Magnesioferrite-olivine rock and monticellite-bearing dunite from the Iwanai-dake alpine-type peridotite mass in

the Kamuikotan structural belt, Hokkaido, Japan

JITSUYA NAGATA

Department of Earth Sciences, Kanazawa University, Kanazawa 920, Japan

Magnesioferrite, monticellite, perovskite and calcium brittle mica occur in the alpine-

type Iwanai-dake peridotite mass in the Kamuikotan structural belt, Hokkaido. These

minerals were not primary phases, but were produced through metasomatic processes at a

later stage of the granulite facies equilibration.

Introduction

Magnesioferrite, monticellite, perovskite

and calcium brittle mica were found in the

Iwanai-dake peridotite mass, an alpine-type

intrusion in Hokkaido. As these minerals

have not been described in alpine-type peri

dotite, their modes of occurrence, chemis

tries and paragenetic relations will be

described in this paper.

The Iwani-dake mass (Fig. 1) is intrud-ed in the Kamuikotan structural belt which is a melange zone consisting of high-pressure metamorphic rocks, low-pressure metamor

phic rocks derived from an ophiolitic suite, and ultramafic rocks (Banno et al., 1978; Asahina and Komatsu, 1979). The Iwanai-dake mass is composed largely of dunite and harzburgite and is almost free from ser

pentinization (Bamba, 1955; Kato, 1978; Niida and Kato, 1978; Arai, 1978). Generally, dunite and harzburgite form layered structures. Small amounts of chromitite are also present. Ultramafic rocks of the Iwanai-dake mass are intensely deformed, and translation lamellae of olivine and orthopyroxene are very common. Accord-

Fig. 1. Locality of the Iwanai-dake peridotite

mass, Hokkaido.

ing to Arai (1978), equilibrium temperature

of ultramafic rocks is estimated to 600 to

700°C using the olivine-Ca-rich clinopyroxene

geothermometer and olivine-spinel geothermometer.

Mode of occurrence, petrography and

mineral chemistry

A large quarry in the center of the

Iwanai-dake peridotite mass contains

exposures of remarkably fresh dunite and

(Manuscript received September 21, 1981)

J. Japan. Assoc. Min,Petr. Econ. Geol.77, 23-31. 1982

24 Jitsuya Nagata

harzburgite.

Magnesioferrite occurs in one magnesio-

ferrite-olivine rock, which consists of single

layer concordant with well-developed rhyth-

mic layering of dunite and harzburgite.

Its thickness does not exceed 20 cm. The

mode of occurrence in the outcrop is shown in Fig. 2. Magnesioferrite-olivine rock consists of magnesioferrite and olivine in equal amounts, and minor amounts of calcium brittle mica (clintonite or xantho-

phyllite) and serpentine. Fine calcium

Table 1. Selected EPMA analyses of minerals in magnesioferrite-olivien rock and

IW 328 dunite from the Iwanai-dake peridotite mass

Fig. 2. Mode of occurrence of the magnesioferrite -olivine rock i

n outcrop

Magnesioferrite-olivine rock and others from the peridotite mass 25

Fig. 3. Range of spinet solid solution. Al-Cr-Fe3+ (A) and Mg-Fez+ (B) atomic ratios of magneisoferrite (solid circle) in magnesioferrite-olivine rock, and of spinel core (solid

diamond) and rim (open diamond) in dunite, specimen IW 328, from the Iwanaidake mass. Tie lines connect core-rim pairs.

brittle mica (10 X 30 ƒÊm) sometimes occurs

in monticellite-bearing dunite. Table 1

shows the representative analyses, and Fig. 3

illustrates the range of spinet solid solu-

tions. Olivine is very magnesian with the

Fo molecule ranging from 96.7 to 98.7 per

cent and has high CaO contents, 0.1 to 0.2

weight percent.

Monticellite was detected in 2 samples

of 400 peridotite sections from the Iwanai

dake quarry examined by electron probe.

One is a dunite, specimen I W 328, and con-

tains monticellite, Ca-rich diopside, pero-

vskite, ferripargasite, Fe3+-rich spinel,

calcium brittle mica, andradite, magnetite,

ilmenite and calcite as well as olivine,

brucite and serpentine. The other is a

dunite, specimen IW 104, and contains

olivine, diopside, chromian spinel, brucite

and serpentine. It occurs about 20 m

above the magnesioferrite-olivine rocks

mentioned previously. Very fine monti-

cellite grains are attached to the rim of

olivine. Specimen IW 328 studied in detail

will be described below.

Dunite, specimen IW 328, is free from

severe serpentinization and usually exhibits

equigranular texture as shown in Fig. 4,

though olivine porphyroclasts do occur.

Selected microprobe analyses of the min

erals in this sample are listed in Table 1.

Olivine, Fo91 .1, is mostly 0.05 to 0.4 mm

Fig. 4. Photomicrograph of IW 328 dunite,

showing equigranular texture. Crossed

nicols. In this photo, all grains are

olivine.

2

6 jitsuyia Nagata

in diameter. Fig. 5 shows the Cs.U contents

in olivine of this sample and those in

monticellite-free dunite and harzburgite

from the Iwanai-dake mass. The CaO

content of the olivine from monticellite-

bearing dunite ranges from 0.1 to 0.4 weight

percent, and is distinctly higher than that of

olivine from monticellite-free dunite, less

than 0.1 weight percent.

Monticellite has several modes of oc

currence, (1) irregular-shaped and 100 to

200ƒÊm across at olivine grain boundary, (2)

as composite grains with diopside about

50ƒÊm across, and (3) as lamellae in porphyro-

clastic olivine (3 mm in diameter) (Fig . 7).

Monticellite of mode (3) is too minute to

analyze with electron probe, and chemical

data for modes (1) and (2) are presented.

CaO content of olivine in grain A _'- eases towards the monticellite-olivine iiterface

(Fig. 6). However, this relationship iay be reversed as in grain B, in which the Ca

content of olivine increases towards monti-

cellite. In Fig. 7, parallel lamellae of mon

ticellite in olivine are seen using a back-

scattered electron image and a CaKa X-ray

image of an olivine grain. Even though:

the crystallographic orientation of the

monticellite in the olivine could not be

determined (we could find only one olivine

porphyroclast to contain such lamellae), it is possible that they are exsolution

The Ca/(Ca+Mg-f-Fe*) atomic ratio

are around 0.49, and the Mg/(Mg-r-Fe*)

ratio are 0.79, which are lower than the

value of coexisting olivine , 0.91 (Fe* is total Fe).

Mode (1) is illustrated in Fig . 6-a which shows a Ca profile across the interface of

monticellite and neighboring olivine . The

Fig. 5. Histograms of CaO contents of olivine

in harzburgite (A) , dunite (B) and monticellite-bearing dunite (IW 328)

(C) from the Iwanai-dake mass . N, number of analyses .

Fig. 6. Compositional profiles across the interface of monticellit e and neighboring

olivine in regard to Ca. Ca ntent of olivine decreases (a) or inci s (b) towards monticellite . Mo; ?? cellite, O1; olivine .

Magnesioferrite-olivine rock and others from the peridotite mass 27

Fig. 8. Al-Cr-(Mg+Fe*) atomic ratios of diopside from the Iwanai-dake mass. Fe*: total iron.

Fig. 7. Backscattered electron image (A) and CaKa X-ray image (B) of an olivine

porphyroclast.

from chemistry alone. Perovskite occurs

as isolated rounded grain and 50,um across

at most. Andradite is included in calcite,

and minute magnetite and ilmenite grains

are scattered all around the thin section.

Discussion

lamellae.

Chemical analyses of diopsides show

that the (Ca-0.5 Al)/(Ca-0.5A1+Mg+Fe*)

ratio, i.e. Ca/(Ca+Mg+Fe*) ratio corrected

for tschermakite substitution, is 0.49 to 0.50

and is higher than those of diopside from

the ordinary dunite and harzburgite of the

same mass, 0.44 to 0.47 (Fig. 8).

Pargasite has a composition :

Na0,9Ca2.0 (Mg3.9Fe2+0.1) (Fe3}0.4T10.1Cr0.1A10.4)

(S16.0A12.0) O22 (OH) 2The range of solid solutions of the

spinel is shown in Fig. 3. Fe3+/(Al+Cr+Fe3+)

ratio at the rim is about 0.35 and is higher

than that of the core, 0.15.

At the margin of spinel, very fine-grain-

ed calcium brittle mica is often attached.

Its small grain size prevents us from deter-

mining whether it is clintonite or xanth-

ophyllite which cannot be distinguished

Most of accessory minerals of the du

nites described above are rare minerals, and

their known modes of occurrence will be

briefly reviewed below.

Magnesioferrite has been known notably

from fumaroles of Vesuvius (Deer, et al.,

1962a) and dolomitic marble at Lhngban,

Sweden (Von de Pijpekamp et al., 1969).

In Japan, magnesioferrite was identified in a

magnetite-bearing pyrite ore of Bessi and

Shingu mines by Curie point measurement

and ore microscopy (Yamaoka, 1962).

Monticellite is riot rare in skarns and

carbonatites, but from mafic and ultramafic

rocks it has been described only from alonite

in the Oka area, Canada (Gold, 1967), and

monticellite peridotite in the Highwood

Mountain alkaline intrusive complex, Montana

(Buie, 1941) .In addition to the occurrences in skarns

28 Jitsuya Nagata

and carbonatites, perovskite has been

described in alnoite (Dawson et al., 1978) and

kimberlite (Mitchell and Clarke, 1976;

Gogineni et al., 1978). In Japan, monti

cellite and perovskite were described from

skarn (Kusachi et al., 1973, 1979).

The most common occurrence of calcium

brittle mica, clintonite and xanthophyllite,

is with talc in chlorite schist, and in metas

omatically altered limestones (Deer et al.,

1962b). In Japan, the known occurrence

of xanthophyllite is limited to skarn (Sekino

et al., 1975).

Genesis of ina,anesioferrite

Two hypotheses may be considered for

the gensis of the magnesioferrite equilibrated

with olivine to form the magnesioferrite-

olivine rock in question.

The first hypothesis that there was

magnesioferrite-olivine rock coexisted with

basaltic magma has the difficulty that

magnesioferrite has been unknown in alpine-

type peridotite. Its formation requires

quite a high f(02), about 10-2 atm at 1300•Ž

(Ulmer, 1969), usually not realized in basaltic

magmas. It appears quite unlikely that

such an oxidized environment is realized

when alpine-type peridotite coexisted with

basaltic magma .

If magnesioferrite was a primary phase,

we should accept that the following exchange

reaction proceeded from the left to right

hand side with falling temperature :

Mg1•E82Fe2+0 .18SiO4+Mg0.84Fe2+0.16Fe3+2O4

(Fo9l olivine)=Mg1.96Fe2+0.04SiO

4+(Fogs olivine)

Mg0.70Fe2+0 .30Fe3+2O4 (1)

We assume that Fo component in olivine of

the primary assemblage is 0.91, the value

normal for olivine from alpine-type peridotite, i

ncluding the majority of the Iwanai -dake

mass. Mg/(Mg+Fe*) and Mg/(Mg+Fe2+)

ratios of olivine and spinel, respecti ?? of the lower temperature assemblage are t ?? values actually observed in the Iwanai-da ?? e mass. The Mg/(Mg+Fe2+) ratio of the assumed

primary magnesioferrite should be 0.84. It follows that Mg-Fe2+ partition coefficient, defined by (Mg/Fe*)o1ivine/ (Mg/Fe2+)spinel changed from 1.9 to 27.0 with falling tem

perature. On the other hand, thermodynami-cal data of relevant solid solutions given by Robie et al. (1978) present the data of Mg-Fe2+ partitioning on the following reaction :

Mg2SiO4+Fe2+Fe3+204

(forsterite) (magnetite)=Fe2+2SiO4+MgFe3+2O4 (2)

(fayalite) (magnesioferrite)which gives the coefficient to be 2.1 at 1200°C and 6.2 at 800°C. Thus, the par

tition coefficients calculated from Robie

et al. (1978) and from equation (1) are con

sistent at high temperature but significantly

inconsistent at low temperature .The second hypothesis that metaso-

matism is responsible also had difficulties . The most important is that even though

chromian spines of the Iwanai -dake peri-dotite is often zoned owing to retrograde

metamorphism , magnetite-rich chromian spinel but not magnesioferrite -rich spinel is formed at the rim . However , magnesio-ferrite-olivine rock has some feat

ures in common with monticellite -bearing dunite such that calcium brittle mica

and highly oxidized minerals occur . In the monticel-lite-bearing dunite , ferripargasite, andradite and also magnetite -rich spinel do occur . T

he fact that the Iwanai -dake peridotite was oxidized after the prima

ry assemblage had been formed , is also possibly supported by the occurrence of ch

romite lam ?? tae in olivine as described by Arai (1978) al

one of these features offers direct evi ?? to

Magnesioferrite-olivine rock and others from the peridotite mass 29

deduce the genesis of magnesioferrite, but

we think at present that the second hypothe-

sis is a little more likely.

Genesis of monticellite

The stability of monticellite + olivine assemblage is one thing and that of mon-ticellite-bearing dunite another. Fig. 9 shows the paragenetic relations of a part of the system CaO-MgO-Si02. Monticellite and enstatite are incompatible and hence if monticeUite of the Iwanai-dake peridotite mass was a primary mineral, i.e. a mineral which coexisted with basaltic magma, the monticellite-free dunite and harzburgite should have equilibrated with quite different magmas. The monticellite-bearing dunites contain olivine with CaO weight percent larger than 0.15, but monticellite-free dunite and harzburgite contain olivine with CaO less than 0.1 weight percent. Therefore, the majority of dunite of the Iwanai-dake mass forms an ordinary dunite-harzburgite complex, without anomalous CaO contents, in which monticellite could not have been a primary phase. Simkin and Smith (1970)

Fig. 9. Phase relation for a portion of the

system CaO-MgO-Si02. Di ; diop

side, En; enstatite, Mo ; monti

cellite, Fo ; forsterite.

showed that olivine from an intrusive per-idotite body contains less than 0.15 weight

percent CaO. Thus, it is quite unlikely that monticellite-bearing dunite, occupying a

quite small portion of the complex, co-existed with basalt equilibrated with the majority of the complex.

We also do not believe that monticellite

could have formed under subsolidus condi-

tions in a closed system because monticellite-

bearing dunite is a rare rock in the Iwanai-

dake peridotite mass. Dunite and harz-

burgite containing olivine with CaO less

than 0.1 weight percent cannot produce

monticellite and also associated olivine, the

latter containing more than 0.15 weight

percent CaO.

Fig. 7 indicates that monticellite

lamellae were exsolved from relatively Ca-

enriched olivine. A compositional profile

of Fig. 6-a suggests that some Ca from

olivine was supplied to form monticellite.

It is suggested, therefore, that the

monticellite was formed by a two-step

process ; the first, enrichment of Ca in olivine, and the second, exsolution and precipitation

of monticellite from Ca-rich olivine : we

do not know if these two processes were

continuous. Monticellite exsolution lamellae

are not observed in equigranular olivine but

only in an olivine porphyroclast preserved

monticellite exsolution lamellae.

The temperatures at which these steps of process took place may be estimated from the solvus of monticellite-forsterite system as determined by Warner and Luth (1973). As monticellite and olivine of the specimen we examined have high Mg/(Mg + Fe*) ratios, multicomponent correction was not applied. The minimum temperature of the formation of the most Ca-rich olivine (Ca/(Ca+Mg+Fe*)=0.006) is 815°C and that of monticellite exsolution

30 Jitsuya Nagata

(Ca/(Ca+Mg+Fe*): monticellite=0.490, olivine = 0.003) is 720°C. These values were

read on a diagram, Fig. 10 in which NMO.

(mole percent of CaMgSiO4 in olivine) was

plotted against 1 /T. The error of temperature estimation is about ±30°C. The uncertainty of geothermometers makes it impossible to

determine whether or not the formation of

monticellite from olivine took place at higher

temperature that the granulite facies equili-

bration, during which homogeneous olivine

and pyroxenes of the Iwanai-dake peridotie

mass were formed.

Thus, the enrichment of Ca in olivine

should have taken place before the final

stage of the granulite facies equilibration.

Although we could not reach at a clear-cut

conclusion, it is possible that the Ca was

supplied to the dunite-harzburgite com-

plex after the stage when the complex

equilibrated with basaltic magma and

before the cooling of it to lower temperature

than the granulite facies, i.e. 600 to 700•Ž

according to geothermometer by Arai (1978),

Genesis of Ca-rich diopside

The monticellite-bearing dunite IW 328 contains diopside, but as is seen in Fig. 8, its composition is distinctly different from those in monticellite-free dunite and harzburgite in the Iwanai-dake peridotite mass. It is natural, though circumstantial, to consider that such diopside with unusu-ally high (Ca-0.5A1)/(Ca-0.5A1+ Mg + Fe*) ratio was formed by the same process as the formation of Ca-rich olivine.

Acknowledgements: This paper is a part of

my Master thesis work that was carried out

at the Department of Earth Sciences , Kana-zawa University. I am deeply indebted

to Dr. S. Arai of the Tsukuba University

for his supervision and encouragement

throught the work and critical review . I wish to express my hearty appreciation to

Prof. S. Banno of the Kyoto University, P

rof. M. Yamasaki and Dr . H. Sato of the Kanazawa University and Dr . K. Yokoyama of the University of Auckland for their

constructive discussions and critical reading

of the manuscript .

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北海道 ・神居古潭構造帯 ・岩 内岳力ンラン岩体に 産する

マゲネシオフェ ライ トーカンラン石岩とモンチセリ カンラン石を含むダナ イ ト

長 田 実 也

北海 道 ・神 居古 潭構 造 帯 ・岩 内岳 カ ン ラン岩 体 か ら,ア ル プ ス型超 苦 鉄 質岩類 として は世 界 で初 めての,マ グ

ネ シオ フ ェ ライ ト に富 んだ 岩石 と,モ ンチ セ リカン ラン 石 を含 む ダ ナ イ ト が見 出 きれ た。 こ うした鉱 物 の形成

は,酸 化 作 用 や岩石 中 のCa/Si比 を増 加 きせ るよ うな交 代 作用 が局 所 的 に起 こった結果 で あ ると考 え られ る。

地 名

Iwanai-dake 岩 内 岳