Occurrence and chemical composition of barian feldspars

in a jadeitite from the Itoigawa-Ohmi district in the Renge

high-P/T-type metamorphic belt, Japan

T. MORISHITA*

Graduate School of Natural Science and Technology, Kanazawa University, Kanazawa 920-1192, Japan

ABSTRACT

Coexisting Ba-rich (Cn56ÿ59Or40ÿ42Ab2An0ÿ1) and Ba-poor (Cn7ÿ15Or83ÿ92Ab1ÿ3An0ÿ1) feldspars

were found in a jadeitite collected from the Itoigawa-Ohmi district in the Renge high-P/T-type

metamorphic belt, Japan. There is an apparent compositional gap in these barian feldspars at Cn15ÿ56 in

the ternary system BaAl2Si2O8ÿKAlSi3O8ÿNaAlSi3O8. Barian feldspars occur as interstitial phases

between subhedral to euhedral jadeite and prehnite, and sometimes cut a jadeite crystal. The occurrence

of barian feldspars combined with the texture and mineral assemblages of the host jadeitite suggest that

barian feldspars were formed from residual fluids after the formation of jadeite and prehnite from

primary aqueous fluids at low-T conditions (<350ëC) and pressure of ~0.6 GPa. The Ba content in the

residual fluids would increase locally during the formation of jadeite because Ba is incompatible with

clinopyroxene. This study supports the suggestion that Ba-rich minerals are not uncommon in jadeitites

and albitites/metasomatized rocks that occur as tectonic blocks in serpentinite-matrix meÂlanges. Barium

is an important minor element in metasomatizing fluids related to the formation of jadeitites and might

have been derived from subducted oceanic crust at the beginning of the subduction.

KEYWORDS: barium, Ba-feldspars, Renge metamorphic belt, Japan, jadeitite.

Introduction

JADEITITE typically occurs as tectonic inclusions in

serpentinite-matrix meÂlanges (e.g. Harlow, 1994;

Harlow and Sorensen, 2005). The dominant

mineral of jadeitites is jadeite (NaAlSi2O6), but

other diverse minerals also occur coexisting with

jadeite. The mineralogy and textural character-

istics of jadeitites support the hypothesis that at

least some jadeitites were formed by extensive

metasomatism or crystallization from ¯uids (e.g.

Coleman, 1961; Harlow, 1994; Johnson and

Harlow, 1999; Miyajima et al., 1999, 2001,

2002; Harlow and Sorensen, 2005). Based on O

and H isotope data, Johnson and Harlow (1999)

suggested that jadeitites and albitites from the

Motagua Valley, Guatemala, are metasomatic

rocks formed from a residual ¯uid after

serpentinization reactions in ultrama®c rocks.

The Itoigawa-Ohmi district, which belongs to

the Late Palaeozoic Renge high-P/T-type meta-

morphic belt (Shibata and Nozawa, 1968;

Nishimura, 1998; Tsujimori and Itaya, 1999;

Kunugiza et al., 2004) (Fig. 1), is the most

famous locality for jadeitite in Japan (Kawano,

1939; Ohmori, 1939). Tsujimori et al. (2000) and

Tsujimori (2002) reported glaucophane eclogite

preserving a progressive transition from the

epidote-blueschist facies to the eclogite facies

from the Itoigawa-Ohmi district. Jadeitites and

eclogites in this area are thought to be tectonic

inclusions in serpentinite-matrix meÂlanges.

Tsujimori (2002) suggested that blueschist-to-

eclogite metamorphism may be related to

subduction of oceanic crust between the Sino-

* E-mail: moripta@kenroku.kanazawa-u.ac.jp

DOI: 10.1180/0026461056910237

Mineralogical Magazine, February 2005, Vol. 69(1), pp. 39±51

# 2005 The Mineralogical Society

Korean and Yangtze blocks of Late Palaeozoic

age. Thus, jadeitites in the Itoigawa-Ohmi district

may record information about ¯uid-rock interac-

tions in subduction environments.

Sr-rich minerals were found uniquely in

jadeitites from the Itoigawa-Ohmi district

(Komatsu et al., 1973; Chihara et al., 1974;

Miyajima et al., 1999, 2001, 2002). Barium has

similar geochemical characteristics to Sr and is a

large-ion-lithophile element (LILE). Ba-rich

minerals have been reported in other jadeitites

or albitites/metasomatized rocks in serpentinite-

matrix meÂlanges (Kobayashi et al., 1987; Harlow,

1995), and were reported as coexisting phases

with Sr-rich minerals in albitite rocks from the

study area (Sakai and Akai, 1994). Coexisting Ba-

rich and Ba-poor feldspars were found in a

lavender-coloured jadeitite collected from the

FIG. 1. Sample locality on simpli®ed geotectonic (upper) and geological (lower) maps of the Itoigawa-Ohmi district

complied by Tsujimori (2002).

40

T. MORISHITA

Itoigawa-Ohmi district. Barian feldspar series

from this locality have not been studied in detail

although the presence of barian feldspars has been

reported from other tectonic blocks in the Renge

Metamorphic Belt by Miyajima et al. (1998). In

this paper, we describe the occurrence and

chemical compositions of these barian feldspars

in the sample studied. The petrogenesis of barian

feldspar in jadeitites is also discussed to provide

insight into element circulation in subduction

environments.

Geological background

The Itoigawa-Ohmi district is located at the

northeastern end of the Renge metamorphic belt,

which is a high-P/T-type metamorphic belt dated

at 330ÿ280 Ma (e.g. Shibata and Nozawa, 1968;

Nishimura, 1998; Tsujimori and Itaya, 1999;

Kunugiza et al., 2004) (Fig. 1). The Renge

schist is distributed as serpentinite-matrix

meÂlanges in a small area along an ENEÿWSW

line from central Kyushu to the Hida Gaien belt

(e.g. Nishimura, 1998; Tsujimori and Itaya,

1999). The Hida Gaien belt is a pre-Jurassic

composite geotectonic unit (Chihara et al., 1979;

Nakamizu et al., 1989; Komatsu, 1990; Tsukada

et al., 2004) and is tectonically bordered by the

Hida metamorphic belt (lowÿmedium-P gneiss

and schist) (Fig. 1). Serpentinite-matrix meÂlanges

contain high-P schists (blueschist and eclogitic

rocks) (Banno, 1958; Chihara et al., 1979;

Nakamizu et al., 1989; Komatsu, 1990;

Nishimura, 1998; Tsujimori et al., 2000;

Tsujimori, 2002). The ultrama®c rocks in

serpentinite-matrix meÂlanges are mainly serpenti-

nized dunite-harzburgite and serpentine-carbonate

rock (Iwao, 1953; Yokoyama, 1985). Chromitites

were also found as boulders in the studied area

(Yamane et al., 1988; Tsujimori, 2004).

The Itoigawa-Ohmi district is located at the

northeastern end of the Renge metamorphic belt

and is well known in Japan for the occurrence of

jadeitite (Kawano, 1939; Ohmori, 1939). Jadeitite

is thought to occur as tectonic inclusions in the

serpentinite-matrix meÂlange. Samples showing

spherical zoning arranged, from inner to outer,

in the order albitite-jadeitite-green jadeitite-

actinolite-host serpentinite, were reported by

Iwao (1953), Shido (1958) and Chihara (1971,

1989). The albitite varies from a few cm to 1 m

wide (Chihara, 1971). Harlow and Sorensen

(2005) examined jadeitite-albitite rocks in the

study area and have not identi®ed albitite as a

core in jadeitite blocks. They interpreted albitite

as a zone or layer and suggested that the albitites

are possible alteration products of jadeitite.

Jadeitites in the studied area are divided into

several types based on differences in colour

corresponding to the mineral assemblages (Iwao,

1953; Chihara, 1971, 1989; Oba et al., 1992;

Miyajima et al., 2001), e.g. blue jade consists

mainly of jadeite, titanian omphacite and sodic

amphibole, lavender jade consists mainly of

jadeite and Ti-bearing jadeite, and green jadeite

consists mainly of jadeite and omphacite.

Yokoyama and Sameshima (1982) ®rst reported

the coexistence of jadeite with omphacite. The

reasons for the differences in colour of jadeitite

were examined by Ou Yang (1984), MeÂvel and

FIG. 2. Sawn surface of the sample studied.

BARIAN FELDSPARS IN JADEITITE

41

KieÂnast (1986), Harlow and Olds (1987), Ouyang

(2001) and Harlow et al. (2003), and were

summarized by Harlow and Sorensen (2005).

Rare Sr-bearing minerals including three new

minerals of itoigawaite (SrAl2Si2O7(OH)´H2O),

rengeite (Sr4ZrTi4Si4O22) and matsubaraite

(Sr4Ti5(Si2O7)O8) have been reported in jadeitite

from the Itoigawa-Ohmi district (Miyajima et al.,

1999, 2001, 2002). These Sr-bearing minerals

usually occur as interstitial phases between

subhedral to euhedral jadeites and/or as veins in

jadeitites. The rengeite and matsubaraite, are both

Sr-bearing Ti or Ti-Zr silicate minerals, and are

associated with titanite, zircon and rutile. Other

Sr-bear ing minera ls such as ohmil i te

Sr3TiSi4O12(OH)2´3H2O, strontio-orthojoaquinite

Na2+xBa4Fe1.5(Sr,Ba,REE,Nb)4ÿxTi4(O,OH)4

(Si4O12)4´2H2O, strontium apatite (Sakai and

Akai, 1994) and niigataite CaSrAl3(Si2O7)

(SiO4)O(OH) were also reported in albitites/

metasomatized rocks associated with serpentinite

from the studied area (Komatsu et al., 1973;

Chihara et al., 1974; Sakai and Akai, 1994;

Miyajima et al., 2003). It is interesting to note that

strontio-orthojoaquinite and lamprophyllite also

contain signi®cant amounts of Ba (Chihara et al.,

1974; Miyajima et al., 2002) and that Sr-bearing

minerals in the albitites/metasomatized rocks

occur associated with Ba-bearing minerals such

as benitoite (Sakai and Akai, 1994).

Sample description

The sample studied was collected as a boulder

from the Omi river (Fig. 1). The sample varies in

colour on a cm scale, with white, pale green to

blue (not green jade) and purple (lavender-

coloured jade) areas (Fig. 2) re¯ecting the

variations in mineralogy. The lavender-coloured

jade occurs as spots, a few cm in size, in both the

white and pale green to blue jade areas (Fig. 2).

The white area is an aggregate of prismatic jadeite

with a very small amount of pectolite. The pale

green to blue area consists of subhedral to

euhedral prismatic jadeite in a prehnite matrix

with minor amounts of pectolite, titanite and

zircon. The lavender-coloured part consists of Ti-

bearing jadeite (up to 0.7 wt.% TiO2) in an

analcime matrix with a small amount of pectolite.

The jadeite varies in size from 10 to 500 mm long

and tends to be slightly smaller in the pale green

to blue area than in other areas. Representative

chemical compositions of jadeites are listed in

Table 1. Aggregates of ®ne-grained titanite

associated with analcime are found at the centre

of lavender-coloured regions. Ti-rich phases

(titanite or pre-existing Ti-rich phase now

replaced by titanite aggregate) might be a source

of Ti for Ti-rich jadeitite. Quartz and albite have

not been found in the studied sample. Pectolite

veins containing small amounts of wollastonite

are found in both the pale green and lavender-

coloured parts of the sample.

Barian feldspar is rare and is sporadically

distributed in the pale green to blue areas of the

sample. It occurs mainly as an interstitial phase

between subhedral to euhedral jadeite and

prehnite, and sometimes cuts subhedral jadeite

crystals (Figs 3, 4). Two types of barian feldspar,

FIG. 3. Occurrence of barian feldspars and related

minerals. (a) Photomicrograph of barian-rich area in

pale green-blue parts. (b) BSE image of a part of a

showing the occurrence of barian feldspars. Br = Ba-rich

feldspar, Bp = Ba-poor feldspar, Jd = jadeite, Pre =

prehnite, and Pec = pectolite.

42

T. MORISHITA

i.e. Ba-rich and Ba-poor, are distinguished in

back-scattered electron (BSE) images and X-ray

intensity images, and occur in direct contact with

each other (Fig. 4). Contact between Ba-rich and

Ba-poor feldspars is usually irregular but is

sometimes linear (Fig. 4). No intergrowth of two

feldspars is found. The Ba-rich feldspar seems to

occur adjacent to jadite and the Ba-poor feldspar

occurs juxtaposed against prehnite although it is

likely that the former is surrounded by the latter

(Figs 3 and 4). The Ba-rich feldspar is more

abundant than the Ba-poor one. Only the Ba-poor

feldspar cuts jadeites, so far (Fig. 4).

Mineral chemistry

The major-element compositions of the minerals

was determined using a JEOL JXA-8800

Superprobe at the Center for Cooperative

Research of Kanazawa University. The analyses

were performed at accelerating voltages of

15/20 kV and beam currents of 15/20 nA, using

a 3 mm diameter beam. ZAF correction proce-

dures were employed. Natural and synthesized

minerals were used as standards as follows: quartz

for Si, KTiPO5 for K and Ti, corundum for Al,

eskolaite for Cr, fayalite for Fe, manganosite for

Mn, periclase for Mg, wollastonite for Ca, jadeite

for Na, and baryte for Ba. No Sr was detected in

barian feldspars or other phases using multiple

qualitative analysis (SrO4~0.5 wt.%), so we did

not analyse for Sr in the studied sample. The

chemical compositions of jadeites and barian

feldspars determined by electron microprobe are

shown in Tables 1 and 2, respectively.

The chemical compositions of jadeites are

different re¯ecting the differences in colour.

Jadeites in the lavender-coloured parts are

higher in TiO2 (0.3ÿ0.7 wt.%) than others

(<0.3 wt.%) (Table 1). Jadeites surrounded by

barian feldspar in the pale green to blue area are

nearly pure jadeite but have a slightly Ca-rich,

probably diopside component-rich, rind (Fig. 4).

The chemical composition from the Ca-rich rind

has never been determined because of its small

size.

FIG. 4. X-ray intensity maps of Ba, Ca, Na and K of Fig. 3b. Br = Ba-rich feldspar, Bp = Ba-poor feldspar, Jd =

jadeite, and Pec = pectolite.

BARIAN FELDSPARS IN JADEITITE

43

As suggested above, the barian feldspars are

divided into two types: (1) celsian component

(Cn)-rich feldspars with compositions Cn56ÿ59

Or40ÿ42Ab2An0ÿ1, and (2) Cn-poor, Or-rich feld-

spars with compositions Cn7ÿ15Or83ÿ92

Ab1ÿ3An0ÿ1. There is no apparent change in the

albite content of feldspars (1ÿ3% Ab) over this

compositional range. A compositional disconti-

nuity is found at Cn15 to Cn55.

Discussion

Chemical characteristics of barian feldspars in the study

area and the effect on them of the P/T conditions

Natural feldspars are normally considered to be

within the NaAlSi3O8±KAlSi3O8±CaAl2Si2O8

ternary system. Barium seems to be a major

additional constituent of K-feldspar under natural

conditions (Deer et al., 1963; Gay and Roy,

1968). In natural samples, Ba plagioclases and Ca

celsians have very restricted compositions and are

exceedingly rare (Gay and Roy, 1968). There are

some data showing variations in the Na content of

barian feldspars (e.g. Vermaas, 1953;

Viswanathan and Kielhorn, 1983; Pan and Fleet,

1991; Chabu and BouleÁgue, 1992). Barian

feldspar is therefore usually considered to

belong to the ternary system BaAl2Si2O8±

KAlSi3O8±NaAlSi3O8. The barian feldspars

studied plot close to the BaAl2Si2O8±KAlSi3O8

line in the ternary system (Fig. 5). The effects of

the presence of Fe and Mg are negligible for the

studied sample. The feldspar compositions plot

close to the ideal line BaAl = (Na, K)Si in the

series KAlSi3O8±BaAl2Si2O8 (Fig. 6), re¯ecting

the low abundance of other elements.

As suggested above, our observations indicated

that the compositional gap exists at Cn15ÿ56 in the

ternary system. However, Gay and Roy (1968)

examined both synthetic and natural barian

feldspars and found that Ba-K feldspars show

complete solid solution at 500ëC and 700ëC at P =

2000 atm. The compositional gap in the Cn

content is, however, not uncommon in natural

samples (e.g. Yoshimura, 1939; Vermaas, 1953;

Gay and Roy, 1968; Viswanathan, 1978;

Viswanathan and Kielhorn, 1983; Pan and Fleet,

1991; Chabu and BouleÁgue, 1992; Harlow, 1994).

TABLE 1. Representative chemical compositions of jadeites.

Lavender White Green to blue (Ba-feldspar area)

J1-8-6 J1-8-21 J2-14-26 J-2-14-25 J2912-26 J4-26-37 J2912-28

SiO2 59.50 59.80 59.77 59.60 58.94 59.61 58.82

TiO2 0.47 0.74 0.29 0.18 <0.06 0.12 0.07

Al2O3 24.57 24.33 25.16 24.56 25.19 24.58 24.48

FeO <0.06 0.25 0.13 0.39 0.12 0.08 0.18

MgO <0.03 0.15 <0.03 0.09 0.05 0.41 0.57

CaO 0.02 0.10 0.05 0.11 0.13 0.69 0.84

BaO n.a. n.a. n.a. n.a. <0.2 n.a. <0.2

Na2O 15.30 15.21 15.42 15.05 15.20 14.83 14.90

K2O <0.02 <0.02 <0.02 <0.02 0.04 <0.02 <0.03

Total 99.85 100.57 100.87 100.01 99.66 100.33 99.87

Number of cations on the basis of 6 oxygens

Si 2.006 2.005 1.997 2.008 1.992 2.002 1.990

Ti 0.012 0.019 0.007 0.005 0.000 0.003 0.002

Al 0.976 0.961 0.990 0.975 1.003 0.973 0.976

Fe 0.007 0.004 0.011 0.002 0.005

Mg 0.000 0.007 0.004 0.002 0.021 0.021

Ca 0.001 0.004 0.002 0.004 0.005 0.025 0.030

Ba

Na 0.999 0.988 0.998 0.983 0.996 0.965 0.977

K 0.001 0.001 0.001 0.001 0.002 0.001

Total 3.994 3.991 3.999 3.991 4.000 3.991 4.001

n.a.= not analysed

44

T. MORISHITA

Sub-solidus relations in barian feldspars have

never been fully understood, particularly in low-T

regimes. The chemical compositions of barian

feldspars from this study were compared with

results in green mica schists from the Hemlo area,

Ontario (Pan and Fleet, 1991), from a Zn-Pb-Cu

deposit from Shaba, ZaõÈre (Chabu and BouleÁgue,

1992) and albite-mica rocks from the Motagua

Valley, Guatemala (Harlow, 1994) where the P/T

conditions of the host rocks were estimated and

the effects of the presence of Fe and Mg are

negligible. Irrespective of some dif®culties, these

data suggest that there is an increase in the Ab

content in barian feldspars with increase in

temperature (Fig. 5). If so, the barian feldspars

from this study crystallized under low-T condi-

tions, probably <350ëC. This is consistent with the

mineralogy of the sample studied (Fig. 7).

Prehnite is common in low-grade metamorphic

rocks, such as prehnite-pumpellyite facies assem-

blages (<350ëC) (e.g. Peacock, 1993).

Furthermore, pectolite typically occurs as a

hydrothermal mineral in cavities and on joint

faces in basic igneous rocks. Pectolite may have a

close genetic association with prehnite and is

produced at the last stage of the cooling history of

basic igneous rocks (Deer et al., 1978). Pectolite

also occurs in serpentinites (e.g. Harada, 1934;

Coleman, 1961). The temperature conditions of

jadeitite formation might be also constrained by

the serpentinite hosts. Serpentinite hosts in the

study area are antigorite + talc, antigorite + talc +

tremolite, meta-olivine + tremolite + talc and

antigorite + talc + carbonate rocks (Yokoyama,

1985). These rocks are stable at T 4500ëC in the

system CaOÿMgOÿSiO2ÿH2O (Spear, 1993)

(Fig. 7). Studies of ¯uid inclusions and oxygen

isotopes in other jadeitites generally yielded

temperatures of ~300ÿ400ëC (Johnson and

Harlow, 1999; Sorensen et al., 2003).

The pressure conditions recorded in the sample

studied are constrained by the reaction: celsian +

TABLE 2. Representative chemical compositions (wt.%) of

barian feldspars.

Ba-poor Ba-rich

SiO2 61.19 58.70 58.26 42.85 43.52

TiO2 <0.06 <0.06 <0.06 0.07 <0.06

Al2O3 18.76 19.50 19.73 24.58 24.21

FeO <0.1 <0.1 <0.1 <0.1 <0.1

MgO <0.04 <0.04 <0.04 <0.04 <0.04

CaO <0.03 0.11 0.05 0.08 0.05

BaO 3.80 6.57 7.50 25.10 26.50

Na2O 0.14 0.17 0.20 0.14 0.15

K2O 14.88 13.66 13.19 5.80 5.50

Total 98.78 98.71 98.92 98.62 99.94

Number of cations on the basis of 8 oxygens

Si 2.937 2.873 2.859 2.397 2.419

Ti 0.003

Al 1.061 1.125 1.141 1.621 1.586

Fe

Mg

Ca 0.006 0.003 0.005 0.003

Ba 0.072 0.126 0.144 0.550 0.577

Na 0.013 0.016 0.019 0.015 0.016

K 0.911 0.853 0.826 0.414 0.390

Total 4.993 4.999 4.993 5.004 4.991

An 0.0 0.6 0.3 0.5 0.3

Cn 7.2 12.6 14.5 55.9 58.5

Ab 1.3 1.6 1.9 1.5 1.6

Or 91.5 85.2 83.3 42.1 39.5

BARIAN FELDSPARS IN JADEITITE

45

H2O = cymrite, This reaction occurs at a pressure

of ~0.5 GPa at a temperature of 350ëC (Graham et

al., 1992) (Fig. 7). Experimental data from Seki

and Kennedy (1964) suggest that K would shift

the cymrite-celsian equilibrium to higher pres-

sures. Other pressure constraints are de®ned by

jadeite-analcime relationships. Although analcime

is closely associated with jadeitite in the lavender-

coloured area of the sample, the lack of analcime

and quartz in the barian feldspar-bearing area

constrain the pressure below the jadeite + quartz =

albite reaction and above jadeite + H2O =

analcime, i.e. a pressure of 0.7ÿ1.0 GPa at a

temperature of 350ëC (Fig. 7). The latter reaction,

however, would be shifted to lower pressures if

SiO2 activity is <1 (Harlow, 1994). Low SiO2

activity is a reasonable condition for quartz-free

jadeitite. It is concluded that the barian feldspar

was formed at a pressure of ~0.6 GPa and a

temperature of <350ëC.

Some considerations on the petrogenesis of barian

feldspar

In the sample studied, euhedral jadeite was found,

in a number of instances, in the prehnite matrix,

from barian feldspar-bearing regions. BarianFIG. 5. A ternary plot of barian feldspars in terms of the

molar fractionation of Ab (albite), Or (orthoclase) and

Cn (celsian). (a) This study, and mica-albite rocks which

occurred as tectonic blocks in serpentinite-matrix

meÂlanges from the Motagua Valley, Guatemala (Har-

low, 1994). (b) Green mica schists from the Hemlo area,

Ontario (Pan and Fleet, 1991). (c) Zn-Pb-Cu deposit

from Shaba, ZaõÈre (Chabu and BouleÁgue, 1992). Broken

lines in b and c show the compositional ranges of barian

feldspars from the literature. Arrows in b indicate the

compositional change from core to margin.

FIG. 6. Correlation between Ba and Al in barian

feldspars.

46

T. MORISHITA

FIG. 7. P/T diagrams for reactions related to barian feldspar-bearing jadeitite petrogenesis compiled by Harlow and

Sorensen (2005). The ®lled star represents the P/T conditions for the formation of barian-feldspar in the rock studied.

The petrogenetic grids for metabasites are from Peacock (1993), at pressures to ~2 GPa, and Katayama et al. (2001)

at higher pressures. The reaction for the formation of jadeitite is from Harlow (1994). Broken lines represent

reactions for antigorite-bearing mineral assemblages in the system CaOÿMgOÿSiO2ÿH2O (Spear, 1993). The P/T

path of the Renge eclogite (Tusjimori, 2002) is also shown. PP = prehnite-pumpellyite facies, PA = pumpellyite-

actinolite facies; GS = greenschist facies; EA = epidote amphibolite facies; AM = amphibolite facies, LBS =

lawsonite blueschist facies; EBS = epidote blueschist facies; Ab = albite; Anal = analcime, Atg = antigorite; Brc =

brucite; Coe = coesite; Fo = forsterite; Jd = jadeite; Ne = nepheline; Qtz = quartz; Tlc = talc; W = H2O

BARIAN FELDSPARS IN JADEITITE

47

feldspars occur not as late-stage veins but as

interstitial phases between jadeite crystal and

prehnite. These textural characteristics support the

suggestion that the barian feldspar-bearing area in

the studied jadeitite, at least, was crystallized

directly from aqueous ¯uids and that barian

feldspars were formed locally from residual

¯uids rather than through local introduction of a

Ba-bearing metasomatizing agent after the forma-

tion of jadeitite. The occurrence of Ba-poor

feldspar, such as that occurring near the Ba-rich

feldspar and cutting the jadeite grain, might

suggest that the Ba-poor feldspar was formed

later than the Ba-rich feldspar.

Based on a cathodoluminescence study of

jadeitites from all major localities including the

study area, Harlow and Sorensen (2005) found no

relics of protolith replacement textures and found

cryptically to rhythmically zoned jadeites.

Rhythmically zoned jadeites in the Motagua

Fault Zone, Guatemala, are characterized by

decreasing jadeite content as they grow (Harlow,

1994). Harlow and Sorensen (2001, 2005)

suggested that cryptically to rhythmically zoned

jadeites might result from direct crystallization

from an aqueous ¯uid. It is interesting to note that

the jadeites closely associated with barian

feldspars have a diopside component-rich rind

(Fig. 4). This supports the idea that the jadeitite

studied here was also crystallized directly from

aqueous ¯uids and that the jedetites associated

with barian feldspar in the pale green to blue area

were formed later than other parts (lavender-

coloured and white-coloured areas). Miyajima et

al. (1999, 2001, 2002) suggested that at least

some jadeitites in the Itoigawa-Ohmi district were

formed from metasomatizing ¯uids because some

jadeites have euhedral shapes within interstitial

phases.

Green and Adam (2003) examined the trace

element characteristics of aqueous ¯uid from

subducted dehydrated ma®c oceanic crust at P =

3.5 GPa and T = 650ëC and 700ëC. They

suggested that the LILE such as Ba, Sr, Cs and

Rb are incompatible with clinopyroxene (ompha-

cite in their experiments). The ionic radius of

these elements would be too large to be accepted

in the six-fold coordinated M2 site in jadeite

(Miyajima et al., 2002). Barium as well as Sr

concentrations in the residual ¯uids are, therefore,

expected to increase during the crystallization of

jadeite from the primary aqueous ¯uids. This is

consistent with the occurrence of other Ba-rich

minerals coexisting with Sr-rich minerals in

jadeitites and albitites/metasomatized rocks from

the study area (Chihara et al., 1974; Sakai and

Akai, 1994; Miyajima et al., 2003). Miyajima et

al. (1999, 2001, 2002) interpreted itoigawaite as a

crystallization product from Sr-rich ¯uids during

the later stages of high-P/T metamorphism and

that rengeite and matsubaraite were formed by

interaction between pre-existing Ti- and/or Zr-

bearing minerals and Sr-rich metasomatizing

¯uids at the same time or at a later stage of

high-P/T metamorphism. They suggested that Sr-

rich metasomatizing ¯uids were formed as

residual ¯uids after the formation of jadeite.

It is noteworthy that Ba-rich minerals occur in

other jadeitites and metamorphic rocks from high-

P/T metamorphic belts as suggested by Harlow

(1995). Ernst (1963) reported Ba-bearing musco-

vite in glaucophane schists from the Sambagawa

high-P/T type metamorphic belt, Japan. Coleman

(1967) reported local Ba enrichment in metaso-

matized rocks included in serpentinite from

Oregon, California and Washington. Wise

(1982) reported strontiojoaquinite and bario-

orthojoaquinite in a metamorphosed basalt,

which is included as a tectonic block in a

serpentinite body from San Benito County,

California. Kobayashi et al. (1987) reported Ba-

rich patches or veinlets in a stronalsite grain in a

jadeitite from the other tectonic blocks in the

Renge metamorphic belt (Oosa-cho, Okayama).

Harlow (1995) reported barian micas in the rims

of mica grains coexisting with celsian or

hyalophane in jadeitites and albitite/metasoma-

tized rocks from a serpentinite-matrix meÂlange

along the Motagua Valley, Guatemala. He

suggested that an increase of the Ba concentration

in the ¯uid caused the Ba-enrichment towards

mica rims. Harlow (1994) also reported an

enrichment of Sr in the rims of zoisite in jadeitites

from the same locality. These facts, coupled with

this study, suggest that Ba and Sr are important

minor constituents in metasomatizing ¯uids

related to the formation of jadeitites/metasoma-

tized rocks in serpentinite-matrix meÂlanges.

The source of Ba in aqueous ¯uids responsible

for the formation of jadeitites is still unclear.

Harlow (1995) suggested that the destabilization

of baryte was caused by a decrease in fO2and by a

sulphate reduction in the serpentinite meÂlange,

because baryte-bearing metasedimentary rocks

were found in serpentinite-matrix meÂlanges

containing jadeitites/metasomatized rocks.

Furthermore, he noticed that Ba enrichments are

common among marine sediments (Vine and

48

T. MORISHITA

Tourtelot, 1970). Tsujimori (2002) suggested that

blueschist-to-eclogite metamorphism recorded in

eclogites in serpentinite-matrix meÂlanges from the

study area (Fig. 7) was caused by subduction of

oceanic crusts. Ultrama®c rocks in the study area

have spinels high in Cr# (= Cr/(Cr + Al) atomic

ratio), 0.7ÿ0.8 (Yamane et al., 1988; Tsujimori,

2004), indicating that they were probably derived

from a surpra-subduction zone, especially of fore-

arc setting (e.g. Arai, 1994). Tsujimori (2004)

investigated geochemical zoning of chromian

spinel and revealed that serpentinites in the

study area derived from the mantle wedge

following metamorphism at low- to mid-T

conditions of probable eclogite or amphibolite

facies. Harlow and Sorensen (2001, 2005)

suggested the possibility that if ¯uids formed by

dehydration due to the blueschist-to-eclogite

transition intruded into ultrama®c rocks, such

¯uids could produce jadeitite because the SiO2

activity was insuf®cient to crystallize albite. It is

probable that Ba-bearing ¯uids forming the

jadeitite studied here were caused by dehydration

of Ba-rich sediments at the early stages of

subduction of oceanic crusts and could be

related to the serpentinization of the mantle

wedge.

Acknowledgements

The author is grateful to H. Miyajima and K.

Takenouchi for their kindness and for information

about the study area, and to T. Tsujimori for his

various comments on jadeitites and related

minerals. K. Tazaki is thanked for EMPA

analyses at Kanazawa University. The construc-

tive reviews by G.E. Harlow and an anonymous

reviewer improved the manuscript. The author is

also grateful to G.E. Harlow for providing a

preprint of the paper by Harlow and Sorensen

(2005). The sample studied was collected during a

®eld course (2003) run by Kanazawa University

and was kindly donated to the author by Yuya

Iwata at that time. Sincere thanks are extended to

all staff and students who participated in the

course.

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[Manuscript received 15 February 2004:

revised 18 January 2005]

BARIAN FELDSPARS IN JADEITITE

51