N2 and CO2 in deep crustal fluids: evidence from the Caledonides of Norway

Chemical Geology, 108 ( 1993 ) 113-132 Elsevier Science Publishers B.V., Amsterdam

N 2

and

CO 2

113

in deep crustal fluids: evidence from the Caledonides of Norway

T o m Andersen a, H h k o n A u s t r h e i m a, Ernst A.J. Burke b a n d S y n n o v e Elvevold c a Mineralogisk-geologisk Museum, Sars gate 1, N-0562 Oslo, Norway b Faculteit Aardwetenschappen, Vrije Universiteit, De Boelelaan 1085, NL-1081 HVAmsterdam, Netherlands c Instituttfor Biologi og Geologi, Universitetet i Tromso, N-9037 Tromso, Norway

(Received 1 February 1993; revised and accepted April 8, 1993 )

ABSTRACT High-pressure metamorphic rocks (eclogites and high-pressure granulites) occur along the entire length of the Norwegian Caledonides, and have formed from a variety of protoliths. In some cases, the relationship between protoliths, highpressure rocks and their later retrogression products have been preserved in-situ. Fluid-inc]usion data suggest a simple correlation between metamorphic grade and metamorphic fluid composition: ( 1 ) Eelogites and high-pressure granulites contain Nz-bearing fluids (pure N2, or mixtures with CO2 or H20, with XN2 > 5%). In some eelogite-facies rocks, CO2-N2 inclusions are associated with aqueous brine inclusions (ca. 30 wt% NaCI ), the two compositions representing immiscible fluids at peak metamorphic conditions. (2) Granulite-facies protoliths and eclogites reworked in the granulite-facies contain pure CO2 or CO2-dominated fluids with less than 2.5% Nz. (3) Rocks retrograded in the amphibolite facies contain H20-NaC1 fluids. Immiscibility between brine and anhydrous N2-CO 2 fluid, and between anhydrous fluid and waterbearing aluminosilicate-melt have taken place in some eclogites. During high-grade metamorphism, nitrogen may be incorporated in minerals, as NH4 substituting for K, or it may occur as N2 in the free fluid phase. The partitioning of nitrogen between minerals and fluids depends upon the water activity and oxygen fugacity during metamorphism, low aH2o and/ or high fo2 partitioning nitrogen to the fluid phase. A rock interacting with a carbonic fluid at granulite-facies PTconditions will be depleted in mineralogically bound nitrogen. In cases where the protoliths of high-pressure rocks have been through a previous, granulite-facies event, a local source for the nitrogen contained in high-pressure fl aid is therefore unlikely.

1. Introduction It is becoming increasingly recognized that fluids play important and multiple roles in metamorphism. Not only will the composition of the metamorphic fluid phase control the position of hydration- and carbonation reactions in P--T--Xfluid space, but the presence or absence of a fluid also controls metamorphic reaction rates. Since metamorphism induces dramatic changes in density and rheology of rocks, the fluid phase affects the dynamics of an orogeny. It is therefore highly important to obtain a better understanding of the fluid-rock interaction history of metamorphic rocks, in particular for rocks of the lower crust, 0009-2541/93/$06,00

High-pressure metamorphic rocks (eclogites, high-pressure granulites) form in the deep continental crust during continent-continent collision and other orogenic processes leading to crustal thickening. Whereas granulites are "dry" rocks, reflecting low water activity during crystallization, eclogite-facies rocks may range from equally anhydrous garnet + omphacite rocks, to compositons rich in hydroxyl-bearing silicate minerals (phengite, amphibole, (clino)zoisite), indicating that a wide range of water fugacities are possible during eclogite-facies metamorphism (e.g. Holland, 1979; Jamtveit et al., 1990). Information on composition and physical properties of the fluid phase (s) involved in

© 1993 Elsevier Science Publishers B.V. All rights reserved.

1 14

high-pressure m e t a m o r p h i s m are in principle obtained by t h e r m o d y n a m i c a l modelling from mineral data, and by the study of fluid inclusions in m e t a m o r p h i c minerals. Ideally, both approaches should be c o m b i n e d in any study. T h e r m o d y n a m i c a l analysis of mineral assemblages will constrain the activities of volatile c o m p o n e n t s in a m e t a m o r p h i c system. Unfortunately, reliable equations of state for immiscible fluids at high pressures and temperatures are lacking, and fluid-induced partial melting processes (e.g. Peterson and Newton, 1989; Andersen et al., 199 l b ) are difficult to model quantitatively. In m a n y cases, these effects make it difficult to translate volatile activities calculated from mineral reactions in the eclogite-facies into realistic fluid compositions (compare Jamtveit et al., 1990 and Andersen et al., 1991b ). On the other hand, the P T ranges of eclogites and high-pressure granulites are not ideally suited for fluid inclusion studies, as decompression during cooling and uplift may lead to internal overpressures in fluid inclusions, with leakage, decrepitation and remobilization of fluid as the consequence (Touret, 1992 ). Nevertheless, by selecting inclusions for analysis only after detailed microscopic examination, and using careful routines of data analysis to ensure the representative nature of the inclusions (Touret, 1987), it has been possible to obtain meaningful fluid inclusion data from several high-pressure granulites (e.g. Coolen, 1982; Elvevold and Andersen, 1993) and eclogites (Andersen et al., 1989, 1990, 199 lb; Philippot and Selverstone, 1991; Klemd et al., 1993). To understand the relationships between fluid composition and m e t a m o r p h i s m , and the way a fluid phase may bring about mineral reactions, it is necessary to study not only eclogites and high-pressure granulites, but also their protoliths and retrograded counterparts. In the present paper, we aim to give an overview of the fluid regime in the deep crust of the Caldonides of Norway, as it can be derived from fluid-inclusion data from eclogites and high-

T. ANDERSENET AL.

pressure granulites, their protoliths and their alteration products, and from data on the nitrogen content in minerals and whole rocks.

2. The regional setting

In the Caledonides of Norway, high-pressure rocks occur along the entire length of the m o u n t a i n chain, from Rogaland in the South, to Spitsbergen in the North (Fig. 1). Except for the ophiolites of the U p p e r Allochtonous, they are found in all tectonostratigraphic units of the Scandinavian Caledonides (Gee et al., 1985 ). Our study includes rocks from the parautochtonous basement of the Western Gneiss Region (WGR; the "type area" of the eclogite facies, Eskola, 1921), the Middle Allochtonous (Bergen Arcs and Kalak Nappe complex), as well as the Hekla Hoek complex of Spitsbergen. The high-pressure processes have affected rocks of different compositions, and their metamorphic mineralogy varies accordingly.

Spilsbergen~j,,!: ~q . ?

I I

~F /":' / " / / /

N

|

Oksfjord

, .. ";'('~ ' - ,, ~ .,

, /' ':;£~-(~ " ' l [

.....: ,..~i' #?~.~:~:,. -:~~ ..... .~y t - ' •:~,; I ,~. , ,~ o/;~ir'?l., :,.,., i .... ,:"~ :.

~,~d'~;®~, '"' !~ --~,....... C~x i ' :zz : >, ~'~'~ ~)~'~: .';~:~:~,~-~.-i}~-.';" .~,..... ~:,~<~,i~:__s. Bo~g~,,A~osv&z-~__~?}#,~.:." /~7~ r~" ~ ?:? R°galand ~ ! ~ i \ i' ~ Fig. 1. Sketch map of the Caledonian nappe-sequence of mainland Scandinavia and Spitsbergen (shaded), with high-pressure rock occurrences studied. (*) Granulite-facies rocks. (*) Eclogite-facies rocks.

N~ AND CO2 IN DEEP CRUSTAL FLUIDS: EVIDENCE FROM THE CALEDONIDES OF NORWAY

3. Fluid inclusions and metamorphic fluid compositions

I 15

from Calanda, Switzerland, whereas 7", measurements of CO2 are reproducible to within + 1.0°C, and low-temperature measurements for N2-rich inclusions ( < - 1 4 7 ° C ) to within _+2 ° C. A Dilor Microdil-28 multichannel laser Raman microprobe (Burke and Lustenhouwer, 1987), at the Free University, Amsterdam, The Netherlands was used tbr quantitative Raman microspectroscopyofselectedfluidinclusions. Isochores for selected fluid inclusions were calculated from microthermometry data and Raman microprobe analyses, using the modified Redlich-Kwong equation of Holloway (1977, 1981 ).

Fluid-inclusion data have been published from several eclogites and high-pressure granulites (Table 1, Fig. 1 ). In the following section, an overview of existing data is presented, together with new data from selected occurrences (see Table 1 ). In Tables 2 and 3, we present new data on fluids in retrograded eclogites

3.1. Methods Microthermometric measurements were made in the laboratories of the MineralogicalGeological Museum, Oslo, Norway, and the Free University, Amsterdam, using Chaixmeca and Linkam microthermometry stages. The routines of calibration used were described by Elvevold and Andersen ( 1993 ). The reproducibility of CO2 melting point measurements is estimated at _+0.1 ° C, from repeated measurements of pure CO: inclusions in quartz

3.2. The protoliths The relationships between high-pressure rocks and their protoliths can be observed only in areas where the high-pressure metamorphic processes have not been fully penetrative. On Holsnoy Island in the Bergen Arcs nappe complex, W. Norway, 900-1000 Ma old, dry gran-

TABLE 1 Fluid composition in high-pressure metamorphic rocks from the Caledonides of Norway Area Rock type Rogaland Meta-eclogite Bergen Arcs Granulite protoliths Eclogite shear zones Amphibolite shear zones Western Gneiss Region Eclogite Reworked eclogite Oksfjord M 1 High-Tgranulite M2 M e d i u m - P granulite M3 High-Pgranulite Spitsbergen Blueschist eclogites Late re-equilibration

P kbar

Fluid composition

References

9

H20, N2

10, 12

8 15-19 >4

800 700 600 +_75

CO2 N2-CO2, N2 H20-NaC1

2, 5 2, 4, 5 3, 6

15-18 -%<10

550-850 ~ 750

H20-N2, N2

CO2, CO2-]NI2

1, 8 1, 9, 12

?

T °C

4-5 5-6 7-8

750-1000 700-750 650

CO2 CO2 COs-N2 (30% N2)

7 7 7

18-24 <4

580-640 300-400

HaO-N2 (?) H20

11, 12 11, 12

References: 1: Andersen et al. (1989), 2: Andersen et al. (1990), 3: Andersen et al. (1991a), 4: Andersen et al. (1991b), 5: Austrheim and Griffin (1985), 6: Austrheim and Robins ( 1981 ), 7: Elvevold et al. (1993) and Elvevold and Andersen (1993), 8: Griffin et al. ( 1985 ), 9: Krogh (1980), 10: Maijer et al. ( 1987 ), 11: Hirajima et al. ( 1988 ), 12: This work.

116

T. ANDERSEN ET AL.

TABLE 2

Microthermometry and micro-Raman analysis of fluid inclusions from reworked eclogites, Western Gneiss Region Sample Incl.

Type

Microthermometry 7hs

~[YS (Tyssedal, Sunnl]ord) A-I H3 A-10 H3 A-21 H3 A-22 H3 A-24 H3? A-26 H47 A-28 H4 -153 A-X A-30 H3 A-32 H3 A-33 H3 A-42 H3 B-4 H3 B-5 H3 B-10 H B-15 H3 B-17 H3 C-I H3 (?-7 H3 C-13 H3 C-14 H3 76008 ( Naustdal ) 1 H3 2 H3 3 H3 RH-I ( Romsdalshorn ) A-6 H3 K-I 1/5 (Kristiansund) D-41 H3 D-42 H3

Ti

Raman Tm

-59.5

-59.0

-58.5

-57.9

-61.3

-60.8

Xcoz

7~

Molar volume (cm3/mole )

(mole%)

XN2 (mole% t

15 9 8 6 2t 43 45 4 ----2 2 3 -3 4 3 4 9

49 53 43 50 45 43 43

-10.0 +1.9 -23.3 -5.9 -26.4 -48.1 -45.5

L L L L L L L

- 15.2 +8.6 -15.8 -14.0 -10.9 -31.0 -23.2 -8.3 -11.0 -35.0 -30.4 -0.1 + 19.0

L L L L L L L L L L L L L

85 91 92 94 79 57 55 96 + + + + 98 98 97 + 97 96 97 96 91

43.6 50.5 43.5 43.8 45 41 42 45.2 45 40 41 47 70

-57.1

-56.9 -56.9

-57.3

-56.6

-16.0 -26.0 -29.0

L L I

+ + +

----

43.4 41.6 41.1

-58.5

-57.5

-2.0

L

+

--

46.8

+7.1 -4.1

L L

+ +

--

49.9 46.2

All inclusions are hosted in quartz, except for those of sample K-11/5, which are hosted in plagioclase. Microthermometric types after Van den Kerkhof ( 1988 ), molar volume properties of CO2 and CO2-N2 mixtures after Angus et al. ( 1972 ) and D a r i m o n t and Heyen ( 1988 ), respectively. + : Present in quantities above the laser-Raman microprobe detection limit (ca. 0.1 mole%) - : Not detected with the laser-Raman microprobe. All T m e a s u r e m e n t s have been corrected to Tm (CO2) = - 56.6 °C.

ulite-facies meta-igneous rocks (anorthosite, mangerite) were reworked in the eclogite-facies at 425 Ma (Cohen et al., 1988; Austrheim, 1990). The high-pressure metamorphism was restricted to shear zones and fluid pathways, leaving large volumes of metastable, Precambrian granulite-facies rocks (Austrheim and Griffin, 1985; Austrheim, 1987). Quartz veins formed during granulite-facies metamorphism contain CO2 inclusions with molar volumes in

the range 37 to 41 cm3/mole, yielding isochores matching the PT conditions estimated from mineral thermobarometry (Table 1, Fig. 2). This fluid phase differs from that associated with Caledonian eclogite- and amphibolite-facies shear zones in the same area (see below), in having lower N2-contents (0-2.5 mole %), and no detectable water (Andersen et al., 1990, 1991 a). On the Oksj~ordpeninsula in Finnmark, N.

1l7

N2 AND CO2 IN DEEP CRUSTAL FLUIDS: EVIDENCE FROM THE CALEDONIDES OF NORWAY

TABLE 3 Microlhermometry and micro-Raman analysis of fluid inclusions from blueschist eclogites, Spitsbergen Sample Incl.

B-I-F A- 13 83JP24-26 9 10 1I 17

Type

Microthermometry Th ( N2 )

Raman .}l\:o2

XN~

Peak position ( c m - ~)

Molar volume (cm 3/mole)

H1

- 156

L

-

+

2328.4

50.1

H1 H1 HI HI

- 173 - 178 -167 -177

L L L L

-

+ + + +

2327.7 2327.7 2327.7

40.7 39.0 43.1 39.3

Microthermometric types after Van den Kerkhof ( 1988 ), molar volumes after Angus et al. ( 1979 ). The Raman peak position of atmospheric N 2 during measurement was 2329.1 c m - ~.

a

b

Holsnoycomplex

N2

Water-saturated solidus 18

20 16

"

12 10

-

!:~:

oc~og~/----~:,~"~ : : Z- Z - - - - _- -_-~5o -

basalt

"

Jd30

....

~

~rg

.~4'

oc~

Arcs _ ~._J

. . . .

.=.:-

R- 6

Arnphibolite-facies shear z o n e s

,,~

CO ~

-

--~'~"7~__n~ : ;

Dksfjord complex ~

350

Granulite-facies

,

450

550

-

650

protolith HO

c

isochores 0

~o~

750

N i .~_

Post-M3fiuid

850

Temperature, °C F~.

Holsnoy PTt-path

: M3fluid

Elksljord PTt-path CO~.

HO M1-M2 fluid

Fig. 2. Fluid evolution of the Bergen Arcs and Oksl~ord complexes. (a): P T evolution. The P T t data for the Bergen Arcs are taken from Austrheim and Robins ( 1981 ), Austrheim and Griffin ( 1985 ) and Andersen et al. (1991a), eclogite and granulite isochores from Andersen et al. (1990). The P T t path for Oksl~ord follows Elvevold et al. ( 1993 ), with isochores from Elvevold and Andersen (1993). The basalt/eclogite solidus is taken from Lambert and Wyllie (1974), and the phase diagram for AI2SiO5 from Holdaway ( 1971 ). (b): Compositional evolution of fluids in the Bergen Arcs, projected to the C O 2 - H 2 0 - N 2 triangle. The fluid compositions are tied to their respective P T c o n d i t i o n s by broken lines. Data are taken from Andersen et al. (1990, 1991a, 1991b). (c): Fluid evolution in the Okst)ord granulites, data from Elvevold and Andersen ( 1993 ). M 1, M2 and M3 relate to metamorphic stages along the PTI path of the Oksfjord complex in (a).

Norway, an event of high-pressure granulite-

were restricted to shear zones (Elvevold et al.,

facies metamorphism affected medium pressure-granulites of probable Precambrian age (Table 1 ); again, the high-pressure processes

1993; Elvevold and Andersen, 1993 ). The fluid evolution before high-pressure metamorphism can be followed through two medium-pressure

1 18

events ( denoted M l and M2 in Fig. 2 ) b y f l u i d inclusions and thermodynamical fluid modelling. M 1 represents an episode of contact metamorphism induced by emplacement of mafic intrusions (Table 1 ), M2 is a major metamorphic event, forming the regional foliation in the area (Elvevold et al., 1993). In this case the modelling and fluid-inclusion approaches agree, suggesting the presence of a CO:-dominated fluid with less than 2-3% water. The agreement between M 1 and M2 PT conditions derived from mineral thermobarometry, and fluid-inclusion trapping pressures derived from isochores (Fig. 2 ) is very good (Elvevold and Andersen, 1993). In he Bergen Arcs granulites (Andersen et al., 1990) and in the Okst]ord M 1 and M2 assemblages (Elvevold and Andersen, 1993), preCaledonian fluid inclusions have been preserved through an overprinting Caledonian high-pressure metamorphism. This indicates that fluid inclusions may, under favourable circumstances, survive quite large shifts in external pressure and temperature away from their isochore, without loosing their molar volume characteristics. Active recrystallization of the host phase may be needed to equilibrate the molar volume of fluid inclusions to higher P lower T external conditions,

3.3. The high-pressurefluids The eclogite-facies shear zones in the Bergen Arcs were formed at ca. 700 ° C, at pressures between 15 and 19 kbar (Austrheim and Griffin, 1985; Van Vyck et al., 1990; Andersen et al., 1991 b; Boundy et al., 1992 ). Depending on the protolith composition, the shear zones have developed different high-pressure mineral assemblages with quartz + phengite + (clino)zoisite + kyanite + amphibole _+ omphacite + garnet. Minor felsic veins containing quartz + omphacite, quartz + phengite or quartz + albite + clinozoisite + phengite occur in the central low-strain parts of the shear zones. The veins are in contact with the eclo-

T. ANDERSEN ET AL.

gite-facies mineral assemblages along their contacts, omphacite and other high-pressure phases are commonly seen to project into the veins. The degree of deformation of the veins varies. Locally, they are completely undeformed, with well-preserved pegmatitic igneous textures, but most commonly they are strongly sheared, together with their surrounding eclogite (Austrheim, 1987; Andersen et al., 1991 b). The veins may thus be considered as minor syn-tectonic intrusions, emplaced during formation of the high-pressure shear zones (Austrheim, 1987 ). The, mineral assemblage quartz + albite + phengite + clinozoisite is stable in peraluminous granitic systems to ca. 19 kbar (Huang and Wyllie, 1986), setting a m a x i m u m pressure limit for the eclogite-facies metamorphism. Based on thermodynamic modelling from mineral analyses, JamWeit et al. (1990) suggested the presence of a water-dominated fluid phase during the eclogite-facies event. However, any hydrous fluid phase injected into a melt-lubricated shear zone at above-solidus pressure and temperature, will dissolve in the aluminosilicate melt. Also, water would be selectively absorbed by amphibole-forming reactions, and would cause partial melting in mafic- to intermediate lithologies (Andersen et al., 1991b). Fluid inclusion textures in undisturbed quartz in granitic veins suggest the presence of two immiscible fluid phases: a brine with ca. 30 wt% dissolved salts, but without detectable Raman-active gas species in the vapour bubbles, and an anhydrous carbonic fluid. In most samples, however, there is no visible water in any fluid inclusions, nor has clathrate formation been observed during microthermometry, suggesting that water was indeed selectively removed from the fluid phase. The residual carbonic fluid phase in the shear zones consists of CO2 and N 2 , with XN2 varying between 5 and 80% in the least disturbed fluid-inclusion assemblages (Fig. 2b), and up to 100% in later secondary inclusions (Andersen et al., 1990, 1991b). Although the car-

N2 A N D CO2 IN D E E P C R U S T A L F L U I D S : E V I D E N C E F R O M T H E C A L E D O N I D E S

a

OF NORWAY

l 19

c

7O

Retrogression of eclogites,

60 -

Western

50 • 40 -

i

20 -

il

,

o

!

,

~

[[-

c'~

-~

-50

-40

-30

-20

-10

0

10

20

30

Homogenization temperature, °C

50 o -5

45 40

.

14

::_

~

8

Ii

13_

6 4

~ 4

350

E 35 25

o~ 20 15

.J-

--"

I

Jd30

--

Jd20

"4, cr Jr o,e, ~

~ ,'

~ o~e :

2 ,o

-~ 30

O Z

JdhO

18 16

lo

........

b

Region

20

i ,,

30

Gneiss

•,o .

.

.

~

. . . . . . 7,

~h~ .

i

---____ ]~

550 750 Temperature, °(.3

o ':'

5 -50

3 s .~, o

£

-30

-10

c

: 10

Homogenization temperature,

30 °C

Fig. 3. Fluid evolution of eclogites in the Western Gneiss RegJ.on. (a): Homogemzation temperatures of CO2 inclusions in quartz, garnet and retrograde plagioclase. Data from Andersen et a]. (1989) and the present study. (b): Homogenization temperature vs. composition for post-eclogJte fluid inclusions from WGR cclogites. Data from Andersen ¢t al. ( 1989 ) and the present study (Table 2 ). (c): The cooling and uplift history of ¢c]ogites from the WGR, simplified from Griffin et al. (1985). The higher-temperature PTt path could correspond to eclogites from the Kristiansund and Hjorugav&g areas, the lower-temperature path to eclogites further cast (inland) or south (Krogh, 1977). The shaded isochore field represents pure CO2 inclusions, the heavy line a 45 mole % N2 inclusion, formed by local mi×ing of N2-rich fluid from pre-existing eclogite-facics fluid inclusions, and pure CO2, introduced during retrogression.

bonic fluid inclusions have considerable densities (molar volumes down to ca. 35 cm3/mole or less; Andersen et al., 1991b), the isochores fall 2-5 kbar short of the estimated PT conditions for the eclogite-facies metamorphism (Andersen et al., 1990). The reason for this may be some stretching of inclusions caused by internal overpressure during cooling and uplift, which leads to increased molar volumes (e.g. Touret, 1992). The presence of coeval brine and carbonic fluid inclusions in close association within single quartz grains, suggests that this reequilibration has not disturbed the actual fluid compositons. In the OksJ~ordcomplex, high-pressure granulite-facies (M3) mineral assemblages (Qtz + Plag + Grt + Ky + Bt + Kfs) have formed at 7-8 kbar and ca. 650°C, along shear zones in quartzofeldspatic gneisses, overprinting the

M 1 and M2 assemblages (Elvevold et al., 1993 ). The fluid composition changed from pure CO2 in the M1 and M2 event, to a CO2N2 mixture with XN2 = 25-30% (Fig. 2 ). Like the M 1 and M2 fluids, the inclusions trapped in the M3 event yield isochores (Fig. 3) with excellent correspondence with the metamorphic conditions derived from mineral thermobarometry (Elvevold and Andersen, 1993). Comparatively few and scattered data are available from the classical eclogites of the WesternGneiss Region. Fluid-inclusion data from the Kristiansund area and from the Hjorungav&g opx-bearing eclogite (cf. Krogh, 1980; Carswell et al., 1985; Griffin et al., 1985 ) suggest that the earliest fluid inclusions, which can be found in quartz grains enclosed by garnet, contain a dense mixture of nitrogen and water (Andersen et al., 1989). Because the gas

] 20

phase of the inclusions cannot be heterogenized in a nitrogen-cooled microthermometry stage, the actual compositon and density of the fluid have not been accurately determined, but can be constrained to ca. 24 mole % Nz, at a molar volume of 20-30 cm3/mole. CO2 is present in minor amounts in early fluids in eclogites from Kristiansund, but has not been observed in corresponding fluids in the Hjorungav~g opx-eclogite (Andersen et al., 1989). In matrix quartz, the eclogite-facies fluid inclusions have been strongly disturbed, and locally mixed with late CO2 fluids (Andersen et al.~ 1989, and below),

3.4. Retrograde fluids Most eclogites have been re-equilibrated at lower pressures and temperatures; in many cases, the alteration processes have affected the eclogites only along their contacts, veins or lowgrade shear zones, i.e. where the high-pressure rock has been in contact with a fluid phase (e.g. Maijer et al., 1987; Andersen et al., 1991a). A widespread feature of retrogression is the replacement of anhydrous high-pressure mineral assemblages (Grt, Cpx, Opx, Kfs) by hydrous minerals (Ms, Bt, Chl), suggesting the influence of aqueous fluids. In other cases, anhydrous eclogite-facies mineral assemblages are replaced by equally dry granulite-facies assemblages. During reequilibration of the highpressure rocks, new fluids have been introduced, whereas the fluids contained in early inclusions have been remobilized and locally mixed with post-eclogite fluids. This may give rise to complex inclusion textures and local intermediate fluid compositions (Andersen et al., 1989); such inclusion assemblages must be treated with extreme care, because usually they do not reflect fluid compositions introduced in one single process, In the Bergen Arcs, post-eclogite reworking can be related to distinct crosscutting amphibolite-facies shear zones formed at P > 4 kbar and T = 600_+ 75 °C (Austrheim, 1978; Aus-

T. ANDERSEN ET AL.

trheim and Robins, 1981; Andersen et al., 199 la). Like the previous high-pressure metamorphism in this complex, the amphibolite-facies processes are restricted to rock volumes in which fluid has been introduced. In these shear zones, aqueous brines were introduced, leading to hydration of dry granulite-and eclogitefacies mineral assemblages. In any shear zone, the composition of the brine changed with time, due to variations in fluid supply and to wall-rock ractions, with salt concentrations from 5 to 50 wt% NaC1. Locally, carbonic fluid inclusions were trapped, apparently as an iramiscible fluid. These inclusions have low nitrogen contents (XN2 < 2.5%), suggesting that the carbonic fluid was derived by remobilization of pre-existing inclusions of a carbonic granulite-facies fluid. In the Western Gneiss Region, the retrograde reaction of a dry eclogite-facies mineral assemblage (garnetI + omphacitic cpxI + quartz ) to form an equally dry granulite-facies assemblage (plagioclase + cpxII _+garnetII ) is of widespread occurrence (e.g. Krogh, 1980). This is a particularly interesting case, because the mineral reactions involved appear to be indifferent to the presence and composition of a free fluid phase. In the retrograded eclogites from several localities in the W G R (Table 1 ), fluid inclusions occur in the newly formed plagioclase, in clinopyroxene (cpxII) garnet and in quartz. The plagioclase contains both randomly scattered inclusions (Fig. 2c in Andersen et al., 1989) and inclusions bound to secondary healed fracture trails. The randomly scattered inclusions fulfill the textural criteria of primary inclusions (Roedder, 1984 ), and must have been trapped from a fluid which was present during the growth of the secondary plagioclase. The inclusions are 5 to 50 # m in size and commonly have euhedral negative crystal shapes, although more irregular "stretched" inclusions do occur. At room temperature, the inclusions contain a single carbonic fluid phase, as well as two or more solids, one of which is strongly bi-

N2 AND CO2 IN DEEP CRUSTAL FLUIDS: EVIDENCE FROM THE CALEDONIDES OF NORWAY

refringent, and has been identified as calcite from its Raman peak at 1085 cm-J. The secondary inclusions are on average smaller ( ~ 10 /~m), and more irregular in shape. They contain the same phase assemblage as the primary inclusions in plagioclase, The clinopyroxene (cpxlI) is most commonly devoid of fluid inclusions, or contains only relics after former fluid inclusions, whose contents has been consumed by reactions with the host mineral. In one sample (RH-II), darkgreen cpxII crystals contain scattered apparently primary multi-phase aqueous inclusions, At room temperature, these inclusions contain several, birefringent crystals in addition to liquid water and a Raman-inactive vapour bubble. The degree of fill is in the range 75-80 volume %. In garnet, fluid inclusions are only moderately abundant; when found, they always form secondary healed-fracture trails. The inclusions have complex vermicular shapes, which appear to be controlled by crystal faces of the garnet. They are intimately associated with solid inclusions of calcite, which in some cases occur withintheinclusioncavities, The highest abundance and most complex inclusion patterns occur in quartz. Most fluid inclusions in quartz are found in healed-fracture trails; isolated inclusions or swarms of randomly scattered inclusions are, however, not uncommon. The homogenization temperatures of CO2 inclusions in W G R eclogites form a simple, although somewhat skewed histogram (Fig. 3a). The corresponding isochores agree with the later medium-pressure granulite- to amphibolire-facies parts of the uplift curves of these eclogites (Fig. 3c). In one of our samples (TYS), some quartz grains contain randomly distributed fluid inclusions with large and random differences in nitrogen contents (Table 2, Fig. 3b). Since CO2 and N2 are fully miscible at the P and Toftrapping, this compositional variation must reflect imperfect local mixing of two fluid phases of

12 1

different origins. The N2-rich component was most probably derived from pre-existing eclogite-facies inclusions, whereas the CO2 may come from a N2-free fluid introduced during retrogression. The isochores of these inclusions (one example shown in Fig. 3c ) overlap with those of the N2-free carbonic fluids, indieating that mixing took place during the granulite-facies to upper amphibolite-facies retrogression. The ,existence of such compositional differences indicates that introduction of a fluid phase at these conditions does not necessarily lead to homogenization of fluid composition at a thin-section scale, or to the complete annihilation of the pre-existing fluid inclusion assemblage. The eclogites in Rogaland, SW Norway (Maijer et al., 1987) are the southernmost occurrences of Caledonian high-pressure rocks in Norway. The eclogites occur as strongly retrograded lenses within amphibolite-facies gneisses, and are crosscut by quartz veins. Garnet, omphacite and phengite of the eclogites have been replaced by amphibole, plagioclase and biotite, during low-pressure re-equilibration in the middle amphibolite facies (Maijer et al., 1987). The quartz veins contain abundant secondary fluid inclusions. From their occurrence and phase contents, two main groups of inclusions are recognized: Early secondary inclusions, containing water with or without small amounts of CO2 or N2. These inclusions form several generations of trails which do not crosscut grain boundaries. These trails and the grain boundaries are in turn crosscut by trails of late secondary inclusions, consisting mainly of nitrogen homogenizing to the vapour phase, and scattered water inclusions. More than one generation of such trails is probably present. None of these fluid inclusions relate to the high-pressure metamorphism, but must have been trapped or redistributed at a late stage of the re-equilibration history of the eclogites. The quartz veins may have formed during

122

T. A N D E R S E N ET A L

a

b

N2 inclusions in eclogites from Spitsbergen Homogenization to liquid

20 18

4

, '

,."

~ 16 ..o 1 4 ---~ ~ 12

-145 -150 -155 -160 -165 -170 -175 -180 C

• ~;~. ~ . ~

,

........

~o

n

6

" x'~e,'~°" "' f l "

-

" 2;a'.4flY

i ' 4'I ' ~

~

.,,.~- ~ "

-

2

Spitsbergen eclogites, water inclusions

~o 8

17 i :

e

170 180 190 200 210 220

Critical Na

350

" "

-

-

-

- .

::::: ~:

3

89.2 c m / m o q e

550

~

'e

....

.... .....

Rogaland 750

T e m p e r a t u r e , °C

:

150 160

~

~, ~ ~, ~, 2 ~ ~ ~'~.~t~

,'"

230

:C

Fig. 4. Fluid evolution of retrograded eclogites from Spitsbergen and Rogaland. (a): Distribution of fluid-inclusion homogenization temperatures for N 2 and H20 inclusions in Spitsbergen low- to medium-temperature eclogites. Data from lhe present study (Table 3 ). (b): PTt evolution of the Spitsbergen eclogites,data from Hirajima et al. ( 1988 ) (petrology) and the present study (isochores). The meta-eclogitefrom Garborg, Rogaland, has re-equilbrated pressures at or below the critical N2 isochore. the amphibolite-facies reworking of the eclogites, and the early secondary aqueous inclusions may relate to the fluid which caused hydration of the eclogite-facies mineralogy. However, a systematic study of fluid inclusions in the Rogaland eclogites remains to be done. The Now-to m e d i u m - t e m p e r a t u r e eclogites on Spitsbergen have also gone through considerable retrogression during uplift. The conditions at the peak of m e t a m o r p h i s m were P = 17-24 kbar and T = 575-645°C, but a distinct retrograde event has been recognized from mineralogical data and from A r - A r systematics of phengite, at P < 5 kbar and T = 3 1 5 400°C (Hirajima et al., 1988). This may be related to the influx of externally derived fluids into the eclogites. Quartz-glaucophane veins and matrix quartz in the eclogites contain randomly scattered and intermixed fluid inclusions of nitrogen and water, appearing to have been trapped simultaneously. Most nitrogen

inclusions homogenize to the liquid, at temperatures from the m i n i m u m temperature reached with the m i c r o t h e r m o m e t r y stage (ca. - 180 ° C) up to the critical point (Fig. 4a, Table 3). Some inclusions could not be heterogenized, indicating that nitrogen inclusions of even higher density are present in the samples. The associated water inclusions have final melting points in the range 0 to - 10 ° C, suggesting moderate salt contents, and show a homogenization temperature frequency maxim u m around + 180°C (Fig. 4a). Isochores calculated from these two populations of inclusions intersect at P ~ 4 kbar and T < 550 ° C (Fig. 4b), which agrees with the low-pressure re-equilibration stage along the uplift curve of Hirajima et al. ( 1988 ). 4. Nitrogen in the rocks Nitrogen may substitute as NH4+ ion in potassium-bearing minerals such as mica, alkali

123

N2 AND CO: IN DEEPCRUSTALFLUIDS: EVIDENCEFROM THE CALEDON1DESOF NORWAY

feldspar and, to a lesser extent, plagioclase (Baur and Wlotzka, 1974; Itahara and Honma, 1979; H o n m a and Itahara, 1981; Duit et al., 1986; Bos et al., 1988; Visser, 1992). To evaluate the role of in eclogite-facies processes,

N2

it is therefore also necessary to study the distri-

bution of nitrogen in the metamorphic rocks, and its behaviour in metamorphic mineral reactions. To do this, we have performed an analytical study on the rocks of the Bergen Arcs, determining the total nitrogen content of selected protolith-eclogite whole rock pairs and mineral separates, 4. l. Methods

Nitrogen contents of whole rocks have been determined with a Perkin-Elmer CHN analyser at the University of Copenhagen (courtesy of professor David Bridgwater); minerals were analysed with a LECO nitrogen analyser at the Norwegian Pulp and Paper Research Institute, Oslo (analyst Ms. Anne Daldorff). Replicate analyses suggest a relative uncertainty of 35% for the whole-rocks and 25% for the mineral separates.

4.2. Nitrogen concentrations Bulk nitrogen contents have been determined in 24 whole-rock samples and 9 mineral separates from granulites and eclogites. The bulk nitrogen concentration in the whole-rock samples is low, in the order of 1 to 15 ppm, which is within the range of crustal igneous rocks (Hall, 1988a,b). Because the concentrations are close to the detection limit of the method, these data should be regarded as semiquantitative. A histogram of whole-rock nitrogen concentrations is shown in Fig. 5a, illustrating overlap within the analytical uncertainty between the eclogites and their protoliths, The phengite separates from eclogites (Fig. 5b) range from 125 to 870 p p m nitrogen; local variations within one single pegmatitic vein

a. 7. 6~T ~ sT/ ~

-q

4~~

~ 31 2 ~

• []

Eclogites Granulites

~],

1~

0

~ |

2

! _J~ 4 6 8 10 12 14 16 ppmnitrogenin whole-rocks

b.

Nitrogen in mineral separates

Granulites K-feldspar 825

Eclogites ppm 250

Phen.qite MICA 1

ppm 370

Biotite

ppm

210

PEGA M2 7/88

870

BO 003

696

315

Amphibolite

PEG 2 AB 7 PEGA M1

HA

470 125 250 373

Fig. 5. ( a ) Nitrogen distribution in whole-rock samples from the Bergen Arcs. (b): Nitrogen contents in mineral

separatesfrom

theBergenArcs.

amount to nearly as much (250-870 ppm; PEGA M1 and M2). Two analyses of potassium feldspar from granulite-facies rocks fall within the lower part of'the concentration range of the eclogite facies phengites (250 and 315 p p m ) . One biotite separate from an amphibolite-facies shear zone in granulite (see Andersen et al., 1991a) shows comparatively low nitrogen content (210 p p m ) , but is still within the range of variation of the eclogite-facies phengites.

4.3. NH4 solubility in minerals Although the stability relationships of ammonium-bearing silicate minerals at high temperatures and pressures are too poorly known to allow quantification of the distribution of nitrogen between minerals and fluids, a qualitative to semi-quantitative assessment of the

124

T. ANDERSEN ET AL.

parameters controlling this process may still be made, using the eclogites of the H o l s n w cornplex in the Bergen Arcs as an example. Despite the elevated N2 content of the fluid phase, the eclogite-facies metamorphism has in this case not led to a detectable increase in nitrogen bound as ammonium in minerals, relative to the protolith (Fig. 5 ). In the granulite-facies protoliths, plagioclase, K-feldspar and biotite are the only likely host phases for nitrogen (Honma and Itahara, 1981 ); the change of host phase to phengite in the eclogites could be expected to cause an increased uptake of nitrogen, because micas generally show a higher solubility of NH4 than feldspars (Baur and Wlotzka, 1974; Honma and Itahara, 1981). Our whole-rock data indicate that this is not the case. The reason for this may be sought for among the parameters controlling NH4 solubility in potassium-bearing silicate minerals, which may include temperature, pressure and volatile activities.

4.3.1. Temperature and pressure The nitrogen content in eclogite-facies phengite ( 125-870 ppm) is more than an order of magnitude higher than the whole-rock contents, but at the same time significantly less than the maximum nitrogen contents reported from amphibolite-facies micas, formed at comparable temperatures. For example, biotites from upper amphibolite-facies rocks in the Bamble sector of South Norway, equilibrated at 700-800°C and 6-8 kbar still contain up to 3000 ppm ammonium (Visser, 1992). Ternperature is therefore unlikely to have been the decisive factor limiting the NH4 content of phengite in the Holsnoy eclogites, The NH4+ ion (6-coordinated radius: 1.61 /k) is larger than K ÷ ( 1.48/~) and Rb ÷ ( 1.52 A; Baur and Wlotzka, 1974; Shannon, 1976), suggesting that ammonium uptake in a potassic mineral may decrease with increasing pressure. It is, however, also unlikely that a roughly two-fold pressure difference (from ca. 8 kbar in Bamble to 15-19 kbar in the Holsnoy com-

plex), may alone reduce the solubility of ammonium in the mica structure by an order of magnitude.

4.3.2. Oxygen fugacity andfluid composition More likely, the combined effects of oxidation state and fluid composition upon the solubility of NH4-bearing components in solid solution in silicate minerals may be controlling the nitrogen distribution. The stability relationships of the pure ammonium analogues of muscovite and alkali feMspar have been studied experimentally (Schultz, 1973; Hallam and Eugster, 1976; Voncken et al., 1987; Bos et al., 1987; Bos, 1990; Voncken, 1990). The breakdown of ammonium-bearing components of muscovite and felspar is controlled by a series of mineral reactions, which are univariant in the Si-A1-NH4-O system (Hallam and Eugster, 1976), but polyvariant in S i - A 1 - K - N - O H. The only reactions which are of any relevance to the present case are: 4(NHaA12A1Si3O10(OH)2)Mu + 302 = 6 K y + 6 Q t z + 2 N 2 + 12H20

(1)

and 4 (NHaA1SiOs)vsp + 30:, = 2Ky + 10Qtz + 2N2 + 9H2 O

(2)

Since these reactions are polyvariant in natural metamorphic systems, they are not potential nitrogen fugacity buffers. However, at a given temperature, pressure and fN2, the reactions may control the solubility of ammonium in muscovite and feldspar in equilibrium with a fluid phase, as a function of oxygen and water fugacities. Figure 6 shows contours of NH4 substitution in feldspar and muscovite in equilibrum with a N2 enriched fluid phase, in coordinates of logau2o and relative logfo2. At anyfo: andfN2, there is a unique maximum amount of NH4

125

N2 AND CO2 IN DEEP CRUSTAL FLUIDS: EVIDENCE FROM THE CALEDONIDES OF NORWAY

log QH2o -2

-1

0

/

Gronulffefacies fluid 5

....

EcIogife? .......... ..................~ "

~f (XNHdm°:'O/ 100 9 ~

(XNH4rr'ax) / ]0

.fl -10

i j/

.,o,

[ Fig. 6. The petrology of nitrogen in the deep continental crust. The model is calculated from reactions ( 1 ) and (2) in the text, assuming that a hypothetical pre-granulite protolith (N-fertile crust) did contain the maximum possible amount of NH4, bound in mica and feldspar. The water activities of granulite-facies and eclogite-facies fluids represent the situation in the Bergen Arcs complex, where the granulites interacted with a water-poor, carbonic fluid, and the eclogites with complex immiscible fluids with higher water activity (Jamtveit et al., 1990; Andersen et al., 1991b). The solubility of ammonium in mica and alkali feldspar is shown as isopleths of fractions of the respective (unspecified) maximum concentrations. As can be seen, the capacity of e.g. phengite formed during an eclogite-facies event to take up nitrogen depends on the change in oxygen fugacity, in case (A), the eclogite mineralogy would be an effective sink for nitrogen, whereas in case (B) no nitrogen would be bound in ammonium components in minerals.

which can be incorporated into the minerals; this "NH4 saturation limit" is obtained only in a system in which aH20 approaches unity, i.e. in minerals coexisting with a fluid Consisting of nearly pure water or with immiscible waterand nitrogen-rich fluids. Reducing the water activity at constantfo2 with all other parameters remaining constant, e.g. by diluting the metamorphic fluid with CO2, will decrease the equilibrium NH4 content of feldspar and mica, and N2 will be liberated to the fluid phase, When a rock with mica and feldspar close to their respective NH4 saturation limits at a given oxygen fugacity level (i.e. "Nitrogen fertile crust" in Fig. 6 ) is exposed to a water-poor

fluid with logaH2o = - 2 at constant oxygen fugacity, the maximum NH4-uptake in feldspar and mica (if stable) will be reduced by more than an order of magnitude (Fig. 6). This indicates that introduction of a fluid phase with low water activity, e.g. during granulite-facies metamorphism, is an efficient way to release nitrogen from minerals, without changing the stable ammonium-free metamorphic mineral assemblage. The interaction of NH4-bearing minerals with a CO2-dominated fluid phase may account for some of the depletion in mineralogically bound nitrogen seen in some prograde granulite facies terranes (e.g. Duit et al., 1986 ).

126

5. Discussion

5.1. Fluid composition and metamorphic grade The fluid-inclusion data from the Caledonides indicate a remarkable correlation between metamorphic grade and fluid composition:

5.1. I. The granulites Granulite-facies rocks contain CO2-dominated fluid inclusions, without detectable free water. This is equally true for protoliths of the high-pressure rocks (i.e. Bergen Arcs and Oks0ord), and for eclogites re-equilibrated to plagioclase + clinopyroxene + garnet assemblages during granulite-facies reworking of eclogites. The Oksfjord case illustrates that the fluid phase in high-pressure granulites is also CO2-dominated, but with a substantial nitrogen content, The data from the Caledonides thus provide another piece of supporting evidence for the correlation between granulite-facies metamorphism and CO2-rich fluids, initially suggested by Touret (1971) in the Bamble sector of S. ~'4orway, and later reported from other granuhte terranes (e.g. Hollister and Burruss, 1976; Swanenberg, 1980; Coolen, 1982).

5.1.2. The high-pressure rocks The fluid phase in inclusions in high-pressure rocks is enriched in nitrogen. In the Oksfjord M3 high-pressure granulite-facies shear zones, the nitrogen content of the carbonic fluid inclusions is rather uniform, at 2530 mole % (Elvevold and Andersen, 1993 ). In the Bergen Arcs, two contrasting fluid compositions have been identified: a carbonic N 2bearing fluid and an aqueous brine, representing immiscible fluid fractions present during formation of the eclogite-facies shear zones, The composition of the carbonic fluid inclusions has evolved by immiscibility and water-

T. ANDERSENET AL.

consuming mineral- and melting reactions (Andersen et al., 199 l b); these inclusions are therefore not simple and unmodified representatives of the fluid phase initially supplied to the shear zones. Some of the eclogites from the Western Gneiss Region contain inclusions with high H20 and N2 contents, and may possibly be better representatives of the initial eclogitefacies fluid than the evolved fluid compositions in the Bergen Arcs. Whether or not such fluids unmix into brine and CO2-N2 fractions will depend upon several factors, such as temperature, initial salt content, and the ability of the actual eclogite mineralogy to hydrate or melt at peak metamorphJic conditions, and thus to consume water from the fluid.

5.1.3. The lower-grademeta-eclogites Eclogites re-equilibrated at amphibolite- or greenschist-facies conditions are characterized by complex inclusion patterns, but water-rich inclusions are abundant in all examples studied (Andersen et al., 1991 a and the present work). These rocks have in common that they have re-equilibrated below the water-saturated solidus of mafic and intermediate rocks (Fig. 2); injection of a water-rich fluid therefore cannot induce melting. Water can, however, be consumed by hydration reactions, replacing primary anhydrous eclogite-facies mineral assemblages by amphibo]le and chlorite. Local mixing of fluids, and possible immiscibility between water (or H20-NaC1 mixtures with low salinity)and nitrogen below the critical curve of the H 2 0 - N 2 system lead to complex fluid-inclusion textures in eclogites reworked at low temperatures and pressures. Both the Rogaland and Spitsbergen eclogites show fluidinclusion textures indicating the simultaneous presence of N2-rich fluicl and water. The N2 in such inclusions was, most likely, inherited from pre-existing inclusions originally trapped at higher pressures, having remained within the rock volume throughout later recrystallization processes.

N2 AND CO2 IN DEEP CRUSTAL FLUIDS: EVIDENCE FROM THE CALEDONIDES OF NORWAY

5.2. Nitrogen distribution The tentative model illustrated in Fig. 6 can be related to the evolution of the eclogite-facies shear zones in the Bergen Arcs. The feldspar of the granulite-facies protolith had low initial a m m o n i u m (e.g. Hall, 1988a,b), or it was depleted in the Grenvillian by reaction with a carbonic fluid, as seen in inclusions in the granulite-facies protolith. The situation prior to the Caledonian eclogite-facies event may be represented by the "granulite" field in Fig. 6. Increasing the water activity by one order of magnitude or more at constantfo2 in the Caledonian, in the precence of a N2-rich carbonic fluid, will produce a mica with NH4 contents approaching the NH4 saturation limit (A in Fig. 6), and this, by consequence, will increase the whole-rock nitrogen content. On the other hand, an increase in water fugacity will not lead to a significantly higher bulk uptake of nitrogen from the fluid, if coupled with increasing oxygen fugacity, so that the shift in activities follows a path parallel to the contour of reaction ( 1 ) (B in Fig. 6). At these conditions; nitrogen will be strongly partitioned to the fluid phase. The similar nitrogen concentrations in eclogite- and granulite-facies whole-rocks and minerals (Fig. 5 ) suggest that this most closely represents the situation in the Bergen Arcs eclogite-facies shear zones.

5.2.1. Origin of nitrogen Despite their semi-quantitative nature, our whole-rock nitrogen data indicate that the Precambrian granulite facies protolith of the Caledonian eclogite-facies shear zones in the Holsnay complex was nitrogen-poor, and thus not a likely source for the N2 in the eclogitefacies fluid. Whether the low nitrogen content is a function of rock composition or a result of earlier metamorphism, subsequent metamorphic processes at a higher water activity are unable to drive more nitrogen out of the already depleted protolith. Possible sources for nitrogen in the Bergen Arc complex include the

127

mantle and nitrogen-rich crustal rocks, such as metasediments subducted to an even greater depth. In the mantle, nitrogen may be bound in diamond, and may possibly substitute in minor amounts in potassium-bearing minerals (phlogopite, amphibole). High-density nitrogen has been reported from fluid inclusions in eclogite-nodules of possible mantle-origin in kimberlite from Yakutia, Siberia (Tomilenko and Chupin, 1983), but no evidence of Nz above the detection limit of the Raman microprobe (ca. 0.1 mole %) has yet been reported from CO2 inclusions in mantle peridotite xenoliths, although actively searched for (see, for example, Andersen et al., 1984; Pasteris, 1987 ). This suggests that crustal rocks are more likely nitrogen sources than the mantle; the existence of such rocks at greater depths is implicated by our data and by the results of Andersen et al. (1990, 1991b). In the Okstjord complex, metasedime,nts are abundant, and nitrogen release from such rocks at a greater depth along the M 3 shear zone system is a likely cause for the N2 present in the high-pressure granulite-facies fluid. On the other hand, the low, but detectable N2-content of the granulite-facies fluid in the Bergen Arcs (XN2 < 2.5 mole %, Andersen et al., 1990) may possibly have been released by reactions like ( 1 ) and (2) during the Grenvillianmetamorphic event.

5.2.2. Nitrogen enrichment in the fluid phase It should be noted that the amounts of nitrogen which can be released to the fluid phase from a volume of rock by reactions ( 1 ) and (2) only amounts to several tens to hundereds of parts per million of the solid rock undergoing metamorphism, depending on the initial rock composition (e.g. H o n m a and Itahara, 1981; Visser, 1992 ). In order to produce a n N zrich metamorphic fluid, it is therefore necessary that (i) large volumes of nitrogen-"fertile" rocks at greater depths undergo devolatilization, (ii) the released fluids are concentrated into pathways such as shear

128

T. ANDERSENET AL. Residualfluid

zoneShear~

Xc% > X .~ >> X.;,o

/ /

Present exposure level

o

._o_ ~ ° + D

e t Amphibc

zones, and (iii) N2 is selectively enriched in the fluid phase. Ifa fluid of deep origin, containing H 2 0 , CO2 and subordinate amounts of N2, is channelled into a system of eclogite- or high-pressure granulite-facies shear zones (Fig. 7), hydration and carbonate-forming reactions at an elevated oxygen fugacity level along the fluid pathway would selectiw~ly c o n s u m e H 2 0 and CO2, yielding a residual fluid relatively enriched in N2. Since peraluminous granite melts may dissolve up to ca. 20 wt% water (Huang and Wyllie, 1986), but probably very little N 2 o r C 0 2 , injection of such melt, or in-situ incipient melting at eclogite-facies conditions may be a particularly efficient way to enrich the residual fluid phase in N2 and CO2 relative to water. (Andersen et al., 1991b).

Carbonate

~-

j C)riginGIfluid:

2 X ,>X,>>X.~

Fig. 7. A hypothetical cross-section through a granuliteeclogite transition in the crust, which may correspond to the case in the Bergen Arcs or in Oksfjord. The interface between eclogite- and granulite domains (or in the Okst]ord case: medium- and high-pressure granulites) is controlled by kinetics rather than by the thermodynamical stability of the respective mineral parageneses. A waterrich fluid is injected into a shear zone from below, i.e. from the mantle or from water- and nitrogen-rich rocks of crustal origin, undergoing degassing at greater depth. During its passage through the shear zone system, the composition of the fluid changes as a function of mineral reactions. Water is consumed by formation of hydrous minerals, and by minor partial melting. CO2 is liberated from fluid inclusions in the granulite-facies protolith, and is consumed by carbonate-forming reactions in the shear zone. Nitrogen can beliberated by the breakdown of ammonium-bearing minerals, if such are present in the protolith (reactions 1 and 2), but may also, under favourable water- and oxygen-fugacity conditions, be consumed by the reverse reactions (Fig. 6).

5.3. Fluids in high-pressure rocks: gases vs. aqueous fluids The correlation between high pressure metamorpism and N2-enriched fluids is not unique to the Caledonides. Apart from the eclogite nodules in kimberlites from Siberia (Tomilenko and Chupin, 1983), high-density nitrogen has been described from eclogites of the Mtinchberg complex, Germany (Klemd, 1991; Klemd et al., 1993). In other eclogites, e.g. in the Alps (Selverstone et al., 1984; Philippot and Selverstone, 1991; Philippot, 1993) and in many blueschists (see review by Touret, 1992), fluid inclusions related to the highpressure metamorphic event are dominated by m o r e o r less complex aqueous brines, without e v i d e n c e o f N2 o r o t h e r g a s e o u s components. Furthermore, aqueous fluids are widespread in high-pressure rocks reworked at amphiboliteto greenschist-facies conditions (e.g. Klemd, 1989; Andersen et al., 1991a; Scambelluri, 1992). The difference in fluid regime between blueschists and subduction zone eclogites formed from oceanic crust o n o n e hand, and eclogites formed in deep parts of continent-

N 2 AND CO2 IN DEEP CRUSTAL FLUIDS: EVIDENCE FROM THE CALEDONIDES OF NORWAY

continent collision zones on the other, is most probably related to their respective tectonic settings. Along an oceanic subduction zone, large volumes of clastic sediments are subducted together with hydrated oceanic crust, representing a considerable reservoir of highly nitrogen-fertile material, and of water. If an aqueous fluid is generated by dehydration reactions, and kept back within the downgoing slab (e.g. Philippot, 1993), the water activity would remain high until dehydration has reached an advanced stage; under such conditions, mineralogically bound nitrogen will be kept back in the solid phases by reactions 1 and 2 going from right to left. In cases such as the Bergen Arcs and the Okstjord M3-granulite, the local rocks have been thoroughly dehydrated prior to high-pressure metamorphism, and nitrogen is accordingly partitioned to the fluid phase, 6. Conclusions In the Caledonides of Norway, a clear correlation between metamorphic conditions and fluid composition can be derived from fluidinclusion data: ( 1 ) Granulite-facies rocks (protoliths and reworked eclogites) contain inclusions of CO2dominated fluid, without evidence of free water. In low- to medium-pressure granulites, the nitrogen content of the fluid is negligible; in the high-pressure granulites of the Oksfjord comlex, 25-30 mole % nitrogen is present. The anhydrous nature of the granulite-facies fluid may have been instrumental in stabilizing the dry mineral assemblages of the rocks. In the Bergen Arcs and Oksfjord complexes, inclusions of carbonic fluid in granulite-facies relics have survived later metamorphism at higher pressure, without losing their identity in terms of density and composition, (2) In eclogites, N2 is present as a characteristic component, in combination with CO2 and/or brine. The fluid inclusion assemblage

| 29

in any given eclogite depends upon mineral hydration reactions and partial melting processes at peak metamorphic conditions. (3) In eclogites re-equilibrated at low temperatures and pressures, complex inclusion patterns containing locally remobilized nitrogen and externally derived brine with variable salinity. The high water activity of the fluid is reflected in the hydrous mineralogy of most low-grade meta-eclogites. The presence of nitrogen in fluid inclusions in a low-temperature, low-pressure metamorphic rock is, however, not sufficient evidence that the rock has been derived from a highpressure protolith, as N2 may occur in certain low-grade settings, not related to high-pressure conditions (Guilhaumou et al., 1981; Darimont et al., 1988 ). The N2 content of a high-pressure metamorphic fluid is the result of distribution of nitrogen between minerals (mica, feldspar) and fluid. The most important controlling parameters are the oxygen and water fugacities of the system. High-pressure metamorphic rocks formed in the deep crust may have densities which are too high to allow the rocks to be brought easily to the surface after their formation. The fluid-induced retrogression processes described from the Norwegian Caledonides lead to replacement of dense, high-pressure mineral assemblages by lower-density equivalents. This may be a necessary requirement for the post-orogenic uplift of terranes containing relict highpressure rocks. The example of the high-pressure rocks in the Caledonides of Norway show that fluid inclusions studies can contribute significantly to the understanding of the behaviour of volatile components in the deep crust, even in rocks formed at P T conditions outside those traditionally regarded as ideal for fluid inclusion studies. Fluid inclusions should therefore be considered in any study of high-grade metamorphic rocks.

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Acknowledgements We want to thank Y. Ohta, E. Krogh and W.L. Griffin for providing samples of highpressure rocks, and David Bridgwater and Anne Daldorff for analytical assistance. The work benefited from financial support from NAVF to T. Andersen and H. Austrheim. Laser-Raman analyses were made possible by the Free University of Amsterdam and by the Dutch Organization for Scientific Research (NWO). The paper has benefited from cornments and discussions by Muriel Erambert and Erling Krogh. Last but not least, special thanks are due to J a c q u e s Touret, who suggested the study of high-pressure fluids in the Caledonides, who has given enthusiastic support to the work, and who invited this contribution.

References Andersen, T., O'Reilly, S.Y. and Griffin, W.L., 1984. The trapped fluid phase in upper mantle xenoliths from Victoria, Australia: implications for mantle metasomatism. Contrib. Mineral. Petrol., 88: 72-85. Andersen, T., Burke, E.A.J. and Austrheim, H., 1989. Nitrogen-bearing, aqueous fluid inclusions in some eclogites from the Western Gneiss Region of the Norwegian Caledonides. Contrib. Mineral. Petrol., 103:153165. Andersen, T., Austrheim, H. and Burke, E.A.J., 1990. Fluid inclusions in granulites and eclogites from the Bergen Arcs, Caledonides of Western Norway. Miner. Mag., 54: 145-158. Andersen, T.. Austrheim, H. and Burke, E.A.J., 1991a. Fluid-induced retrogression of granulites in the Bergen Arcs, Caledonides of W. Norway: Fluid inclusion evidence from amphibolite-facies shear zones. Lithos, 27: 29-42. Andersen, T., Austrheim, H. and Burke, E.A.J., 199 lb. Melt-mineral-fluid interaction in high-pressure shearzones in the Bergen Arc nappe complex, Caledonides of W. Norway: Implications for the fluid regime in Caledonian eclogite-facies metamorphism. Lithos, 27: 187-204. Angus, A., Armstrong, B., deReuk, K.M., Altunin, V.V., Gadetskii, O.G., Chapela, G.A. and Rowlinson, J.S., 1976. International Thermodynamic Tables of the Fluid State, Vol. 3: Carbon Dioxide. Pergamon Press, Oxford, 385 pp. Angus, A., Armstrong, B. and deReuk, K.M., 1979. International Thermodynamic Tables of the Fluid State,

F. ANDERSEN ET AL.

Vol. 6: Nitrogen. Pergamon Press, Oxford. Austrheim, H., 1978. The metamorphic evolution of the granulite rocks of Radoy, with special emphasis on the rocks of the mangerite complex. Thesis, Univ. Bergen, Norway, 265 pp. (unpublished). Austrheim, H., 1987. Eclogitizationoflowercrustalgranulites by fluid migration through shear zones. Earth Planet. Sci. Lett., 81: 221-232. Austrheim, H., 1990. Fluid induced processes in the lower

crust as evidenced by Caledonian eclogitization of Precambriangranulites, Bergen Arcs, Western-Norway. Ph.D. thesis, University of Oslo. Austrheim, H. and Griffin, W.L, 1985. Shear deformation and eclogite formation within granulite-facies anorthositesofthe Bergen Arcs, Western Norway. Chem.

Geol., 50:267-281. Austrheim, H. and Robins, B., 1981. Reactions involving hydration of orthopyroxene in anorthosite-gabbro. Lithos, 14: 275-281. Baur, W.H. and Wlotzka, F., 1974. Nitrogen. In: K,H. Wedepohl (Editor), Handbook of GeochemistD'. Springer Verlag, Berlin. Bos, A., 1990. Hydrothermal element distribution at high temperatures. An experimental study of the partitioning of major and trace elements between phlogopite, haplogranitic melt and vapour. Geol. Ultraiectina, 69: 1-99.

Bos, A., De Haas, G.J.L.M., Voncken, J.H.L., Van der Eerden, A.M.J. and Jansen, J.B.H, 1987. Hydrothermal synthesis of ammonium-phlogopite. Geol. Mijnbouw, 66:251-258. Bos, A., Duit, W., Van der Eerden, A.M.J. and Jansen, J.B.H., 1988. Nitrogen storage in biotite: An experimental study of the ammonium and potassium partitioning between 1M phlogopite and vapour at 2 kb. Geochim. Cosmochim. Acta, 52: 1275-1283. Boundy, T.M., Fountain, D.M. and Austrheim, H., 1992. Structural development and petrofabrics of eclogite facies shear zones, Bergen Arcs, western Norway: implications for deep crustal deformational processes. J. Metamorphic Geol., 10:127-146. Burke, E.A.J. and Lustenhouwer, W.L., 1987. The application of a multichannel laser Raman microprobe (Microdil-28) to the analysis of fluid inclusions. Chem. Geol., 61:11-17. Carswell, D.A., Krogh, E.J. and Griffin, W.L., 1985. Norwegian orthopyroxene eclogites: calculated equilibrium conditions and petrogenetic implications. In: D.G. Gee and B.A. Sturt (Editors), The Caledonide Orogen - - Scandinavia and related areas. Wiley, Chichester, pp. 823-841. Cohen, A.S., O'Nions, R.K., Siegenthaler, R. and Griffin, W.L., 1988. Chronology ot: the pressure-temperature history recorded by a granulite terrain. Contrib. Mineral. Petrol., 98:303-311. Coolen, J.J.M.M.M., 1982. Carbonic fluid inclusions in

Nz AND (?02 IN DEEP CRUSTAL FLUIDS: EVIDENCE FROM THE CALEDONIDES OF NORWAY

granulites from Tanzania - - a comparison of geobarometric methods based on fluid density and mineral chemistry. Chem. Geol., 37: 59-77. Darimont, A. and Heyen, G., 1988. Simulation des 6quilibres de phases dans le systbme CO2-N2: applications aux inclusions fluides. Bull. Min6ral., 111:179-182. Darimont, A., Burke, E.A.J. and Touret, J.L.R., 1988. Nitrogen-rich metamorphic fluids in Devonian metasediments from Bastogne, Belgium. Bull. Mineral., 111: 321-330. I)uit, W., Jansen, J.B.H., van Breemen, A. and Bos, A., 1986. Ammonium micas in metamorphic rocks as exemplified by Dome de l'Agout (France). Am. J. Sci., 286: 702-732. Elvevold, S. and Andersen, T., 1993. Fluid evolution during metamorphism at increasing pressure, carbonicand nitrogen-bearing fluid inclusions in granulites from Oksfjord, north Norwegian Caledonides. Contrib. Mineral. Petrol., in press, Elvevold, S., Reginussen, H., Bjorklund, F. and Krogh, E.J.. 1993. Reworking of deep-seated gabbros and associated contact metamorphic paragneisses in the SEpart of the Seiland Igneous Province, Northern Norway. J. Metamorphic Geol., in press. Eskola, P. 1921. On the eclogites of Norway. Vidensk. Selsk. Skr. I, Mat.-Naturv. K1. 1921 No. 8, 118 pp. Gee, D.G., Kumpulainen, R., Roberts, D., Stephens, M.B. and Zacarisson, E., 1985. Scandinavian Caledonides Tectronostratigraphic Map. Sveriges Geologiska Unders6kning, Ser. Ba, No. 35. Griffin, W.L., Austrheim, H., Brastad, K., Bryhni, I., Krill, A.G., Krogh, E.J., Mork, M.B.E., Qvale, H. and Torudbakken, B., 1985. High-pressure metamorphism in the Scandinavian Caledonides. In: D.G. Gee and B.A. Sturt (Editors), The Caledonide Orogen - - Scandinavia and Related Areas. Wiley, Chichester, pp. 784801. Guilhaumou, N., Dhamelincourt, P., Touray, J.-C. and Touret, J., 1981. Etude des inclusions fluides du syst6me Nz-CO2 de dolomites et de quartz de Tunisie septentrionale. Donn6es de la microcryoscopie et de l'analyse h la microsonde h effet Raman. Geochim. Cosmochim. Acta, 45: 657-673. Hall, A., 1988a. Crustal contamination ofminette magmas: evidence from their ammonium contents. Neues Jahrb, Mineral. Monatsh., 137-143. Hall, A., 1988b. The distribution of ammonium in granires from South-West England. J. Geol. Soc. London, 145:37-41. Hallam, M. and Eugster, H.P., 1976. Ammonium silicate stability relations. Contrib. Mineral. Petrol., 57: 227244. Hirajima, T., Banno, S., Hiroi, Y. and Ohta, Y., 1988. Phase petrology ofeclogites and related rocks from the high-pressure metamorphic complex in Spitsbergen (Arctic Ocean) and its significance. Lithos, 22: 75-97.

131

Holdaway, M.J., 1971. Stability ofandalusite and the aluminum silicate phase diagram. Am. J. Sci., 271: 97131. Holland, T.J.B., 1979. High water activities in the generation of high pressure kyanite eclogites of the Tauern window, Austria. J. Geol., 87: 1-27. Hollister, L.S. and Burruss, R.C., 1976. Phase equilibria in fluid inclusions from the Khtada Lake metamorphic complex. Geochim. Cosmochim. Acta, 40:163175. Holloway, J.R., 1977. Fugacity and activity of molecular species in supercritical fluids, In: D.G. Frazer (Editor), Thermodynamics in Geology. Reidel, Dordrecht, pp. 161-181. Holloway, J.R., 1981. Compositions and volumes of supercritical fluids in the earth's crust. Min. Assoc. Can. Short Course Handb., 6:13-38. Honma, H, and Itahara, Y., 1981. Distribution of ammonium in the minerals of metamorphic and granitic rocks. Geochim. Cosmochim. Acta, 45: 983-988. Huang, W.L. and Wyllie, PJ., 1986. Phase relationships ofgabbro-tonalite-granite at 15 kbar with applications to differentiation and anatexis. Am. Mineral., 71: 301316. Itahara, Y. and Honma, H., 1979. Ammonium in biotite from metamorphic and granitic rocks of Japan. Geochim. Cosmochim. Acta, 43: 503-509. Jamtveit, B., Bucher-Nurminen, K. and Austrheim, H., 1990. Fluid controlled eclogitization of granulites in deep crustal shear zone,,;, Bergen Arcs, western Norway. Contrib. Mineral. Petrol., 104: 184-193. Klemd, R., 1989. P - T evolution and fluid inclusion characteristics of retrograded eclogites, M~inchberg gneiss complex, Germany. Contrib. Mineral. Petrol., 102: 221-229. Klemd, R., 1991. Fluid inclusions in eclogite-facies metasediments from the Mfinchberg Gneiss Complex, NEBavaria (abstract). Plinius 1991 No. 5: 121-122. Klemd, R., Van den Kerkhof, A.M. and Horn, E.E., 1993. High density CO2-N2 inclusions in eclogite-facies metasediments from the Mfincberg gneiss complex, SE Germany. Contrib. Mineral. Petrol., 111: 409-419. Krogh, E.J., 1977. Evidence for a Precambrian continent-continent collision in western Norway. Nature, 267: 17-19. Krogh, E.J., 1980. Compat3ible P-T conditions for eclogites and surroundinggneisses in the Kristiansund area, western Norway. Contrib. Mineral. Petrol., 75: 355380. Lambert, I.B. and Wyllie, P.J., 1972. Melting of gabbro (quartz eclogite) with excess water to 35 kilobars, with geological applications. J. Geol., 80: 693-708. Maijer, C., Hermans, G.A.E.M., Tobi, A.C. and Jansen, J.B.H., 1987. Caledonides and westernmost Precambrian intrusions. In: C. Maijer and P. Padget (Edi-

132 tors), The Geology of Southernmost Norway. An Excursion Guide. Nor. Geol. Unders. Spec. Publ., 1: 99104. Pasteris, J.D., 1987. Fluid inclusions in mantle xenoliths. In: P.H. Nixon (Editor), Mantle Xenoliths. Wiley, Chichester, pp. 691-707. Peterson, J.W. and Newton, R.C., 1989. CO2-enhanced melting of biotite-bearing rocks at deep-crustal pressure-temperature conditions. Nature, 340: 378-380. Philippot, P., 1993. Fluid-melt-rock interaction in mafic eclogites and coesite-bearing metasediments: Constraints on volatile recycling during subduction. In: J.L.R. Touret and A.B. Thompson (Guest-Editors), Fluid-Rock Interaction in the Deeper Continental Lithosphere. Chem. Geol., 107: 000-000. Philippot, P. and Selverstone, J., 1991. Trace element-rich brines in eclogitic veins: implications for fluid cornposition and transport during subduction. Contrib. Mineral. Petrol., 106:417-431. Roedder, E., 1984. Fluid Inclusions. Rev. Mineral. 14, 644 pp. Scambelluri, M., 1992. Retrograde fluid inclusions in eclogitic metagabbros from the Ligurian Western Alps. Eur. J. Mineral., 4:1097-1112. Schultz, R., 1973. Bildung und Stabilit~it der AmmoniumsilikateNH4-MuskovitundNH4-Feldspat. Thesis, Universit~it Karlsruhe. Selverstone, J., Spear, F.S., Franz, G. and Morteani, G., 1984. High-pressure metamorphism in the SW Tauern Window, Austria: P-Tpathsfromhornblende-kyanite staurolite schists. J. Petrol., 25:501-531. Shannon, R.D., 1976. Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides. Acta Crystallogr., A, 32:751-767.

T. ANDERSENET AL. Swanenberg, H.C., 1980. Fluid inclusions in high-grade metamorphic rocks from S.W. Norway. Geol. U1traiectina, 25: 1-147. Tomilenko, A.A. and Chupin, V.P., 1983. Thermobarogeochemistry of Metamorphic Formations. Acad. Sci. SSSR Siberian Branch, Novosibirsk, 524, 200 pp. (in Russian). Touret, J.L.R., 1971. Le facies granulite en Norv6ge m6ridionale, II: les inclusions fluides. Lithos, 4: 423-436. Touret, J.L.R., 1987. Metamorphic fluids: data from fluid inclusions. In: H.C. Helgeson (Editor), Chemical Transport in Metasomatic Processes. NATO ASI Seties, C. 218, Reidel, Dordrecht, pp. 91-121. Touret, J.L.R., 1992. Fluid inclusions in subducted rocks. Proc. Kon. Ned. Akad. Wet., 95: 385-403. Van den Kerkhof, A.M., 1988. The system CO2-CH4-N2 in fluid inclusions: Theoretical modelling and geological applications. Ph.D. thesis, Free University Press, Amsterdam, 206 pp. Van Vyck, N., Valley, J.W. and Austrheim, H., 1990. Oxygen isotope geochemistr~ of granulites and eclogites from the Bergen Arc, SW Norway. Geol. Soc. Am. Abstr. Programs, A347. Visser, D., 1992. On ammonium in upper-amphibolite facies cordierite-orthoamphibole-bearing rocks from Rod, Bamble Sector, South Norway. Nor. Geol. Tidsskr., 72: 385-388. Voncken, J.H.L., 1990. Silicates with incorporation of NH4+ , Rb +, or Cs +. Geol. Ultraiectina, 65:1-91. Voncken, J.H.L., Wevers, J.M.A.R., Van der Eerden, A.M.J., Bos, A. and Jansen, J.B.H., 1987. Hydrothermal synthesis oftobelite, NHaAI3Si3AIOIo(OH)2, from various starting materials and implications for its occurrence in nature. Geol. Mijnbouw, 66: 259-269.