Samuel Angiboust

While tracing the geological provenance of Maya jadeitite-jade is key to understanding Mesoamerican exchange and extraction, distinguishing between the north (NMM) and south (SMM) Motagua sources presents significant challenges. This difficulty is due to shared mineralogical traits and the limitations of applying bulk whole rock trace element determination to invaluable archaeological objects. This study proposes an integrated statistical framework combining existing whole rock reference data with new Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) trace element measurements. We characterized 39 geological reference samples from both NMM and SMM, as well as 16 archaeological fragments from seven Maya sites. Despite complex multi-generation growth and local geochemical heterogeneities revealed by cathodoluminescence and LA-ICP-MS, a Partial Least Squares Discriminant Analysis (PLS-DA) model successfully discriminated between NMM and SMM sources with 89% accuracy using thirteen key trace elements (Th, Zn, La, Hf, Ce, Tm, Yb, Ni, Sr, Lu, Eu, Pr, Er). Application of this model to archaeological samples suggests a predominant exploitation of NMM sources across the Maya Highlands, Lowlands, and Pacific coast, with the exception of the El Tuno workshop, which shows more affinity with SMM. This approach validates LA-ICP-MS as a robust tool for jade provenance studies, overcoming the biases of accessory minerals inherent in whole rock analyses and offering a transferable strategy for archaeometric research.

The Sierra del Convento mélange in southeastern Cuba is one of a limited number of jadeite jade occurrences worldwide. This exhumed, pervasively serpentinized fossil subduction interface hosts tectonic blocks up to several tens of meters in scale that were reworked during intense metasomatism. The flat-lying mélange spans a 300 m thick vertical exposure, progressing from a block-rich lower portion toward a block free, serpentinite-dominated upper region. We conducted structurally-controlled sampling of blocks and matrix to produce a spatially-resolved geochemical and microstructural profile through the thickness of the Convento mélange. Pale green, nearly pure jadeitite constitutes the dominant block population in the southern part of this mélange, and may contain up to 30% epidote and/or white mica by volume. Minor metasedimentary and metamafic block populations, recording variable degrees of HP metamorphism, coexist alongside the jade blocks. We report a newly identified metre-scale zoning within the jadeitite bodies, consistent with that documented in other jade localities. Green jadeitite locally occurs surrounding an older core of zoisitite containing relict jadeite crystals rimmed by omphacite. Green jade is cross-cut by fractures infilled by dark-colored brecciated jade, which is in turn rimmed by a late, pyroxene-free rind composed mainly of weakly foliated phengite + albite ± epidote. The latter facies occupies a similar position to chloritite blackwalls previously described from the Convento jade occurrence. All jade varieties except for these latest phengite-albite rinds and chloritite blackwalls are cross-cut by fractures infilled by jadeite and omphacite. Parts of the main jade bodies exhibit prismatic radial and comb jadeite microstructures, consistent with descriptions of P-type jade, which precipitate directly into open fractures from hydrothermal fluids. However, Convento jade contains paragonite with up to 2 wt.% K2O and jadeite-omphacite exsolution domains brecciated into jigsaw-like fragments recemented by jadeite and/or omphacite. These observations are consistent with at least part of the jade in the Sierra del Convento mélange representing near-total high-temperature metasomatic replacement of high-pressure anatectic trondhjemite protoliths, which originated as partial melts of garnet amphibolite at ~15 kbar, resembling an R-type (replacement) jade paragenesis. To overcome the considerable ambiguity inherent to geochronology datasets from HP igneous and metasomatic rocks from this locality (105 to 115 Ma, U-Pb zircon), we are conducting detailed multi-mineral and multi-system geochronology, including further U-Pb on zircon, titanite, and apatite, Ar/Ar on white mica, and Rb-Sr on white mica. This multi-chronometric approach will establish relative and numerical chronology for the diverse jade facies of the Convento occurrence, resolving timescales of the multiple associated fluidization events within the subduction channel.

Subduction dynamics in the Mediterranean realm is largely controlled by the Eurasia-Africa convergence and by the protracted accretion, since the late Cretaceous, of successive oceanic domains and microcontinents to the overriding Eurasian margin. Among these processes, basal accretion of coherent tectonic slices at the base of the forearc domain is one of the most difficult to investigate, as it occurs at high depth along the subduction interface and is only rarely preserved in the geological record. Yet, basal accretion is of prime importance, as it is suspected to remain active beneath active Mediterranean subduction zones and to contribute to the deformation and topographic signals monitored along them. To assess the spatial and temporal scales of the accretion-controlled forearc dynamics, it is therefore crucial to constrain the sequence of slicing episodes forming deep accretionary duplexes. We address this issue through an integrated structural, petro-metamorphic and geochronological study of a high-pressure/low-temperature paleo-duplex exposed in western Crete and formed along the Hellenic subduction zone during the Oligocene–Miocene. We combine field-based structural mapping, petrological characterization, Raman spectroscopy on carbonaceous material, thermodynamic modelling, Rb/Sr multi-mineral geochronology and zircon (U–Th–Sm)/He thermochronology to identify distinct tectono-metamorphic slices, quantify their peak pressure–temperature conditions and constrain their timing of accretion and exhumation.Our results reveal a dome-shaped nappe stack composed of five tectono-metamorphic units bounded by major shear zones, with a systematic down-stepping of peak temperatures (~450 to ~350 °C), pressures (17–18 to 7–8.5 kbar) and Rb/Sr ages (~26 Ma and ~15 Ma) toward lower structural levels. These indicate five basal-accretion episodes that successively migrated to shallower depths (~55–60 km to ~25–30 km) between the late Oligocene and middle Miocene. Subsequent fast exhumation of the duplex, with rates of ~3-11 mm/yr, decreasing to ~2-4 mm/yr at shallow levels, was mainly accommodated by top-to-the-N and subordinate top-to-the-S detachments associated with trench-perpendicular extension, intermittently overprinted by trench-parallel deformation. This study further suggests a sequence of ~2-3-Myr-long deep slicing events, providing a critical timescale for trackingthe tectonic and topographic signatures of deep mass fluxes along active forearc margins in the Mediterranean region and beyond.

The migration of fluids released during slab-dehydration in deep subduction environments is strongly controlled by deformation and lithological discontinuities such as serpentinized shear zones. However, the geological meaning of geochemical fingerprints and how they relate to transport mechanisms and spatial scales of fluid flux in deformed serpentinite at depth remain poorly understood. We herein focus on three subduction-related mantle sections: an intra-slab serpentinized shear zone (Monviso, Italian Alps), an underplated ultramafic sliver (Zagros suture zone, Iran) and the former base of a mantle wedge (Polar Urals, Russia). Most major and trace element signatures of serpentinites appear rather homogeneous along the transects. In contrast, boron isotopic signatures (δ11B) show systematic variations at several hundred meters scale approaching the main structural boundaries for each locality. A decrease is observed in the Monviso and Urals localities (from c. 25‰ to c. 7‰, and from c. 16‰ to c. 0‰, respectively), while the Zagros section shows an increase from c. 1‰, up to c. 9‰ in the most sheared and serpentinized samples. These variations reflect complex fluid-rock interactions processes including B loss or B addition associated with protolith and fluid variability. This demonstrates that major shear zones exert a first-order control on the serpentinite boron isotopic signature. We combine boron isotopic data with Darcy-based flux models to quantify the volume of rock influenced by paleo-fluid fluxes in deep ultramafic settings. These combined petrostructural and isotopic constraints highlight the importance of fracture-controlled fluid flow in slab-top serpentinites, and yield a time-integrated permeability of the (partly) serpentinized base of the mantle wedge in the range of 10-19 to 10-18 m².

Subduction zone geodynamics have been a primary area of focus since the early days of plate tectonics theory. Initial approaches using analytical solutions sought to understand the fundamental driving forces, moment balances, and energetics governing oceanic plate subduction. Despite the limitations of scarce geophysical imaging, limited geological sampling, and emerging numerical techniques, these studies provided the foundations of modern geodynamics. As numerical techniques improved, power-law rheologies and complex geological processes were integrated into various codes. These provided more realistic simulations to understand a wide variety of regions on Earth, but they are rarely presented in the context of classical physical approaches. Nowadays, a unique opportunity exists to revisit these classical frameworks, aided by improved subducting slab imaging, an excellent geological record, and a deeper understanding of slab dynamics across active and ancient subduction zones.In this work, a simplified but physically transparent framework is developed to revisit the force/moment balance, energetic conditions, and dissipation analyses governing slab dynamics. For this purpose, three representative slab geometries (steep, normal, and flat) were selected based on their dip below 40 km depth, where slab behavior is dominated by internal negative buoyancy and viscous lifting forces from the mantle. Slab pull was estimated by varying density contrasts and thickness, while viscous forces were derived from semi-analytical Stokes flow solutions to resolve pressure distributions along the slab surface. Gravitational potential energy changes were calculated to test whether internal density variations (e.g., eclogitization) and crustal thickening from oceanic plateaus drive changes in geometry.Results show that mantle wedge suction can counteract slab weight in a flat subduction setting, while increased density from eclogitization destabilizes flat slabs and promotes steepening, linking moment balance and buoyancy to the global diversity of slab dips. Total energy dissipation is mainly controlled by mantle wedge flow, with low-angle and flat subduction representing the most dissipative configurations. Once moment balance allows, these tend to evolve toward steeper, more energetically stable states. Slab flattening occurs with increased buoyancy, higher convergence velocities, and greater mantle viscosities, producing flattening within 10–30 Myr. Conversely, reduced buoyancy, slower convergence, and lower viscosities favor steepening and slab rollback on comparable or shorter timescales of 5–10 Myr. This integrated and physically transparent analysis is consistent with the development of arc magmas on western margins of the Americas and provides a clear perspective to explain the diverse slab morphologies observed on Earth.

Jadeitites are commonly found in serpentinite mélanges and form by fluid flow across the subduction interface. Petrological analysis of jadeitites from various localities (Myanmar, Guatemala, Cuba, Russia, and Iran) coupled with structural characterization enabled the identification of successive jadeite/omphacite generations with subordinate amphibole and mica. These parageneses reflect metasomatism coeval with brittle and/or ductile deformation and complex crosscutting relationships. The composition of fluid inclusions (salinity, gas, δ18O, δD) reveals a wide range of fluid species pointing to a diversity of jadeitite-forming metasomatism. In situ trace element analysis and isotopic measurements (δ11B, 87Sr/86Sr, δ18O) indicate a fluid source dominated by altered oceanic crust (AOC) with a minor sedimentary component. Despite marked changes in major element content during protracted metasomatism, trace elements evolve only moderately while isotopes are virtually homogeneous, recording only small variations of fluid composition over time. Jadeitite evolution is strongly related to the ongoing serpentinization of the mantle wedge, promoting a longer fluid time-residence at the interface associated with chemical exchange and pore-pressure build-up. This suggests that (i) First jadeite generations formed by percolation of highly channelized AOC-derived-fluids in a dry mantle wedge, while later generations record fluid interaction with sediments and the serpentinized mantle. (ii) Fluid pulses across the subduction interface and rheological behavior of the near interface mantle wedge are not controlled by drastic changes in the nature of the slab input, but rather by the cooling of the serpentinizing subduction environment. (iii) The re-use of the same fluid pathways above the slab promotes the re-equilibration of isotopic signatures. (iv) Overpressures may build up upon jadeitite formation and promote brittle deformation events. This may lead to switches in deformation style and variations in permeability, thus changing fluid flow mode along the base of the mantle wedge.

The transfer of material from the downgoing plate to the overriding plate at depth exerts a first-order control on the mechanical and thermal evolution of subduction zones. However, the geometry, size, and temporal organization of deep tectonic slices formed during basal accretion remain poorly constrained, due to the limited resolution of geophysical imaging and the rarity of geological analogues preserving deep accretionary architectures formed during continental subduction.Here, we investigate the spatial extent and stacking dynamics of tectonic slices by reconstructing the architecture of the deep paleo-accretionary wedge through the study of the exhumed Phyllite-Quartzite (PQ) nappe, derived from microcontinental protoliths, which was formed by basal accretion during the Oligo-Miocene along the Hellenic subduction zone. We carried out a multidisciplinary study of this now-exhumed PQ nappe, which crops out discontinuously from Crete to the Peloponnese (Greece). Preserved in a fore-arc position and weakly overprinted by later tectonic events, this natural laboratory provides direct access to deep accretionary processes.An integrated petro-structural study conducted across southeastern Peloponnese and Kythira combines detailed mapping, structural analysis, petrological observations, Raman spectroscopy of carbonaceous material, and thermobarometric modeling. This approach allows us to distinguish several tectono-metamorphic sub-units within the PQ nappe stack, each recording a distinct P-T evolution that constrains the depth of basal accretion for successive episodes. Hypotheses of lateral continuity between these sub-units provide first-order constraints on their present-day spatial extent and on the minimum size of individual accretionary slices.In southeastern Peloponnese, two HP-LT sub-units are identified within the PQ nappe stack, while at least two equivalent sub-units are recognized on Kythira. These sub-units record a systematic increase in peak temperature from the base to the top of the HP-LT nappe stack, consistent with successive episodes of basal accretion. Reconstructed P-T conditions indicate that basal accretion occurred at depths of ~50-60 km along the subduction interface. Based on spatial correlations between structurally equivalent HP–LT sub-units exposed in neighboring regions along strike, we infer a minimum present-day lateral continuity of individual accretionary slices. On this basis, deep tectonic slices formed during basal accretion are inferred to currently extend over several tens of kilometers in the trench-perpendicular direction and up to a hundred kilometers along strike.This study provides new quantitative constraints on the depth, lateral extent, and dynamics of tectonic underplating, with direct relevance for the Hellenic margin, where such processes may still be active, and for active subduction zones worldwide.

Basal accretion at active subduction margins occurs through a series of tectonic slicing events at varying depths along the plate interface, shaping the forearc domain. To assess the spatial and temporal scale of the accretion-controlled forearc dynamics, it is crucial to constrain the sequence of basal-accretion episodes that form deep accretionary duplexes. This requires identifying the successive tectono-metamorphic units constituting paleo-duplexes and dating the accretion and exhumation events that expose high-pressure rocks at the surface. This first contribution of two companion papers (this issue) presents a detailed reconstruction of the tectonic and thermal structure of a high-pressure/low-temperature paleo-accretionary duplex in western Crete (Greece) that formed along the active Hellenic margin during the Oligocene-Miocene. Combining field observations, structural measurements and Raman spectroscopy on carbonaceous material (RSCM), we identify five tectono-metamorphic slices (i) bounded by shear zones often reworked during exhumation and (ii) characterized by a down-stepping of peak metamorphic temperatures towards lower structural levels. Our geological and structural mapping reveals the overall geometry of the nappe stack forming a dome-like structure, exhumed beneath major top-to-the-N and subordinate top-to-the-S detachments that accommodated N-S-directed crustal extension. This trench-perpendicular extension was intermittently rotated into an E-W direction (trench-parallel), as evidenced by a newly recognized top-to-the-W ductile-brittle detachment. Minor compressional events did not significantly alter the 3D architecture of the paleo-duplex. Reported RSCM peak metamorphic temperatures of ~350-450 °C from the nappe stack align with the typical temperature range for the downdip limit of the seismogenic zones, suggesting a first-order thermo-mechanical control on the depth of basal accretion along the subduction interface. These findings provide crucial constraints for interpreting the deep-accretion and exhumation dynamics that shaped the long-term evolution of the Hellenic forearc domain.