Coltorti, Massimo

This study provides new mineral chemistry data together with micro-thermometric measurements on fluid inclusions hosted in ultramafic xenoliths (lherzolite, wehrlite, and dunite) brought to the surface by the last Mt. Vulture volcano activity (140 ka; southern Italy), and fed by melilitite-carbonatite magmas. Petrographic evidence and mineralogical compositions of Mt. Vulture xenoliths are consistent with an origin in the upper mantle. Fluid inclusions in rock-forming minerals of lherzolite and wehrlite xenoliths are CO2-dominated. The equilibrium temperature calculated by geothermometric estimates ranges from 1039 C (±36°C) to 1142°C (±15°C), and entrapment pressures of fluid inclusions with post-trapping re-equilibration correspond to the local crust–mantle boundary (32 km depth), and to a shallow reservoir located at 12–14 km depth. These results contribute to constrain the origin of these xenoliths and the depth of storage of magmas erupted from Mt. Vulture, where carbonatite-like metasomatism and mantle-derived CO2 degassing occur.

Nominally anhydrous minerals (NAMs) may contain significant amounts of water and constitute an important reservoir for mantle hydrogen. The colloquial term ‘water’ in NAMs is related to the presence of hydroxyl-bearing (OH􀀀 ) point defects in their crystal structure, where hydrogen is bonded to lattice oxygen and is charge-balanced by cation vacancies. This hydrous component may therefore have substantial effects on the thermoelastic parameters of NAMs, comparable to other major crystal-chemical substitutions (e.g., Fe, Al). Assessment of water concentrations in natural minerals from mantle xenoliths indicates that olivine commonly stores ~0–200 ppm of water. However, the lack of samples originating from depths exceeding ~250 km coupled with the rapid diffusion of hydrogen in olivine at magmatic temperatures makes the determination of the olivine water content in the upper mantle challenging. On the other hand, numerous experimental data show that, at pressures and temperatures corresponding to deep upper mantle conditions, the water storage capacity of olivine increases to 0.2–0.5 wt%. Therefore, determining the elastic properties of olivine samples with more realistic water contents for deep upper mantle conditions may help in interpreting both seismic velocity anomalies in potentially hydrous regions of Earth’s mantle as well as the observed seismic velocity and density contrasts across the 410-km discontinuity. Here, we report simultaneous single-crystal X-ray diffraction and Brillouin scattering experiments at room temperature up to 11.96(2) GPa on hydrous [0.20(3) wt% H2O] Fo90 olivine to assess its full elastic tensor, and complement these results with a careful re-analysis of all the available single-crystal elasticity data from the literature for anhydrous Fo90 olivine. While the bulk (K) and shear (G) moduli of hydrous Fo90 olivine are virtually identical to those of the corresponding anhydrous phase, their pressure derivatives K′ and G′ are slightly larger, although consistent within mutual uncertainties. We then defined linear relations between the water concentration in Fo90 olivine, the elastic moduli and their pressure derivatives, which were then used to compute the sound velocities of Fo90 olivine with higher degrees of hydration. Even for water concentrations as high as 0.5 wt%, the sound wave velocities of hydrous and anhydrous olivines were found to be identical within uncertainties at pressures corresponding to the base of the upper mantle. Contrary to previous claims, our data suggest that water in olivine is not seismically detectable, at least for contents consistent with deep upper mantle conditions. In addition to that, our data reveal that the hydration of olivine is unlikely to be a key factor in reconciling seismic velocity and density contrasts across the 410-km discontinuity with a pyrolitic mantle.

Petrology and fluid inclusions (FI) geochemistry are increasingly used in tandem to constrain the compositional features and evolution of the lithospheric mantle. In this study, we combine petrography and mineral chemistry with analyses of noble gases (He, Ne and Ar) and CO2 in olivine, orthopyroxene- and clinopyroxene-hosted FI, as well as radiogenic isotope (Sr-Nd-Pb) systematics of ultramafic xenoliths collected at La Grille volcano in Grande Comore Island, aiming at better characterizing one of the most enigmatic and controversial portions of the western Indian Ocean lithospheric mantle. Xenoliths have been divided in three groups on the basis of their textural features: Group 1 (Opx-bearing), Group 2 (Opx-free) and Group 3 (Cumulate). Overall, petrographic observations and mineral phase compositions indicate that the sampled lithospheric portion experienced variable degrees of melting (from 5% to 35%), recorded by Group 1 most refractory harzburgites and lherzolites, as well as modal metasomatic processes as evidenced by the crystallization of cpx at the expense of opx in Group 1 fertile lherzolites and wehrlite and by Group 2 xenoliths. Crystallization of slightly oversaturated basic silicate melts seems also to have occurred, as shown by Group 3 xenoliths. A positive trend between temperature and ƒO2 is evident, with Group 2 and 3 xenoliths testifying for hotter and more oxidised conditions than Group 1. The variability of the 4He/40Ar* ratio (0.02–0.39) in Group 1, significantly below typical values of a fertile mantle (4He/40Ar* = 1–5), can be explained by the variable degrees of partial melting coupled to metasomatic enrichment that may account for modifying 4He/40Ar*, as also indicated by the mineral composition. He-Ar-CO2 relationships support the presence of a metasomatic CO2-rich process post-dating the melt extraction and the cumulate formation. The air-corrected 3He/4He isotopic ratios (6.30 to 7.36 Ra) are intermediate between the MORB mantle signature (8 ± 1Ra) and the SCLM (6.1 ± 0.9 Ra). The Ne and Ar isotopic signatures (20Ne/22Ne, 21/Ne/22Ne and 40Ar/36Ar) are consistent with mixing between an air-derived component and a MORB-like mantle, supporting the hypothesis for a lithospheric origin of the Comoros magmas, and arguing against any deep mantle plume-related contribution. This is also corroborated by combining Ne with He isotopes, showing that La Grille ultramafic xenoliths are far from the typical plume-type compositions. Sr-Nd-Pb isotope systematics in opx and cpx from La Grille additionally support a MORB-type signature for the lithospheric mantle beneath the area.

Open-conduit basaltic volcanoes can be characterised by sudden large explosive events (paroxysms) that interrupt normal effusive and mild explosive activity. In June-August 2019, one major explosion and two paroxysms occurred at Stromboli volcano (Italy) within only 64 days. Here, via a multifaceted approach using clinopyroxene, we show arrival of mafic recharges up to a few days before the onset of these events and their effects on the eruption pattern at Stromboli, as a prime example of a persistently active, open-conduit basaltic volcano. Our data indicate a rejuvenated Stromboli plumbing system where the extant crystal mush is efficiently permeated by recharge magmas with minimum remobilisation promoting a direct linkage between the deeper and the shallow reservoirs that sustains the currently observed larger variability of eruptive behaviour. Our approach provides vital insights into magma dynamics and their effects on monitoring signals demonstrating the power of petrological studies in interpreting patterns of surficial activity.

The investigation of the role played by CO2 circulating within the mantle during partial melting and metasomatic/refertilization processes, together with a re-consideration of its storage capability and re-cycling in the lithospheric mantle, is crucial to unravel the Earth's main geodynamic processes. In this study, the combination of petrology, CO2 content trapped in bulk rock- and mineral-hosted fluid inclusions (FI), and 3D textural and volumetric characterization of intra- and inter-granular microstructures was used to investigate the extent and modality of CO2 storage in depleted and fertile (or refertilized) Sub-Continental Lithospheric Mantle (SCLM) beneath northern Victoria Land (NVL, Antarctica). Prior to xenoliths entrainment by the host basalt, the Antarctic SCLM may have stored 0.2 vol% melt and 1.1 vol% fluids, mostly as FI trails inside mineral phases but also as inter-granular fluids. The amount of CO2 stored in FI varies from 0.1 μg(CO2)/g(sample) in olivine from the anhydrous mantle xenoliths at Greene Point and Handler Ridge, up to 187.3 μg/g in orthopyroxene from the highly metasomatized amphibole-bearing lherzolites at Baker Rocks, while the corresponding bulk CO2 contents range from 0.3 to 57.2 μg/g. Irrespective of the lithology, CO2 partitioning is favoured in orthopyroxene and clinopyroxene-hosted FI (olivine: orthopyroxene = 0.10 ± 0.06 to 0.26 ± 0.09; olivine: clinopyroxene = 0.10 ± 0.05 to 0.27 ± 0.14). The H2O/(H2O + CO2) molar ratios obtained by comparing the CO2 contents of FI to the H2O amount retained in pyroxene lattices vary between 0.72 ± 0.17 and 0.97 ± 0.03, which is well comparable with the values measured in olivine-hosted melt inclusions from Antarctic primary lavas and assumed as representative of the partition of volatiles at the local mantle conditions. From the relationships between mineral chemistry, thermo-, oxybarometric results and CO2 contents in mantle xenoliths, we speculate that relicts of CO2-depleted mantle are present at Greene Point, representing memory of a CO2-poor tholeiitic refertilization related to the development of the Jurassic Ferrar large magmatic event. On the other hand, a massive mobilization of CO2 took place before the (melt-related) formation of amphibole veins during the alkaline metasomatic event associated with the Cenozoic rift-related magmatism, in response to the storage and recycling of CO2-bearing materials into the Antarctica mantle likely induced by the prolonged Ross subduction.

Crystal zoning plays a fundamental role in modern volcanology as a key to unravel the geometry and the dynamics of plumbing systems. In this study, a detailed textural and compositional study of clinopyroxene crystals entrained in intrusive, hypabyssal and effusive products from Cima Pape (Dolomites) is coupled with thermobarometric-hygrometricmodels to reconstruct the geometry and evolution of the feeding systembeneath Middle Triassic volcanic edifices. Whole-rock major, trace element distribution and Sr-Nd isotopic signature (87Sr/86Sri = 0.7045–0.7050; 143Nd/144Ndi = 0.51223–0.51228) show that the rocks from Cima Pape are SiO2- saturated and have shoshonitic affinity, and likely belong to the acme of the Mid-Triassic magmatismthat shaped the Southern Alps between 239 and 237.6Ma. Highly porphyritic trachybasaltic to basaltic trachyandesitic volcanic rocks contain a large number of concentric-zoned clinopyroxene crystals. Here, high-Mg# and -Cr2O3, REEdepleted bands (Mg# 80–91; Cr2O3 up to 1.2 wt%) with variable thickness grew between relatively low-Mg# and -Cr2O3 (Mg# 70–77; Cr2O3 < 0.1 wt%) augitic cores and rims. In contrast, the gabbroic to monzodioritic 50- to 300-m-thick sill cropping out belowthe volcanic sequences, though to represent a relic of the shallowest portion of the plumbing system, is mostly made up of unzoned clinopyroxene crystals. Thermobarometric and hygrometric models allowed us to define that a small “mush-type” batchwas located beneath the Cima Pape volcano at depths between 7 and 14 km. Here, augitic clinopyroxene formed in equilibrium with a slightly evolved (basaltic trachyandesitic), H2O-rich melt (Mg# = 43–45; T = 1035–1075 °C; H2O = 2.6–3.8 wt%). Periodic replenishments of the magma batch by primitive (Mg# = 65–70), hotter and relatively H2O-poor (T = 1130–1150 °C; H2O = 2.1–2.8 wt%) basaltic magmas led to the formation of diopsidic bands mantling the already formed augitic cores. Later on, re-equilibration of clinopyroxene with the mixed melt resulted in the formation of low-Mg#, LILE- and LREE-enriched rims. The most Mg-poor micro-phenocrystic clinopyroxene in the volcanic rocks and in the sill records the ultimate and shallowest conditions of crystallization, occurring at T of 975–1010 °C and P comprised between 50 and 150 MPa. Based on the presence of similar zoning in clinopyroxene phenocrysts, a comparison between the Mid-Triassic Cima Pape and active volcanoes was put forward to highlight the potential of studying ancient, entirely exposed volcanic systems for interpreting the feeding systemprocesses acting beneath active volcanoes. At a regional scale, this approach represents a new, powerful tool for investigating the evolution of the Mid-Triassic magmatism in the Southern Alps and shedding light on the interactions between mantle-derived melts and differentiated batches ponding in the crust.

The Comoros archipelago is an active geodynamic region located in theMozambique Channel between East continental Africa andMadagascar. The archipelago results from intra-plate volcanism, the most recent eruptions having occurred on the youngest island of Grande Comore and on the oldest one of Mayotte. Since 2018, the eastern submarine flank of Mayotte has been the site of one of the largest recent eruptive events on Earth in terms of erupted lava volume. On land, the most recent volcanic activity occurred in Holocene on the eastern side of Mayotte, corresponding to the small Petite Terre Island,where twomain and persistent gas seep areas are present (Airport Beach, namely BAS, and Dziani Dzaha intracrateric lake). The large submarine eruption at the feet of Mayotte (50 km offshore; 3.5 km b.s.l.) is associated with deep (mantle level) seismic activity closer to the coast (5–15 km offshore) possibly corresponding to a single and large magmatic plumbing system. Our study aims at characterizing the chemical and isotopic composition of gas seeps on land and assesses their potential link with the magmatic plumbing system feeding the Mayotte volcanic ridge and the recent submarine activity. Data from bubbling gases collected between 2018 and 2021 are discussed and compared with older datasets acquired between 2005 and 2016 from different research teams. The relation between 3He/4He and 13C-CO2 shows a clear magmatic origin for Mayotte bubbling gases, while the variable proportions and isotopic signature of CH4 is related to the occurrence of both biogenic and abiogenic sources of methane. Our new dataset points to a time-decreasing influence of the recent seismo-volcanic activity at Mayotte on the composition of hydrothermal fluids on land, whose equilibriumtemperature steadily decreases since 2018. The increased knowledge on the gas-geochemistry at Mayotte makes the results of this work of potential support for volcanic and environmental monitoring programs

Carbon in the upper mantle controls incipient melting of carbonated peridotite and so acts as a critical driver of plate tectonics. The carbon-rich melts that form control the rate of volatile outflux from the Earth’s interior, contributing to climate evolution over geological times. However, attempts to constrain the carbon concentrations of the mantle source beneath oceanic islands and continental rifts is complicated by pre-eruptive volatile loss from magmas. Here, we compile literature data on magmatic gases, as a surface expression of the pre-eruptive volatile loss, from 12 oceanic island and continental rift volcanoes. We find that the levels of carbon enrichment in magmatic gases correlate with the trace element signatures of the corresponding volcanic rocks, implying a mantle source control. We use this global association to estimate that the mean carbon concentration in the upper mantle, down to 200 km depth, is approximately 350 ppm (range 117–669 ppm). We interpret carbon mantle heterogeneities to reflect variable extents of mantle metasomatism from carbonated silicate melts. Finally, we find that the extent of carbon enrichment in the upper mantle positively correlates with the depth at which melting starts. Our results imply a major role of carbon in driving melt formation in the upper mantle.

The possibility of constraining the composition and evolution of specific portions of the Sub-Continental Lithospheric Mantle (SCLM) by means of an integrated study of petrography, mineral chemistry, and concentrations of volatiles in fluid inclusions (FI) is a novel approach that can provide clues on the recycling of volatiles within the lithosphere. This approach is even more important in active or dormant volcanic areas, where the signature of the gaseous emissions at the surface can be that of the underlying lithospheric mantle domains. In this respect, the ultramafic xenoliths brought to the surface in West Eifel (~0.5–0.01 Ma) and Siebengebirge (~30–6 Ma) volcanic fields (Germany) are ideal targets, as they provide direct information on one of the most intriguing portions of SCLM beneath the Central European Volcanic Province (CEVP). Five distinct populations from these localities were investigated using petrographic observations, mineral phase analyses and determination of He, Ne, Ar and CO2 contents in olivine-, orthopyroxene-, and clinopyroxene-hosted FI. The most refractory Siebengebirge rocks have highly forsteritic olivine, high-Mg#, low-Al pyroxene, and spinel with high Cr#, reflecting high extents (up to 30%) of melt extraction. In contrast, xenoliths from West Eifel are modally and compositionally heterogeneous, as indicated by the large forsterite range of olivine (Fo83–92), the Cr# range of spinel (0.1–0.6), and the variable Al and Ti contents of pyroxene. Equilibration temperatures vary from 870 ◦C to 1070 ◦C in Siebengebirge, and from ⁓900 ◦C to ⁓1190 ◦C in West Eifel xenoliths, at oxygen fugacity values generally between 􀀀 0.5 and + 1.3 ΔlogƒO2 [FMQ]. In both areas, the FI composition was dominated by CO2, with clinopyroxene, and most of the orthopyroxene had the highest concentrations of volatiles, while olivine was gas-poor. The noble gas and CO2 distributions suggest that olivine is representative of a residual mantle that experienced one or more melt extraction episodes. The 3He/4He ratio corrected for air contamination (Rc/Ra values) varied from 6.8 Ra in harzburgitic lithotypes to 5.5 Ra in lherzolites and cumulate rocks, indicating that the original MORB-like mantle signature was progressively modified by interaction with crustal-related components and melts having 3He/4He and 4He/40Ar* values consistent with those published for magmatic gaseous emissions. The Ne and Ar isotope systematics indicated that most of the data were consistent with mixing between a recycled atmospheric component and a MORB-like mantle, which does not necessarily require the involvement of a lower mantle plume beneath this portion of the CEVP. The major element distribution in mineral phases from West Eifel and Siebengebirge, together with the systematic variations in FI composition, the positive correlation between Al enrichment in pyroxene and equilibration temperatures, and the concomitant Rc/Ra decrease with increasing temperature, suggest that the SCLM beneath Siebengebirge represented the Variscan lithosphere in CEVP prior to the massive infiltration of melts/fluids belonging to the Quaternary Eifel volcanism. In contrast, West Eifel xenoliths reflect multiple heterogeneous metasomatism/refertilisation events that took place in the regional SCLM between ~6 and ~ 0.5 Ma.

Plutons in crustal shear zones may exploit inherited structures, interfere with strain localizing or be deformed passively. To constrain the relative timing of such tectono-magmatic relationships in natural settings is not always straight-forward. We here present sandbox-type analogue model experiments simulating magma emplacement into simple and transtensional crustal shear zones to test the diagnostic potential of pluton shape with respect to timing and setting. Observations based on surface deformation and intrusion shape exemplify the interplay between evolving and inherited tectonic structures and magma uprising. We observe markedly asymmetric intrusions in association with dikes reflecting the regional stresses, fault pattern and finite strain field. At the same time, the presence of an intrusion modifies the tectonic evolution, but only transiently, resulting in short-lived faults, reactivation and inversion. Diagnostic attributes include the pluton’s aspect ratio, its orientation and amplitude as well as dike association. Accordingly, syn-tectonic intrusions show the highest pluton amplitudes, but only intermediate elongation compared to other scenarios. They are oriented parallel to Riedel shears in simple shear, respectively to the compression direction in transtension. Post-tectonic intrusions are least elongated, have medium amplitudes and exploit Riedel shears. Pre-tectonic intrusions are characterized by lowest amplitudes but the highest aspect ratios and are parallel to the finite elongation direction. Intrusions in transtensional shear zones are generally of less elongate than those in simple shear zones. Experimental results are tested against observations from natural examples validating the diagnostic potential of pluton shape for the timing and the tectonic setting of the emplacement.