Abstract. In this study we report the synthesis of single crystals of burbankite, Na3Ca2La(CO3)5, at 5 GPa and 1073 K. The structural evolution, bulk modulus and thermal expansion of burbankite were studied and determined by two separate high-pressure (0–7.07(5) GPa) and high-temperature (298–746 K) in situ single-crystal X-ray diffraction experiments. The refined parameters of a second-order Birch–Murnaghan equation of state (EoS) are V0= 593.22(3) Å3 and KT0= 69.8(4) GPa. The thermal expansion coefficients of a Berman-type EoS are α0= 6.0(2) ×10-5 K−1, α1= 5.7(7) ×10-8 K−2 and V0= 591.95(8) Å3. The thermoelastic parameters determined in this study allow us to estimate the larger density of burbankite in the pressure-temperature range of 5.5–6 GPa and 1173–1273 K, with respect to the density of carbonatitic magmas at the same conditions. For this reason, we suggest that burbankite might fractionate from the magma and play a key role as an upper-mantle reservoir of light trivalent rare earth elements (REE3+).
Les serpentinites sont les roches produites par l’hydratation de la peridotite au niveau du plancher oceanique. L’antigorite est la phase de haute temperature et haute pression appartenant au groupe mineral des serpentines, pouvant contenir dans sa structure jusqu’a 13 wt% H2O, et permet ainsi le transfert de quantites considerables d’eau dans le manteau, a travers les processus de subduction. Sa destabilisation est fonction du chemin thermique emprunte par la plaque plongeante. Durant cette these nous avons etudie deux cas de figure pour la deshydratation de l’antigorite menant soit a la liberation des fluides dans le coin mantellique et a la production des magmas d’arc, soit au transfert de l’eau a plus grandes profondeurs).Dans un premier temps, des experiences de deshydratation d’antigorite naturelle ont ete conduites sur la presse multi-enclumes a 3 GPa et entre 600 et 900°C. Les conditions oxydantes ou bien reductrices ont ete controlees par le dispositif experimental (four en graphite ou en chromite de lanthane). Cette etude a permis de caracteriser les produits de deshydratation de l’antigorite dans un systeme chimique representatif des systemes naturels ainsi que de contraindre l’etat redox des reactions associees. En effet, les resultats mis en avant par cette etude montrent une fO2 equivalente au tampon Quartz-Magnetite-Fayalite (QFM) +5. Un tel potentiel oxydant des fluides issus de la deshydratation de l’antigorite soutient l’hypothese de l’oxydation de la source mantellique des magmas d’arcs, presentant des rapports Fe3+/Fetotal plus eleves que les basaltes de ride medio-oceanique par exemple.Dans un second temps, nous nous sommes interesses aux modalites de transfert de l’eau dans le manteau profond. L’antigorite naturelle a cette fois ete destabilisee a de plus fortes pressions allant de 6.5 a 10 GPa pour des temperatures comprises entre 500 et 850°C. Ces resultats experimentaux, ainsi qu’une analyse geometrique des relations de phases dans le system FMASH selon la methode de Shreinemaker, ont mis en avant des modifications dans le diagramme de phase pour un systeme ultramafique hydrate en comparaison des etudes precedentes. En effet, la phase A est communement decrite comme le produit de destabilisation de l’antigorite a haute pression, tandis que la phase E n’apparait qu’a des profondeurs plus importantes. Nos resultats suggerent, dans le systeme naturel enrichi en aluminium et en fer, une stabilite continue des phases hydratees, suivant la transition antigorite > phase E > phase A pour des temperatures inferieures a 750°C. Cette etude a egalement permis d’affiner les estimations des quantites d’eau pouvant etre stockees dans les assemblages de mineraux hydrates stables dans la lithosphere plongeante (slab). Dans le cas des plaques plongeantes relativement froides (<750°C a 8-10 GPa) le transport de l’eau par le biais des « Dense Hydrous Magnesium Silicates » (DHMS) phase A et phase E soutient l’hypothese de l’hydratation de la zone de transition dans le manteau.
Abstract We report the synthesis, at 7 GPa and 923 K, and the thermoelastic characterization, up to 16 GPa and 850 K, of a single crystal of Mg-sursassite, Mg5Al5Si6O21(OH)7. In situ high-pressure and high-temperature single-crystal diffraction allowed the study of structural variation at non-ambient conditions and the determination of bulk elastic properties. The refined parameters of a second-order Birch-Murnaghan equation of state (BM-II EoS) are V0 = 446.02(1) Å3 and KT0 = 135.6(7) GPa. The thermal expansion coefficients of a Berman-type EoS are α0 = 3.14 (5) × 10−5 K−1, α1 = 2.50(16) × 10−8 K−2, and V0 = 445.94(3). For comparison, the P-V EoS is determined for a natural sursassite sample, ideally Mn4Al6Si6O22(OH)6. The refined parameters of BM-II EoS [V0 = 470.2(3) Å3, KT0 = 128(4) GPa] indicate that composition has a minimal effect on elastic properties. The similarity of density and bulk properties of Mg-sursassite if compared to olivine and other anhydrous mantle minerals suggests that this phase could be overseen by geophysical methods.
We report an overview of the crystal structures of carbonates determined ab-initio with X-ray single crystal diffraction techniques at mantle conditions. The determined crystal structures of high-pressure polymorphs of CaCO3 have revealed that structures denser than aragonite can exist at upper and lower mantle pressures. These results have stimulated the computational and experimental research of thermodynamically stable polymorphs. At lower mantle conditions, the carbonates transform into new structures featuring tetrahedrally coordinated carbon. The identification of a systematic class of carbonates, nesocarbonates, cyclocarbonates, and inocarbonates reveals a complex crystal chemistry, with analogies to silicates. They provide fundamental input for the understanding of deep carbonatite melt physical properties. The possible polymerization of carbonate units will affect viscosity, and the reduced polymerization in crystal structures as a function of oxidation state could suggest that also oxidation state may affect the mobility of deep carbonatitic magmas. Finally, we report two high-pressure structures of mixed alkali carbonates, which reveal that these compounds may form wide solid solutions and incorporate a sensible amount of vacancies, which would allow incorporation of high-strength elements and therefore play an important role for geochemical element partitioning in the mantle.
CaSiO3 polymorphs are abundant in only unique geological settings on the Earth’s surface and are the major Ca-bearing phases at deep mantle condition. An accurate and comprehensive study of their density and structural evolution with pressure and temperature is still lacking. Therefore, in this study we report the elastic behavior and structural evolution of wollastonite and CaSiO3-walstromite with pressure. Both minerals are characterized by first order phase transitions to denser structures. The deformations that lead to these transformations allow a volume increase ofthe bigger polyhedra, which might ease cation substitution in the structural sites of these phases. Furthermore, their geometrical features are clear analogies with those predicted and observed for tetrahedrally-structured ultra-high-pressure carbonates, which are unfortunately unquenchable. Indeed, wollastonite and CaSiO3-walstromite have a close resemblance to ultra-high-pressure chain- and ring-carbonates. This suggests a rich polymorphism also for tetrahedral carbonates, which might increase the compositional range of these phases, including continuous solid solutions involving cations with different size (Ca vs. Mg in particular) and important minor or trace elements incorporation.
From the Mid-Oceanic-Ridge to the subduction trench, hydration of peridotite minerals in the upper part of the oceanic lithosphere produces hydrous phases such as serpentine. Because of its high-water content (13 wt% H2O) this mineral family is of particular interest for water fluxes. Depending on the thermal path followed by the lithosphere while sinking into the mantle, antigorite destabilization can either lead to fluid release in the mantle wedge or water transfer to deeper levels. During this thesis we conducted experimental investigations of antigorite dehydration in the framework of these two scenarios.First, we investigated antigorite dehydration under conditions relevant to slab water release, known to trigger partial melting and to generate arc magmatism. Multi-anvil experiments were conducted on a natural serpentinite sample, at 3 GPa and between 600 and 900°C under different redox conditions. We were able to constrain phase assemblages produced by antigorite dehydration as well as the fO2 of such reactions to 5 units above the FMQ (Fayalite-Magnetite-Quartz buffer). These results support the oxidizing character of slab released fluids, that could explain the oxidized character of arc magmas compared to Mid-Oceanic-Ridge basalts or Oceanic-Island basalts.The second experimental work conducted during this thesis allowed to refine phase equilibria involving antigorite and the Dense Hydrous Magnesium Silicates (DHMS) phase A and phase E, in a realistic chemical composition for hydrated ultramafic system. Antigorite destabilization was performed between 6.5 and 10 GPa, for temperatures in the range phase E>phase A for the aluminous and iron-rich hydrated peridotite system. This study allowed the refinement of water budgets that can be stored in relatively cold slabs (<750°C at 8-10 GPa), supporting the hypothesis of water survival down to the transition zone.
