Precipitation of ordered dolomite via simultaneous dissolution of calcite and magnesite: New experimental insights into an old precipitation enigma

7International audienceIn the present study, we demonstrate that ordered dolomite can be precipitated via simultaneous dissolution of calcite and magnesite under hydrothermal conditions (from 100 to 200°C). The temperature and high-carbonate alkalinity have significantly co-promoted the dolomite formation. For example, when high-purity water was initially used as interacting fluid, only a small proportion of disordered dolomite was identified at 200°C from XRD patterns and FESEM observations. Conversely, higher proportion of ordered dolomite, i.e. clear identification of superstructure ordering reflections in XRD patterns, was determined when high-carbonate alkalinity solution was initially used in our system at the same durations of reaction. For this latter case, the dolomite formation is favorable therefrom 100°C and two kinetic steps were identified (1) proto-dolomite formation after about five days of reaction, characterized by rounded sub-micrometric particles from FESEM observations and by the absence of superstructure ordering reflections at 22.02 (101), 35.32 (015), 43.80 (021), etc. 2thetha on XRD patterns; (2) proto-dolomite to dolomite transformation, probably produced by a coupled dissolution-recrystallization process. Herein, the activation energy was estimated to 29 kJ/mol by using conventional Arrhenius linear-equation. This study provides new experimental conditions to which dolomite could be formed in hydrothermal systems. Temperature and carbonate alkalinity are particularly key physicochemical parameters to promote dolomite precipitation in abiotic systems

Experimental investigation of the stability of Fe-rich carbonates in the lower mantle

International audienceThe fate of carbonates in the Earth's mantle plays a key role in the geodynamical carbon cycle. Although iron is a major component of the Earth's lower mantle, the stability of Fe-bearing carbonates has rarely been studied. Here we present experimental results on the stability of Fe-rich carbonates at pressures ranging from 40 to 105 GPa and temperatures of 1450-3600 K, corresponding to depths within the Earth's lower mantle of about 1000-2400 km. Samples of iron oxides and iron-magnesium oxides were loaded into CO2 gas and laser heated in a diamond-anvil cell. The nature of crystalline run products was determined in situ by X-ray diffraction, and the recovered samples were studied by analytical transmission electron microscopy and scanning transmission X-ray microscopy. We show that Fe-(II) is systematically involved in redox reactions with CO2 yielding to Fe-(III)-bearing phases and diamonds. We also report a new Fe-(III)-bearing high-pressure phase resulting from the transformation of FeCO3 at pressures exceeding 40 GPa. The presence of both diamonds and an oxidized C-bearing phase suggests that oxidized and reduced forms of carbon might coexist in the deep mantle. Finally, the observed reactions potentially provide a new mechanism for diamond formation at great depth

Evolution des microstructures des serpentinites en contexte convergent: effet du degré de métamorphisme et de la déformation.

This manuscript presents a petrological, spectroscopic, TEM, pluridisciplinary approach of the evolution of serpentine microstructures as a function of metamorphic grade and deformation in order to constraint their behavior in convergent context. Experimentally, the four main varieties of serpentine minerals are regularly compressing up to 10 GPa at ambient temperature. Pressure increase is mostly adjusted by reducing interfoliar space. No pressure-enhancement of hydrogen bonding is evidenciated in serpentine minerals. Cubean and alpine serpentinite analysis confirmed the high pressure stability of antigorite. Its microstructures vary with metamorphic grade. High grade antigorites do not display intracrystalline polysomatic disorder which characterize low grade antigorites. We propose that a second step in ordering microstructures is reached under eclogitic facies conditions by eliminating most of microstructural defects, but only in exceptionally well-preserved samples. Serpentinites are extremely sensitive to retrogressed deformation. In alpine serpentinites, low-grade deformation is characterized by brittle behavior at the crystal-scale. Under higher metamorphic grade, deformation seems to be accommodated by pressure solution processes, which are likely to persist up to eclogitic conditions. Planar defects observed in antigorite microstructures suggest that (001) gliding of sheets partly contributes to accommodate the deformation.Ce mémoire présente une approche pluridisciplinaire (pétrologie, spectroscopie Raman et MET) de l'évolution des microstructures des serpentinites avec les conditions métamorphiques et la déformation afin de préciser leur comportement en contexte de convergence. Expérimentalement, les différentes variétés de serpentines se compriment de façon régulière jusqu'à 10 GPa à température ambiante. L'augmentation de pression est principalement ajustée par réduction de l'espace interfoliaire. Les liaisons hydrogène liant les feuillets ne sont pas renforcées avec la pression. L'analyse d'échantillons de serpentinites de HP-BT provenant de Cuba et des Alpes a confirmé que l'antigorite est la variété stable à haute pression. Les microstructures de ce minéral varient avec le degré métamorphique. Les antigorites de haut degré, au delà du faciès des schistes bleus, ne présentent pas le désordre polysomatique intracristallin qui caractérise les antigorites de bas degré. Une second étape de mise en ordre des microstructures est proposée sous les conditions éclogitiques par l'élimination des défauts microstructuraux. Mais les serpentinites étant très sensibles à toute déformation rétromorphique, cette mise en ordre n'a pu être observée que sous des conditions de préservation exceptionnelle (Cuba) et n'a pas été confirmée dans les échantillons alpins. Dans les serpentinite alpines, la déformation se manifeste à bas degré métamorphique (transition SV-SB) par des structures cassantes à l'échelle du cristal. Sous des conditions PT plus sévères, la déformation est accommodée principalement par des phénomènes de dissolution recristallisation qui semblent persister jusqu'aux stades éclogitiques. Les défauts plans observés dans les antigorites suggèrent aussi une contribution à la déformation par glissement des feuillets le long des plans (001)

Iron partitioning and the selfoxidation of the lower mantle

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Nanostructure of serpentinisation products: Importance for water transport and low-temperature alteration

International audienceThe hydration of mantle rocks occurs at mid-ocean ridges, in subduction zones and in ophiolites where it strongly modifies the properties of the oceanic lithosphere. The nanostructure of the reaction products is poorly constrained, but it is very important, as it controls fluid transport during solid volume increase, and it influences phase reactivity during further fluid/rock interaction at low temperature. We image contacts between olivine and its hydration products at the nanoscale, in a dunite collected during the Oman Drilling Project that underwent nearly isochemical serpentinisation and solid volume increase. Olivine first reacts to form a ~ 100 nm thick coating composed of a brucite/serpentine mixture, suggesting isochemical serpentinisation at this scale too. Lizardite columns ~ 1 μm wide replace this mixture at its margin. The columns are embedded in a brucite-rich assemblage containing a high density of nanopores that may favor fluid transport during reaction. We interpret these observations as a nanometre-scale, two-step process. Fluid pathways are formed rather than clogged by reaction products thanks to mass transfer at a scale limited to less than 100 nm. The preservation of the fluid pathways during solid volume increase explains the observed high extents of reaction. The presence of brucite in high porosity regions may explain its preferential reaction during low temperature fluid/rock interaction

Evolution des microstructures des serpentinites en contexte convergent (effet du degré de métamorphisme et de la déformation)

CLERMONT FD-BCIU Sci.et Tech. (630142101) / SudocFONTAINEBLEAU-MINES ParisTech (771862302) / SudocRENNES-Géosciences (352382209) / SudocSudocFranceF

Serpentinites in an Alpine convergent setting: Effects of metamorphic grade and deformation on microstructures.

Alpine antigorite serpentinites associated with eclogites were investigated to determine if they can be used as indicators of the tectono-metamorphic conditions during subduction and exhumation processes. The detailed petrology of serpentinites sampled in the Monviso massif (Western Alps, Italy) was combined with a transmission electron microscopy study. Alpine serpentinites display a degree of serpentinization close to 100%. Antigorite is the main mineral present, forming non-pseudomorphic textures in the various studied samples and exhibiting a homogeneous chemical composition with limited cationic substitutions. Considering its oceanic origin, the Alpine serpentinite in the Monviso massif formed a lizardite + chrysotile assemblage that recrystallized under greenschist-facies conditions into poorly ordered antigorite, with a modulation wavelength showing significant variations at the crystal scale. Under blueschist-facies conditions, the modulation wavelength of antigorite becomes regular. Thus, periodic antigorites can be related to high-grade conditions, while poorly ordered antigorites characterize lower metamorphic grade. In the present study, we failed to observe any elimination of structural defects with increasing metamorphic grade. While around 50% of the antigorite crystals are highly ordered, it seems that this ordering is at least partly obliterated by retrogressive deformation. Antigorite displays strong evidence of deformation-sensitivity, and the observed microstructures can be directly related to the mechanical behaviour of serpentinites in subduction zones. We investigated the deformation-induced microstructures in serpentinites collected in the Erro-Tobbio eclogitic unit (Ligurian Alps, Italy), which appear to preserve prograde and retrograde structures formed during subduction. According to the microstructural evidence, shearing is accommodated by brittle and/or ductile deformation mechanisms. Collected samples were fractured at different scales (cm to nm) and have a well-developed schistosity characterized by a strong crystallographic fabric. With increasing metamorphic grade, the brittle behaviour gives way to pressure-solution, which persists up to eclogite-facies conditions. The common obliteration of high-grade microstructures in antigorite, as observed in the Monviso serpentinites, results from continuous recrystallization of this mineral during retrogressive deformation

Amorphous Calcium–Magnesium Carbonate (ACMC) Accelerates Dolomitization at Room Temperature under Abiotic Conditions

The challenge to produce dolomite CaMg(CO3)2 at low temperature (20–35 °C) over laboratory time scales so far has remained unsuccessful, which has led to long-lasting scientific debates in the last two centuries. This mineral exerts a major control on the natural carbon dioxide sequestration into various sedimentary, basaltic, and mantellic rocks. The present study reports on specific abiotic conditions that allow the precipitation of disordered dolomite, high Mg calcite, and high Ca magnesite at room temperature over time scales of hours to days. Here we show that an amorphous calcium magnesium carbonate (ACMC) phase accelerates dolomitization at room temperature. ACMC is initially precipitated by mixing a carbonate (HCO3–/CO32– = 1; pH ∼10.3 ≈ pKa2) alkaline solution with a Mg-Ca ionic solution (Mg molar fraction between 0 and 1). Then, time-resolved in situ Raman spectroscopy monitored the transformation of ACMC into Mg-rich carbonate minerals. The initial Mg molar fraction controlled both the reaction mechanism (e.g., nature of transient crystalline phases) and the kinetics. Nanosized crystallites with short-range order, called disordered dolomite CaMg(CO3)2, precipitated following a complex reaction pathway. First, nesquehonite (MgCO3·3H2O: nucleation time 2.5 h) and then disordered dolomite (CaMg(CO3)2: nucleation time 3.2 h) followed by monohydrocalcite (CaCO3·H2O: nucleation time 3.4 h) formed from ACMC transformation. Nesquehonite and monohydrocalcite are transient phases that nourish the slow precipitation of disordered dolomite, which reached a spectral equilibrium after 7 days of reaction. The direct transformation of ACMC into disordered dolomite was also measured. Our experimental results demonstrate that disordered dolomite precipitates at room temperature when an ideal Mg/Ca ratio, high carbonate alkalinity, and high ionic concentration are reached in abiotic systems. This result suggests the possibility of a physicochemical rather than biotic control on the formation of disordered dolomite at low temperature in several geosystems