Chemical Divides and Evaporite Assemblages on Mars
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Earth and Planetary Science Letters 241 (2006) 21–31 www.elsevier.com/locate/epsl Chemical divides and evaporite assemblages on Mars Nicholas J. Tosca *, Scott M. McLennan Department of Geosciences, State University of New York at Stony Brook, Stony Brook, NY 11794-2100, USA Received 31 August 2005; received in revised form 13 October 2005; accepted 17 October 2005 Available online 28 November 2005 Editor: K. Farley Abstract Assemblages of evaporite minerals record detailed physical and chemical characteristics of ancient surficial environments. Accordingly, newly discovered regions of saline minerals on Mars are high priority targets for exploration. The chemical divide concept of evaporite mineral formation is used successfully to predict evaporite mineralogy and brine evolution on Earth. However, basaltic weathering largely controls fluid compositions on Mars and the robust predictive capabilities of terrestrial chemical divides cannot be used to interpret Martian evaporites. Here we present a new chemical divide system that predicts evaporite assemblages identified in SNC-type meteorites, ancient evaporites discovered on Meridiani Planum by the Opportunity rover, and Mars Express OMEGA data. We suggest that a common fluid type that has been buffered to different pH levels by basaltic weathering controls the variability among Martian evaporite assemblages and that evaporite mineralogy and brine evolution is essentially established by the initial composition of the dilute evaporating fluid. D 2005 Elsevier B.V. All rights reserved. Keywords: Mars; Mars exploration rovers; evaporites; geochemistry; sulfates; carbonates 1. Introduction which are markedly different than the assemblages in- ferred for Meridiani Planum sediments [6]. The charac- The influx of new data from the Mars Exploration terization of evaporative environments is important Rover (MER) and Mars Express (MEx) missions has because, just as they do on Earth, assemblages of saline significantly changed the current views of surficial envir- minerals hold the most potential for constraining the onments on Mars. Sulfate salt minerals have been iden- characteristics of ancient aqueous fluids once present at tified at many locations across the planet, manifested in the Martian surface and subsurface. In addition, geolog- soil analyses as well as in an ancient layered terrain [1– ical settings that contain saline minerals are commonly 5]. In several of these localities, most notably Meridiani the host of fossil biosignatures on Earth [7] and accord- Planum, an intricate sedimentary record has been pre- ingly, are targets of high priority for future exploration by served [5]. Additional evidence for saline mineral for- both orbiting and landed missions. mation on Mars comes from alteration mineral For terrestrial evaporite systems, Hardie and Eugster assemblages identified in the SNC-type meteorites, [8] demonstrated that the single most important param- eter responsible for the variety of brine compositions and evaporite mineral assemblages is the composition * Corresponding author. Tel.: +1 631 632 1936; fax: +1 631 632 of the dilute water at the onset of concentration. In 8240. countless terrestrial examples (where diagenetic modi- E-mail address: [email protected] (N.J. Tosca). fication of chemical sediments is insignificant), the end- 0012-821X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2005.10.021 22 N.J. Tosca, S.M. McLennan / Earth and Planetary Science Letters 241 (2006) 21–31 product of evaporative concentration can be traced back Planum, and OMEGA-derived evaporite mineral assem- to the initial chemistry of the evaporating fluid. With blages could have evolved from a similar parent fluid, evaporite mineral precipitation creating turning points but simply buffered to different pH levels by basaltic (or bchemical dividesQ) in the geochemical evolution of weathering. As a result, two distinct geochemical envir- brines, a robust scheme has been developed to place onments for the production of saline minerals on Mars constraints on the aqueous conditions necessary for the have been delineated: (1) acid–sulfate dominated envir- formation of evaporites on Earth [8,9]. onments and (2) mixed carbonate–sulfate dominated A simple definition for the concept of chemical environments. Utilizing new data returned from MER divides states that of two ions precipitated as a salt and MEx missions in combination with thermodynamic mineral, one will increase in solution and one will de- modeling and other geochemical constraints, we suggest crease in solution as evaporation proceeds [8,9]. This that the results obtained by the Opportunity rover point variance in ion concentration is determined by the molar to a geochemical process which may have been common ratio of the two ions in solution compared to the ratio across the Martian surface, but is extremely rare on present in the salt. For example, upon gypsum precipi- Earth—saline mineral formation from acidic fluids de- tation, if the Ca to SO4 molar ratio in solution is greater rived from basaltic weathering. than that of gypsum, SO4 will eventually decrease to the point of exhaustion while Ca will continue to increase. 2. Controls on solution chemistry Alternatively, the inverse may take place if Ca is less than SO4, resulting in a Ca-deficient fluid. This chemical Analogous to the Earth’s surface, solute acquisition divide created by gypsum precipitation largely deter- at the Martian surface will be controlled mainly by the mines the chemical evolution and subsequent precipita- chemical weathering of crustal materials. The resulting tion pathway of the evaporating fluid. Therefore, the two fluid chemistry, however, differs markedly from the most important factors that will determine the chemical majority of dilute inflow waters on Earth that lead to evolution of evaporating brines, as well as the character brine formation. Compared to terrestrial dilute surface and sequence of saline precipitates, are: (1) the chemistry waters, Martian fluids are enriched in Mg, SiO2, Ca and of the dilute fluid being evaporated and (2) the nature of most importantly, under acidic conditions, Fe [10–12]. the precipitates creating chemical divides. These two Fluid composition is a critically important factor to factors appear to be fundamentally different for Martian assess because the final outcome of evaporative con- evaporative systems when compared to terrestrial evap- centration is often sealed at the time dilute waters have orative systems. Accordingly, chemical divides for ter- acquired their solutes [13]. restrial evaporites are not transferable to Martian In general, chemical weathering reactions will be environments. It is imperative then, that chemical driven by acid input and the consumption of this acidity divides be evaluated for these unique systems in order by mafic mineral dissolution will ultimately cause the pH to provide useful constraints on the saline mineral assem- to increase. The most significant sources of acidity at the blages expected at the Martian surface as well as the Martian surface are the influx of volcanic gases rich in chemical evolution of resulting brines. SO2, other minor volatile species such as Cl, and a Here, we present a new system of chemical divides predominantly CO2 atmosphere [14,15]. The three that is likely to be encountered during the production of major anions to be considered for Martian fluids then À 2À À saline mineral assemblages at the Martian surface. The are HCO3 ,SO4 , and Cl . Given that the cation chem- resulting scheme for brine evolution on Mars captures istry will be set by chemical weathering reactions, the the variability of saline mineral assemblages expected, most important variable to consider in assessing a wide their precipitation pathways, and the major brine types range of fluid compositions ultimately leading to evap- formed as a result of evaporative concentration. Our orite production is the relative proportion of these three results establish a broadmapQ for the interpretation of major anions. The relative proportions are closely related saline mineral assemblages identified at the Martian to pH. surface and place constraints on the aqueous conditions A simple way to account for the combined effects of necessary for their formation. The system of chemical acidity from various anion sources is to first consider À 2À divides developed here for the Martian surface indepen- both HCO3 and SO4 equilibria. Beginning with a 2À À dently predicts the saline mineral assemblages observed SO4 and Cl free solution at equilibrium with a high in SNC-type meteorites as well as assemblages inferred pCO2 atmosphere, acidity is largely controlled by car- from MER and MEx data. The new chemical divide bonate equilibria. For pure water, pH as a function of system also demonstrates that the SNC-type, Meridiani pCO2 can be calculated in a straightforward manner N.J. Tosca, S.M. McLennan / Earth and Planetary Science Letters 241 (2006) 21–31 23 using equilibrium thermodynamics. For water contain- Table 1 ing some amount of dissolved solute where the pH has Cation proportions used in thermodynamic calculations increased from cation exchange by mineral dissolution, Cation Concentration Relative percent À 2À (Amol/kg water) carbonate alkalinity ([HCO3 ]+2[CO3 ]) will increase as CO becomes more soluble. However, if the same Mg2+ 497 47 2 2+ water is simply titrated with acid (analogous to the Fe 356 34 Ca2+ 165 16 addition of H2SO4 or HCl from