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We experimentally investigated the dissolution of forsterite, enstatite and magnesite in graphite-saturated COH fluids, synthesized using a rocking piston cylinder apparatus at pressures from 1.0 to 2.1 GPa and temperatures from 700 to 1200 °C. Synthetic forsterite, enstatite, and nearly pure natural magnesite were used as starting materials. Redox conditions were buffered by Ni-NiO-HO (ΔFMQ = − 0.21 to − 1.01), employing a double-capsule setting. Fluids, binary HO-CO mixtures at the P, T, and fO conditions investigated, were generated from graphite, oxalic acid anhydrous (HCO) and water. Their dissolved solute loads were analyzed through an improved version of the cryogenic technique, which takes into account the complexities associated with the presence of CO-bearing fluids. The experimental data show that forsterite + enstatite solubility in HO-CO fluids is higher compared to pure water, both in terms of dissolved silica ( mSiO = 1.24 mol/kg versus mSiO = 0.22 mol/kg at P = 1 GPa, T = 800 °C) and magnesia ( mMgO = 1.08 mol/kg versus mMgO = 0.28 mol/kg) probably due to the formation of organic C-Mg-Si complexes. Our experimental results show that at low temperature conditions, a graphite-saturated HO-CO fluid interacting with a simplified model mantle composition, characterized by low MgO/SiO ratios, would lead to the formation of significant amounts of enstatite if solute concentrations are equal, while at higher temperatures these fluid, characterized by MgO/SiO ratios comparable with that of olivine, would be less effective in metasomatizing the surrounding rocks. However, the molality of COH fluids increases with pressure and temperature, and quintuplicates with respect to the carbon-free aqueous fluids. Therefore, the amount of fluid required to metasomatize the mantle decreases in the presence of carbon at high P- T conditions. COH fluids are thus effective carriers of C, Mg and Si in the mantle wedge up to the shallowest level of the upper mantle. [ABSTRACT FROM AUTHOR]

To constrain the thermodynamics and melting relations of the siderite–magnesite (FeCO3–MgCO3) system, 27 piston cylinder experiments were conducted at 3.5 GPa and 1170–1575 °C. Fe-rich compositions were also investigated with 13 multi-anvil experiments at 10, 13.6 and 20 GPa, 1500–1890 °C. At 3.5 GPa, the solid solution siderite–magnesite coexists with melt over a compositional range of XMg (=Mg/(Mg + Fetot)) = 0.38–1.0, while at ≥10 GPa solid solution appears to be complete. At 3.5 GPa, the system is pseudo-binary because of the limited stability of siderite or liquid FeCO3, Fe-rich carbonates decomposing at subsolidus conditions to magnetite–magnesioferrite solid solution, graphite and CO2. Similar reactions also occur with liquid FeCO3 resulting in melt species with ferric iron components, but the decomposition of the liquid decreases in importance with pressure. At 3.5 GPa, the metastable melting temperature of pure siderite is located at 1264 °C, whereas pure magnesite melts at 1629 °C. The melting loop is non-ideal on the Fe side where the dissociation reaction resulting in Fe3+ in the melt depresses melting temperatures and causes a minimum. Over the pressure range of 3.5–20 GPa, this minimum is 20–35 °C lower than the (metastable) siderite melting temperature. By merging all present and previous experimental data, standard state (298.15 K, 1 bar) thermodynamic properties of the magnesite melt (MgCO3L) end member are calculated and the properties of (Fe,Mg)CO3 melt fit by a regular solution model with an interaction parameter of −7600 J/mol. The solution model reproduces the asymmetric melting loop and predicts the thermal minimum at 1240 °C near the siderite side at XMg = 0.2 (3.5 GPa). The solution model is applicable to pressures reaching to the bottom of the upper mantle and allows calculation of phase relations in the FeO–MgO–O2–C system. [ABSTRACT FROM AUTHOR]

The study of the composition of primary melts during anatexis of high-pressure granulitic migmatites is relevant to understand the generation and differentiation of continental crust. Peritectic minerals in migmatites can trap droplets of melt that forms via incongruent melting reactions during crustal anatexis. These melt inclusions commonly crystallize and form nanogranitoids upon slow cooling of the anatectic terrane. To obtain the primary compositions of crustal melts recorded in these nanogranitoids, including volatile concentrations and information on fluid regimes, they must be remelted and rehomogenized before analysis. A new occurrence of nanogranitoids was recently reported in garnets of mylonitic metapelitic gneisses (former high pressure granulitic migmatites) at the bottom of the prograde metamorphic sequence of Jubrique, located on top of the Ronda peridotite slab (Betic Cordillera, S Spain). Nanogranitoids within separated chips of cores and rims of large garnets from these migmatites were remelted at 15 kbar and 850, 825 or 800 °C and dry (without added H2O), during 24 h, using a piston cylinder apparatus. Although all experiments show glass (former melt) within melt inclusions, the extent of rehomogenization depends on the experimental temperature. Experiments at 850–825 °C show abundant disequilibrium microstructures, whereas those at 800 °C show a relatively high proportion of rehomogenized nanogranitoids, indicating that anatexis and entrapment of melt inclusions in these rocks likely occurred at pressures ≤1.5 GPa and temperatures close to 800 °C. Electron microprobe and NanoSIMS analyses show that experimental glasses are leucogranitoid and peraluminous, though define two distinct compositional groups. Type I melt inclusions correspond to K-rich, Ca- and H2O-poor leucogranitic melts, whereas type II melt inclusions represent K-poor, Ca- and H2O-rich granodioritic to tonalitic melts. Type I and II melt inclusions are found in most cases at the cores and rims of large garnets porphyroclasts, respectively. We tentatively interpret these two distinct melt compositions as suggesting that these former migmatites underwent two melting events under contrasting fluid regimes, possibly during two different orogenic periods. This study demonstrates the strong potential of melt inclusions studies in migmatites and granulites in order to unravel their anatectic history, particularly in strongly deformed rocks where most of the classical anatectic microstructures and macrostructures have been erased during deformation. [ABSTRACT FROM AUTHOR]

The solubility of chromium in chlorite as a function of pressure, temperature, and bulk composition was investigated in the system CrO-MgO-AlO-SiO-HO, and its effect on phase relations evaluated. Three different compositions with X = Cr/(Cr + Al) = 0.075, 0.25, and 0.5 respectively, were investigated at 1.5-6.5 GPa, 650-900 °C. Cr-chlorite only occurs in the bulk composition with X = 0.075; otherwise, spinel and garnet are the major aluminous phases. In the experiments, Cr-chlorite coexists with enstatite up to 3.5 GPa, 800-850 °C, and with forsterite, pyrope, and spinel at higher pressure. At P > 5 GPa other hydrates occur: a Cr-bearing phase-HAPY (MgAlCrSiO(OH)) is stable in assemblage with pyrope, forsterite, and spinel; Mg-sursassite coexists at 6.0 GPa, 650 °C with forsterite and spinel and a new Cr-bearing phase, named 11.5 Å phase (Mg:Al:Si = 6.3:1.2:2.4) after the first diffraction peak observed in high-resolution X-ray diffraction pattern. Cr affects the stability of chlorite by shifting its breakdown reactions toward higher temperature, but Cr solubility at high pressure is reduced compared with the solubility observed in low-pressure occurrences in hydrothermal environments. Chromium partitions generally according to $$X_{\text{Cr}}^{\text{spinel}}$$ ≫ $$X_{\text{Cr}}^{\text{opx}}$$ > $$X_{\text{Cr}}^{\text{chlorite}}$$ ≥ $$X_{\text{Cr}}^{\text{HAPY}}$$ > $$X_{\text{Cr}}^{\text{garnet}}$$ . At 5 GPa, 750 °C (bulk with X = 0.075) equilibrium values are $$X_{\text{Cr}}^{\text{spinel}}$$ = 0.27, $$X_{\text{Cr}}^{\text{chlorite}}$$ = 0.08, $$X_{\text{Cr}}^{\text{garnet}}$$ = 0.05; at 5.4 GPa, 720 °C $$X_{\text{Cr}}^{\text{spinel}}$$ = 0.33, $$X_{\text{Cr}}^{\text{HAPY}}$$ = 0.06, and $$X_{\text{Cr}}^{\text{garnet}}$$ = 0.04; and at 3.5 GPa, 850 °C $$X_{\text{Cr}}^{\text{opx}}$$ = 0.12 and $$X_{\text{Cr}}^{\text{chlorite}}$$ = 0.07. Results on Cr-Al partitioning between spinel and garnet suggest that at low temperature the spinel- to garnet-peridotite transition has a negative slope of 0.5 GPa/100 °C. The formation of phase-HAPY, in assemblage with garnet and spinel, at pressures above chlorite breakdown, provides a viable mechanism to promote HO transport in metasomatized ultramafic mélanges of subduction channels. [ABSTRACT FROM AUTHOR]

Metasomatic amphibole-eclogite sequences grew in selvages of quartz veins from the Marun-Keu complex (Polar Urals, Russia) during high-pressure metamorphism. Relicts of a pre-metasomatic eclogite- facies assemblage are present in the wallrock layers as irregular patches. Wallrock interstitial quartz trails lying at a high angle to reaction fronts provide evidence for grain-scale pore channelisation which may be produced by intergranular-fluid compositional gradients parallel to the quartz trails. Disequilibrium at vein-wallrock scale is inferred from wallrock mineral heterogeneity and from variable initial Sr isotope ratios in mineral separates. Mass-balance calculations between relicts and wallrock assemblages reveal chemical imbalances caused by open system-behaviour with two way mass-transfer. The vein-wallrock system registers a prograde history from 408–434°C (relicts) to 526–668°C (vein precipitates). Vein and metasomatic assemblages formed during a single fluid-rock interaction process, implying high heating rates (≥100 °C/Ma), which could result from heat advection by large-scale fluid circulation. [ABSTRACT FROM AUTHOR]