Mineral replacement rate of olivine by chrysotile and brucite under high alkaline conditions
Highlights
► Complete olivine replacement by chrysotile–brucite under alkaline conditions. ► Preservation of external shape of olivine grains. ► Simple and novel quantification method of serpentinization rate by TG analyses. ► Significant influence of olivine starting grain size on serpentinization rate.
Introduction
Serpentine minerals (chrysotile, lizardite and antigorite) are widespread in Earth's oceanic lithosphere and are frequently found in chondrites and other extraterrestrial objects. Serpentine mineralization is of great interest in several fields of research. Serpentinized rocks present a great enrichment in trace elements compared to primary mantle rocks [1], [2], [3], [4], [5], [6]. Serpentine appears as a vector for trace elements between crustal and mantle reservoirs [2], [5], [7], [8]. Experimental studies have tested the influence of major–minor (e.g. Fe, Ni) and/or trace elements (e.g. Li) on the growth of serpentine [9], [10], [11], [12], [13], [14]. This kind of synthesis experiments presents a great interest in medical research due to the asbestos toxicity that can be induced by inhalation of magnesium silicates' fibers including chrysotile [15], [16], [17], [18]. Serpentine minerals are also crucial for sequestration of CO2 due to its availability and sequestration capacity [19], [20], [21]. Indeed a lot of studies are looking for technologies that could possibly contribute to reduce carbon dioxide emissions. Geological sequestration and ex-situ mineralization of CO2 using serpentine could be one of the most efficient methods considering the enormous quantity of serpentine on Earth [22].
In meteorites, serpentine minerals are directly linked to aqueous alteration processes and reaction conditions (e.g. [23] and references therein). In the oceanic lithosphere, serpentines result from interaction between mantle rocks (peridotite composed by olivine and pyroxenes) and hydrothermal fluids, generally with high fracturation dynamic [24], [25]. Olivine alters along grain boundaries and fractures to produce a mesh texture that preserves the original olivine morphology at the grain scale [26], [27]. This olivine replacement by serpentine is best explained by coupled dissolution–precipitation processes [28], [29], [30]. This re-equilibration process leads to the replacement of one pristine mineral by a secondary mineral (or assemblage) with a lower solubility in the fluid. Replacement occurs at the fluid/solid interface maintaining the original external grain shape (pseudomorphism). During alteration, a secondary porosity is commonly produced due to volume difference between pristine and secondary minerals, material loss during dissolution and grain fracturation [24], [25]. Secondary porosity enables the fluid to move interstitially towards the reaction front until the complete mineral replacement reaction. In oceanic lithosphere, peridotite replacement, and consequent element redistribution associated with this alteration, is primarily controlled by the physico-chemical conditions of the hydrothermal fluid (temperature, pressure, fluid speciation, and pH). Fluids escaping from deep sea hydrothermal vents show a large variety of composition and pH, reflecting a large range of possible physico-chemical conditions. Amongst them, alkaline fluids with high pH were collected in some hydrothermal fields (e.g., [31], [32]).
Numerous experimental studies were conducted to reproduce serpentinization in hydrothermal context [33], [34], [35], [36], [37], [38], [39], [40], [54] and explain serpentine growth [41]. Kinetic appears faster under alkaline conditions [40], [42] but few recent studies have addressed the role of pH on serpentinization kinetic, particularly in alkaline conditions.
In the present experimental study, we have investigated the process and kinetics of olivine serpentinization in alkaline hydrothermal conditions (pH=13.5, measured at 25 °C). Experimental products were characterized using XRD, FESEM and FTIR. The serpentinization rate was determined using a simple and novel method based on thermogravimetric analyses (TGA/DTGA). This demonstrates that serpentinization is much faster under alkaline conditions referring to previous studies at comparable conditions [35], [36], [43] and can lead to a total replacement of <30 μm olivine in less than 30 days and 90 days for 30<particle size<56 μm.
Section snippets
Materials and methods
Millimetric grains of olivine San Carlos (Fo91) were crushed using a Fritsh Pulverisette 7 micro-crusher. Three classes of grain/particle size (particle size<30 μm, 30<particle size<56 μm and 56<particle size<150 μm) were 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
Serpentinization reaction under alkaline conditions
Secondary minerals were identified by XRD and FESEM (Fig. 1, Fig. 2) and they were quantified by TGA (Fig. 3). Under alkaline conditions, olivine is replaced by chrysotile and brucite, independently on the starting grain size of olivine. No other minerals were detected and/or observed during this alteration reaction.
FESEM micro-imaging has revealed a clear evolution of particle size and morphology of crystal faces during serpentinization advancement (Fig. 2). Serpentine nucleation at
Conclusion
Olivine alteration was investigated under alkaline conditions for different starting grain sizes at 200 °C. In this study, we were able to follow complete olivine replacement by an assemblage of chrysotile and brucite.
Thermogravimetric analyses were used to investigate the dehydroxylation of hydrated phases and thus the serpentinization extent as a function of time. Based on this innovative approach, we were able to estimate punctually the serpentinization advancement and a kinetic
Acknowledgments
The authors are grateful to the French National Center for Scientific Research (CNRS) and University Joseph Fourier (UJF) in Grenoble for providing the financial support. R. Lafay was supported by a Ph.D grant from French education ministry. The authors are grateful to IPAG institute where infrared measurements were performed. We thank Martine Lanson for help in the chemical laboratory.
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2021, Precambrian ResearchCitation Excerpt :As observed from the textural and mineralogical study of modern abyssal peridotite serpentinisation, in rock-dominated systems, initial olivine breakdown is marked by development of a ferrobrucite + serpentine (lizardite/chrysotile), without magnetite development (e.g., Bach et al., 2006). Lack of magnetite in completely serpentinised abyssal peridotites has also been attributed to serpentinisation at temperatures ≤~200 °C, similarly resulting in an assemblage of Fe-rich brucite and serpentine (initially lizardite or chrysotile; e.g., Evans, 2004; Lafay et al., 2012; Schwartz et al., 2013; Klein et al., 2014). The ramification of this is that with superimposed metamorphism, reaction between serpentine and the ferrobrucite will produce an olivine composition that is near identical with the original protolith olivine composition (Fig. 13).