Chemical divides and evaporite assemblages on Mars

doi:10.1016/j.epsl.2005.10.021

Earth and Planetary Science Letters

Volume 241, Issues 1–2, 15 January 2006, Pages 21-31

https://doi.org/10.1016/j.epsl.2005.10.021 Get rights and content

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.

Introduction

The influx of new data from the Mars Exploration Rover (MER) and Mars Express (MEx) missions has significantly changed the current views of surficial environments on Mars. Sulfate salt minerals have been identified at many locations across the planet, manifested in soil analyses as well as in an ancient layered terrain [1], [2], [3], [4], [5]. In several of these localities, most notably Meridiani Planum, an intricate sedimentary record has been preserved [5]. Additional evidence for saline mineral formation on Mars comes from alteration mineral assemblages identified in the SNC-type meteorites, which are markedly different than the assemblages inferred for Meridiani Planum sediments [6]. The characterization of evaporative environments is important because, just as they do on Earth, assemblages of saline minerals hold the most potential for constraining the characteristics of ancient aqueous fluids once present at the Martian surface and subsurface. In addition, geological settings that contain saline minerals are commonly the host of fossil biosignatures on Earth [7] and accordingly, are targets of high priority for future exploration by both orbiting and landed missions.

For terrestrial evaporite systems, Hardie and Eugster [8] demonstrated that the single most important parameter responsible for the variety of brine compositions and evaporite mineral assemblages is the composition of the dilute water at the onset of concentration. In countless terrestrial examples (where diagenetic modification of chemical sediments is insignificant), the end-product of evaporative concentration can be traced back to the initial chemistry of the evaporating fluid. With evaporite mineral precipitation creating turning points (or “chemical divides”) in the geochemical evolution of brines, a robust scheme has been developed to place constraints on the aqueous conditions necessary for the formation of evaporites on Earth [8], [9].

A simple definition for the concept of chemical divides states that of two ions precipitated as a salt mineral, one will increase in solution and one will decrease in solution as evaporation proceeds [8], [9]. This variance in ion concentration is determined by the molar ratio of the two ions in solution compared to the ratio present in the salt. For example, upon gypsum precipitation, if the Ca to SO₄ molar ratio in solution is greater than that of gypsum, SO₄ will eventually decrease to the point of exhaustion while Ca will continue to increase. Alternatively, the inverse may take place if Ca is less than SO₄, resulting in a Ca-deficient fluid. This chemical divide created by gypsum precipitation largely determines the chemical evolution and subsequent precipitation pathway of the evaporating fluid. Therefore, the two most important factors that will determine the chemical evolution of evaporating brines, as well as the character and sequence of saline precipitates, are: (1) the chemistry of the dilute fluid being evaporated and (2) the nature of the precipitates creating chemical divides. These two factors appear to be fundamentally different for Martian evaporative systems when compared to terrestrial evaporative systems. Accordingly, chemical divides for terrestrial evaporites are not transferable to Martian environments. It is imperative then, that chemical divides be evaluated for these unique systems in order to provide useful constraints on the saline mineral assemblages expected at the Martian surface as well as the chemical evolution of resulting brines.

Here, we present a new system of chemical divides that is likely to be encountered during the production of saline mineral assemblages at the Martian surface. The resulting scheme for brine evolution on Mars captures the variability of saline mineral assemblages expected, their precipitation pathways, and the major brine types formed as a result of evaporative concentration. Our results establish a “roadmap” for the interpretation of saline mineral assemblages identified at the Martian surface and place constraints on the aqueous conditions necessary for their formation. The system of chemical divides developed here for the Martian surface independently predicts the saline mineral assemblages observed in SNC-type meteorites as well as assemblages inferred from MER and MEx data. The new chemical divide system also demonstrates that the SNC-type, Meridiani Planum, and OMEGA-derived evaporite mineral assemblages could have evolved from a similar parent fluid, but simply buffered to different pH levels by basaltic weathering. As a result, two distinct geochemical environments for the production of saline minerals on Mars have been delineated: (1) acid–sulfate dominated environments and (2) mixed carbonate–sulfate dominated environments. Utilizing new data returned from MER and MEx missions in combination with thermodynamic modeling and other geochemical constraints, we suggest that the results obtained by the Opportunity rover point to a geochemical process which may have been common across the Martian surface, but is extremely rare on Earth—saline mineral formation from acidic fluids derived from basaltic weathering.

Section snippets

Controls on solution chemistry

Analogous to the Earth's surface, solute acquisition at the Martian surface will be controlled mainly by the chemical weathering of crustal materials. The resulting fluid chemistry, however, differs markedly from the majority of dilute inflow waters on Earth that lead to brine formation. Compared to terrestrial dilute surface waters, Martian fluids are enriched in Mg, SiO₂, Ca and most importantly, under acidic conditions, Fe [10], [11], [12]. Fluid composition is a critically important factor

Approach and methods

A series of evaporation calculations based on equilibrium thermodynamics was performed in this study to construct a new system of chemical divides applicable to the Martian surface. Thermodynamic calculations of mineral solubility upon evaporation were performed with the Geochemist's Workbench^® software (GWB) [17]. The Pitzer ion interaction approach was used for all activity coefficient calculations. All Pitzer parameters and mineral solubility data used are the same as those reported in Tosca

Chemical divides at the Martian surface

Fig. 2 depicts the major chemical divides, precipitation pathways, and brine compositions encountered during the evaporation of fluids where the initial chemistry is controlled by basaltic weathering under acidic conditions. Our thermodynamic calculations suggest that upon evaporative concentration (moving downward on Fig. 2), the first major precipitate formed is dependent on the proportion of HCO₃⁻ to SO₄²⁻ in the fluid. Accordingly, the first major chemical divides encountered in this system

Discussion and implications

Chemical divides have been applied to numerous terrestrial evaporative environments placing powerful constraints on their geochemical evolution. As a result, the chemical divide concept has demonstrated a robust predictive ability for the precipitation of major saline mineral assemblages from a wide range of fluid compositions. Of the numerous applications on Earth, chemical divides have been used to interpret the evolution of seawater composition over geologic time [30], [31] as well as the

Acknowledgements

The authors would like to thank Joel Hurowitz, John Grotzinger, Andrew Knoll, Rebecca Parsons, and the Athena Science Team for thorough reviews and helpful discussions. The authors would also like to thank two anonymous reviewers for insightful comments.

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Citation Excerpt :
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Chemical divides and evaporite assemblages on Mars

Abstract

Introduction

Section snippets

Controls on solution chemistry

Approach and methods

Chemical divides at the Martian surface

Discussion and implications

Acknowledgements

Geochim. Cosmochim. Acta

Geochim. Cosmochim. Acta

Geochim. Cosmochim. Acta

Chem. Geol.

Icarus

Earth Planet. Sci. Lett.

Geochim. Cosmochim. Acta

Earth Planet. Sci. Lett.

Chemistry of rocks and soils in Gusev from alpha particle X-ray spectrometer

Science

Sulfates in Martian layered terrains: the OMEGA/Mars express view

Science

Sulfates in the north polar region of Mars detected by OMEGA/Mars Express

Science

Chemistry of rocks and soils at Meridiani Planum from the alpha particle X-ray spectrometer

Science

In-situ evidence for an ancient aqueous environment at Meridiani Planum, Mars

Science

Alteration assemblages in Martian meteorites: implications for near-surface processes

Space Sci. Rev.

Microbes and mineral precipitation, Miette Hot Springs, Jasper National Park, Alberta, Canada

Can. J. Earth Sci.

The evolution of closed-basin brines

Saline Lakes

Acid–sulfate weathering of synthetic Martian basalt: the acid fog model revisited

J. Geophys. Res.—Planets