Fluid inclusion and mineral isotopic compositions (HCO) in eclogitic rocks as tracers of local fluid migration during high-pressure metamorphism

Earth and Planetary Science Letters, 114 (1993) 431-448 Elsevier Science Publishers B.V, Amsterdam

431

[CLJ

Fluid inclusion and mineral isotopic compositions (H-C-O) in eclogitic rocks as tracers of local fluid migration during high-pressure metamorphism Serge Nadeau

a Pascal Philippot ,b and Fran~oise Pineau a

Laboratoire de G~ochimie des Isotopes Stables, Universitd Paris 7, IPGP, CNRS URA 196, 2 place Jussieu, 75251 Paris" cedex 05, France b C N R S - U R A 736, Laboratoire de Pdtrologie Mdtamorphique, Universitd Paris 7, 4 Place Jussieu, 75252 Paris cedex 05, France Received February 11, 1992; revision accepted December 3, 1992

ABSTRACT Eclogite facies metagabbros from the Monviso ophiolitic complex (Italian Western Alps) provide a unique opportunity to trace fluid migration processes in a portion of the oceanic crust that has undergone subduction at a depth of > 40 km. We have determined the omphacites 6180 and the abundance, 6D and 613C of hydrous and carbonaceous compounds present in whole rocks which are believed to trace the residual phases of what was mobilized in the original rocks during subduction. Prograde dehydration reactions and eclogitization of hydrothermally altered oceanic metagabbros was accompanied by approximately 90% fluid loss. The remaining fluid was trapped as primary water-rich fluid inclusions in omphacite megacrysts that developed at the expense of the precursor magmatic pyroxene. Deformation of the eclogitic rocks resulted in continuous recycling of fluid between mylonites and omphacite veins without further fluid loss from the host ductile shear zone. Oxygen isotopes of omphacite (omp) and hydrogen isotopes of water in the fluid inclusions (FI), analyzed in low-strain rocks, mylonites and undeformed/deformed veins show marked variations, 81SOomp and 6DFI values ranging from + 3.0 to + 5.3%o and from - 3 1 to -93%o, respectively. Detailed isotopic analysis of several individual vein-wallrock pairs show that the scale of isotopic equilibration is of the order of one centimeter. Therefore, the eclogitic minerals and fluids filling the veins are concluded to be locally derived. Our results argue against recent models which suggest large-scale mass flushing of isotopically homogeneous fluids during subduction zone metamorphism. On average, the H20 contents and 6DFl value are within the upper mantle range. The carbon has been inherited from the metamorphic transformation of the original carbon present in the oceanic crust and has a mean 6~3C value of - 24.2 + 1.2%o. Two carbonaceous components can be recognized, condensed carbon which represents the major carbon species and carbonate daughter crystals present in fluid inclusions. 61SOomp values are significantly lower than those reported for mantle minerals and are similar to pyroxene and whole-rock values from hydrothermally altered oceanic crust. It is suggested that the isotopic imprint of the Monviso eclogitic minerals and fluids represents the signature of mid-ocean ridge hydrothermal alteration and subduction zone eclogitization processes. The rocks studied could be the missing link between altered oceanic gabbros and eclogitic xenoliths. In addition, the range in 618Oomp values recorded in the different microstructural domains covers most of the 61So values reported in type A, B and C eclogitic xenoliths, implying that caution must be exercised when using oxygen isotopes as an indicator of the origin of eclogitic xenoliths.

1. Introduction Several petrological and geochemical models have e m p h a s i z e d the p o t e n t i a l role of fluids, rel e a s e d b y d o w n g o i n g s u b d u c t e d p l a t e s , in t h e

* Corresponding author.

generation

of

island

arc

magmas

and

mantle

w e d g e m e t a s o m a t i s m [1-3] a n d also t h e recycling o f v o l a t i l e s i n t o t h e m a n t l e [4,5]. L a r g e v o l u m e s o f f l u i d a r e c o n s i d e r e d t o b e r e l e a s e d by m e c h a n ical c o m p a c t i o n a n d b y p r o g r a d e m e t a m o r p h i s m o f h y d r a t e d o c e a n i c c r u s t a n d s e d i m e n t s in t h e s h a l l o w e r l e v e l s o f s u b d u c t i o n z o n e s (e.g. 0 - 3 0 k m ) [6]. T h e f l u i d s t h a t a r e r e l e a s e d at o c e a n i c

0012-821X/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved

432

S. N A D E A U

trenches migrate upwards in sedimentary wedges [e.g. 7]. Although a growing body of evidence supports the idea that a free fluid phase is present on a regional scale during subduction zone metamorphism [8], little is known about the nature and abundance of the fluids present, or about the mechanisms and scale of fluid transport during eclogite facies metamorphism. Although studies of fluid migration in highpressure low-temperature (HP-LT) terrains are still in their infancy, recent results from the Catalina schist complex, California, suggest large-scale fluxes of isotopically homogeneous fluids in intensively deformed subduction mdlanges (containing blocks of blueschist and eclogitic rocks) exhumed from depths of 20-40 km [3,9]. In contrast, fluid inclusion and petrological studies of particularly well-preserved eclogitic mylonites and veins from Monviso, Western Alps [10], and

45 °_

E T AL.

in interlayered tuffaceous and mafic rocks from the Eastern Alps [11], have emphasized that fluid flow is limited to a local scale, thus arguing against extensive fluid migration at minimum depths of 40-60 km. Burg and Philippot [12] have evaluated different modes of vein formation and scales of fluid migration during the exhumation of interlayered metagabbros that equilibrated successively under eclogitic, blueschist and greenschist facies conditions. They suggest that fluidrock chemical-mechanical interactions occur on small scales by diffusional mass-transfer processes controlling local segregation during eclogitic metamorphism at > 40 kin. During blueschist and greenschist facies metamorphism at depths from 35 to 10 km the interactions occur on large scales by channelized and buoyancydriven upward fluid flow, which is accommodated by hydraulic fracture propagation and cataclasite

(

Pennine European

2

b

front! plal

3 4

44 ° _

I

I

I

6°

7°

8°

Fig. 1. S c h e m a t i c m a p of the W e s t e r n Alps. 1 = A u s t r o - A l p i n e n a p p e s ; 2 = I n t e r n a l P e n n i n e Z o n e , a = I n t e r n a l Crystalline Massifs, b = schistes lustr~s, c = m a i n ophiolite complexes; 3 = E x t e r n a l P e n n i n e Z o n e ; 4 = E x t e r n a l Crystalline Massifs.

433

FLUID INCLUSION AND MINERAL ISOTOPIC COMPOSITIONS

development. Thus the blueschist to eclogite facies transition may be the site of a major change in the fluid flow mechanism and the physical properties of the fluid. The aim of this study is to decipher the variations in fluid and minerals chemistry at eclogite facies P - T conditions in samples from the Monviso meta-ophiolitic complex (Fig. 1), whose deformational history is well documented. Fluid inclusion compositions, daughter mineral assemblages, and isotopic compositions of hydrogen in water-rich fluid inclusions, oxygen in omphacite, and carbon in carbonaceous phases present, are

used to test the hypothesis made by Philippot and Selverstone [10] of limited fluid migration processes in the Monviso eclogites. These results are then compared with volatile contents and isotopic values in eclogitic and peridotite xenoliths.

2. Geology

2.1 Regional geology The rocks studied are eclogites from the Monviso massif in the Italian Western Alps (Fig. 1). This massif is thought to be a remnant of the

'

A~nealed domai•

'

Deformed vein

Fig. 2. Schematic drawing showing the five different types of omphacite (omp) with respect to microstructures in the Monviso eclogitic shear zone (see text). FI= fluid inclusions. (a) Omphacite 1 megacryst in the low-strain rocks. (b) Dynamically recrystallized omphacite 2 in the mylonite that developed at the expense of omphacite 1. (c) Annealed omphacite 3 in the mylonite. (d) Oscillatory-zoned omphacite 4 crystals in the undeformed veins. (e) Dynamically recrystallized omphacite 5 in deformed veins.

434

S. NADEAU ET AL.

Neotethys basin that separated the South Alpine plate from the European plate during Jurassic time and then closed by subduction during late Cretaceous-Eocene times [13]. This Eoalpine phase of the Alpine orogeny has produced regional high-pressure low-temperature (HP-LT) metamorphism in nappes of continental and oceanic affinities, stacked one upon another during late Eocene-early Oligocene time [13]. Previ-

ous studies in the Monviso massif showed that it equilibrated successively under decreasing P - T : eciogite facies (P >/1 GPa, 450 ~< T~< 550°C), blueschist facies (P/> 0.7-0.8 GPa, 400 ~
TABLE 1 Mineralogy, fluid inclusion, omphacite ,~l~O, bulk content and isotopic ratios for CO 2 and water extracted by step heating, obtained on the Monviso eclogite facies metagabbros (Fe- and Mg-metagabbros), glaucophane vein and amphibolite facies metagabbros from the Chenaillet massif Sample

Microstruc-

Mineralogy

ture

omp

Fluid inclusions Silicate

gt

ap

X X X

X

ab

rt

(1)

X X X

(103)

X

(21)

X X X

(32)

(2)

61SOornp

Total carbon

Total water

ppm CO.

ppm H2O

~13C

3D

Monviso

Eclogite facies Fe-metagabbros VS- 1 Vi-79 Vi-258 Vi-260 Vi-260 VI-261 Vi-261 Vi-262b Vi-262c Vi-263 Vi-378a Vi-378 Vi-384 Vi-385 Vi-385 Vi-387 Vi-389a Vi-389b Vi-389c

low-strain rock vein vein vein mylonite vein mylonite vein mylonite vein vein mylonite vein vein mylonite vein vein (center) vein (margin) mylonite

X X X X X X X X X X X X X X X X X X X

X X X X

X X X

X

X X X

X

X X X X

33.1 84.7

-28.7 - 26.1

1900 1 501

-72 - 53

3.9(_+ < 0.1) 3.8

176.6 42.5

-26.3 - 29.0

526 388

-34 - 84

<0.1)

225.1

-25.0

1596

-25

< 0.2)

77.2 172.7

- 25.0 -23.1

2 510 1041

- 52 -72

(20) (1)

3.9(_+ 3.2 4.2 5.3(+ 3.1 4.3(_+ 3.9(_+ 4.0(+_ 4.0

72.4 145.0 84.7 406.4

- 12.3 -22.5

1 153 2607 2734 722

-31 -62 -53 63

82.0 47.5

- 26.0 - 21.1

1015 1384

- 50 - 68

13.7

227.4

- 30.0

20 089

- 91

553.0 485.0

- 23.6 - 23.6

21912 19 478

- 53 - 89

(12) (33) (4)

(33) (10) (15) (32)

X X

3.9(+ < 0.1) 3.0

X X X X

(11)

X X X

X

X

(11)

X

(talc _+Mg-chlorite3.7 (talc _+Mg-chlorit~.7

< 0.1) < 0.1) < 0.1)

Eclogite facies Mg-metagabbros VS- 14 VS-23

vein vein

X X

Glaucophane vein Vi-407

vein

X

Chenaillet

Amphibolite facies Fe-metagabbros CHE-80-02 Undeformed metagabbros containing: CHE-129 augite, plagioclase _+actinolite _+chlorite + epidote ± calcite

Mineral abbreviations: omp = omphacite, gt = garnet, ap = apatite, ab = albite, rt = rutile. Fluid inclusion analysis: (1) and (2) refer to the analyses performed in this study and by Philippot and Selverstone [10], respectively; numbers in parentheses correspond to the number of fluid inclusions analyzed in each sample.

FLUID INCLUSION

AND MINERAL

435

ISOTOPIC COMPOSITIONS

Fe-metagabbros occur at the top of the cumulate sequence and are underlain by Mg-rich metagabbros and talc schist layers. These rocks form a 2 km ductile shear zone that consists of highly deformed mylonitic rocks that locally contain small lenses of low-strain metagabbros and which are crosscut by different generations of synkinematic veins [see 15]. Field work suggests the following eclogite volume proportions in the shear zone: 95-98% mylonites, 2-5% omphacite-rich veins and < 1% low-strain gabbros

2.2 Mineralogy and microtextures In this section, we present a summary of the microtextural evolution of the eclogitic metagabbros during foliation development and veining; for a more detailed description the reader is referred to the study by Philippot and Van Roermund [15]. In the eclogite facies metagabbros, the lowstrain domains, mylonites and veins all contain the same mineral assemblage of omphacite, garnet, rutile and apatite. Patchy retrogressive alteration by blueschist and greenschist facies minerals is limited to (1) veins of coexisting phengite, blue and green amphibole, green chlorite and albite, or (2) individual blasts of these minerals overprinting the eclogitic foliation or oriented fibers in monomineralic tension gashes. There are five different types of omphacite and in order to simplify the discussion we refer to these as "omphacite 1", "omphacite 2" etc. In the following we describe each type of omphacite with respect to microstructures in the eclogitic metagabbros (see Fig. 2). In the low-strain rocks, relic mineral textures preserve igneous microstructures. Ilmenite and augite grains (up to 2 cm across) have been mimetically pseudomorphed by rutile and "omphacite 1", respectively. Plagioclase is completely pseudomorphed by microcrystalline omphacite (_+ zoisite and phengite). These different grains are mottled with coronas of garnet. Omphacite 1 contains abundant solid (mainly rutile) and fluid inclusions. In the mylonites, strain-induced deformation accommodated dynamic recrystallization of the "primary" omphacite 1 grains into a fine-grained matrix of "omphacite 2". Omphacite 2 grains are devoid of fluid and

solid inclusions suggesting that fluid (and solid) inclusions were released during the dynamic recrystallization process [10,15]. Locally, static recrystallization of omphacite 2 resulted in a mosaic texture characterized by blocky "omphacite 3" grains. These grains contain small amounts of fluid inclusions oriented parallel to the crystallographic faces of the crystals and lining the overgrowth zones. This suggests that the fluids wetting the omphacite 2 grain boundaries were trapped during the grain growth recovery process [15]. Omphacite (_apat i t e ± rutile ± garnet ± albite) veins crosscut the foliation plane at variable angles. When present within the veins, garnet, rutile and apatite occur as randomly distributed millimeter- to centimeter-scale clusters, whereas albite occurs as small interstitial crystals between vein-omphacite grains (Table 1). Two types of omphacite veins have been identified: undeformed veins (including crack-seal type of vein microstructure [16]) and deformed veins. In the undeformed veins, "omphacite 4" grains (mm to cm in size) contain abundant fluid inclusions. These fluid inclusions were studied in detail by Philippot and Selverstone [10] and are discussed below. In the deformed veins, omphacite 4 crystals dynamically recrystallized into fine-grained "omphacite 5". Omphacite 5 is typically devoid of fluid inclusions implying fluid liberation during dynamic recrystallization of omphacite 4.

3. Sampling and analytical procedures 3.1 Sampling strategy The samples collected for fluid inclusion and stable isotope analyses are listed together with their mineralogy in Table 1. The eclogitic metagabbros were collected over an area of 100 × 30 m. Two Mg-rich omphacite veins from the underlying Mg-metagabbros were also included to compare fluid composition from a different gabbroic lithology. Finally, one glaucophane vein was analyzed to provide constraints on the retrograde blueschist facies isotopic imprint and two samples of undeformed and amphibolitized Ferich metagabbros (CH-80-02 and CH-129) from the Chenaillet massif to record the isotopic signature of the hydrothermally altered gabbroic protolith to the Monviso eclogites [17], (M6vel; pers. commun., 1992; see Fig. 1 for sample location).

436

s. NADEAU ET AL

TABLE 2 S t e p h e a t i n g results o b t a i n e d f o r the M o n v i s o e c l o g i t e facies m e t a g a b b r o s (Fe- a n d M g - m e t a g a b b r o s ) , g l a u c o p h a n e vein a n d t h e C h e n a i l l e t a m p h i b o l i t e facies F e - m e t a g a b b r o s Sample: Microstructure: Size: W e i g h t (g):

VS-1 l o w - s t r a i n r o c k (Fe) 0.315-1.0 mm 2.000

T (°C)

CO 2 (ppm) 8.1 9.5 3.7 3.7 8.1 0.0 0.0

VS-14 vein (Mg) 0.315-1.0 mm 2.000

Vi-79 vein (Fe) 0.315-1.0 mm 2.000

VS-23 vein (Mg) 0.315-1.0 mm 2.000

H20

613C

CO 2

H20

(~13C

(ppm)

(%0)

(ppm)

(ppm)

(%~)

CO 2 (ppm)

H20 (ppm)

15 295 610 425 471 66 18 M

-26.9 - 26.3 -26.8 - 32.5 lost nd nd M

34.8 16.0 8.2 12.3 10.7 0.0 0.0 M

19 341 221 236 167 31 0 M

-26.2 - 22.3 - 28.8 32.4 - 25.6 nd nd M

25.9 9.6 2.8 0.4 6.0 2.8 0.0 M

103 397 350 159 315 60 0 M

2 0 0 - 400 4 0 0 - 600 6 0 0 - 800 800-1000 1000-1200 1200-1350 1350-1450 1450-1550

M

Sample: Microstructure: Size: W e i g h t (g):

Vi-262c m y l o n i t e (Fe) 0.315-1.0 mm 2.000

T (°C)

CO 2 (ppm)

H20 (ppm)

2 0 0 - 400 4 0 0 - 600 6 0 0 - 800 800-1000 1000-1200 1200-1350 1350-1450 1450-1550

6.2 10.3 5.1 7.3 0.0 9.2 4.4 M

12 58 82 94 82 35 25 M

Sample: Microstructure: Size: W e i g h t (g):

Vi-385 vein (Fe) 0.315-1.0 mm 2.000

T (°C)

CO 2 (ppm)

H20 (ppm)

313C (%o)

CO 2 (ppm)

H20 (ppm)

613C (%0)

CO 2 (ppm)

H20 (ppm)

2 0 0 - 400 4 0 0 - 600 6 0 0 - 800 800-1000 1000-1200 1200-1350 1350-1450 1450-1550

39.9 37.0 35.6 24.9 19.4 3.7 2.2 M

20 260 359 280 90 16 16 M

- 25.3 -23.7 -23.1 - 23.3 - 21.3 - 8.1 -8.4 M

8.1 16.1 16.1 12.4 6.2 4.7 8.8 M

34 325 257 238 238 46 15 M

- 25.0 -18.5 - 12.6 - 11.7 8.9 - 3.5 -3.3 M

4.4 48.7 46.1 22.0 15.4 4.4 4.0 M

76 890 954 355 253 57 22 M

Vi-262b vein ( F e ) 1 cm 3 1.370

~13C (%e)

CO 2 (ppm)

H20 (ppm)

- 24.3 - 21.7 - 19.9 nd - 14.5 - 7.6 nd M

38.0 9.4 1.8 0.0 0.0 0.0 35.5 0.0 M

7 413 413 298 196 58 91 25 M

Vi-378a vein (Fe) 1 cm 3 1.334

613C (%o) - 24.2 -24.6 nd nd nd nd - 30.0 nd M

Vi-384 vein (Fe) 0.315-1.0 mm 2.000

~X3C (%~)

CO 2 (ppm)

H20 (ppm)

613C (%o)

CO 2

H20

~13C

CO 2

H20

613C

(ppm)

(ppm)

(%o)

(ppm)

(ppm)

(%o)

- 27.6 - 25.3 - 26.3 - 30.9 nd -31.1 - 34.7 M

4.4 93.9 45.5 19.4 5.1 8.4 M

30 60 79 159 99 99 M

- 28.7 - 25.8 - 27.5 - 28.1 - 26.0 -20.3 M

33.0 11.0 81.0 48.0 28.0 8.8 2.5 12.8 M

11 296 242 242 242 174 289 112 M

- 29.1 - 25.0 - 26.4 - 26.9 - 20.6 -24.9 - 18.1 - 25.9 M

14 17.6 12.2 0.0 1.8 10.4 10.4 9.9 M

96 36 561 585 580 118 34 0 M

Vi-387 vein (Fe) 0.315-1.0 mm 2.000

Vi-389a vein c e n t e r (Fe) 1 cm 3

- 26.1 - 21.7 - 22.9 nd nd -27.3 - 29.2 - 29.7 M

Vi-389b vein m a r g i n (Fe) 1 cm 3

1.202

0.809 ~13C (%c)

CO 2 (ppm)

H20 (ppm)

- 25.5 nd nd - 24.5 - 18.5 - 7.4 -6.9 M

24.6 147.8 71.1 73.3 27.9 2.6 2.2 M

155 646 880 516 387 93 53 M

~13C (%0) - 25.8 - 22.7 -21.5 -24.5 - 18.2 - 6.3 -6.0 M

FLUID INCLUSIONAND MINERAL ISOTOPIC COMPOSITIONS

437

TABLE 2 (continued) CHE- 129 undeformed rock (Fe) 0.2-2.0 mm 0.288

CHE-80-02 undeformed rock (Fe) 0.2-2.0 mm. 0.250

(%c)

CO 2 (ppm)

H20 (ppm)

813C (%o)

CO 2 (ppm)

H20 (ppm)

~13C (%~)

nd nd nd - 30.5 - 29.5 nd M

nd 346 84 54 46 23 M

nd 2 254 7763 7 200 3 005 1690 M

nd - 23.5 -24.5 - 22.7 - 25.0 - 20.5 M

nd 261 83 50 56 35 M

nd 1659 6348 6 926 2958 1587 M

nd - 22.7 -25.3 - 25.8 nd - 23.5 M

Sample: Microstructure: Size: Weight (g):

Vi-389c mylonite (Fe) 1 cm 3 0.932

Vi-407 glaucophane vein 0.315-1.0 mm 2.000

T (°C)

CO 2 (ppm)

H20 (ppm)

613C (%o)

CO 2 (ppm)

H20 (ppm)

~13C

200- 400 400- 600 600- 800 800-1000 1000-1200 1200-1350 1350-1450

63.1 220.0 83.6 31.2 5.2 3.3 M

12 141 190 176 140 63 M

- 27.6 - 26.4 -19.1 nd nd - 23.6 M

nd 5.9 0.0 90.6 122.8 8.1 M

nd 2282 6546 9 436 1294 531 M

Sample numbers, type of microstructure, granulometry (granulo.) and sample weight are given for each sample. Heating steps are given in increasing order of temperature range. All samples were melted, and " M " indicates the temperature step at which melting occurred.

3.2 Analytical methods 3.2.1 Microthermometry and Raman microanalysis Fluid inclusions in omphacite 1, 3 and 4 were analyzed by microthermometry and laser-Raman microspectrometry on = 80 to 120 tim-thick doubly polished sections. Microthermometric measurements were made using a Fluid Inc. freezing-heating stage. Liquid nitrogen was used as the cooling medium. T e m p e r a t u r e s of final melting (Tmf) and of homogenization (T h) were determined at heating rates of ~0.1°C min -1 and 1.0°C min 1, respectively. Tmf and T h measurements were reproducible to + 0.1°C and _+ 1.0°C, respectively. In order to check the accuracy of the microthermometric results obtained by Philippot and Selverstone [10] with those from this study, we repeated 20 out of their 161 Tmf and T h values (Table 1). All 20 Tmf values were reproduced to +0.2°C whereas 5 T h values were reproduced to _+1.0°C and 15 to +2.0°C. These small differences are within analytical uncertainty and do not alter the consistency of both sets of microthermometric results, thus enabling their comparison. R a m a n microanalysis was used to identify daughter minerals in situ in some selected fluid inclusions. Analyses were made on a XY Dilor Microdil-28 multichannel laser-Raman microprobe.

3.2.2 Sample preparation and isotopic methods Rock samples were cut into slabs ( < 5 - 1 0 cm 3) to separate vein materials from host rocks. These slabs were broken into different aliquots (cubiccentimeter size fragments, or crushed and sieved into 0.3-2 m m and < 0.3 m m fractions). Minerals coated by alteration products (mainly surface oxidation) were discarded. The sample cleaning procedure is the same as described in [18]. The 0.3-2 m m fraction was leached ultrasonically for about 5 min in HCI ( 1 N ) to remove carbonates which would interfere with the fluid inclusion study. However, the cubic-centimeter fragment aliquots were not treated with HC1. The sample heating procedure for C and H isotopic analysis is the same as performed for CO2-rich fluid inclusions by Nadeau et al. [18] and the heating steps are given in Table 2. H 2 0 and CO 2 were separated cryogenically by a variable temperature cold trap immersed in liquid n i t r o g e n . H 2 0 was reduced t o H 2 over a hot U furnace (800°C). For all samples, H 2 0 was accumulated (i.e. summing all steps), before being reduced to H 2, and the amounts measured manometrically. The contents and ~13C values of CO z were determined at each step. The step heating technique is regularly used to study the carbon content of mafic rocks [18,19]. The main conclusions that are important to know are given here.

438

S. N A D E A U E T AL.

(1) Between 200 and 600°C, condensed carbon (carbonaceous films [20] and also called reduced carbon) is extracted as quite a pure phase. The only contamination comes from adsorbed carbon (important in powdered samples) which has ~13C generally between - 20 and - 28%o. After 600°C, the purity may not be preserved since there may be significant contributions from other components such as carbonates. (2) " F r e e " carbonates are decomposed between 600 and 800°C. Because carbonates are isotopically the heaviest species, low abundances can be detected during these steps even when there are remnants of condensed carbon especially if the grain size is large. (3) Fluid inclusions are opened at various temperatures depending on their size and density: small inclusions decrepitate at higher temperature than large ones. Daughter carbonate crystals release CO 2 (i) between 600 and 800°C if the inclusions have already been decrepitated at lower temperatures, or (ii) at the same time as decrepitation for higher temperatures. H 2 and CO 2 blanks in the step heating extraction method were measured under the same conditions before sample analysis. The total H 2 blank r'O

rn

was less than 3/~mole with a 8D value of - 7 3 _+ 5%v. The CO 2 blank value at each step was constant at 0.07 + 0.02 /zmole with a 813C value of - 2 8 . 2 _+ 0.5%o. For each step, the CO 2 yield and isotopic accuracy were better than 99% and +_0.1%o, respectively [18]. Oxygen in silicates was extracted by bromine pentafluoride reaction at 600°C. The standards NBS-28 and Circe basalt yielded values of +9.6 and + 5.5 _+ 0.1%o (respectively) compared to the accepted values of +9.6 and +5.7%o, respectively [21]. C, H and O isotopic ratios were measured on a delta E Finnigan Mat(TR) spectrometer. Isotopic ratios are reported in the conventional 3-notation, in permil, relative to S M O W (H and O) and PDB (C).

4. Results 4.1 Fluid inclusion study 4.1.1 Inclusion types and fluid composition In the low-strain gabbros, fluid inclusions in omphacite 1 are 5-30 /zm tubular inclusions, randomly distributed throughout the crystals and preferentially oriented parallel to the [001] axis of

D []

-4

~ _~:~3,.~

[]

O

uu

[]

[] []

[]

[] []

c o~ e~

-8

~J

t ~

~

d~ -

[] []

[]

[]

o

[]

[]

-12

[]

[]

[] []

• -16

"20

~ ~ O m

[] Veins [] Myionites • Lowstrainrocks ,

120

I

140

,

I

160

•

.s:,l(._ •

,

i

,

180

I

200

,

•

r

~m

mVmmlmm•m I

220

,

I

240

Th (°C) Fig. 3. Temperature of final melting (Tr. f) against temperature of homogenization (T h) in omphacite in the Monviso eclogitic Fe-metagabbros.

FLUID INCLUSIONAND MINERALISOTOPICCOMPOSITIONS

439

the grains. This suggests that they are of primary origin. In the mylonites, the fluid inclusions follow the growth zoning in omphacite 3 grains and are therefore primary with respect to the recov-

m

378

ery/growth process. All of the types of primary fluid inclusions described by Philippot and Selverstone [10] occur in the undeformed veins (omphacite 4). These are evenly distributed clus-

385

_ []

=13

o

407

o 4 818 0

~..,~.~2 389

[[

~

,

f

~l

I

14

5 (%~) SMOW

407 ~

389

l-q (T-r-q ,

'

i

0.05

0.10

~

0.15

0. 0

/

0.25

~,

0.30

CHE-129

C.E.SO-O2 [ ] 2.00

2.10

2.20

HzO (% weight)

262 [] []

~ ] CHE-129

-30

CHE-80-02 262 / [] ~ 407

~ 389 ~

-i0

-70

-'90

5 D (%~) snow 389 I 0

I

I 100

~ I

[~ 2 0

389 I

I

I

300

389 1 400

I

500

I

I

600

COg (ppm weight)

Fig. 4. Mineral and volatile isotopic and concentration results. (a) ~ S o values of omphacite in eclogitic samples and of glaucophane in sample Vi-407. (b) Total H20 content in fluid inclusions in eclogitic samples and of hydrous phases in samples Vi-407 (glaucophane) and CHE-80-02 and CHE-129 (chlorite + amphibole). (c) SD values of fluid inclusions in eclogitic samples and of hydrous phases in samples Vi-407 (glaucophane) and CHE-80-02 and CHE-129 (chlorite+amphibole). (d) Total CO 2 content. Tie-lines relating individual vein-wallrock pairs in direct contact to each other (Vi-261, Vi-262 and Vi-389) and within 5 cm of each other (Vi-378 and Vi-385). Abbreviations: C = vein center, M = vein margin. Monviso samples: 1 = low-strain rock (Fe), 2 = mylonite (Fe), 3 = vein (Fe), 4 = vein (Mg), 5 = glaucophane vein. Chenaillet amphibolite facies samples: 6 = undeformed rock (Fe).

440

ters in omphacite cores, fluid-inclusion bands lining oscillatory-growth zones and crack-seal microcracks. At room temperature, all fluid inclusions contain three phases (liquid + vapor + one to several daughter minerals). In all inclusions, the first melting temperatures (Tmi) range between - 24.5 and - 20.0°C, which is consistent with H 2 0 , NaC1 and KCI being the major components. Temperatures below -22.9°C (the eutectic temperature in the system H 2 0 NaCI-KC1) point to the presence of additional dissolved divalent or more highly charged ions. This is in agreement with Raman microanalysis and morphological inspection of the daughter minerals which indicate that calcite and pyrite are present in omphacite 1 inclusions. Calcite, pyrite, halite a n d / o r sylvite, and rutile a n d / o r sphene (as well as undetermined daughter phases) occur in omphacite 3 inclusions. The fluid inclusions in vein omphacite 4 contain halite, sylvite, calcite, dolomite, albite, anhydrite a n d / o r gypsum, barite, baddeleyite, rutile, sphene, Fe oxides, pyrite and monazite [10].

4.1.2 Fluid salinity and density Final melting (Tmf) and homogenization (T h) behavior of all inclusions are shown in Fig. 3. The inclusions in omphacite 3 and 4 show a similar range of Tm~.and T h values, in striking contrast to the omphacite 1 inclusions. Tmf ranges between - 1 8 . 3 and -12.8°C in omphacite 1 inclusions and between - 1 4 . 9 and - 2 . 9 ° C in omphacite 3 and 4 inclusions. On heating, the liquid + vapor phases homogenized to liquid in all inclusions. T h ranges between 190 and 246°C in omphacite 1 inclusions (peak at = 208°C) and between 115 and 240°C in omphacite 3 and 4 inclusions (peaks at ~ 150 and 190°C). Further heating up to 500°C led to the decrepitation of the fluid inclusions before dissolution of any of the daughter minerals. The differences in Tmf and T h values argue for two populations of brines, of different salinities and densities, filling omphacite 1 and omphacite 3 + 4 inclusions, respectively. The differences in T h values recorded are probably due to the trapping of different generation of inclusions at different P - T conditions. Indeed, omphacite 1 clasts replace magmatic augite grains. This stage of

s. NADEAU

E T AL.

static replacement of the precursor assemblage may have occurred prior to the deformation event that caused foliation development and veining. This implies that omphacite 1 inclusions may represent a stage of entrapment predating the entrapment of omphacite 3 and 4 inclusions. Two possible explanations for omphacite 1 inclusions are (i) that they are exsolution inclusions that precipitated from the host mineral during omphacite replacement of augite, or (ii) that they formed from fluids derived from the breakdown of amphibole and chlorite, which are know to partly or entirely replace the magmatic pyroxene in the altered gabbroic protolith ([17], see below).

4.2 Stable isotopes

4.2.1 Oxygen in silicate minerals Oxygen isotopic ratios obtained on the various types of omphacite are given in Table 1 and represented in Fig. 4a. Oxygen isotopes in veinomphacite 4 and 5 show a wide range o f 618Oomp values from +3.0 to +5.3%o. Although less marked, oxygen isotopes in mylonite-omphacite 2 and 3 are also heterogeneous with 618Oomp values ranging from +3.1 to +4.0%o. Omphacite 1 in the low-strain gabbro shows a 6180omp value of + 3.7%o. Variations in oxygen isotope values have also been recorded between omphacite 3 and 4 in two individual vein-mylonite pairs (Vi-378 and Vi-385) for which the vein and wallrock have been sampled within 5 cm of each other (8180omp differences of 0.7%o for Vi-378 and of 2.2%o for Vi-385). In contrast, oxygen isotopes in ompbacite 3 and 4 in three individual vein-mylonite pairs taken in direct contact to each other yielded identical 6~8Oomp values (undeformed veins Vi261 and Vi-389, and deformed vein VI-262). In addition, two different fractions (aliquots Vi-389a and Vi-389b) of the same vein Vi-389 show identical ~8Oom p values; these two fractions correspond to large omphacite grains (up to 2 cm across) from the vein center and to small omphacite fibers ( < 1 ram) from the vein margin, respectively. The omphacite in Mg-rich veins shows similar oxygen isotopic v a r i a t i o n s (6180omp = +3.7 to +4.7%o) than those observed in Ferich omphacite veins.

FLUID INCLUSION

AND MINERAL

4.2.2 1420 contents and isotopic composition (a) Fluid inclusions in eclogitic rocks. The H 2 0 contents and isotopic compositions obtained by step heating on whole-rock samples are given in Tables 1 and 2. Representative step heating patterns are shown in Fig. 5. Release of water in all eclogitic rocks occurs between 400 and 1350°C with most being released below the melting point. Since most H 2 0 was released at T < 1200°C and the beginning of OH-diffusion in pyroxene was shown by infrared spectroscopy to occur at T > ll00°C [22], we assume that most H 2 0 is derived from fluid inclusions (FI). The HzOFI contents in omphacite veins (0.053-0.273%) overlap with the one in the lowstrain gabbro (0.190%) and are generally higher than in mylonites (0.039-0.072%). 6D n values of water in the veins cover a larger range ( - 3 1 to 500

441

ISOTOPIC COMPOSITIONS

Vs-I low strain rock (Fe)

a

- 9 3 % 0 ) than in the mylonites ( - 6 3 to - 8 4 % o ) and low-strain gabbro ( - 7 2 % 0 ) . There is no difference between the centre and margin of the undeformed vein (Vi-389a-b). Although the host mylonite (Vi-389c) contains much less water, the t~DF1 value of - 6 3 % o is close to the average 6DFI value of - 5 9 % o in the vein. The deformed vein Vi-262 has much less HzOFI than the other veins and has a t~DFI value very different from its host mylonite. There is no significant difference between the Mg-rich and Fe-rich omphacite veins. (b) Hydrothermal alteration and retrograde blueschist. The two samples of amphibolite facies metagabbros from the Chenaillet massif, which represent remnants of hydrothermally altered oceanic crust [17], yielded 1.95 and 2.19% H 2 0 with 6D values of - 8 9 and - 5 3 % o , respectively

Vi-262c mylonite (Fe)

b

H2 0

--

I [

400

a'c

.60

300

0

-20

I

& -40

CO2 200 (ppm weight) 14

(~

C

Vi-385 vein (Fe)

d

Vi-387 vein (Fe) 13

H20 & 300 CO z (ppm weight)

0

-20

IC" m

~

-40

!

o

&

6o (°/oo)

-60

.2o_ 100

(%0)

-80

0

200

;SD

-60

100

400

&

.!

=

-80

.0 400

800

T (°C)

1200

400

800

1200

1600

T (°C)

Fig. 5. Typical step heating patterns showing H20 and CO2 contents (scale on left-hand side) and 313C and 6D values (scale on the right hand-side) against temperature of extraction.

442

s. N A D E A U ET AL.

(Table 1). The water is derived from amphibole and chlorite that replace the magmatic pyroxene. In the Monviso, late-stage blueschist facies glaucophane-vein Vi-407 (gl) yielded 1.90% H z O g 1 with a 6DgI value of - 9 1 % o and a 318OgI value of + 13.7%o.

of - 2 4 . 6 _+ 0.5%o. The isotopic distribution obtained through all temperatures shows that condensed carbon is the most abundant carbonaceous species. (2) Carbonates can be detected in the fraction 600-800°C in samples Vs-23, Vi-387, Vi-389 and Vi-407, but their exact 6~3C could not be determined. (3) At temperatures higher than 1200°C very little CO 2 is extracted. There is no relationship between concentration and isotopic composition. Five samples have 8 to 13.5 ppm CO2, associated with high 613C ( - 3 . 4 to -7.6%c), whereas another eight samples have 3 to 35 ppm CO 2 with 613C very similar to the condensed carbon extracted before 600°C. The later group represents condensed carbon liberated only at very high temperature because of its position deep inside the crystal. The former group of CO 2 is considered as the best representation of the daughter carbonates isotopic composition, enclosed in fluid inclusions. Figure 5 shows that associated 813C of the three temperature fractions does not correlate with concentration. The carbonate daughter minerals, associated with eclogitization (extracted at

4.2.3 Carbon distribution and isotopic composition Bulk carbon content and isotopic ratios are given in Table 1 and detailed results on step heating are given in Table 2. Typical step heating patterns are shown in Fig. 5. Using our experimental protocol and our interpretations of step heating results, we are able to make several comments on steps grouped in three fractions: 200 to 600°C, 600 to 1200°C and 1200 to 1550°C. The distribution of their respective 813C relative to the amount of CO 2 extracted is represented in Fig. 6. (1) Between 18 and 346 ppm of CO 2 was extracted between 200 and 600°C. This amount is more than can be attributed to adsorption (only a few ppm C) on such a large granulometry and is thus considered to be condensed carbon. It has typical isotopic signatures with a m e a n ¢~13C value

0 LEGEND [] LT fraction []

-10

[]

[] O

O

MT fraction HT fraction Bulk Carbon extracted

(.) -20 "°o

O')

0~. -30

[]

0

0

O

~0, ,.....0:~ Chenaillet gabbros

Glaucophane vein

-40 0

100

200

300

400

500

600

ppm CO 2 Fig. 6. ~13C-ppm COz diagram related to temperature step groups. LT, MT and HT correspond to low, med~ and high temperature fractions. Grey area shows the range of 613Cvalues that prevail in most of the eclogiticmetagabbros in, endently of the concentration. The high 8~3Cvalues correspond to carbonate daughter mineral extracted at HT.

FLUID INCLUSION AND MINERAL ISOTOPIC COMI'OSI]'IONS

high temperature), are strongly enriched in ~3C, the medium temperature steps (800-1200°C) representing mixing between the two reservoirs. 5. Discussion

5.1 Fluid origin and scale of)quid flow The occurrence of omphacite veins in the eclogitic ductile shear zone indicates that fracturing and dissolution/precipitation of material was important during deformation. Because identical mineralogies (omphacite, garnet, rutile, apatite) are found in both the veins and the surrounding mylonites, the crack initiation and propagation that led to veining are attributed to microstructural transformations accommodating foliation development and not to dehydration reactions. Similarly, fluid liberation from host .omphacite 1 megacrysts is supposed to have occurred during crystal plastic flow, forming a wet fluid film along dynamically recrystallized omphacite 2 grain boundaries in the mylonites. Relics of such microstructures can still be found within annealed omphacite 3 crystals. Heterogeneous deformation resulted in the superposition of layers of different grain size and fluid content that caused local strain and porefluid pressure partitioning [15]. These differences could have initiated microcrack development and veining. Recognition of undeformed and deformed veins implies that the process described above is cyclic in nature as the same free-fluid phase can be released and trapped several times during omphacite crystal plastic flow and veining (see [15] for further details). The chemical evolution of the fluids during the transformation from low-strain gabbros to mylonites and veins is recorded in fluid inclusions by shifts in Tm~ values and by differences in daughter mineral contents. Mylonites and veins show similar fluid composition and the vein + mylonite fluid population may be considered as "macroscopically" homogeneous. However, marked fluid compositional differences have been recognized between adjacent vein minerals and within a single grain, implying that the composition of the fluids filling the veins (and the mylonites?) is heterogeneous on a microscopic scale [10].

443

Variations in oxygen isotope ratios between the different microstructural domains (6180om p = + 3.0 to + 5.3%o) and within a single microstructural domain (variations of 2.3%o and of 1%~ between the different veins and mylonites, respectively) indicate that the rocks studied did not equilibrate during eclogite facies metamorphism. Similarly, recognition of oxygen isotope variations between the vein-wallrock pairs sampled at 5 cm from each other suggests that the scale of isotopic equilibration was very small. Since there are no differences between the undeformed and the deformed veins and their respective adjacent wallrock, homogenisation was probably at the 1 cm scale. 6DFI in omphacites 1, 3 and 4 shows similar type of variations as that described above for oxygen implying that 6D was also heterogeneous on a cm-scale. However, a major difference can be seen when considering the deformed v e i n wallrock pair Vi-262. In contrast to the homogeneous ( ~ 1 8 0 o m p values, the 6D w values range from - 3 4 % o in the vein to - 8 4 % o in the adjacent mylonitic wallrock (Table 1). This may be related to kinetic fractionation (diffusion related) during the partial leakage of the fluids during dynamic recrystallization of omphacite 4 into omphacite 5. The carbon content behaves in a similar way; the deformation process produced a wet fluid film which reacted with condensed carbon. The 613C of the carbonate in some veins indicates that the CO 2 extracted from this condensed carbon was strongly enriched in ~3C. This suggests that these reactions are associated to complex adsorption-desorption and oxydation reactions controlled by kinetic isotopic fractionation rather than by equilibrium isotopic reactions. 5.2 Fluid loss in a subducting gabbroic crust and mantle metasomatism Volatile-bearing phases form in the oceanic crust at mid-ocean ridges [24-31] as a result of hydrothermal circulation and metamorphism, under conditions of high temperature and low pressure. Estimates of the volatile content of the oceanic crust are highly variable. In this study, we use the estimates of Hart and Staudigel [4] for the first kilometer of the basaltic layer (7% H 2°

444

s. N A D E A U ET AL.

and 3% CO2), of Peacock [23] for the gabbroic layer (1% H 2 0 and 0.1% CO 2) and of Hart and Staudigel [4] and Javoy et al. [5] for the whole oceanic crust (0.7-2% H 2 0 and 0.1-0.7% CO2). Assuming that the Chenaillet metagabbros represent the hydrothermally altered gabbroic protolith of the Monviso eclogites [17], Fig. 7 shows that eclogitization of the Monviso metagabbros was accompanied by about 90% loss of water and carbon. This suggests that most of the volatiles stored within hydrous phases in the oceanic gabbroic protolith were released during prograde metamorphic reactions prior to the ~ 40 km depth. Deformation of the rocks during eclogite facies metamorphism resulted in the redistribution of the remaining fluid between the different microstructural domains, without further fluid loss from the shear zone. The bulk rock fluid estimates of the Monviso eclogites (shown as M in Fig. 7) are 0.059 + 0.015% H~O and 220 p p m CO 2 (using Table 1 and rock volumes in section 2.1). The bulk 6DFI value of ca. --70%e is within the range of

mantle-derived materials (-6(1 to - 8 0 % c ) . The bulk ~13C value is ca. - 2 5 % 0 which is outside of the range of most mantle materials. The results presented above are of critical importance in constraining fluid processes during H P - L T metamorphism. Indeed, although it seems well established that a slab component is present in arc-related magmas [2], debate remains as to whether this component is a fluid or a partial melt and whether it is derived from subducted sediments or altered oceanic crust. We suggest that up to 90% of the fluid present in the hydrothermally altered protolith of the Monviso eclogites was released before the 40 km depth. During eclogite facies metamorphism, deformation resulted in continuous fluid redistribution between the different microstructural domains, thus suggesting that the fluid could have been retained at a depth of > 40 km in the slab. This together with the recognition of limited fluid flow argues against the release of large volumes of fluid from subducting oceanic crust or, in other words, that when the eclogitic transition is corn-

10.0

[] HSBC

1.0

JPAOC

CO 2 (% weight) 0,

PKGC

@ D2

oo, led 0.001

.

0.01

.

.

.

.

.

.

I

0.1

. . . . . . . .

np

I

1.0

i

.

.

.

.

.

.

©6

A8

D7

V?9

04

.

10.0

(% weight) Fig. 7. H~O and CO 2 contents in Monviso and Chenaillet metagabbros, mantle peridotite, eclogite xenoliths and oceanic rocks dredged at Hess Deep. H 2 0 and CO~ contents for oceanic crust materials are represented by letters: HSBC for the first kilometer of basaltic crust from Hart and Staudigel [4], PKGC for the gabbroic layer from Peacock [23], HSOC and JPAOC for the whole oceanic crust from Hart and Staudigel [4] and Javoy et al. [5]. Arrows refer to the processes described in the text: open arrow, eclogitization; solid arrow, mylonitization and veining under eclogitic conditions. (a) Oceanic crust metagabbros: l = different samples of Monviso eclogitic metagabbros, 2 bulk-Monviso eclogite, 3 - Chenaillet amphibolite facies metagabbro. (b) Xenoliths (data from [18] and unpublished results): 4 = anhydrous peridotite, 5 = amphibole peridotite, 0 = amphibole pyroxenite, 7 = eclogite. (c) Hess Deep oceanic rocks [25]: 8 = dolerite, 9 gabbro.

FLUID INCLUSION

AND MINERAL

445

ISOTOPIC COMPOSITIONS

pleted, fluids (in opposition to melts?) may be inefficient metasomatic agent. 5.3 Magmatic vs. oceanic crust origin of eclogite xenoliths Low 6~80 values reported in ophiolitic and seafloor gabbros have been attributed to the high

temperature hydrothermal alteration of the oceanic crust by deep circulating fluids at low w a t e r / r o c k ratios [24-35]. In the Monviso eclogites, omphacite yields a bulk filSOomp value of + 3.5%o, which is similar to pyroxene and wholerock values from hydrothermally altered oceanic crust but significantly lower than the common 6ISo value of 5.5_+0.5%o accepted for upper mantle minerals.

MANTLE VALUES

[261 <

°I [...

~

[ ] Eclogite (mantle melt)

Continents

[] Eelogite (oceanic crust)

Z 0

.

[ ] Eclogite (continental)

iiiiiiiiiiilili

[ ] Gabbros

• B ................

Z '
Basalts

~i:!!iiiii::l~ Bellsbank [41]

Type 11 [~,iiiiiiiiii:-IType X .,mmhfimmn,uuudii"iiiiiil I Roberts Victor Mine [39] nmmmm.nm....m iii'i~ii~im..n,m.mm.~ Roberts Victor Mine [38]

Greenstone [24,33]

[...

Hole 504B [25,34] Onverwacht [32] Xigaze [30] Samail [31,35] ~

Pinde [291

Macquarie Island [28]

0

.~

~ I 0

I

~ 4

Chilean [26]

Monviso (this study) I I I I 8 12

8180

I 16

(°/oo) Fig. 8. Comparison between the 61~Oomp values from Monviso eclogites with the 6180 values reported in minerals from crustal and eclogitic xenoliths, oceanic crust rocks (basalts, dolerites and gabbros) from ophiolite complexes and dredged samples. References to the data are given in parentheses.

446

s. NADEAU ET AL.

Comparison between our oxygen isotopic resuits on omphacite and those reported in eclogitic xenoliths and crustal eclogites was also made possible because omphacite and garnet are known to be in isotopic equilibrium [45]. This comparison is shown in Fig. 8. Low 3ISO values in omphacite from crustal and xenolith eclogitic rocks have been attributed to the eclogitization of deeply seated magmatic intrusions in the continental crust or altered seafloor rocks [33,36-39]. Type B and C eclogite xenoliths, attributed by their O-Sr-Nd isotope systematics to subducted oceanic rocks [39-42], cover a similar range of 6 ~SO values as obtained in omphacite from Monviso. The omphacite-rich vein Vi-385 in Monviso, however, has a value that overlaps with type A eclogite xenoliths interpreted to correspond to eclogitic mantle melt compositions [43,44]. If vein Vi-385 with a high ,~MSOo.,p value ( + 5.3%o) and omphacite crystals displaying spectacular oscillatory zonations (see figs. 3a and 4 in [10]) was subducted in the mantle and later sampled by a kimberlite as for eclogitic xenoliths, the chemical and textural characteristics of Vi-385 would be attributed to a eclogitic mantle melt, although the vein is of metamorphic origin.

~l~O,,mv

6. Conclusions

(1) In the Monviso eclogites, the fluid composition varies on the mm-scale, ~ S O and 6D vary on the era-scale. This suggests that the eclogitic vein-minerals and fluids were derived locally, arguing for limited fluid flow during eclogite facies metamorphism. This is in direct contrast with recent isotopic studies from other high pressure tcrrains, where regional-scale mass flushing by isotopically homogeneous fluids has been suggested (e.g. Catalina schist complex, [3] and references therein). (2) The bulk H 2 0 and CO 2 in the different microstructural domains show that, although fluid was lost during omphacite crystal plastic flow, it went into the in-situ formation of veins and thus retained at depth in the slab. Any large fluxes of fluids from the subducting oceanic crust into the overlying mantle wedge must have occurred prior to the eclogitic conditions (40 km deep in the present case), that is during prograde blueschist a n d / o r amphibolite facies metamorphism.

(3) The Monviso eclogitic rocks preserve the 61SO signature typical of oceanic crust which has been hydrothermally altered at high temperature. This implies that eclogitic metamorphism, plastic deformation, local fluid migration and veining did not alter the 6180 values of the oceanic crustal materials subducted into the mantle. (4) Omphacite from the different microstructural domains shows oxygen isotopic values that cover most of the range of 6JSO values reported in types A, B and C eclogitic xenoliths. Care must be taken when using the A, B and C type of classification for the origin of eclogitic xenoliths. (5) The Monviso eclogites may represent the missing link between subducted gabbros and eclogitic xenoliths that bear low oxygen isotopic values. Acknowledgements

S.N. acknowledges FCAR (Qudbec Government, Canada) for a postdoctorate fellowship held in Paris. Many thanks to Agnbs Rejou-Michel for guidance on the oxygen extraction line. Discussions with M. Javoy, P. Agrinier, H.L.M. van Roermund and E. Wilmart during the course of this work are greatly appreciated. Special thanks to S. Boyd for his critical and efficient help during revision. The manuscript benefited from thorough reviews by J. Selverstone and T. Tingle. This is CNRS-INSU-DBT contribution No. 470 and IPGP contribution No. 1242. References 1 P.J. Wyllie, Magmas and volatile components, Am. Mineral. 64, 469-500, 1980. 2 J.B. Gill, Orogenic Andesites and Plate Tectonics, Springer-Verlag, New York, 39(1 pp., 198[). 3 G.E. Bebout, Field-based evidence for devolatization in subduction zones: implications for arc magmatism, Science 251,413-416. 1991. 4 S.R. Hart and H. Staudigel, Isotopic characterization and identification of recycled components, in: Crust/Mantle Recycling at Convergence Zones, S.R. Hart and E. Giilen, eds., pp. 15-28, Kluwer Acad. Pub., Dordrecht, 1989. 5 M. Jaw~y, F. Pineau and P. Agrinier. Volatiles and stable isotopes in recycling, in: Crust/Mantle Recycling at Convergence Zones, S.R. Hart and k. Giilen, eds., pp. 121 138, Klnwer Acad. Pub., Dordrecht, 1989. 6 M. Cloos and R.L. Shrcvc, Subduction-channel model of prism accretion, melange formation, sediment subduction. and sediment erosion at convergent plate margins: 2. implications and discussion, Pageophys. 128, 501-545, 1988.

FLUID INCLUSION AND MINERAL ISOTOPIC COMPOSITIONS

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