Simultaneous precipitation of magnesite and lizardite from hydrothermal alteration of olivine under high-carbonate alkalinity
Introduction
The physicochemical reactions at the solid–fluid interfaces play a crucial role in the global cycle of major and trace elements in the Earth and other telluric planets. In this way, chemical weathering, metamorphic reactions, diagenetic reactions, hydrothermalism, volcanic activity, and crystal–melt reactions are important non-limited physicochemical processes that shape the Earth's surface and sub-surface. However, many physicochemical and textural aspects of these so-called rock–fluid interactions are still poorly understood. For example, when mantle peridotite is tectonically exposed with (sub-)surface fluids (e.g. seafloor water and meteoric water), the olivine and pyroxene anhydrous minerals contained in peridotite are far-from-equilibrium with respect to fluid composition. In this way, numerous physicochemical reactions at peridotite–fluid interfaces can take place such as hydration (–OH incorporation or serpentinization) and carbonation processes if the required temperature and fluid compositions are enough to activate these reactions; both most important processes directly related to natural H2 and abiotic methane production via redox reactions and the formation of other non-limited secondary minerals as it has been observed in various natural hydrothermal sites (e.g. Logatchev, Rainbow and The Lost City) (e.g. Charlou et al., 2002, Allen and Seyfried, 2004, Ludwig et al., 2006, Seyfried et al., 2007, Klein et al., 2009, McCollom and Bach, 2009, Rudge et al., 2010, Seyfried et al., 2011). Such reducing systems may represent analogs to early Earth environments and may provide insights into requirements for the emergence of life, probably initiated at the sea floor (e.g. MacLeod et al., 1994, Charlou et al., 2002, Früh-Green et al., 2003, Kelley et al., 2005). The field monitoring and ex-situ characterization have revealed complex fluid chemistry and generally low pH (from 2.8 to 4.3) and high temperature (from 275 to 365 °C) in the expelled fluids from various studied ultramafic-hosted hydrothermal systems at the Mid-Atlantic Ridge (Charlou et al., 2002). Conversely, the expelled fluids at the Lost City field and other sites for example in continental serpentinization systems (e.g. Samail Ophiolite in Oman) are highly alkaline (pH > 9) and lower temperatures have been monitored/determined (from 55 to 90 °C) (Kelley et al., 2001, Früh-Green et al., 2003, Ludwig et al., 2006, Kelemen et al., 2011). These surprising measurements and the recent discovery of spectacular carbonate towers at the Lost City hydrothermal field have stimulated interest in the role of serpentinization and carbonation processes on the production of hydrogen- and methane-rich fluids and on the biological communities that may be supported in these environments (Früh-Green et al., 2003, Kelley et al., 2005, Schrenk et al., 2013). Moreover, at the present time, the ex-situ and in-situ carbonation of mafic and ultramafic rocks (e.g. basalts and peridotite), extensively available in the oceanic crust and ophiolites, have been proposed as a promising solution to mitigate the global warming of Earth's atmosphere related to excessive anthropogenic and natural CO2 emissions; because Mg-, Ca- or Fe-carbonates resulting from mineral carbonation of CO2 can remain stable at the geological time scales as frequently observed in the Earth surface and/or sub-surface (e.g. Seifritz, 1990, Lackner et al., 1995, Bachu, 2000, Kaszuba et al., 2003, Xu et al., 2004, Kaszuba et al., 2005, Gerdemann et al., 2007, IPCC (Intergovernmental Panel on Climate Change), 2007, Kelemen and Matter, 2008, Oelkers et al., 2008, Matter and Kelemen, 2009, Montes-Hernandez et al., 2009a, Montes-Hernandez et al., 2009b, Kelemen et al., 2011, Schwarzenbach et al., 2013). In this general context, numerous experimental studies concerning the serpentinization or carbonation of peridotite (or single olivine) have been recently performed using batch, semi-continuous or flow-through reactors in order to better understand the reaction mechanisms and kinetics, reaction and cracking propagation from the grain boundaries, nature and role of secondary phase formation, potential of hydrogen production, potential for mineral sequestration of CO2 and role of P, T, pH, solid/fluid ratio and fluid chemistry (e.g. Wunder and Schreyer, 1997, James et al., 2003, Giammar et al., 2005, Bearat et al., 2006, Seyfried et al., 2007, Andreani et al., 2009, McCollom and Bach, 2009, Garcia et al., 2010, King et al., 2010, Daval et al., 2011, Hövelmann et al., 2011, Klein and Garrido, 2011, Marcaillou et al., 2011, Bonfils et al., 2012, Lafay et al., 2012, Malvoisin et al., 2012). However, the competitive and/or coexistence between serpentinization and carbonation at peridotite–fluid interfaces have been rarely investigated at the laboratory scale, remarking that serpentinization and carbonation of peridotite, leading to precipitation of serpentine (e.g. lizardite and chrysotile) and magnesite (or hydrated Mg carbonates), could occur simultaneously in natural hydrothermal systems if the interacting solution is supersaturated with respect to both minerals. For this simple reason, the main goal of this present study was focussed to determine the simultaneous precipitation of serpentine and magnesite from hydrothermal alteration of olivine under high-carbonate alkalinity. For this particular case, specific experimental conditions were used (200 °C, saturation vapor pressure (≈ 16 bar), solution/solid weight ratio (= 15), olivine grain size (< 30 μm) and high-carbonate alkalinity solution (1 M NaHCO3)). These experimental conditions were selected with help of previous-experimental studies, investigating independently the serpentinization or the carbonation of olivine (e.g. Giammar et al., 2005, Bearat et al., 2006, Seyfried et al., 2007, Garcia et al., 2010, King et al., 2010, Daval et al., 2011, Hövelmann et al., 2011, Marcaillou et al., 2011, Bonfils et al., 2012, Lafay et al., 2012, Malvoisin et al., 2012). High-purity synthetic chrysotile and serpentinized olivine (chrysotile + brucite mineral + small amount of residual olivine) obtained in our laboratory were also used as starting solids in complementary-similar experiments in order to determine their reactivity under high-alkalinity. As expected, the chrysotile was slightly altered and brucite quickly transformed to magnesite at the investigated conditions. Various analytical tools such as X-ray diffraction (XRD), Field Emission Gun Scanning Electron Microscopy (FESEM), Thermogravimetric analyses (TGA/SDTA) and Fourier Transform Infrared Spectroscopy (FTIR) were used to characterize the solid products. TGA analyses and the respective 1st derivative curves were particularly used to determine with high accuracy the temporal variation of magnesite and serpentine during olivine alteration.
Section snippets
Olivine grains
Millimetric grains of olivine San Carlos (Fo91) were crushed using a Fritsh Pulverisette 7 micro-crusher. One class of grain/particle size (particle size < 30 μm) was isolated by sieving. The samples were washed three times using high-pure water in order to remove the ultrafine particles that possibly stuck at grain surfaces during crushing step. Optical and electron microscopy was performed to control the initial state/appearance of olivine surfaces. On the other hand, high specific surface area
Mineral composition of products
The conventional analytic techniques (XRD, TGA, FTIR and FESEM) have revealed that the hydrothermal alteration of olivine using high-carbonate alkalinity solutions, i.e. enriched with CO2 (S1 and S2 solutions), concerns the competitive formation of magnesite and serpentine, in other words, competitive carbonation and serpentinization processes during olivine alteration was clearly observed. As expected, both solutions (S1 and S2) have revealed a very similar effect on the olivine alteration
Reaction steps
In a previous recent study, we reported that the serpentinization of San Carlos olivine under high-hydroxyl alkalinity “or high-basic conditions” (pH = 13.5 ex-situ measured at 20 °C) takes place via mineral replacement of olivine by chrysotile and brucite assemblage, i.e. a spatial and temporal coupling of dissolution and precipitation reactions at the interface between olivine and chrysotile–brucite minerals, leading to preservation of external shape of olivine grains (Fig. 6a). For more
Coexistence of carbonation and serpentinization processes: from experimentation to natural systems
In the last decades, the serpentinization of olivine have been intensively investigated at the lab scale in order to determine the reaction mechanisms and kinetics, the reaction and cracking propagation from the grain boundaries, its potential for hydrogen production and its implications on the early Earth life, i.e. its role on the abiotic formation of organic molecules (MacLeod et al., 1994, James et al., 2003, Seyfried et al., 2007, McCollom and Bach, 2009, Hövelmann et al., 2011, Marcaillou
Implications for in-situ carbonation of peridotite for CO2 storage
Unregulated CO2 emissions into the Earth's atmosphere (about 22 × 109 ton CO2 year− 1), caused mainly by fossil fuel combustion, have led to concerns about global warming. To maintain the atmospheric CO2 level below 500 ppm, CO2 emissions will have to be stabilized at current levels, although they are forecast to double over the next 50 years (Allwood et al., 2010). Captured from individual industrial sources and long-term geological storage are realistic and available ways of reducing CO2 emissions
Conclusion
The coexistence of serpentinization and aqueous carbonation of ultrabasic rocks has up to now not been investigated at laboratory scale and various questions still remain unanswered concerning its mechanistic pathways in natural systems, mainly under high alkalinity. In response to this scientific gap, this study provides new insights on competitive serpentinization and aqueous carbonation of olivine under high-carbonate alkalinity. In this way, we quantified a retarding process of serpentine
Acknowledgments
The authors are grateful to the French National Center for Scientific Research (CNRS/INSU), the University Joseph Fourier in Grenoble and ANR French research agency (ANR CORO and ANR SPRING projects) for providing financial support.
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