sign in sign up
<<
Home , Leucitite, Magma, Nephelinite, Volcanic gas

Mineralium Deposita ,2001) 36: 490±502 DOI 10.1007/s001260100185

ARTICLE

Jacob B. Lowenstern in and implications for hydrothermal systems

Received: 15 September 1999 / Accepted: 12 April 2001 / Published online: 6 July 2001 Ó Springer-Verlag 2001

Abstract This review focuses onthe solubility, origin, presence of CO2 caninduceimmiscibility both withinthe abundance, and degassing of carbon dioxide ,CO2)in magmatic volatile phase and in hydrothermal systems. ±hydrothermal systems, with applications for Because some metals, including gold, can be more vol- those workers interested in intrusion-related deposits of atile invapor phases thancoexistingliquids, the pres- gold and other metals. The solubility of CO2 increases ence of CO2 may indirectly aid the process of with pressure and magma alkalinity. Its solubility is low metallogenesis by inducing phase separation. relative to that of H2O, so that ¯uids exsolved deep in the tend to have high CO2/H2O compared with ¯uids evolved closer to the surface. Similarly, CO2/H2O will typically decrease during progressive decompres- Introduction sion- or -induced degassing. The temper- ature dependence of solubility is a function of the After H2O, carbondioxide is the secondmost common speciationof CO 2, which dissolves inmolecular form in in volcanic exhalations ,White and Waring 1963; ,retrograde temperature solubility), but exists Symonds et al. 1994). In exsolved from basaltic as dissolved carbonate groups in ,prograde). magmas, CO2 sometimes exceeds water in concentration Magnesite and dolomite are stable under a relatively ,e.g., Gerlach 1980; Giggenbach 1997), presumably due wide range of conditions, but melt just above the to its greater abundance in some mantle source regions. solidus, thereby contributing CO to mantle magmas. 2 The presence of CO2 inmagmatic systems, combined Graphite, diamond, and a free CO2-bearing ¯uid may be with its relatively low solubility, results inearly exsolu- the primary carbon-bearing phases in other mantle tionof vapors that are rich inCO 2 ,Holloway 1976). source regions. Growing evidence suggests that most Once exsolved, this low density vapor phase has im- CO is contributed to arc magmas via recycling of sub- 2 portant implications for the ascent and eruption of CO2- ducted and its overlying sediment blanket. bearing magmas ,Papale and Polacci 1999). Many Additional carbon can be added to magmas during workers have noted evidence for CO2-rich ¯uids ina magma±wallrock interactions in the crust. Studies of variety of intrusion-related ore deposits, particularly ¯uid and melt inclusions from intrusive and extrusive those associated with Au and lithophile enrichment igneous rocks yield ample evidence that many magmas ,Newberry et al. 1995; Goldfarb et al. 1998; Thompson are vapor saturated as deep as the mid crust ,10±15 km) et al. 1999). Insuch systems, CO 2 may be derived from and that CO2 is anappreciable part of the exsolved the magma, and even from a mantle source, although vapor. Such is the case inboth basaltic andsome there are other means for CO2 to enter ore-forming magmas. Under most conditions, the presence of a CO2- systems such as decarbonation reactions in wallrock and bearing vapor does not hinder, and in fact may promote, dissolutionof CO 2 and carbonate from crustal the ascent and eruption of the host magma. Carbonic hydrothermal systems. ¯uids are poorly miscible with aqueous ¯uids, particu- This review focuses onCO inmagmas, providing larly at high temperature and low pressure, so that the 2 anintroductiontothe behavior of CO 2-bearing vapors withinmagmas, andas they enterhydrothermal sys- tems. First, it outlines current knowledge about the solubility of carbonspecies insilicate melts, thencon- J.B. Lowenstern U.S. Geological Survey, Mail Stop 910, tinues with a summary of carbon in the mantle and 345 Middle®eld Road, Menlo Park, CA 94025, USA mantle melts, and proceeds to discuss CO2 ininter- E-mail: [email protected] mediate and silicic magmas. Next, it concentrates on 491 the compositionof exsolved magmatic vapors andthe gens). She found that the e€ect of Ca2+ was most pro- + + 2+ 2+ e€ect of CO2 on magma properties, ascent, eruption, nounced, followed by K ,Na ,Mg ,andFe in and intrusion. Finally, it provides a brief summary of descending order of importance. In contrast, CO2 does CO2 inhydrothermal andgeothermal systems, and not react with the melt as it dissolves in molecular form, speculates onthe importance of CO 2 insome ore- so that there should be little variationinCO 2 solubility forming systems. This synthesis of current knowledge infelsic magmas. Moreover, ona molar basis, CO 2 about CO2, and its role in magmas, attempts to provide solubility changes little with melt composition along the economic with a background that can be tholeiitic ± join ,Blank and Brooker 1994; used when interpreting the role of CO2 in intrusion- see also Fig. 1). related deposits.

Solubility as a function of pressure and temperature The solubility of CO2 in melts Inall studied magma types, the solubility of CO 2 in- CO2 dissolution creases with pressure at a nearly linear rate ,Fig. 1). Temperature e€ects are not so simple, as they are de- To understand the systematics of CO2 behavior inig- pendent on the mechanism of incorporation of the CO2 neous systems, one must ®rst consider how CO2 dis- inthe melt ,Stolper et al. 1987). Inrhyolitic magmas, the solves in magmas and the variables that in¯uence its solubility of CO2 clearly decreases as temperature in- solubility ,see Blank and Brooker 1994 for an excellent creases ,see Fig. 4 from Fogel and Rutherford 1990). review). Experimental studies in the 1960s and 1970s This is consistent with the typical inverse correlation recognized that CO2 solubility innatural melts is betweenmolecular gas solubility andtemperature, about anorder of magnitude less thanthat of water whether inroom-temperature , hydrothermal ,e.g., Wyllie and Tuttle 1959; Kadik et al. 1972). In the ¯uids, or rock melts. Exceptions occur when the gases late 1970s and 1980s, experimentalists were able to use dissolve by interacting with the melt to form charged vibrational to determine the speciation of complexes ,Fig. 2). For example, the solubility of within silicate melts and obtain more quanti- ,which exists only in oxidized magmas) increases with tative solubility estimates. Mysenet al. ,1975) used 14C temperature ,Baker and Rutherford 1996), as does car- beta-track mapping techniques to study the solubility bonate solubility in albite melts ,Stolper et al. 1987) and of CO2 in a variety of natural melt compositions and basaltic melts ,Mysenet al. 1975). Ingeneral, though, found that it dissolves both as carbonate groups and the temperature e€ect onsolubility is very small com- molecular CO2. Later, Fine and Stolper ,1985) explored pared with the e€ects of melt compositionandpressure the incorporation of CO2 insodium aluminosilicate ,Blank and Brooker 1994). glasses, and found that it dissolved both as molecular CO2 and Na±CO3 ionic complexes, with the latter in- creasing along with Na2O onthe Na 2O±SiO2 join. In subsequent studies, researchers have found that CO2 dissolves solely as carbonate groups in basaltic melts including ma®c alkaline magmas such as , , and ,Blank and Brooker 1994). Incontrast,solely molecular CO 2 is found in rhyolites ,Fogel and Rutherford 1990). Intermediate melt com- positions ,) have both species present ,Mysen et al. 1975; King et al. 1996), as is the case for evolved silica-undersaturated magmas such as ,Blank and Brooker 1994). Because CO2 reacts with the melt as it dissolves to form carbonate groups in ma®c melts, its solubility is highly dependent on the nature and abundance of vari- ous cations; in particular, Ca, K, and Na. As a result, carbonate is far more soluble in alkaline basalts than tholeiites ,Blank and Brooker 1994). Dixon ,1997) de- rived a parameter ,P), which she used to model the re- Fig. 1 Solubility of total CO2 ,molecular + carbonate) as a func- lationship between carbonate solubility and melt tionof pressure for four melt compositions, ppmw parts per million compositionfor tholeiitic to leucititic compositions.The by weight). , , and tholeiitic basalt calculated from variable P, which correlates positively with carbonate compositions and parameters given in Holloway and Blank ,1994) at 1,200 °C. Rhyolite curve from experimental data of Fogel and solubility, decreases as a function of silica and alumina Rutherford ,1990) at 1,150 °C. The solubility of CO inrhyolite concentrations. It also increases with alkalis and alkaline 2 and tholeiitic basalt are very similar. The CO2 solubility increases elements ,and therefore with non-bridging oxy- both with pressure and with magma alkalinity 492

Fig. 2 Molecular CO2 vs. carbonate concentrations for CO2- saturated albite melt ,after Stolper et al. 1987). Inalbite, CO 2 dissolves as both carbonate and molecular CO2. Bulk solubility increases with pressure. The carbonate to molecular CO2 ratio Fig. 3 Solubility plot for system rhyolite±H2O±CO2 at 675 °C. increases sympathetically with temperature Lines labeled with units of pressure represent isobars that display solubility of H2OandCO2 as function of ¯uid composition; intersections with X- and Y-axes represent H2OandCO2 solubility, Solubility as a function of the presence respectively ± all other points being mixtures. Isopleths labeled of other volatiles with units of XH O display the compositionof vapor inequilibrium with the melt. Melt2 compositions ,as deduced from silicate melt Blank et al. ,1993) showed that Henry's Law applies for inclusions, ¯uid inclusions, etc.) can be plotted to calculate the the H O±CO ±rhyolite system at pressures <1 kbar and minimum pressure at which the melt would be saturated with a 2 2 vapor phase ,saturation pressure). The corresponding isopleth magmatic temperatures. Ina melt±vapor system where would indicate the composition of vapor that would exsolve the vapor contains two components ,in this case CO2 and H2O), additionof onegas dilutes the other gas, decreasing its activity and consequently decreasing its ationof with unusual magma compositions,car- concentration ,solubility) in the melt. In other words, bonatites and strongly silica-undersaturated magmas additionof water to a melt±CO 2 system will decrease the such as nephelinites and melilitites). Except when created solubility of CO2. Insuch a case, Henry'sLaw holds for at unusually deep mantle depths where diamond is stable the melt, and the activity of a constituent is proportional ,Fig. 4; see Haggerty 1999), such magmas will most to its concentration, which is decreased by addition of typically be created due to melting of carbonated mantle. another component. This concept is illustrated in Fig. 3, Wyllie and Huang ,1976) showed that the reaction: where the solubility of CO2 at ®xed temperature and 2Mg2SiO4 ‡ CaMgSi2O6 ‡ 2CO2 pressure varies with vapor compositioninthe H 2O± CO2-melt system. There is some controversy whether , 4MgSiO3 ‡ CaMg CO3†2 1† this Henrian behavior in the melt continues at high pressures ,>5 kbar; e.g., Mysenet al. 1975) due to the is favored at mantle pressures ,Fig. 4). At pressures possible depolymerizing e€ect of water on the melt >28 kbar ,OlafssonandEggler 1983), magnesite structure ,cf. Stolper et al. 1987). ,MgCO3) is the stable crystalline carbonate phase, even in -bearing compositions ,Dalton and Wood 1993). The carbonate-forming reactions destabilize the calci- CO2 in the mantle um-rich ,cpx) and silica-poor minerals ,) within the mantle. Therefore, when is melted, result- The ®rst introduction of CO2 into ma®c magmas occurs ing liquids are rich in and and are during of the mantle. The presence of low insilica, as is observed insilica-undersaturated mantle-derived carbon is most obviously associated with magmas ,nephelinites, melilitites), , and continental and oceanic rifts where deep melting events ± all magma types commonly associated with create carbon-rich, silica-undersaturated magmas. Bailey high abundances of CO2 ,Brey 1978). Later experimen- and colleagues ,e.g., Bailey 1987; Bailey and Macdonald tal studies have shownthat solid carbonatesshould be 1987; Bailey and Hampton 1990) have repeatedly argued present in mantle at temperatures below that tectonic zones are accompanied by unusually 1,200 °C and at oxidation states above QFM ,the carbon-rich magmas, as corroborated by gas-rich ±fayalite± bu€er) minus 0.5 ,Dalton and , CO2-rich volcanic gases, and the associ- Wood 1995). 493 idotites were held at 1,000 °C and decompressed at rates equivalent to 90 km/h prior to rapid quenching; yet in all cases the dolomite broke downcompletely. Berg ,1986) described calcite±brucite intergrowths, which may also represent relict mantle carbonate phases that did not survive their ascent to the surface. Given such rapid breakdown, it is not surprising that mineralogical evidence for the host phase for CO2 inthe mantleis quite rare. Typically, such evidence consists of either remnant carbonate melts or carbonate inclusions within rigid hosts. Inmantle with lower oxidationstates or higher temperatures, carbonis likely to be presenteither ina ¯uid phase, as diamond ,at >50 kbar), or as graphite. Holloway ,1998) presented a model for production of mid-ocean-ridge basalt from a graphite-saturated mantle under relatively reducing conditions. At low pressures, carbonate minerals are not stable and will react to form CO2 vapor. Roedder ,1965) showed convincing evidence for nearly pure-phase CO2 vapor exsolved inthe deep Fig. 4 Stability of carbonic phases in lherzolitic mantle ,the CaO± crystallization environment of numerous basaltic mag- MgO±SiO2±CO2 system). The CO2, dolomite, and magnesite ®elds mas. Of 72 occurrences of olivine-bearing nodules found are after DaltonandWood ,1993), with ±CO and 2 in basalt , 64 contained high-density CO2-domi- peridotite±dry solidus from Wyllie ,1978). Shield and oceanic nated ¯uid inclusions that must have been trapped at geotherms from Bailey ,1985). Note that both the shield and oceanic geotherms would pass through the ®eld of carbonated depths of 10±15 km. Many olivine from the mantle provided that the mantle f is suciently high to stabilize host lavas also contained similar inclusions, indicating O2 carbonate ,i.e., >QFM minus 0.5; Dalton and Wood 1995) that the magmas were saturated at great depth with CO2-rich ¯uids.

Although melting will tend to deplete the mantle with respect to carbon, there are a number of processes that Estimates of CO2 concentrations in magmas can lead to subsequent enrichment in volatiles. Studies of mantle , particularly those erupted with al- Evidence for the presence of CO2 incrustal magmas is kali basalts, indicate that the mantle can undergo me- not limited to inclusions in mantle and cognate xeno- tasomatism by enriched ¯uids or melts liths. During the past 20 years, experimental and pet- ,e.g., Bailey 1987; Menzies and Hawkesworth 1987; rologic studies of igneous volatiles have led to a far Stolz and Davies 1988). In many cases, the ¯uid re- greater understanding of the abundance of dissolved and sponsible for these changes is interpreted to be CO2-rich, exsolved gases in magmatic systems. Although concen- based onthe presenceof CO 2-rich ¯uid inclusions and trations of CO2 insilicate glasses had beenestimated by reactionassemblages ,Dautria et al. 1992). Carbonated techniques such as 14C beta-track counting and mano- melts have been documented in peridotite nodules from metric techniques, it was not until Fourier transform the Kerguelen Islands by Schiano et al. ,1994), who infrared ,FTIR) spectroscopy became widely used that found carbonate-rich melt inclusions and concluded that quantitative techniques for measurement of CO2 con- they represented metasomatic liquids. centrations were developed ,Fine and Stolper 1985; Evidence for carbonate phases within mantle Stolper et al. 1987). Subsequently, studies of experi- peridotite nodules was ®rst documented by McGetchin mental run products, quenched natural glasses and sili- and Besancon ,1973), who found small carbonate in- cate melt inclusions were begun ,Dixon et al. 1988; clusions within pyropes. Amundsen et al. ,1987) later Newman et al. 1988; Anderson et al. 1989; Fogel and described quenched carbonate-bearing melts within spi- Rutherford 1990). The latter are small blebs of silicate nel peridotite xenoliths from Spitsbergen. Later studies melt ,oftenquenched to glass) that are trapped inphe- onthese samples by Ionovet al. ,1993) identi®edmag- nocryst hosts during pre-eruptive crystallization. As- nesium-rich dolomites, implying equilibrium with man- suming no post-entrapment processes have changed the tle minerals. Most of the carbonate minerals in these composition ,see Lowenstern 1995), these rocks showed evidence for breakdown by reaction with features provide information on the melt composition peridotite. Such breakdownshould be commonandthe and dissolved volatiles at the time of trapping. In this preservation of any relict mantle carbonate minerals is section, I review the recent literature on measured CO2 likely to be extremely rare. Canil ,1990) found that Eq. concentrations in natural melts, and what has been ,1) is readily reversed at magmatic temperatures and learned about the nature and abundance of CO2-bearing sub-mantle pressures. In his experiments, synthetic per- igneous ¯uids in the middle to upper crust. 494 Alkalic basalts magmas from Mt. Erebus, Antarctica, which ranged in composition from basanite through evolved phonolites. Given the apparent presence of solid carbonates, CO2- They found very high dissolved CO2 concentrations in bearing ¯uid, and graphite in di€erent parts of the melt inclusions trapped within olivine phenocrysts; as mantle, magma generation will result in CO2-bearing much as 0.7 wt%, corresponding to saturation pressures liquids. For example, carbonate is the ®rst mineral phase of 3±4 kbar giventhe measured H 2O concentrations of to melt whenthe peridotite solidus is exceeded at 1.15±1.75 wt%. As inthe Vesuvius magmas, evolved 40 kbar ,Canil and Scarfe 1990) and this would also be magmas related to the deep ma®c melts are depleted in expected at lower pressures ,Fig. 4). The CO2 concen- volatiles, re¯ecting crystallization and chemical evolu- trations in partial melts from carbonate-bearing mantle tionina shallow where CO 2,H2O, and peridotite will be dependent on its bulk carbonate con- S are lost to the atmosphere and/or overlying hydro- tent and the extent of melting. In many cases, initial thermal system. Eschenbacher et al. ,1998) conclude that partial melts might be extremely carbonrich, but would volatile saturation and degassing occurs throughout the be diluted as melting proceeded. Such a trend was re- continuous fractionation from basanite through phon- cently observed by Dixon et al. ,1997), who looked at olite. dissolved CO2, both measured and calculated, for a range of basalt compositions from the submarine North Arch volcanic ®eld, north of Oahu ,). They argue MORB and ocean island basalts that a homogeneous mantle source was variably melted ,1.6±9.0% batch melting), producing liquids ranging A large amount of work has been done on volatile with progressive melting from to basanite to concentrations in MORB glasses dredged from the alkaline olivine basalt ,AOB). Initial volatile contents ocean¯oor. Johnsonet al. ,1994) review some of the were assumed to correlate with P2O5 and were calculated pertinent literature and describe the analytical diculties to range from 6.9 wt% CO2+H2O in the nephelinites to of volatile analysis in bulk glass samples. In general, 1.8 wt% inthe AOBs ,with CO 2/H2O by wt=2.5 in MORB glasses are H2O-undersaturated, but contain both). The mantle source region was inferred to be ¯uid vesicles, requiring saturation with a CO2-rich gas at the absent, although the host phase for carbon was not depth of emplacement. Concentrations of CO2 are stated and batch melting was assumed. The volatile higher for bulk manometric analyses than for spot an- content of the unmelted source mantle was estimated to alyses ,e.g., FTIR spectroscopy), presumably because be 525‹75 ppmw ,ppm by weight) H2Oandthey are a€ected by the presence of CO2 invesicles 1,300‹800 ppmw CO2. Uponascent,the magmas ,Moore et al. 1977). Ina study of lavas with numerous would not have begun degassing ,i.e., reached their CO2-rich bubbles ,``popping rocks''), Pineau and Javoy saturation pressures) until reaching depths of 25±40 km, ,1983) showed that the di€erence between the carbon two to three times shallower thantheir source region. isotopic compositionof CO 2 from vesicles and CO2 glass inlavas dredged from the ¯oor at dissolved inMORB glass could be attributed to isotopic 4 km depth had degassed, but the bulk rocks still fractionation due to during ascent and that contained several wt% total volatiles due to the abun- the magnitude of fractionation implied CO2 concentra- dance of CO2-rich vesicles, consistent with the high tions of 0.2 to 1.0 wt% in the mantle source region. In calculated volatile contents. Similar results were found general, H2O concentrations in MORBs correlate with by Moore et al. ,1977), who discovered that vesicles ina K2O, and are interpreted to track the degree of melting variety of submarine basalts contained more than 95% inthe source region.The CO 2 does not correlate with CO2, requiring saturation with CO2 at water depths of major or trace elements, presumably due to the e€ect of 5 km and greater. degassing ,Johnson et al. 1994). The highest dissolved CO2 contents measured in Estimates of initial dissolved CO2 concentrations in natural glasses come from silicate melt ,glass) inclusions parental Kilauean ocean island basalts ,Hawaii) range in olivine from ma®c basalts. Because the inclusions are from a minimum of 0.2 wt% ,Anderson and Brown oftentrapped at great pressure ,several kbar), these 1993) to 0.3 wt% ,Greenland et al. ,1985) to 0.6 wt% melts have degassed far less thanbasalts emplaced at ,Gerlach and Graeber 1985). Anderson and Brown depths of 3 to 4 km below sea level ,0.3±0.4 kbar). ,1993) estimated saturationpressures for melts inKi- Marianelli et al. ,1999) analyzed glass inclusions in po- lauean glass inclusions that ranged up to 5 kbar, al- tassium-rich from Vesuvius, , that have as though most appeared to grow at much lower much as 3 wt% dissolved H2O and 0.3 wt% dissolved pressures. The 5 kbar estimate is consistent with the data CO2, requiring saturation with a volatile phase at pres- of Roedder ,1965), who found CO2-dominated ¯uid sures up to and over 5 kbar. These magmas fed a shal- inclusions in both olivine phenocrysts and cumulate low magma chamber at a depth of 1±2 km, where they xenoliths, requiring saturation with a CO2-rich vapor degassed and crystallized to form a -rich phase at depth. At Reunion Island, Bureau et al. ,1998) with very low dissolved CO2 interpreted ¯uid and melt inclusion compositions to ,<500 ppmw) and less than 1 wt% dissolved H2O. imply that magmas were volatile-saturated with a CO2- Similarly, Eschenbacher et al. ,1998) studied alkalic rich ¯uid at pressures of at least 5 kbar. 495

Arc basalts and andesites found evidence for appreciable CO2. Using infrared ,FTIR) spectroscopy, Newmanet al. ,1988) undertook Recently, several studies have explored the dissolved CO2 the ®rst study to quantify the abundances of both CO2 concentrations within ma®c arc magmas. Sisson and and H2O dissolved in natural silicic glasses. Analyzing Bronto ,1998) worked on high-magnesium basalts from quenched fragments ejected from the throat of the 1983 eruption of Galunggung, on the volcanic front the Mono Craters ,California) vent, they found evidence in Java, Indonesia. They found low H2O concentrations for a degassing trend whereby glasses with successively ,0.21 to 0.38 wt%) in olivine-hosted melt inclusions, but lower CO2 and H2O were created as the magma rose elevated CO2 ,as much as 750 ppmw) and S ,as much as through its conduit. The trend could be modeled by 2,200 ppmw), indicating that the inclusions had not closed-system degassing from a magma that started out leaked and were instead trapped during ascent through with 1.2 wt% CO2 and 5 wt% H2O inthe lower crust. the crust at depths of 3 to 6 km. Presumably, the magmas Lower initial CO2 concentrations would have resulted in were saturated with a CO2-dominated vapor during signi®cantly di€erent trajectories on a plot of H2O vs. much of their ascent through the crust, although the in- CO2. clusions were trapped at low pressure, after much de- Following the lead of Newman et al. ,1988), other gassing occurred. Sisson and Bronto ,1998) argue that researchers soonbeganusingFTIR spectroscopy to the high-magnesium basalt was created by upwelling and measure the H2OandCO2 concentrations in silicate melt pressure-release melting of the , without inclusions. Anderson et al. ,1989) provided the ®rst re- signi®cant input from ¯uids and magmas derived from liable CO2 concentrations in silicate melt inclusions from the subducted slab. This accounts for the extremely wa- any magma. Studying the 760 ka eruption of the rhyo- ter-depleted nature of the magma, which would be un- litic Bishop Tu€, they found evidence for hundreds of expected for basalts incorporating slab-derived ¯uids. ppmw CO2 dissolved in melt inclusions as well as higher Such a slab-in¯uenced melt was studied by Roggen- water concentrations than were generally accepted for sack et al. ,1997), who found that basaltic melt inclusions silicic melts ,>6 wt%). Anderson et al. ,1989) calculated from Cerro Negro ,Nicaragua) had elevated concentra- saturationpressures above 2 kbar. They concluded that tions of dissolved H2O ,as much as 6 wt%) and CO2 the host quartz crystals were forming in a vapor-satu- ,some >1,000 ppmw), yielding estimated saturation rated melt with vapors containing as much as 45 mol% pressures as high as 6 kbar ,i.e., 21 km). Studied bas- CO2. Later work by Wallace et al. ,1995) found that alts that erupted explosively in1992 are interpreted to dissolved CO2 concentrations were negatively correlated have stagnated and crystallized at greater depths than with incompatible elements such as uranium, and that those that erupted e€usively three years later. The high the slope of the correlationcould be explainedby the CO2 ,and H2O) ¯ux from arc magmas with a signi®cant mass fractionof gas presentinvarious parts of the slab component is consistent with the conclusions of magma chamber. To date, the study of Wallace et al. Sano and Williams ,1996), who estimated that only ,1995) is the only one that has used melt inclusions ,and about 10% of the CO2 degassed from -zone CO2 concentrations) to estimate the amount of gas volcanoes was derived from the mantle and that a major present in a magma chamber prior to eruption. part of the CO2 ¯ux was recycled from the subducted Lowenstern ,1994) studied volcanic equivalents to the oceanic crust and its overlying sediments. Because sub- 23 Ma Pine Grove molybdenum deposit ,Utah) ductionzonesproduce almost two thirds of the volca- and found quartz-hosted melt inclusions with 6±8 wt% nogenic carbon ¯ux, crustal recycling represents a major H2O and as much as 960 ppmw CO2. Calculated satura- source of carbon to the atmosphere ,Sano and Williams tion pressures ranged as high as 4.3 kbar, requiring that 1996; Alt and Teagle 1999). Similarly, Giggenbach ,1996) volatile exsolutionbeganat depths of 16 km duringas- asserted that the volatile budget of arcs follows a steady cent to the shallow regions ,2±3 km) where the porphyries state, whereby a major part of the CO2 and N2 ¯ux is were emplaced. Other studies showed appreciable CO2 recycled via breakdownof sedimentsfrom the subducted dissolved in rhyolitic magmas, including the work of slab and these gases return to the surface through arc Wallace and Gerlach ,1994) on the 1991 eruption of . Presumably, much of the recycled carbon Pinatubo ,Philippines), and that of Gansecki and comes from carbonaceous sediments and ooze, and ma- Lowenstern ,1995) on the 0.6 Ma Creek Tu€ rine limestone ,Wyllie and Huang 1976; Plank and ,Yellowstone, Wyoming). Lowenstern et al. ,1997) found Langmuir 1993; Alt and Teagle 1999), given that car- relatively low CO2 ,40±60 ppmw) and H2O ,2 to 3.5 wt%) bonate phases subducted with basalt are likely to break concentrations in an A-type rhyolite from the Afar rift of downat low pressures ,CanilandScarfe 1990), before Eritrea. They concluded that the low concentrations were they canreach the depth of magma generation. due to shallow entrapment pressures ,0.4±0.9 kbar), which was consistent with geologic constraints. A number of studies of silicic eruptions have found Silicic magmas no evidence for CO2 in -hosted melt inclu- sions. Such eruptions include the 1912 eruption at the Although rhyolitic and dacitic magmas are often as- Valley of TenThousandSmokes ,Lowenstern1993),the sumed to contain only H2O-rich gas, many studies have climactic 6.8 ka Crater Lake eruption,Baconet al. 496 1992), and the 74 ka eruption of the Toba Tu€ ,Chesner Crustal sources of carbon 1998). The absence of detectable CO2 inthe melt in- clusions does not mean that CO2 is absent from the and silicic volcanic rocks are more likely to have magma, given the ready partitioning of CO2 into a vapor an origin through crustal melting than their ma®c coun- phase. Lowenstern and Mahood ,1991) found no de- terparts, and CO2 concentrations in rhyolites and tectable ,<50 ppmw) CO2 insilicate melt inclusions may be augmented by partial melting and assimilation of from peralkaline rhyolites at , Italy. Never- crustal materials. The formation of skarns often includes theless, CO2 was detected invapor bubbles foundwithin breakdown of carbonate-bearing rocks by in®ltration of melt inclusions ,Lowenstern et al. 1991). Because the water-rich magmatic ¯uids ,Einaudi et al. 1981; Meinert magma chamber was located at shallow depths ,2± et al. 1997). Such a process is certainto add CO 2 to the 3 km), little CO2 was present in the melt phase and magma, while creating decarbonated calc-silicate assem- nearly all had partitioned into the vapor. Lowenstern blages and carbonate-rich melts ,Lentz 1999). Graywac- et al. ,1991) concluded that the CO2-bearing bubbles kes, calcareous sandstones, and many hydrothermally were present in inclusions where both melt and a coex- altered rocks may contain sucient carbon to provide a isting vapor phase had been trapped simultaneously. source of CO2 to crustal melts and ascending ma®c Insome systems, CO 2 may be virtually absent due to magmas. One diculty in identifying this source of car- long-term, open-system degassing of the magma cham- bon in silicic magmas is the lack of igneous phenocrysts ber and consequent loss of the least soluble gases. Evi- that incorporate carbon. With the exception of a small dence for such open-system degassing has been inferred carbonate component in ¯uorapatite, CO2 is partitioned by the behavior of and sulfur as mea- almost entirely into outgassed ¯uids and has a very low sured in melt inclusions, matrix glass, and coexisting abundance in analyses of volcanic rocks and igneous in- sul®de minerals ,Mandeville et al. 1998). As such, CO2 trusions ,White et al. 1999). Where present, such carbon could be distilled out of magmas by long-term loss of may be sub-solidus innature,White et al. 1999). As a vapor, combined with little input of new CO2 from ei- result, only direct sampling of magmatic gases is likely to ther wallrock or associated magmas. provide insight into the source of CO2 inmagmas. Un- fortunately, at this point, most isotopic data for carbon in is from ma®c systems. Evidence for carbonic ¯uids in granites

Fluid inclusion studies of unaltered granites have pro- Can vapor-saturated magmas ascend vided similar evidence of vapor-saturated silicic magmas through the crust and erupt? in the intermediate to shallow crust. Frost and Touret ,1989) studied coeval saline and CO2-rich ¯uid inclu- Magma ascent sions in the Laramie complex and concluded that silicate melt, nearly pure CO2, and hydrosaline melt As discussed above, many recent studies provide evi- coexisted at a late magmatic stage of this complex. dence for CO2-rich vapors inmagmas withinthe middle Calculated isochores allowed them to infer that the in- to upper crust. It is a commonly held belief that vapor- clusions were trapped at 3 kbar and 950 °C. The coex- saturated magmas cannot ascend through the crust istence of immiscible silicate melt, CO2-rich vapor, and without freezing, and that addition of CO2 to a magma hydrosaline melt was ®rst shown by Roedder and Coo- should also cause it to crystallize. However, the presence mbs ,1967) for granitic blocks ejected during eruptions of vapor is not always likely to impede the movement of at Ascension Island and has since been studied experi- magmas through the crust. mentally by Shinohara et al. ,1989; see also Shinohara Tuttle and Bowen ,1958) stated that H2O-saturated 1994). Thomas and Spooner ,1992) studied CO2-bearing anatectic magmas that reside at the haplogranite ternary ¯uid inclusions in quartz crystals from the Tanco gran- minimum cannot rise through the crust because any loss itic and found that the high-temperature of H2O via degassing would move the solidus to higher ``magmatic'' inclusions contained dominantly H2O temperatures, resulting in crystallization and solidi®ca- ,95%) and CO2, and that this ¯uid unmixed at low tion. This statement has been corroborated by numerous temperatures to form a vapor phase and a dense brine subsequent studies, but it overlooks the probability that phase. Frezzotti et al. ,1994) found evidence that the most anatectic melts are created at depths where water from the Deep Freeze Range of Antarctica saturationwould require >10 wt% dissolved H 2O. If were vapor saturated at a pressure of 3 kbar at 750 °C magmas instead contain less dissolved water, as has with a ¯uid having aH2O=0.5, with the remainder con- typically been inferred in many of the studies listed sisting mostly of CO2. Nabelek and Ternes ,1997) in- above, they should be able to ascend through the crust terpreted CO2-rich ¯uid inclusions from the Harney without impediment, even if they become saturated with Peak ,Black Hills, South Dakota) to be primary a multicomponent vapor containing other gases in ad- magmatic vapors trapped at 3.5 kbar inequilibrium ditionto H 2O. with a granitic melt containing 3.5 wt% dissolved H2O Because CO2 dissolves inits molecular form insilicic and 1,500 ppmw dissolved CO2. melts at crustal pressures ,Blank et al. 1993), it has little 497 e€ect on melt structure and controls phase equilibria ¯uids. Just as the presence of a vapor phase does not primarily through dilution of coexisting vapors and necessarily hasten magma crystallization, it also does not consequent reduction of H2O activity. Holloway ,1976) imply immediate eruption. Shinohara et al. ,1995) pre- showed that closed-system equilibrationof silicic magma sented a model whereby bubble-rich magmas undergo with CO2-bearing vapor can bu€er the H2O concentra- open-system degassing at shallow depths ,2±3 km) to tionof the melt, causingP H2O to stay constant, or in- create Climax-type molybdenum porphyry deposits. In crease, during magma ascent. This process is illustrated such a scenario, the magma is suciently volatile-rich inFig. 5 ,diagram similar to those inAndersonet al. that it canoutgas as a porous opensystem without fur- 1989 and Lowenstern 1994). The trends for decom- ther bubble growth and expansion, allowing magma that pression degassing show that ascent will have minimal loses its bubbles to sink relative to the volatile- and e€ect ondissolved H 2O concentration and, therefore, on molybdenum-bearing magma rising from depth. This melt structure and phase relations. In other words, if convective model would work best for volatile-rich degassing does not cause the dissolved H2O content to magmas; they would be less eruptible because they can change appreciably, then there will be no impetus to- become permeable ,see Eichelberger et al. 1986) and lose wards crystallization. In fact, slow adiabatic ,isenthal- exsolved gas at greater depths thanless gas-rich magmas. pic) ascent of the magma alone will be likely to cause a As discussed above, thermodynamics and phase dia- small temperature increase ,Waldbaum 1971). Thus, grams predict that rising hydrous magmas should crys- during decompression, the vapor-saturated magma will tallize before reaching the surface if ascent took place not necessarily crystallize and can rise into the upper under equilibrium conditions. If the vapor-saturated crust ,4±6 km depth). Once the magma rises to shallow magma represented in Fig. 5 were to rise to depths where regions, the 1±4 km depths associated with epithermal pressures are less than2 kbar, thenthe vapor phase and porphyry ore deposits, subsequent ascent will likely would become increasingly H2O-rich as water is drawn force degassing of H2O-rich ,but still CO2-bearing) ¯u- out of the melt. At low pressures, degassing has a pro- ids. This process will inevitably begin to a€ect the soli- nounced e€ect on dissolved H2O concentration and dus and to induce crystallization. should inhibit magma ascent by inducing crystallization. Even so, we know from modern volcanologic studies that vapor- ,and H2O-) saturated magmas canand do rise and Magma eruption erupt from relatively shallow magma chambers ,<11 km depths; e.g., Pinatubo, Bishop Tu€, Cerro Negro, and all Often, magma does not erupt but instead ponds in the the other volcanic events summarized above). The crust to crystallize as intrusions. The creation of a lode rhyolitic melts that form glassy lavas, such as obsidian gold deposit from a magmatic source requires that at domes, although clearly H2O-saturated during much of least some magma remains behind in the crust and un- their ®nal ascent to the surface ,Eichelberger et al. 1986; dergoes devolatilizationto release the metal-bearing Newmanet al. 1988; Taylor 1988), are able to rise and erupt without signi®cant crystallization and can ¯ow for kilometers over the Earth's surface as supercooled liq- uids. Their ability to do so is a result of the sluggish crystallizationkinetics of silicic melts andthe decrease in caused by the presence of bubbles in magmas ,Bagdassarov and Dingwell 1992). The most likely situationwhere the presenceof CO 2 would impede magma ascent would be where a CO2- poor magma was created inthe mantleor deep crust at a relatively low temperature. As it rose through the crust, if there were a mechanism to add large amounts of CO2 to the magma ,e.g., by assimilation/melting of carbo- naceous sediments or limestone; Lentz 1999), the re- sulting CO2-rich vapor phase would draw water out of the melt, potentially causing the magma to cross its Fig. 5 Solubility plot for the system rhyolite±H2O±CO2 at 750 °C. Lines labeled with units of pressure represent isobars that display solidus and crystallize ,Fig. 6). solubility of H2OandCO2 as function of ¯uid composition ,see Fig. 3). Isopleths not shown ,see Fig. 3). Trend parallel to isobars shows e€ect of crystallizationonvapor-saturated melt at 4.6 kbar The degassing process starting with 2,000 ppmw CO2 and 5.0 wt% H2O. Decompression- induced degassing trends show paths of isothermal ascent, in equilibrium with the vapor ,closed system) or allowing created Giventhe evidencefor CO 2-bearing vapors in a wide vapor to escape ,open). Numbers along paths list mol% H2Oin range of crustal magmas, it becomes important to vapor ,remainder being CO2). The decompressing magma should understand the general chemical behavior and char- degas without signi®cant crystallization until it reaches virtual acteristics of such vapors. One generality is that early- H2O-saturation ,X-axis). At that point, magma ascends as a supercooled , as inall silicic eruptions exsolved vapors will always be more CO2-rich thanthose 498

CH4/CO2 ratios may be appreciable at the temperatures expected for hydrothermal mineralization in intrusion- hosted gold deposits ,300±450 °C). Another important concept to consider is the e€ect of CO2 on unmixing of the volatile phase, particularly in silicic magmas where temperatures may not exceed 800 °C. Under such conditions, halides and some sul- fates have extremely low solubility inCO 2-bearing va- pors. For example, inthe H 2O±NaCl system at 700 °C, ¯uids with more than5 wt% NaCl will unmixto form immiscible vapor and brine phases at any pressure lower than about 1.1 kbar ,Souririjan and Kennedy 1962). However, additionof 20 wt% or more CO 2 will cause such a ¯uid to unmix at any pressure lower than about 5 kbar ,Bodnar and Sterner 1987; Joyce and Holloway 1993). Increasing temperature will only increase the likelihood of unmixing ,as long as the temperature is below the melting point of halite; 800 °C at 1 bar). This Fig. 6 Pressure versus temperature diagram for the granite solidus means that the conditions where silicic magmas are ,thin lines) and the decarbonation of calcite ,thick lines: ®gure after saturated with both a vapor and brine ,hydrosaline melt) Holloway 1976). A vapor-saturated magma near its solidus and will be commoninthe middle to upper crust. Giventhe with a inthe vapor equal to 0.1 would be supercooled ,relative CO2 di€erent partitioning behavior of metals between melt, to its solidus) by about 35 °C if the magma were ¯ooded with vapor, and brine ,Candela and Piccoli 1995), the pres- enough CO2 to move the vapor-phase activity of CO2 to 0.25 ,arrow). This would be possible, for example, by heating siliceous ence of CO2 will increase the likelihood of fractionation limestones above 600 °C, causing calcite ,cc) + quartz ,qtz)to of metals among ¯uids with di€erent densities and form wollastonite ,wo). This reactionwould eventuallyslow down transport properties. due to the productionof CO 2 exsolved later. As the magma rises and/or cools, the low CO2 in hydrothermal ¯uids and surface manifestations solubility of CO2 relative to H2O will cause it to parti- tion preferentially into any exsolving vapor phase. The Although this review focuses primarily onCO 2 in earliest-formed vapors may also be ,relatively) rich in magmas, it is appropriate to discuss some general con- other magma-insoluble gases such as the noble gases and cepts that describe the evolutionandbehavior of car- N2 ,Giggenbach 1997). Both closed- and open-system bonic ¯uid in hydrothermal systems. As magmatic degassing of any natural silicate melt will result in pro- vapors, brines, and supercritical ¯uids leave the magma gressively more H2O-rich vapors with time. This will be and enter hydrothermal systems, they can potentially act the case whether degassing is caused by depressurization as mineralizing ¯uids. Just as in the magma, the presence or crystallization,``secondboiling''). If bubble forma- of CO2 will promote immiscibility and creation of a tionis delayed until the magma reaches shallow depths, vapor phase that will incorporate sulfur gases and noble partitioning into the vapor may be dominated by H2O, gases as well as CO2 and steam. swamping any CO2 inthe system. Magmas canalso Pressure decreases will tend to cause boiling or e€er- contain little CO2 if their di€erentiation is protracted vescence of CO2-andH2S-bearing vapors. Such boiling and magma-insoluble gases are distilled away over time. events may be most dramatic in the high-temperature Evidence for open-system degassing has been noted by ,300±450 °C) environments envisioned for intrusion- Mandeville et al. ,1998) and is discussed above in the hosted Au deposits. At high temperatures, rock loses sectiononsilicic magmas. strength and becomes ductile, which spurs annealing and CO2 is not the only carbon-bearing species that will reductionof rock permeability. Pressure canbuild until be present in magmatic emanations. Concentrations of rapid ¯uid-release events force brittle fracturing, some- as much as 0.4 wt% CO occur insome hot, low f times causing pressure to decline from lithostatic to hy- O2 basaltic gases ,Table 3 of Symonds et al. 1994), about drostatic gradients ,Fournier 1999). Such catastrophic 5% of the abundance of CO2. More typically, CO is two depressurizationwill force boilingof hydrothermal so- to four orders of magnitude less abundant than CO2, lutions and release of CO2-rich vapors. Sheeted hydro- particularly in andesitic and dacitic gases and those with thermal veins may result from repetition of these f >QFM. ,CH ) is extremely rare involcanic pressure ¯uctuations with their associated boiling, min- O2 4 gas and is usually attributed to contamination with eral precipitation reactions, and subsequent annealing. geothermal and low-temperature crustal gases ,Symonds At temperatures above 300 °C, CO2 strongly parti- et al. 1994) or initial oxidation states well below QFM. tions into the vapor phase and has a relatively low sol- However, as magmatic ¯uids re-equilibrate at lower ubility inaqueous ¯uids ,Rimstidt 1997). The vapor can temperatures, methane concentrations will increase, and be relatively unreactive because carbonic acid weakly 499 dissociates at temperatures above 300 °C ,Bischo€ and tance of carbonic ¯uids in transporting ore-forming Rosenbauer 1996; Giggenbach 1997). As temperatures metals. Perusing a recent compilation on melt/vapor/ fall, however, carbonic acid ,H2CO3) more readily dis- brine partition coecients ,Candela and Piccoli 1995), it sociates, allowing it to react with rock to leach cations becomes apparent that little is known about the role of and form bicarbonate-rich geothermal ¯uids ,Bischo€ CO2 as a ligand, and little ®rm evidence exists that CO2 and Rosenbauer 1996). Even at low temperatures, CO2 plays a direct role inore formation.Seward andBarnes has only limited solubility in geothermal waters. For ,1997) demonstrate that based on Lewis acid±base be- example, at 200 °C, a 1 mol% solutionof CO 2 inpure havior of metals and ligands, one would not predict H2O has a partial pressure of 66 bar ,data from Chapter metal±carbonate complexes to be particularly stable. 4 of Henley et al. 1984), whereas that for water is 16 bar. Where experimental studies have explored the parti- Thus, at or below 82 bar, a CO2-dominated vapor will tioning of metals into CO2-rich ¯uids, most have con- exsolve from this solutionas bubbles. Additionof other cluded that increasing concentrations of CO2 tend to gases or the presence of NaCl in solution will increase decrease metal vapor/melt partitioncoecients,e.g., the vapor pressure and thus the pressure at which bub- Webster et al. 1989). bles will form. At the Broadlands geothermal area, New A few studies, however, have documented apparent Zealand, evidence for CO2-saturated geothermal ¯uids mobilizationof metals by CO 2-rich ¯uids, primarily in extends to depths of 2,500 m ,p. 54, Henley et al. 1984). the deep crust. Wendlandt and Harrison ,1979) dem- As a result, CO2 gas oftenaccumulates inthe shallow onstrated the ability of carbonate-rich melts to transport crust and can result in hydrothermal and geothermal LREE and discussed the importance of LREE enrich- well-®eld ,e.g., Hedenquist and Henley 1985). ments in fenites and metasomatized mantle. Keppler and Volcanic regions have long been known as subject to Wyllie ,1990) found limited, but signi®cant, uranium CO2 buildup. The 1986 disaster at , Camer- mobility inhigh-temperature magmatic ¯uids at mod- oon, evidently occurred due to deep volcanic degassing erate crustal pressures ,2 kbar). They noted that in- and resultant accumulation of CO2-saturated water in creasing CO2 concentrations in the ¯uid were the deep lake, followed by catastrophic overturnand accompanied by higher uranium concentrations. In a release of a large, CO2 gas cloud, killing over 1,700 petrographic study of tungsten-mineralized rocks, Hig- people ,Kusakabe 1996). At Mammoth , Cal- gins ,1980) noted a correlation between CO2-rich ¯uid ifornia, ongoing di€use degassing of 530 t/day CO2 has inclusions and tungsten concentrations. However, ex- caused about 30 ha of forest to die and is a continuing perimental studies have yet to document this relation- source of concern ,Farrar et al. 1995, 1999). The gas ship. appears to emanate from a vapor-rich gas reservoir lo- We are, therefore, left with a preponderance of cated inseismically active zoneswithin3 km of the sur- studies that show evidence for CO2-bearing, ¯uid in- face ,Julianet al. 1998). The isotopic compositionof the clusions in gold-bearing intrusions and the apparent lack gas is consistent with derivation from magma and the of experimental evidence that CO2 acts as a ligand for tree-kill beganafter a 1989 seismic swarm that was ap- most ore-associated metals. One potential explanation parently related to intrusion of a basaltic ,Farrar for the apparent discrepancy would be simply that CO2 et al. 1995). On Mt. Etna, Italy, residents have long been is a very common gas in igneous and hydrothermal aware of di€use percolationof CO 2 through the volcanic systems and should be ubiquitous regardless of whether edi®ce, resulting in CO2-charged and dif- it has a role inthe ore-formingprocess. Appreciable fuse degassing that constitutes a signi®cant percentage of CO2/H2O ratios ,e.g., >0.1) could indicate that the the total CO2 released by the ,Allard et al. 1997). magma did not undergo a protracted period of di€er- Clearly, CO2 behaves as a highly volatile species, both entiation and degassing, which would have caused loss withinmagmas andinsuperjacenthydrothermal sys- of the more magma-insoluble components such as CO2. tems. In magmas, there are no crystalline components Deep degassing, and input of volatiles from underlying that incorporate CO2 or carbonate groups ,except for a magmas could also result inthe presenceof appreciable very minor amount in ) and CO2 solubility is CO2 in¯uids derived from the ore-hostingmagma. It is generally low in the melt. In hydrothermal systems, CO2 also likely that CO2 acts as a fugitive agent in the hy- is not highly soluble under most conditions and precip- drothermal ¯uid, readily causing unmixing of the ¯uid to itates as carbonate minerals primarily at lower temper- create a separate vapor phase. Heinrich ,1995) summa- atures. As a result, CO2 canexsolve from hydrothermal rizes evidence for vapor-phase transport of copper in a waters and groundwaters to escape to the surface. number of intrusion-hosted deposits, and especially in tindeposits. Audetat et al. ,1998) give anevenmore striking example of vapor-phase transport of copper and Metal partitioning into CO2-bearing ¯uids boron, presumably by sul®de complexes. They stress that gold should also be complexed by sul®de inthe Giventhe evidencefor CO 2-bearing ¯uids in magmatic vapor phase. If this is the case, the presence of CO2 will systems and, in particular, associated with gold-bearing hastenthe creationof a vapor phase causingH 2S, and intrusions ,Newberry et al. 1995; Thompson et al. 1999 possibly HCl, to move into the vapor and act as ligands and references therein), one must ascertain the impor- for some metals. At the same time, e€ervescence of CO2 500 from the ¯uid will result ina decrease inP and an Brey G ,1978) Origin of olivine melilitites± chemical and experi- CO2 increase of pH in the remaining liquid, causing impor- mental constraints. J Volcanol Geotherm Res 3:61±73 Bureau H, Pineau F, MeÂtrich N, Semet MP, Javoy M ,1998) A melt tant mineral precipitation reactions ,e.g., Simmons and and ¯uid inclusion study of the gas phase at Piton de la Christenson 1994) and destabilization of ligands such as Fournaise volcano ,ReÂunion Island). Chem Geol 147:115±130 gold bisul®de ,Seward 1973). Therefore, although the Candela PA, Piccoli PM ,1995) Model ore±metal partitioning from melts into vapor and vapor±brine mixtures. In: Thompson JFH e€ect of CO2 on mineralization may not be direct, this commongas plays a key role inthe creationandevo- ,ed) Magmas, ¯uids and ore deposits. Mineral Assoc Canada Short Course 23, pp 101±127 lution of metal-bearing vapors in ore-forming systems. Canil D ,1990) Experimental study bearing on the absence of carbonate in mantle-derived xenoliths. Geology 18:1011±1013 Acknowledgements Sally Newmanprovided code adapted to cal- Canil D, Scarfe CM ,1990) Phase relations in peridotite+CO2 culate Figs. 3 and 5. I appreciate helpful reviews by Terry Gerlach, systems to 12 GPa: implications for the origin of and Je€ Hedenquist, Larry Meinert, Stuart Simmons, and Tom Sisson. carbonate stability in the Earth's upper mantle. J Geophys Res 95:15805±15816 Chesner CA ,1998) Petrogenesis of the Toba Tu€s, Sumatra, In- donesia. J Petrol 39:397±438 References Dalton JA, Wood BJ ,1993) The partitioning of Fe and Mg be- tween olivine and carbonate and the stability of carbonate Allard P, Jean-Baptiste P, D'Alessandro W, Parello F, Parisi B, under mantle conditions. Contrib Mineral Petrol 114:501±509 Flehoc C ,1997) Mantle-derived and carbon in Dalton JA, Wood BJ ,1995) The stability of carbonate under up- groundwaters and gases of Mount Etna, Italy. Earth Planet Sci per-mantle conditions as a function of temperature and Lett 148:501±516 fugacity. Eur J Mineral 7:883±891 Alt JC, Teagle DAH ,1999) The uptake of carbonduringalteration Dautria JM, Dupuy C, Takherist D, Dostal J ,1992) Carbonate of oceancrust. Geochim Cosmochim Acta 63:1527±1535 in the lithospheric mantle: peridotitic xenoliths Amundsen HEF, Grin WL, O'Reilly SY ,1987) The lower crust from a melilitic district of the Sahara basin. Contrib Mineral and upper mantle beneath northwestern Spitsbergen; evidence Petrol 111:37±52 from xenoliths and . Tectonophysics 139:169±185 Dixon JE ,1997) Degassing of alkalic basalts. Am Mineral 82:368± Anderson AT Jr, Brown GG ,1993) CO2 and formation pressures 378 of some Kilauean melt inclusions. Am Mineral 78:794±803 Dixon JE, Stolper E, Delaney JR ,1988) Infrared spectroscopic Anderson AT Jr, Newman S, Williams SN, Druitt TH, Skirius C, measurements of CO2 and H2O inJuande Fuca Ridge basaltic Stolper E ,1989) H2O, CO2, Cl and gas in Plinian and ash-¯ow glasses. Earth Planet Sci Lett 90:87±104 Bishop rhyolite. Geology 17:221±225 DixonJE, Clague DA, Wallace P, Poreda R ,1997) Volatiles in Audetat A, GuÈnther D, Heinrich CA ,1998) Formation of a alkalic basalts from the North Arch , Hawaii: magmatic±hydrothermal ore deposit: insights with LA±ICP± extensive degassing of deep submarine-erupted alkalic series MS analysis of ¯uid inclusions. Science 279:2091±2094 lavas. J Petrol 38:911±939 BaconCR, NewmanS, Stolper E ,1992) Water, CO 2, Cl, and F in Eichelberger JC, CarriganCR, Westrich HR, Price RH ,1986) melt inclusions in phenocrysts from three Holocene explosive Non-explosive silicic volcanism. Nature 323:598±602 eruptions, Crater Lake, Oregon. Am Mineral 77:1021±1030 Einaudi MT, Meinert LD, Newberry RJ ,1981) Skarn deposits. Bagdassarov NS, Dingwell DB ,1992) A rheological investigation Econ Geol 75th Anniv Vol, pp 317±391 of vesicular rhyolite. J Volcanol Geotherm Res 50:307±322 Eschenbacher AJ, Kyle PR, Lowenstern JB, Dunbar NW ,1998) Bailey DK ,1985) Fluids, melt, ¯owage and styles of eruption in Melt inclusion investigation of the volatile behavior of frac- alkaline ultrama®c . Trans Geol Soc S Afr 88:449± tionating alkaline magma, , Ross Island. Ant- 457 arctica. In: Vanko DA, Cline JS ,eds) PACROFI ± Pan Bailey DK ,1987) Mantle metasomatism: perspective and prospect. American Conference on Research on Fluid Inclusions, Pro- In: Fitton JG, Upton BGJ ,eds) Alkaline igneous rocks. Geol gram and Abstracts 7, p 30 Soc Spec Publ 30, pp 1±13 Farrar CD, Sorey ML, Evans WC, Howle JF, Kerr BD, Kennedy Bailey DK, HamptonCM ,1990) Volatiles inalkalinemagmatism. BM, King C-Y, Southon, JR ,1995) Forest-killing di€use CO2 Lithos 26:257±165 emission at as a sign of magmatic unrest. Bailey DK, Macdonald R ,1987) Dry peralkaline liquids and Nature 376:675±678 carbon dioxide ¯ux through the Kenya . In: Mysen BO Farrar CD, Neil JM, Howle JF ,1999) Magmatic carbondioxide ,ed) Magmatic processes: physicochemical processes. Geochem emissions at Mammoth Mountain, California. US Geol Surv Soc Spec Publ 1, pp 91±105 Water Resour Invest Rep 98±4217 Baker LL, Rutherford MJ ,1996) Crystallizationof anhydrite- Fine G, Stolper E ,1985) The speciation of carbon dioxide in bearing magmas. Trans R Soc Edinburgh 87:243±250 aluminosilicate glass. Contrib Mineral Petrol 91:105± Berg GW ,1986) Evidence for carbonate in the mantle. Nature 121 324:50±51 Fogel RA, Rutherford MJ ,1990) The solubility of carbondioxide Bischo€ JL, Rosenbauer RJ ,1996) The alteration of rhyolite in in rhyolitic melts: a quantitative FTIR study. Am Mineral CO2 charged water at 200 and 350 °C: the unreactivity of CO2 75:1311±1326 at higher temperatures. Geochim Cosmochim Acta 60:3859± Fournier RO ,1999) Hydrothermal processes related to movement 3867 of ¯uid from plastic into brittle rock in the magmatic±epither- Blank JG, Brooker RA ,1994) Experimental studies of carbon di- mal environment. Econ Geol 94:1193±1211 in silicate melts: solubility, speciation, and stable carbon Frezzotti ML, Di Vicenzo G, Ghezzo C, Burke EAJ ,1994) Evi- behavior. In: Carroll MR, Holloway JR ,eds) Volatiles dence of magmatic CO2-rich ¯uids inperaluminousgraphite- inmagmas. Volatiles inmagmas. Rev Mineralvol 30, pp 157± bearing leucogranites from Deep Freeze Range ,northern Vic- 186 toria Land, Antarctica). Contrib Mineral Petrol 117:111±123 Blank JG, Stolper EM, Carroll MR ,1993) Solubilities of carbon Frost BR, Touret JLR ,1989) Magmatic CO2 and saline melts from dioxide and water in rhyolitic melt at 850 °C and 750 bar. Earth the Sybille Monzosyenite, Laramie Anorthosite complex, Wy- Planet Sci Lett 119:27±36 oming. Contrib Mineral Petrol 103:178±186 Bodnar RJ, Sterner SM ,1987) Synthetic ¯uid inclusions. In: Ulmer Gansecki CA, Lowenstern JB ,1995) Pre-eruptive volatile compo- GC, Barnes HL ,eds) Hydrothermal experimental techniques. sitions of the Lava Creek Tu€ magma, Yellowstone Wiley Interscience, New York, pp 423±457 Volcanic Field. Trans Am Geophys Union, Eos 76:F665 501

Gerlach TM ,1980) Chemical characteristics of the volcanic gases Lentz DR ,1999) genesis: a reexamination of the role from Nyiragongo and the generation of CH4-rich ¯uid of intrusion-related pneumatolytic skarn processes in limestone inclusions in alkaline rocks. J Volcanol Geotherm Res 8:177±189 melting. Geology 27:335±338 Gerlach TM, Graeber EJ ,1985) The volatile budget of Kilauea, Lowenstern JB ,1993) Evidence for a copper-bearing ¯uid in Hawaii. Nature 313:273±277 magma erupted at the Valley of TenThousandSmokes, Alaska. Giggenbach WF ,1996) Chemical composition of volcanic gases. Contrib Mineral Petrol 114:409±421 In: Tilling RI, Scarpa R ,eds) Monitoring and mitigation of Lowenstern JB ,1994) Dissolved volatile concentrations in an ore- volcanic hazards. Springer, Berlin Heidelberg New York, forming magma. Geology 22:893±896 pp 221±256 Lowenstern JB ,1995) Applications of silicate melt inclusions to the Giggenbach WF ,1997) The origin and evolution of ¯uids in study of magmatic volatiles. In: Thompson JFH ,ed) Magmas, magmatic±hydrothermal systems. In: Barnes HL ,ed) Geo- ¯uids and ore deposits. Mineral Assoc Canada Short Course 23, chemistry of hydrothermal ore deposits, 3rd edn. Wiley, New pp 71±99 York, pp 737±796 Lowenstern JB, Mahood GA ,1991) New data on magmatic H2O Goldfarb RJ, Phillips GN, Nokleberg WJ ,1998) Tectonic setting contents of pantellerites, with implications for petrogenesis and of synorogenic gold deposits of the Paci®c Rim. In: Ramsay eruptive dynamics at Pantelleria. Bull Volcanol 54:78±83 WRH, Bierlein FP, Arne DC ,eds) Mesothermal gold miner- Lowenstern JB, Mahood GA, Rivers ML, Sutton SR ,1991) Evi- alizationinspace andtime. Ore Geol Rev 13, pp 185±218 dence for extreme partitioning of copper into a magmatic vapor Greenland LP, Rose WI, Stokes JB ,1985) As estimate of gas phase. Science 252:1405±1409 emissions and magmatic gas content from Kilauea Volcano. Lowenstern JB, Clynne MA, Bullen TD ,1997) Comagmatic A- Geochim Cosmochim Acta 49:125±129 type and rhyolite from the Alid volcanic center, Haggerty SE ,1999) A diamond trilogy: superplumes, superconti- Eritrea, northeast Africa. J Petrol 38:1707±1721 nents and supernovae. Science 285:851±860 Mandeville CW, Sasaki A, Saito G, Faure K, King R, Hauri E Hedenquist JW, Henley RW ,1985) Hydrothermal eruptions in the ,1998) Open-system degassing of sulfur from Krakatau 1883 Waiotapu geothermal system, New Zealand: their origin, as- magma. Earth Planet Sci Lett 160:709±722 sociated , and relation to precious metal mineralization. Marianelli P, Metrich N, Sbrana A ,1999) Shallow and deep res- EconGeol 80:1640±1668 ervoirs involved in magma supply of the 1944 eruption of Heinrich CA ,1995) Geochemical evolution and hydrothermal Vesuvius. Bull Volcanol 61:48±63 mineral deposition in Sn ,±W ± base metal) and other granite- McGetchin TR, Besancon JR ,1973) Carbonate inclusions in related ore systems: some conclusions from Australian exam- mantle-derived pyropes. Earth Planet Sci Lett 18:408±410 ples. In: Thompson JFH ,ed) Magmas, ¯uids and ore deposits. Meinert LD, Hefton KK, Mayes D, Tasiran I ,1997) Geology, Mineral Assoc Canada Short Course 23, pp 203±220 zonation and ¯uid evolution of the Big Gossan Cu±Au skarn Henley RW, Truesdell AH, Barton PB Jr, Whitney JA ,1984) deposit, Ertsberg District, IrianJaya. EconGeol 92:509±534 Fluid±mineral equilibria in hydrothermal systems. Rev Econ Menzies MA, Hawkesworth ,eds) ,1987) Mantle metasomatism. Geol 1 Academic Press, London Higgins NC ,1980) Fluid inclusion evidence for the transport of Moore JG, Batchelder JN, Cunningham CG ,1977) CO2-®lled ves- carbonate complexes in hydrothermal solutions. Can J Earth icles inmid-oceanbasalt. J VolcanolGeotherm Res 2:309±327 Sci 17:823±830 MysenBO, Arculus RJ, Eggler DH ,1975) Solubility of carbon Holloway JR ,1976) Fluids inthe evolutionof graniticmagmas: dioxide in natural nephelinite, tholeiite and melts to consequences of ®nite CO2 solubility. Geol Soc Am Bull 30 kbar pressure. Contrib Mineral Petrol 53:227±239 87:1513±1518 Nabelek PI, Ternes K ,1997) Fluid inclusions in the Harney Peak Holloway JR ,1998) Graphite±melt equilibria during mantle Granite, Black Hills, South Dakota, USA: implications for melting: constraints on CO2 inMORB magmas andthe carbon solubility and evolution of magmatic volatiles and crystalliza- content of the mantle. Chem Geol 147:89±97 tionof leucogranitemagmas. Geochim Cosmochim Acta Holloway JR, Blank JG ,1994) Application of experimental results 61:1447±1465 to C±O±H species in natural melts. In: Carroll MR, Holloway Newberry RJ, McCoy DT, Brew DA ,1995) Plutonic-hosted gold JR ,eds) Volatiles inmagmas. Rev Mineralvol 30, pp 187±230 ores in Alaska: igneous vs. metamorphic origins. In: Ishihara S, Ionov DA, Dupuy C, O'Reilly YO, Kopylova MG, Genshaft YS Czamanske GK ,eds) Proceedings of the Sapporo international ,1993) Carbonated peridotite xenoliths from Spitsbergen: im- conference on mineral resources of the NW Paci®c Rim. Japan, plications for trace element signature of mantle carbonate me- 4±8 October 1994. Soc Resour Geol Japan, Spec Issue vol 18, tasomatism. Earth Planet Sci Lett 119:283±297 pp 57±100 Johnson MC, Anderson AT Jr, Rutherford MJ ,1994) Pre-eruptive NewmanS, EpsteinS, Stolper E ,1988) Water, carbondioxide and volatile content of magmas. In: Carroll MR, Holloway JR ,eds) hydrogenisotopes inglasses from the ca. 1340 A.D. eruptions Volatiles inmagmas. Rev Mineralvol 30, pp 281±330 of the Mono Craters, California: constraints on degassing Joyce DB, Holloway JR ,1993) An experimental determination of phenomena and initial volatile content. J Volcanol Geotherm the thermodynamic properties of H2O±CO2±NaCl ¯uids at high Res 35:75±96 pressures and temperatures. Geochim Cosmochim Acta 75:733± OlafssonM, Eggler DH ,1983) Phase relationsof amphibole, am- 746 phibole±carbonate and phlogopite±carbonate peridotite: pet- JulianBR, Pitt AM, Foulger GR ,1998) Seismic image of a CO 2 rologic constraints on the asthenosphere. Earth Planet Sci Lett reservoir beneath a seismically active volcano. Geophys J Int 64:305±315 133:F7±F10 Papale P, Polacci M ,1999) Role of carbondioxide inthe dynamics Kadik AA, Lukanin OA, Lebedev YB, Kolovushkina EY ,1972) of magma ascent in explosive eruptions. Bull Volcanol Solubility of H2OandCO2 in granite and basalt melts at high 60:583±594 pressures. Geochem Int 9:1041±1050 Pineau F, Javoy M ,1983) Carbon isotopes and concentrations in Keppler H, Wyllie PJ ,1990) Role of ¯uids in transport and frac- mid-oceanridge basalts. Earth PlanetSci Lett 62:239±257 tionation of uranium and thorium in magmatic processes. Na- Plank T, Langmuir CH ,1993) Tracing trace elements from sedi- ture 348:531±533 ment input to volcanic output at subduction zones. Nature King PL, Forneris J, Holloway JR, Hervig RL ,1996) C±H±O 362:739±742 speciation in andesite glasses prepared at high pressure and Rimstidt JD ,1997) Gangue mineral transport and deposition. In: temperature. Trans Am Geophys Union, Eos 77:F802 Barnes HL ,ed) Geochemistry of hydrothermal ore deposits, Kusakabe M ,1996) Hazardous crater lakes. In: Tilling RI, Scarpa 3rd edn. Wiley, New York, pp 487±515 R ,eds) Monitoring and mitigation of volcanic hazards. Roedder E ,1965) Liquid CO2 inclusions in olivine-bearing nodules Springer, Berlin Heidelberg New York, pp 573±598 and phenocrysts from basalts. Am Mineral 50:1746±1782 502

Roedder E, Coombs DS ,1967) Immiscibility ingraniticmelts, Holloway JR ,eds) Volatiles inmagmas. Rev Mineralvol 30, indicated by ¯uid inclusions in ejected granitic blocks from pp 1±66 Ascension Island. J Petrol 8:417±451 Taylor BE ,1988) Degassing of rhyolitic magmas: isotope Roggensack K, Hervig RL, McKnight SB, Williams SN ,1997) evidence and implications for magmatic±hydrothermal ore de- Explosive basaltic volcanism from Cerro Negro volcano: posits. In: Taylor RP, Strong DF ,eds) Recent advances in the in¯uence of volatiles on eruptive style. Science 277:1639±1642 geology of granite-related mineral deposits. Can Inst Min Sano Y, Williams SN ,1996) Fluxes of mantle and subducted car- Metall Spec Vol 39, pp 33±49 bon along convergent plate boundaries. Geophys Res Lett Thomas AV, Spooner ETC ,1992) The volatile geochemistry of 23:2749±2752 magmatic H2O±CO2 ¯uid inclusions from the Tanco zoned Schiano P, Clocchiatti R, Shimizu N, Weis D, Mattielli N ,1994) granitic pegmatite, southeastern Manitoba, Canada. Geochim Cogenetic silica-rich and carbonate-rich melts trapped in Cosmochim Acta 56:49±65 mantle minerals in Kerguelen ultrama®c xenoliths: implications Thompson JFH, Sillitoe RH, Baker T, Lang JR, Mortensen JK for metasomatism in the oceanic upper mantle. Earth Planet Sci ,1999) Intrusion-related gold deposits associated with tungsten± Lett 123:167±178 tin provinces. Miner Deposita 34:323±334 Seward TM ,1973) Thiocomplexes of gold and the transport of Tuttle OF, BowenNL ,1958) Originof graniteinthe light of gold inhydrothermal ore solutions.Geochim Cosmochim Acta experimental studies in the system NaAlSi3O8±KAlSi3O8±SiO2± 37:379±399 H2O. Geol Soc Am Mem 74 Seward TM, Barnes HL ,1997) Metal transport by hydrothermal Waldbaum DR ,1971) Temperature changes associated with adia- ore ¯uids. In: Barnes HL ,ed) Geochemistry of hydrothermal batic decompressioningeological processes. Nature ore deposits, 3rd edn. Wiley, New York, pp 435±486 232:545±547 Simmons SF, Christenson BW ,1994) Origins of calcite in a boiling Wallace PJ, Gerlach TM ,1994) Magmatic vapor source for sulfur geothermal system. Am J Sci 294:361±400 dioxide released during volcanic eruptions: evidence from Sisson TW, Bronto S ,1998) Evidence for pressure-release melting . Science 265:497±499 beneath magmatic arcs from basalt at Galunggung, Indonesia. Wallace PJ, Anderson AT Jr, Davis AM ,1995) Quanti®cation of Nature 391:883±886 pre-eruptive exsolved gas contents in silicic magmas. Nature Shinohara H ,1994) Exsolution of immiscible vapor and liquid 377:612±616 phases from a crystallizing silicate melt: implications for chlo- Webster JD, Holloway JR, Hervig RL ,1989) Partitioning of rine and metal transport. Geochim Cosmochim Acta lithophile trace elements between H2OandH2O+CO2 ¯uids 58:5215±5221 and topaz rhyolite melt. Econ Geol 84:116±134 Shinohara H, Iiyama JT, Matsuo S ,1989) Partition of Wendlandt RF, Harrison WJ ,1979) Rare earth partitioning be- compounds between silicate melt and hydrothermal solutions I. tween immiscible carbonate and silicate liquids and CO2 vapor: Partitionof NaCl±KCl. Geochim Cosmochim Acta results and implications for the formation of light rare earth- 53:2617±2630 enriched rocks. Contrib Mineral Petrol 69:409±419 Shinohara H, Kazahaya K, Lowenstern JB ,1995) Volatile trans- White AF, BullenTD, Vivit DV, Schulz MS, Clow DW ,1999) The port in a convecting magma column: implications for porphyry role of disseminated calcite in the chemical weathering of Mo mineralization. Geology 23:1091±1094 rocks. Geochim Cosmochim Acta 63:1939±1953 Sourirajan S, Kennedy GC ,1962) The system H2O±NaCl at White DE, Waring GA ,1963) Data of geochemistry, 6th edn. elevated temperatures and pressures. Am J Sci 260:115±141 Chapter K, Volcanic emanations. US Geol Surv Prof Pap 44-K Stolper E, Fine G, Johnson T, Newman S ,1987) Solubility of Wyllie PJ ,1978) Mantle ¯uid compositions bu€ered in peridotite- carbondioxide inalbitic melt. Am Mineral72:1071±1085 CO2±H2O carbonates, amphibole and phlogopite. J Geol Stolz AJ, Davies GR ,1988) Chemical and isotopic evidence from 86:687±713 lherzolite xenoliths for episodic metasomatism of the Wyllie PJ, Huang WL ,1976) Carbonation and melting reactions in upper mantle beneath Southeast Australia. In: Menzies MA, the system CaO±MgO±SiO2±CO2 at mantle pressures with Cox KG ,eds) Oceanic and continental : similarities geophysical and petrological applications. Contrib Mineral and di€erences. Oxford University Press, Oxford. J Petrol Spec Petrol 54:79±107 Issue pp 303±330 Wyllie PJ, Tuttle OF ,1959) E€ect of carbondioxide onthe melting Symonds RB, Rose WI, Bluth GJS, Gerlach TM ,1994) Volcanic- of granite and . Am J Sci 257:648±655 gas studies: methods, results and applications. In: Carroll MR,