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# Insights into Ceres's evolution from surface composition

Julie Castillo-Rogez, Marc Neveu, Harry Mcsween, Roger Fu, Michael Toplis,
Thomas Prettyman

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## To cite this version:

Julie Castillo-Rogez, Marc Neveu, Harry Mcsween, Roger Fu, Michael Toplis, et al. Insights into Ceres's evolution from surface composition. Meteoritics and Planetary Science, 2018, 53 (9), pp.1820-1843. 10.1111/maps.13181. hal-02410961

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https://hal.science/hal-02410961v1

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# Insights into Ceres's evolution from surface composition

Julie CASTILLO-ROGEZ<sup>1*</sup>, Marc NEVEU<sup>2,3</sup>, Harry Y. McSWEEN<sup>4</sup>, Roger R. FU<sup>5</sup>, Michael J. TOPLIS<sup>6</sup>, and Thomas PRETTYMAN<sup>7</sup>

*(Received 13 July 2017; revision accepted 13 June 2018)*

**Abstract**—Inspired by the recent results of the Dawn mission, thermodynamic models of rock alteration and brine evaporation have been used to help understand the conditions under which water-rock interaction took place within the dwarf planet Ceres. This analysis constrains Ceres's early history and offers a framework within which future observations may be interpreted. A broad range of alteration conditions have been simulated using the Geochemist's Workbench and PHREEQC software, associated with the FREZCHEM model that constrains the consequences of freezing the liquid phase in equilibrium with the observed mineralogical assemblage. Comparison of the modeling results with observed surface mineralogy at Ceres indicates advanced alteration under a relatively high fugacity of hydrogen, a conclusion that is consistent with predictions for, and observations of, large ice-rich bodies. The simulations suggest production of methane that could help regulate the redox environment and possibly form clathrate hydrates upon freezing of the early ocean. The detection of localized occurrences of natrite (sodium carbonate) at the surface of Ceres provides key constraints on the composition of fluids that are necessarily alkaline. In addition, the combined hydrothermal and freezing simulations suggest that hydrohalite may be abundant in Ceres's subsurface, similar to Earth's polar regions. The global homogeneity of Ceres's surface, made of material formed at depth, suggests a large-scale formation mechanism, while local heterogeneities associated with impact craters and landslides suggest that some form of sodium carbonate and other salts are accessible in the shallow subsurface.

## INTRODUCTION

The *Dawn* Visible and Infrared Mapping spectrometer (VIR) has revealed that the surface of Ceres is dominated by a mixture of Mg-serpentine, ammoniated clays, Ca/Mg carbonates, and a darkening agent whose nature is unknown (De Sanctis et al. 2015). The global distribution of this material shows only minor lateral variations in spectral properties, which is interpreted as evidence for an in situ origin (Ammannito et al. 2016). VIR's detection of ammoniated minerals is robust and supported by a

previous ground-based detection (King et al. 1992) and by complementary studies (Beck et al. 2015; Berg et al. 2016). Based on experimental production of ammoniated clays and comparison with the VIR data, these clays are most likely to be a magnesium-rich smectite, such as saponite (Ehlmann et al. 2018). The VIR observations also indicate the global presence of 5–15 vol% of low-solubility carbonates (e.g., calcite, magnesite, dolomite). Complementary observations obtained from the SOFIA observatory in the mid-infrared by Vernazza et al. (2017) support the upper end of that range of abundance.

<footnote n="1"><sup>1</sup>Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109, USA</footnote>

<footnote n="2"><sup>2</sup>School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85281, USA</footnote>

<footnote n="3"><sup>3</sup>NASA Postdoctoral Management Program Fellow, NASA Headquarters, Washington, District of Columbia 20546, USA</footnote>

<footnote n="4"><sup>4</sup>Department of Earth and Planetary Sciences, The University of Tennessee in Knoxville, Knoxville, Tennessee 37996, USA</footnote>

<footnote n="5"><sup>5</sup>Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138, USA</footnote>

<footnote n="6"><sup>6</sup>IRAP, Université de Toulouse, CNRS, UPS, 31003, Toulouse, France</footnote>

<footnote n="7"><sup>7</sup>Planetary Science Institute, Tucson, Arizona 85719, USA</footnote>

<footnote>*Corresponding author. E-mail: julie.c.castillo@jpl.nasa.gov</footnote>
The follow-on detection of Na₂CO₃ as the dominant mineral forming the bright features observed in Occator crater's central region (De Sanctis et al. 2016), and the fact that Na₂CO₃ is found elsewhere, including the flanks of the extruded feature called Ahuna Mons (Zambon et al. 2016), implies the existence of low-eutectic brines, such as chloride solutions at depth (Ruesch et al. 2016). Indeed, sodium carbonates are more soluble than Ca/Mg carbonates and on Earth the former are typically found in evaporites associated with volcanic (CO₂-rich) lakes. Sodium carbonate and bicarbonate (NaHCO₃) have also been found in Enceladus's plumes (Postberg et al. 2011) and interpreted as evidence for the presence of alkaline conditions in Enceladus's ocean, currently or in the past (Zolotov 2007; Marion et al. 2012). The bright deposits on Ceres also include ammonium salts and indirect evidence for the presence of chlorides, observations that are also consistent with the studies of Zolotov (2007) and Marion et al. (2012). Sodium carbonate has also been found in several other sites across Ceres's surface (Carrozzo et al. 2018). Taken together, these observations suggest that Na₂CO₃ (or a possible precursor, such as NaHCO₃ or its hydrates) is available in Ceres's subsurface, either exposed by small impacts and mass wasting processes or mobilized by impact-induced heating (Bowling et al. 2016).

While hexahydrite (MgSO₄·6H₂O) was initially suggested as responsible for the bright spots based on reflectance observations at visible wavelengths (Nathues et al. 2015), the hyperspectral observations by the VIR instrument do not support that interpretation (De Sanctis et al. 2016) and experimental study of its stability at low pressure preclude its occurrence (Bu et al. 2018). Nevertheless, the state of sulfur in large icy bodies is an important question because it determines the fate of alkalis and alkaline earth metals in the formation of salt species, which can significantly influence the eutectic temperature of the ocean.

In addition to the VIR data, Dawn's Gamma Ray and Neutron Detector (GRaND) provides critical constraints on the chemical composition of Ceres's surface. For example, iron [Fe] is reported at a level of 16 ± 1 wt%, suggesting the presence of iron-rich materials, such as magnetite, pyrrhotite, and/or ferrihydrite (Prettyman et al. 2017). However, the fact that this value of [Fe] is slightly subchondritic (with respect to CI chondrites) implies modest ice-rock fractionation, perhaps the result of physical differentiation (e.g., convective upwelling of fines combined with sedimentation of coarser Fe-rich particles at depth) (Prettyman et al. 2017). Comparisons between ice stability models and GRaND counting data (Prettyman et al. 2017) indicate the regolith is fine grained, consistent with meteorite analogs.

<table><caption>Table 1. Observational constraints on Ceres's surface mineralogy, from De Sanctis et al. (2015, 2016), Raponi et al. (2018), and Kaplan et al. (2018) for the organics, and elemental composition from Prettyman et al. (2017, 2018). Concentrations of Fe and K are for Ceres ice-free regolith composition.</caption><thead><tr><th></th><th></th></tr></thead><tbody><tr><td>Average surface mineralogy</td><td>Abundances (vol%)</td></tr><tr><td>NH<sub>4</sub>-clays</td><td>~6–7</td></tr><tr><td>Serpentine (antigorite used in spectral fits)</td><td>~9–10</td></tr><tr><td>Darkening agent</td><td>~60–80</td></tr><tr><td>Carbonate (magnesite, dolomite, calcite)</td><td>5–15</td></tr><tr><td>Occator</td><td>Abundances (vol%)</td></tr><tr><td>Na<sub>2</sub>CO<sub>3</sub></td><td>At least 30</td></tr><tr><td>Evidence for ammonium salt (NH<sub>4</sub>Cl)</td><td>Up to ~5 wt%</td></tr><tr><td>Ernutet Region</td><td>Abundances (vol%)</td></tr><tr><td>Organics, with a predominant aliphatic signature and matched by chondritic insoluble organic material</td><td>Several % to 10s %</td></tr><tr><td>Element</td><td>Abundances</td></tr><tr><td>Water equivalent hydrogen</td><td>~27 wt% (17% in hydrated minerals; 10% water ice within a meter of the surface)</td></tr><tr><td>Iron</td><td>16 ± 1 wt%</td></tr><tr><td>Potassium</td><td>410 ± 40 µg g<sup>-1</sup></td></tr><tr><td>Carbon</td><td>Data permit concentrations in excess of CI chondrites (&gt;3.5 wt%)</td></tr></tbody></table>

The purpose of the present study was to constrain Ceres's chemical evolution based on the observations returned by Dawn to date (Table 1) (see also McSween et al. [2018] for a review). Of particular interest, we aim to assess the implications of the presence of ammoniated clays, and to constrain the redox and pH conditions of the large-scale environments in which the two types of detected carbonates, CaMg-rich and natrite, formed, noting that the latter is not found in carbonaceous chondrites. This study also assesses, in a self-consistent manner, the evolution in the chemistry of both the aqueous solution and residual solid and tracks the fate of sulfur and its availability relative to chlorine for the formation of salt species. The results lead to predictions of other mineral species that may be present within Ceres and provides a point of comparison with icy satellites that share certain mineralogical characteristics (e.g., Zolotov 2007; Glein et al. 2015).

## MODELING APPROACH AND INPUT PARAMETERS

Geochemical evolution during alteration is addressed using a two-step modeling approach. In the
first step, the aqueous conditions in which Ceres's surface mineralogy formed are simulated, and then the freezing of the solution in equilibrium with the observed surface mineralogy is considered. Results may then be compared with previous predictions for large icy satellites and observations at Europa and Enceladus, and used to constrain Ceres's geophysical evolution.

## Methodology

We run the commercial software Geochemist's Workbench (hereafter referred to as GWB) (Bethke 2008) to model Ceres's chemistry at thermodynamic equilibrium. This software uses the thermodynamic database thermo.tdat (https://student.gwb.com/thermo.php) developed by the Lawrence Livermore National Laboratory, which is the default data set for GWB. It allows the computation of activity coefficients with a Debye-Hückel model and includes equilibrium constants for 624 minerals and the main gas species H₂O, CO₂, CH₄, H₂S, N₂, S₂, and O₂. Species that are known to not form in the aqueous environment considered for this study have been suppressed to avoid confusion. For example, the antigorite polymorph of Mg-serpentine tends to be expressed preferentially in the GWB output, even though it is a high-pressure form of chrysotile and lizardite, which are actually expected. Another example is the expression of pyrite over pyrrhotite. A similar situation is found with other software and databases (e.g., PHREEQC; Neveu et al. 2017). Furthermore, we validated our modeling with the PHREEQC software (Parkhurst and Appelo 2013) based on the more general study applied to icy worlds by Neveu et al. (2017). The thermodynamic data used for PHREEQC calculations are those compiled in the core10.dat database file distributed with PHREEQC at https://wwwbrr.cr.usgs.gov/projects/GWC coupledd/ phreeqc. Briefly, this database was developed from PHREEQC's llnl.dat compilation, benchmarked against databases of similar extent and complemented with organic solids and solutes, ammoniated minerals, low-temperature ammonium salts, updated data for phyllosilicates, and molar volumes for minerals and solutes. Details on the construction and validation of this database are provided by Neveu et al. (2017).

The Geochemist's Workbench has the capability to track how mineralogy evolves as a function of the progress of the reaction toward equilibrium (final state). For the case of Ceres, the occurrence of Mg-serpentine and high abundance of carbonate both suggest aqueous alteration was advanced (McSween et al. 2018) and consistent with the most aqueously altered CI chondrites. Hence, in this paper we assume the system has reached equilibrium, although more accurate

<table><caption>Table 2. Parametric space covered in this study.</caption><thead><tr><th>Parameter</th><th>Range</th></tr></thead><tbody><tr><td>T (°C)</td><td>0–120</td></tr><tr><td>Partial pressure H<sub>2</sub> (log (bar))</td><td>-20 to +5</td></tr><tr><td>CO<sub>2</sub> (wt% volatiles)</td><td>0–10</td></tr><tr><td>Water-to-rock ratio</td><td>0.1–100</td></tr><tr><td>Chlorine (mass)</td><td>0.2× to 2× CI value (680 mg kg<sup>-1</sup>)</td></tr><tr><td>NH<sub>3</sub> (wt% volatiles)</td><td>0–5 (either accreted or desorbed from organics)</td></tr><tr><td>Rock composition</td><td>CI or CM chondrite</td></tr></tbody></table>

estimates of the modal composition, clay composition (e.g., nature of the cations), and forms taken by iron and other metals would help to confirm this assumption.

The database used in GWB is not valid at temperatures beyond about 120 °C. In particular, the fate of ammonia is not properly handled for higher temperatures. However, Neveu et al. (2017) show that the temperatures relevant to Ceres are more likely <50 °C, consistent with geophysical modeling (Travis et al. 2018). Hence, the temperatures considered in this study are <100 °C.

In a second stage, we track the freezing of the solution obtained from aqueous alteration by importing the output of GWB into the FREZCHEM software (Marion and Grant 1994). This software is particularly suitable for modeling the geochemical evolution of cold solutions and was recently upgraded to handle ammonia (Marion et al. 2012). FREZCHEM cannot handle all the species dealt with by GWB but is sufficient to track the evolution of the most abundant solutes.

The parametric space surveyed in this study is broad, as described in Table 2. We assume that Ceres's material evolved from the interaction of anhydrous rock with volatiles that melted on a global scale upon heating by decay of short-lived radionuclides. The detection of natrite suggests that Ceres's bulk chemistry, environment, and/or alteration pathways differ from those that operated on chondritic parent bodies (e.g., Zolotov 2009).

## Input Parameters

### Composition of Accreted Rock

The nature of the original accreted rock is uncertain, but aqueously altered carbonaceous chondrites are plausible protoliths (McSween et al. 2018). Recent advances in isotopic chemistry suggest an origin of CI chondrite parent bodies beyond 3 AU (Alexander et al. 2015; Kruijer et al. 2017; Scott et al. 2018). Since the detection of ammonia at Ceres also
points to an origin of Ceres's material beyond 3 AU (De Sanctis et al. 2015), with a corresponding enrichment in volatiles, we take the dry composition of CI chondrites as the reference composition for this work. To test the robustness of our chemical evolution models with respect to the assumed bulk composition, we also run our models starting with an average CM chondrite composition. Geochemically, CM chondrites are depleted in major volatile species (i.e., H, C) with respect to the CI, indicating that CI chondrites accreted more volatiles than CM chondrites. This absence of volatiles leads to a slight (~25%) enrichment in the absolute abundance of refractory elements such as Mg and Al as measured today (e.g., Wasson and Kalllemeyn 1988). On the other hand, this geochemical depletion in CM relative to CI does not involve only typical ice-forming elements such as H and C, but is also observed for a wide range of cosmochemically volatile elements (i.e., those that have condensation temperatures <~1000 K). Indeed, elements such as Na, K, S, and Cl can show depletions of up to a factor of 6 in CM relative to CI (Wasson and Kalllemeyn 1988) through a mass-balance effect. All of these differences may have implications for geochemical evolution: for example, depletion in the heat source K may be of importance, while a limited supply of water may affect the extent of aqueous alteration.

## Ammonia

The detection of ammonia-rich material on Ceres (De Sanctis et al. 2015), which is not typically seen in chondrites, requires revisiting initial conditions for these models. Multiple origins have been proposed for the ammonia. It could have been accreted in the form of ammonia hydrates supplied from cometary material in the context of the pebble model (De Sanctis et al. 2015). An alternative source may be soluble organics, such as amino acids, or ammonia desorbed upon heating of other organics (McSween et al. 2018). Ammonium salts have been found in CI chondrites and could have been provided in this form as well as (soluble) amino acids and insoluble organic matter (e.g., Peltzer et al. 1984; Pizzarello and Shock 2014). If Ceres formed beyond 15 AU, then nitrogen could have accreted primarily in the form of nitrogen ice N₂ (Lodders 2003). In this study, we explore accretion of nitrogen both in the form of ammonia (hydrates or released from organics) and in the form of N₂.

## Carbon

We vary the amount of accreted CO₂ as a free parameter. This CO₂ could have been supplied with the cold material that brought in the ammonia (De Sanctis et al. 2015), but it is also likely that carbon was

accreted, at least in part, in the form of organics. It is likely a major component of the dark agent in Ceres's regolith is some form of amorphous or graphitized carbon (Hendrix et al. 2016; Kaplan et al. 2018; McSween et al. 2018; Prettyman et al. 2018). McSween et al. (2018) and Prettyman et al. (2018) discuss issues with magnetite being the only darkening agent, leaving carbon as an important contributor. Furthermore, Prettyman et al. (2018) describe bounds on carbon and interpretations of nuclear spectroscopy data, which allow a lot of C to be present in the regolith (Table 1).

The thermodynamic database used with the GWB software (thermo.tdat) does not contain organic species, but the core10.dat database for use with PHREEQC does and is used in particular for those simulations. We do not track the production of soluble organics in the hydrothermal environment, although they have been suggested to have occurred in carbonaceous chondrite parent bodies (e.g., Schulte and Shock 2004) and seem to be involved in the production of carbonates (see McSween et al. 2018). Hence, it is possible that neglecting organics results in underestimating the amount of carbonates produced upon aqueous alteration.

## Environmental Parameters

The temperature and partial pressure of hydrogen, referred to as pH₂ in the remainder of this paper, are varied to simulate a large variety of environments, from hydrothermal conditions at high pressure (reducing) to local, surficial conditions. The pH is not imposed, but is instead an output of the simulations.

The temperature is varied between 0 and 120 °C. For the examples shown in this paper, we use a reference temperature of 15 °C, following Travis et al. (2018), who modeled hydrothermal circulation in Ceres's early history and found this temperature to prevail. The temperature is assumed constant throughout each test, which is a reasonable assumption considering that the time scale of chemical alteration is likely much shorter than that of ocean freezing (e.g., Castillo-Rogez and McCord 2010; Travis and Schubert 2015).

The pressure in the interior of Ceres can reach up to ~145 MPa. The GWB simulations are run at a pressure of 1 bar, while the PHREEQC simulations cover a broad range reaching pressures of 100 MPa, which occur at depths of ~170 km. We refer the reader to Fig. 4 based on the modeling by Neveu et al. (2017). This figure shows that the effect of pressure on the overall chemistry is negligible within this range.

The water-to-rock ratio (W/R) is a way to illustrate the extent to which the rocky material is exposed to “fresh” water, providing indirect information on the efficiency of hydrothermal circulation. For example, a
$W/R = 5$ means that 1 kg of rock is exposed to about 5 kg of water during hydrothermal circulation. CM chondrite material is characterized by a $W/R$~0.35–0.6, and CIs are thought to have experienced $W/R$ up to 2.5 (Clayton and Mayeda 1999). Ceres has a potential for more pervasive hydrothermal circulation, considering its size and water abundance. We simulate these conditions by testing a broad range for $W/R$, between 0.1 and 100.

Examples of GWB input and output files are presented in the Appendix.

## EARLY HYDROTHERMAL CHEMISTRY INSIDE CERES

This section starts with activity diagrams, followed by an example of a simulation that reproduces the global surface mineralogy of Ceres and a survey of the range of environmental conditions that are compatible with the global surface composition. We show that the surface composition provides evidence for extensive aqueous alteration that approaches equilibrium, and we present the fate of key elements.

### Activity Diagrams

In order to illustrate the results, we begin by considering the activities of key species in pH-redox space and as a function of temperature (Fig. 1). In this way, the dominant thermodynamically stable form of each element can be visualized as a function of pH and redox conditions. We assume the various components of the system (unspecified solutes) have equal chemical activity of 0.001 for the sake of illustration.

For nitrogen-bearing species, ammonium dominates the aqueous nitrogen budget at $pH \le 9.5$ and log $pH_2 \ge -8$ (Fig. 1a) for a temperature of 15 °C. The equilibrium is shifted to pH~9 and log $pH_2 > -7$ at 50 °C and pH~7.5 and log $pH_2 > -5$ at 100°C. Dolomite is also expected to form for similar conditions, for a pH between 8 and 11 and log $pH_2 < -4$ (Fig. 1b), while bicarbonate is the dominant form of carbonate in solution (Figs. 1b and 1c). Hence, the occurrence of ammonium and dolomite constrains the environmental conditions in Ceres's early ocean to first order. The total abundance of carbonates is also a function of the water-to-rock ratio as illustrated in the Reproduction of Surface Mineralogy section.

In a system where aqueous alteration of rock rich in minerals containing relatively reduced iron is taking place, serpentinization releases significant amounts of hydrogen upon oxidation of iron in silicates, which in a closed system drives the chemistry toward the reducing end (top) of the diagrams shown in Fig. 1. For high $pH_2$ (log $pH_2 > -6$), reduction of carbon species may

lead to a significant production of methane, provided that the kinetics are favorable (see the Fate of Key Elements and Composition of the Remaining Solution section).

The partial pressure of $H_2$ in Ceres's ocean could range from <0.1 to >500 bars. At these conditions, sulfur is present in reduced form (Fig. 1d) and iron is partitioned between magnetite and sulfides (Figs. 1e and 1f). The stability of other elements is discussed in the Dependence on Environmental Parameters section.

### Reproduction of Surface Mineralogy

The surface mineralogy of Ceres can be reproduced for a specific but broad range of conditions that are characterized by a relatively high partial pressure of hydrogen (Fig. 1). This indicates that the redox conditions are representative of a chemically closed environment where $H_2$ produced from aqueous alteration could accumulate under hydrostatic pressure, rather than escape to space. This is consistent with expectations for objects of Ceres's size (McKinnon and Zolensky 2003). Cronstedtite (Fe-serpentine) forms in the early phase of the aqueous alteration process, but is converted into Mg-serpentine and saponite clays, while the iron is released to combine with sulfur and oxygen (Howard et al. 2011). Cronstedtite is generally found in CM chondrites where aqueous alteration is partial. An example of mineralogical evolution is presented in Fig. 2. It reproduces the observed serpentine, clays, carbonates, as well as magnetite and pyrrhotite as iron-rich minerals. As can be appreciated from this figure, more than 90 wt% of the secondary assemblage is composed of the four minerals saponite, antigorite (or, in practice, another Mg-serpentine polymorph), magnetite, and troilite.

### Clay Composition

Clays produced in the simulations are preferentially saponites, consistent with observations of CI chondrites (e.g., Howard et al. 2011). The most abundant saponite is Na-rich prior to exchange with ammonium. K-saponite is expressed over a broad range of conditions at a minor level (2–3 wt%). Mg-rich saponite is also expressed under certain environmental conditions discussed below. It is important to note that none of our simulations produce brucite. That compound was suspected to be present on Ceres based on ground-based spectral observations (Milliken and Rivkin 2009), but not found in the VIR data. Brucite requires a low fugacity of $CO_2$, which is not consistent with the system modeled in this study that involves a substantial amount of $CO_2$.
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Fig. 1. Activity diagrams for key species found by VIR observations in the pH-redox space obtained with the act2 software component of the Geochemist's Workbench. Each panel indicates which compounds are in dominant form in the system, i.e., in an abundance higher than any other forms of a particular element. The reference temperature and pressure are 15 °C and 10 bar. Unspecified solute activities are taken as 0.001, except where noted. The thermodynamic properties of solutes are referenced at infinite dilution (activity tends toward 0), whereas those of solids are referenced to the pure phase (activity = 1). For solutes, an activity of 0.001 is small enough that any deviations from ideality of the solution are negligible. Taking activities of 0.001 is an arbitrary choice, but it results in solute-solid boundaries corresponding to the conditions at which enough solid dissolves to yield a solute activity of 0.001. Red boundaries between solute predominance fields indicate the log fH₂ and pH conditions at which the activities of solutes on either side of this boundary are equal (to ≈0.001/2 if the stoichiometry of the element of interest is the same for both solutes, and neglecting all other solutes) and such that if only one solute were present (i.e., predominated), its activity would be 0.001. a) Equilibrium in a simple NH₄⁺/NH₃ system. b) Equilibrium of CO₃²⁻ in presence of Na+, K+, Mg++, Ca++, Fe++, HS-, Cl-, NH₄⁺; the activity for Fe++ is 10⁻¹⁰. c) Same but the activity of Na+ is increased to 0.1. d) Equilibrium in a system with sulfur in presence of Na+, K+, Mg++, Ca++, Fe++, Cl-. e) Equilibrium of iron in presence of Na+, K+, Mg++, Ca++, Fe++, Cl-, and HS- with a = 10⁻¹⁰. f) Same for HS- with a = 10⁻⁵. (Color figure can be viewed at wileyonlinelibrary.com.)

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Fig. 2. Evolution of the secondary mineralogy for simulation of a reducing environment obtained with the react software component of the Geochemist's Workbench. T = 15 °C, W/R = 10, log pH₂ = -3, NH₃ = 5 wt%, CO₂ = 2 wt%. (Color figure can be viewed at wileyonlinelibrary.com.)

## Carbonates

Simulations show that carbonates and ammoniated clays can both occur under similar conditions (e.g., Fig. 1). Carbonate is not necessarily produced as a consequence of freezing or evaporation, as initially suggested by Neveu and Desch (2015). Instead, two types of carbonates are found. Those dominated by alkaline earths (like calcite, magnesite, or dolomite) have low solubilities at the conditions of interest and precipitate with the altered rock. The other type is in solution and forms the precursor of sodium bicarbonate. Hence, the two forms of carbonates observed by the VIR team, i.e., low-solubility Mg/Ca observed across the surface (De Sanctis et al. 2015) and Na-carbonate detected at the Occator bright spots (De Sanctis et al. 2016), can originate under the same environmental conditions (the Na-bearing species precipitating later from fluids as detailed below). The affinity of ammonium for chlorine may drive the production of NaHCO₃ from a solution of ammonium,

bicarbonate, sodium, and chloride, in a similar way to that used in the Solvay process that is used for the industrial production of sodium carbonates. Zolotov (2017) showed that the concentration of Na-carbonates found in the faculae could form in the melt generated by the impact that formed the crater.

## Ammoniated Minerals

The dearth of direct measurements of the thermodynamic properties of ammonium minerals (Watkins 1981; Neveu et al. 2017) prevents us from explicitly including them in our GWB model. However, our PHREEQC simulations do account for the thermodynamic properties of two NH₄-bearing species (NH₄-feldspar or buddingtonite, and NH₄-muscovite or tobelite). These simulations produce ammonium minerals (see Fig. 4), resulting from exchange between NH₄⁺ and other cations (mostly K⁺; see below). This result must nonetheless be considered cautiously, given that (1) these thermodynamic properties were not derived from actual measurements, but rather extrapolated from those of the corresponding K-minerals (Watkins 1981); and (2) while these extrapolated thermodynamic data are available only for buddingtonite and tobelite, these minerals only approximate the NH₄-species reported on Ceres's surface: NH₄-montmorillonite and/or NH₄-annite (De Sanctis et al. 2015, 2016).

## Fate of Key Elements and Composition of the Remaining Solution

### Fate of Iron

Most of the iron is in the form of magnetite and sulfides. Very little iron is left in solution (<10⁻⁸ mol kg⁻¹). Magnetite requires a mix of Fe²⁺ and Fe³⁺, i.e., a moderately oxidizing formation environment. Magnetite gets its oxygen from water, which leads to the release of hydrogen (H₂). As discussed in detail below, this hydrogen may combine
with CO₂ to form methane, which in turn acts on the pH₂ and shifts it toward lower values.

The production of methane in reactions that generated CI and CM chondrite mineralogies has been predicted from theoretical modeling (Rosenberg et al. 2001) and inferred from isotopic analysis (Guo and Eiler 2007) and is likely to be a common phenomenon, as predicted for icy satellites (e.g., Glein et al. 2008, 2009). In our geochemical models, magnetite combined with iron and nickel sulfides represents ~20 vol% (~38 wt%).

## Sulfur

The reaction of sulfur with iron and nickel is total and consistent across the whole set of simulations. Hence, there is very little sulfur left in solution and SO₄²⁻ is not expressed. The absence of sulfates contrasts with the results of Neveu and Desch (2015), but the latter have been found to be affected by mis-specification for a few sulfur solutes in the lnl.dat database used with PHREEQC (Neveu et al. 2017). The results obtained by GWB and PHREEQC presented in this paper are now fully consistent. Using the Hubble Space Telescope (HST), Hendrix et al. (2016) explained Ceres's spectrum in the UV by the presence of sulfur and sulfur dioxide (SO₂). However, additional observations by Roth (2018), also with HST, discarded the occurrence of those compounds. Neither elemental sulfur nor SO₂ are found in our simulations.

An important corollary to the chemical evolution leading to the observed material is that a significant fraction of salts is produced from leaching the original rock. The species dominant in solution are presented in Fig. 3 for a case consistent with the observed mineralogy. The salinity of the solution is moderate, at about 50 g of salts per kilogram of water (5 wt%), which is close to the salinity of terrestrial seawater (3.5 wt%). The simulations of Fig. 3 show that the composition of the brine is rapidly buffered to final values.

## Chlorine

Chlorine is the dominant anion in solution because it is not reactive like most of the other species. Instead, it remains entirely in the fluid phase. The conditions are too reducing for chlorine to form chlorates and perchlorates. The latter carry a signature in the near-IR but have not been detected by Dawn's VIR instrument so far. On the other hand, the detection of ammonium, not associated with clays, in combination with sodium carbonates in Occator's central bright spot, may point to the presence of chlorides (De Sanctis et al. 2016; Raponi et al. 2018).

## Nitrogen

In solution, nitrogen is distributed between ammonia, ammonium, and N₂, whose partial pressure is of the order of log pN₂ = +1–2 depending on the amount of CO₂ in the system.

## Alkali Metals and Alkaline Earths

The composition of the aqueous solution originally in equilibrium with Ceres's rocky component is enriched in alkalis and alkaline earth cations, in particular Na, K, and Ca. Potassium is the most soluble, and its extraction from the rock is estimated to be early and almost total, which has interesting implications for thermal evolution (Kirk and Stevenson 1987; Castillo-Rogez and Lunine 2010), as discussed in the Implications and Predictions section. A small fraction of K⁺ remains in the clays, although it may eventually end up in solution through exchange with NH₄⁺. This process, which also applies to sodium and calcium, has been extensively studied on Earth and is found to occur over a very broad range of conditions that encompass those expected on Ceres (Krishnamoorthy and Overstreet 1949). Again, we do not explicitly model that exchange with GWB. However, our PHREEQC simulations suggest that the K-NH₄ exchange appears to be temperature-limited (Neveu et al. 2017) such that ammonium is not substituted at temperatures >50 °C (see Fig. 4).

Under this premise, we simulate the exchange in GWB by decreasing the amount of ammonium in solution commensurately with the amount of exchangeable cations in the clays. The fraction of these alkali and alkaline earth cations in solution is increased accordingly, but it is not possible to predict the extent of the exchange. The fate of these species upon freezing of the solution is described in the Freezing of the Ocean section.

## Carbon Species (Besides Carbonates)

Not all the aqueous oxidized carbon is consumed in the precipitation of carbonates, unless there is very little to begin with (<0.5 wt%). A large fraction could combine with the hydrogen released from serpentinization to form methane, leading to the formation of clathrate hydrates, as explored in the Freezing of the Ocean section. Methane formation via this process moderates the pH and redox, and is key to obtaining the observed mineralogy. Incidentally, the occurrence of CH₄ has been inferred from isotope measurements in carbonaceous chondrite parent bodies (Guo and Eiler 2007). More generally, it has been commonly assumed that CH₄ can occur in serpentinizing environments across the solar system as the consequence of H₂ release (Holm et al. 2015).
<figure><img src="image_12.png" /><figcaption>Fig. 3. Evolution of major elements in solution for the example shown in Fig. 2. (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

However, the kinetics of the conversion of CO₂ into CH₄ are slow. At temperatures of only a few tens of degrees Celsius, such as those that seem to have prevailed inside Ceres (Travis et al. 2018), methane formation might not have been favorable. On the other

hand, Glein (2015) noted the CO₂ conversion into CH₄ may be helped by the presence of catalysts, such as nickel. Hence, it is possible that methane could be present, as suggested by Guo and Eiler (2007), but also likely that CO₂ would remain a dominant species.
<figure><img src="image_13.png" /></figure>

* Ionic strength < 0.1, theoretical upper limit of validity of Debye-Hückel model for solutes

* 0.1 < Ionic strength < 1, within range where model reproduces experimental results

<caption>Fig. 4. Survey of the temperature, pressure, and water-to-rock ratio parametric space for a CM composition, using PHREEQ (see Neveu et al. [2017] for methodology). Assumes 5 wt% NH₃, 2 wt% CO₂, 0.5 mol% S, 520 ppm chlorine to match bulk CI chlorine abundances. (Color figure can be viewed at wileyonlinelibrary.com.)</caption>

We do not focus here on the fate of other carbon compounds. This topic is addressed by Neveu et al. (2017). Results from their modeling, presented in Fig. 4, indicate the formation of a large fraction of graphite upon hydrothermal alteration. This combined with surficial weathering from irradiation may explain why organic absorption bands have proved elusive in the VIR observations so far. Instead Hendrix et al. (2016) suggested that features in Hubble's ultraviolet observations of Ceres may be associated with degraded forms of organics, such as amorphous carbon, space weathered as a result of UV exposure. Such material was proposed as a potential contributor to the “dark-material” inferred by VIR, in addition to magnetite and sulfides (De Sanctis et al. 2015).

## Minor Species

Our simulations also predict a number of species in amounts too small to be detectable by VIR, such as

nickel sulfides, manganese sulfide (alabandite) or carbonate (rhodochrosite), and hydroxyapatite. Simulations with PHREEQC show that uranium and thorium compounds are also found as part of the insoluble product of alteration (see Neveu et al. 2017).

## Dependence on Environmental Parameters

The dependence of the mineralogical assemblage produced from aqueous alteration on input parameters is illustrated in Figs. 4–6. The key parameters determining alteration products are (1) water-to-rock ratio, (2) partial pressure of H₂, and (3) relative abundances of CO₂ and NH₃ in the system.

The mineralogy produced in GWB as a function of the W/R is presented in Fig. 5 for a log pH₂ set to -3, T = 15 °C, P = 1 bar, and two assumptions concerning the amounts of NH₃ and CO₂ assumed in the system. Figure 5a uses 5 wt% CO₂ and 2 wt% NH₃ as an
<figure><img src="image_14.png" /><figcaption>Fig. 5. Evolution of the mineralogy formed under hydrothermal alteration for a CI composition. T = 15 °C, log pH₂ = -3. Top: accreted CO₂ = 5 wt%, NH₃ = 2 wt%; Bottom: CO₂ = 2 wt%, NH₃ = 5 wt%. (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

example of a relatively carbon-rich system. That figure shows the fate of the silicate phases starting from a mix of Mg-serpentine and cronstedtite (Fe-serpentine) to a mix of Mg-serpentine and clays, which then becomes unstable for W/R > 10 being replaced by quartz. For W/R < 1, the alteration is incomplete, e.g., silicates are only partially altered, which is consistent with observations of CM chondrites (e.g., Howard et al. 2015). However, the absence or small amounts of carbonates formed under these conditions is not consistent with the VIR and mid-IR (SOFIA) observations (De Sanctis et al. 2015; Vernazza et al. 2017). The form taken by iron evolves from a mix of magnetite and troilite (iron-rich endmember of pyrrhotite) to an intermediate pyrrhotite combined with siderite for W/R > 5. While siderite has been suggested in the possible fits of the global surface composition by De Sanctis et al. (2015), recent analyses discard this possibility (Palomba, personal communication).

Furthermore, the high W/R model leads to the breakdown of the phyllosilicate into an assemblage of quartz and multiple types of carbonates, in particular siderite (FeCO₃) accompanied by a shift in pH to <8. However, the main discrepancy with the VIR results is the predominance of saponite clays over serpentine, almost by a factor 2, for W/R > 1. In other words, this particular (C-rich) scenario, typical of all scenarios with more than 3 wt% CO₂, may reproduce the observed mineralogy only for a very narrow range of W/R.

The case shown in Fig. 5b uses 2 wt% CO₂ and 5 wt% NH₃. The mineralogical outcome is similar to the previous case for W/R < 1, and the presence of cronstedtite and small abundance of carbonates are rather consistent with the mineralogy of CM chondrites. However, the mineral assemblage shows different trends at higher W/R: Mg-serpentine is found in greater abundance than saponite clays. Carbonates are less abundant than in the previous case, but still present at a level of ~5 vol%, consistent with the VIR observations, for W/R ≥ 2. Magnesite becomes the dominant form of carbonate for W/R ≥ 1. Overall the average mineralogical composition observed by VIR is reproduced for a broad range of W/R (except for the dark component). This difference in behavior is ascribed to the role of NH₃ in keeping the pH high, around 11 over a wide range of W/R = 1–100. This W/R range is consistent with inference of iron fractionation from the GRAND observations (Prettyman et al. 2017).

The dependence of alteration reactions on the abundances of NH₃ and CO₂ is further illustrated in Fig. 6, all other parameters being held constant (P = 1 bar, T = 15 °C, log pH₂ = -3, W/R = 1). Carbon dioxide acts to decrease pH₂ by removing H₂ from the system and driving the redox toward conditions where ammonia is oxidized to N₂ gas. This in turn decreases the amount of NH₄⁺ in solution. The lower pH₂ and lower pH lead to saponite clays dominating over serpentine. Those effects are moderated by the presence of NH₃ that acts to increase the pH. The relative abundance of serpentine over the mixed clays matches the VIR observations for ~<1 wt% CO₂ and ~>2 wt% NH₃. These bounds approximately apply over a broad range of W/R. The exact concentrations in NH₃ and CO₂ also impact the composition of the ocean and determine in particular the amount of NaHCO₃ in solution. Hence, it may be possible to infer further constraints on the accreted amounts of NH₃ and CO₂ from a more thorough assessment of the material exposed on Ceres's surface.

Although Fig. 6 shows results for initial ammonia abundances of up to 10 wt%, at abundances approaching this upper value, the ionic strength of the solution becomes greater than 0.1, the upper limit of
<figure><img src="image_15.png" /><figcaption>Fig. 6. Dependence of hydrothermal mineralogy (insoluble component) as a function of the fraction of NH₃ (top plot) or of CO₂ (bottom plot) accreted by Ceres, expressed in percentage of volatiles accreted (mostly H₂O). Both series assume T = 15 °C, log pH₂ = -5, and W/R = 1. (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

validity of the extended Debye–Hückel (“B-dot”) model for solutes. Above this limit, the models, which are based on solute properties referenced at infinite dilution, become unreliable (Anderson 2005).

Decreasing log pH₂ below -3 is equivalent to increasing the abundance of CO₂, i.e., the predominance of Mg-serpentine over saponite is found only over a narrow range of W/R, as shown in Fig. 5a. A lower bound log pH₂ = -5 is found for ~1 wt% CO₂.

The mineralogical assemblage shows limited dependence on temperature: the overall composition does not change, but the relative abundances of the minerals vary. In particular, clay minerals such as smectites (saponites) tend to dominate at increasing

temperature at the expense of serpentine. This suggests Ceres's surface mineralogy formed at moderate temperature conditions.

The mineralogy is similar between the CI composition here and the CM composition assumed in Neveu et al. (2017) and illustrated in Fig. 4. The produced mineralogy shows a weak dependence on pressure over the range of interest to Ceres. The highest pressure and temperature conditions in Ceres are slightly below the low-grade metamorphic regime.

## FREEZING OF THE OCEAN

We now model the composition produced upon freezing of the ocean, which is encompassed in part in Ceres's crust (Fu et al. 2017). The starting liquid composition is the residual liquid in equilibrium with the mineralogy described in the Early Hydrothermal Chemistry Inside Ceres section and illustrated in Fig. 3. Pressure is expected to drive the fate of the gas species in solution, especially methane (e.g., Marion et al. 2012). An example of FREZCHEM code output is presented in Fig. 7 (see caption for a description). Freezing of the ocean is expected to develop in two stages, illustrated in Fig. 8 for two pressures: 1 MPa (Fig. 8a) and 10 MPa (Fig. 8b). At low pressure, about 90% of the water in the original ocean freezes as pure water with potentially a few ppm of Al(OH)₃ at the grain boundaries and triple junctions. Then NaHCO₃ starts precipitating at 266 K (for a pressure of 1 bar). Upon freezing, the pH of the ocean decreases from ~9.5 to ~7, which is ideal for the precipitation of NaHCO₃. Then the eutectic for several chlorides is met between 247 and 240 K, with little dependence on pressure. Hydrohalite (NaCl·2H₂O) represents about 65% of the volume of salt precipitated at that time. That hydrate is the typical form of sodium in cold solutions (Marion and Kargel 2007) and is commonly found on Earth (e.g., Light et al. 2009). Depending on the relative amounts of carbonate ions and chloride still available, the ammonium may combine with the bicarbonate ion (NH₄HCO₃) or form ammonium chloride (NH₄Cl) (see also Vu et al. 2017). If less HCO₃⁻ is present in solution, then chlorine is shared among ammonium, sodium, and potassium. Altogether these species represent <30 vol% of the material at the beginning of the freezing sequence. So far, ammonium chloride has been found at the top of Occator's Cerealia Facula (Raponi et al. 2018) and Dantu Crater (Stephan et al. 2018). Ammonium carbonate has not yet been found, possibly because it represents too small a fraction of the salts that condense upon freezing of the ocean. Based on the experimental work from Vu et al. (2017), the preferential association of ammonium with chlorides implies the source of the Occator faculae is very rich in
<figure><img src="image_16.png" /><figcaption>Fig. 7. Example of output from the FREZCHEM numerical code. This modeling tracks the products of freezing of an ocean with the composition presented in Fig. 3, for a pressure or 1 bar and assuming fractional crystallization. This representation shows the accumulated abundance of each species as temperature decreases. The starting point for each curve indicates the evolving eutectic temperature of the aqueous system. For example, the majority of water freezes early on while calcite (CaCO₃), dolomite (CaMg[CO₃]₂), and gibbsite (Al[OH]₃) precipitate; followed by the precipitation of magnesite (MgCO₃) and sodium bicarbonate (NaHCO₃), all at temperatures above 250 K. Around 245 K, a number of salt species precipitate, which include hydrohalite (NaCl.2H₂O), ammonium chloride (NH₄Cl), and potassium chloride (KCl). At colder temperature, the remaining water is used to form clathrate hydrate (CH₄.6H₂O). In this particular simulation, the remaining ammonium combines with the bicarbonate ion. The abundance is small so that compound could be present but not detectable. NaCl is the last compound to precipitate, starting at ~220 K. (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

chlorine, ammonium, or both. Thomas et al. (2018) further pointed out a different mineralogical outcome depending on the freezing time scale. In flash-freezing conditions, ammonium chlorides formed preferentially over hydrohalite, whereas Vu et al. (2017) found that hydrohalite is a dominant component of a slow freezing brine. Hence, the relative fractions of these various compounds can inform on the freezing rate of Ceres's crust.

Sulfates do not form in the runs performed in this study. Bu et al. (2018) suggest sulfates cannot be ruled out based on available data. So, if their presence is confirmed on Ceres's surface, then a different set of conditions should be considered. It is also possible sulfates could also be formed from the oxidation of FeS (e.g., Wolfe 2011) under local, oxidizing hydrothermal conditions, for example, by impact-induced heating.

Clathrate hydrates are also a product of freezing due to the presence of carbon dioxide and methane in solution. At low pressure, these clathrates form toward the end of the freezing sequence. Depending on the partial pressures of CO₂ and CH₄, a large fraction of methane could form at warm temperature (starting at 250 K) and become the main form taken by water. However, clathrates are also known to be unstable in the

presence of salts since both species compete for water (e.g., Elwood Madden et al. 2007; Safi et al. 2017). This process was not explored as part of this study.

At a pressure of 10 MPa and higher, clathrates formed preferentially over ice (Fig. 8b). Clathrate hydrates represent an important component of Earth permafrost and are potentially abundant in Mars's ground and Titan's shell (e.g., Chastain and Chevrier 2007; see Mousis et al. 2015) for a review). Hence, their occurrence in Ceres's crust is expected and consistent with the mechanical and density properties inferred by Ermakov et al. (2017) and Fu et al. (2017). However, it is difficult to directly prove the occurrence of clathrates because they are not stable in vacuum at Ceres's surface temperatures (Choukroun et al. 2013) and, assuming very recent exposure, their spectral signature is similar to that of water ice.

The most soluble species KCl, NaCl, and NH₃ accumulate in the liquid and eventually form supersaturated brines at the base of the shell. The eutectic temperature of this mixture is significantly depressed below 200 K, owing to NH₃. However, the exchange of cations with NH₄⁺ in the clays could potentially lead to the full consumption of NH₃ depending on the original abundance of NH₃.
<figure><img src="image_17.png" /><figcaption>Fig. 8. Freezing evolution of Ceres’s early ocean modeled with FREZCHEM and assuming a starting fluid composition in equilibrium with the mineralogy observed by De Sanctis et al. (2015). Top: equilibrium volume fractions of compounds frozen as temperature decreases, starting from 273 K for a pressure of 1 MPa; Bottom: same, for a pressure of 10 MPa. The freezing sequence does not necessarily reflect the final stratification of the ocean as these species significantly differ in densities and Ceres’s early ocean might have been convecting. (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

## IMPLICATIONS AND PREDICTIONS

### Relatively Reduced Mineralogy

The average mineralogy observed at Ceres is consistent with a carbonaceous chondrite precursor and typical of such a protolith altered in an aqueous environment with a water-to-rock ratio greater than 1, in the presence of ammonia and CO₂ under high partial pressure of hydrogen, in a mostly closed environment. The high pH₂ required for the formation of serpentine and ammoniated clays is fully consistent with previous suggestions that large ice-rich bodies could maintain high partial pressures of hydrogen, driving a relatively reduced mineralogy (McKinnon and Zolensky 2003;

Zolotov 2007). The redox/pH conditions are regulated by the presence of CO₂ and NH₃ that determine the relative abundances of serpentine and other clays; the fraction of carbon species in solid form and in solution; and the fate of sulfur in the form of iron, nickel, and manganese sulfides.

These observations and analyses raise the question of the origin of ammoniated silicates in Ceres versus carbonaceous chondrites. While ammonium salts have been identified in CI and CM chondrites (see Gounelle and Zolensky [2014] and references therein; Peltzer et al. 1984), ammoniated clays have proved elusive not only in chondrites but also in interplanetary dust. Yang et al. (1993) made a detailed search for ammoniated silicates in the Kaidun meteorite that bears ammonium salts, but
<figure><img src="image_18.png" /><figcaption>Fig. 9. Grain density of the rocky material as a function of water-to-rock ratio formed upon hydrothermal alteration (insoluble phase) for the two cases presented in Fig. 5. (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

were unsuccessful. Similarly, ammonium salts were reported in the Orgueil meteorite (Gounelle and Zolensky 2014) but not in the clays. That material is fragile and may get destroyed in sample processing and analysis if not specifically searched for (Brownlee and Alexander, personal communications). Alternatively, the absence of ammoniated clay may suggest that the CI/CM parent body environment presented a kinetic barrier to the exchange of ammonium with Na, Ca, or K or that pH₂ was too low for ammonium to be the dominant form of nitrogen. This open question could be resolved by searching specifically for ammonium in clay interlayers in future analyses.

## Rock Grain Density

Estimating the (insoluble) rock grain density is fraught with uncertainties tied to a broad range of parameters. However, it is possible to at least place bounds on this property in order to support future geophysical studies. The grain density depends on the form of the main silicate—in particular serpentine versus other clays—and the host minerals for iron. Indeed, the density of relevant iron minerals varies by almost a factor of two between sulfides and oxides. The density of the rock is low at ~2.3 g cm⁻³ for a water: rock ratio lower than 1, which is consistent with the density found for CI chondrites (Consolmagno et al. 2008). The upper bound depends on the composition of the volatile. For a relatively high abundance of CO₂ over NH₃ (Fig. 9, red curve), the density peaks at 3.4 g cm⁻³ for W/R = 2 and decreases toward

3.2 g cm⁻³ for large W/R values. For our preferred model shown in Fig. 5b, a water:rock ratio greater than 2 translates into a rock grain density range between 2.49 and 2.62 g cm⁻³ (Fig. 9). This density range lies slightly below the mean mantle density inferred from the Dawn observations (Ermakov et al. 2017; Mitri et al. submitted), and suggests the presence of a small porosity fraction (<5%).

It is unclear, however, whether that density estimate is representative of Ceres's rocky mantle or should apply only to the lag deposit observed by Dawn. First, Ceres's surface elemental composition shows evidence for iron fractionation (Prettyman et al. 2017), suggesting an increase in iron content with depth. Also, large particles concentrating at depth during the early differentiation phase (Neveu and Desch 2015) could be subject to limited hydrothermal circulation depending on permeability. On the other hand, mud remaining in suspension in a convective ocean could undergo increased aqueous alteration, although Fig. 5b shows that under a relatively high partial pressure of hydrogen, the mineralogy remains similar for a broad range of water-to-rock ratios. The pH conditions may not have remained constant throughout the body and certain pH buffering agents, like ammonia, may have been heterogeneously distributed. The analysis in this paper also does not take into account the removal of methane from the medium in the form of clathrate hydrates, which could be more efficient at greater pressures. Hence, it is not possible to conclude whether the mineralogy observed on the surface is representative of the deep interior.
## Density of the Frozen Ocean and Remaining Brines

The density of the frozen ocean is estimated from the FREZCHEM runs for the two pressures used in Fig. 8. For clathrates, we assume a density of 1.050 g cm⁻³, which is intermediate between CH₄ and CO₂ clathrates (e.g., Waite et al. 2009; Uchida et al. 1999; Safi et al. 2017). The density of the frozen mixtures of water ice, chlorides, and bicarbonates at low pressure is ~1.175 g cm⁻³ and the higher pressure mixture dominated by clathrate hydrate has a density of ~1.07 g cm⁻³. Since the density inferred for Ceres's crust is ~1.28 g cm⁻³ (Ermakov et al. 2017), the crust requires an additional dense phase, which is most likely rocky material. This corresponds to at least 15 vol% (25 wt%), taking the preferred range of rock density presented in Fig. 9, consistent with the estimates from Ermakov et al. (2017). Averaged over the crust, salt species represent about 10 vol% of the non-ice component of the frozen ocean based on cosmochemical abundances.

Fine rocky material is expected to concentrate with the residual liquid supersaturating in Na⁺, K⁺, and Cl⁻, as well as NH₃. The eutectic of that layer is depressed toward 200 K, which implies that it may never freeze over Ceres's lifetime, at least regionally (Castillo-Rogez and McCord 2010; Neveu and Desch 2015). The density of the residual liquid starts at ~1.05 g cm⁻³ and increases toward 1.18–1.19 g cm⁻³ at 250 K as water ice is removed from the system, but before chlorides start crystallizing. Then the liquid becomes dominated by ammonia and some NaCl and KCl, with a density around 1.04 g cm⁻³, owing to the low density of ammonia.

Methane clathrates are 10% less dense than liquid water, while the densities of bicarbonates and chloride compounds are 50% to 100% denser. Sodium bicarbonate is the densest material (~2.2 g cm⁻³) in the sequence and as such most of it is expected to sink, except for the smallest grains that may remain in suspension, especially if the liquid is convecting. However, the dehydrated form of that compound (Na₂CO₃) is found in abundance on Ceres's surface, for example, in Occator and Ahuna Mons (Zambon et al. 2016), even though it is among the least abundant species produced in the modeling. It is possible that this material is more stable at low pressure, near the freezing front and got embedded in the ice, that it was brought to the surface via endogenic processes, or that it resulted from space weathering of hydrated precursors. However, the abundance of that material found in the Occator deposits requires an exceptional concentration mechanism, like the impact-induced melt reservoir suggested by Zolotov (2017).

## Constraints on Ceres's Geophysical Evolution

The observed mineralogy is consistent with the scenario explored here that Ceres accreted a mixture of ice and rock whose melting triggered pervasive water-rock interaction. The mineralogy requires modest temperatures, similar to those inferred from oxygen isotope thermometry in CI chondrites (Keil 2000). The water-to-rock ratio points to a water abundance consistent with the observed bulk density of Ceres (McCord and Sotin 2005) and hydrothermal circulation, as predicted by Young et al. (2003). It is likely that an episode of hydrothermal activity was triggered by short-lived radioisotope decay, rapidly yielding abundant liquid water, as is thought to have occurred in carbonaceous chondrite parent bodies (e.g., Grimm and McSween 1993). Considering the many similarities with carbonaceous chondrites (McSween et al. 2018), it makes sense to assume that Ceres formed contemporaneously with the former parent bodies, i.e., <4 Myr after the formation of calcium-aluminum inclusions (Fujiya et al. 2012; Jogo et al. 2017) taken as time reference for the decay of ²⁶Al. Other observations, for example geophysical, may help confirm this hypothesis.

The geochemical clues to Ceres's evolution are generally consistent with the geophysical and geochemical scenario discussed by Castillo-Rogez and McCord (2010), Neveu and Desch (2015), and Travis et al. (2018). All these scenarios involved Ceres forming within a few Myr of CAIs such that ²⁶Al decay heat would melt water in much of Ceres's interior. Much of the chemical reactions discussed here could therefore have taken place early on. Contrary to the simplified model by Castillo-Rogez and McCord (2010), modeling of physical differentiation under Ceres's relatively low gravity suggests fine rock grains akin to chondritic matrix would remain suspended for an extended period of time (Neveu and Desch 2015; Bland and Travis 2017), enabling high water:rock ratios. Geochemical models updated from those described by Neveu and Desch (2015) by improving the treatment of sulfur species yield mineralogy consistent with Dawn observations (Fig. 4). Moreover, the composition of fluid in equilibrium with this mineralogy is qualitatively similar with that observed at Occator faculae and Ahuna Mons. This link between ocean chemistry and recently emplaced material is strengthened by our simulations of fluid chemistry changes driven by freezing (Figs. 7 and 8) (see also Quick et al. 2018). However, neglecting further chemical changes by processes such as space weathering, for example, dehydration of hydrated salts, which were not accounted for either by Neveu and Desch (2015) or in the present study, prevent the establishment of a fully
quantitative link between bright surface material and fluid chemistry at this time. In addition, the geochemical models of Neveu and Desch (2015) neglected key salts and clathrates (such as Na-carbonates and gas hydrates) and their geophysical effects, which are thought to be key to explaining Ceres's strong, yet low-density outer layer (Ermakov et al. 2017; Fu et al. 2017).

A different pre-Dawn scenario for the origin of Ceres's surface material assumed that Ceres accreted as processed chondritic material as a whole and devoid of ice (Zolotov 2009; Neumann et al. 2015) or as a consequence of impact-induced hydrothermal processing (Zolotov 2014). These studies promote a highly oxidizing mineralogy that is not only inconsistent with VIR observations of Ceres but is also in discrepancy with the state of knowledge of CI and CM chondrites.

The implications of ammonium exchange with potassium were first suggested by Kirk and Stevenson (1987) and investigated by Engel et al. (1994) who showed that this process could efficiently displace $ ^{40}K $ from the rock phase into Titan's ocean. This idea was explored further by Castillo-Rogez and Lunine (2010), who demonstrated that the loss of K from Titan's rocky mantle could significantly impact its thermal evolution and delay its dehydration. Potassium in Ceres appears to be partitioned between three sources: saponite, solid KCl, and KCl accumulating in a supersaturated solution at the base of the volatile layer. The fractionation of potassium between these three reservoirs is a function of temperature and $W/R$ (see also Neveu et al. 2017), and thus its quantification in Ceres is model-dependent. However, the fraction of K that remains in the rock phase is <50%. This loss of a major heat source likely retarded temperature increase in the mantle, potentially preventing dehydration of the phyllosilicates. Hence, much warmer evolution models presented in Castillo-Rogez and McCord (2010) or Neveu et al. (2015) need to be revisited. Conversely, long-lived $ ^{40}K $ could have played an important role in maintaining warm temperatures in Ceres's shell and potentially promote endogenic activity as suggested by the formation of Ahuna Mons (Ruesch et al. 2016). Future thermal evolution models of Ceres should account for potassium displacement as well as salt and clathrate low thermal conductivities.

Last, we note that the range of pH₂ inferred for Ceres's surface composition is several orders of magnitude lower than the maximum pH₂ achievable in the Ceres, assuming pH₂ = $P_{\text{total}}$. If Ceres's interior were subject to protracted hydrothermal circulation, then one should not necessarily expect surface material to reflect low-pressure conditions. It is possible that part of the H₂ could be lost as a consequence of convection in an open system (e.g., exposure of the ocean via impacts, overturn

of the original crust). Hydrogen could also be removed from the system in the form of CH₄. Indeed, the simulations yield methane fugacities up to $log fCH_4 \sim 2$.

## Predictions of Ceres's Near-Surface Composition

The two types of carbonates detected at Ceres provide complementary information on Ceres's mineralogical composition. They suggest that the hydrothermal liquid in contact with the mineralogical assemblage observed on Ceres's surface was relatively alkaline and should contain an abundant fraction of chlorides and alkali/alkaline Earth metals. If chlorides are confirmed at Ceres, then they should occur in part as hydrohalite, as predicted by FREZCHEM and by analogy with Earth's cold oceans, and, to a lesser degree, as ammonium chloride; the latter has been suggested at Occator (De Sanctis et al. 2016). Recent experimental studies also confirm the coexistence of hydrohalite and ammonium chlorides (Thomas et al. 2018).

Depending on the amount of CO₂ originally accreted in Ceres, water may preferentially freeze in the form of methane clathrate hydrates, keeping the ice fraction low. This possibility has been independently suggested from thermo-mechanical modeling studies (Bland et al. 2016; Fu et al. 2017). Clathrates are fragile and may decompose upon impact and rapid exposure to vacuum (see Choukroun et al. [2013] for phase diagrams). This may produce outgassing features, such as pitted terrains. However, liquid water rapidly exposed to space would also leave pitted terrains (Sizemore et al. 2017).

The occurrence of sodium carbonate has been confirmed in multiple places on Ceres, generally in association with large impacts that can source deep material as well as Ahuna Mons (Zambon et al. 2016). Variability of Ceres's surface points to some layering at depth. It is unclear though whether the sodium carbonate comes from a shallow or deep source. Its abundance in the bright deposits at Occator Crater suggests it may be accessible in Ceres's shallow subsurface. Indeed, the density of that phase ($2.54 \text{ g cm}^{-3}$) is higher than the average density suggested for Ceres's outer shell (Ermakov et al. 2017). Hence, the concentration of that material on the surface requires an exceptional mechanism. A possibility is that it formed in a surficial impact melt reservoir (Bowling et al. 2016; see also Zolotov 2017). A possible alternative is that this material is sourced from a deeper, large-scale brine reservoir (Fu et al. 2017; Quick et al. 2018).

The spectral fits from De Sanctis et al. (2015) require at least 60% of darkening agent (Table 1) that is featureless in the visible-near-IR. Several processes can act together to decrease albedo: small regolith particle size (i.e., as inferred by GRaND; Prettyman et al. 2017)
and intimate mixing with graphite or amorphous carbon. The latter material is predicted to form either as a consequence of hydrothermal processing and/or from exogenic processing (e.g., charged particle bombardment or UV exposure) of organics, as suggested by recent spectroscopic observations in the UV (Hendrix et al. 2015, 2016). Also, in the presence of abundant water, ion irradiation can convert carbon to CO₂ rather than to organics (Hodyss, personal communication). It is not known if this process has been occurring on Ceres. Last, Kaplan et al. (2018) pointed out that organics could be in significant abundance in the regolith but not detectable with Dawn at the ~100 m pixel scale of the VIR observations.

## Contribution to the State of Knowledge of Icy Bodies

McKinnon and Zolensky (2003) showed that a high pH₂ could be reached in carbonaceous chondrite parent bodies and icy satellites, and the same applies to Ceres. The detection of sodium bicarbonate, both at Ceres and Enceladus, is consistent with the chemical evolution expected in relatively large ice-rich bodies (e.g., Zolotov 2007). As a corollary, the mineralogy observed at Ceres is a possible starting point (prior to potential metamorphism) for the rocky mantles of large icy satellites (Ransford et al. 1981). The condensation of ammonia hydrates is expected starting from the Saturn feeding zone and beyond (Hersant et al. 2008), and ammonium exchange with cations has been predicted as well, although the above modeling indicates that potassium is mobile in hydrothermal conditions independently from the presence of ammonium. The detections of sodium bicarbonate, halite, and sylvite in Enceladus's plume dust (Postberg et al. 2011) suggest Ceres falls in the same category of bodies hosting an alkaline ocean for part (or all) of their history. A similar composition has also been inferred for Titan (Brassé et al. 2017). The recent detection of chlorides on Europa (Brown and Hand 2013) suggests that that Cl⁻ may be the dominant salt anion over sulfate, and supports the idea that the presence of the latter on Europa's surface was supplied from Io (Hand and Carlson 2015). The confirmation of abundant chloride on Ceres from future observations could put important constraints on the nebula conditions in the outer solar system. Along the same lines, the absence of brucite on Ceres, which is attributed to a high pCO₂, may suggest that that mineral is also absent on Enceladus.

More generally, few models of the evolution of large icy satellites have properly tracked the compositional evolution of rock and the production of salts upon hydrothermal processing. Hence, the data

point offered by Ceres will further help understand that class of objects in general.

## SUMMARY

Ceres's surface mineralogy provides evidence for pervasive aqueous alteration, suggesting a global alteration event in its deep interior. Key results from this study are:

1. The mineralogy is characteristic of a reducing and alkaline environment. The simultaneous presence of ammonium-bearing clays and of carbonates constrains pH to lie in the range 7–11, and log pH₂ to be greater than about -5. We propose that hydrogen was produced from serpentinization and accumulated under high pressure in a (mostly) closed aqueous system.

2. The presence of ammoniated clays, in similar fractions to serpentine, constrain the temperature of even the early ocean to <50°C. This also suggests a water:rock ratio greater than 2.

3. The preferred model in this study, which matches the Dawn observations for a wide range of conditions, in particular water-to-rock ratio, assumes the accretion of a significant fraction of NH₃ (a few percent with respect to H₂O). This aspect needs to be explored in more detail because of the potential implications for constraining Ceres's origin.

4. Sulfur is expected to be in reduced form, combined with metals, while chlorides and carbonates are the main forms of salts.

5. A very large fraction of the water could be in the form of clathrate hydrates, which supports geophysical interpretations for the abundance of that material in Ceres's crust.

6. A small discrepancy between the rock grain density predicted by these models and the rocky mantle density inferred from the Dawn data points to the potential occurrence of a small fraction of porosity in the mantle.

The relatively reducing conditions, while conducive to the production of carbonates, are similar to those expected in large icy satellites (McKinnon and Zolensky 2003). Indeed, Ceres's salt chemistry resembles that of Enceladus (Postberg et al. 2011) and possibly Europa (Brown and Hand 2013). More generally, these results add to our knowledge of large volatile-rich bodies. The presence at multiple sites of frozen oceanic materials rich in sodium carbonate and ammonium chloride suggests that oceanic material is frozen in the near surface (first 10s of kilometers, and perhaps shallower), consistent with geophysical data (Ermakov et al. 2017; Fu et al. 2017). The occurrence of clathrates predicted in this study is consistent with reports of a mechanically strong yet low-density outer shell. Potential directions
for future work may include an assessment of the
fugacity of hydrogen expected as a function of pressure
and other conditions that might impact the mineralogy
formed at depth; the dependence of the chemical
evolution on the original form of sulfur (H₂S or SO₂)
(see preliminary work by Sekine et al. 2016) and
nitrogen (N₂ or NH₃). Moreover, this study assumed
the native form of the sodium carbonate in Ceres's crust
is sodium bicarbonate, which is consistent with the VIR
observations (Carrozzo et al. 2018). However, alternate
forms of carbonates with different degrees of hydration
may also explain these observations, considering
carbonates dehydrate on a short time scale (Bu et al.
2018). Last, since Ceres's crust encompasses the bulk of
Ceres's original ocean, refined studies of the mineralogy
at salt-rich sites, especially searching for species that
could not be detected by the Dawn mission, can provide
important constraints on the composition and
environmental conditions of that early ocean and the
conditions under which it evolved. As recent studies
have pointed out the role of salts in promoting diverse
organics production (e.g., Brassé et al. [2017] for Titan),
Ceres's oceanic material exposed in many sites offers an
opportunity to better comprehend environmental
conditions and their impact on organic chemistry inside
ocean worlds.

*Acknowledgments*—Part of this work was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract to NASA. All rights reserved. J. C. C. thanks Conel Alexander (DTM, Carnegie), Don Brownlee (U. Washington), Rob Hodyss, Mathieu Choukroun (JPL/Caltech), and Chris Glein (SwRI) for personal communications. The authors are also thankful to Steve Desch and an anonymous reviewer whose comments improved the quality of this manuscript.

*Editorial Handling*—Dr. Thomas McCord

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## Appendix. Geochemist's Workbench—Input and output files.

### Example Input File

Temperature = 15 °C

Reacting amount of liquid H₂O = .1 kg

Initial concentration of Cl⁻ = 680 mg kg⁻¹

log pH₂(gas) = -5

Initial amount of CO₂ in solution = 5 wt%

Initial amount of NH₃ in solution = 2 wt%

Initial amount of SiO₂ in solution = 26.82 g

## Appendix. Continued. Geochemist's Workbench—Input and output files.

### Example Input File

Initial amount of Al₂O₃ in solution = 1.98 g

Initial amount of MnO in solution = 0.25 g

Initial amount of FeO in solution = 28.3 g

Initial amount of NiO in solution = 1.48 g

Initial amount of MgO in solution = 18.78 g

Initial amount of CaO in solution = 1.48 g

Initial amount of Na₂O in solution = 0.87 g
Appendix. *Continued*. Geochemist's Workbench—Input and output files.

Example Input File

Initial amount of K₂O in solution = 0.09 g

Initial amount of P₂O₅ in solution = 0.37 g

Initial amount of H₂S in solution = 5 g

Output Excerpt (Final Step)

Step # 1004 Xi = 1.0000

Temperature = 15.0 °C Pressure = 1.013 bars

pH = 10.413 log fO₂ = -77.509

Eh = -0.4720 volts pe = -8.2555

Ionic strength = 0.135634

Activity of water = 0.999326

Solvent mass = 1.030491 kg

Solution mass = 1.063366 kg

Solution density = 1.034 g cm⁻³

Chlorinity = 0.019514 molal

Dissolved solids = 30916 mg kg⁻¹ sol'n

Hardness = 2.98 mg kg⁻¹ sol'n as CaCO₃

Carbonate = 2.98 mg kg⁻¹ sol'n as CaCO₃

Non-carbonate = 0.00 mg kg⁻¹ sol'n as CaCO₃

Rock mass = 0.091449 kg

Carbonate alkalinity = 10178.58 mg kg⁻¹ sol'n as CaCO₃

Water type = NH₄-CO₃

<table><thead><tr><th>Reactants</th><th>Moles remaining</th><th>Moles reacted</th><th>Grams reacted</th><th>cm³ reacted</th></tr></thead><tbody><tr><td>Al₂O₃</td><td>-3.226e-17</td><td>0.01942</td><td>1.980</td><td></td></tr><tr><td>CO₂(aq)</td><td>-4.243e-15</td><td>0.2382</td><td>10.48</td><td></td></tr><tr><td>CaO</td><td>-6.442e-17</td><td>0.02639</td><td>1.480</td><td></td></tr><tr><td>FeO</td><td>1.428e-15</td><td>0.3939</td><td>28.30</td><td></td></tr><tr><td>H₂S(aq)</td><td>-2.564e-15</td><td>0.1467</td><td>5.000</td><td></td></tr><tr><td>K₂O</td><td>1.680e-17</td><td>0.0009554</td><td>0.09000</td><td></td></tr><tr><td>MgO</td><td>-6.109e-15</td><td>0.4659</td><td>18.78</td><td></td></tr><tr><td>MnO</td><td>-7.326e-17</td><td>0.003524</td><td>0.2500</td><td></td></tr><tr><td>NH₃</td><td>5.727e-15</td><td>0.6156</td><td>10.48</td><td></td></tr><tr><td>Na₂O</td><td>-2.012e-16</td><td>0.01404</td><td>0.8700</td><td></td></tr><tr><td>NiO</td><td>-8.975e-17</td><td>0.01981</td><td>1.480</td><td>0.2173</td></tr><tr><td>P₂O₅</td><td>-6.188e-19</td><td>0.002607</td><td>0.3700</td><td></td></tr><tr><td>SiO₂</td><td>-3.429e-15</td><td>0.4464</td><td>26.82</td><td></td></tr></tbody></table>

<table><thead><tr><th>Minerals in system</th><th>Moles</th><th>log<br>moles</th><th>Grams</th><th>Volume<br>(cm³)</th></tr></thead><tbody><tr><td>Chrysotile</td><td>0.008015</td><td>-2.096</td><td>2.221</td><td>0.8696</td></tr><tr><td>Dolomite</td><td>0.02505</td><td>-1.601</td><td>4.620</td><td>1.612</td></tr><tr><td>Hydroxyapatite</td><td>0.0002679</td><td>-3.572</td><td>0.1346</td><td>0.04276</td></tr><tr><td>Magnesite</td><td>0.05946</td><td>-1.226</td><td>5.013</td><td>1.666</td></tr><tr><td>Magnetite</td><td>0.09121</td><td>-1.040</td><td>21.12</td><td>4.061</td></tr><tr><td>MnHPO₄(c)</td><td>0.003524</td><td>-2.453</td><td>0.5318</td><td></td></tr><tr><td>Ni₃S₄</td><td>0.006603</td><td>-2.180</td><td>2.010</td><td>0.4235</td></tr><tr><td>Saponite-K</td><td>0.005283</td><td>-2.277</td><td>2.070</td><td>0.7389</td></tr><tr><td>Saponite-Mg</td><td>0.03341</td><td>-1.476</td><td>12.79</td><td>4.465</td></tr></tbody></table>

Appendix. *Continued*. Geochemist's Workbench—Input and output files.

<table><thead><tr><th>Minerals in system</th><th>Moles</th><th>log<br>moles</th><th>Grams</th><th>Volume<br>(cm³)</th></tr></thead><tbody><tr><td>Saponite-Na</td><td>0.07857</td><td>-1.105</td><td>30.37</td><td>10.74</td></tr><tr><td>Troilite</td><td>0.1203</td><td>-0.920</td><td>10.57</td><td>2.189</td></tr><tr><td>(Total)</td><td>91.45</td><td></td><td>26.81*</td><td></td></tr></tbody></table>

<table><thead><tr><th>Aqueous<br>species</th><th>Molality</th><th>mg kg⁻¹<br>sol'n</th><th>Act.<br>coef.</th><th>log<br>act.</th></tr></thead><tbody><tr><td>N₂(aq)</td><td>0.6112</td><td>1.659e+04</td><td>1.0000</td><td>-0.2138</td></tr><tr><td>NH₃</td><td>0.4698</td><td>7753.</td><td>1.0000</td><td>-0.3281</td></tr><tr><td>NH₄⁺</td><td>0.1000</td><td>1748.</td><td>0.7288</td><td>-1.1374</td></tr><tr><td>CH₄(aq)</td><td>0.07686</td><td>1195.</td><td>1.0364</td><td>-1.0987</td></tr><tr><td>CO₃⁻</td><td>0.03259</td><td>1895.</td><td>0.3359</td><td>-1.9606</td></tr><tr><td>Cl⁻</td><td>0.01951</td><td>670.4</td><td>0.7394</td><td>-1.8408</td></tr><tr><td>HCO₃⁻</td><td>0.01527</td><td>902.8</td><td>0.7668</td><td>-1.9315</td></tr><tr><td>Na⁺</td><td>0.001960</td><td>43.66</td><td>0.7583</td><td>-2.8280</td></tr><tr><td>HPO₄²⁻</td><td>0.0008060</td><td>74.97</td><td>0.3213</td><td>-3.5868</td></tr><tr><td>K⁺</td><td>0.0001619</td><td>6.135</td><td>0.7394</td><td>-3.9219</td></tr><tr><td>OH⁻</td><td>0.0001570</td><td>2.588</td><td>0.7492</td><td>-3.9295</td></tr><tr><td>Al(OH)₄⁻</td><td>0.0001381</td><td>12.71</td><td>0.7583</td><td>-3.9801</td></tr><tr><td>NaCO₃⁻</td><td>8.885e-05</td><td>7.147</td><td>0.7583</td><td>-4.1715</td></tr><tr><td>HS⁻</td><td>4.138e-05</td><td>1.326</td><td>0.7492</td><td>-4.5086</td></tr><tr><td>PO₄³⁻</td><td>3.387e-05</td><td>3.117</td><td>0.0768</td><td>-5.5850</td></tr><tr><td>NaHCO₃</td><td>2.865e-05</td><td>2.332</td><td>1.0000</td><td>-4.5429</td></tr><tr><td>MgPO₄³⁻</td><td>1.402e-05</td><td>1.621</td><td>0.7583</td><td>-4.9733</td></tr><tr><td>MgCO₃</td><td>1.207e-05</td><td>0.9859</td><td>1.0000</td><td>-4.9184</td></tr><tr><td>NaHPO₄⁻</td><td>4.494e-06</td><td>0.5181</td><td>0.7583</td><td>-5.4675</td></tr><tr><td>Mg⁺</td><td>3.902e-06</td><td>0.09191</td><td>0.4254</td><td>-5.7799</td></tr><tr><td>H₃SiO₄⁻</td><td>2.956e-06</td><td>0.2724</td><td>0.7583</td><td>-5.6495</td></tr><tr><td>CH₃COO⁻</td><td>2.582e-06</td><td>0.1477</td><td>0.7668</td><td>-5.7033</td></tr><tr><td>CO₂(aq)</td><td>1.235e-06</td><td>0.05267</td><td>1.0000</td><td>-5.9083</td></tr><tr><td>SiO₂(aq)</td><td>7.403e-07</td><td>0.04310</td><td>1.0364</td><td>-6.1151</td></tr><tr><td>NaCl</td><td>4.298e-07</td><td>0.02434</td><td>1.0000</td><td>-6.3667</td></tr><tr><td>KHPO₄⁻</td><td>3.222e-07</td><td>0.04217</td><td>0.7583</td><td>-6.6121</td></tr><tr><td>MgHPO₄</td><td>2.980e-07</td><td>0.03473</td><td>1.0000</td><td>-6.5258</td></tr><tr><td>MgHCO₃⁺</td><td>2.533e-07</td><td>0.02095</td><td>0.7583</td><td>-6.7165</td></tr><tr><td>H₂PO₄⁻</td><td>2.286e-07</td><td>0.02149</td><td>0.7583</td><td>-6.7611</td></tr><tr><td>NaOH</td><td>1.095e-07</td><td>0.004245</td><td>1.0000</td><td>-6.9605</td></tr><tr><td>Mn(NH₃)₃⁺⁺</td><td>7.832e-08</td><td>0.008048</td><td>0.3213</td><td>-7.5992</td></tr><tr><td>CaCO₃</td><td>7.415e-08</td><td>0.007192</td><td>1.0000</td><td>-7.1299</td></tr><tr><td>MgCl⁺</td><td>5.218e-08</td><td>0.003022</td><td>0.7583</td><td>-7.4026</td></tr><tr><td>Mn(NH₃)₂⁺⁺</td><td>5.195e-08</td><td>0.004480</td><td>0.3213</td><td>-7.7776</td></tr><tr><td>NaH₃SiO₄</td><td>4.841e-08</td><td>0.005540</td><td>1.0000</td><td>-7.3151</td></tr><tr><td>CaPO₄⁻</td><td>4.088e-08</td><td>0.005351</td><td>0.7583</td><td>-7.5086</td></tr><tr><td>KCl</td><td>3.846e-08</td><td>0.002779</td><td>1.0000</td><td>-7.4150</td></tr><tr><td>MgOH⁺</td><td>3.621e-08</td><td>0.001450</td><td>0.7583</td><td>-7.5613</td></tr><tr><td>FeCO₃</td><td>2.357e-08</td><td>0.002647</td><td>1.0000</td><td>-7.6276</td></tr><tr><td>H₂(aq)</td><td>2.252e-08</td><td>4.400e-05</td><td>1.0364</td><td>-7.6318</td></tr><tr><td>H₂S(aq)</td><td>2.033e-08</td><td>0.0006713</td><td>1.0000</td><td>-7.6919</td></tr><tr><td>Ni(NH₃)₆⁺⁺</td><td>2.001e-08</td><td>0.003119</td><td>0.3501</td><td>-8.1546</td></tr><tr><td>MnCO₃</td><td>1.986e-08</td><td>0.002212</td><td>1.0000</td><td>-7.7020</td></tr><tr><td>FePO₄⁻</td><td>1.683e-08</td><td>0.002460</td><td>0.7583</td><td>-7.8941</td></tr><tr><td>MnNH₃⁺⁺</td><td>1.432e-08</td><td>0.0009990</td><td>0.3213</td><td>-8.3371</td></tr></tbody></table>
Appendix. *Continued*. Geochemist's Workbench—Input and output files.

<table><thead><tr><th>Aqueous species</th><th>Molality</th><th>mg kg<sup>-1</sup> sol'n</th><th>Act. coef.</th><th>log act.</th></tr></thead><tbody><tr><td>S-</td><td>1.410e-08</td><td>0.0004382</td><td>0.3501</td><td>-8.3065</td></tr><tr><td>Ca++</td><td>1.270e-08</td><td>0.0004934</td><td>0.3770</td><td>-8.3197</td></tr><tr><td>Al(OH)3<br>(only species >1e-8 molal listed)</td><td>1.034e-08</td><td>0.0007819</td><td>1.0000</td><td>-7.9853</td></tr></tbody></table>

Mineral saturation states

<table><thead><tr><th>log Q/K</th><th></th><th>log Q/K</th><th></th></tr></thead><tbody><tr><td>Antigorite</td><td>4.6613s/sat</td><td>Dawsonite</td><td>-0.9452</td></tr><tr><td>Pyrite</td><td>2.4909s/sat</td><td>Clinochl-7A</td><td>-1.0120</td></tr><tr><td>Clinochl-14A</td><td>2.4447s/sat</td><td>Muscovite</td><td>-1.0744</td></tr><tr><td>Phlogopite</td><td>2.0832s/sat</td><td>Siderite</td><td>-1.1091</td></tr><tr><td>Ripidolit-14A</td><td>1.6265s/sat</td><td>Boehmite</td><td>-1.4156</td></tr><tr><td>Amesite-14A</td><td>1.6040s/sat</td><td>Chamosite-7A</td><td>-1.4373</td></tr><tr><td>Smectite-Reykjan</td><td>0.6802s/sat</td><td>Saponite-H</td><td>-1.4746</td></tr><tr><td>Gibbsite</td><td>0.3633s/sat</td><td>NiS<sub>2</sub></td><td>-1.5356</td></tr><tr><td>Hydroxyapatite</td><td>0.0000 sat</td><td>Dolomite-dis</td><td>-1.6196</td></tr><tr><td>MnHPO<sub>4</sub>(c)</td><td>0.0000 sat</td><td>Calcite</td><td>-1.6918</td></tr><tr><td>Chrysotile</td><td>0.0000 sat</td><td>NiS</td><td>-1.6988</td></tr><tr><td>Saponite-K</td><td>0.0000 sat</td><td>Ripidolit-7A</td><td>-1.8329</td></tr><tr><td>Troilite</td><td>0.0000 sat</td><td>Talc</td><td>-1.8382</td></tr><tr><td>Ni<sub>3</sub>S<sub>4</sub></td><td>0.0000 sat</td><td>Aragonite</td><td>-1.8578</td></tr><tr><td>Dolomite</td><td>0.0000 sat</td><td>Whitlockite</td><td>-1.8796</td></tr><tr><td>Saponite-Na</td><td>0.0000 sat</td><td>Quartz</td><td>-1.9246</td></tr><tr><td>Magnetite</td><td>0.0000 sat</td><td>Brucite</td><td>-2.0762</td></tr><tr><td>Magnesite</td><td>0.0000 sat</td><td>Tridymite</td><td>-2.1000</td></tr><tr><td>Saponite-Mg</td><td>0.0000 sat</td><td>Fe(OH)2(ppd)</td><td>-2.1934</td></tr><tr><td>Dolomite-ord</td><td>-0.0000 sat</td><td>Chalcedony</td><td>-2.2054</td></tr><tr><td>Pyrrhotite</td><td>-0.0753</td><td>Greenalite</td><td>-2.2280</td></tr><tr><td>Daphnite-14A</td><td>-0.3613</td><td>Graphite</td><td>-2.3928</td></tr><tr><td>Saponite-Ca</td><td>-0.4194</td><td>Phengite</td><td>-2.4276</td></tr><tr><td>Hematite</td><td>-0.4223</td><td>Hercynite</td><td>-2.4406</td></tr><tr><td>Diaspore</td><td>-0.5175</td><td>Cristobalite</td><td>-2.4984</td></tr><tr><td>Annite</td><td>-0.6413</td><td>Kalsilite</td><td>-2.6008</td></tr><tr><td>Goethite</td><td>-0.6742</td><td>Monohydrocalcite</td><td>-2.6664</td></tr><tr><td>FeO(c)</td><td>-0.7291</td><td>Ferrosilite</td><td>-2.6904</td></tr><tr><td>Rhodochrosite</td><td>-0.7524</td><td>Kaolinite</td><td>-2.7028</td></tr><tr><td>Cronstedt-7A<br>(only minerals with log Q/K > -3 listed)</td><td>-0.9321</td><td>Artinite</td><td>-2.9852</td></tr></tbody></table><table><thead><tr><th>Gases</th><th>Fugacity</th><th>log fug.</th></tr></thead><tbody><tr><td>N<sub>2</sub>(g)</td><td>800.8</td><td>2.904</td></tr></tbody></table>

Appendix. *Continued*. Geochemist's Workbench—Input and output files.

<table><thead><tr><th>Gases</th><th>Fugacity</th><th>log fug.</th></tr></thead><tbody><tr><td>CH<sub>4</sub>(g)</td><td>45.06</td><td>1.654</td></tr><tr><td>Steam</td><td>0.01673</td><td>-1.777</td></tr><tr><td>H<sub>2</sub>(g)</td><td>2.796e-05</td><td>-4.553</td></tr><tr><td>CO<sub>2</sub>(g)</td><td>2.601e-05</td><td>-4.585</td></tr><tr><td>H<sub>2</sub>S(g)</td><td>1.166e-07</td><td>-6.933</td></tr><tr><td>S<sub>2</sub>(g)</td><td>3.943e-32</td><td>-31.404</td></tr><tr><td>O<sub>2</sub>(g)</td><td>3.098e-78</td><td>-77.509</td></tr></tbody></table>

<table><thead><tr><th rowspan="2">Original basis<br>total moles</th><th colspan="2">In fluid</th><th rowspan="2">Sorbed<br>moles</th><th colspan="2">Kd</th></tr><tr><th>moles</th><th>mg kg<sup>-1</sup></th><th>mg kg<sup>-1</sup></th><th>L kg<sup>-1</sup></th></tr></thead><tbody><tr><td>Al+++</td><td>0.0388</td><td></td><td>0.000142</td><td>3.61</td><td></td></tr><tr><td>Ca++</td><td>0.0264</td><td></td><td>1.34e-07</td><td>0.00505</td><td></td></tr><tr><td>Cl-</td><td>0.0201</td><td></td><td>0.0201</td><td>670.</td><td></td></tr><tr><td>Fe++</td><td>0.394</td><td></td><td>4.29e-08</td><td>0.00225</td><td></td></tr><tr><td>H+</td><td>0.444</td><td></td><td>1.99</td><td>1.89e+03</td><td></td></tr><tr><td>H<sub>2</sub>O</td><td>58.1</td><td></td><td>57.2</td><td>9.70e+05</td><td></td></tr><tr><td>HCO<sub>3</sub><sup>-</sup></td><td>0.238</td><td></td><td>0.129</td><td>7.38e+03</td><td></td></tr><tr><td>HPO<sub>4</sub><sup>-</sup></td><td>0.00521</td><td></td><td>0.000886</td><td>79.9</td><td></td></tr><tr><td>K+</td><td>0.00191</td><td></td><td>0.000167</td><td>6.15</td><td></td></tr><tr><td>Mg++</td><td>0.466</td><td></td><td>3.16e-05</td><td>0.722</td><td></td></tr><tr><td>Mn++</td><td>0.00352</td><td></td><td>1.74e-07</td><td>0.00901</td><td></td></tr><tr><td>NO<sub>3</sub><sup>-</sup></td><td>1.85</td><td></td><td>1.85</td><td>1.08e+05</td><td></td></tr><tr><td>Na+</td><td>0.0281</td><td></td><td>0.00215</td><td>46.4</td><td></td></tr><tr><td>Ni++</td><td>0.0198</td><td></td><td>2.06e-08</td><td>0.00114</td><td></td></tr><tr><td>O<sub>2</sub>(aq)</td><td>-3.15</td><td></td><td>-2.91</td><td>-8.75e+04</td><td></td></tr><tr><td>SO<sub>4</sub><sup>-</sup></td><td>0.147</td><td></td><td>4.27e-05</td><td>3.85</td><td></td></tr><tr><td>SiO<sub>2</sub>(aq)</td><td>0.446</td><td></td><td>3.88e-06</td><td>0.219</td><td></td></tr></tbody></table>

<table><thead><tr><th rowspan="2">Elemental<br>composition</th><th rowspan="2">Total<br>moles</th><th colspan="2">In fluid</th><th colspan="2">Sorbed</th></tr><tr><th>Moles</th><th>mg kg<sup>-1</sup></th><th>Moles</th><th>mg kg<sup>-1</sup></th></tr></thead><tbody><tr><td>Aluminum</td><td>0.03884</td><td>0.0001423</td><td></td><td>3.610</td><td></td></tr><tr><td>Calcium</td><td>0.02639</td><td>1.340e-07</td><td></td><td>0.005050</td><td></td></tr><tr><td>Carbon</td><td>0.2382</td><td>0.1287</td><td></td><td>1453.</td><td></td></tr><tr><td>Chlorine</td><td>0.02011</td><td>0.02011</td><td></td><td>670.4</td><td></td></tr><tr><td>Hydrogen</td><td>116.9</td><td>116.6</td><td></td><td>1.105e+05</td><td></td></tr><tr><td>Iron</td><td>0.3939</td><td>4.292e-08</td><td></td><td>0.002254</td><td></td></tr><tr><td>Magnesium</td><td>0.4659</td><td>3.157e-05</td><td></td><td>0.7217</td><td></td></tr><tr><td>Manganese</td><td>0.003524</td><td>1.744e-07</td><td></td><td>0.009009</td><td></td></tr><tr><td>Nickel</td><td>0.01981</td><td>2.064e-08</td><td></td><td>0.001140</td><td></td></tr><tr><td>Nitrogen</td><td>1.847</td><td>1.847</td><td></td><td>2.433e+04</td><td></td></tr><tr><td>Oxygen</td><td>59.54</td><td>57.35</td><td></td><td>8.629e+05</td><td></td></tr><tr><td>Phosphorus</td><td>0.005213</td><td>0.0008855</td><td></td><td>25.79</td><td></td></tr><tr><td>Potassium</td><td>0.001911</td><td>0.0001672</td><td></td><td>6.149</td><td></td></tr><tr><td>Silicon</td><td>0.4464</td><td>3.875e-06</td><td></td><td>0.1024</td><td></td></tr><tr><td>Sodium</td><td>0.02807</td><td>0.002146</td><td></td><td>46.39</td><td></td></tr><tr><td>Sulfur</td><td>0.1467</td><td>4.267e-05</td><td></td><td>1.287</td><td></td></tr></tbody></table>