HFSiF4 ratios in volcanic and magmatic gases

Beochimica et Cosmochimics Acta,1973,Vol.37,pp.109to 112.Pergamon Press.Printed inNorthern Ireland

HF/SiFa Ratios in volcanic and magmatic gases P. E. ROSENBERG Department of Geology, Washington State University, Pullmm, Washington 99163 (Received

21 March 1972; accepted in revisedform 16 June 1972)

Abstract-The possible importance of SiF, in volcanic and magmatic gases haa been neglected due to the convention of reporting analyses and basing calculations on the presence of HF. Calculated HF/SiF, ratios for natuml gas compositions serve to justify this convention by showing that SiF, is not a significant F-bearing molecular species at high temperatures. INTRODUCTION BEFORE World War II it was commonly accepted that SiF, was a probable and possibly a significant constituent of volcanic and magmatic gases (e.g. BOWEN, 1933; Although this concept has never been refuted, FENNER, 1933; SHEPARD, 1925,1938). time and the manner in which data have been collected and reported have led to its

neglect. Volcanic gas sampling procedures used to date have not permitted the distinction between F-bearing molecular species. The chemistry of the absorption method (OZAWA, 1968; NAUOHTON et al., 1963) and the construction of gas sampling apparatus entirely or partially of glass are obstacles which have yet to be overcome. Fluorine is reported in gas analyses as HF by convention, the possible presence of SiF, being neglected (e.g. OZAWA, 1968). WHITE and WARING (1963) in their review of volcanic emanations tabulate and discuss F as HP without mentioning SiF,. Calculated compositions of magmatic gases have also been based on the assumption that F is present only as HF (e.g. KRAUSKOPF, 1959). While reporting F as HF is simply a convenience under the circumstances, this assumption has become entrenched to the point that it is now generally accepted as a fact. For example, MUELLER (1970) states that F is present in volcanic gases predominantly as HF molecules citing as evidence analyses from the literature. A re-evaluation of the relative importance of HP and SiF‘, as molecular species in volcanic and magmatic gases appears to be necessary. Sufficient data are available to calculate expected HF/SiF, ratios and thus, to resolve this question on a factual basis. CALCULATION OF HF/SiF,

RATIOS

Although the fractionation of F between magmatic gases, melts and crystals strongly favors the condensed phases (BURNHAM, 1967; MUNOZ and EUBSTER, 1969) small but significant amounts of F may enter the vapor phase. The concentration of F in volcanic and magmatic gases has been estimated, assuming that F is present

only as HF. Values of PHF/Ptota,in magmatic gases are thought to be commonly equal to or less than 1O-4(KRAUSKOPF, 1959; MUELLER, 1970; MUNOZ and EUUSTER, 1969) but may be as much as an order of magnitude higher in some natural environments (MUNOZ and EUBSTER, 1969). Analyses of F-bearing volcanic gases (WHITE and WAKING, 1963) suggest a similar range, but one unusual analysis of gas from Kilauea (MURATA et al., 1964) gave a value of 2 x 10-Z. 109

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ROSENBERU

Fluorides of wide variety of cations are, no doubt, present in volcanic and magmatic gases but their partial pressures are probably very small relative to those of HP and SiF4 due either to comparatively low abundances or low volatilities (KRAUSKOPF,1969). This is borne out by chemical analyses of all detectable constituents of high temperature fumarolic gases from Showa-Shinzan (WHITE and WARING, 1963). Furthermore, most cations of common metals (e.g. Fe2+) are much more volatile as the chloride than as the fluoride (KRAUSKOPF,1969). It seems reasonable to assume that F is present in volcanic and magmatic gases principally as HF and Sip,.

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0

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4

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IO

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LOG m HF/SiF4

Fig. 1. Variations in the molar ratio HF/SiF, with temperature at 1 atm and at 1000 atm as a function of fluorine concentration (PHB/P1). Curve 1, 10-l at 1000 atm; Curve 2, 10-l at 1 atm and lOpa at 1000atm; Curve 3, 10m2at 1 atm and 10msat 1000atm; Curve 4, 10msat 1 atm and lO+ at 1000 atm; Curve 5,10-* at 1 atm and 10v5 at 1000 atm.

The relative proportions of HF and SiF4 in gases derived from magmas are controlled by the reaction, SiO, + 4HF = SiF, + 2H,O. Variables that require consideration in the calculation of HF/SiF, ratios in these gases include F/H,0 ratios in the vapor phase, the equilibrium constant for the above reaction, the activity of silica in the magma, temperature and total pressure. The equilibrium constant (Kp) for the above reaction has been determined experimentally over a wide temperature range at a silica activity of unity and found to be essentially the same for several low pressure polymorphs of silica including silica glass (LENFESTY et al., 1952). Kp inoreases very rapidly with decreasing temperature according to the equation, log Kp(atm) = 5547 - 6383127. Variations in the molar ratio HF/SiF, with temperature have been calculated from these data for a wide range in fluorine concentrations at 1 atm and at 1000 atm assuming ideality (Fig. 1). Since a change in the total F-content of the gas by one

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HF/SiF, Ratios in volcanic and magmaticgases

order of magnitude is equivalent to a change in total pressure of three orders of magnitude, two sets of conditions (e.g. Fig. 1, curve 4, Pm/P1 = 10v4 at 1000 atm and PHB/P1= IO--" at 1 atm) may be represented by a single curve. In these calculations total pressure has been taken to be the sum of the partial pressures of H,O, HF and SiF,. The presence of other constituents (e.g. CO,) will slightly reduce the effective total pressure as defined here. Calculations are based on a silica activity of unity. Variations in usiopin magmas (or rocks) will change the HF/SiF, ratio of the coexisting gases but the effect is relatively small and may be neglected for purposes of this discussion. Discussion AND CONCLUSIONS Despite uncertainties due to assumptions made in these calculations, it seems clear that HF must greatly predominate over SiF, in the vapor phase of magmas. If the PnB/Pt of magmatic gases is equal to or less than 10d4as supposed, the partial pressure of SiF, must be vanishingly small. For example, the magmatic gas composition calculated by KRAUSKOPF(1959) would have an HF/SiF, ratio on the order of ten million to one (Fig. 1, filled square). The ratio would be even higher if this gas were re-equilibrated to 1 atm (Fig. 1, curve 5) at high temperature. However, any concomitant decline in temperature would tend to offset the effect of decreased should increase and become appreciable pressure. With decreasing temperature PaiF, below about 15O’C. The volcanic gas analysis with the highest F-content ever reported (PnF/Pt = 2 x 10-2; MURATAet al., 1964) implies a correspondingly high partial pressure of SiF,. Assuming that its HF/SiF, ratio was quenched in at its approximate emission temperature, SOO”C,and 1000 atm, the ratio would still be greater than 100: 1 (Fig. 1, open square). Re-equilibration of this gas to 1 atm (Fig. 1, curve 3) at lower temperatures would lead to appreciable partial pressures of SiF, below about 300°C but F-rich gases of comparable composition are apparently very rare. SiF, is negligible in magmatic gases and in volcanic gases at high temperatures but may become a significant F-bearing molecular species in volcanic gases if these gases are re-equilibrated at sufficiently low temperatures. Leaching of Si during Fmetasomatism produced by volcanic gases at relatively low temperatures (e.g. 200-3OO’C; NABOKO,1957) provides evidence for such re-equilibration. Fe is also leached during this process (ibid.) suggesting that the behavior of FeF, is similar to that of SiF,. However, F cannot be responsible for the removal of any significant amount of silica from magmas by escape of gases at high temperatures as proposed by PENNER(1933). Although the possible presence of volatile fluorides of cations other than Si (e.g. Fe) has been neglected, the convention of reporting and calculating F in volcanic and magmatic gases as HF appears to be justified at high temperatures. Acknowledgements-This study was supportedby NationalScienceFound&ion The author is indebted to Mrs.ALBINA MELLOT for valuable technical assistance H. D. HOLLANDof Princeton University for helpful suggestions.

Grant GA-4483. and to Professor

NOTE Qualitatively simile conclusionswere reached by HONDA and MIZUTANI(1968) in 8 brought to the

paper attention of the author by an anonymousreviewer. Their paper emphasizes

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relatively low temperature ( <700°C) fumarolic gases and their products and bases its conclusions largely on equilibriumconstants calculatedfrom questionablethermochemicaldata (K~AUSKOPF, 1959, p. 265). Mole fractions of SiF, calculated for the ma&ion CaSiOs + 6HF = CaF, + SiF, + 3HaO from these equilibrium constants, are significantlydifferentthan those presentedhere. Furthermore, the effect of fluorine concentration on HF/SiF, ratios is not taken into account. REFERENCES BO~EN N. L. (1933) The broader story of magmatic differentiationbriefly told. In Ore De@ts of the Western States, Lindgren Volume, pp. 106-126. Amer. Inst. Min. Metal. Eng. BTTRNHAM C. WAYNE (1967) Hydrothermal fluids at the magmatic stage. In Beochemietry of Hydrothermal Ore Deposit8 (editor H. L. Barnes), pp. 3P76. Holt, Rinehart and Winston. FENNERC. N. (1933) Pneumatolytic processes in the formation of minerals and ores. In Ore Deposits of the Western Statea, Lindgren Volume, pp. 58-106. Amer. Min. Metal. Eng. HONDA F. and MIZUTANIY. (1968) Silicon content of fumarolic gases and the formation of a siliceous sublimate. Geochem. J. 2, 1-9. KXXATJSKOPZ K. B. (1959) The use of equilibrium calculations in finding the composition of a magmatic gas phase. In Researches in Geochemistry, Vol. 1 (editor P. H. Abelson), pp. 260278. Wiley. KRAUSEOPF K. B. (1969) Relative volatilities of fluoridesand chloridesor ore-forming metals in magmatic gases. In Problems in Geochemistry (editor N. I. Khitarov) Akad. Nauk. SSR., pp. 155-165. Israel Prog. Sci. Trans. LENFESTYF. A., FARR T. D. and BROSHEERJ. C. (1952) Equilibrium in the system silicon tetrafluoride-water. Ind. Eng. Chem. 44, 1448-1450. MUELLERR. F. (1970) Energetics of HCl and HF in volcanic emanations. Beochim. Cosmochim. Acta 54, 737-744. MUNOZJ. L. and EU~STERH. P. (1969) Experimental control of fluorine reactions in hydrothermal systems. Amer. Mineral. 54, 953-959. M~RATAK. J., ATJLT W. V. and WHITE D. E. (1964) Halogen acids in fumarolic gases of Kilauea volcano. Bull. Volcanol. 27, 3-4. NABOKOS. I. (1957) A case of gaseousfluorinemetasomatism at an active volcano. Geochemistry 452-455. NAUCHTONJ. J., HEALD E. F. and BARNESI. L. (1963) The chemistry of volcanic gases I. Collection and analysis of equilibrium mixtures by gas chromatography. J. Geophys. Res. 68, 539-557. OZAWA T. (1968) Chemical analysis of volcanic gases I. Chemical analysis of volcanic gases containing water vapor, hydrogen chloride, sulfur dioxide, hydrogen sulfide, carbon dioxide, etc. Geochem. Int. 5, 934-947. SHEPARDE. S. (1925) The analyses of gases obtained from volcanoes and from rocks. J. Geol. 33, 289-370. SHEPARDE. S. (1938) The gases in rocks and some related problems. Amer. J. Sci., 5th ser. 85-A, 311-351. WHITE D. E. and WAKINGG. A. (1963) Volcanic emanations. In Data of Ueochemietry, 6th ed., Chap. K., U.S. aeol. Surv. Prof. Paper 440-K, Kl-K29.