Antigorite is considered as the most important source of water in subduction zones, playing a key role during arc magma genesis. Although, these magmas seem more oxidized than midoceanic ridge basalts (MORB), the possible inherent link between the oxidation state of arc magmas and serpentinite-derived hydrous fluids is still not well established. Here, we have performed dehydration experiments of natural antigorite serpentinite containing 5 weight percent (wt%) magnetite at 3 GPa and in a temperature range from 600 to 900 °C using a multianvil apparatus. These experiments aim to reproduce the different stages of H2O release, forming chlorite, olivine and orthopyroxene and water. Our experimental set up permits to preserve the intrinsic oxygen fugacity (fO2) of serpentinites during their dehydration. The new olivine and orthopyroxene which formed in equilibrium with antigorite, chlorite and magnetite have high XMg numbers setting up the oxygen fugacity to high values, between 3.0 and 4.1 log units above QFM (Quartz-Fayalite-Magnetite buffer). Hematite is observed concomitantly with high XMg in olivine, of 0.94-0.97, generally at low temperatures, below 800 °C, in coexistence with chlorite. Once the magnetite is destabilized, upon chlorite breakdown, above 800°C, the oxygen fugacity decreases to 3.7 due to the decrease of the XMg of silicates. This study demonstrates the highly oxidizing nature of the fluids released from antigorite dehydration. Thus, at high pressure and high temperature conditions, fO2-sensitive elements such as carbon and sulfur are expected to be mobilized under their oxidized form, providing an oxidizing context for arc magmas genesis and assuming that they are not completely reduced by their percolation through meta-gabbro, meta-basalts and meta-sediments.

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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 CaCO 3 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.

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+).

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.