Stefano Poli

The Austroalpine Domain in the Eastern Alps consists of Mesozoic (meta)sedimentary cover units and polymetamorphic crystalline basements. The latter were heterogeneously overprinted by the Alpine metamorphism, with some of them still preserving traces of their Paleozoic magmatic and metamorphic evolution. One of the most prominent examples is the Ötztal–Stubai Complex, which extends over approximately 50 × 20 km across northern Italy and western Austria. Despite this complex experienced extensive Variscan high- to medium-grade metamorphism, some relicts of older events related to previous orogenies (i.e. Cadomian) are still preserved. As such, it represents a key natural archive for reconstructing the Paleozoic evolution of the Austroalpine Domain.Within this regional framework, a small area of about 25 km² near Reschen Pass (South Tyrol, NE Italy) is of particular interest, as it hosts potential pre-Variscan geological features that escaped younger metamorphism and deformation. These include the Klopaier Pluton, its contact metamorphic aureole, and associated migmatites, all enclosed within country gneiss characterized by Variscan mineral assemblages and fabrics. The coexistence of these elements offers is key to investigate the timing and relationships between magmatism, metamorphism, and deformation during the early Paleozoic.New detailed field mapping indicates that the Klopaier Pluton is largely undeformed, locally behaving as a rigid body and locally preserving primary intrusive contacts. Relicts of cordierite-bearing assemblages are locally found at the pluton margins and may represent remnants of a contact metamorphic aureole. The surrounding migmatites, predominantly metatexites, are interpreted as the result of syn-intrusion partial melting of the host rocks, later affected by Variscan metamorphism. Pegmatitic dikes are widespread both within the pluton and in the surrounding country rocks. These dikes are likely related to highly evolved fluid-rich melts derived from the main body of the Klopaier Pluton and have previously yielded U–Pb ages between ca. 490 and 413 Ma. These ages suggest that the observed structural configuration was already established during the Ordovician. New U–Pb zircon dating of the Klopaier Tonalite constrains its emplacement to approximately 460 Ma, in good agreement with the ages obtained from the associated pegmatites.Despite these constraints, the relative timing of pluton emplacement and migmatite formation remains unresolved, posing a classic geological “chicken-or-egg” problem: did the pluton intrude into pre-existing, already cooled migmatites, or did its emplacement and associated heat supply trigger partial melting in the surrounding rocks? To address this question, new U–Pb zircon and monazite ages from the migmatites, combined with their geochemical characterization, indicate crystallization and melt-related processes between ca. 460 and 450 Ma. These new data provide crucial constraints on the early Paleozoic tectonometamorphic evolution of the Austroalpine Domain and contribute to a better understanding of pre-Alpine crustal processes in the Eastern Alps.

Melilitites are ultramafic magmas characterized by normative Ca2SiO4, larnite, high FeO* and TiO2. Liquids compositionally close to melilitites were experimentally reproduced from carbonated lherzolites in alkali, Fe and Ti-free model systems at 3.2-3.3 GPa, approx. 1500 °C (Gudfinnsson & Presnall, 2005), at relatively high melt proportions. In complex compositions, MORB-eclogite derived, carbonated, partial melts reacted with a fertile peridotite were proposed at the origin of melilitites (Mallik & Dasgupta, 2013, 2014). The experimental reconstruction of phase relationships along a join olivine melilitite - carbonate revealed that at 3 GPa, clinopyroxene and olivine or garnet are stable on the liquidus (Brey & Ryabchikov, 1994), suggesting that carbonated wehrlites are potential sources for the genesis of melilitites.Here, we explore phase relationships on the high pressure melting of a model wehrlite, initially composed of a mechanical mixture of San Carlos olivine, diopside, aegirine, dolomite, rutile and kyanite. Starting materials were loaded in graphite capsules, inserted in sealed platinum capsules. Vitreous carbon spheres and synthetic diamond grains were adopted for liquid traps. Preliminary experimental results show that at 3 GPa the solidus is located at temperatures lower than 1200 °C. A thick, orthopyroxene-rich layer, with polygonal microstructure, forms at contact with aggregates resulting from quenched liquids, both at 1200 °C and 1400 °C. Estimates of liquid composition are melilititic, with TiO2 approx. 2.5 wt.% on a volatile free basis.Currently available experiments suggest that the solidus is controlled by the reaction dolomite + olivine + clinopyroxene = orthopyroxene + liquid, as suggested in Eggler (1976). This is feasible only if the liquid composition is located on the CaO-rich side of the plane diopside-forsterite-dolomite in the model system CaO-MgO-SiO2-CO2, i.e. on the normative larnite (akermanite) portion of the tetrahedron. Brey G.P. & Ryabchikov I.D. (1994). Carbon-dioxide in strongly silica undersaturated melts and origin of kimberlite magmas. Neues Jahrbuch Fur Mineralogie-Monatshefte, (10), 449-463.Eggler D.H. (1976). Does CO2 cause partial melting in the low-velocity layer of the mantle?. Geology, 4(2), 69-72Gudfinnsson G.H. & Presnall D.C. (2005). Continuous gradations among primary carbonatitic, kimberlitic, melilititic, basaltic, picritic, and komatiitic melts in equilibrium with garnet lherzolite at 3–8 GPa. Journal of Petrology, 46(8), 1645-1659.Mallik A. & Dasgupta R. (2013). Reactive infiltration of MORB-eclogite-derived carbonated silicate melt into fertile peridotite at 3 GPa and genesis of alkalic magmas. Journal of Petrology, 54(11), 2267-23

The Oetztal-Stübai Complex (OSC) of the Eastern Austroalpine domain is a polymetamorphic unit extending between western Austria (Tyrol) and northern Italy (Autonomous Province of Bozen/Bolzano). The OSC is made of crystalline basement consisting of metasedimentary rocks (paragneisses and micaschists) hosting numerous bodies of metagranitoids and metabasic rocks, along with subordinate metacarbonates and ultramafics of igneous origin. The Alpine metamorphism in the OSC increases from the north-west to the south-east (Purtscheller & Rammlmair, 1982). Pre-Alpine evolution of the OSC is testified by the crystallization of Cambrian mafic-to-ultramafic cumulates with MORB-like signatures preserved in pods and layers within the Central Metabasite Zone (CMZ) (Miller & Thöni, 1995; Konzett et al., 2005); an Ordovician high temperature event resulting in intrusions of granitoids and metapelites anatexis (e.g. Winnebach migmatites); occurrence of Variscan eclogites within the CMZ and subsequent pervasive re-equilibration under amphibolite facies conditions. However, structural relationships in the field do not rule out the possibility of a pre-Variscan HP event. Extensive field work, microstructural and petrological analyses, and radiometric dating are being carried out in two key areas, Längenfeld and Reschenpass/Passo Resia. The two areas are similar in the occurrence of two Ordovician intrusions, the Sulztal type-S granite (Längenfeld) and the Klopaier Tonalite (Reschenpass), for which we obtained by U-Pb LA-ICP-MS dating of zircons ages of 482.4 ± 1.5 Ma and 460 ± 0.83 Ma, respectively. In the Längenfeld area, rocks belonging to the CMZ show various degrees of metamorphic reactions progress often resulting in symplectitic relationships. Mafic-to-ultramafic rocks, with exceptionally well preserved cumulitic textures, display the destabilization at high pressure conditions of anorthite-rich plagioclase to omphacite+corundum intergrowths bordered by garnet, replacing plagioclase as confirmed by REE patterns determined by in-situ LA-ICP-MS. Within these cumulates, newly discovered troctolitic layers show corundum-bearing coronas around olivine and granoblastic textures with increased anorthite content in plagioclase rims, implying a static phase of high temperature recrystallization. Associated metacarbonates are characterized by olivine (Mg/(Mg+Fe)=0.95) and Fe-spinel. Additionally, we found evidence of partial melting involving metapelites and eclogites of the CMZ, resulting in corundum-bearing migmatitic gneisses and eclogite-derived melts. The Variscan age of CMZ eclogites has been assessed by Sm-Nd mineral and WR isochrons in Miller & Thöni (1995), but unpublished U-Pb zircon data (Sollner & Gebauer in Hoinkes & Thöni, 1993) points to an age of 497 Ma for this high pressure event. This hypothesis deserves further investigations on account of our field work and newly discovered field relationships, suggesting the existence of a pre-Variscan high temperature event post-dating an older eclogite facies metamorphism. References: Hoinkes G & Thöni M (1993) Pre-Mesozoic Geology in the Alps, 485–494Konzett J et al. (2005) Journal of Petrology 46 (4), 717–747Miller C & Thöni (1995) Chemical Geology 122 (1) 199–225Purtscheller F & Rammlmair D (1982) Tschermaks Petr. Mitt. 29 (3), 205–221

Ocean sediments consist mainly of calcium carbonate and organic matter (phytoplankton debris). Once subducted, some carbon is removed from the slab and returns to the atmosphere as CO₂ in arc magmas. Its isotopic signature is thought to reflect the bulk fraction of inorganic (carbonate) and organic (graphitic) carbon in the sedimentary source. Here we challenge this assumption by experimentally investigating model sediments composed of ¹³C-CaCO₃ + ¹²C-graphite interacting with water at pressure, temperature and redox conditions of an average slab–mantle interface beneath arcs. We show that oxidative dissolution of graphite is the main process controlling the production of CO₂, and its isotopic composition reflects the CO₂/CaCO₃ rather than the bulk graphite/CaCO₃ (i.e., organic/inorganic carbon) fraction. We provide a mathematical model to relate the arc CO₂ isotopic signature with the fluid–rock ratios and the redox state in force in its subarc source.

The system CaCO3-MgCO3 has been used since the '60s for reconstructing the petrogenesis of carbonated lithologies, notably of carbonatite magmas possibly generated in the Earth's mantle. Yet, experimental results at high temperatures and pressures remain contradictory, and a thermodynamic model for the carbonate liquid in this binary is still lacking.We experimentally investigated the melting of aragonite and magnesite to pressures of 12 GPa, and of calcite-magnesite mixtures at 3 and 4.5 GPa, and at variable Mg/(Mg+Ca) (XMg). Results show that the melting of aragonite, and of magnesite have similar slopes, magnesite melting ≈ 30 °C higher than aragonite. The minimum on the liquidus surface is at XMg ≈ 0.35-0.40, 1200 °C at 3 GPa, and 1275 °C at 4.5 GPa, which, when combined with data from Byrnes and Wyllie (1981) and Müller et al. (2017), imply that minimum liquid composition remains approximately constant with pressure increase. We present the first thermodynamic model for CaCO3-MgCO3 liquids, retrieved from the experimental data available. Although carbonate liquids should be relatively simple molten salts, they display large non-ideality and a three-component (including a dolomite component), pressure dependent, asymmetric solution model is required to model the liquidus surface. Attempts to use an end-member two-component model fail, invariably generating a very wide magnesite-liquid loop, contrary to the experimental evidence.The liquid model is used to evaluate results of experimentally determined phase relationships for carbonated peridotites modelled in CaO-MgO-SiO2-CO2 (CMS-CO2), and CaO-MgO-Al2O3-SiO2-CO2 (CMAS- CO2). Computations highlight that the liquid composition in the CMS-CO2 and CMAS-CO2 and in more complex systems do not represent "minimum melts" but are significantly more magnesian at high pressure, and that the pressure-temperature position of the solidus, as well as its dP/dT slope, depend on the bulk composition selected, unless truly invariant assemblages occur. Calculated phase relationships are somewhat dependent on the model selected for clinopyroxene, and to a lesser extent of garnet.Byrnes A.P. and Wyllie P.J. (1981) Subsolidus and melting relations for the join CaCO3-MgCO3 at 10 kbar. Geochim. Cosmochim. Acta 45, 321-328Müller I.A., Müller M. K., Rhede D., Wilke F.D.H. and Wirth R. (2017) Melting relations in the system CaCO3-MgCO3 at 6 GPa. Am. Mineral. 102, 2440-2449.

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 1–3 GPa, 800 °C, and ∆FMQ ≈ −0.5 for the volatiles and solute contents of graphite-saturated fluids in the systems COH, SiO₂–COH ( + quartz/coesite) and MgO–SiO₂–COH ( + forsterite and enstatite). The CO₂ content of fluids interacting with silicates exceeds the amounts measured in the pure COH system by up to 30 mol%, 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.