9Geological, petrographical and petrological characteristics ofultramafic-mafic xenoliths in Kurose and Takashima, northern
Kyushu, southwestern Japan
Shoji ARAI1, Natsue ABE2, Hisatoshi HIRAI3 and Yohei SHIMIZU1
1 Department of Earth Sciences, Kanazawa University, Kanazawa 920-1192, Japan
2 GEMOC, Department of Earth & Planetary Sciences, Macquarie University, Sydney,
NSW 2109, Australia
3 National Institute of Advanced Industrial Science and Technology (AIST) Kyushu,
Tosu, Saga 841-0052, Japan
Abstract : Characteristics of ultramafic xenoliths are quite different between Kurose
and Takashima, which are only 30 km apart each other, in northern Kyushu, the Southwestern Japan arc. The host rock is slightly nepheline-normative alkaline basalt for both
localities, and the age is 1.1 Ma for Kurose and is 3.0 Ma for Takashima. Mantle lherzolite/harzburgite is predominant over Group I rocks (dunite and pyroxenites), andGroup II xenoliths and related black pyroxene megacrysts are almost absent in Kurose.
Mantle peridotite is almost absent and, instead, Group I rocks are prominent both in volume and in lithological variation, from dunite to pyroxenites, in Takashima. Group II
xenoliths and related megacrysts are abundant in Takashima basalt. Group I rocks of
Takashima were penetrated by a silica-oversaturated melt to produce Group III pyroxenites. These rocks were finally invaded by Group II rocks.
This contrast between the two xenolith suites indicates a strong lateral heterogeneity of the uppermost mantle in terms of extent both of Group I dunite-pyroxenites and ofGroup II rocks, in the Southwest Japan arc, especially in northern Kyushu area. Group I
rocks are basically cumulates from some island arc magma(s), and the dunite especiallyis a possible reaction product between peridotite and arc magma. Group II rocks are
deep-seated cumulates from the alkali basalts that had preceded the host basalt in thesame area or the same volcano cluster. The alkali basaltic melts involved in the Group II
formation gave a strong chemical effect (metasomatism) on surrounding Group I rocks.
This complicated and heterogeneous upper mantle has been accomplished at the rim of
the Western Pacific where magmatism of arc and back arc has been active.

1. Introduction
Ultramafic and related xenoliths in magmas give us unrivaled insights into petrological constitution of deep parts of island arcs or continents that are otherwise inaccessible.
The Japanese island arcs especially provide us with ultramafic xenoliths of possible mantle
wedge origin (Takahashi, 1978 ; Aoki, 1987). In contrast to this, almost all localities of ul-

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Shoji ARAI, Natsue ABE, Hisatoshi HIRAI and Yohei SHIMIZU

tramafic xenoliths of the world are located on continental rift zones and oceanic hot spots(Nixon, 1987).
In the southwest Japan arc there have been many localities documented where we canobtain various ultramafic xenoliths from Cenozoic alkali basalts and related volcanics (e.g.,
Takamura, 1973 ; Takahashi, 1978 ; Arai et al., 2000) (Fig. 1). Takashima and Kuroseamong them are located, being only 30 km apart each other, in the northern Kyushu area,and provide quite different xenolith suites from one another. This will give us an excellent
insight into the upper mantle constitution, especially its petrological heterogeneity, beneath
the Japan island arc. Ultramafic xenoliths of Kurose and Takashima were described andbriefly discussed by Kobayashi and Arai (1981), Arai and Kobayashi (1981), Arai and
Hirai (1983) and Arai et al. (2000).
In contrast to the Southwest Japan arc, there are only two volcanoes that provide us
peridotite xenoliths, Megata volcano (Kuno, 1967 ; Aoki and Shiba, 1973 ; Aoki andPrinz, 1974 ; Takahashi, 1978, 1986) of the Oga Peninsula, and Oshima-Oshima volcano(Yamamoto, 1984 ; Ninomiya and Arai, 1992) in the Sea of Japan off Hokkaido. The hostFig. 1. Distribution of Cenozoic alkali basalts in the Southwest Japan arc (modified from Takamura,1973). Note that they often make clusters. Main localities of deep-seated xenoliths are indicated :OK, Oki-Dogo ; ON, On-yama ; AR, Aratoyama ; HM, Hamada ; OG, Noyamadake ; KW,
Kawashimo ; SH, Shingu ; IK, Iki ; KR, Kurose ; and TK, Takashima.

Ultramafic-mafic xenoliths in Kurose and Takashima, southwestern Japan

11

Fig. 2. Topographical maps of Kurose and Takashima. Made from 1 : 25,000 topographical maps of ”
Karatsu” and ”Genkai-jima” issued by the Geographical Survey Institute, Japan.

magmas for the two xenolith suites are of island-arc type, high-alumina basalt and calcalkaline andesite to dacite for Megata volcano (Katsui et al. 1979 ; Aoki and Fujimaki,
1982 ; Sakuyama and Koyaguchi, 1984), and calc-alkaline andesite for Oshima-Oshima
volcano (Katsui et al., 1979).
2. Geological background
There are many localities of Cenozoic alkali basalts and related volcanics that produce
deep-seated rocks as xenoliths (Fig. 1). They usually make volcano clusters, which havedistinct characteristics of age and chemistry of magmas (Uto, 1990 ; Iwamori, 1991). The
oldest of them is Shingu alkali basalt (lamprophyre) dikes on Shikoku Island, of which age
is 17.7 Ma (Uto et al., 1987) (Fig. 1). Kurose and Takashima belong to the youngest onesof them. Takashima alkali basalt belongs to the Higashi-Matsuura volcano cluster, andKurose alkali basalt is isolated from other clusters (Fig. 1).
2.1 Kurose
Kurose (= Black Rock in Japanese) is a reef-like rock of alkali basalt off Genkai-jimaIsland at the mouth of Hakata Bay (Figs. 1 and 2). The Genkai-jima Island is composed of12Shoji ARAI, Natsue ABE, Hisatoshi HIRAI and Yohei SHIMIZU

Cretaceous granite, through which the alkali
basalt erupted at 1.1 Ma (Uto et al., 1993). No
pyroclastics are found around Kurose. The alkali basalt is slightly porous and light gray incolor, frequently showing flow foliations on
the outcrop. Pillow-like structures are occasionally observed. The Kurose alkali basalt isabundant in xenoliths, which are up to 30 cm
across, angular to rounded in shape, and arevarious from feldspathic (granulite) to peridotitic via websterites. The most important is
the almost absence of black pyroxene
megacrysts and related pyroxenites of GroupII in the sense of Frey and Prinz (1978) (Fig.
3). Loosely packed black pyroxenites ofGroup II are rarely found.
2.2 Takashima
Takashima (=Tall Island in Japanese) isa kind of mesa of granodiorite capped by alkali basalt of 3.0 Ma in age (Nakamura et al.,1986) in Karatsu Bay (Figs. 1 and 2). The
Fig. 3. Frequency diagrams of xenolith rockcliff of alkali basalt is not accessible but we
species and K-Ar ages of host basalts
from Kurose and Takashima (eastern
can observe boulders of various volcanics oncoast). Ages from Uto et al. (1993) andthe shore of the islet of Takashima. Alkali baNakamura et al. (1986). Note the differsalt lavas show various lithologies, from fineence between the two localities. harz,
grained black one to coarser-grained grayish
harzburgite ; lherz, lherzolite ; dun,one. It is sometimes highly plagioclase-phyric.
dunite ; wehr, wehrlite ; pyrox, pyroxVent-opening pyroclastics underlie the lava
enites ; granul, granulite.
flows, and directly cover the basement of granodiorite. Xenoliths up to 50 cm across and megacrysts up to 5 cm across are abundantly
included in all kinds of volcanics. Peridotite xenoliths have been selectively eroded to beconcave and, are unfortunately difficult to sample from hard alkali basalt boulders, which
are frequently rounded by wave action.
Peridotites and pyroxenites are predominant over feldspathic rocks ; granulite (rare),gabbros and basement granodiorite, which is considerably digested to be have small druses
inside and glass-rich halo around xenoliths. Variations of frequency of xenolith species areobserved depending on the difference of host rock types (lithologies) (Fig. 4). The pyroclasitics are basically composed of comminuted granodioritic materials and essential fragments with small xenoliths and megacrysts (Fig. 5). Gabbro xenoliths are more abundantUltramafic-mafic xenoliths in Kurose and Takashima, southwestern Japan

13

Fig. 4. Frequency diagrams of xenolith rock species in alkali basaltic lavas at three localities from Takashima. Dun, dunite ; Wehr, wehrlite ; Ol-cpx, olivine clinopyroxenite ; Cpx, clinopyroxenite ;Web, websterite ; Ol-web, olivine websterite ; Gab, gabbro and mafic granulite ; Gran, granodiorite. Group II rocks are obliquely ruled, and crustal rocks are horizontally ruled.

Fig. 5. Frequency diagrams of xenolith rock species in various volcanics of Takashima. 1, lapilli stone ;2, ill-sorted tuff breccia with essential blocks and bombs ; 3, essential blocks and bombs of 2 ; 4,lapilli tuff ; 5, weakly welded tuff ; 6, plagioclase-rich lava.

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Shoji ARAI, Natsue ABE, Hisatoshi HIRAI and Yohei SHIMIZU

relative to ultramafic ones in pyroclastics than in lavas (Figs. 4 and 5).
This possibly implies a kind of fractionation of xenolith species in alkali
basalt magmatic column within aconduit before eruption (Hirai, 1983).
Megacrysts are dominated by pitchblack clinopyroxene and orthopyroxene with subordinate plagioclase (andesine) (Kuno, 1964). Orthopyroxene megacrysts are always surrounded by leucocratic reaction rim.
3. Petrography
Typical mantle peridotite xenoliths from the Japan island arcs areshown in Plate I for comparison. The
textural variety is noteworthy.

Fig. 6. Modal amounts of olivine and pyroxenes in ultramafic xenoliths from Kurose. Open square,
dunite ; open circle, harzburgite/lherzolite ; closed
triangle, pyroxene-rich lherzolite ; diamond, websterite.

3.1 Kurose
The Kurose alkali basalt is vesicular, and has phenocrysts or microphenocrysts of olivine (< 0.5 mm) and titanaugite. Groundmass is composed of euhedral olivine, titanaugite,
plagioclase, magnetite, alkali feldspar (subhedral to anhedral) and zeolites. Xenocrysts ofolivine and orthopyroxene, derived from disaggregated peridotite xenoliths, are also found.
The basalt is moderately silica-undersaturated and has about 6 % of normative nepheline(Table 1). It is higher in Na2O amount relative to K2O than an averaged Cenozoic alkali basalt in the Southwest Japan arc (Takamura, 1973), leading to having high normative Ab
and low normative Or contents.
More than 1,700 xenoliths were examined on the outcrop for the size and rock species
to determine the frequency (volume ratio) of the rock species in the Kurose xenolith suite(Fig. 3). Harzburgite and lherzolite are predominant over dunite-wehrlite, pyroxenites(mostly websterite) and mafic granulites (Fig. 3). The mantle peridotite is moderately foliated and is relatively fine-grained (1 to 2 mm across) (Plate II). It has typically equigranular to porphyroclastic textures (Plate II) : olivine has straight grain boundaries that havenice triple junctions. Clinopyroxene is up to 8 vol. % and cpx/(opx + cpx) volume ratio isaround 0.05, meaning that the peridotite is around the boundary between lherzolite andharzburgite (Arai et al., 2000 ; Fig. 6). Olivine and orthopyroxene commonly show kinkbands and wavy extinction, respectively. Orthopyroxene has only thin exsolution lamellaeof clinopyroxene. Finer-grained pyroxene-rich lherzolite is sometimes found, and its spinel
is light greenish brown indicating its fertile aluminous character (Plate II). Websterites areUltramafic-mafic xenoliths in Kurose and Takashima, southwestern Japan

15

usually free from olivine, and contain green spinel (Plate II).
Granulite is composed of plagioclase, clinopyroxene, orthopyroxene and green spinel (Plate II), andis apparently continuous in mode to
websterite with a decrease of plagioclase and an increase of green
spinel. Clinopyroxene is slightly
dominant over orthopyroxene involume in these rocks. It is noteworthy that olivine does not coexist
with plagioclase in all Kurose xenoliths.
Fig. 7. Modal amounts of olivine and pyroxenes of TakaThe Group II xenoliths areshima xenoliths. Note their wide lithological varimostly cpx-rich websterite (Plate II),
ation.
composedofirregular-shaped
grains up to 2 mm across of clinopyroxene and orthopyroxene. Pyroxenes have exsolution
lamellae with various thickness. Clinopyroxene occasionally have trace amounts of kaersutite and rhoenite blebs up to 50 microns, occasionally associated with orthopyroxene lamellae.
3.2 Takashima
The host rock is alkali basalt (Table 1), containing phenocrysts of olivine, clinopyroxene and plagioclase, and the groundmass of plagioclase, olivine, clinopyroxene, magnetiteand zeolites. Druses are filled with clays, zeolites and calcite (Kobayashi and Arai, 1981).
Almost all xenoliths can be divided into two groups, I and II, in the sense of Frey andPrinz (1978). Group I xenoliths have green clinopyroxene, and their minerals are closely
packed ; Group II ones in contrast have black clinopyroxene, and their minerals are usually coarse and loosely packed. Intermediate ones are, however, found very frequently,
having intermediate-colored (dull green) clinopyroxene. Dunite and clinopyroxenie ofGroup I are predominant over other rock species as xenoliths both in number and size in
Takashima.
3.2.1 Group I
Group I xenoliths are dunite, wehrlite, clinopyroxenites, and websterites (Fig. 7). Olivine to clinopyroxene ratio is apparently gradual from dunite to clinopyroxenite. Clinopyroxene is predominant in volume over orthopyroxene in Group I pyroxenites. Rocks ofGroup I frequently exhibit layered or banded structures.
Dunite and wehrlite are medium-grained and have mosaic-equigranular texture (Plate16

Shoji ARAI, Natsue ABE, Hisatoshi HIRAI and Yohei SHIMIZU

III). Olivine exhibits kink band andwavy extinction, and pyroxenes
scarcely have exsolution lamellae.
Chromian spinel, usually associatedwith clinoyroxene, is brown to opaquein thin section, and varies from 0.2 to
7.7 % in volume. Orthopyroxene isrounded and included in clinopyroxene in wehrlite.
Clinopyroxenites are composedof clinopyoxene with subordinate
amounts of olivine, chromian spineland orthopyroxene. Clinopyroxene ismosaic and shows wavy extinction.
Exsolution lamellae in clinopyroxene
are not so abundant but is slightly
more prominent than in dunitewehrlite. Olivine is smaller in size
than clinopyroxene, and is rounded inshape but is mosaic in olivine-richparts. Orthopyroxene is various in
shape, being coarse-grained andpoikilitically enclosing olivine or being rounded and enclosed by clinopyroxene. Chromian spinel is opaque inthin section.
Websterites are composed of mosaic pyroxenes with exolution lamellae and wavy extinction. Orthopyroxene is sometimes kinked. Olivine isrounded and is also kinked. Chromian spinel is brown to opaque in thin section.
Table 1. Bulk rock chemistry of host alkali basalts ofKurose and Takashima. Cr2O3* & NiO*,
numbers in parentheses are in ppm of Cr andNi, respectively. Conventional wet chemical
analysis by H. Haramura, University of Tokyo. KRh-1, vesicular basalt ; T-119, pale
gray basalt ; T-058, gray basalt ; T-066,
black basalt.

3.2.2 Lherzolite
One lherzolite xenolith of Group I (TKS-10) was found from Takashima (Arai andKobayashi, 1984). It has protogranular to slightly porphyroclastic texture, and has olivine(70 %), orthopyroxene (20 %), clinopyroxene (8 %), spinel (0.2 %), sulfides (0.4 %) andothers (1 %). Olivine and pyroxenes characteristically have numerous minute inclusion
trails. Calcite is frequently found associated with clay minerals, and usually fills cracks orinterstices of primary silicates, indicating secondary origin. It also occurs as rounded orbleb-like inclusions in fresh pyroxenes (Arai and Kobayashi, 1984). Sulfides (pyrite, pyr-

Ultramafic-mafic xenoliths in Kurose and Takashima, southwestern Japan17

rhotite and pentlandite) are also found as composite grains.
3.2.3 Chromitite
Fine-grained chromitite of Group I is occasionally found as thin layers or streaks indunitic rocks (Arai and Abe, 1994) (Plate III). Discrete xenoliths of chromitite are only
rarely found : two of them were found to have so-called nodular texture (Plate III), which
is characteristic of podiform (alpine-type) chromitite (Arai and Abe, 1994). They haverounded aggregates (nodules, up to 5 mm across) of fine-grained chromian spinel associated with pyroxenes, which are embedded in olivine-rich matrix.
3.2.4 Group II
Clinopyroxenites, websterites and orthopyroxenites mainly comprise Group II xenolith suite (Kobayashi and Arai, 1981 ; Fig. 7). They are mostly composed of coarse grainsof pyroxenes that have spongy margins associated with fine-grained aggregates of plagioclase, olivine and spinel (Plate III). Pyroxenes have thick exsolution lamellae or blebs. Orthopyroxene sometimes shows wavy extinction. Olivine, rounded and sometimes kinked, ispoikilitically enclosed by pyroxenes. Chromian spinel is rare and pyrrhotite is sometimes
found. Coarse-grained gabbros of Group II are also found.
3.2.5 Orthopyroxene-rich veinlets (Group III)
Orthopyroxenite with or without plagioclase and clinopyroxene sometimes makeveinlets in dunite and wehrlite xenoliths of Group I (Plate III). Discrete xenoliths of finegrained orthopyroxenite that contain dunite fragments are not rare. They are equivalent toGroup III xenoliths of Kobayashi and Arai (1981) and Arai and Kobayashi (1983). In
these rocks replacement textures of olivine by orthopyroxene are clearly observed underthe microscope. Plagioclase is found in central portions of relatively thick veins. Lightcolored (greenish brown to grayish brown) aluminous spinel is occasionally found. Orthopyroxene is light purplish brown in thin section.
Kobayashi and Arai (1981) reported clinopyroxenite and websterites of Group III.
They are mainly composed of coarse pyroxenes that have prominent exsolution lamellae,
and green spinel. They are different from the orthopyroxenite mentioned above in grain
size but is similar to each other in the abundance and color (light greenish to brownish) of
spinel and the color of pyroxenes (light purplish brown) under the microscope (Kobayashi
and Arai, 1981).
3.2.6 Composite xenoliths
Composite xenoliths (Irving, 1980) are very commonly found in Takashima. Dunite
to clinopyroxenite of Group I is frequently enclosed or veined by black coarse-grained pyroxenite of Group II, indicating the younger generation of the latter. As mentioned above
Group I dunite or wehrlite is sometimes veined by fine-grained orthopyroxenite (Group III).

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Shoji ARAI, Natsue ABE, Hisatoshi HIRAI and Yohei SHIMIZU

Group II rocks are possibly younger than the fine-grained orthopyroxenite (Group III) because the latter appears to be invaded by Group II in the some composite xenolith(orthopyorxenite-veining dunite enclosed by Group II pyroxenite).
4. Mineral chemistry
Minerals were analyzed by electron microprobe (JEOL JXA-50 A) at Chemical
Analysis Center of the University of Tsukuba when two of the authors (S.A. and H.H.) belonged to the University of Tsukuba. Additional microprobe (JEOL JXA-8800) analysis
was done at the Center for Cooperative Research of Kanazawa University. Selected analyses are listed in Tables 2 and 3. Ferrous and ferric irons in spinel were calculated assuming
spinel stoichiometry.
4.1 Kurose
Mineral chemical compositions show narrow ranges in ordinary mantle peridotites(harzburgite and clinopyroxene-poor lherzolite) in accordance with their small modal variations (especially cpx/(opx + cpx) ratio). Pyroxene-rich lherzolites have more fertile mineral chemistry (Table 2).

Table 2. Selected microprobe analyses of minerals in mafic-ultramafic xenoliths from Kurose. opx, orthopyroxene ; cpx, clinopyroxene ; pl, plagioclase ; lam, lamella ; FeO*, total iron as FeO.
nd, not determined. Mg#, Mg/(Mg + total Fe) atomic ratio of silicates, and Mg/(Mg + Fe2+)
atomic ratio for spinel. Cr#, Cr/(Cr + Al) atomic ratio. Ca, Mg & Fe*, respective catonic fractions to (Ca + Mg + total Fe) in pyroxenes. Cr, Al & Fe3+, respective cationic fractions to (Cr +Al + Fe3+) in spinel. Ca, Na & K, respective cationic fractions to (Ca + Na + K) in plagioclase.

Ultramafic-mafic xenoliths in Kurose and Takashima, southwestern Japan

19

Table 2. (continued)

4.1.1 Group I and mafic granulite
Fo content and NiO content of olivine range from 89.9 to 91.2 (Fig. 8) and
0.3 to 0.4 wt%, respectively, in the mantle peridotite. Dunite has a slightly wider
range of Fo content of olivine, from 88.8
to 92.3. Fo content of olivine is lower inpyroxne-rich lherzolite, being from 85.2
to 89.3.
The Mg# (= Mg/(Mg + total Fe)
atomic ratio) of orthopyroxene ranges
from 0.905 to 0.918, being slightly
higher that of olivine in the mantle peridotite. Al2O3 , Cr2O3 and CaO contents
range from 1.8 to 3.5 wt%, 0.5 to 0.6 wt% and 0.8 to 1.1 wt%, respectively. Olivine is very similar in composition between dunite and mantle peridotite, but
Cr2O3 and CaO contents are slightly
lower in the former. The Mg# of orthopyroxene ranges from 0.822 to 0.877
in pyroxene-rich lherzolite, from 0.788
to 0.848 in websterites and from 0.647 to
0.839 in mafic granulites. Al2O3 content
of orthopyroxene varies from 4 to 5 wt%

Fig. 8. Relationships between the Fo content of olivine
and the Cr/(Cr+Al) atomic ratio of chromianspinel in peridotites from Kurose. Peridotites
from Noyamadake (Fig. 1) are also plotted forcomparison. Note that the Fo content of olivine is slightly lower at a given Cr# of spinelin Kurose than in Noyamadake. OSMA,
olivine-spinel mantle array, a spinel peridotite
restite trend by Arai (1994).

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Shoji ARAI, Natsue ABE, Hisatoshi HIRAI and Yohei SHIMIZU

in pyroxene-rich lherzolite, from 4 to 7 wt% in websterite, and from 6 to 7 wt% in mafic
granulite. Cr2O3 content of orthopyroxene ranges from 0.2 to 0.5 wt% in pyroxene-richlherzolite, from 0.1 to 0.3 wt% in websterite, and from 0.1 to 0.2 wt% in mafic granulite.
The Mg# of clinopyroxene ranges from 0.911 to 0.932, which is slightly higher thanthat of coexisting olivine and orthopyroxene in mantle peridotite. Al2O3 and Cr2O3 contents
of clinopyroxene in mantle peridotite range from 1.9 to 3.6 wt% and from 0.6 to 0.8 wt%,
respectively. Clinopyroxene is relatively low in TiO2 (less than 0.2 wt%) and in Na2O (lessthan 0.5 wt%). The Mg# of clinopyroxene ranges from 0.861 to 0.893 in pyroxene-richlherzolite, from 0.788 to 0.867 in websterites and from 0.688 to 0.861 in mafic granulites.
Al2O3 content of clinopyroxene varies from 5 to 6 wt% in pyroxene-rich lherzolite, from 5
to 8 wt% in websterite, and from 6 to 9 wt% in mafic granulite. Cr2O3 content of clinopyroxene ranges from 0.4 to 0.8 wt% in pyroxene-rich lherzolite, from 0.2 to 0.6 wt% in
websterite, and from 0.1 to 0.3 wt% in mafic granulite.
Cr# (= Cr/(Cr + Al) atomic ratio) of spinel ranges from 0.27 to 0.55 in mantle peridotite, being relatively narrow in concordance with the relatively small variation of petrographical characteristics (especially cpx/(opx + cpx) ratio) (Figs. 6 and 8). TiO2 content islower than 0.2 wt%, and Fe3+/(Cr + Al + F e3+) atomic ratio varies from 0.02 to 0.04. The
Cr# of spinel ranges from 0.06 to 0.15 in pyroxene-rich lherzolite, and from 0.02 to 0.1 inwebsterite. The ratio of spinel is lower than 0.02 in mafic granulite.
Plagioclase is calcic and An content varies from 76 to 93 in mafic granulite.
4.1.2 Group II
The Mg# is 0.74 in orthopyroxene and is 0.73 in clinopyroxene
in Group II, being considerably
lower than in Group I (Table 2). Al2O3 content is 5.0 wt% in orthopyroxene and is 6.8 wt% in clinopyroxene. Cr2O3 content is low (< 0.12wt%) and Na2O content is high (1.2
wt%) in clinopyroxene. In contrast
TiO2 content is high both in orthopyroxene (0.3 wt%) and inclinopyroxene (1.2 wt%). Kaersutite has about 5 wt% of TiO2 andMg# of 0.69. Rhoenite contains
about 25 mole % of aenigmatitecomponent and is slightly deficientin Ti (about 0.82 per formula unit).

Fig. 9. Relationships between the Fo content of olivine andthe Cr/(Cr+Al) atomic ratio of chromian spinel indunite from Takashima. Solid circle, discrete
dunite ; open circle, dunite clast in Group II orGroup III. Solid curves with arrow indicate variations within the dunite clasts.

Ultramafic-mafic xenoliths in Kurose and Takashima, southwestern Japan21

4.2 Takashima
4.2.1 Group I
Fo content of olivine in dunite is highly variable from 93 to 84 even for discrete xenoliths (Fig. 9). NiO content of olivine vary positively with the Fo content, while MnO content has a negative correlation. In composite xenoliths with pyroxenites of Group II orGroup III, olivine in dunite parts regularly become Fe-rich toward the boundary (Fig.9).
The variation is relatively sharp toward the dunite-Group III contanct and gradual toward
the dunite-Group II contact. The Fo content of olivine appears to be bimodally distributed,
around 90 and 85. As mentioned below, the Cr# of spinel varies in antipathy with the Fo ofolivine (Fig. 9). Wehrlite and olivine clinopyroxenite have olivine of Fo85.5-87.6 and Fo84.7, respectively.
Chromian spinel in dunite demonstrates a wide range of composition ; the Cr# varies
from 0.4 to 0.8 in discrete dunite xenoliths (Fig. 9). The Cr# of spinel becomes much lower
in metasomatized dunite (Fig. 9). In individual samples coarser-grained spinels tend to behigher in Cr# than finer-grained ones. Chromian spinel in wehrlite and pyroxenites ofGroup I is lower in Cr# on average, < 0.6. Chromain spinel in Group I dunite exhibits large
compositional variations when it is in contact with Group III pyroxenites. TiO2 content ofchromian spinel in dunite is usually lower than 2 wt% ; being < 0.5 wt% if coexisting witholivine of >Fo87 and > 0.8 wt% if coexisting with olivine of <Fo86.
Chemical compositions of clinopyroxene significantly change depending on the lithological variation of Group I rocks. The Mg# varies from 0.93 in dunite to 0.83 in clinopyroxenite, and is most frequently 0.88 to 0.89. The ratio is generally higher than inTable 3. Selected microprobe analyses of minerals in ultramafic xenoliths from Takashima. dun, dunite ;ol, olivine ; cpx’nite, clinopyroxenite ; opx’nite, orthopyroxenite ; mc, megacryst. Other abbreviations are the same as those in Table 2.

22Shoji ARAI, Natsue ABE, Hisatoshi HIRAI and Yohei SHIMIZU

Table 3. (continued)

Groups II and III.
4.2.2 Lherzolite
The lherzolite (TKS-10) has iron-rich mineralogy (Arai and Kobayashi, 1984) (Table3) : olivine is Fo85 in composition. The Cr# of chromian spinel is as low as 0.128, and Al2O3 and Cr2O3 in clinopyroxene are 3.74 wt% and 0.29 wt%, respectively. Carbonate is almost pure calcite.
4.2.3 Chromitite
Chromian spinel is highly chromiferous and its Cr# ranges from 0.6 to 0.8, which isequivalent to the range for typical podiform chromitites (Arai and Abe, 1994). The chromitite xenolith with nodular texture has especially high-Cr# spinel, the Cr# being around 0.8,
which ratio is the same as that of the most typical podiform chromitite.
4.2.4 Group II
As is well known, minerals of Group II rocks are generally richer in Fe, Al and Ti
than those of Group I (Fig. 10). Al2O3 and TiO2 contents of clinopyroxene are 5 to 7 wt%
and 1 to 1.8 wt%, respectively. Its Cr2O3 content is lower than 0.5 wt%. The Al2O3 content
of orthopyroxene ranges from 3 to 5 wt%. The Mg# of pyroxenes ranges mostly from 0.80
to 0.85. Ca/(Mg + total Fe + Ca) atomic ratio of clinopyroxene is around 0.4.

Ultramafic-mafic xenoliths in Kurose and Takashima, southwestern Japan

23

4.2.5
Orhopyroxene-richveinlets(Group III)Rocks of Group III have relatively
Fe-,Al-rich mineralogy except for thinveinlets within dunite. Al2O3 and TiO2
contents of clinopyroxene are up to 9
wt% and > 1 wt%, respectively. The Al2O3 content of orthopyroxene ranges
from 4 to 7 wt%. The Mg# of pyroxenes is lower than 0.8. Pyroxenes areusually lower in Mg# and are higher in
Al2O3 and TiO2 than those of Group II(Fig. 10). The Cr# of spinel is lower
than 0.01. As mentioned above olivine
and spinel in Group I dunite are modified to be Fe-richer and Al-richer, respectively, toward the contact withGroup III pyroxenites. In accordandewith this, pyroxenes and spinel in theGroup III pyroxenites become magne- Fig.10. Relationships between the TiO2 content and Cr2
sian and Cr-rich toward the contact
O3 and Al2O3 contents in clinopyroxene in ultramafic rocks from Takashima and Kurose.
with the dunite. In thin orthopyroxenite
veinlests in dunite of Group I, orthopyroxene is high in Mg# (up to 0.89) and low in Al2O3 (2 wt%).
4.2.6 Megacrysts
Megacrystal pyroxenes are similar in composition to equivalents in Group II pyroxenites (Arai et al., 2000). Plagioclase is mostly andesine (Aoki, 1970).
5. Geothermo-barometry
Equilibrium pressure can not be calculated because there are no good geobarometers
for our rocks. All ultramafic rocks from Kurose and Takashima as the other xenoliths fromSouthwest Japan arc have neither olivine-garnet nor olivine-plagioclase assemblages, indicating they have been equilibrated within spinel lherzolite stability field (Fig. 11). Twopyroxene temperatures are almost the same for the Kurose peridotites to mafic granulite,
Group I and Group III rocks of Takashima. For example Wells’ (1977) temperature is
1,000 to 1,100 ℃ for them all. Equilibrium temperature (Wells, 1977) for Group II rocksof Takashima is 100 ℃ higher, around 1,150 ℃ (Fig. 12).
The Kurose harzburgite/lherzolite has distinctly lower equilibrium temperature than,for example, Noyamadake lherzolite/harzburgite (Arai and Hirai, 1983 ; Hirai, 1986 ; Abe

24

Shoji ARAI, Natsue ABE, Hisatoshi HIRAI and Yohei SHIMIZU

Fig.11. Pressure-temperature relationshipsof peridotite xenoliths from theJapan island arcs. The Kurose
peridotite is within the region of ”
SWJ low-T”. The temperaturesare shown as histograms in Fig.
13. Geotherms of SE Australia
and of Qilin (SE China) are after
O’Reilly and Griffin (1985) and
Xu et al. (1996), respectively.
The phase boundaries are after
O’Neil (1981), Takahashi andKushiro (1983) and Gasparik(1987).

Fig. 12. Histograms of equilibrium temperatures (Wells, 1977) of Takashima and Kurose ultramafic
xenoliths.

Ultramafic-mafic xenoliths in Kurose and Takashima, southwestern Japan25

Fig.13. Equilibrium temperatures (Wells, 1977) of mantle peridotite xenoliths from the Japan island arcs.
Note that the Kurose peridotite has relarively low temperatures. Data of Fukue-jima are from
Umino and Yoshizawa (1996).

Fig.14. Relationships between the f O2 and temperature of peridotite xenoliths from the Japan island arcs.
Oxygen barometer by Ballhaus et al. (1991) and the thermometer by Sachtleben and Seck (1981).
Note that peridotites from the Southwest Japan arc, including the Kurose peridotite, have low f O2
than those from the Northeast Japan arc.

et al., 1999) (Fig. 13). Assuming a single geotherm, the upper mantle has a relatively highgeotherm in the Southwest Japan arc (Abe and Arai, 2001), being higher than those of SE
China (Xu et al., 1996) and SE Australia (O’Reilly and Griffin, 1985) (Fig. 11).
The f O2 (Ballhaus et al., 1991) of the Kurose peridotite is slightly lower than that of
QMF equilibration (Fig. 14). It is noteworthy that the f O2 is higher in Megata peridotite
xenoliths, which are sometimes hydrated, from the Northeast Japan arc than in peridotites,
which are dry, from the Southwest Japan arc (Fig. 14).

26

Shoji ARAI, Natsue ABE, Hisatoshi HIRAI and Yohei SHIMIZU

6. Discussion
6.1 Origin of deep-seated rocks
6.1.1 Mantle peridotite
The Kurose peridotite is very similar in major-element chemistry to some abyssal
peridotite. Na2O content of clinopyroxene is relatively low (< 0.6 wt%) (Arai, 1991, 1994)
and the Cr# of spinel is intermediate, from 0.3 to 0.5 (Arai, 1994) (Fig. 15). The Fo content
of olivine is relatively low as compared at a given Cr# of coexisting spinel (Arai, 1994).
The Kurose peridotite is a low-pressure mantle restite, formed either at mid-ocean ridge but
now captured in arc regime or at in-situ island arc. This kind of peridotite is present atsome part of sub-arc mantle ; possibly at the uppermost mantle of arc with thin crust.

Fig.15. Relationships between the Na2O content of clinopyroxene and the Cr/(Cr+Al) atomic ratio ofchromian spinel in peridotite xenoliths. Note that the Kurose peridotite has relatively low Na2O
content of clinopyroxene.

Pyroxene-rich lherzolite and spinel websterites are possibly metamorphosed
plagioclase-invaded peridotite and troctolite (or olivine gabbro), respectively (Fig. 16).
This should be confirmed by trace-element geochemistry for clinopyroxene and bulk rock.
Incompatibility of olivine with plagioclase in the Kurose xenoliths may support this interpretation.
6.1.2 Group I dunite-wehrlite-pyroxenites
Dunite, wehrlite and pyroxenites of Group I are basically cumulates, forming the ”cumulus mantle” of Takahashi (1978). Their mother magmas or magma may be of arc affin-Ultramafic-mafic xenoliths in Kurose and Takashima, southwestern Japan27

Fig.16. A possible model of the lower crustal rocks beneath Kurose deduced from xenoliths.

ity because of high Cr# (up to 0.8) and low Ti character of chromian spinel in most magnesian dunite (Fig. 9 ; Table 3). The wide range of Fo content of olivine in dunite (84 to 93)
was possibly produced not by fractional crystallization but by metasomatism by the melt
for Group II formation. It may be difficult to crystallize only olivine and spinel during thelarge variation of Mg# of melt upon fractional crystallization.
Possibly presence of podiform chromitite (Arai and Abe, 1994) associated with dunite
suggests a peridotite/melt reaction origin for some dunite (see Arai and Yurimoto, 1994,
1995). The reactant peridotite is possibly similar to the Kurose harzburgite/lherzolite,
which is approapriate to produce podiform chromitites (Arai, 1997). This further suggeststhat the Group I rocks partly form a kind of Moho transition zone (= dunite-dominant zone
between lower harzburgite and upper layered gabbro of ophiolites) (Nicolas, 1989).
6.1.3 Group II pyroxenites and megacrysts
Clinopyroxenes of Group II pyroxenites are similar in chemistry to clinopyroxene
megacrysts (Arai et al., 2000). As megacrsts have been considered to be high-pressure
megacrysts of host alkaline basalts (Irving, 1974), Group II rocks are deep-seated cumulates from alkali basalts, which are akin to the host basalts but are slightly older in age.
Their cognate origin can be denied because their pyroxenes have characteristics indicating
deformation (kink bands) and subsolidus cooling (exsolution blebs and lamellae). This isconsistent with the geologic situation of Takashima and Kurose alkali basalts mentioned
above (Fig. 1). The Takashima alkali basalt belongs to the Higashi-Matsuura volcano clus-

28

Shoji ARAI, Natsue ABE, Hisatoshi HIRAI and Yohei SHIMIZU

Fig.17. A petrological mode of the uppermost mantle beneath the Southwest Japan arc : two contrastingtypes. The Aratoyama-type mantle is characterized by invasion of dense network of Group II
rocks within peridotite, and the Kurose-type one is almost free of Group II rocks. The fomer wasdeveloped beneath monogenetic volcano clusters, while the latter is representative of intact mantle beneath a solitary volcano (Fig. 1).

Ultramafic-mafic xenoliths in Kurose and Takashima, southwestern Japan29

Fig.18. A petrological model of the uppermost mantle beneath northern Kyushu (Kurose-Takashimaarea). The mantle is strongly heterogeneous in terms of extent of Group II invasion, formation of
dunite-pyroxenites of Group I.

30

Shoji ARAI, Natsue ABE, Hisatoshi HIRAI and Yohei SHIMIZU

ter, indicating that preceding activity of similar alkali basalt was highly possible in the upper mantle. On the contrary, the Kurose alkali basalt is solitary, and cumulates of Group II
by consanguineous magma could not been produced. This is consistent with the scarcity ofGroup II xenoliths and black pyroxene megacrysts in the Kurose alkali basalt.
6.1.4 Group III
It is noteworthy that the melt for Group III rock formation was reactive with olivine to
produce orthopyroxene. The pyroxenes are high in Fe, Ti and Al, and spinel is also highly
aluminous. This strongly indicates an activity of Fe-,Ti-and Al-rich silica-oversaturated
melt within the uppermost mantle (or Moho transition zone) of arc. We interpreted that themelt was derived from the then subducting slab.
6.2 Petrological heterogeneity of the upper mantle
Lateral heterogeneity of the upper mantle to lower crust is clearly shown by the difference of xenolith species between Kurose and Takashima, which are only 30 km apart (Araiet al., 2000).
6.2.1 Distribution of Group II rocks
Group II rocks and related megacrysts are almost absent in the Kurose xenolith suite.
This indicates that the distribution of Group II rocks is highly heterogeneous laterally (Araiet al., 2000). The Kurose xenoliths are, therefore, devoid of metasomatism by the melt involved in Group II formation, and are expected to preserve their primary petrological characteristics (Arai et al., 2000). This kind of xenolith suite is very rare in the Southwest Japanarc, where monogenetic volcanoes of alkali basalts usually make clusters (Figs. 1 and 17).
The Group I rocks of Takashima, on the other hand, were possibly affected by themelt for Group II formation (Fig. 18), considering the large amount of Group II xenoliths
associated with Group I xenoliths. It is possible that the negative corelationship between
the Fo content of olivine and Cr# of chromian spinel in Group I dunite is partly a result ofcryptic metasomatism by the melt for Group II formation. This may be suggested by thevariation within dunite pieces captured by Group II pyroxenites (Kobayashi and Arai,
1981).
6.2.2 Distribution of Group I rocks
The possible ”Moho transition zone”, which is mainly composed of Group I rocks, isalso changeable laterally (Fig. 18). The paucity of both lithological variety and relative volume of Group I dunite and pyroxenites in Kurose possibly means that dunite-wehrlitepyroxenites of Group I characteristics are thin or small in volume relative to mantle peridotite beneath Kurose. The equilibrium temperature of Group I rocks from Takashima issimilar to that of the mantle peridotite from Kurose, possibly indicating the similar depth oforigin for the two xenolith suites. The thick cumulus mantle or Moho transition zone of arc

Ultramafic-mafic xenoliths in Kurose and Takashima, southwestern Japan31

origin may mean large activity of arc magma(s), leading to formation of a thick overlying
arc crust.
7. Conclusions
The macmatic activites responsible for the formation of Group I dunite to pyroxenites
and Group III rocks were related with arc magmas and slab-derived melts, respectively.
Group II rocks and megacrysts are deep-seated cumulates and high-pressure phenocrysts ofthe preceding and present host alkali basalts, respectively. The alkali basalt magmatism
was possibly related with opening of the Sea of Japan, a back-arc basin (Uto, 1990 ;Iwamori, 1991). They were derived from mantle diapers that were responsible for theJapan-Sea opening. The petrological lateral heterogeneity of the upper mantle deduced
from the contrast between the two xenolith suites has been possibly made by continuous
magmatic activities of arc and back arc settings of the Southwest Japan arc at the Western
Pacific rim (Arai et al., 2000).
Acknowledgements
One of the authors (S.A.) greatly acknowledges Y. Kobayashi and M. Ikeda for their collaboration for Takashima and Kurose xenoliths, respectively. N. Nishida and K. Kadoshima helped us during the microprobe analysis. This is contribution no. 264 from the
ARC National Key Centre for Geochemical Evolution and Metallogeny of Continents(www.es.mq.edu.au/GEMOC/).
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Shoji ARAI, Natsue ABE, Hisatoshi HIRAI and Yohei SHIMIZU----------and Kushiro, I. (1983). Melting of a dry peridotite at high pressure and basalt magma genesis.
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Fac. Sci. Hokkaido Univ., v.21, p.77- 131.

Ultramafic-mafic xenoliths in Kurose and Takashima, southwestern Japan

35

Plate I
Photomicrographs of typical peridotite xenoliths from the Japan island arcs. The scale is all the same
(Scale bar, 0.5 mm). (a) to (c), from the Northeast Japan arc. (d) to (h), from the Southwest Japan arc.
(a) Type I texture of Ichinomegata lherzolite (I-729). Textures of Types I, II and III are classified by
Abe et al. (1999). Crossed-polarized light. (b) Depleted lherzolite with Type II texture from Ichinomegata (I-708). Note a large kinked olivine porphyroclast (upper). Crossed-polarized light. (c) Harzburgite with type III texture from Ninomegata (N-06) with strongly deformed texture. Crossedpolarized light. (d) Low-temperature type lherzolite from On-yama (ONY-127). The texture is slimiler
to Type I of peridotite from Megata volcano. Note the small grain size compared with the Kurose peridotite. Crossed-polarized light. (e) High-temperature type lherzolite with protogranular texture from
Noyamadake (OG-104). The texture is similar to the protogranular type of Mercier and Nicola (1975).
Note the relatively large grain size and rounded pyroxenes with spongy rim. Plane-polarized light. (f)Weakly deformed lherzolite from Kawashimo (KWS-25) with protogranular texture similar to
Noyamadake one. Crossed-polarized light. (g) Clinopyroxene-poor lherzolite (KWS-11) with porphyroclastic texture. Note the rounded form of pyroxene phorphyroclasts. Plane-polarized light. (h) The
same as (g), with crossed-polarized light. Note the strong kink band in the olivine porphyroclast.
Plate II
Photomicrographs of mafic-ultramafic xenoliths from Kurose, the Southwest Japan arc. (a) Relatively
fine-grained lherzolite (KR 120). Crossed-polarized light. (b) The same as (a). Plane-polarized light.
(c) Harzburgite with porphyroclastic texture (KR 201). Note the absence of exsolution lamellae in orthopyroxene porphyroclast (left). Crossed-polarized light. (d) The same as (c). Plane-polarized light.
(e) Spinel-rich olivine websterite to lherzolite (KR 383). Note the light color of spinel. Plane-polarized
light. (f) Spinel websterite (KR 6). Note the dark green color of spinel. (g) Spinel-bearing granulite(KR 348). Note the symplectic aggregate of spinel and pyroxene. Plane-polarized light. (h) Websteriteof Group II. Note the loose grain boundaries. Crossed-polarized light.
Plate III
Photomicrographs of ultramafic xenoliths and megacrysts from Takashima, the Southwest Japan arc.
(a) Dunite of Group I (T-064). Crossed-polarized light. (b) Clinopyroxenite of Group II (T-179). (c)Dunite (Group I) with orthopyroxene-rich veinlet of Group III (center) (TK-3). Plane-polarized light.
(d) The same as (c). Crossed-polarized light. (e) Orthopyroxene megacryst with fine-grained reactionrim (TK-40). Note the homogeneous appearance and the absence of exsolution lamellae. Crossedpolarized light. (f) Clinopyroxene megacryst (T-064). Note the homogeneous appearance and the absence of exsolution lamellae. Crossed-polarized light. (g) Fine-grained chromitite seam in dunite ofGroup I (T-037). Plane-polarized light. (h) Nodular-textured chromitite. Plane-polarized light.

36

Plate I

Shoji ARAI, Natsue ABE, Hisatoshi HIRAI and Yohei SHIMIZU

Ultramafic-mafic xenoliths in Kurose and Takashima, southwestern Japan

Plate II

37

38

Plate III

Shoji ARAI, Natsue ABE, Hisatoshi HIRAI and Yohei SHIMIZU