multiple electron beam analyses applied to eclogite from the western alps

Multiple Electron Beam Analyses Applied to Eclogite from the Western Alps

Alessandro Borghi1;�, Davide Agnella1, Elena Belluso1, Roberto Cossio2, and Raffaella Ruf®ni3

1 Dipartimento di Scienze Mineralogiche e Petrologiche, Via Valperga Caluso 35, Universit�a di Torino, I-10125 Torino, Italy2 Dipartimento di Scienze della Terra, Via Valperga Caluso 35, Universit�a di Torino, I-10125 Torino, Italy3 C.N.R. ± C. S. Geodinamica delle catene Collisionali, Via Valperga Caluso 35, I-10125 Torino, Italy

Abstract. The occurrence of compositional and

microtextural relics within metamorphic rocks can

provide useful information on the pressure-tempera-

ture history of the host rock. The grain-size of these

microstructures, such as coronitic reaction microtex-

tures, is mostly too ®ne to be detected by optical

microscopy. Therefore, a more detailed analytical

approach is needed. In this paper multiple electron

beam techniques including the acquisition of X-ray

multi-element maps, micro and nano-analysis per-

formed by SEM/EDS and TEM/STEM-EDX systems

were applied to a speci®c petrological problem related

to metamorphism. Fine-grained decompressional reac-

tion microtextures of an eclogitic sample (Mt. Rosa

Nappe, Western Alps) are described and discussed.

Key words: SEM/EDS; TEM/STEM-EDX; X-ray maps;eclogite.

A metamorphic rock can be compared to a thermo-

dynamic system that ful®ls the phase rule

V�CÿP� 2, where V (variance or the degree of

freedom of the system) represents the physical

variables (pressure and temperature) at which the

rock fully re-equilibrated. V depends on the number

of chemical components (C), which are normally

expressed as oxides, and on the number of miner-

alogical phases at the equilibrium (P).

In natural systems, however, it is dif®cult to reach a

thermodynamic equilibrium state as the physical

conditions change more rapidly than the metamorphic

reaction rates [1]. Evidence of this non-equilibrium is

preserved in metamorphic rocks as microtextures and/

or chemical zoning of single crystals. The study of

these microtextural and compositional relics can

provide information that can be used to reconstruct

the geodynamic history of the rock [2].

Eclogites is one of the most important examples of

metamorphic rocks with recorded different P±T

steps in their evolution [3]. They are the metamorphic

products of ma®c protholits (such as basalts or

gabbros) and consist of anhydrous and high-density

mineral phases (Na-pyroxene and garnet) that crystal-

lised at great depth (> 50 km), according to experi-

mental petrology results, e.g. refs. [4, 5].

In particular, eclogites preserve microtextures

related to decompressional reactions which provide

an indication of the different stages of exhumation

undergone by rocks. Generally, the grain size of these

microtextures is too ®ne to be detected by optical

microscopy, therefore their detailed analytical study

requires a multiple electron beam technique approach.

In this paper, the metamorphic evolution of an

eclogite belonging to the Mt. Rosa Massif (Western

Alps) is reconstructed using digitised backscattered

electron images, integrated ± X-ray multi-elemental

maps and micro- and nano-probe analysis performed

by a SEM/EDS and TEM/STEM-EDX systems.

Experimental

BSE Acquisition Image

Backscattered electron images were collected using a scanningelectron microscope (SEM) from Cambridge Instruments (Stereo-scan S-360). The images were collected at the followinginstrumental conditions: Working Distance� 14 mm, probe cur-rent� 1 nA, accelerating potential� 20 kV.

Mikrochim. Acta 132, 479±487 (2000)

� To whom correspondence should be addressed

EDS Microprobe Analysis

Mineral analyses were performed with an energy dispersivespectrometer (EDS) from Link Analytical (QX-2000) equippedwith a SATW Pentafet detector from Oxford Instruments. Anaccelerating potential of 15 kV and 600 pA probe current wereused. An acquisition time of 60 s was selected. Natural silicatesand oxides were used as standards. The ZAF correction methodwas applied throughout. All the analyses were recalculated usingthe M1NSORT program [6]. Mineral compositions are expressedas atoms per formula unit (a.p.f.u.). Estimates of ferric iron ingarnet, pyroxene and amphibole are based on a ®xed number ofoxygen atoms. Representative analyses for garnet, pyroxene,amphibole and plagioclase crystals are reported in Table 1.

X-Ray Multielemental Map

The maps were collected by an EDS microprobe equipped withLink±system mapping digital interface (MDI) board, whichpermits the remote scanning of the electron beam. The scannedarea can be selected by the secondary or backscattered electronimage. The size of the analysed area can be varied as well as thespatial resolution of the map. Every map consists of a matrix of nintensity values with known x and y co-ordinates which areproportional to the concentration of the selected element. Up to 16single elemental maps can be acquired simultaneously. Moredetailed information has been published elsewhere [7]. In order toachieve a gaussian distribution of the elemental K line intensities, a

dwell time of almost 30 ms/pixel is required. It corresponds to ca.800 counts/pixel, using a pentafet detector able to reach ca.20,000 counts/s.

TEM Data Acquisition

Polished sections glued with Lakeside resin on glass wereprepared. Selected pyroxenes (particularly [001] planes, if present)were microdrilled and the standard single-hole copper grids wereglued on. The detached disk-grid assemblies were washed and thenthinned with a Gatan 600 Duomill Ion Thinner working with Ar atroom temperature, at 5 kV, 0.50 mA per gun, 15� and ®nish angleof 12�. The sample preparation was completed with a 300 AÊ carboncoating. TEM analyses were performed with a Philips CM 12transmission electron microscope working at 120 kV.

STEM-EDX Nanoprobe Analysis

Analyses were made with a TEM/STEM Philips CM 12instrument ®tted with an EDAX Si(Li) detector. The analyseswere processed with a PV9100 system for energy dispersivemicroanalysis. EDX was performed with a 200±100 AÊ diameterbeam, 1.0 nA beam current, 50 s counting time and with specimentilting of about 25�. Data were processed by the SUPQ programusing built-in K factors. All the analyses were recalculated usingthe MINSORT program [6]. Mineral compositions are againexpressed as atoms per formula unit (a.p.f.u.). Estimates of ferriciron in pyroxene are based on a ®xed number of oxygen/atoms.

Table 1. Representative microchemical EDS analyses (oxides as wt%) and cationic calculation (atoms per formula unit) for garnet (based on12 oxygens), pyroxene (6 oxygens), amphibole (23 oxygens) and plagioclase (8 oxygens). Mineral symbols according to Kretz [24]. For theamphibole analyses the Na content is partitioned between [M4] and [A] sites

1 2 3 4 5 6 7 8 9 10 11 12 13 14

wt% Grt(core)

Grt(rim)

Cpx I(Agt)

Cpx I(Omp)

Cpx II(Omp)

Cpx II(Omp)

Cpx III(Di)

Cpx III(Di)

Amp II(Bar)

Amp II(Hbl)

Amp II(Prg)

Amp III(Prg)

PI I(Ab)

PI II(Olig)

SiO2 37.35 37.48 53.36 55.28 56.50 56.44 51.54 52.99 49.62 50.63 38.98 43.81 68.36 67.88TiO2 ± ± ± ± ± ± ± ± 0.03 0.47 0.36 0.00 ± ±Al2O3 20.47 20.64 4.24 8.37 9.78 9.64 6.17 3.28 7.94 5.35 17.23 13.76 19.73 17.40FeO 23.98 31.21 12.88 7.66 6.88 6.39 12.04 9.38 15.87 14.22 19.44 14.70 ± ±MnO 5.88 0.00 ± ± ± ± ± ± ± ± ± ± ± ±MgO 0.59 2.00 7.95 7.85 7.44 7.58 12.99 11.79 12.61 13.78 6.93 10.73 ± ±CaO 11.34 8.46 15.40 13.77 12.20 11.74 15.90 20.87 8.62 11.81 10.36 11.34 0.34 4.72Na2O ± ± 5.47 6.80 7.58 7.79 2.33 2.24 3.52 1.87 3.41 2.90 11.70 10.29K2O ± ± ± ± ± ± ± ± 0.31 0.37 1.47 0.64 0.03 0.12

Total 99.60 99.80 99.30 99.73 100.38 99.57 100.97 100.55 98.52 98.50 98.18 97.89 100.15 100.42

Atoms per formula unit

Si 3.001 3.000 1.974 1.989 1.993 1.996 1.878 1.945 7.084 7.336 5.883 6.449 2.983 2.990Al IV 0.000 0.000 0.026 0.011 0.007 0.004 0.122 0.055 0.916 0.664 2.117 1.551 1.015 0.903Al IV 1.939 1.947 0.159 0.344 0.416 0.419 0.143 0.087 0.420 0.250 0.948 0.835 ± ±Ti ± ± ± ± ± ± ± ± 0.004 0.051 0.040 0.000 ± ±Fe3� 0.061 0.053 0.259 0.142 0.099 0.106 0.144 0.127 0.822 0.053 0.453 0.189 ± ±Fe2� 1.551 2.036 0.139 0.089 0.106 0.084 0.223 0.161 1.072 1.670 2.000 1.621 ± ±Mn 0.400 0.000 ± ± ± ± ± ± ± ± ± ± ± ±Mg 0.070 0.238 0.439 0.421 0.393 0.404 0.705 0.645 2.683 2.977 1.559 2.354 ± ±Ca 0.977 0.726 0.611 0.531 0.464 0.449 0.621 0.821 1.319 1.834 1.676 1.789 0.016 0.223Na tot. ± ± 0.393 0.474 0.522 0.539 0.165 0.159 ± ± ± ± 0.990 0.879Na [M4] ± ± ± ± ± ± ± ± 0.681 0.166 0.324 0.211 ± ±Na [A] ± ± ± ± ± ± ± ± 0.293 0.358 0.675 0.618 ± ±K ± ± ± ± ± ± ± ± 0.055 0.068 0.284 0.120 0.002 0.007

480 A. Borghi et al.

Results and Discussion

The described multiple microbeam approach was

applied to solve a speci®c petrological problem. An

eclogite sample from the east slope of the Mt. Rosa

Massif (Western Alps) was selected. A polished thin

section (30 mm) was mounted on a glass slide and

coated with carbon.

Generally, the eclogite-facies rocks of the Western

Alps formed at high pressure (15±20 kbar) and low-

temperature (500±600 �C) [8] related to the subduc-

tion of the Mesozoic oceanic crust under the Insubric

continental crust (see e.g. [9] for more details about

the geodynamic evolution of the Alps). During uplift

towards the surface the eclogitic assemblage was

generally replaced by minerals stable at lower

pressure. Only in relict low-strain rock-volumes can

the high-pressure eclogitic minerals still be found

even if partially replaced by ®ne-grained coronitic

textures. Coronitic texture represents a local equili-

brium state and consists of a metastable mineral phase

completely surrounded by a corona of one or more

new mineral phases crystallised under different

pressure temperature conditions. Coronitic textures

include kelyphyites, consisting of radial ®brous inter-

growth to the reacting interface of both anhydrous

and hydrous mineral assemblages, and symplectites,

composed of globular minerals intergrowing together.

Compared with synthetic materials such as alloys,

these microtextures are generally interpreted as

exsolution reactions [10]. Coronitic textures are well

known in the Alpine metamorphic literature [11, 12].

Microtextural Features

The selected sample consists of a preserved eclogite-

facies mineralogical assemblage including Na-pyrox-

ene, garnet and minor quartz, rutile, zoisite and white

mica. This eclogitic assemblage is partially replaced

by ®ne-grained reaction coronas developed during

different metamorphic events re¯ecting progressive

decompressional conditions. The sample belongs to a

low-strain rock domain, where no evidence of intense

intracrystalline deformation occurs. It shows a

granoblastic structure without any tectonic foliation.

The garnet forms millimetric porphyroblasts com-

pletely surrounded by a reaction corona composed of

amphibole, plagioclase and sometimes biotite (Fig.

1a). Two different generations of Na-pyroxene can be

detected (Fig. 1b). The ®rst consists of coarse (about

1 mm) porphyroblasts showing a characteristic sector

zoning of different chemical composition (Fig. 1c)

and evidence of intracrystalline deformation. In the

core of some crystals, relics of pre-eclogitic amphi-

bole are present (Fig. 1b). The second Na-pyroxene

generation is represented by ®ne (50±100mm) grano-

blasts showing homogeneous microtextural and pet-

rochemical features (Figs. 1b, 1c). At the TEM scale,

this Na-pyroxene generation was found structurally

homogeneous; the selected area electron diffraction

(SAED) patterns parallel to [001] and the high-

resolution (HRTEM) images clearly show the lack of

any defects.

The second Na-pyroxene generation generally

occurs inside or along the boundary of the ®rst Na-

pyroxene generation crystals. It may thus represent

restricted portions of the original Na-pyroxene

porphyroblast recrystallised under eclogitic meta-

morphic conditions.

Both Na-pyroxene generations are pervasively

(sometimes totally) replaced by composite microtex-

tural coronas (Fig. 1d). Generally it is possible to

distinguish an inner (®ner) and an outer (coarser)

coronite portion. The heterogranular size of these

microtextures suggests that the pyroxene breakdown

probably occurred during two distinct metamorphic

stages. Since Na-pyroxene breakdown starts to grow

along grain boundaries and proceeds inwards [10], the

external coarser and the internal ®ner portions

developed during an earlier and a later stage of the

pyroxene exsolution, respectively.

The outer portion of the composite microtexture

consists of a kelyphytic corona composed of acicular

crystals of amphibole alternating with plagioclase

(Fig. 1e). These minerals grew roughly perpendicular

to the rim of the pyroxene crystal. Their grain-size is

about 50mm and becomes coarser moving outwards

where large-sized granoblastic aggregates have devel-

oped. The coarser grain-size of the outer portion can

be explained by the role of ¯uids, which may have

enhanced intercrystalline diffusion thus favouring a

greater dimensional development of the hydrated

paragenesis (amphibole� plagioclase) [11].

The inner and younger portion of coronitic micro-

texture (not always present) shows globular morphol-

ogy and a very ®ne and homogeneous grain-size

of < 10mm (Fig. 1f ). It consists of an anhydrous

paragenesis (Ca-pyroxene� plagioclase) derived from

the destabilisation of Na-pyroxene. This ®ne micro-

texture can better be observed by TEM. Medium

Multiple Electron Beam Analyses Applied to Eclogite from the Western Alps 481

Fig. 1. SEM backscattered digitised images of the coronitic microtextures developed in the eclogite sample during its decompressionalmetamorphic history. Mineral symbols according to [24]


magni®cation images (Fig. 2) reveal an intergrowth

between the two symplectite forming phases.

Slow diffusion of elements relative to the rate of

progress of the reacting interface is held responsible

for the formation of these microtextures. In many

cases, some elements diffuse towards the interface,

and others move in the opposite direction. The type of

diffusion is dominantly grain-boundary diffusion, and

the actual rate of diffusion is largely controlled by

chemical potential gradients [13].

In Figs. 3.1 and 3.2 the qualitative X-ray composi-

tional maps for Na and Mg in a small area of the

kelyphytic corona microtexture are shown. The very

distinct distribution of these two elements is obvious.

In detail, while Na and Mg are homogeneously

distributed in the pyroxene crystal, they are strongly

partitioned in the reaction microtexture, where Na and

Mg are concentrated in the plagioclase and in the

amphibole, respectively. Even in the symplectite

corona a strong partitioning of these elements into

the product phases is shown clearly by the qualitative

X-ray compositional maps of Figs. 3.3 and 3.4. In this

case Na is dominant in the plagioclase lattice whereas

Mg is dominant in the Ca-pyroxene. The transition

Na-pyroxene)Ca-pyroxene�Na-plagioclase has been

studied in detail in eclogites from various geological

environments and is usually explained in terms of

decreasing pressure [14, 15]. According to metallur-

gical concepts, this phase transition is classi®ed as a

discontinuous precipitation reaction [10, 16].

Mineral Chemistry

Garnet. The X-ray qualitative compositional maps of

a garnet porphyroblast of the four elements (Fe, Mg,

Mn and Ca) involved in the exchange processes

within the octahedral site are reported in Fig. 4. The

garnet shows compositional zoning de®ned by a

gradual and concentric chemical variation from core

(Alm40 Sps24 Pyr2 Grs34) to rim (Alm67 Sps0 Pyr10

Grs23). This zoning displays a decrease of the Mn and

Ca contents and consequently an increase of the Fe

and Mg contents towards the rim (Fig. 5). This

compositional pattern re¯ects a growth zoning devel-

oped under prograde and decompressional meta-

morphic conditions. In particular, the type of Mn

zoning has been described in the literature as a `̀ bell

shaped zoning'' and is typical of growth zoning from

low- to medium-grade metamorphic rocks [17].

Pyroxene. The three pyroxene generations distin-

guished on the basis of microtextural observations

revealed different compositional parameters. The ®rst

generation shows chemical sector zoning de®ned by

an omphacitic composition for the darker portion

displayed on backscattered images and an aegirin-

augitic composition for the lighter portion (see Fig.

1c), according to the pyroxene classi®cation [18] (Fig.

6). Sector zoning is well known in pyroxene [19]. This

phenomenon is of special interest because variations

in composition between different sectors of individual

crystals re¯ect local variations in crystal growth

conditions. It can also provide evidence for a low-

temperature miscibility gap between omphacite and

aegirin-augite [19]. We have tentatively interpreted

the described sector zoning as an incomplete cation

re-equilibration of Na-pyroxene under eclogitic P±T

conditions. According to our interpretation the

aegirin-augite micro-domains should represent relict

portions grown during the initial stage of the burial

process.

The second pyroxene generation is chemically

homogeneous and shows an omphacitic composition

similar to the darker portion of the ®rst generation

with a maximum jadeitic content of 42% (Fig. 6). Its

homogeneous and high P compatible composition is

in agreement with microtextural observations and

both suggest crystallisation under eclogitic meta-

morphic conditions.

Fig. 2. TEM image of diopside ± plagioclase grain boundary inthe symplectitic portion of the corona microtexture. Magni®cation34000x


Finally, ®ne crystals of Ca-pyroxene intergrown

with plagioclase in the inner portion of coronitic

symplectite show a diopsidic composition and is

located in the Quad ®eld according to the Morimoto's

classi®cation [18].

In Table 2 representative STEM/EDX nanoprobe

analyses of Na-pyroxene are reported. STEM/EDX

nanoprobe analyses show good agreement when

compared with the SEM/EDS microprobe analyses,

although compared with the EDS analyses (e.g., the

results in Table 1) greater variations in Na content are

observed. This is probably because the Si(Li) detector

in EDX has a lower detection ef®ciency compared

with the Pentafet Si(Li) detector.

Amphibole. Amphiboles are a useful mineral group

for petrochemical study as their compositional varia-

tion, starting from the basic formula of tremolite

[&Ca2Mg5Si8O22(OH)], changes according to three

different cationic substitutions named edenitic

(&� Si�Na[A]�AlIV), tschermakitic (Si�Mg�AlIV�AlVI) and glaucophanic (Ca�Mg�Na[M4]�AlVI). The ®rst two substitutions are

dependent on the temperature whereas the third

depends on the pressure. Therefore, the AlIV and the

Fig. 3. Qualitative X-ray maps for the kelyphytic (1, 2) and symplectitic (3, 4) decompressional microtextures developed around the Na-pyroxene. The images were collected by an EDS system and elaborated with image analysis software. Resolution� 512� 512 pixels; dwelltime/pixel� 30 ms. dark and light areas represent low and high concentrations, respectively. Pseudo-colours have been normalised in eachmap to highlight the zoning


Na[A] contents are proportional to the temperature,

while the AlVI and the Na[M4] increase with pressure.

In the studied eclogite, three different generations

of amphibole have been detected showing a wide

compositional range, depending on the metamorphic

conditions of growth and the microtextural site of

crystallisation. For the nomenclature of the amphibole

the classi®cation proposed by Leake et al. [20] has

been followed.

The ®rst generation consists of relict amphibole

enclosed into Na-pyroxene and plots in the pargasite

®eld near the boundary with edenite (Fig. 7A). The

second amphibole generation grew in the kelyphytic

portion of the composite coronite microtextures.

Fig. 4. Qualitative X-ray maps for a garnet porphyroblast. The images were collected by an EDS system and elaborated with an imageanalysis software. Resolution� 512� 512; dwell time/pixel� 30 ms. dark and light areas represent low and high concentrations,respectively. Pseudo-colours have been normalised in each map to highlight the zoning

Fig. 5. Line traverse along the mapped garnet crystal showing thequantitative variation for octahedral cations (on the basis of 12oxygens) in a 1-dimensional direction (garnet diameter� 1 mm)


Single crystals are chemically zoned, ranging in

composition from a Na-Ca variety (barroisite) at the

core to a Ca-amphibole (tremolite to Mg-hornblende)

at the rim (Fig. 7B). This zoning is marked by a

progressive decrease of glaucophanic substitutions

towards the rim, combined with an increase of tscher-

makitic substitution, suggesting a growth under de-

compressional and prograde metamorphic conditions.

The third generation grew around the porphyro-

blasts of garnet. It is chemically zoned with a

composition ranging from edenite to Fe-pargasite

(Fig. 7A).

Fig. 6. Representative compositions of the different pyroxenegenerations plotted on the classi®cation diagram of [18]. Trianglepyroxene I (light portion in BSD images of Fig. 1); Circlepyroxene I and pyroxene II (dark portion in BSD images of Fig. 1);Square Pyroxene III; Jd jadeite; Ac acmite; Q Ca-Mg-Fepyroxenes

Table 2. Representative STEM-EDX analyses calculated as atomsper formula unit for the different generations of pyroxene. Spot sizediameter ca. 50 nm

1 3 2An Cpx I (Omph.) Cpx II (Omph.) Cpx III (Di)

Si 1.992 1.980 2.020Al IV 0.008 0.020 ±Fe3� ± ± ±Cr IV ± ± ±

Al VI 0.455 0.411 0.120Ti ± ± ±Fe3� 0.055 0.081 ±Fe2� 0.101 0.107 0.240Mn ± ± ±Mg 0.390 0.400 0.593

Fe2� 0.012 0.015 ±Mn ± ± ±Mg 0.045 0.057 ±Ca 0.442 0.456 0.940Na 0.501 0.472 0.075K ± ± ±

Fig. 7. Representative analyses of Ca-amphiboles plotted on theclassi®cation diagram of [20]. The relict amphibole I is located inpargasite ®eld; the amphibole II grown in the kelyphyticmicrotextures on Na-pyroxene is located in the transitional ®eldbetween actinolite and Mg-hornblende, and the amphibole IIIgrown in the coronite microtextures around garnet plots in thetransitional ®eld between edenite and Fe-pargasite. Mineralsymbols according to Kretz [24]


Plagioclase. Pure albite is associated with the

amphibole in the kelyphytic corona (Pl I in Table 1),

whereas oligoclase is intergrown with diopside in the

symplectitic one (Pl II in Table 1).

Metamorphic Evolution

Microtextural and petrochemical data allowed to

reconstruct some steps of the P±T evolution suffered

by the rock-sample selected. Thermobarometric

estimates of the eclogitic metamorphic peak were

calculated from the composition of the mineralogical

phases re-equilibrated under high pressure and low-

temperature conditions. Applying the geothermometer

of Ellis and Green [21] based on the Fe-Mg cationic

exchange between pyroxene and garnet and the

geobarometer of Holland [22] proportional to the

Jadeite molecule content in the Na-pyroxene an

average P±T range of about 480±500 �C for 12±

14 kbar can be inferred for this ®rst metamorphic

event.

The post-eclogitic uplift history can be determined

by the microtextural and chemical data coming from

coronite reactions. During a ®rst decompressional step

the omphacite breakdown evidenced from the amphi-

bole (barroisite to Mg-hornblende)� albite kelyphytic

corona occurred, while garnet remained stable.

According to experimental petrology data [4, 22],

the hydrated Na-pyroxene breakdown developed

under low grade metamorphic conditions at pressures

between 7 and 11 kbar. During a second decompres-

sional step also garnet became metastable and

interacted with the kelyphyte microtexture according

to the dehydrated reaction garnet� tremolite�albite) pargasite� anortite [23], while diopside�plagioclase symplectite around the Na-pyroxene

developed. According to previously mentioned miner-

alogical assemblage, medium temperature (around

600 �C) and low-pressure (< 5 kbar) metamorphic

conditions can be attributed to this second decom-

pressional stage.

Concluding Remarks

Applying a multiple electron-beam-technique-

approach to a speci®c metamorphic petrological

problem, it has been possible to determine pressure

and temperature conditions at which a metamorphic

rock re-equilibrated. The investigation of several

different types of microtextures and mineral assem-

blages in an eclogite sample allowed the petrologic

history to reconstructed and the shape of the P±T

exhumation trajectory to be better understand.

Acknowledgements. This study was carried out with the ®nancialsupport of M.U.R.S.T., grant 60% to A. Borghi and of the CNR ± C.S. Geodinamica Catene Collisionali ± Torino. Two anonymousreferees are thanked for their constructive remarks.

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multiple electron beam analyses applied to eclogite from the western alps

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