Éditeur(s) : HAL CCSD Mineralogical Society of America
Résumé : International audience
In situ mineralization of CO2 in ultramafic rock-hosted aquifers is one of the promising solutions for decreasing CO2 concentrations in the atmosphere. Naturally altered ultramafic rocks suggest that carbonation processes are controlled by local heterogeneities in the structure of the rock and fluid transport at the water-rock interfaces. We studied the role of rock crystallographic anisotropy relative to the global fluid flow direction on the mineralization of CO2 by means of electron microscope analyses from the macro- to the micrometer scale (EBSD-FIB). The sample used for the measurements was a hot pressed olivine core percolated by water enriched in CO2 (pCO2 = 10 MPa) at 180 °C. During the percolation experiment, olivine was dissolved and two types of carbonates, dolomite, and magnesite, were precipitated on olivine surfaces. The results showed that the dissolution of olivine is controlled by its crystallographic properties as shown by the development of etch-pits only on the (010)ol planes and with elongated shapes parallel to the [010]ol axes. In contrast, the precipitation of carbonates is governed by hydrodynamic properties. Carbonates are heterogeneously distributed in the percolated rock. They are mainly located along the moderate (for dolomite) and the minor (for magnesite) flow paths, both oriented parallel to the principal fluid flow direction, which allow carbonates to be supplied with divalent cations (e.g., Ca2+, Mg2+, and Fe2+). In these flow paths, carbonate growth is systematically oriented normal to the flow that facilitates the development of chemical gradients with cationic supersaturation conditions for carbonate precipitation near the walls.In natural systems, the (010)ol planes are parallel to the Moho and the (100)ol planes are vertical; our study suggests that flow of CO2-rich fluids will induce precipitation of carbonates localized along, and preferentially clogging, vertical flow paths while favoring olivine dissolution along horizontal fluid pathways. This dual control of structure and fluid flow on carbonation mechanisms could be an important parameter allowing sustainable CO2 storage in peridotites, while limiting the risks of leakage toward the surface.
ISSN: 0003-004X

hal-01143393
https://hal.archives-ouvertes.fr/hal-01143393
DOI : 10.2138/am-2015-4913

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The coastal sediments of Rhodes in the eastern Mediterranean have recorded transgression-regression cycles that took place during the early Pleistocene. The sedimentary deposits from the Kritika Member of the Rhodes Formation consist in conglomerates, sandstones, siltstones and clays deposited in brackish to shallow marine environments. Faunal associations are dominated by molluscs and reveal rapid ecological changes. Carbon and oxygen isotope ratios of aquatic skeletal carbonates show that these ecological changes were most likely driven by large salinity changes while water temperature remained rather constant at about 22.0 ± 1.5 °C. The tectonic activity of the island rather than glacio-eustatic variations of climatic origin is advocated to be responsible for the ecological and salinity changes and sea-level variations recorded in the sedimentary sequence.
ISSN: 0031-0182

hal-00767568
https://hal.archives-ouvertes.fr/hal-00767568
DOI : 10.1016/j.palaeo.2012.06.009

Éditeur(s) : HAL CCSD National Academy of Sciences
Résumé : International audience
The Isua Supracrustal Belt, Greenland, of Early Archean age (3.81-3.70 Ga) represents the oldest crustal segment on Earth. Its complex lithology comprises an ophiolite-like unit and volcanic rocks reminiscent of boninites, which tie Isua supracrustals to an island arc environment. We here present zinc (Zn) isotope compositions measured on serpentinites and other rocks from the Isua supracrustal sequence and on serpentinites from modern ophiolites, midocean ridges, and the Mariana forearc. In stark contrast to modern midocean ridge and ophiolite serpentinites, Zn in Isua and Mariana serpentinites is markedly depleted in heavy isotopes with respect to the igneous average. Based on recent results of Zn isotope fractionation between coexisting species in solution, the Isua serpentinites were permeated by carbonate- rich, high- pH hydrothermal solutions at medium temperature (100-300 degrees C). Zinc isotopes therefore stand out as a pH meter for fossil hydrothermal solutions. The geochemical features of the Isua fluids resemble the interstitial fluids sampled in the mud volcano serpentinites of the Mariana forearc. The reduced character and the high pH inferred for these fluids make Archean serpentine mud volcanoes a particularly favorable setting for the early stabilization of amino acids.
ISSN: 0027-8424

insu-00674714
https://hal-insu.archives-ouvertes.fr/insu-00674714
DOI : 10.1073/pnas.1108061108

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Preface
ISSN: 0024-4937

hal-00903596
https://hal.archives-ouvertes.fr/hal-00903596
DOI : 10.1016/j.lithos.2013.06.006

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Carbonation of peridotite is a widespread process in nature, with emerging societal and environmental implications through geological storage of CO2. We studied the micro- and nano-scale structures associated with the carbonation of peridotite pervaded by a CO2-rich water and the induced changes of its hydrodynamic properties. Focused ion beam (FIB) nanotomography was coupled with scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations. An experimental sample of dunite core percolated by a CO2-rich fluid (pCO2 = 11 MPa) at 160 °C was compared with a naturally carbonated sample from the Atlantic Ridge (Exp. IODP 304). Results of the 8-days percolation experiment show continuous alteration of olivine into magnesite locally associated with a poorly crystalline serpentine-type mineral referred to as proto-serpentine. The experiment is also characterized by an overall decrease in rock porosity (from 7.7 to 6.4%) and permeability (from 8.5 × 10− 16 to 1.7 × 10− 16 m2). The primary inter-granular porosity (1 to 10 μm) is progressively filled with magnesite, leading to the overall decrease of permeability. At the same time, a secondary small-scale porosity (10 nm to 1 μm) is created at the tip of the dissolution features along the (010) cleavage planes of olivine grains. Carbonate crystals grow within these etch-pit structures behind the dissolution front. Dissolution and precipitation occur at a relative rate that maintains the secondary porosity over the timescale of the experiment (i.e. carbonate growth has to be the slowest one). This process is interpreted as an interface coupled dissolution-precipitation mechanism of olivine carbonation. Proto-serpentine is only formed in highly reduced flow areas, such as small olivine etch-pits (10 to 500 nm wide), which are more isolated from CO2 input. SEM-FIB analyses show the 3D preferential orientation of the dissolution-precipitation planes within a given olivine grain. It suggests the formation of lateral connections between the etch-pits (infra-micrometric “channels”) providing access to reactants and removing reaction products at the reaction sites. The experimental results show that olivine dissolution and magnesite precipitation converge to a quasi-steady state resulting in a constant carbonation rate controlled by reaction kinetics and not by mass transfer for this flow regime. The system self-regulates, independently of the porosity change at least during this incipient stage of olivine carbonation. Similar textures are observed in natural samples collected at 170 m-depth in the vicinity of veinlets driving late fluids down into the cooling oceanic lithosphere. This suggests that similar mechanisms are active in nature and can play an important role in the late, low temperature alteration stages of fractured peridotite. These results also highlight the critical role of nm-scale spatial arrangement of dissolution and precipitation features on mass transfers and on the sustainability of olivine carbonation through coupled dissolution-precipitation processes.
ISSN: 0009-2541

hal-01684733
https://hal.archives-ouvertes.fr/hal-01684733
DOI : 10.1016/j.chemgeo.2017.11.013