Sci. Rep. Kanazawa Univ.
Vol.48, No.1,2, p.9-29, 2004

9

Inhomogeneous spinel in chromitite from the Iwanai-dakeperidotite complex, Hokkaido, Japan: variations of spinel
unmixing texture and chemical composition
Akihiro TAMURA1 and Shoji ARAI2
1 Graduate School of Natural Science and Technology, Kanazawa University, Kakuma,
Kanazawa 920-1192, Japan
2 Department of Earth Sciences, Kanazawa University, Kakuma, Kanazawa 920 -1192,
Japan
Abstract : Spinel ((Mg Fe2+)(Cr Al Fe3+)2O4 in formula) with inhomogeneous textures
were observed in chromitite from the Iwanai-dake complex, in the Kamuikotan zone, Japan, which mainly consists of highly depleted alpine-type peridotite. The spinel inhomogeneous texture is different between in the chromitite layer and in the surrounding peridotites but the variation is gradual. Spinel has a speckled texture (Type A spinel) in thechromitite. Irregular mottling and symplectic textures (Type B spinel) are predominant
in spinel-rich dunite abutting to the chromitite layer. Spinel in the surrounding duniteand harzburgite is homogeneous (Type C spinel), and it is similar in composition tospinel in ordinary peridotite from the Iwanai-dake complex. The inhomogeneous spinel
comprises two phases of Mg-and Al-rich spinel and Fe2＋‐and Fe3＋‐rich spinel. The variations of texture and composition were achieved by an unmixing process due to the miscibility gap of spinel between the Al-rich and Fe3＋‐rich phases below 900°
C. TheIwanai-dake peridotite is almost free from serpentinization and lacks evidence of regional metamorphism after emplacement, which suggests that the unmixing of spinel
took place during a cooling process of the mantle peridotite. The pre-unmixing compositions of Type A and B spinels are estimated as bulk spinel compositions, and they aresystematically correlated with the unmixing texture. We propose that the variation of unmixing texture is controlled by pre-unmixing spinel compositions.

1. Introduction
Spinel group minerals have a wide range of composition reflecting various chemicaland physical conditions of their genesis. Spinel, (Mg, Fe2+)(Cr, Al, Fe3+)2O4, is especiallyemployed as a useful petrogenetic indicator for ultramafic and mafic rocks because its
composition is sensitive to change of temperature, pressure, oxygen fugacity, bulk rockand fluid compositions (e.g., Irvine, 1965). For upper mantle-derived peridotites, its composition may indicate the degree of partial melting and has been used as a guide to classification of peridotites in terms of tectonic setting (e.g., Dick and Bullen, 1984 ; Arai, 1994).
Although the extensive solid solution exists in spinel group minerals at high temperatures

10

Akihiro TAMURA and Shoji ARAI

such as upper mantle conditions, miscibility gaps between aluminum-and ferric iron-richspinels are recognized at lower temperatures such as greenschist, amphibolite or granulitefacies condition (e.g., Loferski and Lipin, 1983). The experimental and thermodynamic
studies have shown a solvus between them (Turnock and Eugster, 1962 ; Cremer, 1969 ;Sack and Ghiorso, 1991). Spinels with a miscibility gap often develop unmixing textures
as described from metamorphosed ultramafic and mafic rocks (Muir and Naldrett, 1973 ;Loferski and Lipin, 1983 ; Lipin, 1984 ; Eales et al., 1988 ; Zakrezewski, 1989 ; Burkhard, 1993). Loferski and Lipin (1983) referred to the scarcity of unmixing spinels frommantle derived peridotites. Unmixing-textured spinels have been reported only from alpine-type peridotite that has undergone regional crustal metamorphism (Lipin, 1984 ; Burkhard, 1993). We found inhomogeneous spinels from a chromitite layer in a mantle-derived
peridotite complex, the Iwanai-dake complex, Hokkaido, northern Japan. In this article we
describes textural and compositional characteristics of the inhomogeneous spinel. Then weshow that the inhomogeneous texture of spinel is caused by unmixing during cooling ofmantle peridotite and that pre-unmixing spinel compositions control the unmixing texture.
In this article, ”spinel” refers to a spinel group mineral generally expressed as (Mg, Fe2+)(Cr, Al, Fe3+)2 O4 (e.g., Haggerty, 1991a). ”Chromitite” refers to the rock rich in ”spinel” (>20% modal ratio) defined above.
2. Geology and petrology of the Iwanai-dake peridotite complex
The Iwanai-dake peridotite complex is located in the southern part of the Kamuikotan
zone. The Kamuikotan zone is distributed discontinuously over 320 km from north tosouth within the Sorachi-Yezo belt of a Mesozoic accretionary complex, the central axial
zone of Hokkaido, northern Japan, and is a tectonic melange which consists of peridotites
and high-and low-pressure metamorphic rocks (Miyashiro, 1961 ; Ishizuka, 1983). Distribution of a large amount of ultramafic rocks serpentinized to various degrees characterizes
the Kamuikotan zone (Fig. 1), and some of them are considered to the basal mantle member of an ophiolite complex (e.g., Asahina and Komatsu, 1979 ; Ishizuka, 1980 ; Ishizuka
et al., 1983). The Iwanai-dake complex, approximately 2 km X 0.5 km in plan, is mainlycomposed of spinel harzburgite and dunite with a small amount of pyroxenites (orthopyroxenite or olivine-orthopyroxenite) and chromitite. They are well exposed at a large
quarry around Mt. Iwanai-dake (964 m) and are almost free from serpentinization (Bamba,
1955 ; Arai, 1978 ; Katoh, 1978 ; Niida and Katoh, 1978). Dunite commonly forms tabular layers or veins that range in thickness from 10 cm to 10 m within massive harzburgite(e.g. Tamura et al., 1999 ; Kubo, 2002). Layers or veins of pyroxenite and chromitite areassociated with dunite in places. The harzburgite and dunite have strong deformed textureswith kink-banded olivine, such as protogranular and porphyroclastic textures of Mercier
and Nicolas (1975). Relics of fluid inclusions (Arai and Hirai, 1985 ; Hirai and Arai,
1987), chromian spinel lamellae (Arai, 1978) and fine inclusions in the olivine grains aremuch more frequently found in dunite than in harzburgite. The compositional variations ofInhomogeneous spinel in chromitite from the Iwanai-dake peridotite complex

11

Figure. 1. Locality of the Iwanai-dake peridotite complex and distribution of the ultramafic rocks in the
Kamuikotan zone, Hokkaido, northern Japan.

12

Akihiro TAMURA and Shoji ARAI

Figure. 2. Chemical compositions of spinel and olivine in ordinary peridotite and chromitite from the
Iwanai-dake complex. Abyssal peridotite field from Dick and Bullen (1984) and Arai (1994).
Forearc peridotite field from Bloomer and Hawkins (1983), Bloomer and Fisher (1987), Ishii
et al. (1992) and Parkinson and Pearce (1998). (a) Mg# (=Mg/(Mg+Fe2+) atomic ratio) vs Cr#(=Cr/(Cr+Al) atomic ratio) of spinel. (b) Fo content of olivine vs Cr# of spinel. Note that, forcoexisting chromian spinel-olivine pairs in harzburgite and dunite, the Cr# and Fo content
have a positive correlation entirely along the olivine-spinel mantle array (OSMA : Arai, 1987,
1994), a spinel peridotite restite trend.

olivine and spinel in peridotite and chromitite are illustrated in Figure 2. The Iwanai-dakeperidotite is characteristically depleted and is similar in mineral chemistry to forearc peridotites rather than to abyssal peridotites (Fig. 2).
3. Studied chromitite
The studied rocks were obtained from a chromitite layer and surrounding peridotites
at the eastern part (outcrop No. IWD580) of the quarry. The lithological sequence of IWD
580 is illustrated in Figure 3a. The samples were collected by drilling up to a distance 100
cm from the chromitite layer into surrounding dunite and harzburgite (Fig. 3a). The chromitite layer, about 5 cm in thickness, appears to be continuous over about 80 cm in length(samples MF5 and 22), and pinches out at one end (Plate I-a,b). Small amounts of subhedral olivine (up to 5 mm in diameter) are enclosed by massive spinel. The boundary between chromitite and dunite is clearly identified as grain boundaries between massive
spinel aggregate and olivine (Fig. 3b). The abutting dunite layers (5 cm in thickness) en-

Inhomogeneous spinel in chromitite from the Iwanai-dake peridotite complex

13

Figure. 3. (a) Sketch of studied section (IWD580). Sample numbers encircled are shown with Fo content of olivine. See plate I-a and b for outcrop. (b) The variation from chromitite layer to aureole dunite is shown by the thin sections of MF 22, 6 and 7. Black stuff is spinel and a lightgrey is almost olivine. Note the antinodular texture in chromitite (MF22) and a decrease ofspinel volume within the aureole dunite (MF6 and 7) from chromitite border toward surrounding dunite.

closing the chromitite layer are richer in spinel than the surrounding dunite and harzburgite,
and are called aureole dunite hereafter (samples MF6, 7, 21, 23 and 24) (Fig. 3a). In the
aureole dunite subhedral and anhedral spinel grains (0.1-1.0 mm in diameter) are interstitial to olivine, and minor amount of chlorite (clinochlore) and andradite are also interstitial
to olivine and/or spinel. It is noteworthy that andradite sometimes occurs in the core ofchlorite. Spinel volume is gradually decreasing with an increase of distance from the chromitite layer. The Surrounding dunite (samples MF1, 3, 8, 9, 10 and 19) and harzburgite(samples MF11, 12, 16 and 27), which are peridotites outside of the aureole dunite, aresimilar to ordinary peridotites from the Iwanai-dake complex. The massive spinel aggregate in the chromitite layer is usually black with translucent yellow or green blebs in thinsection (Plate I-c). Kink band of olivine is less frequent in chromitite than in aureole dunite.
The surrounding dunite and harzburgite have a porphyroclastic texture with fairly deformed olivine and orthopyroxene porphyroclasts. Spinels show a range of color from darkbrown to light reddish-brown.

14

Akihiro TAMURA and Shoji ARAI

4. Spinel textures
Textural and compositional varieties of spinel have been identified by reflected-lightmicroscopic observations (Plate I-IV). In order to simplify the varieties, which are so complicated and gradual, we divided the textures into three types ; Types A and B showing
two, dark-colored and light-colored, phases, and Type C with a homogeneous appearance.
Type A spinel exhibits a speckled texture consisting of a large amount of dark-coloredgranules and a light-colored host (e.g., Plate. I-d, e, II-a, IV-a). The granules are often distributed in linear or concentric arrays. Individual granules are rounded to rectangular andrange from less than 5 μm to 50 μm in diameter. Type B spinel shows irregular mottling or
symplectic texture composed of dark-colored and light-colored phases (e.g., Plate II-e, g,
III-a, e, IV-b). Gradual change of the inhomogeneous texture is achieved by change of theratio of the two phases. Symplectic-textured part sometimes coexists with apparently homogeneous part within single spinel grains (e.g., Plate. III-c, g, IV-c). Type C is mainlycomposed of a dark-colored phase (e.g., Plate IV-g, h) but some of Type C grains are
zoned in dunite aureole (e.g., Plate. III-h, IV-f). Type A spinel is mainly observed in thechromitite layer. Within the aureole dunite, the dominant spinel type changes in the orderof Type A, B, C from chromitite border outward. Only Type C spinel occurs in the surrounding dunite and harzburgite. We summarize that the textural variation of spinel is related not only to lithology of host rock but also to distance from the chromitite layer.
5. Mineral chemistry
Chemical analysis of minerals were carried out with an energy dispersive spectrometer system (Phillips EDAX-9100) attached to scanning electron microscope (AKASHIALPHA-30A) at Faculty of Science, Kanazawa University and with a wavelength dispersive spectrometer system (JEOL EPMA JXA-8800) at Center for Cooperative Research ofKanazawa University. In the microprobe analysis of spinel, Fe is reported as total FeO, andFe2+ and Fe3+ were recalculated by assuming ideal spinel stoichiometry with all Ti being allotted to ulvospinel (Fe2TiO4). Both Fe2O3 and FeO were recalculated along with the resulting new totals. Selected analyses of spinel are listed in Table 1. Figures 4, 5 and 6 illustrate
the compositional relationships between Type A, B and C spinels. The dark-colored phaseis rich in the component of spinel sensu stricto (MgAl2O4) whereas the light-colored phaseis intermediate between magnetite (Fe2+Fe3+ 2O4) and magnesioferrite (MgFe3+ 2O4) components (Figs. 4 and 5a). The compositional variation of Types A, B and C is dependent on
distance from the chromitite layer. For example, within the aureole dunite Type B spinel inchromitite side is lower in chromium content than that in the surrounding peridotite side.
TiO2 contents are usually low (<0.5 wt%) in all spinel phases (Fig. 6a). Some Type B
spinels have relatively high TiO2 content, up to 1 wt%, especially in the light-colored phase.
The NiO content of spinel is higher in the chromitite and aureole dunite than in the surrounding dunite and harzburgite (Fig. 6b). Dark-colored and light-colored phases in TypeA and B spinels contain 0.10-0.54 wt% and 0.56-1.40 wt% of NiO, respectively. The NiO

Table.1. Representative microprobe analysis of spinels. Refer to Figure 3 for sample#. Total* is a new total from FeO and Fe2O3 calculated assuming
spinel stoichiometry．†：thin harzburgite layer within dunite．††＝massive harzburgite. Mg#=Mg/(Mg+Fe2+) atomic ratio ; Cr#=Cr/(Cr+Al) atomic ratio ; R3+= Cr+Al+Fe3+ ; Fet=Fe2++Fe3+.

Inhomogeneous spinel in chromitite from the Iwanai-dake peridotite complex
15

16

Akihiro TAMURA and Shoji ARAI

Figure. 4. Mg# (=Mg/(Mg+Fe2+) atomic ratio) vs Fe3+# (=Fe3+/(Cr+Al+Fe3+) atomic ratio) of Type A, B
and C spinels from IWD580 in the Iwanai-dake complex. Contiguous pairs of Type A and B
spinels are joined by solid line.

Figure. 5. Compositional relations of Type A, B and C spinels from IWD580 in the Iwanai-dake complex in terms of trivalent cations (Cr, Al and Fe3+). (a) Contiguous pairs of Type A and B
spinels are joined by lines. (b) Bulk compositions are estimated from contiguous pairs tied bylines (see text). LL indicates a solvus at 600°
C suggested from unmixing spinels from the
Red Lodge district (Loferski and Lipin, 1983). SG is a solvus calculated from thermodynamics assuming spinels coexisting with Fo95 olivine at 600°
C (Sack and Ghiorso, 1991). Spinelcompositions in the ordinary Iwanai-dake peridotite and chromitite are shown for comparison.

Inhomogeneous spinel in chromitite from the Iwanai-dake peridotite complex

17

Figure. 6. Chemical compositions of spinel from IWD580 in the Iwanai-dake complex. (a) Variation of
Mg# (=Mg/(Mg+Fe2+) atomic ratio) vs TiO2 content. (b) Variation of Mg# vs NiO content.

content of Type C spinel is 0.10-0.35 wt% in aureole dunite and is <0.11 wt% in surrounding peridotite. We estimated the bulk compositions of Type A and B spinels, based on obtained chemistry and areal ratio of the two phases. The areal ratio was analyzed by using
digital images on a public domain Image-J program (website : http : //rsb.info.nih.gov/ij/
index.html). The bulk spinel compositions are listed in Table 2 and plotted in Figures 4 and5b. The variation of Fo content of coexisting olivine from the chromitite layer to surrounding peridotites is shown in Figure 3. The Fo content tends to decrease from the chromitite
layer (Fo95-96) to surrounding dunite and harzburgite (Fo91-92). Olivine in aureole dunite is
Fo92-94.

18

Akihiro TAMURA and Shoji ARAI

Table. 2. Estimated bulk spinel compositions of Type A and B spinel, and areal ratios between contiguous phases. The composition for Type A is an average of 8 domains in a chromitite thin section(MF22). 2s : standard deviation at the 2 sigma. The compositions for Type B are from representative two spinel grains.

6. Discussion
Miscibility gap of spinel has been documented by both experimental and thermodynamic studies (e.g., Turnock and Eugster 1962 ; Sack and Ghiorso 1991). Inhomogeneous
spinels due to immiscibility have been well described from some metamorphosed ultramafic and mafic rocks (Muir and Naldrett 1973 ; Loferski and Lipin 1983 ; Lipin 1984 ;Eales et al. 1988 ; Zakrezewski 1989 ; Burkhard 1993). Compositional relationships of
IWD580 spinels strongly suggest that the inhomogeneous textures in spinel are caused byunmixing due to miscibility gap. Loferski and Lipin (1983) interpreted the inhomogeneityof spinel in meta-gabbro from the Red Lodge Mountain, Montana, as unmixing of Fe-richand Al-rich phases representing a miscibility gap. They have shown a solvus of spinel interms of trivalent cations (Cr-Al-Fe3+) at a temperature of approximately 600°
C (Fig. 5b).

Figure. 7. Schematic diagram for interpretation of relationships between unmixing texture and chemical composition in spinel from IWD580. (a) The
spinel prism and approximate plane for IWD580 spinel compositions (cf. Figures 5 and 6). (b) Hypothetical solvus in the T-trivalent cations(Cr-Al-Fe3+) space. The solvus is estimated from Sack and Ghiorso (1991). (c) (d) (e) Al-Fe3+ binary planes cut through the iso-Cr ratio
planes of (b). Each plane labeled by Cr-free, low-Cr and high-Cr corresponds to Type A, B and C spinel, respectively (See text).

Inhomogeneous spinel in chromitite from the Iwanai-dake peridotite complex
19

20

Akihiro TAMURA and Shoji ARAIAn expected solvus of IWD580 is slightly different in shape from the solvus shown by
Loferski and Lipin (1983) (Fig. 4b). This is due to a difference of Mg# of spinel. The immiscible field on the spinel prism may decrease from the Fe-rich side to the Mg-rich side
(Fig. 7 of Loferski and Lipin, 1983). The temperature should be also a factor controllingthe immiscible field. Sack and Ghiorso (1991) showed differences in solvus at different
temperatures and compositions based on thermodynamic calculations (Fig. 5b). We estimated the equilibrium temperature of IWD580 section. For the chromitite and surrounding
peridotite, 710-900°
C is calculated using calibration of Ballhaus et al. (1991). Arai (1978)
estimated the equilibrium temperature of the Iwanai-dake peridotite, based on the olivinespinel geothermometer (Evans and Frost, 1975), to be 700°
C or higher. Spinel unmixing is3＋
a function of composition and temperature : Fe ‐rich spinel compositions inside the apparent immiscible field in terms of the trivalent cation ratio (Fig. 5b) is common in differentiated gabbros and altered alpine-type peridotites. Lipin (1984) reported spinel with unmixing textures from alpine-type peridotite. The unmixing may have been caused by hightemperature regional metamorphism after serpentinization during or after emplacement(Lipin, 1984). On the other hand, peridotites from the Iwanai-dake complex are almost freefrom serpentinization and have no evidence of deserpentinization. The Iwanai-dake peridotite and IWD580 chromitite have probably undergone neither regional metamorphism
nor serpentinization after emplacement. Therefore, the inhomogeneous texture was obtained during a cooling process of the mantle peridotite upon emplacement. We conclude
that the inhomogeneous texture observed was formed by unmixing during cooling to about
700°
C indicated by Mg-Fe partitioning between olivine and spinel.
The variety of textures in Type A and B spinels (unmixed) and Type C spinel (homogeneous) can be interpreted by differences of initial spinel composition because the texturalvariation is systematically related with the chemical composition. It is postulated that preunmixing spinel composition is represented by estimated bulk spinel composition of unmixing spinel (Table. 2). Let us consider the processes on an ideal phase diagram of spinel(Fig. 7). Each spinel phase and pre-unmixing spinel composition from IWD580 can be approximated on a plane, (Mg0.5,Fe2+ 0.5)Cr2O4-MgAl2O4-(Mg0.5,Fe2+ 0.5)Fe3+ 2O4, in the spinel
prism, and we draw a hypothetical solvus on the plane of trivalent cations (Cr-Al-Fe3+) (Fig.
7a, b). Type C spinel represented by high-Cr spinel is free from unmixing, which suggeststhat this composition is outside of solvus at the blocking temperature (Fig. 7c). According
to Sack and Ghiorso (1991), Type A and B spinels must be single phases as solid solutions
at high temperature (>900°
C). In Figure 7, the pre-unmixing spinel compositions of TypeA and B represent, on the binary system, low-Cr and Cr-free compositions, respectively(Fig. 7d, e). Both compositions enter the immiscible field during cooling. In Type A spinel
dark-colored granules (e.g., Plate I-d, e) could be effectively formed by phase separation of
nucleation growth. The pre-unmixing spinel composition of Type A spinel, of which
MgAl2O4 component is very small in amount relative to (Mg0.5,Fe2+ 0.5) Fe3+ 2O4 component(Fig. 7e), causes discontinuous transformation. This process forms nucleus of MgAl2O4,

Inhomogeneous spinel in chromitite from the Iwanai-dake peridotite complex

21

dark-colored granules observed. On the other hand, Type B spinel does not show nucleation (e.g., Plate II-g, III-d). Pre-unmixing composition of Type B spinel could correspondto inside of spinodal at the blocking temperature (Fig. 7d). Thus we can ascribe the symplectic texture in Type B spinel to the spinodal decomposition of continuous transformation (e.g., Kingery et al., 1976). The textural variations between Types A, B and C werepossibly due to gradual differences of pre-unmixing spinel composition but not to the differences of cooling rate or thermal history.
Unmixing textures of IWD580 spinel are various, speckled or irregular symplectic
texture as shown above, but do not show trellis lamellae texture commonly observed inilmenite-magnetite unmixing (e.g., Haggerty, 1991a). We speculate that this difference isdue to the difference of crystal system of unmixing phases. Ilmenite and magnetite belong
to the hexagonal and cubic systems, respectively. Thus the trellis lamellae texture is probably developed by an anisotropic feature of ilmenite. On the other hand, both phases in
IWD580 unmixing spinel belong to the cubic system. They are isotropic and noncrystallographic-controlled texture should be developed during unmixing.
7. Conclusions and suggestions for further study
Compositional relationship between two spinel phases in this study supports that theinhomogeneous texture in spinel was developed by unmixing process due to immiscibility
at low temperature. The textural variety of unmixing spinel is dependent on composition of
initial pre-unmixing spinel.
If the estimated pre-unmixing composition is appropriate, then a next problem mayarise on the origin of the pre-unmixing spinel. The pre-unmixing spinel composition ismuch higher in ferric iron content than spinel from mantle derived peridotite and chromitite ever reported not only from the Iwanai-dake complex (Fig. 5b) but also from the world(e.g., Arai and Yurimoto, 1995 ; Zhou et al., 1996 ; Melcher et al., 1997). We suspect thata highly oxidized melt had played an important role in the IWD580 chromitite formation.
Because the chromitite formation has been attributed to reaction between mantle peridotite
and melt (e.g., Arai and Yurimoto, 1994 ; Zhou et al, 1994), and the spinel composition
strongly depends on oxygen fugacity in crystallization (e.g. Hill and Roeder, 1974). We
also focus on the irregularity of unmixing texture observed. For example, Type B spinel
grain contains two portions with symplectic texture and homogeneous appearance (e.g.,
Plate. III-b, IV-d). This may be due to initial chemical heterogeneity within the preunmixing spinel phase.
Acknowledgments
We would like to give special thanks to M. Makita and K. Kadoshima for their help infield survey and collecting rocks. A. Toramaru and A. Tsune are gratefully acknowledged
for daily discussion and helpful suggestions. A. Ishiwatari and T. Morishita are thanked for22

Akihiro TAMURA and Shoji ARAI

constructive comments. Y. Shimizu is thanked for assistance with the electron microprobe.
Hatanaka Co. kindly allowed us the field survey in the quarry.
Reference
Arai, S. (1978) Chromian spinel lamellae in olivine from the Iwanai-dake peridotite mass, Hokkaido, Japan. Earth Planet. Sci. Lett., v. 39, p. 267-273.
Arai, S. (1987) An estimation of the least depleted spinel peridotite on the basis of olivine-spinel mantle
array. N. Jb. Miner. Mh., v. 8, p. 347-354.
Arai, S. (1994) Characterization of spinel peridotites by olivine-spinel compositional relationships : Review and interpretation. Chem. Geol ., v. 113, p. 191-204.
Arai, S. and Hirai, H. (1985) Relics of H2O fluid inclusions in mantle-derived olivine. Nature, v. 318, p.
276-277.
Arai, S., and Yurimoto, H. (1994) Podiform chromitites of the Tari-Misaka ultramafic complex, Southwestern Japan, as mantle-melt interaction products. Economic Geology, v. 89, p. 1279-1288.
Asahina, T. and Komatsu, M. (1979) The Horokanai ophiolitic complex in the Kamuikotan Tectonic
Belt, Hokkaido, Japan. Jour. Geol. Soc. Japan, v. 85, p. p. 317-330.
Ballhaus, C., Berry, R. F. and Green, D. H., (1991). High pressure experimental calibration of the olivine-orthopyroxene-spinel oxygen geobarometer : implications for the oxidation state of the upper mantle. Contrib. Mineral. Petrol ., v. 107, 27-40.
Bamba, T. (1955) Petrological study on the Iwanaidake peridotite mass. Bull. Geol. Committ. Hokkaido,
v. 29, p. 7 (in Japanese).
Bloomer, S.H. and Fisher, R.L. (1987) Petrology and geochemistry of igneous rocks from the Tonga
Trench-A non-accreting plate boundary. Jour. Geol ., v. 95, p. 469-495.
Bloomer, S.H. and Hawkins, J.W. (1983) Gabbroic and ultramafic rocks from the Mariana trench : anisland arc ophiolite. In : D.E. Hayes (Ed.) The Tectonics and Geologic Evolution of Southeast Asian
Seas and Islands : Part II. Geophys. Monogr., American Geophysical Union, v. 27, p. 294-317.
Burkhard, D.J.M. (1993) Accessory chromium spinels : Their coexistence and alteration in serpentinites.
Geochim. Cosmochim. Acta, v. 57, p. 1297-1306.
Cremer, V. (1969) Die Mischkristallbildung im System Chromit-Magnetit-Hercynit zwischen 1000 und
500°
C. N. Jb. Mineral. Abh., v. 111, p. 184-205 (in German with English abstract).
Dick, H.J.B. and Bullen, T. (1984) Chromian spinel as a petrogenetic indicator in abyssal and alpinetype peridotites and spatially associated lavas. Contrib. Mineral. Petrol ., v. 86, p. 54-76.
Eales, H.V., Wilson, A.H. and Reynolds, I.M. (1988) Complex unmixed spinels in layered intrusions
within an obducted ophiolite in the Natal-Namaqua mobile belt. Mineral. Deposita, v. 23, p. 150157.
Evans, B.W., and Frost, B.R. (1975) Chrome-spinel in progressive metamorphism a preliminary analysis.
Geochim. Cosmochim. Acta, v. 39, 959-972.
Haggerty, S.E. (1991a) Oxide textures-a mini-atlas. In D.H. Lindsley, (Ed.) Oxide Minerals : Petrologic
and Magnetic Significance. Reviews in Mineralogy, Mineralogical Society of America, Washington,

Inhomogeneous spinel in chromitite from the Iwanai-dake peridotite complex

23

D.C., v. 25, p. 129-219.
Haggerty, S.E. (1991b) Oxide mineralogy of the upper mantle. In : D.H. Lindsley, (Ed.) Oxide Minerals : Petrologic and Magnetic Significance. Reviews in Mineralogy, Mineralogical Society ofAmerica, Washington, D.C., v. 25, p. 355-416.
Hill, R., and Roeder, P. (1974) The crystallization of spinel from basaltic liquid as function of oxygen
fugacity. Jour. Geol ., v. 82, p. 709-729.
Hirai, H. and Arai, S. (1987) H2O-CO2 fluids supplied in alpine-type mantle peridotites : electron petrology of relic fluid inclusions in olivines. Earth Planet. Sci. Lett., v. 85, p. 311-318.
Irvine, T.N. (1965) Chromian spinel as a petrognetic indicator. Part 1. Theory. Can. Jour, Earth Sci., v.
2, p. 648-672.
Ishii, T., Robinson, P.T., Maekawa, H. and Fiske, R. (1992) Petrological studies of peridotites from diapiric serpentinite seamounts in The Izu-Ogasawara-Mariana forearc, LEG125. In : P. Fryer, J.A.
Pearce, L.B. Stokking et al., (Eds). Proc. ODP, Sci. Results, College Station, Texas (Ocean Drilling
Program), v. 125, p. 445-485.
Ishizuka, H. (1980) Geology of the Horokanai Ophiolite in the Kamuikotan Tectonic Belt, Hokkaido, Japan. Jour. Geol. Soc. Japan, 86, p. 119-134 (in Japanese with English abstract).
Ishizuka, H., Imaizumi, M., Gouchi, N. and Banno, S. (1983) The Kamuikotan zone in Hokkaido, Japan : Tectonic mixing of high-pressure and low-pressure metamorphic rocks. Jour. Metamor. Geol .,v. 1, p. 263-275.
Katoh, T. (1978) The Sarugawa ultrabasic massif in Kamuikotan belt, central axial zone of Hokkaido.
Earth Science (Chikyu Kagaku) , v. 32, p. 273-279 (in Japanese with English abstract).
Katoh, T. and Nakagawa, M. (1986) Tectogenesis of ultramafic rocks in the Kamuikotan tectonic belt,
Hokkaido, Japan. Monogr. Assoc. Geol. Collab. Japan, v. 31, p. 119-135 (in Japanese with English
abstract).
Kingery, W.D., Bowen, H.K., and Uhlmann, D.R. (1976) Introduction to Ceramics, 2nd edition. A
Wiley-Ineterscience Publication, New York. 1056 p
Kubo, K. (2002) Dunite formation processes in highly depleted peridotite : case study of the Iwanaidake
peridotite, Hokkaido, Japan. Jour. Petrol ., v. 43, p. 423-448.
Lipin, B.R. (1984) Chromite from the Blue Ridge Province of North Carolina. Am. Jour. Sci., v. 284, p.
507-529.
Loferski, P.J. and Lipin, B.R. (1983) Exsolution in metamorphosed chromite from the Red Lodge district,
Montana. Am. Mineral ., v. 68, p. 777-789.
Melcher, F., Grum, W., Simon, G., Thalhammer, T.C., and Stumpfl, E.F. (1997) Petrogenesis of theophiolitic giant chromite deposits of Kempirsai, Kazakhstan : a study of solid and fluid inclusions
in chromite. Jour. Petrol ., v. 36, p. 1419-1458.
Mercier, J.C. and Nicolas, A. (1975) Textures and fabrics of upper mantle peridotites as illustrated byxenoliths from basalt. Jour. Petrol ., v. 16, p. 454-487.
Miyashiro, A. (1961) Evolution of Metamorphic Belts. Jour. Petrol ., v. 2, p. 227-331.
Muir, J.E. and Naldrett, A.J. (1973) A natural occurrence of two-phase chromium-bearing spinels. Can.
Mineral ., v. 11, p. 930-939.

24

Akihiro TAMURA and Shoji ARAI

Niida, K. and Katoh, T. (1978) Ultramafic rocks in Hokkaido. Monogr. Assoc. Geol. Collab. Japan, v.
21, p. 61-81 (in Japanese with English abstract).
Parkinson, I.J. and Pearce, J.A. (1998) Peridotites from the Izu-Bonin-Mariana forearc (ODP Leg 125) :evidence for mantle melting and melt-mantle interaction in a supra-subduction zone setting. Jour.
Petrol ., v. 39, p. 1577-1618.
Sack, R.O. and Ghiorso, M.S. (1991) Chromian spinels as petrogenetic indicators : Thermodynamics
and petrological applications. Am. Mineral ., v. 76, 827-847.
Tamura, A., Makita, M. and Arai, S. (1999) Petrogenesis of ultramafic rocks in the Kamuikotan belt,
Hokkaido, northern Japan. Memoir. Geol. Soc. Japan, v. 52, p. 53-68 (in Japanese with English abstract).
Turnock, A.C. and Eugster, H.P. (1962) Fe-Al oxides : Phase relationships below 1000°
C. Jour. Petrol .,v. 3, p. 533-565.
Zakrezewski, M.A. (1989) Chromian spinels from Kusa, Bergslagen, Sweden. Am. Mineral ., v. 74, p.
○

448-455.
Zhou, M.F., Robinson, P.T., and Bai, W.J. (1994) Formation of podiform chromitites by melt/rock interaction in upper mantle. Mineral. Deposita, v. 29, 980-101.
Zhou, M.-F., Robinson, P.T., Malpas, J., and Li, Z. (1996) Podiform chromitites in the Luobusa
ophiolite (southern Tibet) : implications for melt-rock interaction and chromite segregation in theupper mantle. Jour. Petrol ., v. 37, 3-21.

Inhomogeneous spinel in chromitite from the Iwanai-dake peridotite complex

25

PLATES
Mode of occurrence of IWD580 chromitite and the textural variety of spinel. Sample MF22 is fromchromitite layer, samples MF6, 7, 22 and 23 are from aureole dunite and sample MF1 and 3 are fromsurrounding peridotite (see Fig. 3).
Plate I (Outcrop and MF22)(a) and (b) An outcrop of IWD580 in the Iwanai-dake complex.
(a) The chromitite layer (black) pinches out at left side.
(b) Subhedral olivine (grey dots) in chromitite layer (black) and spinel concentration in aureole dunite.
(c) Photomicrograph of massive spinel (black) in MF22. White dots in spinel are yellow or green incolor. Plane-polarized light.
(d) Reflected-light photomicrograph of (c). Note that some dark-colored granules correspond to white
dots in (c).
(e)-(h) Photomicrographs in reflected light.
(e) Granules (grey) and host (white) are spinel sensu stricto and Mg-rich magnetite in composition, respectively. Granules are distributed irregularly but often show linear or concentric arrays.
(f)-(h) Close up of (e). Note that the granules are various in size. Stars in (e) to (h) indicate the same
position for reference. Spinel with this inhomogeneous texture in MF22 is classified as Type A.
Plate II (MF 6)
Reflected-light photomicrographs of spinel in MF6. The pictures are arranged from (a) to (h) from thechromitite side outward. (a) to (d) show textures of Type A spinel and the others show those of TypeB spinel.
(a) and (b) Speckled texture in subhedral spinel.
(c) Speckled texture is predominant but irregular texture exists at the rim .
(d) Close-up of (c)(e) Irregular mottling texture. Note that light-colored phase is predominant.
(f) Close-up of (e) showing a dark-colored phase at the rim.
(g) Fine irregular mottling texture.
(h) Close-up of (g) showing the symplectic texture at the rim.
Plate III (MF 7)
Reflected-light photomicrographs of spinel in MF7. The pictures are arranged from (a) to (h) in orderof distance from chromitite. (a) to (g) exhibit Type B spinel pattern and (h) is the Type C spinel texture.
(a) Irregular mottling texture. Note the irregular shape of the dark-colored phase.
(b) Irregular mottling texture. Note the dark-colored phase at the rim.
(c) Mottling and symplectic texture distributed irregularly. Note that the light-colored phase is predominant(d) Close-up of (c)(e) Symplectic texture existing between light-and dark-colored parts with homogeneous appearance.
Note that the dark-colored phase is predominant.
(f) Close-up of (e) showing gradual transition from symplectic texture to homogeneous texture.
(g) Fine symplectic patten and light-colored appearance at the rim.
(h) Dark-colored homogeneous phase with a light-colored phase at the rim.
Plate IV (MF 23, 24, 1 and 3)
Reflected light photomicrographs of spinel. (a) and (b) show Type A spinel texture, (c) shows Type B
spinel texture, and (e) to (h) correspond to Type C spinel.
(a) and (b) Speckle texture in spinel aggregates from aureole dunite (MF23).
(c) Symplectic texture between light-and dark-colored homogeneous phases (MF24).
(d) A close-up of (c).
(e) and (f) Homogeneous-looking spinel in MF24. Note the light-colored phase only exists as spot
and rim.
(g) and (h) Homogeneous spinel in MF1 and MF3, respectively.

26

Akihiro TAMURA and Shoji ARAI

Plate I (Outcrop and MF22)

Inhomogeneous spinel in chromitite from the Iwanai-dake peridotite complex

Plate II (MF 6)

27

28

Akihiro TAMURA and Shoji ARAI

Plate III (MF 7)

Inhomogeneous spinel in chromitite from the Iwanai-dake peridotite complex

Plate IV (MF 23, 24, 1 and 3)29