Series Coordinator: R. Raeside - Geochemistry - Virginia Tech

The cover photo shows a water-rich melt inclusion in pegmatite quartz fromEhrenfriedersdorf/Germany, re-homogenized at 600°C and 1 kbar using the cold-seal pressurevessel technique. The vessel was pressurized with CO 2 . The melt inclusion glass contains 25wt%H 2 O determined with Raman spectroscopy. The results are given in the paper by Thomas,Kamenetsky and Davidson - example b of Table 2. The yellow color corresponds to 25 wt.% waterand the dark green color at the basis corresponds to about 0 wt.% (quartz). The blue ridge at theright side comes from a small secondary fluid inclusion trail perpendicular to the quartz chip.The water distribution image was taken with the confocal high resolution Raman device LabRamHR800 using the 488 nm excitation Laser line. The image is composed of 750 single points, each2µm. Behind each point is a complete Raman spectrum taken in the frequency range from 2800 to4000 cm –1 . For the quantification of the water we used the integral intensity of the broadasymmetric H 2 O-OH Raman band between 3100 and 3750 cm –1 .DistributionCopies of Volume 36 and preceding volumes are obtainable from:The Business ManagerMineralogical Association of CanadaP.O. Box 78087Meriline Postal Outlet1460 Merivale RoadOttawa, OntarioCanada K2E 1B1e-mail: canmin.mac.ottawa@sympatico.cawww.mineralogicalassociation.caSeries Coordinator: R. RaesideDepartment of GeologyAcadia UniversityWolfville, Nova ScotiaCanada, B4P 2R6Fax: +1 902-585-1816e-mail: rob.raeside@acadiau.caISBN 0-921294-36-0Copyright 2006 – Mineralogical Association of CanadaPrinted in Canada

MELT INCLUSIONS IN PLUTONIC ROCKSMineralogical Association of CanadaShort Course Series Volume 36Edited byJames D. WebsterDepartment of Earth and Planetary Sciences,American Museum of Natural History,Central Park West at 79th Street,New York, NY 10024-5192, USAShort Course delivered in association with the annual meeting of the Geological Association of Canada andthe Mineralogical Association of Canada, Montreal, Quebec, 13–14 May, 2006.

TABLE OF CONTENTSPrefaceix1. Melt inclusions in plutonic rocks: petrography and microthermometry 1Robert J. Bodnar, James J. Student2. Application of secondary ion mass spectrometry to the determination of traditional andnon-traditional light stable isotopes in silicate melt inclusions. 27Graham D. Layne3. In situ laser ablation-ICP-MS chemical analysis of melt inclusions and prospects forconstraining subduction zone magmatism. 51Thomas Pettke4. Melt inclusion record of magmatic immiscibility in crustal and mantle magmas.Vadim S. Kamenetsky 815. Crystallized melt inclusions in gabbroic rocks. 99Ilya V. Veksler6. Melt inclusions in carbonate-rich magmas. 123Ilya V. Veksler, David Lentz7. Magmatic processes and volatile phase generation in porphyry-type environments: A laserablation–ICP–MS study of silicate and sulfide melt inclusions. 151Werner E. Halter, Christoph A. Heinrich8. Silicate melt inclusions in felsic plutons: a synthesis and review. 165James D. Webster, Rainer Thomas9. Understanding pegmatite formation: the melt and fluid inclusion approach 189Rainer Thomas, James D. Webster, Paul Davidson10. Fluid and melt inclusions in the sub-volcanic environments from volcanic systems:Examples from the Neapolitan area and Pontine Islands, Italy 211Benedetto De Vivo, Annamaria Lima, Vadim S. Kamenetsky, Leonid V. Danyushevskyiii

CHAPTER 1: MELT INCLUSIONS IN PLUTONIC ROCKS: PETROGRAPHY ANDMICROTHERMOMETRYR. J. BodnarFluids Research Laboratory, Virginia Tech,Blacksburg VA 24061, USAE-mail: rjb@vt.eduandJames J. StudentDept. of Geology, Central Michigan University,Mt. Pleasant, MI 48859, USAE-mail: stude1jj@cmich.eduINTRODUCTIONMelt inclusions (MI) are small droplets ofmelt now containing some combination of crystals,glass and vapor that are trapped in crystals formedin magmas. Over the past few decades, the study ofMI has matured to become an accepted technique toinvestigate melt evolution in volcanic systems(Clocchiatti 1975, Roedder 1979, Lowenstern 1995,2003, Sobolev 1996, Frezzotti 2001, Hauri et al.2001, Anderson 2003, Schiano 2003). MI involcanic rocks are commonly large (>50 µm),glassy and contained in fresh and transparentminerals, and are generally easy to identify. MI involcanic rocks generally provide consistent andreasonable results concerning the chemistry of meltsat depth and provide the best tool available forassessing the volatiles in magmas (Anderson 1973,1974, Lowenstern 1994, 1995, 2003). The best MIare often found in the most rapidly cooled parts ofpyroclastic deposits and glassy rinds of lavas, whichare not so easy to find (A.T. Anderson, Jr., personalcommunication, 2006).Compared to studies of MI in extrusiverocks, there are many fewer studies of MI fromplutonic rocks, and there is still some uncertaintyconcerning the interpretation of data obtained fromthese samples. Roedder (1984) noted, “The lack ofevident silicate melt inclusions in many igneousintrusive rocks, particularly those formed at greaterdepths, is puzzling.” Roedder further stated, “Oneof the major problems in the study of meltinclusions in intrusive rocks is the difficulty inrecognizing the inclusions.” MI in intrusive rocksare usually small (5–20 µm) compared to those inextrusive rocks. Thomas et al. (1996) noted thatabout 80% of the MI in granitic rocks from theErzgebirge, Germany, were smaller than 20 µm,with a mean diameter of about 10 µm. R. Thomas(personal communication, 2006) also noted acorrelation between inclusion size and volatilecontent, with more volatile-rich inclusions reaching200 µm in diameter. Thomas also suggested that MIin Precambrian rocks are generally smaller thanthose in younger rocks. MI in plutonic rocks arecommonly completely crystalline and contain adistorted bubble that is not always recognizable,and are much more likely to have reequilibratedfollowing entrapment, owing to slow cooling andcontact with subsolidus hydrothermal fluids.This chapter summarizes progress in thedevelopment of techniques to study MI fromplutonic/intrusive rocks and focuses on MI frommore silicic (granitic) environments (Table 1-1).Veksler (2006) discusses crystallized MI fromgabbroic rocks, and Veksler & Lentz (2006)consider MI in carbonatites and related rocks. Theapplication of MI in understanding pegmatiteformation is discussed by Thomas et al. (2006).In a more general sense, this chapterfocuses on MI that have experienced significantdevitrification or crystallization following entrapment,or subsolidus aqueous alteration. This discussionthus applies to MI from the plutonic environment,as well as inclusions from the volcanic orextrusive environment that have undergone slowcooling and/or hydrothermal alteration.IN THE BEGINNING ……The first detailed discussion of MI within arigorous geological and petrological context was bySorby (1858). In his classic 1858 paper on fluid andmelt inclusions, Sorby introduced the section onMineralogical Association of Canada Short Course 36, Montreal, Quebec, p. 1-25.1

MELT INCLUSIONS IN PLUTONIC ROCKS: PETROGRAPHY AND MICROTHERMOMETRYmelt inclusions with the statement “The formationof crystals from a state of igneous fusion is in everyrespect analogous to what takes place when crystalsare formed in water”. Later he stated “There is thusa most perfect analogy between glass- and stonecavitiesand fluid-cavities in every respect exceptthe nature of the included substances”. Theimplication of this statement is that assumptionsapplied to the interpretation of FI should also applyto MI. Sorby referred to MI containing glass (± avapor bubble) as glass-cavities; those in which themelt has crystallized are referred to as stonecavities.Sorby equated the stone-cavities toaqueous FI that are cooled to low temperature andcontain ice plus various salts and hydrates that wereoriginally in solution.Sorby examined both glassy andcrystallized MI and arrived at interpretations thatare still valid today. Cameras were unavailable toSorby to photograph features observed with themicroscope, and he drew illustrative sketches ofinclusions that he observed in thin sections andgrain mounts. Several of Sorby’s drawings ofcrystallized MI from plutonic environments areshown in Figure 1-1. Much of Sorby’sunderstanding of MI is based on studies ofFIG. 1-1. Drawings of crystallized melt inclusions from Sorby (1858). The number above each drawing refers to the figurenumber in the original publication. Sorby did not include a scale as in modern publications but, rather, indicated thenumber of times the inclusions are magnified in linear dimensions, indicated by the number in parentheses following eachdescription. (A) Crystallized melt inclusion in a crystal of iron silicate from a copper slag (X1600); (B) Crystallized meltinclusion in pyroxene from a blast furnace slag (X400), (C) Crystallized melt inclusion in feldspar in a xenolith fromVesuvius, Italy (X500); (D) Crystallized melt inclusion in quartz from Ponza, Italy (X400); (E) Crystallized melt inclusionin trachyte from Ponza, Italy (X800); (F) Altered melt inclusion from a porphyry from Arthur’s Seat, near Edinburgh, UK(X400); (G) Crystallized melt inclusion in quartz from an elvan near Penrhyn, Cornwall, UK (X250); (H) Crystallized meltinclusion in quartz from the granite at St. Austell, UK (X1000); (I) Altered melt inclusion from a porphyry from Arthur’sSeat, near Edinburgh, UK (X200); (J) Crystallized melt inclusion in quartz from an elvan near Penrhyn, Cornwall, UK(X800); (K) Crystallized melt inclusion in quartz from a coarse-grained granite near Cape Cornwall, UK (X800); (L)Crystallized melt inclusion in quartz from the granite at St. Austell, UK, with radiating fractures (X600).3

MAGMATIC PROCESSES AND VOLATILE PHASE GENERATION IN PORPHYRY-TYPE ENVIRONMENTSinclusions in slags from iron furnaces in England(Fig. 1-1A, B). Sorby also described many naturalcrystallized MI, including “stone-cavities” from theisland of Ponza (Italy) (Fig. 1-1D, E). Fedele et al.(2003) studied similar samples from Ponza andheated the MI to produce a homogeneous glass thatwas then analyzed by electron microprobe. Theresults were used to develop a model for igneouspetrogenesis in the Ponza trachyte.Sorby was among the first to recognizethat coexisting FI and MI indicated entrapment in avolatile-saturated magma. Today melt-volatileimmiscibility is recognized as an important processin many magmatic–hydrothermal systems (Roedder1992), ranging from shallow granites (Frezzotti1992) to porphyry copper deposits (Bodnar 1995,Kamenetsky et al. 1999, Davidson & Kamenetsky2001) to orthomagmatic Au–Te deposits such asCripple Creek, CO (Webster 2004). Referring toinclusions trapped in a volatile-saturated melt,Sorby concluded “Some crystals might be depositedfrom solution in the highly heated water, and catchup small portions of the fused stone [to form meltinclusions], whilst others might be formed by thecrystallization of the melted stone, and catch upsmall portions of liquid water [to form aqueousinclusions]”. He went on to state “… we may, Ithink, conclude that the crystals would containglass- [glass melt inclusions] or stone-cavities[crystallized melt inclusions], and perhaps gas- andvapour-cavities [fluid inclusions], ....” Today theoccurrence of coexisting MI and FI that weretrapped from the same magma is considered to beunequivocal evidence for melt–volatileimmiscibility in the magma, as was documented atAscension Island by Roedder & Coombs (1967)based on coexisting MI and halite-rich FI. In fact,one could conclude that an assumption of volatilesaturation in the absence of coexisting MI and FI ishighly questionable.PETROGRAPHIC ANALYSIS OF MELTINCLUSIONS IN PLUTONIC ROCKSSorby noted that MI in plutonic rocks areoften difficult to identify because “In the quartz ofvery coarse grained granites the stone cavities[crystallized melt inclusions] are generally obscureand of irregular shape….” and “Those in thefeldspar are often so much obscured by the partialdecomposition of that mineral, that it is difficult todistinguish them from small decomposed patches[of feldspar]…” (see Fig. 1-1F-L). More recentworkers have also discussed the difficulty inrecognizing completely crystallized MI in plutonicrocks (Roedder 1984, Yang & Bodnar 1994), andMI that have been altered by later hydrothermalprocesses (Frezzotti 1992, Varela 1994, Student &Bodnar 2004). Altered, crystallized MI areespecially common in the porphyry copper deposits(Student & Bodnar 2004), where the rock hasundergone subsolidus alteration associated withhydrothermal activity during formation of the oredeposit. In this environment, planes of secondaryaqueous FI commonly intersect the MI, and thecompositions of the MI reflect alteration by thehydrothermal fluid (Frezzotti 1992, Student &Bodnar 2004).Crystallized MI usually appear as opaque,poorly defined patches at low magnification (Fig.1-2). At higher magnification, the inclusions appearto be dark masses of crystals lacking sharp bordersbetween the MI and the host (Fig. 1-3). Undercrossed polars the inclusions often show a fewbright spots representing birefringent daughterminerals, or an overall sparkly appearance in finergrainedinclusions. In many cases it may be difficultto distinguish between crystallized MI and trappedsolids in the host (Rapien et al. 2003). The largestMI in plutonic rocks are generally less than several10s of micrometres in longest dimension, whereasMI up to several hundred micrometres are notuncommon in extrusive rocks.FIG. 1-2. Quartz phenocryst from Stage IV granodioritefrom the Tyrone, New Mexico, porphyry copperdeposit (Student & Bodnar 2004). The crystal isapproximately 2 mm across. The phenocryst containsabundant crystallized MI, including hourglassinclusions (Anderson 1991), large, randomlydistributed MI (larger opaque areas in crystal) andnumerous, small MI along a growth zone near theouter edge of the crystal.4

MAGMATIC PROCESSES AND VOLATILE PHASE GENERATION IN PORPHYRY-TYPE ENVIRONMENTSFIG. 1-3. Examples of melt inclusions in quartz phenocrysts from Stage IV granodiorite from the Tyrone, New Mexico,porphyry copper deposit (Student & Bodnar 2004). All of the inclusions occur in the same phenocryst, which isapproximately 2 mm across (see Figure 1-2). The scale bar, shown in (Q) represents 30 µm and applies to all images. MIthat occur along growth zones (G) are generally smaller than those that are randomly distributed throughout the crystal.Some MI (K, P, Q) appear to have nucleated on solid inclusions in the phenocryst; other melt inclusions (M–O) haveanomalously large vapor bubbles, suggesting either entrapment of vapor along with melt or later reequilibration. Some MI(D, E) have fractures that extend into the surrounding quartz.Many workers have discussed processesthat lead to the formation of crystallized MI(Roedder 1979, 1984, Lowenstern 1995, Frezzotti2001). In general, for a given inclusion size, theslower the cooling rate, the more likely that the meltin the inclusions will crystallize. Similarly, for agiven cooling rate, larger inclusions are more likelyto crystallize compared to smaller inclusions. Inaddition to cooling rate and size, the composition ofthe melt may affect the crystallization behavior ofMI (Roedder 1984). Student & Bodnar (1999) notedthat synthetic MI trapped under H 2 O-saturatedconditions in the quartz-saturated haplogranitesystem were often partially to completelycrystallized (Fig. 1-4), even though the sampleswere quenched from formation conditions (720°Cand 200 MPa) to room temperature in a fewminutes. These workers suggested that the high H 2 Ocontents of the melts, and the fact that the meltswere saturated in H 2 O at the time of trapping, mighthave promoted crystallization during cooling.Inclusion SelectionThe most important aspect of any MI or FIstudy is determining which inclusions to study andwhether those inclusions record the physical andchemical conditions at the time of trapping. Assuch, the timing of inclusion trapping relative toformation of the host phase, and the position of thehost within the overall paragenesis, must beconstrained. There is surprisingly little discussion inthe literature concerning the temporal classificationof MI. Sobolev & Kostyuk (1975) described theuse of MI in studies of magmatic crystallization,5

R. J. BODNAR & J.J. STUDENTNatural (top) and synthetic (bottom)crystallized MI. The natural MI is from a quartzphenocryst in Stage IV granodiorite from the Tyrone,New Mexico, porphyry copper deposit (Student &Bodnar 2004). The synthetic MI is in quartz and wastrapped at 720°C and 200 MPa under H 2 O-saturatedconditions (Student & Bodnar 1999).FIG. 1-4.FIG. 1-5. Photomicrograph of primary MI outlininggrowth zones in a pyroxene phenocryst from the WhiteIsland, New Zealand, volcano (from Rapien et al.2003).and summarized the different temporal occurrencesof MI. These workers distinguished between zonaland azonal primary MI. Zonal inclusions define agrowth zone and there is little debate concerningtheir primary origin. [Note, however, that a primaryorigin does not guarantee that the inclusions recordthe physical and chemical conditions of crystalgrowth, as the inclusions may have reequilibratedfollowing entrapment, as described in more detailbelow.] Zonal inclusions are common in someminerals, such as nepheline, plagioclase (Halter etal. 2004b) and pyroxene (Yang & Scott 2002,Rapien et al. 2003) (Fig. 1-5), but are less commonin other minerals such as olivine (Anderson 1974)and zircon (Thomas et al. 2002).Isolated or randomly distributed inclusionsthat cannot be associated with a specific growthfeature are referred to as azonal inclusions (Fig.1-6). Such isolated inclusions are generallyconsidered to be primary if no evidence offracturing or mineral dissolution (that could allowsecondary inclusions to form) is observed. Sobolev& Kostyuk (1975) emphasized that a negativecrystal shape is not in itself sufficient evidence for aprimary origin of MI, as has also been noted for FI(Roedder 1984, Bodnar 2003a). Some minerals,such as quartz (Anderson et al. 2000, Halter et al.2004a) commonly contain both zonal and azonal MI(Fig. 1-2). Several workers (Yang & Scott 2002,Student & Bodnar 2004) have noted that within agiven phenocryst zonal inclusions tend to beconsiderably smaller than azonal MI.Within the FI community, much effort hasbeen devoted to the issue of inclusion selection. Aprocedure has been developed that allows one to beconfident that the FI selected are related to theprocess being studied, and that the inclusions havenot been affected by later events. The first step is toidentify a group of FI that were all trapped at thesame “time” and, by inference, at the sametemperature and pressure and from fluids of thesame composition. This group of inclusions,representing the most finely discriminated trappingevent that can be identified based on petrography, isreferred to as a fluid inclusion assemblage or FIA(Goldstein & Reynolds 1994). The amount of timerepresented by an FIA will vary, depending on thegeologic environment. Thus, the amount of timerepresented by FI trapped along a growth surface ofa halite crystal forming as a result of diurnaltemperature variations in a sabkha environment isless than 24 hours. Similarly, the amount of timerequired to heal a fracture in a high temperature6

MELT INCLUSIONS IN PLUTONIC ROCKS: PETROGRAPHY AND MICROTHERMOMETRYFIG. 1-6. (Top) Quartz phenocryst from the Bishop Tuff(upper left) containing numerous randomly distributed(azonal) glassy MI. (Bottom) Transmitted lightphotographs of melt inclusions (MI) in zircons fromthe Quottoon Igneous Complex, British Columbia,Canada (from Thomas et al. 2002).hydrothermal environment to produce a plane ofsecondary FI might be on the order of days to weeks(Brantley 1992, Sterner & Bodnar 1984, Bodnar &Sterner 1987).Dowty (1980) and Bacon (1989) compiledcrystal growth rates from melts for variousminerals. In general, growth rates fall in the rangefrom about 10 –2 to 10 –11 cm/s. These growth rateswould require 0.1 sec to 10 11 sec (3.17 x 10 3 years)to trap a 10 µm MI. Tomiya & Takahashi (2005)estimated average growth rates for plagioclase andpyroxene in the magma beneath Usu volcano(Japan) of 0.1 to 0.7 µm/a. Thus, an FIA composedof 10 µm MI along a growth zone in a plagioclaseor pyroxene phenocryst from this system wouldrepresent a minimum of about 15–100 years. Thisamount of time assumes a constant growth rate forthe host crystal. However, MI may be preferentiallytrapped during episodes of anomalous and rapidcrystal growth associated with changingtemperature and/or pressure and/or volatile contentin the magma (Roedder 1984), effectivelydecreasing the amount of time represented by theinclusion assemblage.It is important to emphasize that the fluidinclusion assemblage definition implies nothingabout the temporal relationship between theinclusions and the host mineral. An FIA may becomposed of either primary or secondary orpseudosecondary inclusions, The FIA concept doesnot constrain when the inclusions were trappedrelative to growth of the host mineral (other thanthat they must have been trapped during or after thehost formed). Thus, secondary (orpseudosecondary) FI along a well-defined fracturerepresent an FIA that was formed after precipitationof the bulk of the mineral in which those inclusionsoccur.While the FIA approach to selectinginclusions to study is used routinely in studies of FI,an analogous method has not been developed forMI. However, because similar petrographictechniques are (and should) be used to identifygroups of MI that were trapped at the same time, wepropose the term melt inclusion assemblage (MIA)to describe a group of MI trapped at (essentially)the same time and, by analogy, at the sametemperature and pressure and from a melt of thesame composition. Thus, MI along an individualgrowth zone in a phenocryst, such as those shownin Figures 1-2 and 1-5, represent MIAs containingprimary MI. Yang & Bodnar (1994) studied MI inquartz from deep crustal granodiorite plutons in theGyeongsang Basin, South Korea, and identifiedvarious MIAs based on petrographic analysis of thesamples. One type of MIA occurred near theinterface between plagioclase and quartz crystalsthat extended into vugs, while other MIAs wereobserved along growth surfaces near the outerportions of quartz crystals (Fig. 1-7). In both cases,petrographic evidence for simultaneous trapping ofthe MI in the MIA was conclusive (although there issome uncertainty as to whether the MIA at theplagioclase–quartz interface represents primary orsecondary MI). Compared to FI, secondary MI aremuch less common. Thus, at a minimum the MI in agiven phenocryst can be considered to be an MIAcomposed of inclusions that were all trapped duringthe (unknown) amount of time required for thephenocryst to grow.7

R. J. BODNAR & J.J. STUDENTevaluating the quality of data obtained from MI is toidentify MI that represent a MIA, as describedbelow (Fig. 1-8).If MI do not occur in growth zones oralong fractures, it is sometimes possible to groupMI that show similar petrographic characteristicsand, by inference, trapped a melt of the samecomposition. Anderson et al. (2000) noted thatquartz phenocrysts from the Bishop Tuff containclear, faceted MI in the interior, and round, brownMI close to the edge of the crystal. These workersused this zonal arrangement to infer the sequence oftrapping – thus the clear MI would be assigned toone MIA, and the brown MI to a later MIA, bothassociated with growth of the crystal.Recently, many workers have successfullyused cathodoluminescence (CL) analysis to identifyFI that belong to the same FIA (Landtwing et al.2005). Peppard et al. (2001) used CL to revealgrowth zones in quartz from the Bishop Tuff, andused this information to identify MI with a commonorigin. CL is a powerful technique that can be usedto reveal growth textures in phases that show nosuch features during normal petrography, and its usein MI studies is expected to increase.FIG. 1-7. Schematic representation of the distribution ofmostly crystallized MI in quartz from deep crustalgranitoid intrusions from the Gyeongsang Basin, SouthKorea. Abundant small (12 µm) MI show random orisolated distribution (B). The outer growth zone of thequartz is decorated by numerous MI (C). Some MI inthe outermost portion of the crystal contain devitrifiedglass (D). The scale corresponds to the crystal only –MI have been drawn larger for illustrative purposes.(from Yang & Bodnar 1994).It is relatively easy to assign MI alonggrowth surfaces or other interfaces to the sameMIA. However, many minerals or phenocrystscontain only one or a few melt inclusions, and theseare commonly distributed irregularly within thegrain (Fig. 1-6). Where two or more randomlydistributed MI occur within a given crystal,petrographic observations usually do not providesufficient information to determine if the inclusionsrepresent an MIA, i.e., were trapped at the sametime and at the same temperature and pressure andfrom a melt of the same composition. Thisdistinction is important because the first step inMICROTHERMOMETRYIn FI studies, once a FIA has beenidentified, the next step is to determine if theinclusions record the physical and chemicalconditions of trapping. To address this, the FIAmust be tested to determine if the inclusions adhereto “Roedder’s Rules”, which are based on criteriafirst proposed by Sorby (1858). According toRoedder (1984), FI record the original trappingconditions if the following requirements are met:1. the inclusions trap a single, homogeneous phase,2. nothing has been added to or removed from theinclusion following trapping,3. the inclusion represents an isochoric (constantvolume) system.Note that the requirement of constant volumeapplies to the volume of the original cavity in whichthe fluid was trapped. During cooling of a MI toroom temperature some (considerable?) amount ofmaterial may precipitate on the inclusion walls,resulting in a volume that is apparently smaller thanthe original MI volume at the trapping conditions. Ifthis material is incorporated back into the meltwhen the MI is heated to homogenization (see Fig.1-9), the condition of constant volume is satisfied.The condition would not be satisfied if the hostmineral surrounding the MI deformed8

MAGMATIC PROCESSES AND VOLATILE PHASE GENERATION IN PORPHYRY-TYPE ENVIRONMENTSIdentify a Melt Inclusion Assemblage (MIA) based on petrographyStopNoConsistent Phase Relations?NoYesHomogenize MI:Consistent Microthermometry?YesAnalyze MINoWhy do MI have variablephase relations and/ormicrothermometry?Mixed TrappingReequilibrationin natureReequilibrationin the laboratoryMI do not representan MIAInterpret ResultsFIG. 1-8. Flow chart showing the steps that should be followed to ensure that data obtained from MI represent conditions inthe magma at the time of trapping. The first step is to identify MI that were trapped at the same time, and at the sametemperature and pressure and from a melt of the same composition, and thus represent a melt inclusion assemblage (MIA).Then, petrographic, microthermometric and chemical data are evaluated to test for reequilibration of the MI after trapping.FIG. 1-9. Series of photomicrographs showing the behavior of two crystallized MI in quartz from the Red Mountain, Arizona,USA, porphyry copper deposit during heating. Evidence of melting is first observed at about 675°C. At 790°C theinclusions contain a vapor bubble and a small feldspar crystal (not visible). At 810°C both the vapor bubble and feldspardisappear (from Student & Bodnar 2004).9

MAGMATIC PROCESSES AND VOLATILE PHASE GENERATION IN PORPHYRY-TYPE ENVIRONMENTSplastically during heating to produce a volume thatwas larger than the original MI volume (Bodnar2003b).How does one test the inclusions in a FIAto determine if “Roedder’s Rules” are satisfied?Studies of FI have shown rather conclusively thatwhen FI re-equilibrate following entrapment themicrothermometric and chemical properties of theFI show a wide range compared to inclusions thathave not changed (Bodnar 2003b). Thus, if all ofthe inclusions within an FIA have the same numberof phases and in the same volume proportions whenobserved at room temperature, and if thetemperatures of phase changes are the same in all ofthe inclusions in the assemblage (indicating that theinclusions all have the same composition), one canbe reasonably certain that the inclusions record theoriginal formation conditions.While one can usually determine if all ofthe FI in an FIA contain the same phases and are inthe same volume proportions, this is usually notpossible with crystallized MI because the individualphases within the MI cannot be discerned (see Fig.1-3). As such, crystallized MI must be homogenizedand analyzed to determine if all of the inclusions inthe MIA have the same composition and phasebehavior during heating. Techniques that arecommonly used to analyze MI include electronmicroprobe, SIMS (Layne 2006), FTIR, Ramanspectroscopy and laser ablation ICP–MS (Pettke2006, Halter et al. 2006). All of these techniques(except LA–ICP–MS; Halter et al. 2002) require ahomogeneous glass, thus necessitating that the MIbe heated to homogenization and quenched.Several different techniques have beenused to homogenize MI. These techniques canbroadly be divided into those in which the MI areheated at one atmosphere confining pressure andthose in which the MI are heated under an elevatedconfining pressure. These techniques may be furtherdivided based on whether the MI are heated continuouslyin one step or are heated incrementally.The most commonly used technique tohomogenize MI is to heat the inclusions at oneatmosphere in a microscope-mounted heating stage(Clocchiatti 1975, Magakyan et al. 1993,Lowenstern 1994, Reyf 1997, Fedele et al. 2003).The heating is usually conducted in an inert orreduced gas (He±H 2 ) atmosphere to preventoxidation of the sample. Magakyan et al. (1993)heated MI in clinopyroxene from boninite in a oneatmosphere stage and report an error of ±20°C forhomogenization temperatures (T h ) in the 900–1400°C range. These workers heated the sample ata rate of 30–40°C/minute for temperatures above900°C and held the sample at the maximumtemperature for 7 minutes before quenching. Theynoted that slower heating rates or longer time at themaximum temperature resulted in an increased T hand oxidation of the sample. Lowenstern (1994)heated MI in quartz phenocrysts from Pantelleria,Italy, in a one atmosphere heating stage at a rate of50°C up to 600°C. The sample was held at 600°Cfor 10–20 minutes, then heated to 700°C andmaintained at this temperature for 10 minutes. Thesample was then heated in 50°C increments with 10minute equilibration periods after each step up to800°C. At higher temperatures 25°C heating stepswere used. Lowenstern (1994) noted that theseheating rates were likely too rapid, resulting inobserved T h that were 25–75°C higher than theactual temperatures. Reyf (1997) heated small (3–5µm) MI in quartz from a granite in a microscopeheating stage, using heating steps of 30–50°C, andholding the sample at temperature for 1.5–2 hoursafter each step. Although this technique involvedlong heating experiments, the results wereconsistent with previous work that suggested hostgranites were hypersolvus and crystallized outsideof the region of immiscibility in the Ab–An–Orsystem at 50–100 MPa. Massare et al. (2002)studied MI in olivine phenocrysts and found thatwhen heated at one atmosphere the T h increasedsystematically with time as a result of deformationof the host crystal and loss of H 2 O from the MI.Danyushevsky et al. (2002) conducted a detailedstudy of the effect of heating schedule on themeasured T h . They found that T h decreases withdecreasing heating rate and approaches a constantvalue. Using this kinetic technique, the “correct”heating rate for each host phase and melt compositioncan be determined. Using faster heating rateswill result in measured T h that are too high, whileusing heating rates that are slower than that atwhich the T h levels off increases the likelihood thatthe inclusion composition will change as a result ofdiffusion of components out of (or into?) theinclusion (Qin et al. 1992, Danyushevsky et al.2000, Gaetani & Watson 2000). One of the mostdetailed studies of the effect of heating rate,inclusion size and volatile content on thehomogenization behavior of MI is that of Thomas(1994a). Thomas used this information to develop amethod to determine a minimum homogenizationtemperature for any given MI, which he interpretedto represent the trapping temperature of the MI.10

MELT INCLUSIONS IN PLUTONIC ROCKS: PETROGRAPHY AND MICROTHERMOMETRYRather than heating MI in a microscopemountedheating stage, some workers heatedinclusions at one atmosphere in a tube furnace(Yang & Bodnar 1994, Webster et al. 1997, Raia etal. 2000, Thomas & Webster 2000, Stockstill et al.2005). While this technique is similar to heating atone atmosphere in a microscope-mounted stage, itdoes not allow MI to be monitored continuouslyduring heating to determine temperatures of phasechanges and to look for “anomalous behavior”during heating. Some workers heated the sample inone step to the final temperature (Thomas &Webster 2000). Other workers (Yang & Bodnar1994, Student & Bodnar 2004) heated the samplesincrementally and observed the sample under themicroscope between heating steps to monitor themelting behavior (Fig. 1-9).Heating MI at one atmosphere pressure,either in a microscope-mounted stage or in a tubefurnace, works well for MI with low concentrationsof volatiles. However, if the MI contain significantamounts of H 2 O (or CO 2 ?) the inclusions willcommonly decrepitate before T h is reached, owingto the high internal pressures generated duringheating. As a result, many workers heat crystallizedvolatile-rich MI only until the solid phases havedissolved, producing an inclusion that contains meltplus vapor bubble (Fig. 1-10), or heat the MI in apressure vessel (Skirius et al. 1990, Webster &Duffield 1991, Anderson et al. 2000, Audétat et al.2000, Thomas et al. 2002, Student & Bodnar 2004)or in a pressurized microscope stage (Massare &Clocchiatti 1987). This technique minimizesdecrepitation of volatile-rich MI, althoughAnderson et al. (2000) reported that some CO 2 waslost from MI as a result of heating at 800–900°Cunder ≈200 MPa of Ar pressure for 20 hours.Student & Bodnar (1999) investigated theeffect of heating technique on the observed T h ,using synthetic silicate MI trapped at known P–Tconditions. These workers found that, regardless ofthe technique used, T h increased with increasinginclusion size (Fig. 1-11), suggesting thathomogenization of MI is a diffusion-controlledprocess, as previously argued by Thomas (1994a)and Thomas et al. (1996). MI heated in one step atone atmosphere in a tube furnace produced T h thatmost closely matched known temperatures.Inclusions heated continuously in a one atmospherestage with a heating rate of 1°C/minute were about10–15°C higher than the correct temperature, andthose heated continuously at 3°C/minute were about25°C too high (Fig. 1-12).An additional complication associated withcrystallized MI from magmatic–hydrothermal oredeposits is that the host phase is often altered and/orcrosscut by numerous planes of aqueous FI. Manyof these planes intersect MI, and promote thedecrepitation of MI during heating. Student &Bodnar (2004) tested various methods tohomogenize MI in quartz phenocrysts containingabundant planes of secondary FI, including heatingin a microscope heating stage, heating in a oneatmosphere vertical tube furnace, and heating underpressure in a cold-seal pressure vessel.The method that proved most satisfactoryto homogenize crystallized MI in phenocrysts withabundant planes of aqueous FI involved heatingFIG. 1-10. Melting sequence of a crystallized MI in plagioclase in trachyte from Ponza, Italy. At room temperature theinclusion consists of a mass of intergrown, fine-grained crystals ± glass. By 1092°C most of the silicate minerals havemelted leaving only a mass of fine-grained opaque minerals, which melt between 1092 and 1197°C. After quenching, theinclusion contains a homogeneous glass and a vapor bubble (from Fedele et al. 2003).11

R. J. BODNAR & J.J. STUDENT8708603°C/min1°C/minHomogenization Temperature (°C)850840830820810800TrappingTemperature800 ± 5 °C53°C/minThermal CyclingT h = 819-839 °Cn=16Tube FurnaceT h = 800 ± 10 °Cn=18Frequency4 3 217907801°C/minT h = 806-853 °C n=163°C/minT h = 819-863 °C n=160 10 20 30 40 50 60 70 80Inclusion Area ( m2 )90 100 110FIG. 1-11. Effect of inclusion size and heating technique on the measured homogenization temperature (T h ) of syntheticsilicate MI. The T h ranges for heating in a tube furnace (at one atmosphere) and the thermal cycling experiments areshown by the shaded boxes, along with the experimental conditions. These experiments did not consider inclusion size.A histogram of frequency versus T h for the thermal cycling experiment is inset along the right side of the diagram.Homogenization temperature as a function of inclusion area (as viewed through the microscope) for continuous heatingexperiments (1°C/min and 3°C/min) are indicated with unfilled and filled circles, respectively. Black vertical tie lines linkdata for the same inclusion. The solid line drawn through the data points for the continuous heating experiments shows thegeneral relationship between inclusion size and T h , with T h approaching the trapping temperature as inclusion sizeapproaches zero. The T h for the tube furnace experiment coincides most closely with the known trapping temperature(from Student & Bodnar 1999).12

MELT INCLUSIONS IN PLUTONIC ROCKS: PETROGRAPHY AND MICROTHERMOMETRY400350(a)Entrapment P-TestimateHaplograniteminimum curvePressure (MPa)30025020015010050Trapping Conditions800 °C and 200 MPaFluid inclusion T h11.9 wt% NaCl fluid isochoreMelt inclusionT h range200 300 400 500 600 700 800 900 1000Temperature °C230(b)3 °C/min Thermal CyclingPressure (MPa)2202102003 °C/min Continuous Heating1 °C/min Continuous HeatingTube Furnace11.9 wt% NaClfluid isochoreTrapping Conditions800 ± 5 °C, 200 ± 5 MPa190750 800 850Temperature °CFIG. 1-12. Estimated P–T formation conditions for coexisting melt and aqueous synthetic inclusions, calculated usingvapor/melt T h for the four different heating experiments. The top diagram shows the P–T estimate for the ≈3°C/minthermal cycling heating experiment, showing the aqueous inclusion liquid/vapor curve and isochore to illustrate the"intersecting isochore" technique (Roedder & Bodnar 1980) used to determine formation pressure. The bottom diagramshows the P–T estimates for four different heating experiments shown in Figure 1-11 (from Student & Bodnar 1999).13

MAGMATIC PROCESSES AND VOLATILE PHASE GENERATION IN PORPHYRY-TYPE ENVIRONMENTSsamples under an elevated confining pressure.Phenocrysts were placed in a 5 mm long, 5 mm ODplatinum capsule and loaded into a pressure vessel.A small hole was punched into the capsule bottomand top to permit the argon pressure medium tofreely enter the capsule at run conditions to preventcollapse of the capsule and possible crushing of thesample. The vessel was sealed and pressurized withargon to 30–50 MPa, then lowered into a preheatedfurnace. Once the pressure reached 100 MPa duringheating, argon was continuously bled off such thatthe sample was heated isobarically to the final runtemperature.The technique for homogenizing MI in apressure vessel involves heating in one step to someelevated temperature and holding the sample at thistemperature for a sufficient amount of time toassure homogenization of a significant proportion ofthe inclusions. In general, the smaller inclusions inthe sample homogenize first (at lower temperatureand/or shorter run durations). Thomas et al. (1996)observed similar behavior. To determine theminimum run time and temperature required tohomogenize a significant portion of the inclusionswhile at the same time minimizing the number ofMI that decrepitate, a few phenocrysts from onesample were heated incrementally and examinedafter each heating step to determine thehomogenization progress (Fig. 1-9). In mostsamples, a large portion of the MI homogenizedover a range of a few tens of degrees – at this pointthe heating experiment was stopped. If heating werecontinued in an attempt to homogenize allinclusions in the phenocryst, those with lower T hwould have decrepitated. Once the minimum T h wasdetermined in this way, several phenocrysts fromthe same sample were heated to that temperature inone step and held for 24 hours.Heating under pressure significantlyreduced decrepitation of MI. However, even underthese conditions the larger inclusions (greater thanabout 30–50 µm) still decrepitated. Other workers(Sterner & Bodnar 1989, Skirius et al. 1990,Schmidt et al. 1998) have previously shown thatheating FI or MI under confining pressure eliminates(or minimizes) decrepitation. Additionally,Massare & Clocchiatti (1987) reported that, whenMI are heated in a pressurized microscope heatingstage, T h of rhyolitic MI in sanidine decrease by70°C/100 MPa in the temperature range 560–850°C.As noted above, MI containing H 2 O-richcompositions often decrepitate, even when heatedunder an elevated pressure. Decrepitation resultsbecause the internal pressure in the MI exceeds thestrength of the host mineral (Bodnar 2003b).Thomas (1994b) described qualitatively the P–Tpath followed by H 2 O-rich MI during heating to thesolidus and during melting, and Student & Bodnar(1996) quantified the effect of H 2 O on the P–T pathfollowed by MI during cooling (or heating). Thepartial molar volume of H 2 O in hydrous melts isless than the molar volume of H 2 O in the vaporphase, and this difference becomes greater at lowerpressures where the molar volume of H 2 O is large.As a result, as H 2 O exsolves from the melt phase inan MI of constant volume (i.e., assumes the volumeof the host phase does not change with changingtemperature and pressure), the pressure in theinclusion increases. For example, if an MI istrapped on the H 2 O-saturated solidus at 50 MPa and782°C, H 2 O will begin to exsolve from the melt asthe sample cools and the MI begins to crystallizefeldspar and quartz (Fig. 1-13A). [This assumes thatphase equilibrium is maintained during cooling.While synthetic MI do appear to maintainequilibrium during heating and cooling, it isunknown whether this applies to natural MI.] Thepressure in the MI will continue to increase from 50MPa as crystallization proceeds and the P–T pathfollows the H 2 O-saturated solidus, reaching amaximum pressure of about 150 MPa when the MIis completely crystallized at 708°C (Fig. 1-14A,point II). With further cooling the MI will followthe isochore corresponding to the H 2 O density in thecrystallized inclusion (path II – I, Fig. 1-14A). Thehigh pressures generated as the MI cools along theH 2 O-saturated solidus may cause the quartz hostsurrounding the MI to fracture with loss of H 2 O.The relative pressure increase during coolingdecreases with increasing trapping pressure owingto the smaller difference between the partial molarvolume of H 2 O in the melt and the molar volume ofthe free H 2 O phase. Thus, the pressure in a MItrapped on the H 2 O-saturated solidus at 200 MPaand 682°C increases to only about 230 MPa as thesample cools along the H 2 O-saturated solidus (Fig.1-14B).Water-rich MI may not decrepitate duringcooling in nature, owing to the elevated confiningpressure. However, when heated in the laboratory tohomogenize the MI in preparation for later analysis,the MI will follow the reverse of the P–T pathfollowed during cooling in nature. Thus, the MItrapped on the H 2 O-saturated solidus at 50 MPa and782°C will generate an internal pressure of about14

MAGMATIC PROCESSES AND VOLATILE PHASE GENERATION IN PORPHYRY-TYPE ENVIRONMENTSFIG. 1-13. Calculated phase behavior of crystallized MI that trapped an H 2 O-saturated melt in the haplogranite system at50 (A) and 200 (B) MPa. The calculated pressure in the MI during cooling from trapping conditions (and heating tohomogenization) assumes a partial molar volume for H 2 O in the melt of 22 cm 3 /mole. The heating sequences A & Bcorrespond to the P–T paths A & B, respectively, shown in Fig. 1-14, and the heating increments labeled I – V correspondto the P–T points I – V on Fig. 1-14. See text for additional details. (from Student & Bodnar 1996).FIG. 1-14. Calculated P–T path during heating of crystallized MI that trapped an H 2 O-saturated melt in the haplogranitesystem at 50 (A) and 200 (B) MPa. The calculated pressure in the MI during cooling from trapping conditions (and heatingto homogenization) assumes a partial molar volume for H 2 O in the melt of 22 cm 3 /mole. The P–T paths shown on A & Bcorrespond to the heating sequences A & B, respectively, shown in Fig. 1-13, and the P–T points labeled I – V correspondto the heating increments I – V on Fig. 1-13. See text for additional details. (from Student & Bodnar 1996).15

R. J. BODNAR & J.J. STUDENT230 MPa before melting begins (i.e., before the pathintersects the H 2 O-saturated solidus) (point II, Fig.1-14A). Once melting begins, the path follows theH 2 O-saturated solidus and the pressure in theinclusion decreases with continued heating as H 2 Odissolves into the melt phase.SUMMARY OF PETROGRAPHY ANDMICROTHERMOMETRYThe first step in any MI study is to identifya melt inclusion assemblage, or MIA, thatrepresents a group of inclusions that were alltrapped at the same time. By extension, thisrequirement implies that all of the MI in theassemblage trapped a melt of the same compositionand at the same temperature and pressure. It isimportant to emphasize that a MIA is identifiedbased initially on petrographic analysis of thesample, and not on MI compositions. Subsequentanalysis of the MI in the assemblage can help toconfirm that the inclusions being studied do indeedrepresent a MIA, and to distinguish between thoseMI that trapped a single, homogeneous melt andmaintained the original composition during latercooling in nature and heating in the lab, and thosethat have not.After an MIA has been identified, the nextstep is to determine if the MI record the originalphysical and chemical conditions of trapping. ForFI, this is most easily accomplished based onobservation of consistent phase relations in all theinclusions within the FIA, and is confirmed byconsistent microthermometric results from all FI inthe assemblage. For crystallized MI, it is generallynot possible to determine the phase relations of theMI owing to the poor optics of MI and host mineral.In this case, the inclusions within the MIA shouldbe tested for consistency in microthermometric andcompositional data. As an example, Figure 1-15shows a quartz phenocryst from the Red Mountain,Arizona, porphyry copper deposit with a zone ofprimary MI trapped along a growth (or possiblyresorbtion) surface. A portion of this phenocrystwas heated under 100 MPa confining pressure asdescribed above, and the phase relations of 86 MIwere monitored. It was previously determined(Student & Bodnar 2004) that the MI were trappedon the H 2 O-saturated solidus at about 810°C. Thus,when heated, the last daughter mineral (in this casea feldspar crystal) and the vapor bubble shoulddisappear at the same temperature (i.e., similar tothe MI in Fig 1-13A), which also represents thetrapping temperature. Thirty-seven of the 86inclusions from the MIA showed this behavior (Fig.1-15), indicating that these 37 inclusions trappedonly the H 2 O-saturated melt phase and did notreequilibrate after trapping. This conclusion isbased on the fact that any change in compositionand/or volume of the MI would have producedvariability in the mode and temperature ofhomogenization (Bodnar 2003b). Thirty of the 86inclusions contained a feldspar crystal of varyingFIG. 1-15. Results of heatingexperiments on 86 crystallized MIfrom a single growth (orresorbtion) zone in a quartzphenocryst from the RedMountain, Arizona, porphyrycopper deposit. See text fordetails.16

MAGMATIC PROCESSES AND VOLATILE PHASE GENERATION IN PORPHYRY-TYPE ENVIRONMENTSsizes after heating to 810°C. These inclusions areinterpreted to have trapped H 2 O-saturated melt plusa feldspar crystal. Twelve of the MI contained bothfeldspar and a vapor bubble after heating to 810°C,indicating that these inclusions either trappedfeldspar and vapor along with the melt, orreequilibrated, following entrapment. Theremaining 7 MI had various combinations of melt,crystals and vapor bubble present after heating to810°C, again suggesting mixed trapping and/orreequilibration. Thus, analyses of any of these 49MI that did not show simultaneous dissolution offeldspar and vapor bubble at 810°C would provide amelt composition that is not representative of themelt that was present at the time of trapping.Random selection of MI in this MIA would provideerroneous (and misleading) information if thebehavior of the MI during heating was notmonitored before analyses were conducted.Observations of MI behavior during laboratoryheating thus provide a test of assumptions regardingthe timing of inclusion trapping and possiblereequilibration.The protocol that one should follow in theselection of MI for study is outlined in Figure 1-8.To summarize, one should first select two or moreMI that define a melt inclusion assemblage (MIA),based on petrographic examination of the sample.MI in the same growth zone or along a healedfracture are examples of MI occurrences that couldbe used to define an MIA. If MI cannot be relatedto a MIA, one should reconsider whether a MI studyshould be undertaken, as it is unlikely that one willbe able to argue convincingly that the resultsobtained represent the melt that was present at thetime of MI formation. After an MIA is identified,the MI should be examined to determine if theycontain consistent phase relations (Fig. 1-8). If not,the MIA should be abandoned because the MI haveeither trapped mixtures of phases or have reequilibratedafter trapping, or both. In either case, itis unlikely that the MI will provide usefulinformation concerning the physical and chemicalconditions in the magma. If the phase relationscannot be determined at room temperature becauseof poor optics or because the phases are too finegrained,the MI should be heated to homogenization.Assuming that the inclusions trapped a single,homogeneous melt phase, all MI should showsimilar temperatures of phase changes duringheating. If the MI show variability in the order inwhich various phases disappear (i.e., solids andvapor bubble) and/or in the temperatures ofphase changes, the MIA should be abandonedbecause, as with the example of inconsistentphase relations described earlier, the MI do notrepresent the original melt that was present at thetime of trapping. Those MI in the MIA that showsimilar phase behavior and temperatures of phasechanges, such as the 37 MI in the Red Mountainsample described above, are most likely to havetrapped the melt that was present and have not reequilibratedfollowing trapping. It is theseinclusions, and only these inclusions, that should beselected for further analysis and interpretation. Byapplying these simple tests and selecting onlyinclusions that satisfy the criteria outlined above,one can obtain data that accurately reflectconditions in the magma at the time that the MIwere trapped.MELT INCLUSIONS IN PORPHYRY-TYPEDEPOSITSA plutonic environment in which MI havecontributed significantly in recent years is inunderstanding ore-forming processes in porphyrytypedeposits (see Student & Bodnar 2004 forreview). Porphyry copper deposits are associatedwith epizonal siliceous intrusions emplaced atconvergent margins. While FI studies haveimproved our understanding of the nature and roleof magmatic–hydrothermal fluids in porphyrysystems (Roedder 1971, Nash 1976, Bodnar 1995,Beane & Bodnar 1995, Roedder & Bodnar 1997,Davidson & Kamenetsky 2001), it has been onlyrecently that workers have begun to study the meltMI in these systems.Student & Bodnar (2004) summarized aprotocol for studying crystallized MI in samplesthat have undergone extensive subsolidus hydrothermalalteration, such as those in porphyry copperdeposits. The technique produces glassy MI with ahomogeneous composition that are amenable toanalysis by a variety of techniques, includingsynchrotron XRF, PIXE, SIMS, electron microprobe,Raman spectroscopy, FTIR spectroscopy andlaser ablation ICP–MS. Importantly, the ore metalcontent of MI representing different stages in themagmatic history can be determined, and relativedifferences between pre-, syn- and post-mineralizationsamples (Fig. 1-16) can be compared withmodels for metal partitioning between melt andcoexisting magmatic–hydrothermal fluids (Candela& Holland 1986, Candela 1989, 1997, Cline &Bodnar 1991, Lynton et al. 1993: Candela & Piccoli1995).17

MAGMATIC PROCESSES AND VOLATILE PHASE GENERATION IN PORPHYRY-TYPE ENVIRONMENTSFIG. 1-16. Comparison of Zn and Cu concentrations in MI from the porphyry copper deposits at Red Mountain, Arizona, andTyrone, New Mexico, with concentrations in MI from the White Island volcano, New Zealand. Metal concentrations inpre-mineralization quartz latite at Red Mountain and at White Island are higher than those in syn- and post-mineralizationMI from Red Mountain and Tyrone. (from Student & Bodnar 2004).AquartzBquartzRM QL MIMI RM D2Ty QM MIphyllicphyllicRMD2potassicalbiteorthoclase albiteorthoclaseFIG. 1-17. Compositions of MI from the porphyry copper deposits at Red Mountain, Arizona (A), and Tyrone,New Mexico (B). Hydrothermal fluids associated with phyllic alteration in porphyry copper deposits lie alongthe quartz–potassium feldspar (orthoclase) join near the quartz apex, whereas fluids associated with potassicalteration lie near the orthoclase apex. MI compositions at Red Mountain produce a trend (arrows, Fig. 1-17A)that projects towards the composition of fluids associated with phyllic alteration, whereas MI at Tyrone showtrends (arrows, Fig. 1-17B) that project to both phyllic and potassic alteration fluids. (from Student & Bodnar2004).18

MELT INCLUSIONS IN PLUTONIC ROCKS: PETROGRAPHY AND MICROTHERMOMETRYAs noted above, MI in porphyry copperand related deposits are often intersected by one ormore planes of FI. It is thought that most of thesefractures occur at subsolidus conditions associatedwith hydrothermal alteration and may affect thecompositions of MI. This interpretation issupported by the observation that compositions ofMI fall along trends that project to compositions offluids associated with potassic and/or phyllicalteration in porphyry copper deposits (Fig. 1-17).An interesting application of MI is tocompare the geochemistry of magmatic systemsthat host (or have the potential to host) economicmineralization and those that are barren or subeconomic.As an example, Rapien et al. (2003)studied MI from the White Island, New Zealand,volcano and compared the results to bulk rockcompositions in productive and barren porphyryintrusions. These workers concluded that the WhiteIsland magma has the potential to generateeconomic porphyry copper-type mineralization, butthat the magmatic system has not evolved to theproductive stage, i.e., the magmatic–hydrothermalsystem is too “young”. Similarly, Audétat & Pettke(2003) studied MI and coexisting FI from twobarren plutons in New Mexico, USA. Theyconcluded that the absence of mineralization wasrelated to the low salinity of the exsolvingmagmatic fluids, resulting in less efficientextraction of metals from the melt. Grancea et al.(2001) studied MI in mineralized and barrenintrusions in Romania and found that MI inmineralized systems were enriched in S and had alower Al/(K+Na+2Ca) compared to barrensystems.Kamenetsky et al. (1999) studied mixedsilicate glass and crystalline silicate–sulfate–carbonate–sulfide–oxide inclusions from theDinkidi Cu–Au porphyry deposit, Philippines. Theinclusions are enriched in ore metals, and theyinterpreted the inclusions to have originally formedearly in the ore-forming process as a result ofimmiscibility. Harris et al. (2003) observedcoexisting silicate MI and high-salinity and vaporrichFI in magmatic-hydrothermal quartz veins fromthe Bajo de la Alumbrera porphyry copper deposit,Argentina. The close association of these threedifferent inclusion types was interpreted torepresent melt–aqueous fluid immiscibility, andcompositions of the coexisting inclusions were usedto calculate bulk partition coefficients for ore metalsbetween the melt and magmatic aqueous phase.Schmitt et al. (2002) studied MI in peralkalinegranites from the Amis Complex in Namibia. TheMI are enriched in Nb and REE and proved that theperalkaline composition and rare metal enrichmentsare primary magmatic features and not the result oflater hydrothermal activity.SUMMARYThis chapter attempts to summarize amethodology to recognize, select and study MI inplutonic rocks. In general, MI from this environmentare small, mostly to completely crystallized,and difficult to recognize during normalpetrographic observations.Crystallized MI must be homogenized toconfirm that all MI in a melt inclusion assemblageshow similar modes and temperatures of phasechanges and, thus, likely trapped samples of theoriginal homogeneous melt and did not reequilibratefollowing entrapment. The techniquethat proves most reliable to homogenizecrystallized, volatile-rich MI is to heat the samplesin one step in a pressure vessel under an elevatedconfining pressure.The most important take-home messagefrom this chapter concerns the selection of MI tostudy. Numerous studies of natural and synthetic FIand MI show clearly that the best evidence thatinclusions have trapped a single homogeneousphase and have not reequilibrated followingentrapment is if all of the MI or FI in anassemblage show the same phase behavior andtemperatures of phase changes. Trapping mixturesof phases, leakage, or change in the volume of theinclusion, all result in a wide range in phaserelations and temperatures of phase changes. Thus,if a group of MI that were all trapped at the sametime (MIA) show similar phase behavior andtemperatures of phase changes (includinghomogenization), one can have a high level ofconfidence that the MI provide information on thephysical and chemical environment at the time oftrapping.ACKNOWLEDGEMENTSDuring the past decade many students,colleagues and visitors to the Fluids ResearchLaboratory at Virginia Tech have contributed to ourunderstanding of melt inclusions in plutonicenvironments. We wish to acknowledgecontributions from Andreas Audétat, ClaudiaCannatelli, Benedetto DeVivo, Luca Fedele, JohnMavrogenes, Maria Rapien, Nobu Shimizu, KarenStockstill, Csaba Szabo, Jay Thomas, Rainer19

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