The Manda Hararo rift segment in the Afar depression (Ethiopia) represents a key location to study the timing, style, and evolution of volcanic activity within an active continental rift close to the Ocean-Continent Transition (OCT) stage. Despite its importance, the southern part of this segment has remained poorly documented. In this study, we provide a detailed mapping based on a combination of medium-resolution spectral imagery and a unique dataset of very-high-resolution Pléiades images. We combine this map with volcanological observations, major and trace element analyses, and cosmogenic 36Cl surface exposure dating to investigate the very last eruptions and volcanic history of two sub-segments (Gumat Mali and Gablaytu). Our results show that the last volcanic activity occurred at Gablaytu between ~3.20 and 1.50 ka and at Gumat Mali between ~1.90 and 0.61 ka. Petrological data indicate that the Gumat Mali sequence was driven by reservoir recharge with new, less differentiated magma that hybridized with a more evolved mush, enhancing magma mobilization. The Gablaytu sequence records progressive magma differentiation and plagioclase assimilation and/or accumulation. For both sub-segments, these processes control magma evolution prior to eruption and indicate a complex transcrustal plumbing system characterized by magma storage, recharge, and interaction within vertically connected reservoirs. The documentation of these historical and recent volcanologic activities also help to draw potential regional hazards for populations settled nearby these volcanic centres, including parts of Semera district, which should consider both effusive episodes from axial fissures and potentially extremely energetic phreatomagmatic phase, that characterize the initiation of the historical sequence at Gumat mali.Our observations support the recognition of two self-consistent sub-segments within the wide Central Afar magmatic system with their own associated plumbing system: the Gumat Mali sub-segment located in the direct prolongation of the Dabbahu segment corridor and the en-échelon lateral Gablaytu sub-segment along the Manda and Unda Hararo Hararo axis. This illustrates the intimate links between tectonic structures, magma storage and upward migration in controlling the expression of first and second order segmentation in such zones of the Afar depression close to the OCT.

We investigated geothermal gases from Homa Hills, a carbonatitic complex situated along an adjacent branch of the Kenyan rift system, using neon, argon, krypton, xenon and nitrogen isotopes. Large quantities of gas were sampled in Giggenbach-type bottles (Giggenbach, 1975) and analyzed by dynamic mass spectrometry to resolve isotopic variations at high precision (0.01-0.1 parts per thousand; Seltzer and Bekaert, 2022; Bekaert et al., 2023; 2024). Neon and nitrogen isotope compositions are consistent with parental magmas being derived from the convecting mantle. Xenon isotopic data present ubiquitous enrichments (relative to air) of 129Xe from the decay of extinct 129I (T1/2 = 15.7 Myr) and 131-136Xef from fissions of 238U (T1/2 = 4.468 Myr) and/or 244Pu (T1/2 = 82 Myr). We also find slight excesses of 128Xe (relative to 130Xe and air), which could be due to subsurface isotopic fractionation during e.g., diffusive transport fractionation (DTF) and gravitational settling. However, the 128Xe excesses are not accompanied by correlated Kr isotope excesses and plot off the empirical fractionation line defined from several other locations worldwide (Bekaert et al., 2023). Instead, a detailed isotope deconvolution suggests the occurrence of either chondritic Xe (with mantle 130Xe consisting of up to 22 % of chondritic 130Xe) or recycled Xe from the Archean atmosphere could explain the observed Xe isotope signatures. The latter possibility would have profound implications for models of mantle-surface exchange throughout Earth history. The fission spectra indicate a predominantly 238U origin for fissiogenic Xe, with contribution of 244Pu-derived Xe being negligible within uncertainties, implying extensive mantle degassing during the Hadean and Archean eons. The 129Xe*/136Xe* ratio (where * indicates non-atmospheric excesses of Xe isotopes) of Homa Hills samples correlates with other tracers of mantle/crust contributions such as He, Ar and N isotopes. Variations in 129Xe*/136Xe* among the different gases sampled at Homa Hills is mainly the result of contribution from fissiogenic Xe produced in uranium-rich crustal material. Therefore, this ratio may constitute a robust tracer of mantle-crust interactions. Given available high precision data (Bekaert et al., 2023; 2024; this work) together with mantle-derived rock data, 129Xe*/136Xe* appears homogenous in the convecting mantle, and comparable to values observed at mantle plumes. Such homogeneity is in sharp contrast with light noble gas systematics and may call for whole mantle convection and a core origin for He and Ne..

The Afar depression at the northern end of the East African Rift system is presently experiencing the final stage of continental break-up and progressive onset of steady magmatic spreading. The Magmatic Rift Segments in Afar broadly analogous to those observed within the mid oceanic ridges, offer the opportunity to study both mantle and crustal processes. Investigating the crustal architecture of those magmatic segments represents a key aspect to decipher fundamental parameters that control focussing of magmatic and tectonic activity during the generation of magmatic crust. Here, we present the typical organization of a 32 km long subsegment of the Manda Hararo magmatic rift system, with fissural activities symmetrical to an apparent mid segment magmatic reservoir and establish geochronology of the last eruptive history. We combine field investigations, precise mapping of volcanological and tectonic features, cosmogenic 36Cl exposure dating and geochemical analysis of lavas to constrain the temporal frame and the dynamics of magmatic processes. Our results show that the recent historical volcanic events (~ 500 to 2000 years) are sourced from calderas and fissures representing an alternating sequence of effusive and explosive (block fields) activities related to a coherent rifting episode along a single self-consistent magmatic sub-segment. Those recent fissural flows resurfaced a large portion of the segment and emplaced on older thick pahoehoe flows with a rather long lag-time of about 75 kyr separating the two episodes. Strongly contrasted geochemical signatures are also observed between those two volcanic episodes, with more differentiated and trace elements enriched basalts for the recent one, compared to the older one which are characterized by a unusual depleted signature. These new results for the Central Afar Manda Hararo rift have important implications for: (i) the local hazards along the segments, and (ii) the volcano-tectonic organization of the segment with coexistence of contrasted melt reservoirs on the underlying transcrustal plumbing system.

The Earth surface, where life develops and stands, is strongly affected by denudation which is the sum of physical erosion and chemical weathering. Denudation impacts soil formation and agriculture, affects the relief stability and, at the geological time scale, controls the atmospheric CO2 via the weathering of silicates and the production of sediments that later bury organic matter in the oceans. In the context of global warming, it is particularly important to predict how denudation will change and hence impact the Earth Surface where we live. This requires to understand the links between past climate variability and denudation changes, especially during the Quaternary when Earth experienced rapid climate oscillations of amplitude similar to what is expected in the future due to anthropic impact. To reach this goal, quantitative estimate of past denudation rates during the Quaternary are needed especially in Volcanic island located in tropics because here silicate weathering and hence CO2 consumption is particularly efficient. In this study, we reconstruct Quaternary paleo-denudation rates in Santoa Antao, one of the largest islands of the Cabo Verde archipelago that is located in the Atlantic ocean 800 km off the coast of Senegal. To reconstruct the paleo-denudation rates we measured in situ cosmogenic 3He concentrations in ancient fluvial sediments stored in deep entrenched valleys across the island. The depositional ages of sediments were determined by dating using Ar/Ar adjacent volcanic layers (pumices or basalt lavas). For comparison between all data, paleo-denudation rates are normalized to modern 3He derived denudation rates across the same drainage basin obtained from the analysis of modern river sand in a previous study. This yields to a 0-550ka record of paleo-denudation rates that is compared to climate variations to discuss the potential links between the two.

Iceland is a location of geological interest due to the combination of upwelling mantle plume and divergent plate boundary, which resulted in the formation of its extensive surface area (>100,000 km(2)) that rises above sea-level. This unique setting facilitates assessing the role of the underlying mantle plume and tectonic activity on crust-forming processes. Helium isotopes provide a useful tool in this regard, as they can identify physical processes and resolve deep and shallow fluid sources in the crust. In Iceland, the highest He-3/He-4 for geothermal fluids are found in Vestfir & eth;ir with values up to 29 R-a (where R-a is the He-3/He-4 of air), more than 110 km away from current active rift zones. Such locations are key to understand the extent of mantle degassing processes associated with the high buoyant Icelandic mantle plume. Other off-rift regions, such as most of North Iceland, have not been extensively investigated, despite the widespread presence of geothermal activity. Although North Iceland has been volcanically inactive for the past 0.8 Ma, severe earthquake hazards associated with mature and partially on-land transform zones have occurred, rendering the monitoring of the full tectonic-hydrogeochemical system of societal importance. Our study in North Iceland aimed to (i) assess temporal variations in helium isotopic signatures in low-T geothermal water and their relationship with regional earthquakes, (ii) diminish the helium isotope data gap in geothermal fluids of this region, and (iii) elucidate both local and regional processes controlling the He isotope systematics in this region as a case study for other off-rift contexts on Earth. In order to achieve these goals, we report helium isotope time series data collected from June 2020 to October 2022 from a borehole in Hafral ae kur, A & eth;aldalur valley (95 samples collected on a near-weekly basis), along with an isotope survey (delta H-2-He-3/He-4-delta C-13(TDIC)-delta O-18-delta S-34(SO4)) of North Iceland geothermal fluids (T < 130 degrees C, n = 36 samples). The results indicate a large regional variability in helium isotope ratios (4 to 27 R-a) that is comparable to the entire range evident in geothermal fluids across Iceland (similar to 1 to 29 R-a) where the maximum He-3/He-4 signature is among the highest measured in geothermal fluids from oceanic and continental hotspots globally. Several processes, both on regional and local scales, are needed to account for this large range: (i) influence of a deeply-derived mantle flux evidenced by a high He-3/He-4 mantle component, degassing via fault systems, (ii) release of local radiogenic helium components, potentially associated with seismic events along the Dalv & iacute;k Lineament, and (iii) local groundwater mixing, for example evident at the Hafral ae kur site and documented by periodic M > 5 seismic events. The estimated magmatic helium flux for the entire study region is comparable to that of the mid-ocean ridge, where mantle-derived melts intruded in the crust are actively degassing, confirming the large-scale degassing of the Iceland plume. As basalts and their source materials can be affected by radiogenic additions and temporal variations, we postulate that geothermal fluids may better reflect the maximum present-day He-3/He-4 plume signature.

The Asal Rift, situated within the Afar region, presents a unique opportunity to study continental rifting and ongoing break-up mechanisms. Here we present a comprehensive study based on volatile element contents of magmas within the Asal rift's segment. Samples were gathered from various volcanic sub-segments within the active part of the Asal rift, and a subset was collected to document the successive steps of the recent 1978 Ardoukoba eruption. Altogether our new sampling is offering a chronological framework crucial to understanding this magmatic system and the eruptive sequences. Quenched pyroclastic deposits (scoria) were used as they are more likely to preserve the magma's volatile content without significant degassing upon cooling at surface than lava flows. By analyzing over 400 melt inclusions within plagioclase and olivine crystals of 15 new samples, we provide insight into the pre-eruptive volatile content of Asal magmas. The melt inclusions volatile contents (H2O, CO2, Cl, and S) were quantified through SIMS analyses at CRPG. Sample preparation for SIMS measurements was carefully achieved to avoid any contamination or volatile loss. After estimation of the volatile migration through the shrinkage bubbles of the melt inclusions by Raman spectroscopy, and correction from post-entrapment crystallization processes, we reconstructed the initial magmatic volatile content at the reservoir depth. This content, combined with detailed petrographic study, field observations, and new dating, allows us to propose a comprehensive picture of the Asal Rift plumbing system architecture, and to discuss its spatial variability and temporal evolution. The wealth of data allowed us to highlight a relative homogeneity of the reservoir volatile contents over the recent Asal rift segment erupted magmas, and to discern the depth range of the Asal igneous reservoir that spans from approximately 5 km to 25 km (based on solubility models of VESIcal (1)). Furthermore, volatile data, combined with in-situ major and trace element analysis, provides insights into magma differentiation, degassing, and into the volatile content of the mantle source. We investigate 3 potential steps of degassing and their effect on volatile, trace and major element: within the plumbing system (during the differentiation), during magma ascent, and at the surface during the eruption (unlikely given the sampling method). Finally, our study focused on the 1978 Ardoukoba eruption within the Asal rift. With a sampling of the entire eruptive sequence, we were able to highlight the evolution of the volatile content during this eruption, showing that the initial differentiated magma reservoir underwent a recharge event before eruption. In conclusion, this new in-situ high-resolution volatile, trace and major element extended dataset 1/ delineated the extensive depth range of the magmatic reservoir, 2/ allows a better understanding of the dynamics of magma reservoirs that feed continental rift systems, and 3/ provides new constraints on the magma evolution, differentiation, and degassing at depth within such a system. (1) Iacovino, K., Matthews, S., Wieser, P. E., Moore, G. M., & Bégué, F. (2021). VESIcal Part I: An Open‐Source Thermodynamic Model Engine for Mixed Volatile (H2O‐CO2) Solubility in Silicate Melts. Earth and Space Science, 8(11).

We present a new stratigraphy of a 50 × 20 km area of the Ethiopian Plateau near Seladingay in northern Shewa province, some 160 km northeast of Addis Ababa. Situated near the rim of the plateau where the Afar depression funnels southwest into the Ethiopian rift, the area consists of > 1200 m of basalt lavas and interbedded rhyolitic volcanics. We describe three detailed stratigraphic sections and establish stratigraphic units on the basis of lithology and thin-section petrography, placed in the context of the regional stratigraphic framework and existing geochronology. We identify and name five new formations, each a package of either basaltic or rhyolitic units. Interlayered sedimentary strata and paleosols attest to intervals of volcanic quiescence. Likely initiating in the Paleogene, volcanism in our study area differs from the coeval Ethiopian Traps both in terms of lava composition and mechanism of magma genesis and extends into the Miocene ( 15 Ma). From this, we determine that the first four of the volcanic formations in the central Ethiopian Plateau record a unique episode of volcanism in East Africa.

Mineralogical and geochemical studies have been undertaken on the Triassic to Lower Jurassic Adigrat Sandstone of the Blue Nile Basin of central Ethiopia to infer its source rock type, paleoweathering, and paleoclimatic history. The Adigrat Sandstone occurs at the basal section of the Mesozoic sedimentary formation and unconformably overlays the Neoproterozoic–Paleozoic crystalline rocks, or locally, the Karroo sediments in the northern Blue Nile Basin. A mineralogical study reveals that quartz(Q), feldspars(F), and lithic fragments(L) are the framework grains of the sandstone. On the QFL diagram, the plot of the modal composition of the sandstone mainly falls within the feldspathic arenite and quartzose arenite fields. The geochemical data of the lower section of the sandstone mainly falls within the arkose and subarkose fields, whereas the upper section data falls within the quartzose and sublithic arenite fields. Mineralogical and geochemical weathering indices indicate that the provenances of the Adigrat Sandstone were exposed to pronounced weathering intensity, where the lower part of the sandstone was controlled by arid to semi-arid conditions, whereas the upper section was linked to humid to semi-humid(tropical to subtropical) climatic conditions. Mineralogical and geochemical data also indicate that mafic to intermediate basement rocks were the primary source rocks of the sediment. Perhaps the sediment was assumed to have been reworked by multi-cyclic sedimentary processes. The discriminant function diagram, the REE pattern, La/Th vs. La/Yb, and the Th–Hf–Co plot are consistent. A comparison of provenance studies for the Adigrat Sandstone in the Blue Nile Basin and the Mekele outlier of northern Ethiopia indicates that the sediment of the former is highly sorted, experienced higher weathering intensity, and compositionally derived from mafic to intermediate crystalline rocks. On the other hand, the sediment of the latter is essentially a weathering product of felsic rocks.

Intense hydrothermal activity hosted by regional tectonic structures occurs in the Eastern part of the Pyrenees. Helium isotope ratios (3He/4He) in hot springs along the T & ecirc;t (0.033-0.099 Ra) and the Tech (0.171-0.375 Ra) faults indicate different signatures from purely crustal to slightly contaminated by magmatic helium. 3He/4He ratio increased towards the East, consistently with the observed thinning of the continental crust. These results suggest localised and not interconnected hydrothermal systems at fault scale. The origin of magmatic helium contamination is discussed in the light of the Gulf of Lion geodynamic evolution and data from comparable orogenic hydrothermal systems in the Alps.

Abstract Most of the current helium (He) reserves originate from fortuitous discoveries, mainly made during oil and gas exploration in sedimentary basins. As helium generation depends on U and Th α‐decay, old geological provinces gather key ingredients for high He accumulation. However, numerous He‐rich springs have also been documented in much younger rocks, such as Variscan granites (320–250 Ma). These latter discoveries question the current exploration guidelines and require revisiting some of the longstanding paradigms. Here, is investigated He migration along a major fault rooted in the Corso‐Sardinian batholith (France). Two thermal springs, Caldanelle and Guagno‐Les‐Bains, show significant outgassing activities of crustal sourced He with concentrations up to 1.45 vol% and flow rates of 110 m3 STP 4He/year. Besides He, the gas phase is dominated by N2 (≈98 vol%) and minor CH4. Based on a survey employing multidisciplinary methodologies, it is revealed that (a) Variscan rocks represent efficient 4He source rocks, (b) the main source of He comes from the underlying Eo‐Variscan basement, (c) A deeply rooted fault and dense fractures networks drain the He, (d) the helium loss is limited, (e) faults and fractures may act as partial traps, and finally (f) the presence of an efficient trap could promote a He‐rich reservoir with high flux but low reserves. In that sense, young post‐orogenic granites represent promising helium plays. The geological context in which Caldanelle and Guagno‐Les‐Bains are embedded is ubiquitous in European Variscan batholiths. This case study is therefore intended to serve as a guide for helium exploration and to provide insights into helium behavior within a Variscan geological context.

We investigate both the deep crustal structure of the Western and Central Alps orogenic wedge and the timing and amount of convergence accommodated since 32 Ma. The new structural interpretations are based on the most recent geophysical models (Vs and Vp tomography mainly) coupled to geological surface information. We show that first-order similarities in collision kinematics can be described from the Western to the Central Alps. After the subduction-collision transition (37-32 Ma), from around 32 Ma and until 22-20 Ma, the shortening consists of distributed deformation throughout the doubly verging orogenic wedge. From around 20 Ma until recent times, the orogen was controlled by localized west- or northwest-verging thrusts below the External Crystalline Massifs. This probably witnesses localization processes in the proximal European crust (i.e., below the Penninic Frontal Thrust) on a 10 Myr timescale. These structures (both distributed and localized ones) root in middle- to lower crustal low velocity (Vs) zones interpreted as a thick shear zone acting as a deep, crustal decollement. The low seismic velocity is most probably controlled by active fluid circulations, structural anisotropy, and/or metamorphic Alpine paragenesis (amphibolite facies). Thus, the 10 Myr timescale may correspond to characteristic time for the localization processes within the deep, ductile decollement.Along-strike significant differences from Western to Central Alps can also be highlighted. Beyond collisional magmatism and amphibolite facies metamorphism only present in the Central Alps, kinematical differences can be quantified. In the Western Alps, after the first phase of collision, at around 20 Ma, the orogenic wedge consisted in a West-verging wedge while in the Central Alps, North- and South verging structures remained active. These differences imply significant contrasts in terms of convergence rates that can be quantified through balanced cross sections with realistic inherited Mesozoic structures. In Central Alps, convergence rates were about 1.2 +/- 0.2 cm/yr from 32 to 22 Ma and about 0.3 +/- 0.1 cm/yr from 22 to 0 Ma. This strongly suggests that before collision s.s., i.e. before 32 Ma, the convergence rate was higher than 1.2 cm/yr.While similarities in terms of structural styles and kinematics in both parts of the orogen most likely reflect crustal rheology and localization processes, the differences allow discussing the influence of both the inherited Mesozoic structure and the kinematics of Adria after the subduction phase.

The origin of methane in hydrothermal fluids has long been a subject of debate – whether it is abiotic or biotic. In this study, we aim to unravel and quantify the sources of CH4 in active hydrothermal systems by adopting a holistic approach analyzing well characterized high-temperature hydrothermal fluids (∼230–310 °C) in Iceland. We employ a broad variety of geochemical and isotope indicators, encompassing chemical and isotope compositions of the targeted fluids. These signatures are then compared with results from chemical and isotope kinetic models and data from sedimentary-hosted hydrothermal systems. Carbon species in these fluids include CO2 (2.60–184 mmol/kg), CH4 (2.39·10−4–0.325 mmol/kg), dissolved organic carbon (4.78·10−3–0.112 mmol/kg), and CO (1.89·10−6–4.16·10−4 mmol/kg). Carbon and helium isotopes suggest a relatively uniform mantle-derived source of CO2 (δ13C-CO2: −4.80 to −1.50 ‰, CO2/3He: 1.49·109–4.14·1010 14C-CO2: 0.11–2.42 pMC). Methane, in contrast, has multiple sources. Overall, chemical equilibria among carbon species (CO2, CH4, CO) is not attained, suggesting kinetic controls. Tritium content (<0.8–1.42 TU) and hydrologic constraints indicate relatively short hydrothermal fluid residence times (∼5–200 years), with occasional inputs from older water components. Within this short timeframe, CH4 concentrations vary from lower, to significantly higher than those calculated using CO2 reduction kinetics. The isotope composition (δD-CH4: −172 to −138 ‰, δ13C-CH4: −32.0 to −24.6 ‰; 14C-CH4: 0.36–11.54 pMC) and geochemical and isotope modeling suggest that the majority (>80–90 %) of CH4 originates from a radiocarbon inactive source, i.e. mantle CH4, reduction of mantle CO2 and/or old organic matter, with relatively small contributions from both marine (<20 %) and terrestrial (<10 %) dissolved organic carbon. Measured isotopic compositions of CH4 do not match those expected for mantle-derived CH4 as well as values generated from reduction of mantle-derived CO2. Instead, differences in δD-CH4 and δ13C-CH4 values exist between systems fed by meteoric water and those fed by seawater, challenging the assumption of a uniform CO2 source and invariable reaction mechanisms. Differences between systems are best explained by variable extent of thermal decomposition and primary variations in the isotope composition of marine and terrestrial organic matter. Also, δD-CH4 and δ13C-CH4 values in meteoric water-fed systems closely resemble those in the Öxarfjördur sedimentary-hosted systems. In summary, our data supports a predominant thermogenic origin of CH4 in both seawater and terrestrial hydrothermal fluids in Iceland. The source of organic matter appears to be a combination of modern dissolved organic carbon and older sedimentary deposits. In addition, some of the hydrothermal systems studied (Krafla, Reykjanes, Theistareykir) which are characterized by low CH4 concentrations, may contain a significant portion of CH4 that may originate from CO2 reduction.

Central Afar (Ethiopia) is an active example of the final stages of continental rifting. The Stratoid magmatic series (ages between 5 and 1 Ma) were emplaced in a large fissural volcanic province, following an episode of thinning by normal faulting and detachment at 5-6 Ma (Stab et al., 2016). The Gulf Basalt series (0.9-0.4 Ma) later emplaced in more restricted areas attesting for the localisation of the deformation. Current active magmatic axes are even more localized and the most recent lava geochemistry attests for very little crustal contamination (Ayalew et al., 2018) along with recent dyking episodes. This suggests that Central Afar is currently in a late syn-rift stage, possibly close to continental break-up with divergence accommodated by magmatic accretion. The detailed study of the tectono-magmatic evolution of the region will allow us to better constrain the break-up processes active during volcanic margin formation.Our new mapping of Central Afar has consisted in defining Stratoid sub-series to better follow the interplay between magmatism and deformation during continent-ocean transition. This map is supported by field data, new mapping using satellite multispectral images, and new Ar/Ar dating. We defined three new units: the old Stratoid (5-3 Ma), the intermediate (3-2 Ma) and the young Stratoid (2-1 Ma). This mapping shows that the localisation processes started during the old Stratoid emplacement, which we interpret as an equivalent of Seaward Dipping Reflectors described in magma-rich margins. The detailed mapping of the normal faults in Central Afar is used to quantify the amount of deformation through space and time and discuss the mechanism of divergence accommodation (dyke vs normal faults) in order to track the timing and controlling parameters of the eventual switch from rifting to break-up processes. In the next future, we will study the chemical signature of each series to determine the evolution of magma sources and conditions of melting during the Stratoid phases we defined. Moreover, new dates will provide much needed data on this volcanic series's continuous vs discrete (with pulses) nature.

Abstract This paper presents new stratigraphic correlation and naming for younger basalt lavas (<24 Ma) and enclosed rhyolites near the intersection of the southern Red Sea rift and the Ethiopian rift valley, based upon three volcano-stratigraphic sections located over about 30 kilometres, and they bracket the timing of eruption of the upper sequence with existing geochronology. The correlations are based on the petrography (thin sections) of ca. 32 samples taken from these three sections, combined with outcrop description. The stratigraphy is proposed as 5 new formations, partially placed into an existing stratigraphic framework, based on the dominant unit-basalt or rhyolite in the package of sequences. These new stratigraphic columns also include non-volcanic units (interlayered sedimentary strata, paleosols and weathering surfaces), which are not usually described in the volcanic pile, showing that there was more complexity in this succession than previously known. This new stratigraphic framework will encourage more geochronology and geochemistry studies that are sorely needed.

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 integral O2 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.

The in situ (U‐Th‐Sm)/He and U/Pb laser‐ablation double‐dating procedure is a valuable method that can provide a large dataset relatively efficiently in contrast with conventional bulk helium thermochronometry. In this study, we evaluate the potential age error associated with the double ablation procedure and report the in situ (U‐Th‐Sm)/He double‐ablation dating of 249 zircons from the Fish Canyon Tuff locality. With LA‐ICP‐MS pseudo‐depth profiling and 3D numerical modelling, we show that the concentric double‐ablation procedure in minerals with U‐Th‐Sm zoning can generate a significant (U‐Th‐Sm)/He age error (positive or negative), resulting in over‐scattering and/or an offset of the mean age. Pseudo‐depth profiling is insufficient to predict the individual age error, partly because of the superimposed ablations. To evaluate the consequence of this inherent bias, we confront a synthetic age distribution to the error expected for U‐Th‐Sm zoned zircons analysed with double‐ablation (U‐Th‐Sm)/He thermochronometry. As expected, a strong age bias causes the spreading of peak ages, downgrading the original signal. Yet, the throughput of the ablation‐based method can allow intra‐ and inter‐sample peak age identification and comparison, and the coupling of (U‐Th‐Sm)/He and U/Pb ages extends our ability to deconvolute a multimodal age spectrum.