CEMENT IN MARTIAN VOLCANICLASTIC ROCKS. D. G. Towell1 and A. Basu1, 1Department of Geological Sciences, Indiana University, Bloomington, USA ([email protected]; [email protected])

Introduction. There is a consensus activities of SiO2, and activity ratios of that volcanic activity, action of water and (Na++K+)2/H+ or even that of Ca+/H+ suffi- wind, impact cratering, and cryogenic proc- ciently in the pore fluids of volcaniclastic ma- esses have shaped the surface of Mars [1-3]. terial to precipitate [e.g., 9-11]. We Older pyroclastic deposits and impact com- conclude that it is normal and common on minuted volcanic material would be buried Earth for a sequence of volcaniclastic sedi- under newer volcanic material. If water is ments to have zeolite as a secondary . stored as permafrost near the surface, any Absence of authigenic zeolites could indicate heating by volcanism or impact, would melt a a low-pH and dry or nearly dry system unless part of the permafrost and set up a convec- other non-volcanogenic such as cal- tion-cell-like flow [4]. The flow will be most cite or dolomite are dissolved from interca- efficient in the volcaniclastic layers because of lated sedimentary sequences providing a ready their higher porosity and permeability relative source for CaCO3 cement. to volcanic rocks. Existence of liquid water Mars. A few have speculated on zeo- and the flow, however, will be transient de- lite as one product of Martian pedogenic pro- pendent upon the longevity of the heat source. cesses [12-14]. Extensive bedded zeolite If so, the temperature and the water-rock ratio deposition on Mars may also be likely, which in the circulation system will decrease with may store a large amount of CO2 and have a time from an initial high, favoring early disso- profound effect on evolution of the Martian lution of magmatic phases and subsequent atmosphere[15]. The activity of CO2 in Mar- precipitation of diagenetic phases. Such di- tian diagenetic fluids would affect the cement agenesis will lithify buried volcaniclastic debris that would bind Martian sediments. in Mars. On the basis of examples from Earth A sub-permafrost aquifer beneath the and on thermodynamic grounds, we argue that melting isotherm at the base of the Martian zeolite is the likely cement in Martian volcani- cryosphere enables the cycling of water be- clastic rocks. tween the near-surface crust, atmosphere, and Earth. “Zeolites are ubiquitous con- polar caps [16]. We assume that the chemical stituents in the vugs and cavities of ” composition of the rocks in this groundwater and are probably the most abundant cement in aquifer is that calculated for the “soil-free volcaniclastic rocks in the Earth [5]. Whereas rock” [17]. The composition closely approxi- the vug filling and veins of zeolites are likely mates volcanic andesite (62% SiO2) with a to occur in Martian basalts, much larger CIPW normative content of 37% plagioclase amounts are expected in volcaniclastic sedi- (An39Ab61) approximating the composition of ments. Zeolitization of tuffs and other pyro- andesine, 21% quartz, 18% diopside, 17% clastic material, probably in alkaline playa hypersthene, and 4% orthoclase. lakes, has produced thick economic deposits Thermodynamic Considerations. We [e.g., 6,7]. More ubiquitous, however, is the are investigating the relative stability of pla- occurrence of diagenetic and very low-grade gioclase with respect to zeolites in the postu- metamorphic zeolite in volcaniclastic rocks lated groundwater aquifer on Mars. We select [8]. Hydrolysis of volcanic glass raises pH, the calcic zeolites heulandite and be- ZEOLITE CEMENT IN VOLCANICLASTICS: D. G. Towell and A. Basu

cause they are common terrestrially and ther- 10.25. Consequently, the excess dissolved silica modynamic data are available [18]. A highly in the groundwater would thermodynamically simplified chemical equilibrium between an- favor alteration of plagioclase to heulandite. orthite and heulandite can be written as fol- It should be noted that heulandite (and lows: stilbite) are zeolites that contain more silica than the proposed feldspar source (i.e. dis- anorthite silica solved silica is a reactant). In contrast, if CaAl2Si2O8 + 5H4SiO4 (aq) similar calculations are made for the reaction « of albite (NaAlSi3O8) to form the zeolite anal- heulandite water cime (NaAlSi2O6·H2O), silica is a product of CaAl2Si7O18·6H2O + 4H2O(aq) the reaction because analcime is undersatu- Stilbite differs from heulandite primarily in rated in silica (i.e. contains less than albite). containing additional structural H2O and both However, the equilibrium constant in this case minerals exhibit extensive solid solution be- becomes large and the zeolite (analcime) is tween calcium and . Standard Gibbs still favored over the feldspar (albite) under o free energies of formation at 25 C, 1 bar [18] the expected aqueous conditions. for plagioclase and heulandite were composi- CONCLUSION: Although other po- tionally corrected to approximate those of the tential (and more complex) reactions and as- normative plagioclase from the “soil-fee rock” semblages have not been investigated and the [17] and a theoretical heulandite with the kinetics of the above reactions on Mars are same Ca/Na atomic ratio (0.64). The value not known, the stability and ubiquity of zeo- obtained for the Martian plagioclase (ande- lites as observed in terrestrial volcanic systems o -1 sine) is DG form = -3827.2 kJ·mol and for the on Earth and as inferred on Mars, is rein- o -1 heulandite, DG form = -9708.7 kJ·mol . Free forced by the thermodynamic calculations. energies of formation for water and aqueous REFERENCES [1] Clifford et al., (Ed) silica are -237.1 kJ·mol-1 and -1307.5 kJ·mol-1, (1997) Conf. Early Mars, LPI Contrib 916, 87 p [2] respectively. The calculated value of the Wilson and Head (1994) Rev. Geophys, 32, 221-263 [3] Crown and Greeley (1993) JGR Planets, 98, 3431- equilibrium constant (Keq) for the above reac- 51.25 3451 [4] Gulick (1998) JGR Planets, 103, 19365- tion is 10 . If we assume that the water is 19387 [5] Mumpton (1977) Min. Soc. Am. Short low in dissolved solids (thermodynamic activ- Course, 4, 1-17 [6] Tsitsishvili et al. (1992) Natural ity @ 1.00), then the activity of dissolved silica Zeolites, Harwood, 295p.[7] Andreis (1998) Abstracts when equilibrium is attained between plagio- 15th Intn’l. Sed. Cong, 140 [8] Coombs (1954) 1/5 Trans. Royal Society NZ, 82 Part 1, 65-109 [9] Hay clase and heulandite = aH4SiO4 = (1/Keq) = 10-10.25 . (1993) Natural Zeolites (Ming & Mumpton, Eds), 3- 13 [10] Stamatakis et al., (1998) Abstracts 15th Intn’l. The solubility of quartz in neutral wa- Sed. Cong, 740 [11] Hall (1998) JSR, 68, 739-745 o -4 ters at 25 C, 1 bar is approximately 10 m and [12] Allen et al., (1982) JGR B, 87, 10083-10101 [13] that of amorphous silica is substantially Ming and Gooding (1988) LPI Tech Rpt 88-07, 124- greater, 10-2.8m [19]. The presumed Martian 125 [14] Gibson et al. (1983) JGR, 88, A912-A928 andesitic rocks and detritus are quartz norma- [15] Jakes and Rajmon (1998) LPSC XXIX CD-ROM, tive and should contain quartz, and initially, Abstract 1627 [16] Clifford (1993) JGR, 98, 10973- 11016 [17] Rieder et al. (1997) Science, 278, 1771- after volcanism, may have contained amor- 1774 [18] Robie & Hemingway (1995) USGS Bulle- phous silica. In either case, the dissolved sil- tin 2131. [19] Krauskoff & Bird (1995) Introduction ica in the Martian groundwater would likely to Geochemistry, McGraw, 88-90. contain much more dissolved silica than the plagioclase-heulandite equilibrium value of 10-