Fluids in Metamorphic Rocks

Ž .Lithos 55 2001 1–25www.elsevier.nlrlocaterlithos

Fluids in metamorphic rocks

J.L.R. Touret)

Department of Petrology, Vrije UniÕersiteit, De Boelelaan 1085, 1081HV Amsterdam, Netherlands

Received 1 September 1999; accepted 4 April 2000

Abstract

Basic principles for the study of fluid inclusions in metamorphic rocks are reviewed and illustrated. A major problemrelates to the number of inclusions, possibly formed on a wide range of P–T conditions, having also suffered, in most cases,extensive changes after initial trapping. The interpretation of fluid inclusion data can only be done by comparison withindependent P–T estimates derived from coexisting minerals, but this requires a precise knowledge of the chronology ofinclusion formation in respect to their mineral host.

The three essential steps in any fluid inclusion investigation are described: observation, measurements, and interpretation.ObserÕation, with a conventional petrographic microscope, leads to the identification and relative chronology of a limited

Ž .number of fluid types same overall composition, eventually changes in fluid density . For the chronology, the notion of GISŽ .Group of synchronous inclusions is introduced. It should serve as a systematic basis for the rest of the study.

Ž .Microthermometry measurements, completed by nondestructive analyses mostly micro-Raman , specify the compositionand density of the different fluid types. The major problem of density variability can be significantly reduced by simpleconsiderations of the shape of density histograms, allowing elimination of a great number of inclusions having suffered lateperturbations. Finally, the interpretation is based on the comparison between few isochores, representative of the wholeinclusion population, and P–T mineral data. Essential is a clear perception of the relative chronology between the differentisochores. When this is possible, as illustrated by the complicated case of the granulites from Central Kola Peninsula, a goodinterpretation of the fluid inclusion data can be done. If not, fluid inclusions will not tell much about the metamorphicevolution of the rocks in which they occur. q 2001 Elsevier Science B.V. All rights reserved.

Keywords: Metamorphic rocks; Fluid inclusions; Microthermometry; Granulites

1. Introduction

Most metamorphic reactions involve a fluid phase,which plays a major role for a number of rock-for-ming processes: element transport, kinetics of crystalgrowth or evolution, control of external variables,

) Tel.: q31-20-444-7270; fax: q31-20-646-2457.Ž .E-mail address: [email protected] J.L.R. Touret .

such as activity or partial pressure of volatile compo-nents, etc. In recent years, the petrology of this fluidphase has become one of the most active fields ofresearch in metamorphic petrology. It can be ap-proached from a number of ways: thermodynamicalmodeling of heterogeneous mineral equilibria, evalu-ation of fluid flow through rocks, analysis of thestable isotope signature left by transient episodes of

Žfluid percolation, evaluation of geophysical data e.g.

0024-4937r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved.Ž .PII: S0024-4937 00 00036-0

( )J.L.R. TouretrLithos 55 2001 1–252

.electrical conductivity , etc. All these techniques re-quire completely different instruments and concepts,to the point that most research groups specialize inone or two of them, often ignoring the others. Butthere is at least one way of approaching directly this

Ž‘now-missing fluid phase’ the expression is from.the late Phil Orville , namely through the study of

fluid remnants preserved as inclusions in minerals. Inrecent years, this scientific discipline has progres-sively gained a recognized place. It was born — atleast in its modern form — almost simultaneously in

Ž .France Poty, 1969 and in the former Soviet UnionŽ .Dolgov et al., 1967 , through a combination ofseveral factors, most of them not directly related, asfollows:

v The development of ready-to-use heating andespecially freezing stages, firstly conceived for thestudy of ore deposits, but very soon applied to allother rock types.

v Widespread use of equilibrium thermodynamics.Supported by the immense task done by experimen-tal mineralogists and petrologists, this has led to atrue quantification of metamorphic petrology, no-tably through a more and more precise reconstruc-tion of P–T conditions followed by metamorphicrocks through time.

v The interest in inclusions has been raised by theŽ‘unexpected discoveries’ the word is from the late.Prof. Winkler, Gottingen of fluid remnants in rocks

or minerals where the mineral composition wouldnot have given any indication on their possible oc-currence. The best-known example is probably thewidespread occurrence of CO in granulites, but2

Žother cases could be mentioned as well N in2.eclogites or diamond, etc. .

These results have shown that paying some atten-tion to fluid inclusions in metamorphic rocks wasindeed of great interest. But, at the same time, itbecame also obvious that this type of study was noteasy. A number of difficulties were immediatelyapparent: small size, but great number of inclusionsin any rock sample, multiplicity of inclusion genera-tions, possible fluid leakage or ‘post-trapping’

Ž .changes Roedder, 1984 . This indicates a great com-plexity of fluidrmineral interaction processes inmetamorphic rocks, and for a number of workers allthese problems have thrown some doubts on thereliability of fluid inclusions, in general. This is

probably the reason why, despite the fact that thebasic principles of their studies have been well knownfor at least a quarter of a century and that a numberof observations do not require any other instrumentthan a conventional petrographical microscope, fluidinclusions still continue to be almost ignored in mostbasic petrology textbooks, with few significant ex-

Ž .ceptions e.g. Spear, 1993 . This attitude is not justi-fied: fluid inclusions are part of the rock, they oc-cupy roughly the same volume as most accessoryminerals; therefore, they are as deserving of study asany rock-forming mineral. After all, the fact thatthey are fluid is just a question of reference tempera-ture: if observation could be done close to absolutezero, everything would be solid! But too manypetrologists hesitate to engage in this type of study,estimating that they will have to spend a great dealof time and effort for a questionable result. I do notwant to claim here that everything is possible, andthat any sample can be completely understood withinminutes. Fluid inclusion studies in high-grade meta-morphic rocks have been considered as especially

Ž .difficult for a long time Roedder, 1981 . Theserocks, however, present a decisive advantage, namelythe possibility to record extensive P–T informationfrom the composition of coexisting minerals. Thisleads to a specific method of fluid inclusion studies,which has progressively developed in the lastdecades, and which has already been presented in a

Žnumber of publications notably Touret, 1977, 1981,.1987 . Most of these papers, however, remained

rather theoretical, and I have preferred to adopt herea more practical approach, with a number of exam-ples directly taken from the working documents usedin the lab, but never reproduced in final publications.All these examples correspond to high-grade rocks,notably granulites and eclogites. Not only becausethese rocks correspond to a field of research where Ihave been active for many years, but also becausethey may contain exceptionally well-preserved inclu-sions, much easier to study than in many low-graderocks. For the preservation of ‘good’, easy-to-studyinclusions, the most important factor is not the depthat which rocks have been buried, but the conditionswhich prevailed during the crystallization of therock-forming mineral assemblage and, above all, theway by which the rocks have been brought back tothe Earth’s surface.

( )J.L.R. TouretrLithos 55 2001 1–25 3

1.1. The first step: sample selection in the field

A fundamental aspect of fluid inclusion studiesrefers to the notion of scale transfer. In a given rock,each inclusion, typically few microns in size, consti-tutes an independent, isolated system, which basi-cally may give two sets of data: fluid compositionand molar volume. They will be used for the defini-tion of a fluid isochore, extrapolated to high P–Tconditions and compared to independent estimatesderived from coexisting minerals. The extrapolationmay be extreme, covering sometimes a temperaturerange in excess of 10008C, requiring a very rigorousworking procedure. Any studied inclusion must becarefully located within the host crystal, the hostcrystal within a thin section, the thin section withinthe hand specimen, and finally the hand specimenitself within the field exposure. Every step of thisscale transfer may involve significant errors, notablyon the representativity of the studied objects, but anyresearcher should be fully convinced of the fact thatthe real fluid inclusion study already begins in thefield. Whatever the sophistication of later investiga-tions, the quality of the final results will largelydepend on the attention given to field sampling. Theimportance of getting good samples is essential: withthe exception of rocks which are foreign to their

Žimmediate environment e.g. some eclogites lensesembedded in low-grade rocks, all xenoliths in vol-

.canic rocks , any sample intended for fluid inclusionstudies requires a very detailed study of the outcropor exposure. Not only all different lithologies shouldbe carefully recorded, but also all kind of microstruc-tural information, corresponding to ductile as well asto brittle deformation. For instance, in relativelymassive rocks, trails of late inclusions are oftenparallel to major joints or open fracture directions. Inprinciple, the sampling should be done by the personwho will make the investigation, as he will pay muchmore attention to the type of microstructural datathat might be relevant for the final interpretation.With some experience, any fluid inclusion studentwill immediately spot which mineral has the bestchances of containing good inclusions, and he willselect the samples accordingly: the color of the

Žquartz, for instance, can be very helpful not too.milky, but also not too clear . I know some cases

where resampling, specifically intended for fluid in-

clusion studies, has resulted in an order of magnitudeincrease in the size and number of workable inclu-sions.

Each case must be approached independently,according to local conditions and settings; there are,however, a number of basic rules, which are easy tofollow, like the following:

v In metamorphic rocks, fluids are present eitherŽ .in veins segregations , typically made of a dominantŽ .mineral mostly quartz or calcite , or in rock-forming

minerals within the massive rock. There is a distinctchange in the relative importance of both modes ofoccurrence with increasing metamorphic grade: inlow-grade rocks, segregations are dominant, whereasthe opposite tends to be true in high-grade rocks.Whenever possible, both sites should be sampled andprecisely compared. This requires a very precisestudy of the geometry and the mineralogy of theveins, which can change drastically over short dis-tances. If some veins contain idiomorphic crystals

Ž .growing in open cavities alpine-type veins , thesemust evidently require special attention. They are thebest place to identify primary or pseudo-secondaryinclusions, formed at the time of the opening of thevein. But then the idiomorphic crystals themselves,as well as the succession of different mineral phases,must be investigated with the greatest care. In some

Žfamous examples from the Western Alps e.g..Camperio, Wagner et al., 1972 , veins have remained

active over a wide range of P–T conditions, withnotably sudden pressure drops corresponding to in-cremental openings of the fissure. Each opening maycorrespond to a renewed fluid influx, leading to a

Žgreat variety of phenomena mineral growth or disso-.lution, ‘phantoms’ or ‘faden’ structures , which need

to be fully understood before starting the fluid inclu-sion study.

v The study of large veins is a very specializedtask, more related to ore geology than to metamor-phic petrology. Monomineralic veins occur oftenalong major fault or shear zones, witnessing thepassage of fluids issued from a very distant source.In general, they are not or only in partial equilibriumwith the immediate surrounding, with evidence ofretrogression or growth of new, low-temperature

Ž .mineral phases carbonates, zeolites, etc. . This intro-duces almost always a great complexity, with a greatnumber of different fluid pulses, which may be very


variable over short distances. The study of individualinclusions in this type of environment might be verydeceiving. Only techniques that can provide a bulk

Žsignature stable isotopes, crush-leach analyses of.relatively large samples may give some results, with

always the risk of mixing up fluids of very differentŽage and origin. Smaller veins as an order of magni-

.tude, of centimetric to decimetric scale are far moreinteresting, notably when they contain the same min-eral assemblage as the surrounding rocks: andalusite-or sillimanite-bearing segregations in metapelitesmight be taken as a typical example. In this case, thesegregation belongs to the metamorphic assemblage,and it require to be studied — and sampled — withthe same care as the rest of the outcrop. Note that, asa rule, relatively large veins are rather deceiving,especially when filled with conspicuously milky

Ž .minerals quartz . This color is precisely given bymillions of inclusions, too small and too abundant tobe correctly studied. Relatively clear quartz is moreinteresting but if the vein is massive the fluid evolu-

Žtion within the vein itself typically from the edge.towards the center of the vein might be difficult to

trace. Experience shows that the best veins are rela-Ž .tively small cm size . These can easily be compared

to the surrounding rock, especially when they con-Žtain, besides the dominant mineral in most cases,

.quartz , some mineral phases indicative of givenŽ .P–T conditions Al-silicates, zeolites, etc . The study

Žof larger veins may be rewarding best example todate remains the systematic study of Alpine clefts,

.Mullis et al., 1994 , but it may take years to get aŽclear picture of overall results almost a lifetime in

the case of the famous cleft of Camperio, Tessiner.Alps, Wagner et al., 1972 .

v Even if they contain, in general, less spectacularinclusions, the study of massive rocks might befinally less complicated — at least less time-con-suming — than the study of veins, only for the

Žsmaller size of the reference system typically: hand.specimen . Similar rules of sample selection can be

Ž .applied notably avoiding milky crystals , as soon asŽthe rock texture is coarse enough average crystal

.size, about 0.5 to 1 cm . Besides all kinds of possibleŽmicrostructures foliation, schistosity, microfolding,

.jointing special attention should be given to anyevidence of retrogradation. There is a rather generalrule that the latest events in terms of metamorphic

assemblage, notably a partial retrogression of peakmetamorphic minerals, will correspond to the great-est number of inclusions. In many granulites, forinstance, metamorphosed at more than 700–8008C,most visible inclusions must have been formed be-low about 3008C. Note, however, that this does notsystematically indicate, contrary to some statementsŽ .Lamb et al., 1987 , that the fluid contained in theseinclusions has been introduced from outside, inde-pendently from the granulite metamorphism. It mayalso be that these ‘late’ inclusions correspond to thelocal reequilibration of fluids already present for along time in the rock system, sometimes even since

Ž .the premetamorphic stage Vry and Brown, 1991 .I will end these general considerations by stress-

ing the importance of selecting key exposures, stud-Ž .ied and sampled carefully in great detail for all

lithologies. This represents a somewhat different ap-proach than for ‘classical’ metamorphic petrology,which normally addresses to the reconstruction ofP–T conditions, and therefore concentrating on

Ž .‘good’ garnet-bearing! mineral assemblages. Mostmetamorphic petrologists finding a thick quartzitecontaining a small layer of garnet or staurolite-bearing metapelite in the field will tend to concen-trate on and, in many cases, only sample the garnet-bearing unit. For fluid studies, both protoliths areequally important, and they should be sampled withthe same care and attention. As a working rule, acomplete study of an exposure, which will result inthe selection of up to about 10 samples, should

Ž .require at least a complete day of field work,maybe more if some elaborate sampling methodŽ .drilling is required or if lithologies are very vari-able. Always remember to carefully orientate thesample: the geometrical information contained in theorientation of the ‘fluid inclusion plane’ may be

Ž .extremely important Lespinasse and Pecher, 1993 .´It is extremely frustrating to discover at a late stageof the investigation a prominent direction whoseorientation cannot be defined because of the lack ofa measurement which would have taken few secondsat the time of the field sampling. Note, however, thatif the fluid population appears to be interestingenough, these 10 samples may represent months ofwork in the lab, with thousands of dedicated

Ž .measurements microthermometry , and conclusionswhich can be extrapolated to a regional scale.


1.2. Back to basics: the fundamental principle offluid inclusion studies

Compared to any rock-forming minerals, fluidŽinclusions pose a number of specific problems small

size, variability, possibility of evolution after initial.formation , but they present also a decisive advan-

tage: they give not only the chemical composition ofthe trapped fluid system, but also its molar volume.Both sets of information are essentially different, butcomplementary: the composition indicates if a fluidis in equilibrium with a given mineral assemblage ornot. Then, if this condition is fulfilled, the molarvolume will specify the P–T conditions at whichthis equilibrium has been reached. The interpretationrelies on a number of assumptions, all related to thelack of variations of a number of factors between theP–T conditions at which a set of minerals hasequilibrated and the present-day situation, at whichthe inclusion composition and molar volume aremeasured: no variation of the volume of the cavity,no leakage, no reaction within the fluid system. Inpractice, all these hypotheses rely on the ‘isochoric’behavior of the fluid inclusion system, expressed bythe familiar P–T box intersection criterion: the fluidisochore should pass through the P–T field defined

Ž .by the rock mineral assemblage Fig. 1 . It is evi-dent, however, that this condition is important, butneither necessary nor sufficient. The intersection,often rather loosely defined because of possible er-

Žrors in P–T estimates if the P–T box is bigenough, it will be intersected by almost any iso-

.chore! , may be coincidental. This may notably hap-pen when a fluid is introduced from outside, withouta notable influence on the mineral assemblage. Alter-natively, the fluid might well be trapped along a‘good’ isochore, but outside the P–T box of mineralequilibration. In total, the ‘intersection condition’must only be taken as a first, important indication,which must be complemented by a number of other

Ž .arguments Touret, 1987 .

( ) ( )1.3. Not one P–T box intersection , but at leastthree conditions

When an inclusion is hermetically sealed, it con-tains a given mass of a fluid, under any physical

Ž .state liquid, solid, vapor, supercritical fluid . If theinclusion volume remains constant, then pressure

within the inclusion will be related to the tempera-ture by the fluid isochore, in first approximation a

Ž .straight line in the P–T space. Fig. 1 .If inclusions are formed at the time of the crystal-

lization of a given mineral assemblage, if the fluidtrapped in the inclusion is representative of the freefluid phase existing at these P–T conditions, and ifno perturbation has later occurred, then the fluidisochore must correspond to the P–T conditionsdefined by the mineral assemblage; all these condi-tions are required for the theoretical justification ofthe intersection criterion. It would be evidently pos-sible to refine this approach by solving the isochoreequation, namely by knowing at which precise pointof the isochore the inclusion has been formed. Thereare indeed some possibilities, such as intersecting of

Ž .the isochores by another univariant PsF T rela-Ž .tion e.g. geothermal gradient , crossing different

isochores, finding in the fluid system ‘internal’ fluidŽthermometer, see a more elaborate discussion in

.Touret, 1981 . But all these procedures are so loadedwith potential uncertainties or problems that theyhave been very rarely used and, as a matter of fact,they tend to disappear from the current fluid inclu-sion literature. The P–T box isochore intersectionremains the only condition, too often directly usedfor the interpretation of fluid inclusion data withoutfurther justification.

It is, however, evident that the inverse problem,namely inferring from an intersecting isochore thatthe inclusion fluid represents the metamorphic freefluid phase, is not straightforward: the intersectioncan be coincidental, the inclusion may have beenformed on the isochore, but outside of the P–T box,etc. Evaluating the reliability of the fluid containedin an inclusion is always a difficult problem, whichrequires much thinking and an elaborate discussion.

The best probability to identify fluids in equilib-rium with a given mineral assemblage that I will call‘synmetamorphic fluids’ require not only one condi-tion, namely the isochorerP–T box intersection,but, at least, two more as follows:

Ž .i Inclusions should be primary with respect tothis mineral assemblage. This should notably be thecase for the minerals which have been used for the

Ždefinition of metamorphic P–T conditions best ex-.ample: garnet . Note, however, that secondary inclu-

sions may also be used, if — but it requires demon-


Ž .Fig. 1. Isochoric principle of fluid inclusion interpretation, with corresponding ‘model histograms’ Insert . Top: One fluid generation, withŽ .possible post-trapping changes during the retromorphic evolution. Tr. in.: Initial trapping fluid isochore 1, in mineral G , Tr. f.: Final1

Ž X Y .closure of the inclusion 2, 2 , 2 , depending upon the P–T path, in mineral G . Hatched: Safety corridor, in which the inclusion will be2

preserved at any external P–T conditions. Bottom: Case of several fluid generations, trapped during the retrograde evolution. P–T paths:PIspseudo-isochoric, ITDs Isothermal decompression, IBCs Isobaric cooling.

stration! — microfracturing and fracture healing hasoccurred at constant pressure and temperature.

Ž .ii The fluid composition of the inclusion mustbe compatible with those predicted from the mineralassemblage. Any significant deviation, for instance,the fact that most inclusions in granulites are anhy-drous, whereas a finite H O pressure is imposed by2

the possible occurrence of some hydrous mineral

phases, should be interpreted in terms of isochoreŽcorrection see the discussion on the case of Central

.Kola granulites .The question whether post-trapping changes have

occurred after initial formation of the inclusion isobviously of fundamental importance. There is ageneral belief, distinctly expressed by some of the

Žgreatest names in the science of inclusions e.g.


.Roedder, 1981 , that ‘deep’ inclusions, formed athigh P–T conditions, will suffer more changes thaninclusions formed at shallower levels. In fact, asimple look at Fig. 1 shows that this is not the case,and that the driving force for post-trapping evolutionwill be only dependent on the relative trajectories ofthe fluid isochore, on one hand, and on the metamor-phic P–T path, on the other hand. This ‘drivingforce’ is directly expressed by the difference be-

Ž . Žtween fluid in the inclusion and solid outside of.the inclusion pressures. At any temperature, the first

one will be given by the isochore, the second one bythe metamorphic P–T path. If both are equal orsufficiently close, as in the ‘pseudo-isochoric’ P–T

Ž .path Fig. 1 , no pressure difference, no gradient forfluid leakage or volume changes. The inclusion,whatever its depth of formation, will remain per-fectly safe until the surface.

If differences between both trajectories are largeenough to induce pressure differences exceeding thestrength of the host mineral, the inclusion will ex-plode in the case of fluid overpressure, implode for

Ž .fluid underpressure Fig. 1 . This will result inmarked modification in the shape of the cavity, aswell as in the formation of a tridimensional setŽ .cluster of smaller inclusions. This is what we call‘transposed’ or ‘decrepitated’ inclusions. Since the

Ž . Žpioneering work of Lemmlein 1945 also Lemm-.lein and Kliya, 1952 , the typical shapes and pattern

of secondary inclusions after decrepitation are rela-tively well known, essentially from a great amount

Žof experimental work Naumov et al., 1966; Leroy,.1979, and many others . The group of R.B. Bodnar

at Virginia State University, USA, notably has sub-mitted artificial inclusions to realistic P–T trajecto-ries, and he has obtained shapes remarkably similar

Ž .to natural inclusions Vityk and Bodnar, 1995a,b .This experimental work has also documented a dif-ference between ‘exploded’ inclusions, star-like fromwhich are issued a number of aligned small inclu-sions, and ‘imploded’ ones, which show more acircular shape surrounded by a rim of minute cavitiesŽ .Boullier, 1999 . In this last case, the initial volumeof the cavity is grossly diminished, whereas it tendsunderstandably to increase in the case of explodedinclusions. The extreme limit of size reduction corre-

Žsponds to the ‘collapsed’ inclusions Touret and.Huizenga, 1999 , in which the inclusion cavity is

completely squeezed around one or several micro-crystals initially floating in the inclusion fluid. Thedegree of collapse is so great that it is obvious that,if these crystals had not been there, the cavity wouldsimply have disappeared without leaving any trace ofits former occurrence.

It would of course be extremely interesting toknow at which pressure difference the cavity willstart to evolve. In this case, if we could find in agiven sample inclusion formed before and after thedecrepitation event, we could precisely constrain the

Ž .shape of the retrograde P–T path Fig. 1 . Thisproblem has also been approached during experi-ments, mostly to recognize that so many factors are

Žinvolved nature of the fluid and host mineral, sizeand shape of the inclusions, absolute P–T condi-

.tions, etc. , that it is hopeless to expect quantitativeresults. The definition of about 1–2 kb for the safetycorridor of inclusion during retrogression, which hadbeen proposed on the basis of early experimentsŽ .notably by Leroy, 1979 , can only be taken as avery rough order of magnitude. Some CO inclu-2

sions in mantle xenoliths, apparently well protectedwithin the enclosing basalt, could withstand internalfluid overpressures in excess of 10 kb!

If a careful observation of the inclusion shapemay indeed provide a number of interesting data, itis also very important to realize that perturbations,whatever they are, may also occur without leavingany visible evidence. Inclusions may recover a per-fect equilibrium shape, but the fluid isochore willpass way out of the relevant P–T box. Isochorepassing well below the P–T box are relatively fre-quent, a direct evidence of important fluid leakage.But isochores passing above the box have also beenfound. In this case, the volume of the inclusionsmust have decreased, without loss of the fluid con-tent. The process must not be spontaneous, it re-quires some energy which can only come from localdeformation. A spectacular case corresponds to the‘superdense’ inclusions observed in the mobilisate of

Ž .some migmatites van den Kerkhof and Olsen, 1990 .

1.4. Organization of the inÕestigation: obserÕation,measurement, interpretation

Any fluid inclusion investigation involves threesuccessive steps, which must be carefully evaluated


and planned: observation, measurements and inter-pretation. Firstly, the obserÕation, with conventionalpetrographical microscope, should identify a limitednumber of inclusions, to be analyzed by specificallyadapted techniques, from the thousands of inclusionspresent in any sample. The second step correspondsto measurements, involving always microthermome-try, eventually completed by direct chemical analy-

Ž .ses mostly Raman . Finally, the interpretation isdone by comparing few selected inclusion data, as-sumed to be representative of the whole inclusionpopulation, with independent mineral P–T esti-mates. All these steps are interrelated, with constantfeedback and mutual improvement: the first observedinclusion might lead to the beginning of an interpre-tation, which needs to be carefully checked for fur-ther confirmation. It is essential to separate clearlythese three steps, as well as to define precisely eachobjective. The importance of this organization isexemplified by the number of inclusions concerned,decreasing by several orders of magnitude at each

Ž .step. From the thousands if not millions of inclu-Žsions occurring in any sample, only few tens some-

.times few hundreds can possibly be measured, andŽthe final interpretation will only rely on few typi-

.cally less than five single inclusions. This enormousfocussing of the objectives needs to be done in asomewhat rational way, leaving, however, enoughflexibility to cope with the unexpected. This workingprocedure –at least the one used in our group — willbe illustrated by worked examples, showing workingdocuments which, normally, are never published in

Žscientific papers often to my regret, as many contain.a wealth of interesting information! . But, before

going into details, it is necessary to stress the impor-tance of the sample preparation. Much time andeffort will be done in vain, if the object put under themicroscope does not meet a minimum of qualityrequirements.

1.5. Some introductory remarks: the importance of( )the quality of the Fluid Inclusion Section FIS

After a careful selection of the sample in the field,the importance of the next step, namely the fabrica-

Žtion of the standard FIS or Fluid Inclusion Plate,.FIP , should not been underestimated. A FIS is a

double polished loose plate, of the size of normalŽ .thin section approximately 3=5cm , but signifi-

cantly thicker. The quality of the polishing, the idealŽ .thickness typically between 90 and 120 mm , de-

pending on the transparency of the host crystal andthe inclusion size and abundance, are very importantfactors. Every lab has its own technique, and everyworker has experienced how frustrating can be theoccurrence of too many scratches, loose grains ofpolishing materials, or, even worse, artifact inclu-

Žsions caused, notably, by the binding media fluid.immiscibility in epoxy can be very spectacular! .

These points, however, are so obvious that they donot deserve any further comments. More important isto remember a number of important recommenda-tions as follows.

v Before making the FIS, a rather long, expensiveoperation, it is always rewarding to observe carefullya normal thin section at high microscope magnifica-tion. Polished thin sections are better than covered

Žones, but all can be used. Small inclusions less than.5 mm are preserved, and if no interesting inclusion

is seen, there is not much hope that a specificpreparation will do better.

v Pay the greatest attention to the mineral textureand composition, at the scale of the thin section:Whenever possible, mineral analyses used for infer-ring P–T data should be done on the preparation

Žused for fluid inclusion studies alternatively, on a.polished thin section very close from the FIS .

v For microthermometric measurements, the sec-tion will be broken into small pieces and, in manycases, the rock-forming minerals will be studiedalmost grain by grain. Is it essential to keep a goodrecord of the whole FIS, in order to replace thebroken fragments or isolated mineral grains in theiroriginal position. We use normally high-quality pho-tographs of the whole section, approximately en-larged to A4 format. This is rather expensive and, inmany cases, cheaper alternatives may give very goodresults. For instance, when the mineral grains are

Žcoarse enough and sufficiently transparent or con-.trasted , just put directly the FIS on a magnifying

copy machine and enlarge by successive steps, un-less the required format is reached. Modern tech-nique offers a number of other possibilities, such asoptical scanner, computer images by digital cameras,etc. In any case, the final image, as the one illus-


Ž . ŽFig. 2. Example of a fluid inclusion section FIS , which first must be studied globally during the observation stage conventional.petrographic microscope , then broken in small pieces for microthermometric measurements. All inclusions identified in the broken pieces,

Ž .even if very small, must be eventually replaced into their original context. B emplacement of the drawing of Fig. 4. Dabie Chan eclogite,Central China, Sample 94M44, Bin Fu.

trated in Fig. 2, can be used as a base for actuallymapping the distribution of fluid inclusions, conve-niently grouped and labeled in terms of fluid types.The procedure that we use in our group is describedin some more details below.

2. Observation with conventional microscope

The objective of the microscope observation, be-fore any measurement, is more than a simple cursoryidentification of the inclusions. It should aim at thedefinition of fluid types and, above all, at a relativechronology of the different inclusion generations. Inthe discussion of the fundamental principles of inclu-sion interpretation, I have stressed the importance ofthis chronology. It will be sufficient here to repeatthat the only clue is observation, and that any inter-pretation will be severely limited if no reliable resulthas been obtained for this essential part of the study.It is easy to find in the literature a number ofstatements where authors claim that ‘they did theirbest, but that finding out this chronology was impos-sible’. May be their samples were not adapted to thistype of investigation and, if so, why did they choose

them? But I have also the impression that, in manycases, not enough time and efforts have been spent.A partial chronology is always possible and I couldmention a number of cases where, after a first,hopelessly complicated and negative impression, alogical organization has progressively emerged,sometimes after days of careful, painstaking workbehind the microscope. Good observation requirestime and effort. It will never be done in vain,provided that it follows some strict rules for the

Ž .evaluation and the reproduction drawings of theresults.

2.1. Definition of fluid types. The importance ofcorrect drawings

A fluid type is defined as a set of inclusionsŽ . Žhaving roughly the same chemical composition e.g.

.low-salinity aqueous, CO -rich, etc. , eventually2Žvariable density in nature, a sufficiently great num-

.ber of inclusions will never have the same density! ,approximately trapped at the same time. The term‘approximate’ indicates indeed that the precise tim-ing of the inclusion formation is rarely known. Itmay also covers a number of different issues: it may


be that the fluid type has occurred only briefly as afree volatile phase at the time of initial inclusionformation, but that a number of inclusions havesuffered post-trapping changes. Alternatively, several

fluid pulses of the same composition have occurred,but at variable P–T conditions. In any case, thenotion of timing is essential, and it requires to beconsidered with the greatest care.

Ž . Ž .Fig. 3. Camera lucida drawing of ‘cluster’ top and ‘trail-bound’ inclusions bottom , showing evidence of post-trapping perturbationsŽ .Swanenberg, 1980, all samples from RogalandrWest Agder, Norway . Top left: Cluster of biphase H O–N inclusions, with variable2 2

phase ratios. Upper right: Decrepitation cluster composed of monophase CO inclusions. Bottom left: Poorly defined trails composed of2Ž . Ž . Ž .rearranged carbonic inclusions. A Transposition of a pre-existing N–S trail by decoration of subgrain boundaries S , B Array of

.decrepitation clusters . Bottom right: Advanced transposition of a pre-existing NW–SE oriented trail into short, subsidiary N–S trails.


Compared to rock-forming minerals, fluid inclu-sions are short-lived objects. Their formation is in-stantaneous, or at least very short at geological timescale. Since the early days of fluid inclusion studies,a basic distinction exists between primary and sec-ondary inclusions, according to criteria abundantly

Ž .discussed in the literature notably Roedder, 1984 . Iwill not insist on these criteria, but recall that, inmassive rocks, the basic distinction is between iso-

Žlated, clustered groups of few, typically 10–20.neighboring inclusions, in the volume of the FIS

Ž .and trail-bound inclusions Fig. 3 . Trail-bound in-clusions, occurring in the surface of a former microc-racks, are certainly secondary. Both terms are practi-cally equivalent, with the further indication that trailscan conveniently be divided between intra- and inter-

Ž .grain trails Touret, 1981 , an indispensable step forŽstarting the chronology of secondary inclusions see

.below . Isolated inclusions are, in principle, primary,but this term should only be applied if additional

Žcriteria are available direct relation with the growth.features of the host minerals . Otherwise, better keepŽ .the pure descriptive name isolated , eventually a

more neutral term, if you want to stress the timeŽ .difference with secondary inclusions e.g. early .

Clusters may have quite different origins, eitherŽneighboring isolated cavities in this case, no differ-.ence with isolated inclusions , or, very often, inclu-

sions formed by transposition of a former, largercavity. As it is impossible to distinguish betweenthese two cases without the knowledge of the fluid

Ž .density, the neutral term cluster should be keptthroughout the phase of observation, and all cate-

Ž .gories isolated, cluster, trail-bound strictly main-tained for the rest of the study. Not much interest toidentify carefully these different categories, if laterall are mixed during microthermometric measure-ments.

The working method for a good observationshould meet a certain number of elementary require-ments and follow some basic rules as follows.

v A major objective is to select a limited numberof inclusions, later analyzed by microthermometry or

Žany other nondestructive technique Raman, FTIR,.Synchroton radiation, etc. ; selected inclusions must

be found back at any time, either for a properlocation within the analytical instrument, or after, forcontrol or further investigation. Not a trivial problem

Ž .for small objects few mm in size evenly distributedin the volume of the FIS.

v Contemporaneous fluid types, which by defini-tion have a different composition, can only occurtogether if they correspond to immiscible fluids. Thisimportant conclusion can be tested, not only from thephase relations of the different fluid systems, butalso morphologically, from the occurrence of neigh-boring domains containing preferentially one fluid.Note, however, that alternative hypotheses may alsooccur, notably fluid incoming from different sourcesand along different pathways, as well as a localcontrol of the fluid composition by fluidrmineralinteraction. In most cases, careful observation allowsto distinguish between these different possibilities, asillustrated below for the case of Dabie Shan eclogiteŽ .Fig. 6 .

v Trail-bound inclusions are as a rule far moreabundant that early, isolated or clustered cavities.There are also those for which the relative chronol-ogy is the most easy to decipher, from intersectioncriteria and structural orientation of the microcracks.They must be investigated first, then ignored, allow-ing the observer to concentrate on older inclusions.

In conclusion, the identification of the differentfluid types and of their relative chronology is a keyaspect of any fluid inclusion studies. It requiresmuch time and effort, and it must be supported bydetailed, accurate illustrations. Partial conclusions

Žmight be rather subjective e.g. trail intersection.criteria , requiring constant crosschecking, reevalua-

tion and discussion.Figs. 4–6 illustrate the working method that we

Žuse in our group, involving the notion of GSI see.below . These cases have been selected, not only to

give some typical examples of the distribution offluid inclusions in metamorphic rocks, but also toshow the type of conclusions which can be derivedfrom a careful observation of these drawings: trans-position of an early trail along a new direction in thecase of Fig. 4, mode of crack healing in Fig. 5, andvariation of fluid composition according to the na-ture of host mineral in Fig. 6.

Let us remark that, for evaluating the distributionof inclusions in three-dimensional space, drawingsare much better than photographs, sharp only in avery narrow focussing range at the high magnifica-tion normally imposed by the small inclusion size


Ž . ŽFig. 4. Drawing Working document of a GSI, following the procedure used at the Fluid Lab., VU Amsterdam. Dabie Shan eclogite,.Sample Bin Fu, Location B, Fig. 2 . The inclusions are disposed along a former trail oriented X–Y, strongly transposed along the

Z-direction. All inclusions are reequilibrated during this transposition. Near each measured inclusion, the number indicates the homogeniza-Ž . Ž .tion temperature T , always to liquid in 8C . Each of these inclusions is separately drawn on a working sheet for microthermometrich

Ž .results, with the indication of all measurements initial and final melting, homogenization and the volume ratio of different phasesŽ .liquidrvapor at room temperature.


Ž .Fig. 5. Another example of GSI, with indication of the microthermometric T measurements. CO inclusions in a plane of microfractureh 2Ž . Ž .trail-bound, secondary inclusions . Near each measured inclusion, the number indicates the homogenization temperature T , always toh

Ž . Ž . 3liquid in 8C . For a pure fluid, T indicates immediately the fluid density. Iso-density lines thin contours are labeled in grcm . They showh

some high-density ‘highlands’ within a lower density environment. This pattern strongly suggests that crack healing has occurred in anŽambience of fluid density decrease from a given isochore, pressure decrease or temperature increase, evidently very unlikely in the present

. Ž .case . Quartz vein in gold deposit, Sierra Leone Barrie and Touret, 1999 .

Ž .typically =25 or =50 objectives . Then, even fornonborn artists, exact drawings are easily made bythe ‘grid’ procedure, namely replacing in one micro-

Ž .scope ocular the reticule by a grid we use 10=10 ,identical to the one printed on the drawing sheet.Any serious microscope dealer should be able toprovide this type of very simple grid. If not, checkby colleagues from biology or micropaleontology, orfrom any discipline using counting procedures of

Ž .small particles cells, microfossils in a fluid medium.These drawings represent a set of inclusions seen

within a single field of microscope. They require tobe done as long as the distribution of the differentfluid type has not been properly understood, a taskwhich can be rather long for a complex sample. Lesstime, however, that it may look at first sight toinexperienced people. After some training, most

Ždrawings can be completed in 1 or 2 hours includ-.ing the one represented in Fig. 4 , not much com-

pared to the time which will be spent for microther-mometric measurements. Any observer will also ex-perience that such a detailed study, rarely done formost petrological investigations, is very rewarding,not only for fluid inclusions, but also for the study ofmany other small features, such as accessory miner-als or very fine microstructures. For those, the FISprovides much better observation conditions than anormal thin section, e.g. for the appreciation of theidiomorphic shape of small mineral phases, and itshould be in very common use in all branches of

Žpetrology notably in structural petrology, so easierto appreciate the orientations when you can observe

.in three dimensions! .

2.2. Notion of Groups of Synchronous Inclusions( )GSI . Fluid chronology

Besides the knowledge of the actual inclusiondistribution, drawings are the only way to reach the

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Ž .Fig. 6. Final presentation of fluid inclusion data in quartz crystals from the Dabie Shan eclogite, China Sample 94M55, Bin Fu, same rock type than in Fig. 2 . Left: GeneralŽ . Ž .view of part of the FIS, characterized by the trace of a prominent shear zone black with white dots, 6 . All studied inclusions occur in quartz 5, heavy contours , mostly

Ž . Ž . Ž . Ž . Ž .enclosed in the various rock-forming minerals: Garnet 1 , Omphacite 2 , Amphibole 3 , and Epidote 4 . Besides the shear zone 6 , the rock show also a number of oblique,Ž . Ž . Ž . Žparallel lines of fractures 7 . In all studied quartz crystals, indication of the dominant fluid type: High-salinity brine W1 , low-salinity aqueous W2 , Gaseous pure

. Ž . Ž . Ž . Ž .CO sG2 , mixed aqueous–gaseous GW . Right: Actual distribution of inclusions GIS in quartz enclosed within different mineral hosts: Garnet a , Omphacite b ,2Ž . Ž . Ž . Ž . Ž . Ž .Amphibole c , Epidote d . Numbers: homogenisation temperatures in a CO inclusionssG2 , final melting temperatures in b, c and d W1 fluids . G2 fluids occur only in2

Ž .domains within or very close to the shear zone. This, together with the fact that most G2 inclusions are trail-bound a , suggests a late CO introduction, from an external source.2Ž . Ž .The same conclusion yields for W2 low salinity aqueous fluids, mostly disposed along trails roughly parallel to the lines of fractures 7 . W1 fluids, on the other hand, are more

Ž . Ž . Žclustered, as seen in b, c and d . Their composition salinity is more or less constant within a given mineral host, but varies strongly between different minerals T -y258C inm.omphacite, about y158C in amphibole or epidote, would be less than y108C in garnet . This suggests that W1 fluid composition has been controlled by local fluid–mineral

Ž .interaction, at the time of the crystallization of the enclosing host Fu et al., submitted .


notion of GSI, a most important notion in any rock,in general, and in metamorphic rocks, in particular.

ŽA GSI corresponds to a limited number typically.between 10 and 20 of inclusions formed at the same

time, which will serve as the test population toevaluate the homogeneity of microthermometric dataŽ .notably T . The most obvious case for synchronoush

inclusions correspond to a secondary trail, and in thisrespect the notion of GSI, introduced in a recent

Ž .publication Fonarev et al., 1998 , correspondsŽ .closely to FIP Fluid Inclusion Plane or Fluid Inclu-

Ž . Žsion Assemblage FIA of American authors Gold-.stein and Reynolds, 1994 . Semantics is not very

important, and any acronym can be used. I onlyprefer the name GSI, because the bidimensionalsurface of an inclusion trail is often more complexthan a plane, also because it emphasizes the impor-tance of timing in fluid inclusion making. A typicalexample of GSI is shown in Fig. 5. It corresponds toa plane of secondary inclusions in a quartz vein from

Ža gold deposit in Sierra Leone Barrie and Touret,.1999 .

Once the GSI have been clearly identified, estab-lishing the chronology of trapping is just a questionof patient, critical observation. The first distinction is

Ž .evidently between isolated primary and secondaryŽ .trail-bound inclusions. Further refinement requiresthe following.

v For the primary inclusions, a detailed analysis ofŽ .the mineral textures: old deformed grains, recrystal-

lized subgrains, etc. Even if inclusions can be foundŽin a number of other host minerals garnet, pyroxene,

.feldspars, etc. , quartz remains by far the mineral inwhich inclusions are the most abundant. But it is alsothe mineral which suffers most easily deformationandror recrystallization, on a wide range of P–Tconditions. It must consequently be studied with thegreatest attention, and any technique able to revealits internal structure, notably cathodoluminescence,

Ž .can be extremely helpful Behr, 1989 . Cathodolumi-nescence for quartz is, however, much more difficultthan for other minerals, notably carbonates, but itsuse should become almost mandatory in a near fu-ture.

v In high-grade rocks, we do systematically searchŽ .fluid inclusions in quartz included in other rock-forming minerals, notably garnet. These quartz crys-tals have commonly escaped any deformation, and

they may contain few, but really primary inclusions.ŽFig. 6 shows a spectacular case eclogites from

.Dabie Shan, China , but there are a number of otherŽexamples in the literature Blom, 1988; Vry and

.Brown, 1991 which shows the interest of this typeof research.

v Ž .For secondary trail-bound inclusions, findingout the chronology relies essentially on intersectioncriteria of planar structures, with the additional com-plexity that these structure are not continuous, butformed by a regular grid of more or less identical

Ž .inclusions shape and size . I have illustrated inŽ .earlier publications Touret, 1977, 1981 some cases

which are frequently observed: replacement of aninclusion fluid by another while preserving the shape,partial fluid mixtures, disappearance of few inclu-sions within the trail, etc. Possibilities are almostunlimited and, in many cases, results are apparentlycontradictory: Fluid A, apparently younger than B ina given mineral grain, seems to be older in another.It is our experience, however, that after careful ob-servation, these uncertainties progressively decrease,until a firm conclusion can finally be reached. Everyresearcher has experienced an initial feeling of ap-parent chaos, which becomes more and more orga-nized during the course of the observation and mi-crothermometric measurements.

Much work is presently done by the Nancy groupŽLespinasse, 1984; Lespinasse and Cathelineau,

.1990 on the microstructural study of inclusion planeorientation. Measurements are easy, either by elabo-

Ž .rate techniques image analysis , or more simply, byestimating under the microscope the orientation and

.the dip of the plane by changing the focussing level .Measurements of orientation are very easy for sub-vertical trails, but then the complete study requires aset of three orthogonal FIS. The universal stage,almost abandoned by the petrologists, could findhere a new field of application. It is again an impor-tant new field of research, too neglected by structuralgeologists.

In conclusion, the rigorous methodology de-scribed above, which may look tedious, time con-suming, is absolutely necessary to get a properpreparation of the microthermometry measurements.It allows, in most cases, to identify a few major fluidtypes, typically between 2 and 5, which need to be

Ž .correctly labeled. Neutral names Type I, II, III, etc.


Žmay be misleading for instance, successive numbersimplies almost instinctively some chronological or-

.der . Moreover, they make the reading of any paperrather difficult, notably because they do not give anyindication on the composition. Conversely, a namewhich tends to give the complete composition be-

Žcomes rapidly impossible H O–NaCl–CO –CH2 2 4.inclusions, etc. . Some recommendations have been

Ž .proposed in the literature Boiron et al., 1992 , buteveryone is free to choose the best label adapted tohis case, provided that it is clear, informative and notmisleading. We tend in Amsterdam to privilege let-

Ž . Žters 2 maximum for the first definition Ws.aqueous, G for gas, M for mixed aqueous gazeous ,

Ž .eventually supplemented by indices W1, etc. forsubtypes, with, in most cases, a chronological conno-

Ž .tation W1 older than W2, etc. . An example of thislabeling is given in Fig. 6.

(3. Measurements microthermometry, eventually)completed by Raman analyses

3.1. Some general considerations

Once the fluid types have been identified, a ‘rea-sonable’ number of inclusions must be selected, inorder to measure the fluid properties: compositionand molar volume. As for any fluid inclusion studies,this is done by measuring the temperature of phase

Ž .transition, final melting T , eventually supple-m

mented by Raman analyses, for the composition,Ž . Ž .homogenisation T for the density molar volumes .h

Then, from these measurements, which may amountto hundreds of inclusions in a single sample, fewŽ .typically less than 5 ‘representative’ isochores —in fact, inclusions — are selected, which can be

Žcompared to mineral P–T data Fig. 1 for the the-.ory, 9 for a practical example . This last phase of

interpretation is, in fact, the justification of the wholestudy: only then it is possible to verify if the fluidstrapped in inclusions are compatible with the hostmineral assemblage and to what extent they havebeen modified by post-trapping changes. In otherwords, it is only at the very end of the study if wefinally know if all the efforts made were reallyworthwhile. Note, however, that all steps of the

Ž .study observation, measurement, interpretation ,

even if clearly defined, must be done almost simulta-neously: a few measurements are normally neededfor the characterization of the different fluid types, ifonly to distinguish between gaseous and empty in-clusions. It is also advisable to start some kind ofinterpretation, as soon as some microthermometricparameters are known, just to have an idea how fluidinclusions relate to the rest of the rock history. Thesepreliminary attempts, however, must be done withgreat care and critics, in order to avoid any biasedinterpretation. In a rock where many inclusions arepresent, it is too easy to select only those which give‘good’ results, namely isochores, which for one rea-son or another, just pass through the relevant mineralP–T box. Another danger, especially for beginners,is to sit down behind the stage and accumulate data,hoping that some light will come some day from thedark. In almost all cases, the night becomes unfortu-nately even darker, and then the most frantic effortsare done, just to ‘recuperate’ all the hours spent invain. Accumulating microthermometric data is anactive part of the whole research, not a passivereading of numbers on a digital screen while listen-ing to a walkman or a compact disc.

Measured inclusions must be precisely identifiedŽ .on the GSI drawings e.g. Figs. 4 and 5 and, in

principle, a careful drawing of each measured inclu-sion must be done on a separate sheet, before startingthe microthermometric run. This procedure is abso-

Ž .lutely necessary for two-phase LrV inclusions atroom temperature. For inclusions which homogenizeto liquid, pressure increases very rapidly after ho-mogenization. This may cause irreversible changesŽ .leakage, volume increase of the cavity , which areindicated by an increase of the volume of the gasbubble when returning to initial conditions. But anychange can only be appreciated if the relative vol-ume of the different phases had been carefully evalu-ated before heating. For monophase inclusions, thisrather tedious procedure might look somewhat un-necessary. But it can be very helpful, especiallywhen starting measurements on a new fluid type.Many phase transition phenomena are not obviousŽ .e.g. sublimation , and there are so many possibili-

Žties clathrate melting, complicated phase transition.in purely gaseous inclusions, etc. that it is difficult

to predict what will happen, sometimes for fewŽseconds e.g. melting and homogenisation in dense


.CO inclusions . ‘False’ phase transitions may also2

be a major problem: during freezing, the stage mayshow minor movements, which need to be constantlycorrected by playing with the fine focussing of themicroscope andror the X–Y movement of the stage.This may induce optical effects, notably some brightpoints in subspherical cavities, which may be mis-taken for gas bubbles. Experience is of course ofmajor importance, as it the need to only retain clear,repetitive phenomena. But the best way to gain thisexperience is to make precise drawings of any sup-posed phenomenon at the time of its appearance.This will be greatly facilitated if a good sketch of theinclusion contour is already available. Recordingmicrothermometric runs on videotape may also behelpful, but the optical resolution of most recorder ismuch less than the eye, and accumulating video’sbecome rapidly extremely boring. Again, quickdrawings are much faster, more precise and, finally,far more informative.

3.2. Representation of compositional data: composi-tional diagrams and histograms

The basic problem is clear: how to reduce thegreat variability of any inclusion population to fewrepresentative values. The notion of GSI is the first,most important starting point: if inclusions belongingto the same generation already show a great disper-sion, mixing many generations will only complicatethe situation. But some GSI may contain too fewmeasurable inclusions and, in most cases, it is neces-sary to compare GSI from different parts of theinvestigated samples. This can only be done by somestatistical treatment of the data, through some kind ofvariation diagrams or frequency histograms. Statis-tics are used in many disciplines, in a simple orelaborate form. Compared to neighboring specialitiesŽ .e.g. fission track , we can only realize that thestatistical treatment of fluid inclusion data is still inits infancy. Far too often, ‘representative’ values arefreely chosen, sometimes without giving the most

Želementary information number of measurements,.range of instrumental error, etc. . It is impossible to

discuss here this important problem at length, but Iwill briefly comment on the most frequently usedmodes of presentation: density histograms and com-positionrdensity diagrams.

3.3. Density histograms

Data from inclusions belonging to fluid types witha well-defined, relatively constant composition arecommonly presented in the form of density his-tograms, which for pure fluids are, in fact, T his-h

Ž .tograms. I have already shown Fig. 1 how simpleŽconsiderations on the histogram shape e.g. distortion

.towards high temperatures may detect some pertur-bations, helping to concentrate on the most signifi-cant inclusions. This approach can be developed by

Ž .the notion of ‘model’ histograms Touret, 1987which, roughly speaking, corresponds to the predic-tion of some histogram shape from a given hypothe-sis. An example is the modeling done by Darimont

Ž .and Coipel 1982 on necking-down processes, ac-cording to simple hypotheses on the temperature ofinclusion fragmentation. Others attempts have been

Ž .done for boiling fluids Touret, 1987 , and there isno doubt that this approach could be considerablydeveloped. In general, however, most papers do notattempt any kind of statistical treatment. Data areloosely accumulated in temperature intervals, and the‘representative’ value is freely chosen from a more

Ž .or less clear maximum peak on the histogram. Itmight be difficult to do it otherwise, but, at least,some basic information should always been given asfollows.

v Always indicate the total number of measure-ments. Even if it can almost never said that a givenhistogram is truly representative of the total numberof inclusion contained in the investigated sample,500 measurements give more confidence than 5 or10.

v A ‘peak’ should be defined by a sufficientlyhigh number of values, namely it must clearly differ-entiate from the background. This is normally donein a rather subjective way, which, in fact, determinesthe number of measurements to be done. Sometimes,a peak is clear after few 10s of measurement, inother occasions the number has to be much greater.The only way to feel it is to build up the histogramprogressively, when the measurements are beingdone.

v Ž .In most stages notably in the Linkam , mi-crothermometric results are expressed with a preci-sion of 0.18C. This is far to be the real precision ofthe measurements, and the data are collected within


temperature classes, broad or thin in function of theoverall variation range: about 18C for T , up to 5 orm

even 108C for T . As seen on Fig. 7, the size of theh

temperature class has a great influence on the shapeof the histogram. This parameter should be carefullyevaluated and discussed before selecting the bestmode of presentation.

Fig. 7. The importance of temperature classes for the shape of Th

histograms. Each histogram contains the same number of mea-Ž . Žsurements 50 , measured with the same precision results given.within 0.18C , but grouped in T classes from 0.258C in the top,

0.58C in the middle and 18C in the lower histogram. Arrows:Ž .indication of the selected values if any for further interpretation,

notably the selection of ‘representative isochores’.

v Finally, whatever the motivation, the selectedvalues should be clearly indicated on the histogram,

Ž .in the form of arrows above the histogram Fig. 7 .This is the only way by which the reader can rapidlyevaluate the degree of confidence that he can attachto this selected value.

Besides histograms, it might be very instructive tohave an idea of the spatial distribution of the densi-ties in a given histogram, notably within a plane ofsecondary inclusions. This can be done by indicatingT on the GSI drawings and, when some orderingh

appears, by drawing iso-density contours by interpo-lation of the measured values. An example is givenin Fig. 6. It shows that, in the plane of the GSI,relatively high-density inclusions correspond to some‘islands’ within a lower density environment. Such adensity change can either occur by decreasing pres-

Ž .sure at relatively constant temperature or by in-creasing temperature at constant pressure. Needlessto say, the first hypothesis is the most probable forhealing a microcrack.

When comparing different GSI, the number ofhistograms becomes rapidly a major problem: thedifferent histograms must be precisely compared,with, notably, the same temperature scale. But afigure with, let us say, more than 10 histograms ispractically unreadable. We prefer to show only therange of variation of each histogram, supplementedby a number of ‘boxes’ which refers to the fluidtypes occurring in the different samples. This proce-dure is illustrated in Fig. 8, which we will use for anexample of interpretation.

3.4. Compositional diagrams

When the composition of the different fluid typesmay vary, principles discussed above remain valid,but the situation is more complicated. The differentfluid types must be identified on a diagram composi-tionrdensity, in which, normally, the last variableŽ .density shows the greatest variation range.

This mode of representation is applicable to anyfluid system, but it simplifies in the case of binary

Žaqueous, referred to the H O–NaCl system wt.%2.NaCl equivalent salinities in aqueous fluids . X–V

corresponds then to a T rT diagram, widely usedm h

for the representation of aqueous fluids. Each fluid


ŽFig. 8. T data for CO -rich inclusions in granulites from the Central Kola peninsula, presented along the concept of GSI Fonarev et al.,h 2. Ž .1998 . Each horizontal line represents a GSI, in one of the nine studied samples explanation of sample numbers, see text . Each symbol on

Ž .a GSI line corresponds to a T measurement. ‘Boxes’ F1a, F1b, F1–2, F2, F2a : Fluid types, corresponding to a given metamorphic phase:hŽ .F1a and b: M1 metamorphism, F2: M2 metamorphism, F1–2: transitional. F2a: primary inclusions in M2 garnet. 1 to 7, I to V arrows : Th

data retained for the definition of the isochores drawn in Fig. 9.

Ž .types corresponds to a more or less well-defineddomain in this diagram. Like for T histograms, it ish

possible to predict which trend will be given bysome phenomena, either geologically interestingŽ . Ž .fluid mixing or not fluid leakage . Fluid types arenormally defined by loosely delimitated domains onthe T rT diagram, drawn subjectively by the re-h m

searcher. It would be possible to get more precisionby contouring lines of iso-values, but this is veryrarely done.

The precision attached at the definition of thedifferent fluid types is very variable, depending onthe homogeneity of the different inclusions and thetime and effort that the investigator can devote totheir study. But, in any case, the database must bewell identified: how many inclusions have been mea-

sured, in which location, etc. The different types andgeneration of inclusions must be correctly discrimi-nated. Again, the inclusions retained for the defini-tion of the representative isochores must be preciselyidentified, in principle, by a clearly visible signŽ .arrow on the diagram. Inclusion populations arecommonly variable that every researcher is free tochoose the values that he considers to be the best,but he must indicate precisely where they are andwhat are the motivations for his choice.

(4. Interpretation: a detailed example Central)Kola Peninsula granulites, Fonarev et al., 1998

Once the fluid types have been clearly identifiedŽ .and a small number typically less than about 5


isochores have been selected, then the real, finaldiscussion can only start. It consist basically toevaluate the respective importance and source ofeach fluid phase present at any moment of the rockhistory, as well as the interaction that it may havehad with the rock-forming minerals. Note that acorrect appreciation of this problem will consider-ably simplify the apparent diversity of inclusions in agiven rock sample, leading to the very wrong as-sumption that any fluid can be present anywhere.

Ž .When, in a recent abstract Johnson et al., 1996 , Iread that ‘CO -rich inclusions are abundant in green-2

Žschist, amphibolite and granulite settings’ in fact, in.all metamorphic terranes , and that their ‘formation

and preservation of carbonic-rich inclusions is aninescapable by-product of deformation and post-de-formation annealing’, I cannot avoid the overall im-pression that this presentation of facts does not cor-respond to the reality that I have seen in hundreds ofsamples from all parts of the world as follows:

v CO -rich inclusions may occasionally be found2

in rocks from most metamorphic grades, but notoverall, and with major differences between the dif-

Žferent metamorphic grades; in lower grades notably.greenschist-facies , almost exclusively along major

Žshear-zones like, incidentally some great Alpinestructures like the Simplon fault zone, the example

.chosen by Johnson et al., 1996 , in higher gradesabove the beginning of migmatites melting, with asudden and marked increase when entering the gran-ulite domain.

v Post-metamorphic annealing lead indeed to thedisappearance of most inclusions, with aqueous flu-ids disappearing more easily than carbonic ones. But,in general, this is done by selective H O leakage2Ž .Bakker and Jansen, 1994 , resulting in a CO -en-2

richment from a H O–CO mixture. This mecha-2 2

nism, however, which can be documented from theremaining CO density, is by no means an ‘inescapa-2

ble’ way of producing CO inclusions. They will not2

be there if no CO had already been present in the2

initial fluid.v Rocks showing best evidence of deformation

and post-metamorphic annealing are eclogites. Theseare conspicuously poor in CO inclusions, except2

Žwhen inherited from former granulites Andersen et.al, 1990, 1993 or introduced from outside along

Ždeformation structures and shear zones Fu et al.,

.submitted . An illustration of this mechanism is givenin Fig. 6. In this example, it is relatively easy, onlyfrom the mode of occurrence of the different fluid

Žtypes, to distinguish between internal fluids W1,.high salinity brines produced by local fluid–mineral

interaction at peak metamorphic conditions, and ex-Žternal fluids W2, low-salinity aqueous and G2,.CO -rich fluids introduced from outside along shear2

zones or microfractures. The next step would be ofcourse to analyze the inclusions to document thefluid–mineral interaction, but this can only be doneat the scale of the single crystal, by laser ablation orany other kind of spot, punctual analysis. Alternativetechniques, such as crushing and leaching, wouldmix different fluid generations and give very ques-tionable results.

Each metamorphic terrane represents a specificproblem, and it is obviously impossible to discuss

Žhere all possible cases in the recent issues of FluidInclusion Research, the invaluable reference basis forany inclusionist, several 10s of references are dealing

.with metamorphic rocks every year . Some cases arerelatively simple. As for the eclogite example dis-cussed in Fig. 6, the case of the gold-bearing fromSierra Leone, presented in Fig. 5, is relativelystraightforward, at least for a partial interpretation.The figure shows a former microfracture, invaded bya CO -rich fluid. The fracture has healed, and the2

fact that isolated, high-density islands remain withina lower density environment indicates that healinghas been accompanied by a decrease in the fluiddensity. From any point along the initial isochore,this density decrease can only occur either by pres-

Žsure decrease or by temperature increase or by a.combination of both . Temperature increase is very

unlikely, but not pressure decrease, known to bealmost systematic during the formation of quartz

Žveins at medium metamorphic grade e.g. the case of.alpine veins, Mullis et al., 1994 . Pressure decrease

must therefore be favored in the present case. Itcould even be quantified if the metamorphic temper-ature of the vein formation could be estimated fromsome adequate mineral assemblage, e.g. chlorite ormicas.

Regional problems are as a rule more compli-cated. Main aspects of the discussion always involvethe two steps that we have identified, namely theselection of few isochores and their comparison with


P–T mineral data. I will illustrate the details of thisŽdiscussion on a recent example Granulites from the

.Central Kola peninsula, Fonarev et al., 1998 , from awell-investigated region which is probably morecomplicated than most metamorphic domains of acomparable metamorphic grade.

4.1. Regional setting

The Archean metamorphic rocks occurring in theCentral part of the Kola Peninsula, northeastern Baltic

. ŽShield , contain a number of igneous enderbite,. Žbasic granulites or sedimentary metapelites, banded.iron formation protoliths, all metamorphosed to

granulite grade. The metamorphic evolution, whichhas taken place between about 3 and 2.5 Ga, is

Žcomplex, with three major metamorphic events M1,.M2, M3, respectively clearly identified in the rock

Žtexture and in the field M1: peak regional assem-blage, M2 and M3 more local, related to major

.shear-zones . All three stages can be identified insome samples, and they have been calibrated interms of P and T from a consistent system of

Žgeothermometers and barometers T in 8C, P in

.kbar : M1: Ts670"20, Ps5.1"0.5; M2: 565"Ž .15, 4"0.5; M3: 500"20, 3.2"0.4 Fig. 9 . In the

investigated region, all rocks have been metamor-phosed under the same conditions. There are nosignificant differences in the P–T values recordedby the mineral assemblages between sheared andunsheared rocks. Differences are only to be noticedin the extension of each stage, M2- and M3-mineralassemblages being more developed in sheared rocks.All rocks contain a number of fluid types, notably

Žthe familiar ‘granulite fluids’ high-density CO and2.high-salinity brines , as well as less common fluids,

Ž .notably N especially in BIF and CH4. The discus-2

sion and interpretation of fluid inclusion data willhere be based here exclusively on the CO -rich2

inclusions, by far the most abundant and typical inall studied samples.

4.2. Selection of representatiÕe isochores

Fluid inclusions have been studied in nine repre-sentative samples, covering all major lithotypes.These samples have also been chosen because they

Ž .Fig. 9. P–T interpretation of the Central Kola granulites Fonarev et al., 1998 . Left: first attempt based on a loose CO isochore selection2Ž . Ž .1 to 7 , based on extreme T values andror isochores passing through a given metamorphic episode. Right: final published interpretation,h

Ž . Ž .based on a rigorous selection of ‘representative’ isochores I to V . Details of isochore identification: see text . 8 and VI: N isochores. M1,2Ž .M2, M3: Successive metamorphic episodes. d q or y : Pressure difference recorded between fluid inclusions and contemporaneous

Ž . Ž .metamorphic mineral assemblage best estimate: between M2 and V . Double thin arrow: Metamorphic P–T path, heavy line arrow s fluidP–T path during the metamorphic evolution.


show a clear indication of the three metamorphicstages at the scale of the thin section. They comprise

Žmetapelites sillimanite–garnet biotite plagiogneisses:.15r310.4, 7g, 14r117.4, 15r181.6 or gneiss: 3 h ,

Ž . Ž .enderbites 4 h, 7 m and BIF 15r397.6, 4ar328.4 .For the CO -rich inclusions, T results are reported2 h

in Fig. 8 presented along the GSI concept. Each GSIŽ .two to six for a given sample corresponds to ahorizontal line in the diagram, showing the overallT variation range. Most GSIs contain primary orh

Ž .pseudoprimary inclusions isolated or clustered .Special attention has been given to inclusions withinor close to good P–T mineral indicators, especially

Žgarnets whenever possible, primary inclusions ingarnet or isolated inclusions in quartz inclusions

.within garnet .In principle, each GSI should be represented by a

Ž .T histogram, as done in the Fonarev et al. 1998h

publication. All these partial histograms would havecomplicated hopelessly the figure. For the purpose ofthe present discussion, the direct representation of all

Žmicrothermometric results each symbol in Fig. 8.corresponds to a T measurement is sufficient toh

illustrate these two major conclusions:Ž .i The extreme variability of different GSIs. Some

Ž .e.g. sample 15r181.6 are extremely well defined,with T variations not exceeding about 108C, othersh

show a variation range of more than 508C. In thelatter case, it is rather obvious that the lowest densi-

Ž .ties highest T , correspond to post-trapping pertur-hŽ .bations leakage , as they are completely different

Ž .from most other measurements e.g. Sample 7g .This hypothesis could suggest that, for any GSI,

Ž .most representative less perturbed values are to-wards the high-density side of the diagram, but asdiscussed below the story is certainly far more com-plicated.

Ž .ii In average, most T values cluster betweenh

y15 and y258C. They would clearly correspond toŽ .a maximum peak on a composite histogram. It

would certainly not be a mistake to choose thesevalues as representative for the whole region. How-ever, the discussion can be considerably elaboratedby taking into account the more precise relationshipsbetween the inclusions and the metamorphic stages,as well as compositional complexities which mayinfluence the position of an isochore for a givenhomogenisation temperature.

4.3. Fluids related to a giÕen metamorphic episode( ) ( ) (‘boxes’ in Fig. 8 : F1 stage M1 and F2 stage

)M2 fluids

The fact that some sensible P–T mineral indica-tors, notably garnet, can easily be identified at thescale of the thin section is of considerable help forthe identification of fluids related to a given meta-morphic event. In general, most minerals have equi-librated at M1. Inclusions in these minerals are

Ždesigned as F1, subdivided in F1a average, most.common values and F1b for a limited group of

inclusions, exclusively occurring in BIF, which showsignificantly lower T . A closer examination hash

shown that these low homogenisation temperaturesresult from the fluid composition, namely the occur-rence of N , up to 35 mole% in some inclusions. For2

the definition of the isochore, this compositionaleffect has to be considered, and then it can be seenindeed that the lowest T , used for the definition ofh

isochore I, corresponds to a higher molar volume,Ž .therefore is below F1a isochores pure CO in the2

Ž .P–T space Fig. 9 .F2 relates to inclusions in M2 minerals. Espe-

cially important is the subgroup F2a, which corre-Žsponds to primary inclusions in M2 garnet see Fig.

.9 in Fonarev et al., 1998 . F1–2 corresponds toinclusions transitional between M1 and M2. All thesetypes and subtypes are indicated by the ‘boxes’represented in Fig. 8.

The selection of ‘representative isochores’ is doneŽ .on this diagram arrows in Fig. 8 . Note that the

Žextreme values, for a pure CO fluid y458C to2.q308C , would correspond to isochores covering the

complete field of geological interest. Then, it wouldbe easy to select an isochore which fits with anyworking hypothesis. I must recognize that it is a littlebit what had been done in a first attempt, correspond-ing to isochores defined by circled arabic numbers

Ž .on Fig. 8 1 to 7 , drawn on the left diagram of Fig.9. But two major arguments indicate that some as-pects of this random, spontaneous selection wereerroneous, or at least misleading as follows:

v There is absolutely no reason to privilege anyisochore exactly passing through the best estimates

Ž .of a given metamorphic event 4, 5 and 7 .v Ž .For the highest densities lowest T , the com-h

positional effect must imperatively be taken into


account: it makes no sense to consider a mixturecontaining up to 35 mole% N as ‘equivalent pure2

CO ’.2

Therefore, we have chosen for the final publishedŽinterpretation the isochores I to V Roman numbers,

.Figs. 8 and 9, right , with the following definition: I:Ž .average range for F1 typical M1 fluid , II: lowestŽ .T , but high N content 35 mole% , III: Transitionalh 2

ŽF1–2, IV: Average F2, and V: F2a primary inclu-. Ž .sions in M2 garnet . Isochore VI and 8 corresponds

Ž .to a different fluid pure N , therefore it is not2

indicated on Fig. 8.

4.4. Interpretation: successiÕe episodes of isobariccooling, followed by sudden decompression

Ž .A quick look to Fig. 9 right shows indeed that,Ž .in general, M1-related isochores I to III are above

Ž . Ž .higher pressures M2 isochores IV and V . Thisrelative position fits well with the overall character-istics of both metamorphic events: M1 occurs athigher P–T conditions than M2. Pure N fluids,2

Ž .however, isochore VI , even if related to M1, recordmuch lower pressure than CO fluids. Such a behav-2

ior is rather common in N -rich inclusions: nitrogen2

obviously diffuses more easily than CO through any2

host mineral.A closer look, however, reveals a number of

problems. For the reference to a given metamorphicevent, the best-constrained isochore is V, corre-sponding to primary inclusions in a well-char-acterized M2 garnet. But this isochore passes signifi-cantly below M2, recording at M2 temperature afluid pressure lower by at least 1 kb than the meta-morphic pressure. Again, this is a rather generalfeature, observed in many granulites, or granulite-re-lated metamorphic rocks: when syn-metamorphic in-clusions are well identified, the fluid pressurerecorded in inclusions is almost systematically lowerthan metamorphic pressure, typically by about 1–2

Ž .kb e.g. Touret and Huizenga, 1999 . The classicalexplanation is selective water leakage. Almost allgranulites contain some hydrous phases, stable atpeak metamorphic conditions, which impose a finiteH O pressure in the metamorphic fluid. In the case2

of the Kola granulites, it can be estimated that thequantity of H O in the synmetamorphic fluid should2

be at least 10% molar for M1, significantly more atthe lower temperature M2 stage. This water is nolonger present in the inclusions, and indeed theeasiest explanation is that it has disappeared byselective leakage, as indicated by a number of exper-

Ž .imental studies Bakker and Jansen, 1994 . The sameexplanation may be given for isochore II, also well

Ž .below its metamorphic condition M1 . But, then,isochore I, corresponding to the average fluid forM1, passes above M1 ‘box’, by about 0.5 kb for theM1 reference temperature of 6708C. Moreover, thereis absolutely no reason to believe that water leakage,evidenced for M2 on the basis of F2a inclusions, hasalso not taken place during M1. This would displacethe ‘real’ position of isochore I towards higher pres-sures, at least to 2–3 kb above peak metamorphicpressure for M1.

The only way to cope with this situation is toinvoke a period of isobaric cooling after M1, duringwhich either new inclusions are formed at corre-sponding P–T values, or former inclusions are resettowards higher densities. This type of evolution hasbeen well documented in various low-to-intermediate

Žpressure granulites from Southern India Nilgirigranulites, Touret and Hansteen, 1988; Srikkantappa

. Žet al., 1992 , Finland West Uusimaa Complex,.Touret and Hartel, 1990 as well as in migmatites

Ž .van den Kerkhof and Olsen, 1990 . Note that den-sity resetting, if any, imposes a departure from the‘constant-volume’ principle: in order to increase thedensity, the volume has to become smaller withoutfluid leakage. This supposes that some external

Ž .source of energy provided by deformation? must beavailable.

In any case, M1 and post-M1 isochores are farwith the high-pressure field, but F2 fluids needs tobe close to M2 conditions. This can only be achievedby a sudden decompression, which may incidentallyexplain the strong leakage observed in some inclu-

Žsions of F1 GSIs wide range of T towards rela-h

tively high temperatures, e.g. sample 15r397.6 inFig. 8.

ŽThe fact that isochore IV average M2 isochore,in principle, somewhat later than the truly primary

.isochore is above V, suggests that the same type ofevolution may have occurred after M2. These con-siderations explain the P–T path which has beenindicated on Fig. 9. Note that isochore IV passes


through M3, but this intersection is purely coinciden-tal, without any geological meaning.

5. Conclusion

Many aspects of fluid research are now at thecenter of geological interest: models of fluid flowthrough rocks, chemical studies of fluidrrock inter-action processes, fluid signatures indicated by stableisotopes, etc. In this panoply of scientific techniques,the study of fluid inclusions should occupy a recog-nized place. A remarkable collective effort duringthe past decades has provided us with all indispens-able tools. Much work has been done to improve thetechnology or theoretical knowledge of fluid systemsin most common geological conditions. In principle,inclusions can now be studied in any rock type,notably in metamorphic rocks where fluids play suchan important role. Still, the number of well-studiedoccurrences remain rather limited. We must knowenter in a phase of systematization, during which anumber of further ‘unexpected discoveries’ will cer-tainly be done. It must always be remembered, how-ever, that the best analytical tool, the most elaboratetheoretical interpretation will only give poor resultsif applied on a ‘bad’ inclusion. For the study of thesesmall objects, observation is essential, with the goodold microscope, to-day like a century ago, remainingthe most important instruments. In few years time,we can still predict decisive technological advances,notably for the complete analysis of the fluid inclu-

Ž .sion content notably by LAS-ICP-MS . But progressin analytical capacities results always in more timeand money spent on the analyzed object, resulting inthe present case to the decrease of the number ofinclusions which will be possibly investigated. Thiswill only make more important the necessity to havea rational selection of analyzed inclusions, accordingto the principles that I have attempted to present inthis paper.

Acknowledgements

The work done in Amsterdam would not havebeen possible without the support of the Vrije Uni-

Ž .versiteit and NWO ALW , the Dutch organization

for the development of scientific research. But anessential part is due to the exceptional quality of thetechnical and supporting staff, W. Koot et al. for thepreparation of the Fluid plates, E.A.J. Burke, W.Lustenhouwer and S. Kars for the Lab. MicroanalyzeŽmicrothermometry, Raman and Electron probes,

.SEM . ‘Our’ way of studying inclusions in metamor-Ž .phic and magmatic rocks has slowly developed

through the work of all the researchers who havebeen at the VU, mostly for the preparation of theirPhD thesis. Just to mention some names: A.M. vanden Kerkhof, T. Hansteen, M. Ploegsma, E.J. Zwart,Ph. Muchez, J.M. Huizenga, M. Moree, L. Bolder-Schrijver, Bin Fu, and many others. T. Andersen andM.L. Frezzotti are members ‘a part entiere’ of this` `group, and I thank them for having forced me to puton paper some aspects of our collective experience,despite the pressure of present-day academic life.

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