0.00 0.00 2.93 0.00 0.00 0.00 0.00 K20 2.35 0.00 0.00 0.02 0.00 J.21 0.04 0,00 0.00 0.00 0,82 0.02 0.00 0.02 0.00

Total 99.01 100.03 100.32 99.41 100.45 99.07 99.35 99.57 99 .49 98.43 99.31 97.56 99.38 98.58 99.22 DTOGM 20 0 910 890 20 0 1150 1150 30 30 890 870

Adjacent olivine-chromite analyses have similar DTOGM values, olivine was analyzed 21~30 gm from chromite. T(MgO) is the quenching temperature of the sampte based on the MgO of the interstitial glass (Helz and Thornber 1987). FeO"= total Fe for glass and olivine, FezO3 is calculated assuming stoichiometry for chromite, CHR=chromite, OL=olivine, Gl=glass, ND=not determined, DTOGM: for embedded chromite= shortest distance (gin) from the edge of the chromite to the olivine-groundmass contact, for olivine=shortest distance (gm) from analysis spot to the edge of the olivine phenocryst

00 z LIJ (3 >- X O

C O

u')

z O P < L)

D R I L L H O L E K I79 -5

Fe+3

~~ oo o ~

Fe+2

�9 �9 �9 �9

~176176 o o

/

I I I I

Mg , f \

e r u p t i o n s c o r i a ~ j

o K179 -5 -309 ,4 o ~ o o o

o o

o . �9 �9 ee K I 7 9 - 5 - 1 6 3 , 0 5~ � 9

0

1 1 .

1 0 ,

9 ,

8

7

6

5

4

3

2

6

3 �84

Ti

2

1

- - . ,

I I I I

I I I I 1

0 2 0 0 4 0 0 6 0 0 8 0 0

~ o o o ~ 1 7 6 0 ./ o= . : /

Cr

I I I I

t

\ \

~o~ 9_o_ _~ - ; . � 9 � 9 1 4 9

AI

I I I I

2 0 0 4 0 0 6 0 0 8 0 0

DISTANCE TO G R O U N D M A S S (~am)

Fig. 8. Composition of chromite inclusions at varying depths in olivine from KI79-5. Original liquidus chromite compositions are shown by the eruption scoria field (dashed lines). Chromite analyses of the quenched lower crust, K179-5-309.4, are plotted as open circles. Closed dots are chromite analyses from sample KI79-5- 163.0 halfway through the lava lake

Mg# Ol iv ine

90

85

80

75

70 20

o �9

0 0 �9 �9

~.o:.

o o

I I

30 40

Mg#

)<

• ERUPTION S C O R I A

�9 1960+1961

e 1975

* 1979+1981 T>lO00~

O 1979+1991 T<1000~

I

50 60 70

C h r o m i t e

Fig. 9. Composition of adjacent olivine and chromite. Mg ~ = Mg + z x 100/(Mg + 2 + Fe + 2). Olivine analyses were done approxi- mately 30 gm from each chromite crystal

15

in olivine, the Mg =~ of coexisting chromite in samples

cooled to below 1000~ C before quenching is generally

lower than the Mg # of a coexisting chromite in samples

quenched from above 1000 ~ C, indicating temperature-

dependent partition coefficients.

Discussion

Compositional variability o f chromite

The narrow range of composition exhibited by liquidus

chromites from rapidly quenched eruption scoria ap-

pears to be an inherent feature of the Kilauea Iki lavas.

Wright (1973) suggested that the complex chemical vari-

ation in the erupted Iavas of Kilauea Iki could be ex-

plained by mixing of two magmas and Helz (1987b)

presented evidence for progressive mixing and equilibra-

tion of an '~ old" shallowly stored magma and a "new"

magma from a deeper source (45-60 kin). Differing

amounts of mixing of these two magmas could produce variable liquidus chromite compositions. In eruption

scoria, individual chromite crystals are homogeneous

and their composition does not depend on physical loca-

tion. There is no systematic compositional difference be-

tween chromite entirely surrounded by glass and chro-

mite embedded in olivine, and no dependence on depth

of inclusion in olivine. Rapid quenching on eruption

ensured that the liquidus compositions of the chromite

were preserved. Liquidus chromite compositions are not preserved

in the lava lake where there are systematic and progres-

sive changes in composition over time. The fact that

class 1 olivine phenocrysts have not grown while resident

in the lava lake means that the observed changes in the

chemical composition of chromite crystals embedded in

them were necessarily effected by reequilibration

through olivine and cannot be the result of progressive

olivine growth and inclusion of continuously evolving

chromite crystals. Two simultaneously occurring pro-

cesses of reequilibration are thought to be responsible

for the observed compositional changes in olivine and

chromite: reequilibration of both with the residual melt

and internal reequilibration between them. Both olivine

and chromite lose Mg and gain Fe, therefore the compo-

sitional changes cannot be attributed solely to their in-

ternal reequilibration. Chromite is able to communicate through olivine with the residual melt exchanging Mg,

Cr, and AI for Fe and Ti.

Factors controlling chromite composition

The amount of chemical change in chromite is a function of the time elapsed since eruption, the cooling history

of the sample, the extent of differentiation of the intersti-

tial melt, and the position of the chromite in olivine.

The first three factors are related because as time in-

creases, the lava lake cools and crystallizes, and the inter- stitial melt evolves along liquid-descent lines which are hybrids of perfect equilibrium and perfect fractional

16

crystallization (Helz 1987 a). Hetz found that, even where

significant amounts of melt are present, the melt compo-

sition varies widely on the scale of a thin section and

is extremely dependent on the local bulk composition.

This illustrates the lack of convection within the crystal-

lizing front. Compositional changes in olivine and chro-

mites are driven by the changing melt composition (i.e.,

changing chemical potential gradients). The observed re-

lationship between position and composition of chro-

mite within olivine is a result of the time- and tempera-

ture-dependent nature of the processes driving the com-

positional change.

Mechanism of reequilibration

The reequilibration process between chromite inclusions

embedded in olivine and the residual melt involves diffu-

sion of cations through olivine where diffusion is defined

to be the transport of matter in response to composition-

al or temperature gradients (Freer 1981). Transport can

occur through olivine either by volume diffusion

(through the lattice) or along easier routes such as melt-

filled microfractures and discontinuities (dislocations).

Evans and Moore (1968) suggested that compositional

changes in chromite from the 1965 Makaopuhi lava lake

could be explained by the outward diffusion of Mg, A1,

Cr and the inward diffusion on Fe +2, Fe +3, Ti through

olivine. Data from Kilauea Iki support Evans and

Moore's original suggestion and can be used to assess

the importance of the different diffusion pathways.

There are experimental data on Fe § Mg interdif-

fusion coefficients in olivine (Buening and Buseck 1973;

Misener 1974; Nakamura and Schmalzreid 1984) so the

hypothesis that volume diffusion through olivine is re-

sponsible for the observed Fe + 2 - M g changes in chro-

mite can be tested. Cationic diffusion through olivine

is assumed to be the rate-determining step for chromite

reequilibration, diffusion through chromite is ignored

on the basis that the volume of chromite to olivine is

small and diffusion coefficients in spinel are 60 to 100

times faster than in olivine (Wilson 1982), hence the

homogeneity of the chromite inclusions. Table 2 con-

tains the interdiffusion distances calculated at various

olivine compositions and temperatures using xa~Dt (x = diffusion distance, D = interdiffusion coefficient, t =

time). Observed F e § diffusion distances can be

visually estimated from Fig. 6. Note that the "observed

diffusion distances" depend on the sensitivity of the ana-

lytical method used to detect compositional change and

mean distances detectable by electron microprobe. In

the 1960 + 1962 samples, chromites up to ~ 300 gm with-

in olivine have changed in composition. For both the

1975 and 1979+ 1981 samples the deepest chromite in-

clusions have changed Mg and Fe +2 compositions ( ~

1200 gm for 1975 and ~1900 gm for 1979+1981). The

calculated distances from experimentally determined dif-

fusion coefficients (Table 2) are of the right order of

magnitude to account for the observed diffusion dis-

tances.

Table 2. Calculated Fe +2- Mg interdiffusion distances for Kilauea Iki samples quenched 1 2 years, 16 years and 20-22 years after eruption

T Fo D Diffusion distances (~ C) (mole %) (x l0 -11 cm2/s)

1960+ 1961 1975 1979+1981 (gm) (gin) (gin)

1200 86.6 4.08 360--510 1440 1600-1700 1200 71.6 5.20 400-570 1620 1810-1900 1100 86.6 0.936 150-250 690 750 800 1100 71.6 1.38 210-290 830 930 980 1000 86.6 0.407 110-160 450 510- 530 1000 71.6 0.659 140-200 580 640- 680

The calculated distances use interdiffusion coefficients (D) along the c-axis of olivine (Buening and Buseck 1973, Table 2) and as- sume the samples were held at constant temperatures before quenching. This is not realistic for the lava lake samples but the calculations give rough estimates of diffusion distances. All diffu- sion coefficients were determined at log fo2=-12 which is low for samples taken after 1967

Several assumptions have been made in the interdif-

fusion calculations, and complications in applying the

results to Kilauea Iki require comment. The diffusion

calculations are based on the assumption that the sam-

ples were held at a constant temperature (constant D)

before quenching which is not realistic for lava lake sam-

ples. Upper crust samples from the lava lake cooled by

variable degrees before quenching and their cooling-

rates varied widely, being slow ( ~ 3-6 ~ C/year) in the

melting range but extremely fast (up to hundreds of de-

grees/year) when below the solidus. Offsetting the de-

crease in D with temperature is an increase in D as the

lava lake olivine evolves to compositions richer in Fe,

A further complicating effect is the shift in foe of the

Kilauea Iki melt from quartz-fayalite-magnetite (QFM)

in 1959-1962 to nickel-nickel oxide buffer (NNO) or

slightly higher in 1967-1981 (Appendix). Volume diffu-

sion through a mineral depends on lattice defects (Buen-

ing and Buseck 1973). Increasing f% causes oxidation

and non-stoichiometry (cation vacancies) in olivine re-

sulting in larger F e + Z - M g interdiffusion coefficients

(Nitsan 1974; Smyth and Stocker 1975). Actual 1975

and 1979 + 1981 diffusion distances are probably greater

than the calculated ones, other factors being equal, be-

cause of the increase in fo2. Yet another factor to consid-

er is that diffusion is anisotropic, the calculated distances

use D for the c-axis which is the fastest diffusion direc-

tion in olivine.

Modeling the observed diffusion distances from ex-

perimental data is difficult because of the many simulta-

neously and continuously changing variables within the

lava lake. However, by calculating the diffusion dis-

tances at various temperatures and olivine compositions

the results should roughly bracket the range of appro-

priate conditions. Despite the shortcomings of the diffu-

sion calculations, the fact that the calculated distances

are the right order of magnitude to explain the observed

distances makes lattice interdiffusion through olivine a

17

viable mechanism for F e + 2 - M g reequilibration be-

tween chromite inclusions and the residual melt.

Virtually no experimental data exist for diffusion of

Cr, Ti, A1, and Fe + 3 through olivine but some tentative

speculations can be made. In contrast to the homoge-

neous eruption scoria olivine phenocrysts, olivines from

the lava lake are detectably zoned with respect to Cr

and Ti as well as Fe and Mg. The Cr zoning in olivine

and decreasing Cr/(Cr +A1) of zoned chromite exposed

to the melt are both consistent with the observed trend

of decreasing Cr in the residual melt which is mainly

a result of clinopyroxene crystallization. These observa-

tions suggest than volume diffusion of minor elements

through olivine occurs and is driven by the changing

composition of the residual melt. The depth in olivine

to which changes in chromite composition can be de-

tected using the 1979+1981 data are ~200 ~tm for Cr

and Ti, and ~900 gm for A1 and Fe § (estimated from

Fig. 6). Using these values, empirical diffusion coeffi-

cients can be calculated: D ~ 1 x 10-11 cm2/s for A1 and

Fe +3 and D ~ 6 x 10 -13 cm2/s for Cr and Ti. Thesc dif-

fusion coefficients are approximate, order-of-magnitude

estimates that may be only strictly applicable to the par-

ticular set of changing conditions found in Kilauea Iki

lava lake. Relative diffusivities of cations through olivine

as determined above are: DMg ~ Dve+ 2 > DA1 ~ DFe + a >~ Dcr~Dxi. This ranking corresponds to what might be predicted based on charge-balance considerations. Inter-

diffusion of Fe+Z--Mg involves no charge imbalance

whereas diffusion of trivalent cations and Ti + 4 does.

In their study of cationic diffusion through olivine,

Jurewicz and Watson (1988) could only measure A1 dif-

fusivities in high-pressure olivines because A1 concentra-

tions were below detection limits in one-atmosphere oliv-

ines. They found that Fe and AI diffusivities were the

same within analytical error and suggested that A1 diffu-

sion depends on Fe or occurs through an Fe § lattice

defect mechanism. The Kilauea Iki data support their

findings, Fe + 3 and A1 diffusivities appear to be coupled

at atmospheric pressures as well as at high pressures.

The possibility that chromite reequilibration was fa-

cilitated by melt-filled microfractures or other disconti-

nuities in the olivine phenocryst cannot be completely

dismissed, especially as decorated dislocation loops have

been observed in the lava lake olivines. However, volume

diffusion, particularly of Fe + 2 Mg appears to be a like-

ly mechanism for the observed compositional changes.

Implications for layered intrusions

The compositional modification of chromite seen in Ki-

lauea Iki lava lake has important implications for

layered intrusions and generally for any molten body where primary crystals reequilibrate with the residual

melt. Cumulate rocks with large residual porosities (orthocumulates and mesocumulates) are most likely to

be affected and chromite within these rocks should not

be used as indicators of liquidus chromite compositions

even where they are totally enclosed in olivine.

Several studies have centered on the changes in cu-

mulate mineral compositions in layered intrusions that

are caused by reequilibration with the residual melt. In

orthocumulates of the Jimberlana Intrusion, high and

variable Ti and Fe contents of chromite inclusions within

olivine led Roeder and Campbell (1985) to postulate that

chromite had reequilibrated with the intercumulus melt

through olivine. An interesting result of the Jimberlana

study was that chromite embedded in bronzite does not

appear to have changed in composition and so the pro-

cess of reequilibration by cationic diffusion through the

host mineral may be specific to olivine. Chalokwu and

Grant (1987) found that cumulus olivine in mesocumu-

lates and orthocumulates from the Partridge River intru-

sion reequilibrated with the Fe-rich intercumulus liquid

to compositions (Foal-Fo71) vastly different from their

original ones (Fos~-Fo79) with the extent of reequilibra-

tion dependent on the amount of intercumulus liquid.

If chromite inclusions are present in these olivines then

they probably have similar reequilibration trends as the

Kilauea Iki chromites.

There are many differences between Kilauea Iki lava

lake and large layered intrusions, the most significant

of which is probably the differing time-scales involved

in their crystallization. This does not invalidate the com-

parison and, in fact, the longer time-scales and generally

slower cooling rates in large intrusions must allow a

greater degree of reequilibration than is possible in a

small lava lake. All primary cumulus crystals in orthocu-

mulate rocks are likely to have reequilibrated with the

fractionating residual liquid in a similar manner despite

whether the rocks formed in lava lakes, lava flows, small

intrusions or large intrusions. In a series of calculations

Barnes (1986) found that the crystallization of trapped

intercumulus liquid causes significant compositional

changes in the existing cumulus minerals. The exact

trends of compositional change will depend on the liq-

uid-descent lines.

By examining the direction of compositional change

of Mg and Fe § in coexisting olivine and chromite it

should be possible to discriminate between shifts in

composition caused by subsolidus reequilibration as op-

posed to shifts caused by reequilibration with a residual

liquid. Subsolidus reequilibration in an olivine-chromite

cumulate requires the compositional changes of the two

minerals to be in opposite directions. If chromite is volu-

metrically insignificant compared to olivine then, as sub-

solidus recquilibration proceeds, chromite will become

Fe-rich and olivine adjacent to it will become Mg-rich

(Wilson 1982). In contrast to this, both olivine and chro-

mite can become Fe-rich in the lava lake because the

reequilibration process is not a closed system between

the two minerals but involves the residual melt as well.

Conditions such as low melt viscosities, low thermal gra- dients and vigorous convection favour the formation of adcnmulates with low residual porosities (Camp-

bell 1987). The only compositional changes that chro- mite inclusions in olivine are likely to exhibit in adcu-

mulates are those associated with subsolidus reequilibra-

tion.

18

Conclusions

Chromi te inclusions in olivine f rom the lava lake exhibit

striking composi t ional changes over the 22 years since

eruption. The changes are the result o f a two simulta-

neously occurr ing processes: reequilibration o f chromite

with the residual melt by cationic diffusion th rough oliv-

ine and reequil ibration between chromite and olivine.

For the condit ions in the lava lake (a tmospheric P, fo2 ~

NNO, T~ 1200~ ~ C) each cat ion has a characteris-

tic relative diffusivity th rough olivine, these are DMg~

DFe + 2 ~" DA1 ~ DFe + ~ ~ Dcr ~ DTi. Diffusivities o f Cr and

Ti are several orders o f magni tude lower than those o f

Fe + 2 - M g .

In a geologically insignificant a m o u n t o f time, chro-

mite inclusions in Kilauea Iki olivines have changed to

composi t ions completely distinct f rom their original li-

quidus ones. A l though the time-scales in other intrusive

bodies may be different this same effect mus t occur

wherever the evolving residual liquid remains in contac t

with cumulus olivine. It is unlikely that chromite in or th-

ocumulates and mesocumula tes f rom layered intrusions

will retain their original composi t ions even if they are

completely sur rounded by olivine and they should no t

be used as representative o f the liquidus composi t ion.

A final commen t is that this s tudy has shown that

an indirect technique for measur ing difficult-to-deter-

mine diffusion coefficients, such as A1 in low-pressure

olivines, may be feasible by moni to r ing easily detectable

changes in inclusions embedded at variable depths with-

in the mineral in question.

Appendix

Sample no. Depth Nature of sample (m)

Glass-quenching Other temperature temperature information (T~o, ~ C) ~

Phase assemblage No. of Comments chromite analyses

Iki-22 ] 1216 Iki-7 ] 1188 Iki-ll Eruption scoria 1185 Iki-21 1197 Iki-33 1188

KI-113 6.9511960 drill core 1092 KI-168 9.11 ~ 1961 drill core 1094

/

KI-184 10.00J1961 drill core 1027

KI75-1-136.5

KI75-1-139.3 KI75-1-141.5 KI75-I-143.8 KI75-1-144.9

KI79-3-134.8 KI79-3-140.3 KI79-3-144.9 KI79-3-150.4

KI79-3-165.4 KI79-3-166.1 KI79-3-169.1 KI79-3-171.9 KI79-3-172.9 KI79-1-175.0 KI79-1-184.5 KI79-1-200.7

KI79-1-203.6

KI79-5-t63.0

KI79-5-309.4

41.60 I

42.46] 43.13~1975 drill core 43.83|center oflake 44.16)

41.09' 42.76 44.16 45.84

50.41 50.63 1979 drill core 51.54 ~center of lake 52.40 52.70 53.34 56.24 61.17

62.06.

49.48 t 1979 drill core

94.30Jnorth edge oflake

1064

1085 1093-1096 1114 1119

1061 1070 1078 1100 1110 1096 1116

(1135)

1138

L+o l+chr 13 L+o l+chr 10 L+o l+chr 10 L+o l+chr t4 L+o i+chr 10

L + o l + 2 px+pl+ox 16 pig; il L + o l + 2 px+pl+ox 16 pig; il L + o l + 2 p x + p l + o x + a p 7 pig; il+mt;

fo2~QFMb

L + o l + 2 px+pl+ox 13 pig; il+mt; fo2 ~ NNO

L + o l + 2 px+pl+ox 10 pig; il L + o l + c p x + p l + o x 9 il L+o l+cpx+p l 7 L+ ol+cpx+pl 6

T>400 ~ C (t.c.) a o1+2 p x + p l + o x + a p 9 pig T>500 ~ C (t.c.) o1+2 p x + p l + o x + a p 4 pig T>600 ~ C (t.c.) o1+2 p x + p l + o x + a p 11 pig T> 700 ~ C (t.c.); o1+2 p x + p l + o x + a p 4 pig; il+ fpsb +rot; T= 825-885 (mt-il) b fo2 ~NNO b

L + o l + 2 px+pl+ox 10 pig L + o l + 2 px+pl+ox 8 pig L + o l + 2 px+pl+ox 10 pig L + o l + c p x + p l + o x 9 il+ fpsb L+o l+cpx+p l 10 L + o l + c p x + p l + o x 8 il+ fpsb L+o l+cpx+p l 6

Interpolated L + ol + chr + cpx + pl 10 temperature

L+o l+ch r+cpx+p l 9

T=990-995 ~ C (mt-il) b L + o l + 2 p x + p l + o x + a p 8 opx; il+ fpsb +sp; fo~ ~ NNO

T>410 ~ C (t.c.)" o1+2 p x + p l + o x + a p 12 basal chili sample

Notes on individual samples analysed in this study. Abbreviations: L = liquid; o l = olivine; chr = chromite; p x = pyroxene; cp x = clinopyrox- ene; o p x = orthopyroxene; pig = pigeonite; p l = plagioclase; o x = oxide; nat = magnetite; il = ilmenite; ap = apatite; sp = spinel. Cpx refers to augite where it is the only pyroxene present, aTemperature obtained by downhole thermocouple measurement (Hawaiian Volcano Observatory, unpublished data), u T--fo~ values for mt-il assemblage using Spencer and Lindsley (1981). CGlass-quenching temperatures obtained using calibration of Helz and Thornber (1987). aTemperature profile in Hardee et al. (1981)

Appendix (continued)

19

Sample no. Depth Nature of sample Glass-quenching Other temperature

(m) temperature information

( T ~ o , ~ C) ~

Phase assemblage No. of

chromite

analyses

Comments

KI81-1-145.1 44.23'

KI81-1-150.4 45.84

KI81-1-162.9 49.65

KI81-1-167.4 51.02

KI81-1-173.1 52.76

KI81-1-184.8 56.33

KI81-1-189.4 60.14

KI81-1-197.3 62.60

KI81-1-209.8 63.95

KI81-1-230.4 70.22

KI81-1-249.7 76.11

KI81-1-265.1 80.80

KI81-1-281.3 85.74

KI81-1-290.0 88.39

KI81-1-297.6 89.79

KI81-1-306.7 93.48

.1981 drill core

center of lake

(970)

(1068)

1093

1122

1135

1141

1137

(1129)

1122

1117

1107

1087

T>148 ~ C (t.c.) d

T> 286 ~ C (t.c.)

T> 500 ~ C (t.c.)

T> 600 ~ C (t.c.)

T by CaO content of glass c

Interpolated

temperature

Interpolated

temperature

o1+2 p x + p l + o x + a p 10

o1+2 p x + p l + o x + a p 12

o1+2 p x + p l + o x + a p 5

ol+2 p x + p l + o x + a p 10

L + o l + 2 p x + p l + o x + a p 7

L + o l + 2 p x + p l + o x + a p 11

L + o l + c p x + p l + o x 12

L + o l + c p x + p l 10

L+ o l+chr+cpx+pl 8

L + o I + c h r + c p x + p l 8

L + o l + c h r + c p x + p l 10

L + o l + c p x + p l 7

pig

pig

pig

pig

opx

il + fpsb

L + o l + c p x + p l 8

L + o l + c p x + p l 9

L + o l + c p x + p l + o x 9 fpsb

L + o l + c p x + p l + o x 4 opx; fpsb

Acknowledgements. This paper is based on the MSc research of

PAHS who was supported by a Natural Sciences and Engineering

Research Council of Canada postgraduate scholarship while at

Queen's University. David Kempson and Alan Grant are thanked

for help with the manuscript. Thorough reviews by Ray Binns

and Ian Campbell greatly improved the final text.

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