Xiaozhi Yang

Teneurs en eau et compositions isotopiques de H, O et Li des minéraux des granulites de la croûte continentale inférieure de l'Est de la Chine: Pour la première fois, une étude par spectroscopie infrarouge à transformée de Fourrier et par microsonde ionique des minéraux majeurs de la croûte inférieure et des péridotites mantelliques a été entreprise afin de mieux caractériser les mécanismes d’incorporation et les teneurs en eau de ces minéraux nominalement anhydres, et de déterminer leurs compostions isotopiques en H, O, et Li pour retracer les échanges latéraux et horizontaux de l’eau dans la lithosphère continentale profonde. Les résultats montrent que: (1) Les minéraux nominalement anhydres, comme les pyroxènes et plagioclase, dans les granulites de la croûte inférieure contiennent de l’eau en trace essentiellement sous forme hydroxyles et accessoirement sous forme moléculaire, avec des concentrations (exprimées en poids H2O) allant de 200 à 2330 ppm pour les clinopyroxènes, de 60 à 1875 ppm pour les orthopyroxènes, de 65 à 900 ppm pour les plagioclases. Les teneurs calculées pour chaque roche totale d’après sa composition minéralogique et la teneur en eau des minéraux varient de 155 à 1100 ppm. (2) Les teneurs en H2O des minéraux majeurs et en roche totale de la croûte continentale inférieure sont manifestement plus élevées que celles du manteau lithosphérique sous-jacent, suggérant des variations verticales de la quantité d’H2O dans la lithosphère continentale profonde. Un tel contraste peut affecter de façon notable le comportement rhéologique de la lithosphère continentale. (3) Les rapports isotopiques de l’oxygène des pyroxènes étudiés, et probablement les roches totales, exprimés en [delta]18OSMOW , vont de ~ 4,5 à 12,5‰. Ceci indique la contribution de matériaux recyclés de la croûte continentale durant la pétrogenèse des échantillons ayant un TM18O élevé. (4) Les minéraux de la granulites sont caractérisés par des valeurs de élevées TMD, avec des valeurs de -80~-10‰ exprimées en [delta]DSMOW; les minéraux sont le plus souvent en équilibre les uns avec les autres lorsque l’on considère leurs compositions isotopiques moyennes. (5) Les compositions isotopiques du Lithium mesurée dans les minéraux de nos échantillons de granulites, exprimées en [delta]7Li par rapport à Lsvec, varient de -13 à +4.7 ‰. Ces valeurs sont donc pour la plus part inférieures à celles mesurées sur les MORB (2 – 6‰). La dispersion des valeurs reflètent l’hétérogénéité de la source des granulites, et les valeurs bassent resultent probablement de la perte par diffusion de Li pendant la mise en place des liquides silicatées provenant du manteau dans la croûte inférieure. (6) La grande hétérogénéité des teneurs en eau et en Lithium, et des compositions isotopiques de H-O-Li indique l’absence de circulation de fluide pervasive au travers de la croûte inférieure, qui aurait pour effet de supprimer les hétérogénéités à petite échelle et de les diminuer fortement à grande échelle

The electrical structure of Earth’s interior, resolved by using geophysical surveys, is key to understanding its composition, dynamics and relevant properties. Electrically anomalous zones in the upper mantle have been frequently observed, yet the origin remains debated. The geophysically imaged electrical anomalies cannot be properly interpreted if the constraints on the mantle materials from petrological and geochemical surveys and mineral physics experiments are not combined. Studying mantle samples has revealed widespread heterogeneities in their mineral constituents, elemental compositions and thermodynamic properties, in addition to the local occurrence of melts and fluids. The heterogeneities are macroscale, ranging on the levels of meters to kilometers. Four conductive candidates have been identified for the electrical anomalies by using laboratory experiments under mantle conditions, including olivine owing to its oxidized state (but not water), lithologies (such as pyroxenites, eclogites and phlogopite-bearing assemblages due to enriched Fe, water and/or F), partial melt and aqueous fluids. Such materials are able to cause electrical anomalies in a variety of settings that are geophysically detectable, if connected forms, rational fractions and/or suitable temperature and redox states are spatially maintained along certain direction(s). Hydrous minerals except phlogopite (within their stability fields) and non-silicate minerals such as graphite, sulfides and carbonates are usually hard to produce mantle electrical anomalies. Mantle macroscale heterogeneities cause heterogeneous electrical structures. Geophysically imaged electrical anomalies in the upper mantle are intimately related to its petrological and geochemical evolution.

A machine learning study on the age and major and trace elements of a large suite of basalts indicates the presence of oxidized primitive arc basalts since ~0.8 Ga. This may eliminate the possibility of a genetic redox link between the sub-arc mantle and the Neoproterozoic oxygenation event.

Quartz is an important mineral constituent of the continental crust, which commonly contains trace amounts of water as hydroxyl in the lattice. Despite considerable efforts to document the roles of pressure, temperature, and chemistry on water storage in quartz, the effect of redox state, a key thermodynamic factor in Earth sciences, has not been well established. We assessed the redox effect on water solubility in quartz at 2 and 10 kbar and 800°C, by equilibrating two natural gem‐quality crystals with water over a range of redox conditions. Recovered samples show infrared absorption bands at 3,100–3,600 cm⁻¹. Dehydration experiments at 1200°C (room pressure) suggest that the two bands at ∼3,197 and 3,293 cm⁻¹ are linked to Si‐O overtones, with the other bands to hydroxyl. The water solubility, ∼3.7–32.2 ppm wt. H₂O, is redox‐independent, differing from other minerals such as feldspar, pyroxenes, garnet, rutile, and olivine in which the water solubility is significantly redox‐dependent. This provides evidence that the water incorporation in quartz is not controlled by polyvalent elements and/or other redox‐linked mechanisms. Considering the roles of Al, Li, and B in incorporating water and its low solubility over remarkable variations in the abundances of those elements in most natural samples, quartz is not a major water carrier compared to other nominally anhydrous minerals (NAMs) in the crust. The partitioning of water between quartz and other crustal NAMs such as feldspar, pyroxenes, rutile, and garnet is redox‐influenced, and is unlikely to be constant in the crust.

High conductivity anomalies in the shallow mantle are frequently attributed to minor partial melt (basalt or carbonatite) in the olivine-dominated peridotites. Conductivity of a melt-mineral mixture depends on the configuration of melt that could be affected by grain size of the constitutive mineral(s), but this has rarely been explored. Here, we provide experimental evidence using a conductive carbonatite analog and olivine that the bulk conductivity decreases systematically with increasing olivine grain size. The required amount of melt for producing the geophysically resolved high conductivities in the asthenosphere is much greater than previously assumed. We suggest that the effect of partial melt on many conductive regions in the asthenosphere is small. Instead, the electrical anomalies (especially those away from mid-ocean ridges) originate more likely from subsolidus solid assemblages in the upper mantle. This reconciles well the geochemical and petrological constraints of the shallow mantle with its geophysically determined electrical properties.

Electrical conductivity of water-rich omphacite and garnet in eclogite was measured at 1 GPa and 200–800°C in a piston cylinder press and by a Solartron-1260 impedance/gain-phase analyzer at 10⁶-1 Hz frequency. The water content of pre-annealed omphacite and garnet was 775–2,000 and 705–1,460 ppm H₂O, respectively. Sample chemistry and water contents remained unchanged during conductivity runs. At otherwise identical conditions, the conductivity of both minerals increases with both temperature and water content, and the water content exponent is ∼1.45 and 1.12 for omphacite and garnet, respectively. The activation enthalpy is ∼70 kJ/mol for omphacite and 84 kJ/mol for garnet and is broadly independent of sample water content. Combining with previous work, the conductivity dependence of omphacite on water content differs between water-rich and water-poor conditions, due to different types and mobility of water in samples that are closely related to its incorporation mechanism; in contrast, the conductivity dependence of garnet with a similar type of water is comparable over a wide range of water contents. The estimated bulk conductivity of eclogite at water-rich conditions is very high, up to ∼0.01–0.1 S/m at 600–900°C. Geophysically resolved high resistivity of subducting crusts at 70–120 km depth suggests extremely low water contents of omphacite and garnet in the eclogitized slab. The data provide support to the model based on omphacite and garnet conductivity at water-poor conditions that the amount of water recycled by crust subduction to the deep mantle is probably limited.

Garnet, an important mineral component in Earth's interior, usually contains minor water as structural OH, but experimental data on OH incorporation in the mineral as a function of chemical and thermodynamic parameters remain rare. In particular, the role of oxygen fugacity (fO₂) that is variable in the Earth has not been well assessed. We examine the effects of fO₂ on the incorporation of OH at 1–3 GPa and 800–1000°C, by H‐annealing gem‐quality spessartine‐rich, grossular‐rich, and pyrope‐rich garnets. For each type of annealed garnet, the OH peak positions and spectral shapes are the same, suggesting the same OH incorporation mechanism(s). Under identical conditions, spessartine‐rich and grossular‐rich garnets are rich in OH, with solubility orders of magnitudes greater than for pyrope‐rich garnets. Over the studied conditions, the solubility of OH increases with pressure and temperature but decreases with fO₂. The negative correlation of fO₂ and OH solubility is probably due to the Fe³⁺ to Fe²⁺ reduction at more reduced conditions and the coupled OH incorporation in the structure. The effects of fO₂ on OH solubility are more significant in Fe‐rich garnets. The ascertained small OH solubility in peridotite garnets, together with comparisons with relevant studies, suggest that mantle garnet is unlikely to be a major water carrier. In combination with literature‐reported positive correlation of fO₂ and OH solubility in olivine, the partition coefficient of OH between garnet and olivine in the upper mantle is expected to be redox‐dependent.