Sandro Conticelli

Nephelinites are a distinctive group of silica-undersaturated, strongly alkaline ultrabasic volcanic rocks that are occasionally associated with carbonatites. Their genesis has been attributed to the melting of a mantle source enriched by either recycled crustal materials or other metasomatic agents. This study investigates olivine nephelinites and constrains their mantle source from the West Qinling orogen, Central China—a locality noted for its unique occurrence of rocks previously classified as kamafugites or melilitites in China. Olivines in these rocks are categorized into Group I mantle xenocrysts (Fo_89–92) and Group II magmatic crystals (Fo_81–88). In situ δ¹⁸O values of the Group II magmatic olivines (5.04 ± 0.25‰; 1σ, n = 67) are slightly lower than those of Group I mantle xenocrysts (5.26 ± 0.20‰,1σ, n = 199). Measured ¹⁸⁷Os/¹⁸⁸Os ratios for these nephelinites are moderately to highly radiogenic (0.138–0.682), with age-corrected γOs values varying from +6.57 to +409. Magmatic olivines with elevated 100 × Mn/Fe (mean = 1.65), 100 × Ni/Mg (mean = 0.70), and high 100 × Ca/Fe (mean = 2.33) ratios fingerprint a metasomatized mantle peridotite source. Binary mixing indicates that the observed radiogenic Os isotopes can be explained by the incorporation of <30% melts/fluids derived from a subducted oceanic slab. Our findings suggest that mantle metasomatism, driven by recycled oceanic slabs, played a major role in alkaline enrichment observed in the West Qinling magmas, and may provide new insights into the crust–mantle interaction.

The renewed start and funding of the CARG project in volcanic areas have enabled new surveys and refinements of data on the volumes and extents of ignimbrites across the Roman Magmatic Province (RMP). To date, the investigation has focused particularly on the Roccamonfina volcano (Sheets 416 Sessa Aurunca and 417 Teano) and the Bracciano caldera (Sheet 364 Bracciano). The project is enhancing field data from areas already surveyed in past decades, while integrating new models and technologies to obtain more accurate quantifications of erupted magma volumes and a consequent re-evaluation of eruption magnitudes. Preliminary results indicate that the volume of some ignimbrites increases by more than one order of magnitude, suggesting that many other ignimbrites within the RMP may have been significantly underestimated, such as the Brown Leucitic Tuff and the White Trachytic Tuff pertaining to the Roccamonfina volcano. This reassessment potentially characterizes the RMP as an ignimbrite flare-up system, comparable to some of the largest and most impactful volcanic provinces worldwide, such as the Taupo Volcanic Zone. In this framework, new field and literature data, borehole stratigraphy, and GIS-integrated methodologies were combined to refine the bulk volume, areal extension, and magnitude of a case-study ignimbrite, with the aim of developing a standardized procedure for computing and integrating field surveys applicable to all ignimbrites.

Magmatism’s role in continental rupture and ocean formation remains a critical question in Earth Sciences. The Red Sea, where Arabia is rifting from Nubia, offers an ideal setting to explore this process. This study analyses geochemical and isotopic data from gabbros and basaltic dikes in the Tihama Asir complex, formed during early Red Sea rifting (Late Oligocene). The results show that asthenospheric melts assimilated ancient lower crust before rising into shallow magma chambers with minimal upper crustal contamination. Rising asthenosphere driven by the Afar plume and by extensional stresses thermally weakened the lower crust, allowing it to decouple from the mantle, causing depth-dependent deformation. The generated asthenospheric melts underplated and intruded the thinning continental crust protracting continental rifting by re-thickening of the crust and accommodating extension. This study provides the evidence of tholeiitic underplating beneath thick continental crust during early rifting, substantially postponing the onset of seafloor spreading.

Monte Amiata is a small linear Pleistocenic volcano emplaced in a short period of time (between 303 ka and 299 ka) and located in Southern Tuscany. It is made monotonously by silicic massive lava flows and exogenous domes, grouped in two main volcanic periods: the Bagnore Synthem and the Monte Amiata Synthem. Massive lavas and domes host rounded to sub-rounded Mafic Magmatic Enclaves (MMEs). They are abundant in the youngest period of activity (i.e., Monte Amiata Synthem), increasing, apparently in the most recent eruptions. Volcanic rocks and their MMEs range in composition from trachydacites to olivine latites. Here, we present the results of our detailed study about the distribution, origin, and evolution of the mafic magmatic enclaves hosted by Monte Amiata lavas and domes in the frame of the evolution of the short-lived volcanic activity at surface. In order to do this, fieldwork observations were made, thin sections were studied under the petrographic microscope and chemical and isotopic analyses on whole MMEs and their forming minerals were performed. Fieldwork observations, supported by image analysis, showed a variation of the outcropping percentage abundance of the magmatic enclaves within the Monte Amiata domes. The estimated mafic enclaves outcropping abundance percentage varies between ca. 5.5% for La Montagnola dome and ca. 0.8% in the case of the Pratini dome. This suggests a variable degree of interaction between the magmas involved in the mingling process and, in some cases, it remarks the occurrence of the mechanical fractionation. Through the petrographic investigation of the magmatic enclaves five groups of enclaves can be identified. They are diversified by type of phenocrysts, micro-phenocrysts and microliths, the degree of porphyricity and the presence and abundance of xenocrysts (such as, for example, sanidine, biotite, plagioclase and orthopyroxene). Their chemical composition range from olivine latite, with enclaves characterized by abundant xenocrysts of sanidine (Or75-Or82), anhedral orthopyroxene (Mg# = 0.42-0.72), anhedral biotite (Mg# = 0.40-0.686), and copious plagioclase phenocrysts (An89-An44) with sieved texture, to potassic trachybasalt, where the most representative enclaves group is marked by the occurrence of phenocrysts of clinopyroxene (Mg# = 0.15-0.92) and abundant acicular biotite to phlogopite (Mg# = 0.30-0.82). A similar group is also characterized by the presence of olivine phenocrysts and micro-phenocrysts (Fo90-Fo56), often entirely surrounded by acicular microcrystals of phlogopite or biotite. The other two more mafic groups are respectively made up of: i) aggregates of large and abundant clinopyroxene crystals (Mg# = 0.85-0.91) with a groundmass characterized mainly by plagioclase with a feathery texture; ii) large phenocrysts of olivine (Fo87-Fo53) and clinopyroxene (Mg# = 0.61-0.93) and a groundmass rich in microliths of olivine and plagioclase euhedral, as in the case of the most mafic sample among those sampled. Several traces of disequilibrium processes among the crystals of the magmatic enclaves, such as sanidine, orthopyroxene, olivine crystals were observed.

We explore the connections between crustal shortening, volcanism, and mantle dynamics in the Atlas of Morocco. In response to compressional forces and strain localization, this intraplate orogen has evolved far from convergent plate margins. Convective effects, such as lithospheric weakening and plume‐related volcanism, contributed in important ways to the building of high topography. We seek to better understand how crustal and mantle processes interacted during the Atlas' orogeny by combining multiple strands of observations, including new and published data. Constraints on crustal and thermal evolution are combined with new analyses of topographic evolution, petrological, and geochemical data from the Anti‐Atlas volcanic fields, and a simple numerical model of the interactions among crustal deformation, a mantle plume, and volcanism. Our findings substantiate that: (a) crustal deformation and exhumation accelerated during the middle/late Miocene, contemporaneous with the onset of volcanism; (b) volcanism has an anorogenic signature with a deep source; (c) a dynamic mantle upwelling supports the high topography. We propose that a mantle plume and the related volcanism weakened the lithosphere beneath the Atlas and that this favored the localization of crustal shortening along pre‐existing structures during plate convergence. This convective‐tectonic sequence may represent a general mechanism for the modification of continental plates throughout the thermo‐chemical evolution of the supercontinental cycle.

The origin of Italian kamafugites and lamproites is a matter of debate, not least due to their “crustal signature” displayed by trace element compositions and isotopic ratios, but also due to puzzling geodynamic significance. We combine in situ EMPA and LA‐ICP‐MS analyses with in situ analyses of oxygen isotopes (SIMS) on olivine from the Pleistocene San Venanzo kamafugites and Torre Alfina lamproites. Lamproitic olivine shows extremely high Mg# and Ni concentrations whereas Ca and Mn concentrations are low. Their δ¹⁸O_V‐SMOW values are very high up to +11.5 ‰. In kamafugites we recognize three genetically different olivine groups: (a) phenocrystic one with high Mg#, very low Ni, high Ca and Mn. Values of δ¹⁸O_V‐SMOW are up to +10.9 ‰; (b) melt‐related xenocrystic grains that compositionally resemble lamproitic olivine; (c) skarn‐related almost pure forsterite of extreme δ¹⁸O_V‐SMOW ∼27 ‰, with negligible amounts of minor and trace elements. The melting and crystallization conditions of Italian kamafugites and lamproites indicate compositionally heterogeneous mantle sources on very small scales. Distinct geochemical features of the olivine macrocryst populations observed in kamafugite point to a range of processes occurring both within the magma storage and transport system. We suggest that the diversity of metasomatic agents was involved in mantle processes on local scales, coupled with magma mixing and/or the uptake of xenocrysts during magma ascend.

Melts with rhyolite compositions originate from partial melting of crustal rocks or extensive differentiation of basaltic melts, at temperatures in the range of 800 °C. Accordingly, they are confined to the shallow continental crust. Nevertheless, experimental studies have demonstrated that dacite-rhyolite melts can be generated at higher temperature (> 1000°c) and pressure (>2 GPa), by partial melting of continental crustal lithotypes, but direct evidence for their occurrence has never been found. This implies that rhyolite melts may be produced at mantle conditions either by subduction of sedimentary material or exhumation of subducted continental crust. Ephemeral rhyolite melt inclusions were found preserved in peridotite xenoliths from Tallante (Betic Cordillera, southern Spain) that are remnants of a supra-subduction mantle wedge. Here, the interaction of silica-rich melts with peridotite generated hybrid mantle domains, characterized by the occurrence of millimetre-sizes felsic veins with crust-like Sr-Nd-Pb-O- isotope compositions. The “Tallante” composite xenoliths were found among a wide population of peridotitic xenoliths, and display extreme compositional and isotopic heterogeneities both within the ambient peridotite and within the felsic veins. The latter consist of orthopyroxene, plagioclase, and quartz, and they are separated from the surrounding peridotite by an orthopyroxene-rich reaction zone. In their mineral phases, rhyolite glass inclusions and interstitial films associated to quartz crystals were observed. Petrological evidence and thermodynamic modelling indicate that rhyolite melts were originated by partial melting of near an-hydrous garnet-bearing metapelites at temperatures above 1000 °C. Partial melting was likely triggered by near-isothermal decompression during rapid exhumation of previously subducted crustal slivers. The melts reacted with the ambient lithospheric mantle at lower temperature (900 °C) and produced orthopyroxene, followed by plagioclase, quartz, and phlogopite. On the basis of chemical characteristics, it is hypothesized that potassic (HK-calc-alkalic to shoshonitic) and  ultrapotassic magmas may originate from metasomatic mantle sources generated from the interaction of crustal rhyolitic melts with mantle peridotite.

The application of U-Pb dating method performed on calcite has exponentially increased over the last years, since constraining the age of the crystallization for such syn-kinematic minerals, would provide a specific timing of faults movement. The potential gain of this approach is evident but the robustness of the U-Pb method performed on calcites has been not yet systematically tested.  Here, we firstly demonstrated that a mineral/fluid interaction indeed affects the regression of the 238U/206Pb and 207Pb/206Pb data-points and therefore the age; and secondly, we propose an innovative application of U-Pb dating method and a new strategy to identify and reject analytically robust isochrons.We explore 36 samples, combining U-Pb dating performed with different methods along with carbon and oxygen stable isotopes compositions measured on the same fibres of calcite. The extremely high precision 207Pb/206Pb measured by Thermal Ionisation Mass Spectrometry  revealed that every sample experienced a certain degree of fluid interaction. We find no correlation between 238U/206Pb and the spread in δ18O. The higher spread in δ18O is instead coupled with a remarkable scattered data-points that yield U-Pb ages calculated with the different methods on the same samples with a large variability. In conclusion, our study demonstrates that great care must be taken when considering radiometric ages made on calcite since LA-ICP-MS large uncertainties might obscure the isotopic reorganization.

The occurrence of rhyolite melts in the mantle has been predicted by high pressure-high temperature experiments but never observed in nature. Here we report natural quartz-bearing rhyolitic melt inclusions and interstitial glass within peridotite xenoliths. The oxygen isotope composition of quartz crystals shows the unequivocal continental crustal derivation of these melts, which approximate the minimum composition in the quartz-albite-orthoclase system. Thermodynamic modelling suggests rhyolite was originated from partial melting of near-anhydrous garnet-bearing metapelites at temperatures ~1000 °C and interacted with peridotite at pressure ~1 GPa. Reaction of rhyolite with olivine converted lherzolite rocks into orthopyroxene-domains and orthopyroxene + plagioclase veins. The recognition of rhyolitic melts in the mantle provides direct evidence for element cycling through earth’s reservoirs, accommodated by dehydration and melting of crustal material, brought into the mantle by subduction, chemically modifying the mantle source, and ultimately returning to surface by arc magmatism.

Most orogenic belts are close to convergent plate margins. However, some orogens are formed far away from plate boundaries, as a result of compressional stress propagating within plates, basal loading, or a combination of thereof. We focus on the Atlas of Morocco, which is such an intraplate orogeny and shows evidence of mantle driven uplift, and plume-related volcanism. How these processes interact each other is still poorly constrained and it provides clues about intraplate stress propagation, strain localization, and lithospheric weakening due to mantle dynamics. We present three sets of observations constructed by integrating previous data with new analyses. Crustal and thermal evolution constraints are combined with new analyses of topographic evolution and petrological and geochemical data from the Anti-Atlas volcanic fields. Our findings reveal that: i) crustal deformation and exhumation started during middle/late Miocene, contemporaneous with the onset of volcanism; ii) volcanism has an anorogenic signature with a deep source; iii) a dynamic deep mantle source supports the high topography. Lastly, we conducted simple numerical tests to investigate the connections between mantle dynamics and crustal deformation. This leads us to propose a model where mantle upwelling and related volcanism weaken the lithosphere and favor the localization of crustal shortening along pre-existing structures due to plate convergence.