The Mars Science Laboratory (MSL) mission landed in Gale Crater, Mars, on 6th August 2012, and has explored the Yellowknife Bay area. The detailed mineralogical and sedimentological studies provide a unique opportunity to characterise the secondary fluids associated with this habitable environment

Bridges, J. C.

Conrad, P.

Kelley, S. P.

Leveille, R.

Mangold, N.

Martín-Torres, F.

McAdam, A.

Schwenzer, S. P.

Westall, F.

Wiens, R. C.

Zorzano, M.-P.

English

Open Research Online (The Open University)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsFluids, evaporation and precipitates at Gale CraterConference or Workshop ItemHow to cite:Schwenzer, S. P.; Bridges, J. C.; Leveille, R.; Wiens, R. C.; Mangold, N.; McAdam, A.; Conrad, P.; Kelley, S.P.; Westall, F.; Mart´ın-Torres, F. and Zorzano, M.-P. (2015). Fluids, evaporation and precipitates at Gale Crater. In:46th Lunar and Planetary Science Conference, 16-20 Mar 2015, The Woodlands, TX, USA.For guidance on citations see FAQs.c© 2015 The AuthorsVersion: [not recorded]Link(s) to article on publisher’s website:http://www.hou.usra.edu/meetings/lpsc2015/pdf/1441.pdfCopyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyrightowners. For more information on Open Research Online’s data policy on reuse of materials please consult the policiespage.oro.open.ac.ukFLUIDS, EVAPORATION AND PRECIPITATES AT GALE CRATER, MARS.  S.P. Schwenzer1, J.C. Bridg-es2, R. Leveille3, R.C. Wiens4, N. Mangold5, A. McAdam6, P. Conrad6, S. P.Kelley1, F. Westall7, J. Martín-Torres8,9 and M-P. Zorzano10. 1Dept. of Physical Sciences, CEPSAR, Open University, Milton Keynes UK; su-sanne.schwenzer@open.ac.uk; 2Space Research Centre, Dept. of Physics and Astronomy, University of Leicester, UK; j.bridges@le.ac.uk; 3Canadian Space Agency, St-Hubert, QC, J3Y 8Y9, Canada; 4Space Remote Sensing, Los Alamos National Laboratory, Los Alamos, , USA; 5Laboratoire Planétologie et Géodynamique de Nantes, LPGN/CNRS  and Université de Nantes, , France;  6NASA Goddard Space Flight Center, Greenbelt, MD, USA; 7Centre de Biophysique Moléculaire, CNRS, Rue Charles Sadron, 45071 Orléans Cedex 2, France; 8Instituto Anda-luz de Ciencias de la Tierra, Spain, 9Division of Space Technology, Luleå University of Technology, Kiruna, Swe-den, 10Centro de Astrobiología (INTA-CSIC), Spain.   The Mars Science Laboratory (MSL) mission land-ed in Gale Crater, Mars, on 6th August 2012, and has explored the Yellowknife Bay area [1]. The detailed mineralogical and sedimentological studies provide a unique opportunity to characterise the secondary fluids asociated with this habitable environment.   Clay mineral formation at Gale Crater, Mars:  The Yellowknife Bay formation is a sedimentary lake bed and fan deposit sequence [1] that consists of con-glomerates [2], and fine grained sediments including mudstones [3]. Within this sequence a complex set of diagenetic features is observed. The sequence begins wtih the early deposition of the sediment, followed by clay mineral formation and a variety of contemporane-ous or later diagenetic events [4], including ‘raised’ ridges alongside clays, voids, sulfate-filled voids and late sulfate veins [5-7]. We have modeled the condi-tions necessary to form the observed clay minerals [3] from the Portage host rock composition present at the Gale site and concluded that dissolution of 70 % Por-tage amorphous phase, 20 % olivine, and 10 % Portage whole rock can form clays of the observed compostion [8]. Here we present the fluids that are in equilibrium with the newly forming minerals and investigate, whether those fluids are capable of forming the ob-served sulfate veins.  Methods:  We use CHIM-XPT [9] to model altera-tion mineral formation conditions from rocks at Gale crater .We use titration models (details in [8]). For the model presented here we calculated reactions of 70 % Portage amorphous component [10,11], 20 % olivine, and 10 % Portage whole rock [12] at 10 °C. We ex-tracted the mineral forming fluid at W/R of 1000 and 100. Here W/R is mass of initial fluid over reacted rock. Fluids:  Clay formation reactions [8] produce clays as observed at the Gale site at a model W/R between 1000 and 100. Fluids in contact with the newly forming minerals are generally dilute, but at 100 are more con-centrated than at 1000 for most species (Fig. 1).  They are of Na-Cl nature with S-species, SiO2(aq), K and Ca being the next most abundant dissoved solids.  Fig. 1. Species concentrations in the fluid accom-paning clay formation during incongruent dissolution of Portage soil (70 % amorphous component, 20 % olivine, 10 % whole rock) at 10 °C.   Evaporation: We modeled the evaporation of this fluid (Fig. 2). The initial deposit is SiO2, followed by sulfate species and at the highest concentrations rock salt.  Sulfate species transition from gypsum to anhy-drite with decreasing water activity in the system. Car-bonate precipitation is minor, because the initial model assumed separation from the surface and atmosphere, and only minor amounts of dissolved CO2.  Carbonates? The precipitation of minor carbonate is in accordance with the non-detection of carbonates in the clay-rich sediments [3]. However, a lake in con-tact with the atmosphere could take up CO2 depending on CO2 partial pressure, which would in today’s Mars atmosphere lead to a CO2-concentration of 0.02 mmol/L and potentially some carbonate precipitation in a well mixed lake.  Gypsum or anhydrite? The hydration state of the observed sulfate is not necessarily the same as upon precipitation. Experiments have shown that the initial precipitate at low temperatures is almost always gyp-sum, but that a transition to bassanite or anhydrite may occur as early as within hours after the initial precipita-tion [13, 14].  With a low partial pressure of H2O in the current Martian atmosphere [15], some of the dehydration might have occured as recently as during drilling.  1441.pdf46th Lunar and Planetary Science Conference (2015)Amounts of silica and sulfate. Mass balance calcu-lations show that it is possible to explain the high SiO2 areas by precipitation of SiO2 from the fluid that formed the clays either in a fractionated precipitation event or during re-dissolution of previously precipitat-ed sulfates. Only minor evaporation is needed, if a fluid from clay formation at W/R 1000 evaporates. This means one kg of fluid contains 10 g of dissolved host rock, which precipitates 7.6x10-4 moles (or 0.013 mg) SiO2. The same amount of dissolved host rock can precipitate as much as 5.2 x10-4 moles - or 0.004 mg - CaSO4.  Scenarios for Gale Crater: From our models, two scenarios of the vein-forming stage of diagenesis are possible: 1) If the fluids reached the fractures and evaporated in place, a ‘dirty’ sulfate precipitate would result, with the sulfate vein containing some silica and halite (Fig. 3). Depending on fluid flow in the vein and evaporation, fractionation of the ascending fluid might occur and the precipitate might become more pure sul-fate over time and/or space.   Fig. 3. Scenario of vein formation directly from the clay forming fluid.  2) In analogy to the Watchett Bay site (Upper Tri-assic, UK, [16]), a two step process seems plausible. The initial fluid evaporates, e.g. in the shallow subsur-face or from a ground water-fed lake. Such a lake could receive water from the subsurface diagenetic fluids (groundwater) and from surface runoff (as seen by the conglomerates [2]). Evaporation of this water – with ions derived from the basaltic-composition rocks of the wider Gale area – results in a layer of ‘dirty’ sulfates during a dry period [1]. After burial of that layer by more (lake) sediments upwelling fluids preferentially dissolve the sulfates in the initial layer and deposit this refined sulfate in the veins (Fig. 4). The second scenar-io is in line with the observation of pure Ca-sulfate and the complex nature of sedimentation and diagenetic processes observed at Yellowknife Bay [1-7].   Fig. 4. Scenario in analogy of the Watchett Bay site (Upper Triassic, UK, [16]): a first layer of ‘dirty’ sul-fate is deposited and later re-dissolved, which produc-es pure sulfate veins - as observed at Gale.  Outlook: Sulfates have been observed in several layers along the traverse, most recently at Pahrump. From orbit, higher concentrations of sulfates have been observed in the layers within Mt. Sharp above the cur-rent location [17]. Evaporation of a brine derived from alteration of basaltic rock compositions could explain such concentration, especially if repeated evaporation and re-filling of a lake occured. References: [1] Grotzinger, J. P., et al. (2014) Science, 343: DOI: 10.1126/science.1242777. [2] Williams et al. (2013) Sci-ence, DOI: 10.1126/science.1239505. [3] Vaniman D. T. et al. (2014) Science, 343: 10.1126/science1243480.  [4] McLennan, S. M. et al. (2013) Science, 343: doi: 10.1126/science.1244734. [5] Leveille, R. J. et al. (2014) JGR Planets, DOI: 10.1002/2014JE004620. [6] Siebach, K. L. et al. (2014) JGR, DOI: 10.1002/2014JE004623. [7] Stack, K. M. (2014) JGR, DOI: 10.1002/2014JE004617. [8] Bridges et al. (2014) JGR, DOI: 10.1002/2014JE004757. [9] Reed, M.H. et al. (2010) User Guide for CHIM-XPT: A Program for Computing Reaction Processes in Aqueous-Mineral-Gas Systems and Minplot Guide. University of Oregon, Eugene, Oregon. [10] Morris, R.V. et al. (2013) 44th LPSC, abstr. #1653; Houston.  [11] Morris, R.V. (2014) 45th LPSC abstr. #1319; Houston. [12] Gellert, R. (2013) 44th LPSC Abst. #1432, Houston. [13] Ossorio et al. (2014) Chemical Geology, 386: 16–21. [14] Carbone et al. (2008) Eur. J. Mineral, 20: 621–627. [15] Bish, et al. (2003), Icarus 164: 96–103. [16] Philipp, S. L. (2008) Geological Magazine, 145 (6), 831–844.  [16] Thomson B.J. et al. (2011) Icarus, 214, 413. Fig. 2. Evaporation of the fluid forming clay miner-als at 10 °C from incongruent dissolution of Por-tage soil.  Left: Full range of water activities,  Right: Mineral precipitates when the system ap-proaches dryness. 1441.pdf46th Lunar and Planetary Science Conference (2015)

Fluids, evaporation and precipitates at Gale Crater

Fluids, evaporation and precipitates at Gale Crater

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