High-pressure melting of carbonated eclogite and experimental constraints on carbon recycling and storage in the mantle

International audienc

Role of iron and reducing conditions on the stability of dolomite + coesite between 4.25 and 6 GPa - a potential mechanism for diamond formation during subduction

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Diffusion of Sr in fluorphlogopite determined by Rutherford backscattering spectrometry

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Melting of fluorphlogopite-plagioclase pairs at 1 atmosphere

International audienceMelting experiments on fluorphlogopite + plagioclase (An68) pairs have been performed at 1 atmosphere,as a function of temperature and run duration. Run products include, in addition to melt, spinel, forsterite and a newlyformed calcic plagioclase. The solidus was located between 1100 and 1150°C and the amount of melt producedincreases sharply at 1200°C. From this temperature, liquids are compositionally zoned between the two mineral interfaces,and the mica reacts out after sufficient time indicating the existence of a critical temperature of melting locatedbetween 1175 and 1200°C. These two temperature domains are also characterized by differences in plagioclasereaction textures, melt proportions and reaction rates.Textural and Sr isotopic studies of the plagioclases show that at all temperatures the first step of the melting reactionis characterized by congruent dissolution of the starting plagioclase followed by precipitation of the new plagioclase.At all temperatures the melt compositions are initially located on the intersection of the cotectic line and the startingplagioclase-fluorphlogopite tie-line. Below the critical temperature of melting these melts are metastable undercooledliquids. Above the critical temperature of melting there is an array of stable melt compositions located on the plagioclase-fluorphlogopite tie-line. In both cases the melt compositions evolve with time toward more albite-rich compositionsin conjunction with the new plagioclase precipitation. The system fluorphlogopite-plagioclase thus providesan example where liquid compositions vary with time although plagioclase-melt interface equilibrium is satisfied.This is due to the involvement of a solid solution in the reaction.Above the critical temperature of melting, the rate-controlling process for melting is chemical diffusion in the melt.Below the critical temperature of melting the rate-limiting step is the formation of the metastable undercooled melt.The sluggishness of the melting reaction in this temperature regime is explained by the difficulty of forming such amelt

Kinetics of melting of fluorphlogopite-quartz pairs at 1 atmosphere

International audienceMelting experiments on fluorphlogopite + quartz pairs have been performed at I atmosphere, as a functionof temperature and run duration (up to 2 months). Run products incJude, in addition to melt, the humite-groupmineral chondrodite, and enstatite. The solidus was located between 1100 and I 150°C and the amount of melt producedincreases sharply at 1250°C. From this temperature liquids are compositionally zoned between the two mineralinterfaces and liquid immiscibility occurs at 1300°C and above.The variations of the chemical composition of the melts in space and time indicates that melting proceeds by theindividual dissolution of each of the reactants. In the case of the mica, this reaction is kinetically controlled by masstransport in the melt. In the case of quartz, surface reactions are dominant, as shown by the steady silica enrichmentof the melt at the quartz interface with increasing run duration. Surface reactions in quartz are slower than atomic diffusionin the melt, and than nucleation and growth of cristobalite within the reacting quartz.These experiments also show that, when the equilibrium melting temperature is overstepped, there is not a uniqueliquid composition in the charge as long as both reactants are present. This state of disequilibrium persists until oneof the reactants is entirely consumed. The melt zonation provides the driving force for dissolution because it allowsconstant removal from the interface of the species liberated at the crystal interface. For minerals whose interface reactionsare rapid (such as the mica) dissolution is controlled by diffusion of species into the melt. This process may bethe cause of chemical disequilibrium between melt and residue, because dissolution rates can be faster than chemicalexchange between crystals and melt

Melting in the mantle in the presence of carbon: Review of experiments and discussion on the origin of carbonatites

Carbon emission at volcanic centers requires a constant balance between output (mostly by volcanism, either at plate boundaries or intraplate) and input (mostly at trench settings) of carbon from and to the Earth's mantle. The form of carbon that resides in the mantle is controlled by depth (pressure) and oxygen fugacity, the latter in turn depending on the depth and the concentration of iron in the mantle. In the shallow, lithospheric mantle, carbon is likely to be present in the oxidized form of CO2 (except under cratons where carbon is reduced to graphite or diamond). Below approximately 90 km, in the asthenosphere, the oxidized form of carbon is carbonate, either mineral or melt, depending on the thermal regime. At depths greater than approximately 150 km, the asthenospheric mantle is too reducing for carbon to stay in its oxidized form and only diamond is present, unless there is sufficient hydrogen to form reduced C–H fluids. Hence, the region located in the depth range of 90 to 150 km deep is where carbonatitic melts can most likely be produced and impregnate the surrounding mantle through metasomatism. The upper bound of this region is called the carbonate ledge. This limit prevents carbonate (either solid or molten) from ascending because of degassing and CO2 liberation. The lower bound is a redox front where redox melting (that is, melting caused by oxidation) may take place in an ascending portion of carbon-containing mantle. Carbonatite eruptions and presence of carbonate mineral inclusions in deep-seated diamonds provide evidence that these boundaries can be trespassed in some cases.An analysis of the experimental data that has bearing on silicate melting in the presence of carbon further shows that the carbonate ledge is a melting curve with a negative or flat Clapeyron (dP/dT) slope. In the carbonated ultrabasic (peridotite) systems, the carbonate ledge is located between ~ 2–3 GPa. The ledge divides the pressure–temperature space into a region of low-pressure silicate melt production, and a high-pressure region where carbonatites can be produced. Carbonatitic melts in equilibrium with mantle peridotite have compositions close to dolomitic (approximately equal amounts of Ca and Mg) with a general trend of becoming markedly more magnesian with increasing pressure. Calcic carbonatites may be stable at pressures 80 mol% CaCO3) melts can be produced, making the melting of carbonated eclogites an appealing scenario for the genesis of calcio-carbonatites in the Earth's mantle. Comparison with modeled pressure–temperature paths of subducted oceanic lithosphere shows that fusion of carbonated eclogite at depths shallower than 200 km should be expected for hot (Cascadian-type) subduction thermal regimes. On the other hand, in the case of cooler thermal regimes (Honshu-type, for instance), subducted carbonates may be stable to greater depths in Earth at trench settings, depending on the bulk composition of the system.Furthermore, high-pressure experiments show evidence of a continuum among carbonatitic, kimberlitic, melilititic, and basaltic liquids, for increasing melting degree of carbonated peridotite. This continuum has not been documented in the case of fusion of carbonated eclogite. It may be present, however, when certain sediments are fused, although the silicate melts are granitic to rhyodacitic instead of being kimberlitic in composition. Additional high-pressure work on phase relations in the simple binary system CaCO3–MgCO3 and specific focus on oxide solubility in the vapor phase have the potential to further clarify phase relations on complex silicate–carbonate systems at mantle conditions

Control of redox state and Sr isotopic composition of granitic magmas: a critical evaluation of the role of source rocks

International audienceThe current underlying assumption in most geochemical studies of granitic rocks is that granitic magmas reflect their source regions. However, the mechanisms by which source rocks control the intensive and compositional parameters of the magmas remain poorly known. Recent experimental data are used to evaluate the ‘source rock model’ and to discuss controls of (1) redox states and (2) the Sr isotopic compositions of granitic magmas.Experimental studies have been performed in parallel on biotite-muscovite and tourmaline-muscovite leucogranites from the High Himalayas. Results under reducing conditions ( = FMQ – 0·5) at 4 kbar and variable suggest that the tourmaline-muscovite granite evolved under progressively more oxidising conditions during crystallisation, up to values more than four log units above the FMQ buffer. Leucogranite magmas thus provide an example of the control of redox conditions by post-segregation rather than by partial melting processes.Other experiments designed to test the mechanisms of isotopic equilibration of Sr during partial melting of a model crustal assemblage show that kinetic factors can dominate the isotopic signature in the case of source rocks not previously homogenised during an earlier metamorphic event. The possibility is therefore raised that partial melts may not necessarily reflect the Sr isotopic composition of their sources, weakening in a fundamental way the source rock model

Bordures réactionnelles d'olivine dans un gabbro du Skaergaard : premiers résultats d'une approche expérimentale

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