Franco Rolfo

Accurate estimation of uniaxial compressive strength (UCS) is essential in geotechnical engineering, but conventional UCS testing is destructive and time-consuming. This study investigates the effects of mild acid exposure and freeze–thaw (F–T) cycling on the relationships between P-wave velocity (Vp) and UCS, density (ρ) and UCS, and Vp and selected physical properties in Apulian dolostone. Thirty-four cylindrical specimens were tested, comprising ten untreated specimens, five specimens in each acid-treatment group (3, 7, and 28 days), five specimens subjected to 10 F–T cycles, and four specimens monitored progressively during F–T cycling. Chemical weathering was simulated by static immersion in sulfuric acid (initial pH ≈ 5) without solution renewal and pH adjustment. Measurements included Vp, density, UCS, apparent porosity, and water absorption, together with thin-section petrography using optical microscopy and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM–EDS), SEM morphology, and X-ray diffraction (XRD) mineralogical analysis (four thin sections per treatment group; one representative specimen per treatment for SEM and XRD). Under the investigated conditions, both weathering processes produced moderate changes in the physical, ultrasonic, and mechanical properties, accompanied by minor microstructural modifications but without detectable mineralogical phase changes. The treatments also modified the empirical relationships between Vp, UCS, and the measured physical properties. These findings indicate that weathering influences the reliability of Vp-based UCS estimation for Apulian dolostone under the investigated conditions and provide new insights into the early-stage deterioration of this material.

Investigating the carbonate preservation efficiency (CPE) of continental crust is crucial to understand the global carbon cycle, which requires constraints on initial carbonate abundances (ICAs) of crustal rocks. To link Mg isotopes to ICAs, we present elemental and Mg isotopic data for Himalayan carbonate‐bearing and carbonate‐free metasedimentary rocks. Given no evident melt extraction or external‐fluid infiltration, ICAs of these samples can be independently estimated by elemental data. Despite different carbonate species in the protoliths, all the samples show congruent relationship between their δ ²⁶ Mg and ICAs, owing to the elevated carbonate δ ²⁶ Mg and Mg/Ca in protoliths of calcite‐rich samples resulting from diagenetic processes. When collated with literature data, we suggest the observed correlation here can be applied to most carbonate‐bearing (meta‐)sedimentary rocks. Based on a steady state box‐model, we constrained the modern net carbonate accretion flux ( Tmol/year) and the average time‐integrated CPE (∼%) for continental crust.

Orogenic degassing is emerging as a potentially relevant source of carbon dioxide (CO₂) from the continental crust. However, the processes of carbon mobilization are still poorly explored. Here, we use thermodynamic modeling to investigate the decarbonation of sediments metamorphosed under high geothermal gradients. Our modeling shows that immiscible CO₂-rich vapors and hydrosaline brines are generated at these conditions, with different properties and mobility through the crust. The CO₂-rich fluid fraction could rapidly rise toward the surface without interacting with the host rocks by carbo-fracturing the host rocks or through deep faults. The denser hydrosaline brines likely permeate the source rocks. When applied to the active Himalayan orogen, these observations reconcile measured CO₂fluxes at the surface and positive conductivity anomalies associated with micro-seismicity at depth. Our modeling shows that the continental crust represents a relevant reservoir of CO₂that can be efficiently degassed during hot collisions.

Although calcareous pelites are important constituents of sediments involved in orogenic processes, their prograde metamorphism is significantly less studied than that of pure pelites with negligible amounts of modal calcite. This paper presents mineral equilibria modelling in the system MnO–Na₂O–K₂O–CaO–FeO–MgO–Al₂O₃–SiO₂–TiO₂–H₂O–CO₂, with the aim of constraining the prograde evolution of calcareous pelites in collisional orogenic settings. A suite of model bulk‐rock compositions is used to investigate the influence of different proportions of calcite in the protolith on (a) the equilibrium assemblages at different pressure, temperature and fluid composition (P–T–X(CO₂)) conditions; (b) the melt fertility and (c) the fluid evolution and the main decarbonation reactions occurring during prograde metamorphism of calcareous pelites. In spite of being purely theoretical, the reliability of the modelling is tested by comparing the predicted assemblages with those observed in a wide set of natural samples from the Himalayan metamorphic core. Comparison between the predicted and the modelled assemblages demonstrates that even a small amount of calcite in the calcareous pelitic protoliths has a strong influence on the final mineral assemblages and compositions, with potential effects on their melt productivity. Specifically, it appears that up to ~800°C, the melt productivity of calcic metapelites remains low, and melt production occurs gradationally because it is mostly controlled by continuous biotite dehydration melting reactions, rather than by muscovite breakdown. Moreover, the study demonstrates that calcareous pelites could be non‐negligible CO₂‐source rocks in orogenic settings, and that in such contexts, an internal buffered behaviour is likely for most of them.

The North Dabie complex zone (NDZ), central China, is a high-T ultrahigh-pressure (UHP) metamorphic terrane. It underwent a complex evolution comprising of multistage metamorphism and multiple anatectic events during the Mesozoic continental collision, characterized by granulite-facies overprinting and a variety of migmatites with different generations of leucosomes. In this contribution, we carried out an integrated study including field investigation, petrographic observations, zircon U-Pb dating, and whole-rock element and Sr-Nd-Pb isotope analysis for the migmatites in the NDZ and their leucosomes and melanosomes. As a result, four groups of leucosomes have been recognized: Group 1 (garnet-bearing leucosome), strongly deformed leucosomes with coarse-grained peritectic garnet; Group 2 (amphibole-rich leucosome), weakly deformed to undeformed amphibole-rich leucosomes with coarse-grained peritectic amphibole and no garnet; Group 3 (amphibole-poor leucosome), weakly deformed to undeformed amphibole-poor leucosomes with minor fine-grained amphibole; Group 4 (K-feldspar-rich leucosome), K-feldspar-rich leucosomes mainly composed of coarse-grained quartz, plagioclase and K-feldspar. Zircon SHRIMP and LA-ICPMS U-Pb dating suggest that the Group 1 leucosomes formed at 209 ± 2 Ma whereas the rest of the leucosome groups (Groups 2–4) occurred between 145–110 Ma, in response to decompression under granulite-facies conditions during the early stage of exhumation, and to heating during post-orogenic collapse, respectively. Furthermore, the garnet-bearing leucosomes were resulted from fluid-absent anatexis related to biotite dehydration melting, while the other three groups of leucosomes were formed during large-scale fluid-present partial melting and coeval migmatization. This migmatization comes from heating from the mountain-root removal and asthenosphere upwelling, together with the influx of fluids derived from country rocks at mid-upper crustal levels. However, all the leucosomes and melanosomes display Pb-isotopic compositions similar to those observed for the NDZ UHP rocks (eclogites and granitic gneisses), suggesting a common source from the Triassic subducted Neoproterozoic lower-crustal rocks. In addition, the Cretaceous partial melting and migmatization began at 143 ± 2 Ma with three age-peaks at 133 ± 3 Ma, 124 ± 3 Ma and 114 ± 7 Ma, respectively.

Corundum-bearing anatectic aluminous rocks are exposed in the deeply subducted North Dabie complex zone (NDZ), of Central China. The rocks consist of corundum, biotite, K-feldspar and plagioclase, and show clear macro- and micro-structural evidence of anatexis by dehydration melting of muscovite in the absence of quartz. Mineral textures and chemical data integrated with phase equilibria modeling, indicate that coarse-grained corundum in leucosome domains is a peritectic phase, reflecting dehydration melting of muscovite through the reaction: Muscovite = Corundum + K-feldspar + Melt. Aggregates of fine-grained, oriented, corundum grains intergrown with alkali feldspar in the mesosome domains are, instead, formed by the dehydration melting of muscovite with aluminosilicate, through the reaction: Muscovite + Al-silicate = Corundum + K-feldspar + Melt. P-T pseudosections modeling in the Na₂O-CaO-K₂O-FeO-MgO-Al₂O₃-SiO₂-H₂O-TiO₂ system constrains peak pressure-temperature (P-T) conditions at 900–950 °C, 9–14 kbar. The formation of peritectic corundum in the studied rocks is a robust petrographic evidence of white mica decompression melting that has occurred during the near-isothermal exhumation of the NDZ. Combined with P-T estimates for the other metamorphic rocks in the area, these new results further confirm that the NDZ experienced a long-lived high-T evolution with a near-isothermal decompression path from mantle depths to lower-crustal levels. Furthermore, our new data suggest that white mica decompression melting during exhumation of the NDZ was a long-lasting process occurring on a depth interval of more than 30 km.

Fluid–earthquake interplay, as evidenced by aftershock distributions or earthquake-induced effects on near-surface aquifers, has suggested that earthquakes dynamically affect permeability of the Earth’s crust. The connection between the mid-crust and the surface was further supported by instances of carbon dioxide (CO₂) emissions associated with seismic activity, so far only observed in magmatic context. Here we report spectacular non-volcanic CO₂ emissions and hydrothermal disturbances at the front of the Nepal Himalayas following the deadly 25 April 2015 Gorkha earthquake (moment magnitude M_w = 7.8). The data show unambiguously the appearance, after the earthquake, sometimes with a delay of several months, of CO₂ emissions at several sites separated by > 10 kilometres, associated with persistent changes in hydrothermal discharges, including a complete cessation. These observations reveal that Himalayan hydrothermal systems are sensitive to co- and post- seismic deformation, leading to non-stationary release of metamorphic CO₂ from active orogens. Possible pre-seismic effects need further confirmation.