Introductionto aqueous fluid systems - Geochemistry - Virginia Tech

Bodnar RJ (2003) Reequilibration of fluid inclusions. In I. Samson, A. Anderson, & D. Marshall, eds.Fluid Inclusions: Analysis and Interpretation. Mineral. Assoc. Canada, Short Course 32, 213-230.CHAPTER 8. REEQUILIBRATION OF FLUID INCLUSIONSRobert J. BodnarThe Bubble FactoryVirginia TechBlacksburg, VA 24061 USArjb@vt.eduINTRODUCTIONA fluid inclusion provides microthermo -metric data that can be used to infer trappingconditions only if the inclusion satisfies severalcriteria (Roedder 1984). First, the inclusion musttrap a single, homogeneous phase. Secondly, theinclusion must remain a constant volume systemafter trapping (excluding reversible elasticvolume changes) and, thirdly, nothing can beadded to, or removed from the inclusion followingtrapping. These criteria are here referred to as"Roedder's Rules", and must be met if one wishesto use inclusions to reconstruct the thermobarichistory of the host rocks. If the inclusion volumechanges, or if anything is added to or lost fromthe inclusion following trapping, the inclusion issaid to have reequilibrated. Reequilibration canoccur in nature during continued burial or upliftof the rock to the surface, or reequilibration mayoccur during sample preparation and/or datacollection in the laboratory. Less often consideredby inclusionists is the possibility that fluidinclusions in samples collected from surfaceoutcrops may have experienced elevated temperaturesduring exposure to the sun or to forest fires,or may have undergone freezing in colderclimates. Especially for low temperatureinclusions, these temperature fluctuations mayhave been sufficient to cause reequilibration.As a result of numerous studies of bothnatural and synthetic fluid inclusions over the pastfew decades, we now have a fairly comprehensiveunderstanding of the reequilibration process.Before I discuss the physical and chemical factorsthat affect the reequilibration process, it is firstnecessary to introduce the various terms that areused to describe fluid inclusion reequilibration. Inthe fluid inclusion literature, reequilibration is ageneral term that has been used to describe fluidinclusions that have changed volume, or have lostor gained components, or both. Stretching is aterm that is generally used to describe fluidinclusions whose volume has changed with littleor no discernible loss of fluid from the inclusion(Fig. 8-1). The term was first introduced byLarson et al. (1973) to describe permanent,nonelastic deformation of the walls of inclusionsin fluorite and sphalerite as a result ofoverheating. Stretching usually results when aninclusion reequilibrates in a low strain rateenvironment in which the host phase deformsplastically, and is more common in softerminerals than in harder minerals. Any materialthat is lost from the inclusion occurs as a result ofdiffusion or leakage along dislocations (Wilkinset al. 1981, Bodnar & Bethke 1984, Wilkins1986, Bakker & Jansen 1991, 1994, Hall &Sterner 1993, Vityk et al. 2000).With increasing strain rate, fractures maydevelop in the host adjacent to inclusions, withpartial loss of fluid by advection along themicrofractures. This type of reequilibration isreferred to as leakage or partial decrepitation.Sometimes, the extent of fracturing is so intensethat all fluid is lost, leaving an empty, dark cavityin the host phase. This is referred to asdecrepitation. Note that the resulting inclusionsare probably not sensu stricto empty but, rather,likely contain a low-density version of theoriginal fluid (water vapor or CO 2 gas, forexample). The inclusions are referred to as beingempty because no fluid phase is observed duringpetrographic or microthermometric examination.The term decrepitation was introducedby researchers in the former Soviet Union todescribe the explosive release of inclusioncontents during heating. In the early days of fluidinclusion research, before the development ofmicroscope heating/cooling stages and highqualitylong-working distance microscopeobjectives, workers would heat mineral samplesin a furnace with a sensitive microphone attached.The microphone records acoustic emissions (i.e.,noise) associated with decrepitating fluidinclusions, and the decrepitation temperatureswould be related to inclusion trappingtemperatures (incorrectly, in many cases!).213

FIG. 8-3. Relationship between inclusion size and the pressure required to reequilibrate fluid inclusions inquartz under one atmosphere confining pressure. Inclusion volumes shown on the lower abscissa wereconverted to inclusion diameter assuming a spherical geometry and are shown on the top abscissa (modifiedfrom Bodnar et al. 1989).where V is the inclusion volume in µm 3 .Assuming a spherical geometry, this translates toabout 635 bars for a 5 µm inclusion, and 200 barsfor a 50 µm inclusion. Bodnar & Bethke (1984)also observed a clear relationship betweeninclusion size and the internal pressure required tostretch fluid inclusions in sphalerite, but wereunable to accurately calculate the inclusionvolumes because the vapor bubbles were notspherical at room temperature, precludingapplication of the technique described by Bodnar(1983). According to their data, the pressurenecessary to stretch an inclusion smaller thanabout 5-10 µm in sphalerite is about 800 bars,whereas an inclusion 50 µm in diameter requiresabout 300-400 bars of internal pressure (Fig. 8-2).The relationship between inclusion size and thepressure required to initiate stretching is less welldefined for inclusions in barite (Ulrich & Bodnar1988), although the trend indicates higherpressures for smaller inclusions, consistent withother minerals studied. Prezbindowski & Tapp(1991) showed that the size-pressure relationshipobserved in fluid inclusions is consistent withtheoretical models of crack propagation and stressconcentration surrounding flaws (fluid inclusions)in minerals.Prezbindowski & Larese (1987)conducted an experimental study of stretching offluid inclusions in calcite and found that largerinclusions stretch more (i.e., the homogenizationtemperature increases by a larger amount) thansmaller inclusions subjected to the same P-Thistory. Based on the results of Bodnar & Bethke(1984), which indicate that the amount ofstretching (change in the homogenizationtemperature) for a group of inclusions thatinitially had the same homogenizationtemperature can be related to the temperature atwhich stretching begins, the results ofPrezbindowski & Larese (1987) would suggestthat the larger inclusions began to stretch beforethe smaller inclusions. Goldstein (1986) observeda similar size-pressure relationship for naturallyreequilibrated inclusions in low temperaturecalcium carbonate cements. However, Hall et al.(1993) found no correlation between inclusionsize and ease of stretching during an experimentalstudy of calcite, although they did notice thatinclusions closer to the mineral surface were more217

prone to stretching compared to those deeper inthe sample.Inclusion shapeA secondary factor that determines howeasily a fluid inclusion will reequilibrate isinclusion shape. Bodnar et al. (1989) noted thatthe average pressure required to reequilibratefluid inclusions in quartz increased by about 400bars as the inclusion shape varied from veryirregular, with many reentrants or "tentacles", toone that was spherical or negative-crystal shaped.Goldstein (1986) noted a similar dependence oninclusion shape for calcite, as did Bodnar &Bethke (1984) for inclusions in fluorite andsphalerite. The relationship between inclusionshape and the amount of pressure required toinitiate reequilibration is generally consistent withtheoretical deformation models which indicatethat stress is concentrated at sharp corners, andthat the stress approaches infinity at crack tips(ends of tentacles in irregularly-shapedinclusions) or at the corners of angular inclusions(Prezbindowski & Tapp, 1991). However, the factthat most angular inclusions do not spontaneouslystretch or decrepitate during heating suggests thatwhile theoretical models of deformation providevaluable insights into the reequilibration process,they may be less useful than empirical models forpredicting the reequilibration behavior of fluidinclusions.Fluid CompositionLittle information is availableconcerning the role of fluid composition in thereequilibration process. However, one wouldexpect that fluid inclusions containing watermight reequilibrate more easily than those that arewater-free, owing to the presumed highersolubilities of host minerals in aqueous fluidscompared to, for instance, carbon dioxide. Hall &Wheeler (1992) conducted a preliminary study ofthe relationship between fluid composition andinclusion decrepitation and found that 3 µmdiameter inclusions containing H 2 O-NaClrequired a lower pressure (3.0 kbars), andinclusions containing H 2 O-CO 2 a higher pressure(4.2 kbar), compared to pure H 2 O inclusions (3.4kbars) of the same size. The role that fluidcomposition plays in reequilibration is an area ofresearch that should be given high priority infuture studies.HOW DO WE RECOGNIZE INCLUSIONSTHAT HAVE REEQUILIBRATED?Often, one may be able to infer thatreequilibration has occurred based onpetrographic observations of fluid inclusions andthe surrounding host mineral. If the fluidinclusions within a fluid inclusion assemblage(FIA) satisfy "Roedder's Rules" described aboveand have not reequilibrated, all of the inclusionsin the FIA will contain the same phases, and inthe same volume proportions, when observed atroom temperature. Thus, an FIA composed oftwo-phase (liquid + vapor) inclusions in whichthe liquid-to-vapor ratio is identical in allinclusions might indicate that reequilibration hasnot occurred. But, one must use care indetermining whether the inclusions have the sameliquid-to-vapor ratio. For example, consider anFIA containing inclusions with a salinity of 5 wt.% NaCl and which homogenize at 150°C. If someof the inclusions reequilibrate by stretching suchthat the new homogenization temperature is175°C, the volumes of the reequilibratedinclusions would have increased by 2.6 volumepercent (see Bodnar & Bethke 1984, p. 153). If aspherical vapor bubble in one such inclusion hada diameter of 6 µm before stretching, the vaporbubble diameter after stretching would be about6.6 µm! Inclusions with smaller bubbles wouldshow proportionately smaller increases in bubblediameter after stretching. It is unlikely that onewould be able to distinguish between twoinclusions in the same FIA with vapor bubblediameters that differ by only 0.6 µm or less. Thus,an FIA could contain fluid inclusions, some ofwhich had stretched by 25°C and others of whichhad not stretched (or stretched by a smalleramount), and still appear to have consistentliquid-to-vapor ratios. However, this differencewould become obvious during microthermometricanalysis of the inclusions, as described below.Finally, it should be emphasized that, whereasmost reequilibration results in a "smearing" ofdata to produce a wide range in homogenizationtemperatures, in some environments the reequilibrationprocess may progress to the extent that theinclusions completely equilibrate at some new P-T condition, resulting in fluid inclusions withconsistent phase ratios and microthermo metricresults, even though these are not representativeof the original formation conditions (Vityk &Bodnar 1995a, 1998).218

FIG. 8-5. Evolution in the shape of fluidinclusions in quartz during reequilibration atconditions of Pinternal > Pconfining. Thelength/width ratio of the inclusion indicated bythe arrow gradually decreases with time toproduce an equant negative-crystal-shaped fluidinclusion after being held at a temperature of370°C and an internal pressure of 400 bars for5621 hours. The scale bar equals 50 µm.(modified from Bodnar et al., 1989).reequilibration, and the presence of equant ornegative-crystal shaped inclusions in a sampledoes not necessarily indicate that the inclusionshave reequilibrated. With increasing departure ofthe confining pressure from the inclusionisochore, the largest inclusions begin toreequilibrate as fractures are generated at sharpcorners of the inclusion and extend into thesurrounding host (Kotel'nikova 1994, Vityk et al.1996) (Fig. 8-6). Some of the fluid that escapesalong these fractures may be trapped in thesurrounding quartz to produce a halo of verysmall fluid inclusions adjacent to the fracture. Asthe difference between the inclusion pressure andthe confining pressure increases further,progressively smaller inclusions begin toreequilibrate. Barker (1995) used the evolution inshape of inclusions reequilibrated underconditions of internal overpressure to identifythose inclusions that had experienced postentrapmentmodifications in the Caledonian ØseThrust, Norway. Detailed TEM studies byBoullier et al. (1989), Bakker & Jansen (1994)and Vityk et al. (2000) show an increaseddislocation density surrounding reequilibratedinclusions in quartz (Fig. 8-7). This is associatedwith diffusive loss of water into the quartz host,which results in hydrolytic weakening of thequartz and facilitates later fracturing andadditional water loss.Fluid inclusions in quartz thatreequilibrate under conditions of internalunderpressure (confining pressure greater than theinternal pressure) show some distinct differencescompared to those reequilibrated under conditionsof internal overpressure. The first changeobserved in these inclusions is that the inclusionwalls, which are initially smooth, becomecorrugated and rough (Pêcher 1981, Sterner &Bodnar 1989, Bakker & Jansen 1991) (Fig. 8-8).Associated with this change is the development ofa halo of secondary inclusions surrounding theoriginal inclusion. Bakker & Jansen (1991, 1994)and Vityk et al. (2000) suggested that fluid fromthe original inclusion (before reequilibration)migrated into the surrounding quartz alongdislocations and was later trapped to form thesecondary inclusion halo. Compared to conditionsof internal overpressure, in which the largestinclusions begin to reequilibrate first (at thelowest amount of overpressure), the smallestinclusions reequilibrate most easily under220

FIG. 8-8. Evolution of fluidinclusions in quartz afterreequilibration at conditionsof Pinternal

FIG. 8-9. Relationship between the intensity ofreequilibration features and the size and pressuredifferential (Pconfining - Pinternal) for aqueousinclusions in quartz from the Bayanovoo granitein central Mongolia. The inclusions werereequilibrated at 500°C and pressures of 1 (A-C),2 (D-F), 3 (G-I), 4 (J-L) and 5 (M-O) kbar for 7days. At run conditions, all of the inclusions hadan internal pressure of about 500 bars. Withincreasing pressure differential the smallerinclusions begin to reequilibrate first, followedby increasingly larger inclusions as the pressuredifferential increased from about 500 bars (A-C)to 4.5 kbar (M -O). (from Vityk et al. 1994).Bodnar (1995a, 1998) are consistent with thissuggestion.Goldstein (1986) examined fluidinclusions from recent calcium carbonate cementsthat had formed at the surface and were neverdeeply buried. He compared these to inclusions insimilar cements that had formed at the surface butwere later buried to significant depths andexposed to temperatures well above 100°C. Therecent samples that had never been buriedcontained mostly all-liquid inclusions, in additionto liquid + vapor inclusions that had trapped waterplus air. Deeply buried samples showed a widerange in homogenization temperatures andsalinities, reflecting continued reequilibrationduring burial and exposure to fluids of differentsalinities as burial progressed. Goldstein (1986)concluded that the deeply buried cements wereunlikely to provide any useful informationconcerning original formation conditions, butcould prove useful for interpreting burial porefluid history. Barker & Goldstein (1990) used thefact that inclusions in calcite reequilibraterelatively easily to suggest that inclusions incalcite from sedimentary basins and geothermalfields might be used to estimate the maximumburial conditions.One misconception that many workershave concerns the reproducibility ofmicrothermometric data as a means of testingwhether or not reequilibration has occurred.Workers often report that homogenizationtemperatures of inclusions are reproducible whenmeasured multiple times. These workers theninterpret this to indicate that the inclusions havenot reequilibrated. However, a reproducible Th isnecessary but not sufficient to prove that aninclusion has not reequilibrated. As noted byBodnar & Bethke (1984), stretched fluidinclusions in fluorite gave reproducible (±0.2°C)homogenization temperatures, even though thesetemperatures were ˜40°C higher than the Thbefore the inclusions were stretched by laboratoryheating. Larson et al. (1973) also noted that someinclusions in fluorite gave reproducible Th afterfour heating runs, even though some of thesetemperatures were as much as 70 degrees Celsiustoo high (i.e., the inclusions had been stretched by70 degrees Celsius). Thus, reproducibility ofhomogenization temperatures is a necessarycriterion to confirm that inclusions have notreequilibrated, but is not in itself sufficient toprove that reequilibration has not occurred.Compositional ReequilibrationCompared to volumetric reequilibration,compositional reequilibration is less wellunderstood and more difficult to recognize. Most223

compositional reequilibration involves the loss (orgain) of hydrogen or water (OH - ) from theinclusion (Hall & Sterner, 1995). In meltinclusion studies, other components, includingsodium and some trace elements, can be lost frominclusions during heating. Melt inclusionreequilibration is not considered here.Pure carbon dioxide (or carbon dioxiderich)inclusions are common in medium- to highgrademetamorphic environments. Based onthermodynamic considerations, water-free (orwater-poor) fluids often are not in equilibriumwith the mineral assemblage in the rocks, and thishas led many workers to invoke post-trappingreequilibration of the inclusions to explain thehigh CO 2 contents. Hollister (1990) hassuggested that CO 2 inclusions in metamorphicrocks are produced when water leaks out oforiginally water-rich inclusions during plasticdeformation, to produce pure CO 2 , or CO 2 -rich,inclusions. Bakker & Jansen (1991) were able toreproduce this process experimentally. Water waslost from H 2 O-CO 2 inclusions duringreequilibration, resulting in inclusions more CO 2 -rich than the original inclusions. Alternatively,Lamb et al. (1987) and Lamb (1990) suggestedthat the "primary appearing" CO 2 inclusions ingranulites were actually trapped duringretrogression and are not associated with peakgranulite grade conditions.Owing to the small size of the hydrogenion, hydrogen is able to diffuse through mostmineral structures very easily. Mavrogenes &Bodnar (1994) showed experimentally that fluidinclusions several tens of micrometers beneath thesurface of a quartz crystal will reequilibrate to theexternal hydrogen fugacity in only a few hundredhours at 600-800°C, or in only a few tens ofthousands of hours (several years) at temperaturesof 400°C. As part of this work, these workerswere able to confirm that the common failure ofsulfide daughter minerals to dissolve duringheating is the result of post-entrapment hydrogenloss from the inclusions. These workers were ableto reverse this process experimentally anddissolve chalcopyrite daughter minerals - thesephases did not dissolve before hydrogen wasexperimentally diffused into the inclusion at hightemperature and pressure. Hall et al. (1991)attributed the presence of methane in fluidinclusions from the Ducktown, Tennessee, USA,massive sulfide deposits to hydrogen diffusioninto the inclusions during uplift and cooling. Theextremely negative hydrogen isotopic (D/H)composition of the fluid inclusions was consistentwith enrichment in the hydrogen isotope, as thelarger deuterium diffuses through quartz at amuch slower rate. Hall & Bodnar (1990)suggested that the small amount of methane oftenreported in fluid inclusions from granulites is theresult of hydrogen diffusion into the inclusionsfollowing trapping, consistent withthermodynamic properties in the C-O-H system.Hall & Sterner (1993) conductedexperimental studies of water loss from aqueousfluid inclusions. As a result of water loss, thesalinity of the inclusions increased by 22 %.Vityk et al. (2000) conducted similar experimentsand were able to increase the salinity by a fewtenths of a weight percent NaCl as a result ofwater loss. Audétat & Günter (1999) notedcharacteristic reequilibration textures associatedwith fluid inclusions in quartz from the MoleGranite, Australia, and attributed variable andhigh salinities in the inclusions to loss of water toproduce "sweat haloes" around deformedinclusions. It is likely that ranges in salinities ofapparently coeval inclusions in some magmatichydrothermal ore deposits, such as the porphyrycopper deposits, are due at least in part to waterloss from the inclusions following entrapment.One could interpret the discussion aboveto indicate that inclusions reequilibrate by anincrease in volume with no fluid loss (stretching)or by loss of fluid through leakage ordecrepitation. In practice, these variousmechanisms of reequilibration likely operatetogether, with perhaps one or the otherdominating, depending on the environment inwhich reequilibration is occurring. Thus, Vityk etal. (2000) note that stretching of fluid inclusionsin quartz requires the diffusive loss of fluid fromthe inclusion into the surrounding quartz host toinitiate the stretching process. However, normalpetrographic observations would not reveal thepresence of water-bearing dislocations, and onemight assume that the change in homogenizationtemperature was due solely to volume change.Based on our experience and observations, webelieve that in all, or certainly most, examples ofreequilibration, both volume change and fluid loss(or gain) occur. Possible exceptions mightinclude, for example, the reequilibration of "pure"CO 2 inclusions. Wanamaker & Evans (1989)224

noted that following heating to generate highinternal pressures, the density of CO 2 inclusionsin olivine had decreased. Assuming that CO 2cannot diffuse through olivine at these conditions,these workers attributed the density decrease tostretching (volume increase), in agreement withtheir measurements of inclusion diameters beforeand after heating.Necking DownMost fluid inclusions that we observetoday do not have the same shape as when thefluid was originally trapped in the mineral.Following entrapment in the host phase, mostinclusions change shape through a processreferred to as "necking down" that involvesdissolution and re-precipitation of the host phaseto reduce the surface area and produce manysmaller inclusions from the original largerinclusion. For example, when a mineral fracturesduring or after growth, some fluid may enter thefracture as shown in Figure 8-10a. As neckingdown proceeds, the fluid in the fracture becomesisolated into progressively more and smaller fluidinclusions. During this process, the inclusionshape tends to become more regular. This processis often referred to as maturation in the fluidinclusion literature (cf. Bodnar et al., 1985). Ifthis process goes to completion, the formerfracture plane will be decorated by numerousspherical or negative-crystal shaped fluidinclusions, often showing a decrease in inclusionsize towards the original fracture tip (Fig. 8-10d).If necking down occurs at thetemperature and pressure of trapping of theoriginal fluid, the resulting inclusions will allshow similar phase relations as well asmicrothermometric behavior that is consistentwith the PTX trapping conditions. Indeed, theprocess described above and illustrated in Figure8-10 is identical to that used to trap synthetic fluidinclusions according to the technique of Sterner &Bodnar (1984). Accordingly, fractured quartzcores are placed into high temperature autoclavesalong with a fluid of known composition. Duringthe constant-temperature experiment, fluid entersthe fracture and the fracture heals as shown inFigure 8-10 to produce numerous synthetic fluidinclusions trapped at constant and known PTXconditions. The microthermometric behavior ofthe resulting inclusions is consistent with theknown formation conditions. In this case, neckingFig. 8-10. Schematic representation of theformation of secondary fluid inclusions in afracture by necking down. Fluid enters the openfracture (a) and begins to dissolve material fromthe walls and re -precipitate new mineral matterto heal the fracture, isolating small pockets offluid in the process (b). With continued healingthe originally irregularly shaped fluid inclusions(c) become more regular in shape and evolvethrough maturation to produce a plane decoratedby numerous negative-crystal shaped fluidinclusions (d). (Modified from Roedder, 1962).down is simply the process by which inclusionsmature and does not involve reequilibration of thetype discussed in this chapter.Inclusions that undergo necking down atconstant temperature and pressure, and in the onephasefluid field, are of no concern in discussions225

of reequilibration processes that lead to incorrectmicrothermometric results. However, if thetemperature and/or pressure change during thereequilibration process, or if two or more fluids ±solids are present in the fracture during neckingdown, the resulting inclusions likely will provideincorrect compositional and microthermometricdata. Figure 8-11 is a schematic representation ofnecking down in a decreasing temp erature systemin which phase separation to produce a vaporbubble occurs during cooling. The originalinclusion at time T 0 contains only liquid. Duringcooling to temperature T 1 a vapor bubblenucleates in the inclusion. During cooling fromtemperature T1 to T2, the original inclusion necksdown to produce two inclusions. One inclusion(inclusion a) contains the larger vapor bubble thatformed during initial cooling, and the otherinclusion contains only a small vapor bubble thatformed after the two inclusions were separated.With continued cooling, the larger inclusionundergoes necking down again to produce twosmaller inclusions, one containing the bubble thatwas present at the moment necking occurred(inclusion b) and the other (inclusion c)containing a smaller bubble that nucleated afternecking during cooling from T 2 to T 3 . At roomtemperature (T 4 ) the three inclusions show verydifferent liquid to vapor ratios, and wouldhomogenize at different temperatures. Thus, if anFIA is comprised of inclusions that haveundergone necking down in a changing temper-Fig 8-11. Necking down of an originally large,liquid -filled inclusion (T 0 ) to produce threesmaller inclusions. None of these inclusionswould record the correct formation temperature.See text for details. (Modified from Roedder,1962).ature and/or pressure system, or in the presence oftwo or more phases, the fluid inclusions will showvariable phase relations and a range inhomogenization temperatures. Such inclusionsshould be avoided during microthermometricstudies if the goal is to determine the original P-Tformation conditions.REEQUILIBRATION OF MELTINCLUSIONSCompared to fluid inclusions,compositional reequilibration of melt inclusions ismuch more common. Anderson (1974) was one ofthe first to consider the reliability of meltinclusions and the various post-entrapmentchanges that might lead to reequilibration.Nielsen et al. (1998) address the relativeadvantages of microscope heating versus batchheating of mineral grains to homogenize meltinclusions, and also discuss volatile loss andcontamination of melt inclusions from alterationproducts. Manley (1996) discussed the use of meltinclusion morphology as an indicator of thetiming of inclusion formation during the eruptionevent. Qin et al. (1992) developed a generaltheoretical model for diffusive reequilibration ofmelt inclusions, and applied this model toevaluate water loss from melt inclusionsfollowing entrapment, as well as reequilibrationof trace element concentrations in melt inclusions.Gaetani & Watson (2000) evaluated Fe-Mgexchange between melt inclusions and hostolivine, and Danyushevsky et al. (2000) providedata from natural melt inclusions in olivine thathave reequilibrated by Fe-loss.A complete consideration of the meltinclusion reequilibration process is beyond thescope of this chapter. The reader is referred to thesources listed above, as well as references therein,for a more complete discussion of this problem.HOW TO AVOID LABORATORY RE-EQUILIBRATION OF FLUID INCLUSIONSFor most fluid inclusions, compositionalreequilibration during laboratory studies isunlikely if care is taken during data collection.Exceptions include hydrogen loss (or gain) by theinclusions during microthermometry andcompositional reequilibration of melt inclusionsduring heating. The main type of reequilibrationthat one has to be concerned with duringlaboratory studies of most fluid inclusions is226

volumetric reequilibration. Fluid inclusions tendto reequilibrate volumetrically when the internalpressure exceeds the strength of the host mineraland, as noted above, this value depends on themineral hardness as well as the inclusion size andshape and fluid composition. In most cases,heating to some temperature above roomtemperature (and above the homogenizationtemperature) is required to generate pressuressufficiently high to cause most inclusions toreequilibrate. Low temperature (Th =150°C)aqueous inclusions may also reequilibrate duringcooling studies to measure ice-meltingtemperatures. In this case, formation of ice duringcooling, which occupies approximately 9 % morevolume than the liquid water it formed from, cangenerate high pressures and cause the inclusion tostretch. Lawler & Crawford (1983) coined theterm "freeze stretching" to describe thisphenomenon.In the early days of fluid inclusionstudies, most polished sections were preparedusing Canada balsam or Lakeside cement or someother adhesive that required the sample to beheated to temperatures approaching 100°C. Inmany cases, little care was taken to control thistemperature and samples may have been heated toseveral hundred degrees to mount the sample on aglass slide. Such high temperature would causefluid inclusions to stretch before they were placedinto the fluid inclusion stage, resulting inerroneous microthermometric data. Today, mostworkers take care to minimize heating of thesample during polishing, and often use adhesivessuch as Superglue that sets at room temperatureand requires no heating. In some extreme casesinvolving very low temperature inclusions, thesamples must be stored in a temperaturecontrolledroom and transported in ice to avoidstretching of the inclusions (cf. Wilson et al.,2003).Most workers believe that reequilibrationin the laboratory is not a problem as long as theinclusions are not heated above thehomogenization temperature. This is because therate of pressure increase in the inclusion duringheating is much lower before the inclusion hashomogenized than it is after homogenizationoccurs. And, for gas-free aqueous inclusions inmost minerals, the internal pressure is unlikely toexceed a few hundred bars during heating, as longas the homogenization temperature is below about400°C. For higher homogenization temperatures,the internal pressure can become sufficiently highto reequilibrate inclusions in even some of themore resistant minerals, such as quartz. Forexample, a fluid inclusion containing a 25 wt.%NaCl solution will have an internal pressure ofabout 900 bars at a homogenization temperatureof 600°C (Bodnar & Vityk, 1994). According toequation (1) above, this pressure is sufficient tocause decrepitation of inclusions in quartz largerthan about 25 micrometers (Bodnar et al., 1989).Many who study fluid inclusions frommedium- to high-grade metamorphic environmentshave experienced inclusion decrepitationbefore homogenization occurs. This happensbecause most inclusions from this environmentcontain significant amounts of carbon dioxide,resulting in elevated pressure in the inclusions,even at room temperature. As an example, anH 2 O-CO 2 inclusion containing 37 mole percentCO 2 and homogenizing at 263°C would have aninternal pressure of 2880 bars at homogenization(Sterner & Bodnar, 1991). This pressure wouldcause decrepitation of all inclusions larger thanabout 3 µm in quartz (equation (1) above), andany inclusions larger than this would decrepitateduring heating, before the homogenizationtemperature is reached. Schmidt et al. (1999)described application of the HydrothermalDiamond Anvil Cell as a pressurized fluidinclusion stage to eliminate (or minimize)decrepitation of high pressure inclusions duringheating to homogenization.In some cases it may simply not bepossible to heat (or cool) a fluid inclusion withoutcausing reequilibration to occur. Not every fluidinclusion is able to provide usefulmicrothermometric data. What is most importantto the fluid inclusionist is to be able to recognizeinclusions that reequilibrate during laboratorystudies. This can best be accomplished byfollowing one simple rule during microthermo -metric analysis: Thou shalt collect no data fromany fluid inclusion that has not been observedcontinuously during the very first heating (orcooling) run. If one obeys this rule, theprobability of collecting data from inclusions thathave reequilibrated during laboratory analysis issignificantly reduced.227

Barite at one atmosphere confining pressure.Econ. Geol. 83, 1037-1046.VITYK, M.O. & BODNAR, R.J. (1995a) Do fluidinclusions in high grade metamorphic terranespreserve peak metamorphic density duringretrograde decompression? Am. Mineral. 80,641-644.VITYK, M.O. & BODNAR, R.J. (1995b): Texturalevolution of synthetic fluid inclusions in quartzduring re -equilibration, with applications totectonic reconstruction. Contrib Mineral Petrol.121, 309-323.VITYK, M.O. & BODNAR, R.J. (1998): Statisticalmicrothermometry of synthetic fluid inclusionsin quartz during decompression. ContribMineral Petrol 132, 149-162.VITYK, M.O., BODNAR, R.J. & DOUKHAN, J-C.(2000): Synthetic fluid inclusions: XV. TEMinvestigation of plastic flow associated with reequilibrationof synthetic fluid inclusions innatural quartz. Contrib Mineral Petrol 139, 285-297.VITYK, M.O., BODNAR, R.J. & DUDOK, I.V.(1995): Natural and synthetic reequilibrationtextures of fluid inclusions in "MarmaroshDiamonds": Evidence of refilling underconditions of compressive loading. Europ JMineral. 7, 1071-1087.VITYK, M.O., BODNAR, R.J. & DUDOK, I.V.(1996): Fluid inclusions in "MarmaroshDiamonds": Evidence for tectonic history of theFolded Carpathian Mountains, Ukraince.Tectonophysics 255, 163-174.VITYK, M.O., BODNAR, R.J. & SCHMIDT , C.S.(1994): Fluid inclusions as tectonothermobarometers:relation between pressure-temperaturehistory and reequilibration morphology duringcrustal thickening. Geology 22, 731-734.WANAMAKER, B.J. & EVANS, B. (1989):Mechanical re-equilibration of fluid inclusions inSan Carlos olivine by power law creep. ContribMineral Petrol . 102, 102-111.WILKINS, R.W.T. (1986): The mechanisms ofstretching and leaking of fluid inclusions influorite. Econ. Geol., 81, 1003-1008.WILKINS, R.W.T., BIRD, J.R. & EWALD, A.H.(1981): Observations on deformationmicrostructures and fluid inclusions in protonirradiatedhalite. N. Jb. Miner. Abh. 141, 240-257.WILSON, N.S.F., CLINE, J.S. & AMELIN, Y.V.(2003) Origin, timing and temperature ofsecondary calcite-silica mineral formation atyucca mountain, Nevada. Geochim CosmochimActa 67, 1145-1176.231

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