Fuel for Plate Tectonics

International audienc

Water quantification in olivine and wadsleyite by Raman 2spectroscopy and study of errors and uncertainties

International audienceThe study of nominally anhydrous minerals with vibrational spectroscopy, despite its sensitivity, tends to produce large uncertainties (in absorbance or intensity) if the observed dispersion of the values arising from the anisotropy of interaction with light in non-cubic minerals is not assessed. In this study, we focused on Raman spectroscopy, which allows the measurement of crystals down to few micrometers in size in back-scattered geometry, and with any water content, down to 200 ppm by weight of water. Using synthetic hydrous single-crystals of olivine and wadsleyite, we demonstrate that under ideal conditions of measurement and sampling, the data dispersion reaches ±30% of the average (at 1σ) for olivine, and ±32% for wadsleyite, mostly because of their natural anisotropy. As this anisotropy is linked to physical properties of the mineral, it should not be completely considered as error without treatment. By simulating a large number of measurements with a 3D model of the OH/Si spectral intensity ratio for olivine and wadsleyite as a function of orientation, we observe that although dispersion increases when increasing the number of measured points in the sample, analytical error decreases, and the contribution of anisotropy to this error decreases. With a sufficient number of points (five to ten, depending on the measurement method), the greatest contribution to the error on the measured intensities is related to the instrument’s biases, and reaches 12 to 15% in ideal cases, indicating that laser and power drift corrections have to be carefully performed. We finally applied this knowledge on error sources (to translate data dispersion into analytical error) on olivine and wadsleyite standards with known water contents to build calibration lines for each mineral in order to convert the intensity ratio of the water bands over the structural bands (OH/Si) to water content. The conversion factor from OH/Si to ppm by weight of water (H2O) is 93108±24005 for olivine, 250868±45591 for iron-bearing wadsleyite, and 57546±13916 for iron-free wadsleyite, showing the strong effect of iron on the spectral intensities

Distribution and transport of hydrogen in the lithospheric mantle: A review

International audienceThe minerals constituting the Earth's upper mantle are nominally anhydrous silicates (NAMs). However they do contain hydrogen as a trace element, decorating point defects in their crystalline structure. Experimental petrology and mineralogy have quantified the maximum concentration under several compositional and thermodynamic conditions, but systematic studies on the hydrogen concentration in minerals from mantle-derived rocks have only recently been carried out. Here, we have compiled the distribution of hydrogen in upper mantle peridotite xenoliths, from which several conclusions can be drawn. NAMs from peridotite xenoliths contain a few ppm wt. H2O in their structure. From the current database, the hydrogen concentrations in olivine regularly increase with increasing depth. The amount of hydrogen in NAMs from peridotite xenoliths from subduction contexts is not higher than in other geological context for similar temperature and pressure conditions. The highest hydrogen concentrations is found in peridotitic olivines from cratonic mantle, and are likely due to the depth of origin. The increasing hydrogen concentration in olivine with increasing depth is likely controlled by the increase of H partitioning into olivine at the expense of orthopyroxene as imposed by a decrease in Al content in opx with depth. However, the sparse data could also indicate that the bulk hydrogen concentration slightly increases with depth > 150 km. In this case, it would suggest, locally (Udachnaya for example), a possible increase in water fugacity due to fluid saturation. Even if the most abundant mineral in mantle rocks is olivine, the bulk hydrogen concentration in peridotites is controlled by the amount of hydrogen stored in pyroxenes. However, hydrogen concentration in olivine remains crucial for consequences on physical properties such as rheology and electrical conductivity. Kinetics of hydrogen transport is reviewed and hydrous melt/fluid percolation appears necessary to homogenize the hydrogen distribution at km-scale. Sampling of natural rock specimens is currently biased (e.g., in favor of optically attractive samples) and needs improvement, which will be achieved by increasing sample diversity (all type of grain sizes, lithologies, geological settings) and size of rock samples, as well as advances in analytical techniques. Acquisition of high quality data will be achieved by studying the co-existing minerals in mantle specimens, exploring each sample by linear and mapping measurements, and using appropriate FTIR calibrations for polarized and unpolarized radiation

Mantle rain toward the Earth's surface: A model for the internal cycle of water

International audienceThe internal or deep water cycle controls the volume of the oceans at the surface of the Earth. The advent of subduction 2–3 billion years ago initiated the transport of water back to the Earth's interior. With one ocean mass injected into the deep mantle over the last 2–3 billion years, some mantle regions must have become saturated and thus turned into a deep source of water. The mantle transition zone (MTZ) between 410 and 660 km depths is unlikely to be a source of hydrous melt, because its minerals can integrate several thousand ppm of water. On the contrary, the low-velocity layer (LVL) lying above the 410 km-discontinuity is one such source. As proposed by the “Transition-Zone-Water-Filter Model”, the LVL is ubiquitously formed by the global uplift of the hydrous MTZ as a counter flow of subduction of slabs into deeper regions. The seismic signature of the LVL is compatible with the presence of 0.5–1% melt. This melt is produced by dehydration melting during upwelling of the mantle transition zone (MTZ) containing 2200(300) ppm wt H2O, which corresponds to 0.6 ocean mass stored today in the MTZ. Hydrous silicate melt can be gravitationally stable just above the 410 km discontinuity. We propose, here, that at the upper limit of the LVL it becomes buoyant, especially where the mantle is particularly hot and/or hydrous. Once it becomes buoyant, the melt can percolate rapidly upwards through the mantle. As a consequence, the olivine-bearing mantle (OBM) could be almost saturated in water, due to the presence of upwelling hydrous melt. On its path, the melt may be responsible for the seismic low-velocity zones at mantle depths of between 80 and 150 km. It could also be a source for refertilisation of the lithospheric mantle. Based on this model, there should be ~1.0 oceanic mass (OM) stored in the upper mantle today. Secular cooling of the mantle implies an increased capacity of the OBM minerals to store water. The related decrease of oceans' mass at the Earth's surface is estimated to ~20% per one billion years

Water storage capacity in olivine and pyroxene to 14 GPa: Implications for the water content of the Earth's upper mantle and nature of seismic discontinuities

International audienceExperiments were performed under water-saturated conditions in the MFSH (MgO-FeO-SiO2-H2O) and MFASH (MgO-FeO-Al2O3-SiO2-H2O) systems at 2.5, 5, 7.5 and 9 GPa, at temperatures from 1175 to 1400 °C and H2O initial abundance of 0.5-5 wt%. One experiment was performed at 13.5 GPa at a temperature of 1400 °C in the MFSH system. Water contents were analyzed by Fourier transform infrared spectroscopy. Results show that Al contents in olivine and pyroxene in equilibrium with an aluminous phase decrease significantly with increasing pressure and decreasing temperature. The incorporation of Al enhances water incorporation in olivine and pyroxene, but only at pressures of 2.5 and 5 GPa. At 7.5 GPa (i.e. 225 km depth) the pyroxene is monoclinic, indicating that in a hydrous mantle the orthoenstatite to clinoenstatite phase transition occurs at shallower depths than previously thought, which is more consistent with the Lehmann discontinuity than with the X discontinuity. The partitioning of water between pyroxene and olivine in the MFASH system decreases from a value of 2 at 2.5 GPa (80 km depth) to 0.9 at 9 GPa (270 km depth). At 13.5 GPa and 1400 °C, the water content of olivine is 1700±300 ppm wt H2O. The water partition coefficient between coexisting wadsleyite and olivine equals 4.7±0.7. We conclude that the water storage capacity of the upper mantle just above the 410 km discontinuity is of 1500±300 ppm wt H2O. If we assume that the Low Velocity Layer observed near 350 km is due to mantle melting, we can constrain the water content of the mantle at that depth to be ∼850±150 ppm wt H2O. This new value is four times higher than previous estimates for the mantle source of Mid Oceanic Ridge Basalts. Finally, comparison of the depth ranges of the L and X seismic discontinuities and the water storage capacity of the upper mantle suggests that the L-discontinuity (180-240 km) is concomitant with a kink in the water storage due to the orthorhombic to monoclinic phase transition in enstatite, while the X-discontinuity (240-340 km) coincides with a kink in the water storage capacity due to dehydration of garnet

Universal calibration of Raman spectroscopy for the analysis of volatiles in glasses of variable composition

International audienceThe amount and distribution of volatiles (water, carbon dioxide …) in magmas represent key parameters for the understanding of magma processes and dynamics within volcanic plumbing systems. Micro-Raman spectroscopy is an excellent technique for accurate determination of volatile contents in magmas, as it combines several advantages. The technique is non-destructive and requires minimal sample preparation before the analysis. Its high lateral and in-depth spatial resolution is crucial for the study of small objects and samples that are chemically and texturally heterogeneous at the small scale (microns). Moreover, the high confocality allows analysis of sample regions not exposed to the surface and 3D mapping. We present a universal calibration of Raman spectroscopy for quantification of volatiles in silicate glasses. The proposed method is based on internal calibration, i.e., on the correlation between the glass water content and the ratio between the areas of the water and silicate Raman bands. Synthetic glasses with variable major element compositions (basaltic, andesitic, rhyolitic, dacitic ..) bearing different H2O (up to 7 wt%) and CO2 contents are used as standard glasses. Natural silicate glasses, mainly in the form of melt inclusions, are used to test the goodness of the proposed method. In addition to quantification of volatiles in glass, in bubble-bearing melt inclusions we perform micro-Raman spectroscopy investigation of gas-bearing bubbles for accurate determination of total volatile contents in melt inclusions

Thermodynamics of water solubility and partitioning. In Water in nominally anhydrous minerals

International audienc

Pressure and temperature dependence of H solubility in forsterite: An implication to water activity in the Earth interior

International audienceWe performed experiments at 2.5, 6 and 9 GPa, and temperatures ranging from 1000 to 1400 °C in enstatite saturated conditions in theMgO–SiO2– H2O system using a multi-anvil apparatus, and determined the dependence of OH solubility in forsterite as a function of pressure and temperature. The abundance of OH in forsterite was determined using polarized Fourier transform infrared spectroscopy. The results show that OH solubility in forsterite increases with temperature only at 2.5 GPa. At 6 and 9 GPa, the OH solubility reaches a maximum at temperatures of 1175 to 1250 °C, depending on pressure, and then decreases at higher temperatures. Such behaviour is explained by the change of water activity in the fluid due to dissolution of silicate component. Using OH concentrationsmeasured on subsolidus samples we determined the thermodynamic parameters of OHincorporation in forsterite, such as change in internal energy and entropy.We find almost constantΔE of 37.1±6.7 kJ/mol andΔS of 82.8±6.8 J/mol/K. These parameters together with a ΔVsolids of 10.6 cm3/mol are used to calculate the fugacity of the real fluid (i.e. water+silicates) and compare it to the fugacity of pure water. Our results imply that estimates ofwater storage capacity atmantle depths greater than 80 km and above ~1250 °Cshould be substantially reduced compared to models that assume that water is a pure fluid

Equation of state and post-stishovite transformation of Al-bearing silica up to 100 Gpa and 3000 K

International audienc