Tuesday, March 29, 2011

Review of "Contemporaneous deposition of phyllosilicates and sulfates: ..."

This is a review of

Baldridge, A. M., S. J. Hook, et al. (2009). "Contemporaneous deposition of phyllosilicates and sulfates: Using Australian acidic saline lake deposits to describe geochemical variability on Mars." Geophysical Research Letters 36.

This paper came out a year before the Hurowitz et al. (2010) article that I reviewed above, and really deserves to be recognized for first discussing the topic of Fe hydrolysis as a source of acidity in martian groundwater systems, especially since the Hurowitz article does not reference it. This paper is one of the few good studies of a terrestrial analog. It is good because it uses the analog to make new and interesting hypotheses for Mars rather than studying an analog that seems similar to Mars and then not saying much of anything.

The paper examines the geochemistry of acid saline lakes in western Australia and notices that this geochemical behavior could also be happening on Mars. In the Australian lakes, saline reducing groundwater interacts with basement rocks to produce relatively Fe(II)-rich groundwater with neutral pH (6-8). However, when this water rises to the surface it becomes oxidized and is acidified through Fe hydrolysis reactions:

2Fe2+ + 1/2O2 + 5H2O ==> 2Fe(OH)3 + 4H+

The source of the acidity is either from diagenetic pyrite, sulfides in the basement rock, or oxidation of H2S. Note that none of these mechanisms create acidity through weathering of Fe-silicate minerals.

When this groundwater reaches the surface it mixes with fresh water from surface runoff and produces a series of minerals including kaolinite, sulfates, chlorides, opaline silica, Fe-oxides, jarosite, gypsum, etc.. The pH gradients created exist both with depth and laterally with the central portion of the lakes being higher pH along with the subsurface.

Applying this information to Mars, Baldridge et al. suggest that the Andrews-Hanna groundwater models would likely result in reduced neutral to high pH fluids - similar to the groundwater below the australian lakes. However, this groundwater may become oxidized and acidified as it rises to the surface if it contains enough Fe(II). They note that these environments may contain large geochemical gradients making it possible to precipitate phyllosilicates and sulfates contemporaneously.

This poses an interesting alternative to the "Bibring Hypothesis" which states that Mars went through an early phyllosilicate period with alkaline fluids then was later dominated by acidic sulfate forming fluids. I've always felt that this was way too oversimplified, and the Baldridge et al. paper clearly articulates a good reason why by showing that geochemical systems are complicated and it isn't crazy to have a single aqueous system capable of forming all of the minerals at the same time.

However, I think the Baldridge et al. paper makes the same mistake as the Hurowitz et al. (2010) paper does. On the bottom of page 4 is the following quote:
The long aquifer flow paths would also promote dissolution of Fe-bearing volcanic glasses and silicates, thereby enriching the water in Fe2+ ions. As observed in australia, Fe2+ transported in solution would eventually oxidize and precipitate Fe3+ phyllosilicates and/or oxides, while generating acidity in the upward flowing waters.

Here the idea is that you can generate acidity by dissolving Fe(II) bearing silicate minerals. As mentioned in my post about the Hurowitz article, this reaction when viewed in total is pH neutral:
2FeO(pyx, ol) + H2O + 0.5O2 --> 2FeO(OH)

It is important to keep in mind that in the Australian lakes, and in geochemical models, the only way to generate acidity through Fe hydrolysis is if you have dissolved sulfide minerals or added acidity to the solution at some point. This is the requirement for the groundwater models of Mars. However since we do not show a large enrichment of Fe over Mg and Ca in the Meridiani sediments it seems unlikely that iron sulfides have been the source of the sulfur. So in order for the groundwater models to obtain acidity, they need to incorporate large amounts of SO2 into the aquifers during rainfall and runoff. Furthermore a large portion of this acidity must be neutralized through weathering of Fe(II) rich silicates rather than Ca or Mg-rich silicates which will permanently neutralize the acidity.

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