By using the sedimentological constraints together with ChemCam and APXS major element analyses of representative basaltic and alkaline compositions of the Gale Crater rocks and soil, and the CheMin and SAM results during the first 300 sols, we establish an equilibrium thermochemical model for the subsurface mineral reactions in the Yellowknife Bay sediments of Gale Crater. This model envisages reaction of a pore water (Gale Portage Water (GPW), see Methods) with the enclosing detrital sediment. In our model, we primarily study the production of clay through the inhomogeneous alteration of a Rocknest‐type host rock, within which olivine and amorphous material are the predominant alteration phases, because both of which are relatively reactive compared to other phases. We also consider other host rock end‐members (see Methods). There is clear evidence from terrestrial analogue environments such as altered Icelandic basalts and tuffs that olivine and glassy material are the most reactive phases [e.g., Bishop et al ., 2002 ]. We use the thermochemical model to provide a way of understanding of the secondary minerals of Gale Crater that is complementary to the field observations made by the Curiosity team. Starting with unaltered rocks and soils found in the area, we aim to calculate a realistic mixture of dissolving minerals within those rocks and soils that reacted to form the secondary, clay‐bearing assemblage during diagenesis. This will also help to decide whether some of the phases are detrital or authigenic or a mixture of both.

Chemically, the APXS analyses of other Gale Crater rocks have established the presence of a range of compositions. These include Fe‐rich basaltic sediment as shown by the in situ analyses at Yellowknife Bay and the Portage soil analysis [ Schmidt et al ., 2014 ; McLennan et al ., 2014 ]. A large range in alkali compositions has been seen in other samples, including a K‐rich alkaline basaltic composition shown by the Jake_Matijevic sample [ Stolper et al ., 2013 ; Schmidt et al ., 2014 ]. Rock samples found within the Rocknest (sols 55–102) and Bathurst_Inlet (sol 54) localities are probable basaltic sediments with alkaline contents intermediate between those of Jake_Matijevic and Portage soil or the Sheepbed mudstones [ Schmidt et al ., 2014 ].

CheMin and APXS analyses of the Portage soil were carried out between sols 55 and 102 at the Rocknest locality. This provides a mineralogical control on the country rock in the Gale Crater region [ Bish et al ., 2013 , and references therein]. Two of the major phases identified by CheMin were forsteritic olivine and an amorphous component together with plagioclase, augite, pigeonite, and minor minerals but no clay [ Bish et al ., 2013 ] (see Table 1 ). The amorphous component is interpreted as being similar to an amorphous component found in Hawaiian basaltic soils [ Bish et al ., 2013 ].

Two drilled samples of the mudstone, at locations named John_Klein and Cumberland, took place between Martian solar days (sols) 180 and 292 of the mission and allowed analysis of material beneath the uppermost, reddish oxidized dust coating. The samples were analyzed in the CheMin instrument by X‐ray diffraction [ Vaniman et al ., 2014 ] (Table 1 ) and by pyrolysis with the Sample Analysis at Mars (SAM) gas chromatograph–mass spectrometer [ Ming et al ., 2014 ] in order to obtain the mineral identities and evolved gas compositions. Both CheMin analyses revealed a saponite in the Sheepbed mudstone, and geochemical observations [ McLennan et al ., 2014 ] suggest only minor chemical alteration of the sediment source region before deposition. The mineralogical and sedimentological observations suggested that Yellowknife Bay has been a habitable environment, with a neutral to alkaline pH and relatively low temperatures of diagenesis [ Grotzinger et al ., 2014 ]. Furthermore, McLennan et al . [ 2014 ] suggested on the basis of major element discriminant diagrams and Chemical Index of Alteration criteria from Nesbitt [ 2003 ] that the Yellowknife Bay formation had very little evidence of chemical mobility associated with the alteration. They conclude that this indicated arid, possibly cold, palaeoclimates with rapid erosion and deposition and low water/rock ratios during diagenesis.

The Sheepbed mudstone has a sharp contact with the overlying 3 m thick succession of the Gillespie and Glenelg members, which contain fluvial sediments [e.g., Grotzinger et al ., 2014 ], with a lower abundance of sulfate veining than Sheepbed. The Yellowknife Bay formation underlies the Hottah Facies conglomerates found across the Peace Vallis alluvial fan [ Williams et al ., 2013 ]. The upper and youngest sediments on the area are unconsolidated, windblown soils, which were investigated with the rover instruments at the Rocknest site [ Bish et al ., 2013 ; Morris et al ., 2014 ].

The 4.5 m thick Yellowknife Bay formation is subdivided into different members with the lowest one, Sheepbed, being an at least 1.5 m thick mudstone, but its lower contact is not visible; its upper contact to the overlying Gillespie member is sharp. The Sheepbed member is a mudstone of overall basaltic chemical composition with ~15% smectite, ~50% igneous minerals, and ~35% X‐ray amorphous material [ Grotzinger et al ., 2014 ]. The observed magnetite is considered to be of authigenic origin [ Grotzinger et al ., 2014 ]. The unit contains abundant nodules, hollow nodules, voids, raised ridges, and sulfate‐filled cracks (Figure 1 ), all of which are associated with the late stages of the diagenesis [ Grotzinger et al ., 2014 ; McLennan et al ., 2014 ]. Chemistry and Camera (ChemCam) analyses also showed that the raised ridges have a Mg‐rich composition (1.2–1.7 times) relative to the surrounding mudstone [ Leveille et al . 2014]. Key textural observations are that the raised ridges postdate the sedimentary layering and sulfate veins postdate the raised ridges. The notably pure Ca‐sulfate composition of the late veins was initially established by ChemCam (Laser Induced Breakdown Spectroscopy) and was confirmed by Alpha Proton X‐ray Spectrometer (APXS) [ McLennan et al ., 2014 ]. Both drilled samples—named John Klein and Cumberland—are within the Sheepbed member [ Vaniman et al ., 2014 ].

At the Yellowknife Bay locality of Gale Crater, the Mars Science Laboratory (MSL) rover Curiosity has identified and analyzed, for the first time on Mars, a set of mudstones. The mudstones record a history of deposition within a fluvio‐lacustrine environment followed by low temperature, in situ diagenesis [ Grotzinger et al ., 2014 ; McLennan et al ., 2014 ; Vaniman et al ., 2014 ]. The composition and mineralogical information preserved in the Gale Crater sediments provide a unique opportunity to determine the nature of the alteration. In particular, we aim to constrain the mineral reactions, Water/rock ratios, pH, and redox conditions associated with the clay‐ and magnetite‐bearing assemblages identified by heMin XRD in the Sheepbed mudstone [ Vaniman et al ., 2014 ]. We base our model on the sedimentological and mineralogical observations of mudstones and soil observed by the rover Curiosity. The mudstones occur in the Yellowknife Bay area of Gale Crater, about 450 m from the Bradbury landing point. The stratigraphy of the area has been extensively studied from orbit and in the rover images. We give a brief summary from bottom to top of the sequence here, but for details, see Grotzinger et al . [ 2014 ], and references therein.

Gale Crater is thought to have formed near the Noachian‐Hesperian boundary with an age of about 3.7 Gyr, and although the exact age of the Gale sediments is not certain, crater counting suggests an ancient age [ Thomson et al ., 2011 ]. K‐Ar dating by the rover Curiosity supports this ancient age by dating a mixture of detrital and authigenic components as found in the Cumberland drill sample to an age of 4.13 ± 0.42 Ga [ Farley et al ., 2014 ].

2 Model Methods and Assumptions

For the thermochemical modeling, we use the program CHIM‐XPT (previously CHILLER) [Reed and Spycher, 2006; Reed et al., 2010], which is a program for computing multicomponent, heterogeneous chemical equilibria. This means that every calculation step calculates equilibrium between the starting fluid and the dissolved rock. Thus, each step can be treated and interpreted independently from the direction from which it was reached, and trends in water/rock (W/R) ratio can be read in both directions, because equilibrium is independently calculated for each step. Step size may vary depending on the requirements of the task, and the calculation is largely independent of the amount of water, since a weight ratio is used and the base unit for the calculation is moles. For convenience, 1 kg (or 55.5 mol) of water is generally the basis for the calculation. The method used here is a batch calculation where precipitates are not fractionated from the system. For details of the code, database and input files, we refer to the handbook for CHIM‐XPT [Reed et al., 2010] for background on the reaction pathway models, in general, relevant to our paper's methodology, especially titration modeling, see example Kühn [2004, chapter 3] and for a discussion on databases and the mathematical‐theoretical background [see, e.g., Ganguly, 2008 and Oelkers and Schott, 2009, especially chapters 1–3]. CHIM‐XPT has been extensively used in terrestrial basaltic environments [e.g., Reed, 1982; Reed, 1983] and for Martian compositions [DeBraal et al., 1993; De Caritat et al., 1993; Schwenzer and Kring, 2009; Bridges and Schwenzer, 2012; Schwenzer et al., 2012a, 2012b; Schwenzer and Kring, 2013; Filiberto and Schwenzer, 2013]. The program requires choices on starting fluid, host rock elemental composition, temperature, and pressure.

For host rock compositions in our modeling, we used a variety of rocks observed by Curiosity (Table 2 and section 2.2) and, in addition, selective mineral reactions in a Portage‐soil type host rock with the amorphous component and olivine. Note that we start with a rock that has 22.4% olivine and no clay minerals (Portage soil; Table 1) and model the alteration minerals as they are found in the mudstones (John Klein and Cumberland drill samples; Table 1), which contain much less olivine (6 and 2%, respectively) and 20 ± 2% phyllosilicates [Vaniman et al., 2014] (Table 1). Temperature was set to 10°C and pressure to 1 bar for the models presented here, following the diagenetic scenario suggested by the sedimentological observations [Grotzinger et al., 2014].

Table 2. Compositions of Starting Rock, Soil, and Single Phase Compositions Amorphous Wt % Jake_M Ekwir Brushed Portage Olivine Portage John Klein Cumberland SiO 2 51.85 46.07 44.88 36.3 41.78 41.01 41.73 TiO 2 0.51 0.90 1.25 2.25 2.07 1.66 Al 2 O 3 16.51 8.43 9.87 0.06 6.95 6.26 6.00 Cr 2 O 3 0.03 0.37 0.51 0.02 1.17 0.90 0.91 FeO 6.43 13.27 12.96 33.2 7.78 0.35 21.45 Fe 2 O 3 1.07 1.64 2.23 15.74 20.70 2.78 MnO 0.14 0.21 0.43 0.63 0.98 0.55 0.54 MgO 3.71 9.83 9.09 29.7 5.72 7.81 8.35 CaO 6.23 6.00 7.62 0.25 6.37 8.97 6.80 Na 2 O 6.47 1.34 2.22 2.96 3.36 2.49 NaCl 1.48 2.99 1.19 1.77 1.44 3.17 K 2 O 2.27 0.63 0.51 0.95 1.00 0.79 P 2 O 5 0.51 1.10 0.98 2.24 1.98 2.02 FeS 2.76 7.23 6.26 3.35 3.60 1.30

Results of calculated equilibrium mineral assemblages are presented in diagrams of mineral abundance versus W/R ratio (mass of rock reacted with the starting fluid). The plotted W/R ratio is thus a progress variable with very limited rock dissolution at the high W/R end and increased rock dissolution at the low W/R end. Note that W/R end represents the amount of rock reacted with the fluid not the total amount of rock present in a given volume of rock on Mars. Original magmatic minerals are observed in the mudstones [Vaniman et al., 2014] (Table 1), which means that the alteration of the rock is incomplete and unreacted material remains. Therefore, for comparison to the overall water to rock ratio as addressed by bulk rock compositional trends [McLennan et al., 2014], assumptions about the amount of reacted rock per total unit volume of rock in Gale have to be made. In other words, high W/R ratios might be indicative of systems, where the water interacts with a limited surface area, and therefore, only a small mass of rock is dissolved in a large mass of water. This occurs for example in a fracture, or on a rock surface exposed to regular precipitation. Low W/R ratio might occur where large rock surfaces react with a volume of water that is stagnant and not exchanged, e.g., in a porous sediment, although our results in succeeding sections suggest an open system with inflowing water, rather than a stagnant situation. The exact amount of precipitate caused by this dissolution is dependent on the species remaining in solution and on the details of minerals precipitated, specifically on structurally bound water or incorporated CO 2 . The amount of precipitation increases from a few milligram at high W/R to about 1 g at W/R of 1000 and on the order of 10 g at W/R of 100. We model between W/R of 1 and 100000 but only show 10 to 10000 for most of the runs. Higher W/R is unlikely within a sediment, but the lowest W/R would also produce phases with less H 2 O than phyllosilicates. W/R therefore describes the environment (freshwater inflow at high W/R in contrast to stagnant fluids with no fresh inflow at the low W/R), but at the same time reaction progress, because in a stagnant situation, more host rock will react over time, especially at low temperatures, where reactions are slow.

2.1 The Starting Fluid Composition: Gale Portage Water In order to model a realistic starting fluid representative of water associated with diagenesis in the Yellowknife Bay sediments, we start with adapted water (AW). This is the fluid used in our previous Mars studies [see Schwenzer and Kring, 2009]. It is a dilute aqueous solution with species concentrations based on warm fluids venting from a terrestrial basaltic environment—the Deccan Traps [Minissale et al., 2000]. The Deccan Traps were chosen, because of the seawater‐free nature of this environment. The fluid was then adjusted for Martian basaltic compositions by taking the Ca‐concentration of the terrestrial fluid and adjusting the Mg and Fe contents using the Ca/Fe and Ca/Mg ratios observed in Martian rock (shergottite LEW 88516). The solution is initially oxidizing (all S species as SO 4 2−). CHIM‐XPT can be controlled either by the set of O 2 ‐H 2 O‐SO 4 ‐H+ or expressed in terms of HS‐SO 4 ‐H 2 O‐H+. During the reactions, the SO 4 2−/HS− pair controls redox in the fluid [Reed et al., 2010], and a set of 112 different ionic species are typically used to represent the fluid chemistry in each calculation step. Table 3 is a summary representation of the element concentrations. The redox of the system throughout the run is dependent on the Fe2+/Fe3+ ratio of the host rock or soil (see section 2.3.2). Sulfur concentration of the fluid was taken as found in the Deccan Trap fluids [Minissale et al., 2000], and chlorine was used as the charge balance ion. From this, a dilute version was calculated by dividing all species concentrations by 10,000. This reduces the influence of introduced species in our model. Table 3. Fluid Compositions Used in the Modeling AW GPW GPW 185 mbar Cl− 0.587E‐1 5.76E‐3 5.76E‐3 SO 4 2− 0.285E‐2 3.97E‐3 3.97E‐3 HCO 3 − 0.168E‐4 1.68E‐4 0.62E‐2 SiO 2 ‐ 3.49E‐5 3.49E‐5 Ca2+ 0.250E‐2 1.41E‐5 1.41E‐5 Mg2+ 0.205E‐1 1.27E‐8 1.27E‐8 Fe2+ 0.919E‐2 ‐ ‐ K+ ‐ 5.02E‐4 5.02E‐4 Na+ ‐ 9.20E‐3 9.20E‐3 Mn2+ 4.36E‐8 4.36E‐8 Next, solid of Portage soil composition (Table 1) was titrated into this fluid at 50°C and 1 bar to account for a reaction of buried sediment (potentially at a higher geothermal gradient post impact) with the country rock. Portage soil from the Rocknest sand shadow is taken to be representative of average crustal compositions in the vicinity of Gale Crater. The resulting fluid composition at W/R of 100 was separated from the clay precipitate and cooled to 1°C (Figure 2), during which it produced a quartz (or amorphous SiO 2 , depending on kinetics) dominated precipitate (Figure 2). This is a common feature of cooling alteration fluids, and there is evidence for silica‐rich deposits on Mars, probably forming under a variety of temperatures and other conditions [e.g., McAdam et al., 2008; Squyres et al., 2008]. The Gale fluid was again separated from the precipitate, and the ions left in the fluid were considered to be GPW. CO 2 —as a proxy for C‐bearing species—is added as 1.68E‐4 mol HCO 3 −, a concentration that precludes carbonate formation, consistent with MSL results, and used in our previous work [e.g., Schwenzer and Kring, 2009]. All S species in GPW are summarized as SO 4 −. Species with concentrations below 10−10 mol were not considered in this starting fluid composition. Figure 2 Open in figure viewer PowerPoint Portage soil has been reacted with dilute Adapted Water AW at 50°C. The fluid was extracted from the original reaction at W/R (ratio of reacted rock with incoming fluid) of 100 and subsequently cooled to form Gale Portage water GPW, which we use in our model runs for the Yellowknife Bay diagenesis assemblage. (a) Plot of temperature (T in °C) versus ion concentration in 1 kg of water (in mol) of the fluid in equilibrium with the precipitate at different temperatures. Cooling causes precipitation—most noticeably of SiO 2 , Fe, Al and S. (b) Minerals precipitated upon cooling. Main precipitates are quartz (or another SiO 2 phase, depending on reaction kinetics), pyrite, stilbite, and apatite.