Explosive Properties of Water in Volcanic and Hydrothermal Systems

Transcription

1 P R E P R I N T ICPWS XV Berlin, September 8 11, 2008 Explosive Properties of Water in Volcanic and Hydrothermal Systems Régis Thiéry a and Lionel Mercury b a : Laboratoire Magmas et Volcans, UMR 6524, CNRS/Clermont Université r.thiery@opgc.univ-bpclermont.fr b : Institut des Sciences de la Terre d'orléans, UMR 6113, CNRS/Université d'orléans l.mercury@isto-univ.orleans.fr Water is the main explosive agent on Earth. Terrestrial volcanism is mainly produced by two end-member processes, where water plays an important role: (1) first, the depressurization of a H 2 O-rich magma, followed by its violent water exsolution and expansion of a steam-pyroclasts mixture (plinian volcanism), and, (2) the explosive interaction of cold liquid water with hot magma (phreato-magmatism). Thus, volcanological models requires a good assessment of the energetic properties of water and physico-chemical processes. The energetic balance of volcanic eruptions has been reassessed here with the help of the Wagner and Pruss equation of state [1] and by taking into account the irreversibility of steam expansion into the atmosphere. Next, it is shown how other aspects of water explosivity in volcanic and hydrothermal environments can be linked fruitfully to recent concepts about water metastability. Introduction Magmatic, volcanic and hydrothermal processes are associated with the explosive release of energy that is produced essentially by the mechanical work of exsolution and expansion of fluids, mainly composed of water. In particular, most explosive volcanism is typically produced by two types of situations [2]: (1) plinian volcanism and (2) phreato-magmatism. The former is caused by the violent decompression of pressurized gases, which exsolve from a fluid-rich and viscous magma during its ascent through the crust; the latter is produced by the explosive boiling of large quantities of liquid water at the contact of hot magma. Besides these cataclysmic phenomena, one can also mention other eruptive manifestations, less violent, but which are also produced by water: geysering, hydrothermal eruptions [3] and other types of volcanism [2] (strombolian volcanism,...). Thus, water is the main explosive agent on Earth. Therefore, the assessment of the energy released by volcanic eruptions is one of the primary goals of volcanology. The rate of energy release, i.e. the power of explosions, is another important parameter to tackle, and there is a strong need to understand the factors, which control the explosivity of water. For this reason, the energetic properties of water have been already the subject of numerous studies in volcanology and magmatology, both through experimentation and modeling [4-9]. The present contribution tries to address these questions by thermodynamic modeling with the recent reference equation of Wagner and Pruss [1] for water. In the first part, we present the different processes, which transform water into an explosive substance. In particular, we will show how the explosive transformations of water can be linked to its metastability degree. In the second part, the energetic contributions of the different physical transformations of water are quantified. Finally, in the third part, we will give some application examples of our approach to geological problems. What Makes Water an Explosive? An explosion is always the response of a system to a physico-chemical perturbation, which has left it in an energetic, metastable or unstable state. As a consequence, the characterization of water metastability can give us some indications about the explosive nature of these transformations. This work follows a phenomenological approach [10], based on classical thermodynamics and equations of state. Indeed, it is well known that the stability of a fluid is ruled by important relations, which are the consequences of the second law of thermodynamics that are: - the thermal stability criterion: ( S/ T) v > 0, - and the mechanical stability criterion: ( P/ v) T > 0.

2 As a result, the phase diagram of water is subdivided into three main regions: - (1) the instability region, where the stability criteria are not respected by the equation of state of the fluid. Thus, the fluid demixes spontaneously by the rapid phase-separation process of spinodal decomposition [11]. - and the metastable (2) and stable (3) fields, where stability criteria are obeyed. However, in the metastability field, a biphasic liquid-gas association is found to be more stable, and a monophasic fluid is expected to demix by the process of nucleation and phase growth [11]. These three regions are represented in Figure 1 for water in a pressure-temperature diagram. The most interesting elements for our concerns are the two spinodal curves, which delimit the boundaries between the metastable and the unstable regions. The liquid spinodal curve, noted Sp(L), marks the ultimate theoretical limit where a liquid can subsist in a superheated state without boiling. The gas spinodal curve, noted Sp(G), delimits the stability field of metastable supercooled gas, which should relax to equilibrium by condensation. One important point is that the violence of a physical transformation can be linked to its proximity degree to spinodal curves: a system is expected to react all the more explosively when it approaches a spinodal curve. The different physical transformations leading to explosive phenomena are displayed in Figure 1. The first case is illustrated by rapid heating of liquid water at the contact of a magma, which can trigger explosive boiling, when the liquid temperature is abruptly shifted above the spinodal temperature (T sp = C at 1 bar) up to the unstable field. The second case is represented by the sudden decompression of a pressurized reservoir under earth. Depending upon the initial temperature of the fluid, the decompression can perturb the system up to unstable conditions. This is effectively the case of water depressurizations occurring between 320 C and 374 C, where the decompression path cuts the liquid spinodal curve, producing a violent and explosive boiling. Such phenomena are well feared by safety engineers of the chemical industry, where they are known as BLEVE [12,13] (Boiling Liquid Expansion Vapour Explosion). The violence of the reaction can be attributed to spinodal decompositions of liquid water [11]. Cooler liquid reservoirs show a higher resilience to sudden depressurizations and can even bear negative pressures [14]. A pressure drop leads to more or less vigorous boiling. In the case of a transient decompression, the stretched and superheated liquid relaxes rapidly to equilibrium by the collapse of microscopic gas cavities, giving rise to the well-known process of cavitation. The concentration of mechanical energy on these imploding bubbles has important consequences: the pressure reequilibration inside the bubbles (see the upward arrow in Figure 1) shifts the gas into the field of supercooled gases and leads to phenomena, like sonolumininescence, sonochemistry and cleansing of solid surfaces [15]. Figure 1: The P-T phase diagram of water illustrating the different perturbation processes that shift water into metastable or unstable states. The Explosive Energy of Water The energy of volcanic eruptions is provided essentially by three contributions: (1) the exsolution work of water from magma, (2) the expansion work of pressurized steam, and (3) the vaporization work of boiling liquids. The first contribution is an important energy source [16], but is not considered here, as no equation of state has been yet developed for water-magma systems. The second one is always present, and the third one is important only for phreato-magmatic eruptions. One arguable point is related to the assessment of the expansion work of steam, which is always estimated by current volcanological models [2,5] under the hypothesis of reversible transformations. In other words, the decompression energy is calculated by a difference of the internal energy (U) between initial (i) and final (f) states: W = U f U i, 2

3 for an adiabatic and reversible process. However, it is well accepted that this formula overestimates strongly the expansion work. A better solution is to take irreversibility into account through the following relation: W = P atm ( v f v i ), where v i and v f are the molar volumes of water at initial and final states, and the expansion work is calculated against the atmospheric pressure P atm. This is what has been done in this work, where energies have been calculated with the Wagner and Pruss equation of state [1] by using a general package for thermodynamic calculations [17]. Calculated values (in J/g of water) are given respectively for a monophasic gas system (Figure 2) and a biphasic liquid-gas mixture (Figure 3). These values amount to 25% - 30% of those yielded by the reversibility hypothesis, and provide more realistic estimations of eruptive impacts (e.g. mass and velocities of ejecta). (curve G(L)) or saturated liquids (curve L(G)), as a function of the initial saturation temperature T sat. The mechanical energy is calculated either under the reversibility assumption (solid line) or the irreversibility one (dashed line). CP refers to the critical point. The energy produced by isobaric vaporization and heating of initial liquid water, which is an important energetic source in phreato-magmatic and hydrothermal eruptions, can be estimated from: W = (H f U f ) (H i U i ). Corresponding mechanical work can be estimated graphically from Figure 4. For instance, the maximal vaporization work (almost 200 J/g) is produced by the boiling of liquid water at 222 C and 24 bar. Figure 2: A P-T diagram showing: (1) the energy produced by the steam expansion (solid curves), and (2) the mass liquid fraction after expansion (dashed curves). Figure 4: A T-(H-U) diagram to calculate the energy produced by isobaric vaporization and heating of liquid water. The isobaric mechanical work between initial state (A) and final state (B) is given by: W=W B -W A. Geological applications a) Explosivity of Hydrothermal and Volcanic Systems A rough classification of the different types of hydrothermal and volcanic systems can be done in a pressure-enthalpy diagram (Figure 5). This typology is first based on the energetic contents of these systems. From the left to the right (i.e. from the less to the more energetic), one can distinguish: Figure 3: The expansion work (W) produced by the depressurization of saturated steams - liquid-dominated geothermal systems (case A). 3

4 - deep geothermal systems (case B), typically found in the lithocaps of magmatic chambers [18], or in oceanic hydrothermal systems (black smokers of oceanic ridges). - vapour-dominated geothermal systems (case C). - water exsolved by magmas during their ascent through the crust that is at the origin of plinian and vulcanian volcanism (case D). - and superficial waters (case E) which have been heated at the contact of magma (phreato-magmatism). Besides the intensity of the energy transfer involved in these systems, another differentiating point is related to their explosivity degree. This last feature can be estimated by considering the intersections of the expansion pathways followed by water with the spinodal curves and the unstable fields of the phase diagram. Decompression paths, approximated by isentropic expansions, have been drawn in Figure 5. Depending upon the incursion or not of these pathways into the instability field, two contrasted situations can be recognized: subspinodal decompressions and superspinodal ones. Superspinodal decompressions differ from the former by liquid boiling up to the unstable field. As a result, they are featured by explosive boiling (like BLEVE). It can be observed in Figure 5 that most liquid-dominated geothermal systems will exhibit subspinodal decompressions (case A), whereas deeper geothermal ones (case B) will be characterized by explosive superspinodal eruptions. Another explosive situation is represented by phreato-magmatism, where water is brutally shifted from point A to point E through the instability field of water. Case D is also characterized by an explosive exsolution of water from magma (but this phenomenon cannot be featured in Figure 5). Figure 5: An H-P diagram showing the different types of hydrothermal and volcanic systems. The thick dotted curves are the spinodals. The thin dashed curves are isotherms. The thin solid lines are isentropic expansion curves and are labelled by the initial temperature of the fluid at 1000 bar. The shaded area indicates explosive superspinodal decompressions. b) Global Thermodynamic Analysis of Hydromagmatism Another diagram [2,5,6] of interest in volcanology is given in Figure 6. These curves give the amount of mechanical energy produced by the interaction of one gram of magma with a mass m (in grams) of cold liquid water. Such curves point out the key control played by the amount of water in phreato-magmatic (or hydromagmatic) processes. Most explosive conditions are encountered for m between 0.1 and 0.5. Below this optimal range, water is shifted to explosive conditions, but is not abundant enough to drive a large explosion. Above this range, water is not heated strongly enough to behave in an explosive way. Figure 6 presents revised curves, which have been recalculated under the hypothesis of irreversibility with the equation of state of Wagner and Pruss [1]. Again, the explosive character of the magma-water interaction can be assessed from the theoretical peak temperature (T p ) reached by water: when this temperature exceeds the liquid spinodal temperature (T sp = C at 1 bar), a most explosive boiling process can be expected. 4

5 eruptive phenomena are influenced by numerous parameters, the influence of the porosity is a question which merits further investigations. Figure 6: The mechanical energy E m (J/g of magma) produced by the interaction of one gram of magma and m grams of water. Solid curves are calculated for different magma temperatures (from 400 C to 1500 C). Dotted curves indicate the equilibrium temperature (T p ) of magma and water after a first interaction step. The shaded area indicates explosive conditions. c) Boiling of Superheated Liquids in Finely Porous Formations The occurrences of geysers and other hydrothermal eruptions are closely linked to the formation of superheated liquids, either by a temperature increase or a pressure drop. The important superheating degree is a key parameter to create a large destabilization of the system. Thus, it is necessary to consider any factor, which is susceptible to generate superheat in the nature. Interestingly, there are strong evidences that phreato-magmatic eruptions are influenced by the lithology of host rocks: in particular, rocks of low porosity and permeability, like shales and siltstones, tend to favour hydromagmatic eruptions in contrast to highly porous formations, like sandstones [19]. For this reason, it is worthwhile to consider the effect of small pores on the boiling properties of water. This is done by considering (1) the Laplace law and (2) the equality of chemical potentials (µ) of H 2 O between a liquid and a vapour: P vap = P liq + 2σ r, μ liq ( T, P liq )=μ vap ( T, P vap ), where σ is the liquid-gas surface tension coefficient and r is the bubble radius. The corresponding boiling curves calculated in Figure 7 for different bubble radii confirm that a fine porosity can contribute to important superheating. While natural Figure 7: The boiling curves of water in finely porous media. The solid line is the saturation curve, whereas the dotted curves indicate the pressure of liquid. The numbers indicate the bubble radius. d) Instabilities of Supercooled Steams along Water- Magma Interfaces The thin film of vapour, which develops at the interface between magma and liquid water, is known to be affected by numerous fluid instabilities [5,6], like periodic film collapses, Taylor-Rayleigh instabilities and other instabilities (Figure 8). Therefore, such a system is in a strong state of disequilibrium. Moreover, the frequent oscillations of the film thickness (δ) prevents the development of steady heat fluxes from the magma (Q in ) and to the liquid (Q out ). Thus, the system is both anisobaric (P liq P vap ) and anisothermal (T liq T vap ). Nevertheless, an approximate thermodynamic description can be made at the liquid-gas interface from T vap = T liq + ΔT, where ΔT is a positive parameter accounting for the thermal disequilibria between gas and liquid, and from the equality of chemical potentials between both phases: μ liq (T liq, P liq ) = μ vap (T vap, P vap ). The resulting P-T conditions for the steam are given in Figure 9. Calculations have been made for a constant liquid temperature. This shows that the steam is in a metastable supercooled state, which can approach gaseous spinodal conditions under extreme conditions. As a consequence, the film collapse is a phenomenon, which could be ascribed to spinodal conditions. Like cavitation, this creates visible damage [4,5] at the solid surface (distortion and fragmentation of magma and sediments), which 5

6 are of importance in geological formations, called peperites [20]. Acknowledgements This work has benefited from the financial support from the ANR (Agence Nationale de la Recherche) for the project SURCHAUF-JC Literature Figure 8: Schematic illustration of the instabilities generated in the thin steam layer between magma and liquid water of wet sediments. Figure 9: Pressure-temperature diagram, showing the P-T conditions of supercooled steam at the proximity of liquid water (dotted curves). The figures indicate the temperature of liquid. Sp(G) is the gas spinodal curve, and Sat refers to the saturation curve. Conclusion This contribution gives a brief summary of energetic properties of water in hydrothermal and volcanic systems. It provides a phenomenological point of view, based on metastability concepts, to assess the explosivity of water transformations. While this approach can be fruitful, there remains a lot to do, in particular: (1) to determine the spinodal boundaries in water-gas-salts-magma systems, and (2) to gain a more detailed and comprehensive knowledge of destabilization processes of magmawater systems in eruptive phenomena from the molecular scale up to the crustal scale. [1] W. Wagner and A. Pruss: The IAPWS formulation 1995 for the thermodynamic properties of ordinary water substance for general and scientific use. J. Phys. Chem. Ref. Data 31, (2002). [2] L. Mastin: Thermodynamics of gas and steamblast eruptions. Bull. Volcanol. 57, (1995). [3] P. Browne and J. Lawless: Characteristics of hydrothermal eruptions, with examples from New Zealand and elsewhere. Earth-Science Reviews 52, (2001). [4] K. Wohletz: Mechanisms of hydrovolcanic pyroclast formation: Grain-size, scanning electron microscopy, and experimental studies. J. Volc. Geotherm. Res. 17, (1983). [5] K. Wohletz: Explosive water-magma interactions: thermodynamics, explosion mechanisms, and field studies. Bull. Volcanol. 48, (1986). [6] K. Wohletz: Water/magma interaction: some theory and experiments on peperite formation. J. Volc. Geotherm. Res. 114, (2002). [7] B.Zimanowski, G. Fröhlich and V. Lorenz: Quantitative experiments on phreatomagmatic explosions. 48(3-4), (1991). [8] B. Zimanowski, G. Fröhlich and V. Lorenz: Experiments on steam explosion by interaction of water with silicate melts. Nucl. Eng. and Design 155, (1995). [9] B. Zimanowski, R., Büttner, V. Lorenz and H.- G. Häfele: Fragmentation of basaltic melt in the course of explosive volcanism. J. Geophys. Res. 102, (1997). [10] P. Debenedetti: Metastable liquids. Princeton University Press, Princeton, New Jersey (1996). [11] P. Debenedetti: Phase separation by nucleation and by spinodal decomposition: fundamentals. Kluwer Academic Publishers (2000). [12] J. Casal and J. Salla: Using liquid superheating for a quick estimation of overpressure. J. Hazardous Materials A137, (2006). [13] J. Salla., M. Demichela and J. Casal: BLEVE: a new approach to the superheat limit 6

7 temperature. J. of Loss Prevention in the Process Industries 19, (2006). [14] Q. Zheng, D.J. Durben, G.H. Wolf and C.A. Angell: Liquids at large negative pressures: water at the homogeneous nucleation limit. Science 254, 829 (1991). [15] F. Caupin and E. Herbert: Cavitation in water: a review 7, (2006). [16] C. Burnham: Magmas and hydrothermal fluids. In: H. Barnes (Eds): Geochemistry of hydrothermal ore deposits cavitation in water: a review. 7, , Wiley, New York (1979). [17] R. Thiéry: A new object-oriented library for calculating high-order multivariable derivatives and thermodynamic properties of fluids with equations of state. Computers & Geosciences 22, (1996). [18] D. Norton and B. Dutrow: Complex behaviour of magma-hydrothermal processes: role of supercritical fluid, Geochim. Cosmochim. Acta 65, (2001). [19] U. Grunewald, B. Zimanowski, R. Büttner, L.F. Philipps, K. Heide, and G. Büchel: MFCI experiments on the influence of NaClsaturated water on phreato-magmatic explosions. J. Volcanol. Geotherm. Res. 159, (2007). [20] I.P. Skilling, J.D.L. White and J. McPhie: Peperite: a review of magma-sediment mingling. J. Volcanol. Geotherm. Res. 114, (2002). 7