How to Make an Alkaline Lake: Fifty Years of Chemical Divides

Of all the surface environments on our planet, alkaline lakes are among the most distinctive and significant in terms of their biogeochemistry, climatic sensitivity, and associated mineral deposits. But how does the Earth produce alkaline lakes? Fifty years ago, Lawrence Hardie and Hans Eugster hypothesised that the bewildering complexity of non-marine evaporites could be explained by common successions of mineral precipitation events, or chemical divides. Since that time, the chemical divide concept has provided Earth scientists with an enduring framework within which to integrate new advances in mineral–water equilibria and kinetics, sedimentology, and paleoclimatology. These developments are painting an increasingly detailed picture of how alkaline waters form and interact with magmatic and atmospheric CO2, now and in the distant past.</jats:p

Hydrothermal vent fluid-seawater mixing and the origins of Archean iron formation

Precambrian iron formation provides valuable windows onto ancient marine environments, but exactly how these Fe- and Si-rich chemical sedimentary rocks formed has remained a matter of debate. Traditional models envisage Fe(III)-oxide formation through chemical and/or biological oxidation of soluble Fe2+, but sedimentological and textural data show that, throughout much of Archean stratigraphy, the finest-grained Fe(III)-oxides, previously interpreted to reflect primary components, were largely derived through the oxidation of the Fe(II)-silicate mineral greenalite. These observations have formed the basis for an alternative model where Fe2+ and SiO2(aq) combined to precipitate greenalite upon the mixing of hydrothermal fluids with ambient seawater. All models for iron formation genesis invoke hydrothermal alteration of seafloor basalt as the principal source of soluble Fe2+, and its subsequent transport in an anoxic deep ocean. However, the processes and products associated with the venting of near-axis hydrothermal fluids into Precambrian seawater, and implications for Archean iron formation, have not been investigated. In order to test models for Archean iron formation genesis, we used reaction path models to investigate the interactions between ancient hydrothermal vent fluids and anoxic seawater. We assembled available thermodynamic data and executed reaction path models to examine: (1) the equilibration between anoxic SO4-free seawater and basalt/gabbro at subseafloor hydrothermal conditions (400oC and 400-500 bar), (2) the cooling and decompression of these fluids during their ascent through the oceanic crust, and (3) their subsequent mixing with anoxic SO4-free seawater. Our results confirm previous suggestions that the effective lack of SO4 in Archean seawater would have allowed fayalite-magnetite-pyrrhotite-quartz equilibria to buffer fluid chemistry, in turn leading to reaction zone fluids characterised by high Fe/H2S ratios. Cooling and decompression of these reducing fluids upon ascent, and further reaction with mafic wall rock, leads to significant Fe-chlorite and quartz precipitation with little to no sulfide precipitation. As these vent fluids mix with cold seawater directly, or with conductively-heated seawater, mineral assemblages are dominated by pyrite, greenalite, and/or siderite. These results were obtained with no constraints imposed by Archean iron formation, aside from the exclusion of minnesotaite, a demonstrably secondary mineral. The quantity of hydrothermally-derived Fe2+ partitioned into Fe(II)-bearing minerals upon mixing (96-99% of total aqueous Fe), the relative proportion of greenalite precipitated, and its further concentration via plume-driven transport (by virtue of its low density), together imply that the mixing between hydrothermal vent fluids and anoxic seawater may have served as a principal mechanism for generating a large proportion of the mineral mass now preserved as Archean iron formation. These results reconcile long-standing sedimentological and geochemical observations indicating that Archean banded iron formations record a strong hydrothermal influence, and suggest that these unique deposits have the potential to constrain the nature of seafloor-hosted hydrothermal systems on the ancient Earth, and their chemical interactions with seawater.Leverhulme Trust; Gordon & Betty Moore Foundatio

Experimental examination of the Mg-silicate-carbonate system at ambient temperature: Implications for alkaline chemical sedimentation and lacustrine carbonate formation

Despite their clear economic significance, Cretaceous presalt carbonates of the South Atlantic continental margins are not well-described by published facies models. This knowledge gap arises, in part, because the chemical processes that generate distinctive sedimentary products in alkaline, non-marine environments are poorly understood. Here, we use constraints inferred from reported mineralogical and geochemical features of presalt carbonate rocks to design and perform a suite of laboratory experiments to quantify the processes of alkaline chemical sedimentation. Using real-time observations of in-situ fluid chemistry, post-experiment analysis of precipitated solids, and geochemical modeling tools, we illustrate that spherulitic carbonates and Mg-silicate clays observed in presalt carbonates were likely precipitated from elevated pH (~10-10.5) waters with high concentrations of silica and alkali cations typical of intermediate to felsic rocks, such as Na+ and K+. Charge balance constraints require that these cations were not counterbalanced to any significant degree by anions typical of seawater, such as Cl- and SO4 -- , which implies minimal seawater involvement in presalt deposition. Experimental data suggest that, at this alkaline pH, only modest concentrations (i.e., ~0.5-1 mmol/kg) of Ca++ would have been required to precipitate spheroidal CaCO3. Given the rapid rates of CaCO3 nucleation and growth under such conditions, it is unlikely that Ca++ concentrations in lake waters ever exceeded these values, and sustained chemical fluxes are therefore required for extensive sediment accumulation. Moreover, our experiments indicate that the original mineralogy of presalt CaCO3 could have been calcite or aragonite, but the differing time scales of precipitation between CaCO3 and Mg-silicates would have tended to skew the Mg/Ca ratio in solution towards elevated values which favor aragonite. Mg-silicate nucleation and growth rates measured during our experiments suggest that elevated SiO2(aq) and high pH would have limited (to 1-2 mmol/kg) the Mg++ concentrations required to precipitate poorly crystalline Mg-silicates, which, through time, crystallize to minerals such as sepiolite and stevensite. Although our results provide robust constraints on the geochemistry of Mg-silicate-carbonate interactions during alkaline lake sedimentation, they leave open the potential for biological contributions to sedimentation within the presalt basins, as well as the hydrogeochemical mechanisms that maintained a productive carbonate factory of the scale observed along the South Atlantic margins

ALKALINITY IN THEORY AND PRACTICE

The articles in this issue highlight interdisciplinary approaches to the science of alkaline lakes, but one important concept links all of them together: alkalinity. Here, we discuss what alkalinity is, why it is important, and how it is typically measured. We review two different but complementary definitions of alkalinity that offer an intuitive starting point for understanding how this critical parameter responds to biogeochemical processes.</jats:p

Dry, Salty, and Habitable: The Science of Alkaline Lakes

Alkaline lakes are incredibly dynamic, unique, and fascinating biogeo-chemical environments that have remained distinctive features of Earth’s evolving surface over much of its history. Understanding these evaporative surface waters, their exceptionally productive ecosystems, and their rare sedimentary deposits requires an inherently interdisciplinary approach at the intersection of hydrology, geology, and biology. The discipline-spanning articles in this issue evaluate the diverse characteristics that make these dry, salty, and habitable environments so valuable in unraveling the history and evolution of Earth’s surface, and in following the arc of habitability on ancient Mars. Here, in this introductory article, we summarize the characteristics and importance of alkaline lakes with the hope of attracting you, too, to join in our fascination with them.</jats:p

Serpentinization as a reactive transport process: The brucite silicification reaction

Serpentinization plays a fundamental role in the biogeochemical and tectonic evolution of the Earth and perhaps many other rocky planetary bodies. Yet, geochemical models still fail to produce accurate predictions of the various modes of serpentinization, which limits our ability to predict a variety of related geological phenomena over many spatial and temporal scales. Here, we utilize kinetic and reactive transport experiments to parameterize the brucite silicification reaction and provide fundamental constraints on SiO2 transport during serpentinization. We show that, at temperatures characteristic of the sub-seafloor at the serpentinite-hosted Lost City Hydrothermal Field (150°C), the assembly of Si tetrahedra onto MgOH2 (i.e., brucite) surfaces is a rate-limiting elementary reaction in the production of serpentine and/or talc from olivine. Moreover, this reaction is exponentially dependent on the activity of aqueous silica [see manuscript], such that it can be calculated according to the rate law: [see manuscript]. Calculations performed with this rate law demonstrate that both brucite and Si are surprisingly persistent in serpentinizing environments, leading to elevated Si concentrations in fluids that can be transported over comparatively large distances without equilibrating with brucite. Moreover, applying this rate law to an open-system reactive transport experiment indicates that advection, preferential flow pathways, and reactive surface area armoring can diminish the net rate of Si uptake resulting from this reaction even further. Because brucite silicification is a fundamentally rate-limiting elementary reaction for the production of both serpentine and talc from forsterite, our new constraints are applicable across the many environments where serpentinization occurs. The unexpected but highly consequential behavior of this simple reaction emphasizes the need for considering serpentinization and many other hydrothermal processes in a reactive transport framework whereby fluid, solute, and heat transport are intimately coupled to kinetically-controlled reactions

Serpentinization as a reactive transport process: The brucite silicification reaction

Serpentinization plays a fundamental role in the biogeochemical and tectonic evolution of the Earth and perhaps many other rocky planetary bodies. Yet, geochemical models still fail to produce accurate predictions of the various modes of serpentinization, which limits our ability to predict a variety of related geological phenomena over many spatial and temporal scales. Here, we utilize kinetic and reactive transport experiments to parameterize the brucite silicification reaction and provide fundamental constraints on SiO2 transport during serpentinization. We show that, at temperatures characteristic of the sub-seafloor at the serpentinite-hosted Lost City Hydrothermal Field (150°C), the assembly of Si tetrahedra onto MgOH2 (i.e., brucite) surfaces is a rate-limiting elementary reaction in the production of serpentine and/or talc from olivine. Moreover, this reaction is exponentially dependent on the activity of aqueous silica [see manuscript], such that it can be calculated according to the rate law: [see manuscript]. Calculations performed with this rate law demonstrate that both brucite and Si are surprisingly persistent in serpentinizing environments, leading to elevated Si concentrations in fluids that can be transported over comparatively large distances without equilibrating with brucite. Moreover, applying this rate law to an open-system reactive transport experiment indicates that advection, preferential flow pathways, and reactive surface area armoring can diminish the net rate of Si uptake resulting from this reaction even further. Because brucite silicification is a fundamentally rate-limiting elementary reaction for the production of both serpentine and talc from forsterite, our new constraints are applicable across the many environments where serpentinization occurs. The unexpected but highly consequential behavior of this simple reaction emphasizes the need for considering serpentinization and many other hydrothermal processes in a reactive transport framework whereby fluid, solute, and heat transport are intimately coupled to kinetically-controlled reactions

Whole rock basalt alteration from CO2-rich brine during flow-through experiments at 150 °C and 150 bar

Four flow-through experiments at 150 °C were conducted on intact cores of basalt to assess alteration and mass transfer during reaction with CO2-rich fluid. Two experiments used a flow rate of 0.1 ml/min, and two used a flow rate of 0.01 ml/min. Permeability increased for both experiments at the higher flow rate, but decreased for the lower flow rate experiments. The experimental fluid (initial pH of 3.3) became enriched in Si, Mg, and Fe upon passing through the cores, primarily from olivine and titanomagnetite dissolution and possibly pyroxene dissolution. Secondary minerals enriched in Al and Si were present on post-experimental cores, and an Fe2O3-rich phase was identified on the downstream ends of the cores from the experiments at the lower flow rate. While we could not specifically identify if siderite (FeCO3) was present in the post-experimental basalt cores, siderite was generally saturated or supersaturated in outlet fluid samples, suggesting a thermodynamic drive for Fe carbonation from basalt-H2O-CO2 reaction. Reaction path models that employ dissolution kinetics of olivine, labradorite, and enstatite also suggest siderite formation at low pH. Furthermore, fluid-rock interaction caused a relatively high mobility of the alkali metals; up to 27% and 100% of the K and Cs present in the core, respectively, were preferentially dissolved from the cores, likely due to fractional crystallization effects that made alkali metals highly accessible. Together, these datasets illustrate changes in chemical parameters that arise due to fluid-basalt interaction in relatively low pH environments with elevated CO

Magnetite authigenesis and the warming of early Mars

The Curiosity rover has documented lacustrine sediments at Gale Crater, but how liquid water became physically stable on the early Martian surface is a matter of significant debate. To constrain the composition of the early Martian atmosphere during sediment deposition, we experimentally investigated the nucleation and growth kinetics of authigenic Fe-minerals in Gale Crater mudstones. Experiments show that pH variations within anoxic basaltic waters trigger a series of mineral transformations that rapidly generate magnetite and H2(aq). Magnetite continues to form through this mechanism despite high partial pressure of carbon dioxide (pCO2) and supersaturation with respect to Fe-carbonate minerals. Reactive transport simulations that incorporate these experimental data show that groundwater infiltration into a lake equilibrated with a CO2-rich atmosphere can trigger the production of both magnetite and H2(aq) in the mudstones. H2(aq), generated at concentrations that would readily exsolve from solution, is capable of increasing annual mean surface temperatures above freezing in CO2-dominated atmospheres. We therefore suggest that magnetite authigenesis could have provided a short-term feedback for stabilizing liquid water, as well as a principal feedstock for biologically relevant chemical reactions, at the early Martian surface

Magnetite authigenesis and the warming of early Mars

The Curiosity rover has documented lacustrine sediments at Gale Crater, but how liquid water became physically stable on the early Martian surface is a matter of significant debate. To constrain the composition of the early Martian atmosphere during sediment deposition, we experimentally investigated the nucleation and growth kinetics of authigenic Fe-minerals in Gale Crater mudstones. Experiments show that pH variations within anoxic basaltic waters trigger a series of mineral transformations that rapidly generate magnetite and H2(aq). Magnetite continues to form through this mechanism despite high partial pressure of carbon dioxide (pCO2) and supersaturation with respect to Fe-carbonate minerals. Reactive transport simulations that incorporate these experimental data show that groundwater infiltration into a lake equilibrated with a CO2-rich atmosphere can trigger the production of both magnetite and H2(aq) in the mudstones. H2(aq), generated at concentrations that would readily exsolve from solution, is capable of increasing annual mean surface temperatures above freezing in CO2-dominated atmospheres. We therefore suggest that magnetite authigenesis could have provided a short-term feedback for stabilizing liquid water, as well as a principal feedstock for biologically relevant chemical reactions, at the early Martian surface