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The type and kinetics of metamorphic C[O.sub.2]-producing processes in metacarbonate rocks is of importance to understand the nature and magnitude of orogenic C[O.sub.2] cycle. This paper focuses on C[O.sub.2] production by garnet-forming reactions occurring in calc-silicate rocks. Phase equilibria in the CaO-FeO-[Al.sub.2][O.sub.3]-Si[O.sub.2]-C[O.sub.2] -[H.sub.2]O (CFAS-C[O.sub.2]-[H.sub.2]O) system are investigated using P-T phase diagrams at fixed fluid composition, isobaric T-X(C[O.sub.2]) phase diagram sections and phase diagram projections in which fluid composition is unconstrained. The relevance of the CFAS-C[O.sub.2]-[H.sub.2]O garnet-bearing equilibria during metamorphic evolution of calc-silicate rocks is discussed in the light of the observed microstructures and measured mineral compositions in two representative samples of calc-silicate rocks from eastern Nepal Himalaya. The results of this study demonstrate that calc-silicate rocks may act as a significant C[O.sub.2] source during prograde heating and/or early decompression. However, if the system remains closed, fluid-rock interactions may induce hydration of the calc-silicate assemblages and the in situ precipitation of graphite. The interplay between these two contrasting processes (production of C[O.sub.2]-rich fluids vs. carbon sequestration through graphite precipitation) must be considered when dealing with a global estimate of the role exerted by decarbonation processes on the orogenic C[O.sub.2] cycle. Keywords Orogenic C[O.sub.2] cycle * Decarbonation processes * Calc-silicate rocks * Mixed-volatile phase diagram projections * Graphite * Himalaya, Introduction Metamorphic degassing from active collisional orogens supplies a significant amount of C[O.sub.2] to the atmosphere, playing a fundamental role in the long-term (>1 Ma) global carbon cycle (e.g. Kerrick [...]

Magmatic production on Earth is dominated by asthenospheric melts of basaltic composition that have mostly erupted at mid-ocean ridges. The timescale for segregation and transport of these melts, which are [...]

Subduction of the lithosphere at convergent-plate boundaries takes place asymmetrically--the subducted slab sinks downward, while the overriding plate moves horizontally (one-sided subduction). In contrast, global mantle convection models generally predict downwelling of both plates at convergent margins (two-sided subduction). We carried out two-dimensional (2-D) numerical experiments with a mineralogical-thermomechanical viscoelastic-plastic model to elucidate the cause of one-sided subduction. Our experiments show that the stability, intensity, and mode of subduction depend mainly on slab strength and the amount of weak hydrated rocks present above the slab. Two-sided subduction occurs at low slab strength (sin[[phi]] < 0.15, where [phi] is effective internal friction angle), regardless of the extent of hydration. In contrast, steady-state one-sided subduction requires a weak hydrated slab interface and high slab strength (sin[[phi]] > 0.15). The weak interface is maintained by the release of fluids from the subducted oceanic crust as a consequence of metamorphism. The resulting weak interplate zone localizes deformation at the interface and decouples the strong plates, facilitating asymmetric plate movement. Our work suggests that high plate strength and the presence of water are major factors controlling the style of plate tectonics driven by self-sustaining one-sided subduction processes. Keywords: subduction zones, water, numerical modeling, slab dehydration, fluid flow.

Recent work suggests that hydrated partially molten thermal-chemical plumes that originate from subducted slab as a consequence of Rayleigh-Taylor instability are responsible for the heterogeneous composition of the mantle wedge. We use a two-dimensional ultrahigh-resolution numerical simulation involving 10 x [10.sup.9] active markers to anticipate the detailed evolution of the internal structure of natural plumes beneath volcanic arcs in intraoceanic subduction settings. The plumes consist of partially molten hydrated peridotite, dry solid mantle, and subducted oceanic crust, which may compose as much as 12% of the plume. As plumes grow and mature these materials mix chaotically, resulting in attenuation and duplication of the original layering on scales of 1-1000 m. Comparison of numerical results with geological observations from the Horoman ultramafic complex in Japan suggests that mixing and differentiation processes related to development of partially molten plumes above slabs may be responsible for the strongly layered lithologically mixed (marble cake) structure of asthenospheric mantle wedges. Keywords: numerical modeling, subduction, volcanic arcs, mantle wedge structure, subducted crust melting.

Plagioclase rims around metastable kyanite crystals appear during decompression of high-pressure felsic granulites from the high-grade internal zone of the Bohemian Massif (Variscan belt of Central Europe). The development of the plagioclase corona is a manifestation of diffusion-driven transfer of CaO and [Na.sub.2]O from the surrounding matrix and results in isolation of kyanite grains from the quartz- and K-feldspar-bearing matrix. This process establishes Si-undersaturated conditions along the plagioclase-kyanite interface, which allow crystallization of spinel during low-pressure metamorphism. The process of the plagioclase rim development is modeled thermodynamically assuming local equilibrium. The results combined with textural observations enable estimation of equilibration volume and diffusion length for Na and Ca that extends ~400-450 and ~450-550 [micro]m, respectively, around each kyanite crystal. Low estimated bulk diffusion coefficients suggest that the diffusion rate of Ca and Na is controlled by low diffusivity of Al across the plagioclase rim. Keywords Equilibration volume * Coronal structures * Diffusion * Low-pressure granulites * Perple_X * Bohemian Massif

We present experimental results constraining water activity in [H.sub.2]O-[N.sub.2] fluids containing 40-90 mol% water at pressures of 6-13 kbar and temperatures of 680-840 [degrees]C. In the experiments, the displacement of the brucite dehydration equilibrium was used as a gauge of water activity. The experiments were performed in a conventional piston-cylinder apparatus, with NaCl pressure medium and silver azide, Ag[N.sub.3], as a source of nitrogen. Reversals of the dehydration reaction were used to bracket the equilibrium fluid compositions within 3 mol% [H.sub.2]O. Water activities were computed from the equilibrium brucite dehydration conditions in pure [H.sub.2]O as determined by Aranovich and Newton (1996) using thermodynamic data of Holland and Powell (1998). The experimentally derived activities were fit to a van Laar-type equation that reproduces our compositional data with a standard error of 1.6 mol% [H.sub.2]O: RT ln[[gamma].sub.1] = ([X.sub.2])[sup.2]W{[V.sup.0.sub1] [([V.sup.0.sub.2]).sup.2]/[([V.sup.0.sub.1] + [V.sup.0.sub.2])[([X.sub.1][V.sup.0.sub.1] + [X.sub.2][V.sup.0.sub.2]).sup.2]]} where [[gamma].sub.1] is the activity coefficient of [H.sub.2]O, [X.sub.i] is the mole fraction of end-member i (1 = [H.sub.2]O and 2 = [N.sub.2]), [V.sup.0.sub.i] is the molar volume of the pure end-member at the pressure (P) and temperature (T) of interest, and W is analogous to a regular solution parameter. The parameter W was fit as a function of pressure and temperature by the expression W = (A - BT)[1 - exp(-20P)] + C*[P.sup.0.3]T, with A = 40005 J, B = 51.735 J/K, C = 14.848 J/(K*[kbar.sup.0.3]), P in kbar and T in K. With these expressions, activity-concentration relations in [H.sub.2]O-[N.sub.2] fluids can be reconstructed in a broad P-T-X range using any equation of state (EOS) for pure [H.sub.2]O and [N.sub.2]. The activity-concentration relations are similar to the semi-empirical EOS of Duan et al. (2000) and the theoretical EOS of Churakov and Gottschalk (2002a), although the former somewhat underestimates activities within the experimental pressure range whereas the latter appears to overestimate activities of the components at pressure above 20 kbar.