Simone Tumiati

Hydrothermal origin of manganese in the high-pressure ophiolite metasediments of Praborna ore deposit (Aosta Valley, Western Alps)

The manganese ore deposit of Praborna crops out in the Zermatt-Saas unit of the Western Alps, in the St. Marcel valley. It represents a Jurassic ophiolitic sedimentary cover subducted at high-pressure conditions during the Alpine orogeny. Major- and trace-element analyses of representative samples of the ore and the host metasediments were collected to geochemically characterise the deposit. Selected phases (piemontite, braunite, garnet, clinopyroxene, white mica and manganiandrosite) were investigated with ion and electron microprobes to link the mineral chemistry to the bulk-rock chemistry. Compared to shales, Praborna is enriched in Mn (up to 38.7 wt% Mn 2 O 3 ) and in many trace elements (Sc, Co, Ni, Cu, Ge, As, Sr, Ag, Sb, Te, Ba, Tl, Pb and Bi). The bulk-rock REE pattern suggests 20 % hydrogenous and 80 % hydrothermal inputs in the proto-ore. Compared to the shale, the hanging-wall Mn-poor schists share with the Mn ore body the enrichment in Sc, Mn, Co, Sr and Te, suggesting a common enrichment process involving these elements. The REE pattern suggests a sedimentary origin for these schists, which were probably composed of clay mixed with components of volcanic origin. In order to confirm the hydrothermal origin of the Praborna Mn ore deposit, we built up a database of more than 5000 data of modern hydrogenous and hydrothermal oceanic Mn deposits worldwide, adding data of oceanic Mn-rich sediments and of the Ligurian Mn ore deposits, which are thought to be the unmetamorphosed geological equivalent of Praborna. The classic ternary Mn–Fe–(Cu + Co + Ni) diagram, the agglomerative hierarchical clustering and the principal-component analysis, which takes into account a larger set of elements, strongly support the hypothesis of an oceanic hydrothermal origin for manganese in the Praborna and in the Ligurian ore deposits.

The role of C-O-H and oxygen fugacity in subduction-zone garnet peridotites

C-O-H fluids are released by dehydration, partial melting and/or decarbonation of the slab and transferred to the mantle, where they interact with the surrounding rocks, prompting the growth of carbonates, hydrous minerals and C polymorphs. In the pure C-O-H system, C-saturated fluid speciation is a function of the oxygen chemical potential. Therefore, in natural systems, the fluid speciation can be imposed by the redox state of the rock-forming phases. Alternatively, C-O-H fluids may control the bulk oxidation state of the rock system by redox reactions with the mineral phases. We selected three case studies of garnet-bearing ultramafic rocks (Ulten zone, Italy; Sulu, China; Bardane, Norway), which record metasomatic processes driven by C-O-H fluids at the interface between a subducting slab and the overlying mantle wedge. All these rocks contain carbonates (dolomite-only at P , 1.9 GPa at 900 � C, magnesite-only at P. 2.4 GPa at 900 � C, dolomite þ magnesite in between) and hydrous phases (amphibole, phlogopite) equilibrated at some stages in the garnet stability field. The f O2 values, estimated by analysing the Fe 3þ content (skiagite mole fraction) in garnet, indicate that the Ulten and Sulu peridotites record high oxygen fugacities (FMQ to FMQþ2) and a retrograde path with decreasing P and T. The f O2 values obtained for the Bardane garnet websterites, which record a prograde path with increasing T and P, are up to � 2 log units lower than the FMQ. When combined with data for subduction-zone systems (arc lavas and their mantle sources), the studied ultramafic rocks define a trend of decreasing f O2 with increasing pressure. The Bardane websterites contain C-polymorphs in polyphase inclusions, which precipitated from entrapped metasomatic fluids at ultrahigh pressures. The calculated C-O-H fluid phase in equilibrium with the solid phases consists of mixtures of H2O and CO2. Semi-quantitative estimates for the Ulten and Sulu peridotites, in which C-polymorphs have not been found, and petrographic constraints for the Ulten peridotites indicate that the C-O-H component of the fluid could consist of H2OþCO2.

Silicate dissolution boosts the CO2 concentrations in subduction fluids

Estimates of dissolved CO2 in subduction-zone fluids are based on thermodynamic models, relying on a very sparse experimental data base. Here, we present experimental data at 1–3u2009GPa, 800u2009°C, and ∆FMQu2009≈u2009−0.5 for the volatiles and solute contents of graphite-saturated fluids in the systems COH, SiO2–COH (u2009+u2009quartz/coesite) and MgO–SiO2–COH (u2009+u2009forsterite and enstatite). The CO2 content of fluids interacting with silicates exceeds the amounts measured in the pure COH system by up to 30u2009mol%, as a consequence of a decrease in water activity probably associated with the formation of organic complexes containing Si–O–C and Si–O–Mg bonds. The interaction of deep aqueous fluids with silicates is a novel mechanism for controlling the composition of subduction COH fluids, promoting the deep CO2 transfer from the slab–mantle interface to the overlying mantle wedge, in particular where fluids are stable over melts.Current estimates of dissolved CO2 in subduction-zone fluids based on thermodynamic models rely on a very sparse experimental data base. Here, the authors show that experimental graphite-saturated COH fluids interacting with silicates at 1–3u2009GPa and 800u2009°C display unpredictably high CO2 contents.

High-temperature and high-pressure behavior of carbonates in the ternary diagram CaCO3-MgCO3-FeCO3

Abstract We report the thermal expansion and the compressibility of carbonates in the ternary compositional diagram CaCO3-MgCO3-FeCO3, determined by in situ X-ray powder and single-crystal diffraction. High-temperature experiments were performed by high-resolution X-ray synchrotron powder diffraction from ambient to decarbonation temperatures (25–850 °C). Single-crystal synchrotron X ray diffraction experiments were performed in a variable pressure range (0–100 GPa), depending on the stability field of the rhombohedral structure at ambient temperature, which is a function of the carbonate composition. The thermal expansion increases from calcite, CaCO3, α0 = 4.10(7) ×10–5 K–1, to magnesite, MgCO3, α0 = 7.04(2) ×10–5 K–1. In the magnesite-siderite (FeCO3) join, the thermal expansion decreases as iron content increases, with an experimental value of α0 = 6.44(4) ×10–5 K–1 for siderite. The compressibility in the ternary join is higher (i.e., lower bulk modulus) in calcite and Mg-calcite [K0 = 77(3) GPa for Ca0.91Mg0.06Fe0.03(CO3)] than in magnesite, K0 = 113(1) GPa, and siderite, K0 = 125(1) GPa. The analysis of thermal expansion and compressibility variation in calcite-magnesite and calcite-iron-magnesite joins clearly shows that the structural changes associated to the order-disorder transitions [i.e., R3c calcite-type structure vs. R3 CaMg(CO3)2 dolomite-type structure] do not affect significantly the thermal expansion and compressibility of carbonate. On the contrary, the chemical compositions of carbonates play a major role on their thermo-elastic properties. Finally, we use our P-V-T equation of state data to calculate the unit-cell volume of a natural ternary carbonate, and we compare the calculated volumes to experimental observations, measured in situ at elevated pressure and temperatures, using a multi-anvil device. The experimental and calculated data are in good agreement demonstrating that the equation of state here reported can describe the volume behavior with the accuracy needed, for example, for a direct chemical estimation of carbonates based on experimental unit-cell volume data of carbonates at high pressures and temperatures.

Experimental determination of magnesia and silica solubilities in graphite-saturated and redox-buffered high-pressure COH fluids in equilibrium with forsterite + enstatite and magnesite + enstatite

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 1200xa0°C. Synthetic forsterite, enstatite, and nearly pure natural magnesite were used as starting materials. Redox conditions were buffered by Ni–NiO–H2O (ΔFMQ = −u20090.21 to −u20091.01), employing a double-capsule setting. Fluids, binary H2O–CO2 mixtures at the P, T, and fO2 conditions investigated, were generated from graphite, oxalic acid anhydrous (H2C2O4) 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 CO2-bearing fluids. The experimental data show that forsteriteu2009+u2009enstatite solubility in H2O–CO2 fluids is higher compared to pure water, both in terms of dissolved silica (mSiO2u2009=u20091.24xa0mol/kgH2O versus mSiO2u2009=u20090.22xa0mol/kgH2O at Pu2009=u20091 GPa, Tu2009=u2009800xa0°C) and magnesia (mMgOu2009=u20091.08xa0mol/kgH2O versus mMgOu2009=u20090.28xa0mol/kgH2O) probably due to the formation of organic C–Mg–Si complexes. Our experimental results show that at low temperature conditions, a graphite-saturated H2O–CO2 fluid interacting with a simplified model mantle composition, characterized by low MgO/SiO2 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/SiO2 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.

Redox processes and the role of carbon-bearing volatiles from the slab–mantle interface to the mantle wedge

The valence of carbon is governed by the oxidation state of the host system. The subducted oceanic lithosphere contains considerable amounts of iron so that Fe3+/Fe2+ equilibria in mineral assemblages are able to buffer the (intensive) fO2 and the valence of carbon. Alternatively, carbon itself can be a carrier of (extensive) ‘excess oxygen’ when transferred from the slab to the mantle, prompting the oxidation of the sub-arc mantle. Therefore, the correct use of intensive and extensive variables to define the slab-to-mantle oxidation by C-bearing fluids is of primary importance when considering different fluid/rock ratios. Fluid-mediated processes at the slab–mantle interface can also be investigated experimentally. The presence of CO2 (or CH4 at highly reduced conditions) in aqueous COH fluids in peridotitic systems affects the positions of carbonation or decarbonation reactions and of the solidus. Some methods to produce and analyse COH fluid-saturated experiments in model systems are introduced, together with the measurement of experimental COH fluids composition in terms of volatiles and dissolved solutes. The role of COH fluids in the stability of hydrous and carbonate minerals is discussed comparing experimental results with thermodynamic models and the message of nature.

Granulite facies overprint in garnet peridotites and kyanite eclogites of Monte Duria (Central Alps, Italy): clues from srilankite- and sapphirine-bearing symplectites

Peridotites and different types of eclogites occurring in the Monte Duria area (Adula–Cima Lunga unit, Central Alps, Italy) share a common eclogite-facies peak at P1⁄42 6–3 0 GPa and T1⁄4 710–750 C, constrained by conventional thermobarometry and thermodynamic modelling. High-pressure minerals are replaced both in peridotites and in eclogites by lower-P and high-T assemblages. In peridotites, the zirconium titanate srilankite occurs as micrometre-sized crystals in textural equilibrium with spinel, clinopyroxene and orthopyroxene in kelyphites developed between garnet and olivine. By using a new ZrO2–TiO2 solid-solution model, we provide evidence that srilankite is stable in peridotites relative to zirconþ rutile for T>810 C at an assumed P 0 9 GPa, consistent with estimates of T 850 C (at assumed P1⁄4 0 9 GPa) determined for symplectites made of sapphirineþspinelþAl-rich orthopyroxeneþ amphibole found in fractures within garnet. In eclogites, kyanite is replaced by symplectites made of anorthite-rich plagioclaseþ spinel6 sapphirine6 corundum, formed at T 850 C and P1⁄40 8–1 0 GPa, conditions that are coincident with the high-T overprint observed in the peridotites. Thermodynamic modelling coupled with a material-transfer study provides constraints for these sapphirine-bearing symplectites. In these micro-domains, the ‘inert’ components could not fully equilibrate with the surrounding rock, and the locally high Al content promoted the stability of the Al-rich phases (i.e. mosaic equilibrium). This is the first report from the Alps of eclogite-facies rocks of supposed Alpine age showing a granulite-facies metamorphic overprint, which is, in contrast, well documented in the Variscan belt. On these grounds, although the age of the high-pressure and hightemperature stages in the Monte Duria rocks is still not constrained, the possibility that they reached eclogitic and granulitic conditions in pre-Alpine times should be taken into account.