Isabelle Martinez

Mechanism of wollastonite carbonation deduced from micro- to nanometer length scale observations

Abstract The microstructural evolution of CaSiO3 wollastonite subjected to carbonation reactions at T = 90 °C and pCO2 = 25 MPa was studied at three different starting conditions: (1) pure water; (2) aqueous alkaline solution (0.44 M NaOH); and (3) supercritical CO2. Scanning and transmission electron microscopy on reacted grains prepared in cross-section always revealed unaltered wollastonite cores surrounded by micrometer-thick pseudomorphic silica rims that were amorphous, highly porous, and fractured. The fractures were occasionally filled with nanometer-sized crystals of calcite and Ca-phyllosilicates. Nanoscale chemical profiles measured across the wollastonite-silica interfacial region always revealed sharp, step-like decreases in Ca concentration. Comparison of the Ca profiles with diffusion modeling suggests that the silica rims were not formed by preferential cation leaching (leached layer), but rather by interfacial dissolution-precipitation. Extents of carbonation as a function of time were determined by quantitative Rietveld refinement of X-ray diffractograms performed on the reacted powders. Comparing the measured extents of carbonation in water (condition 1) with kinetic modeling suggests that carbonation was rate-controlled by chemical reactions at the wollastonite interface, and not by transport limitations within the silica layers. However, at conditions 2 and 3, calcite crystals occurred as a uniform surface coating covering the silica layers, and also within pores and cracks, thereby blocking the connectivity of the originally open nanoscale porosity. These crystals ultimately may have been responsible for controlling transport of solutes through the silica layers. Therefore, this study suggests that pure silica layers were intrinsically non-passivating, whereas silica layers became partially passivating due to the presence of calcite crystallites

Thermodynamic properties of carbonates at high pressures from vibrational modelling

Simplified vibrational densities of states for five different carbonates are constructed using measured IR and Raman spectra. From the spectroscopic models we calculate thermodynamic and thermoelastic properties of magnesite, calcite, aragonite, dolomite, and siderite. The effects of temperature and pressure on the vibrational frequencies are explicitly introduced into the computations. These spectroscopic models provide high level agreement with the measured values of entropy and heat capacity (within +/- 2%), with the exception of aragonite (within +/- 5% above 600 K) due to its breakdown to calcite: For the molar volumes the agreement is within +/- 0.5 %. The Gibbs free energies of each mineral are then computed in order to obtain pressure and temperature equilibrium conditions for different chemical reactions involving carbonates. Comparing the predicted phase diagrams with those experimentally determined provides an additional constraint on the validity of spectroscopic models and in the values of formation enthalpies.

Impact-induced phase transformations at 50–60 GPa in continental crust: an EPMA and ATEM study

Strongly shocked gneiss fragments from the allochthonous polymict breccia of the 23 Ma old Haughton impact crater were studied by optical microscopy, electron probe microanalysis (EPMA) and analytical transmission electron microscopy (ATEM). The study is focused on the substantiation of shock-induced phase transformations in rock-forming minerals, and their implications for post-shock thermal regimes and impact age dating. n nIn the gneiss fragments, the original layering is largerly preserved, although all the minerals have experienced dramatic phase transformations. Chemically, the various shock-generated decomposition regions still reflect the pre-shock mineralogy. However, significant heterogeneities exist on the 10–100 μm scale. Four different regions of decomposition products can be distinguished (EPMA): (1) shocked biotite-rich regions with strong variations in Si, Al, K, Fe and Mg on the 10 μm scale, (2) shocked quartz with about 3% Al and K and traces of Fe and Mg, (3) a region with Al/>Si ≈ 1, which does not correspond to any pre-shock mineral, and (4) shocked K-feldspar showing a significant deficit in K, and traces of Fe and Mg. On the nanometer scale, a uniform, bimodal reaction pattern emerges: all minerals are transformed to new crystals embedded in a silica-rich glass. The shock-generated crystals are: (1) spinel in shocked biotite, (2)α-quartz, cristobalite, and coesite in shocked quartz, (3) an unknown polymorph of corundum (Al2O3) in feldspar and in the Al/Si ≈ 1 region, and (4) mullite in association with SiO2 glass in shocked sillimanite grains. All crystals range in size between 0.05 and 1.0 μm. Taking into account estimated durations and pressure-temperature conditions for shock-wave passage and post-shock temperatures, the reactions observed can be ascribed to high temperatures during and after pressure release. The different mineral-glass parageneses suggest temperatures in excess of 1400°C. Moreover, given the small size of the newly formed minerals, very rapid cooling down to 1200°C is required.

Experimental investigation of silicate-carbonate system at high pressure and high temperature

Melting and subsolidus relations in the (Mg,Fe)SiO3-(M,Fe)CO3, (Mg,Fe)(2)SiO4-(Mg,Fe)CO3, and (Mg,Fe)O-(Me,Fe)CO2 systems have been investigated at 14, 15, 16 and 25 GPa, 1973 K and 2173 K, using a 1000 t uniaxial multi anvil split sphere apparatus. The iron-magnesium partition coefficients between magnesite and silicates or oxides have been measured in subsolidus assemblages. Iron is always partitioned preferentially in the silicate and oxide phases, the order of increasing partitioning being pyroxene, olivine, silicate perovskite, wadsleyite and magnesiowustite. A thermodynamic model of iron-magnesium distribution between magnesite and all these phases, based on Gibbs free energy minimization, is established. Melting of pyroxene-magnesite and olivine-magnesite pseudo binary systems is eutectic, with eutectic points close to 1973 K and 60 mol % carbonate at 15 GPa in both systems. In the more complex mantle system, it is likely that such melts would form in the transition zone by heating and homogenization of deep subducted carbonates. The melts formed in the olivine-carbonate system are characterized by high Mg+Fe/Si ratios and thus unlikely to be primary kimberlitic magmas, a conclusion in agreement with previous studies in the peridotite-CO2 system, On the other hand, the observed pyroxene-magnesite melts formed at transition zone conditions have Mg+Fe/Si ratios that are comparable to those of natural kimberlites, suggesting that melting of carbonated pyroxenites at high pressures could be a source of kimberlitic magmas.

Electron energy-loss spectroscopy of silicate perovskite-magnesiowüstite high-pressure assemblages

Abstract Silicate perovskite-magnesiowüstite assemblages synthesized from natural olivine in the multianvil press and diamond-anvil cell were studied by electron energy-loss spectroscopy (EELS). Spectra of crystalline silicate perovskite, and its post-amorphization phase, as well as magnesiowüstite were collected at the Fe and Si L2,3 edge, and in the low loss (<50 eV) domain. The technique of line spectra ensuring very low beam doses allows good quality spectra to be collected from crystalline perovskite prior to amorphization and permits characterization of coexisting crystals of perovskite and magnesiowüstite. Spectra at the Si L2,3 edge show that the beam-induced amorphization of silicate perovskite is accompanied by a change from sixfold to fourfold oxygen coordination of silicon atoms. Spectra at the Fe L2,3 edge show that Fe2+ is the major form of Fe in olivine, ringwoodite, and magnesiowüstite, whereas Fe3+ is dominant in crystalline silicate perovskite and its amorphization products. In magnesiowüstite and silicate perovskite observed in contact in these samples, Fe3+ is strongly partitioned into the silicate phase. Careful experimental substraction of zero-loss peak by off Bragg acquisition of electron energyloss spectra allows good quality low loss spectra to be collected from crystalline silicate perovskite and magnesiowüstite. In magnesiowüstite, interband transitions are well characterized, leading to a measured gap of 7.8 eV, in agreement with previous theoretical calculations. Interband transitions at 10 eV and 12.5 eV are also well resolved in crystalline silicate perovskite, leading to a gap of about 9.5 eV.

Massive production of abiotic methane during subduction evidenced in metamorphosed ophicarbonates from the Italian Alps

Alteration of ultramafic rocks plays a major role in the production of hydrocarbons and organic compounds via abiotic processes on Earth and beyond and contributes to the redistribution of C between solid and fluid reservoirs over geological cycles. Abiotic methanogenesis in ultramafic rocks is well documented at shallow conditions, whereas natural evidence at greater depths is scarce. Here we provide evidence for intense high-pressure abiotic methanogenesis by reduction of subducted ophicarbonates. Protracted (≥0.5–1u2009Ma), probably episodic infiltration of reduced fluids in the ophicarbonates and methanogenesis occurred from at least ∼40u2009km depth to ∼15–20u2009km depth. Textural, petrological and isotopic data indicate that methane reached saturation triggering the precipitation of graphitic C accompanied by dissolution of the precursor antigorite. Continuous infiltration of external reducing fluids caused additional methane production by interaction with the newly formed graphite. Alteration of high-pressure carbonate-bearing ultramafic rocks may represent an important source of abiotic methane, with strong implications for the mobility of deep C reservoirs.