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Carbon Dioxide in Magmas and Implications for Hydrothermal Systems

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Carbon Dioxide in Magmas and Implications for Hydrothermal Systems Mineralium Deposita ,2001) 36: 490±502 DOI 10.1007/s001260100185 ARTICLE Jacob B. Lowenstern Carbon dioxide in magmas 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. magma±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 crust 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 crystallization-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 gas in volcanic exhalations ,White and Waring 1963; rhyolites ,retrograde temperature solubility), but exists Symonds et al. 1994). In gases exsolved from basaltic as dissolved carbonate groups in basalts ,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 mantle 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 oceanic crust 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 silicic 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 minerals 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 eect of Ca2+ was most pro- + + 2+ 2+ eect 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 geologists with a background that can be tholeiitic basalt±rhyolite 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 silicate melts Inall studied magma types, the solubility of CO 2 in- CO2 dissolution creases with pressure at a nearly linear rate ,Fig. 1). Temperature eects 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 rock melts is betweenmolecular gas solubility andtemperature, about anorder of magnitude less thanthat of water whether inroom-temperature liquids, 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 spectroscopy to determine the speciation of complexes ,Fig. 2). For example, the solubility of sulfate volatiles 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 eect onsolubility is very small com- molecular CO2. Later, Fine and Stolper ,1985) explored pared with the eects 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 basanites, leucitites, and nephelinites ,Blank and Brooker 1994). Incontrast,solely molecular CO 2 is found in rhyolites ,Fogel and Rutherford 1990). Intermediate melt com- positions ,andesites) have both species present ,Mysen et al. 1975; King et al. 1996), as is the case for evolved silica-undersaturated magmas such as phonolites ,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). Leucitite, basanite, 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 earth 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
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