A global review of Hf-Nd isotopes: New perspectives on the chicken-and-egg problem of ancient mantle signatures

Acknowledgements We are grateful to T. Morishita and C.J. Garrido for their support during the laborious elaboration of the database and writing of the manuscript. We thank A. Sanfilippo for providing data for the individual MOR segments. Constructive comments and suggestions by M. Bizimis, an anonymous reviewer and the editor C. Chauvel greatly improved the manuscript. Romain Tilhac acknowledges grant FJC2018-036729 funded by MCIN/AEI/10.13039/501100011033 and “ESF, Investing in your future” and grant AEI-PID2021-122792NA-I00 funded by MCIN/ AEI/10.13039/501100011033 and “ERDF, A way of making Europe”. This is contribution 1742 from the ARC Centre of Excellence for Core to Crust Fluid Systems (http://www.ccfs.mq.edu.au) and 1511 from the GEMOC Key Centre (http://www.gemoc.mq.edu.au).Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.chemgeo.2022.121039.We present the first global review on the Sm-Nd and Lu-Hf isotope systematics of the mantle; it includes all published data on peridotites and pyroxenites from all tectonic settings (>1100 combined Hf-Nd analyses), as well as previous compilations for oceanic basalts and material such as oceanic and continental sediments. We first provide a comprehensive overview of the main reservoirs and mechanisms accounting for the contrasting variability of radiogenic isotope systematics in the sub-oceanic mantle and the relative homogeneity of its volcanic products, highlighting the paradigm change promoted by the use of Hf isotopes. Secondly, we summarize the different models invoked to explain the decoupling/(re-)coupling of Hf and Nd isotopes. Decoupling above the mantle array is often related to melt-peridotite interaction involving ancient protoliths, whereas coupled Hf-Nd or decoupling below the array are shown to be insufficient criteria to exclude the involvement of such protoliths. The Hf-Nd isotope variability of the SCLM is then addressed using a tectono-thermal classification based on the Global Lithospheric Architecture Mapping (GLAM) project. The extreme variability that characterizes the cratonic SCLM reflects the long-term preservation of depleted signatures overprinted by ancient and recent metasomatic episodes. Refertilized SCLM domains fingerprinted by variably decoupled Hf-Nd isotope systematics record subduction-related processes, which also appears to be instrumental in the recycling of continental material into the convective mantle. We show that there is a critical “chicken-and-egg” question underpinning debates on the spatio-temporal evolution of the SCLM: whether ancient signatures are pre-existing in the lithosphere (e.g. “lithospheric memory” during refertilization) or introduced into the convective mantle (i.e. recycling). Importantly, our compilation shows that fertile lithologies such as pyroxenites can also carry extremely depleted isotopic signatures. In particular, delamination of gravitationally unstable, pyroxenite-rich arc roots represents a volumetrically significant flux of material characterized by ancient radiogenic Hf and basalt-like Nd-isotope compositions that can, once recycled, account for the Hf variability observed between MORB suites. In this context, the characteristic HIMU-like or coupled Hf-Nd signatures observed in garnet-pyroxenite layers from orogenic peridotite massifs probably reflects long-term processing (re-coupling) of recycled lithospheric material in the convective mantle. In contrast, continental dispersal during rifting (± plume-related processes) appears to be mostly limited to buoyant SCLM remnants in the oceanic lithosphere, and these are unlikely to be recycled unless previously refertilized. This work brings a new geodynamic perspective to the ancient signatures identified as chemical and isotopic heterogeneities in the oceanic lithosphere and convective mantle. These conclusions imply that (1) subduction is the main driver of mass transfer between lithosphere and asthenosphere and (2) the long-term evolution of the Earth's mantle and crust are directly linked to convergent plate-tectonic processes, at least since the Archean.Grant FJC2018-036729 funded by MCIN/AEI/10.13039/501100011033 and “ESF, Investing in your future”Grant AEI-PID2021-122792NA-I00 funded by MCIN/ AEI/10.13039/501100011033 and “ERDF, A way of making Europe

Prolonged magmatism and growth of the Iran-Anatolia Cadomian continental arc segment in Northern Gondwana

Much of the crust of Iran and Anatolia, including their oldest exposed rocks, formed during an episode of intense convergent margin (arc) magmatism as a result of subduction of oceanic lithosphere beneath northern Gondwana from ca 620 Ma to ca 500 Ma, the Cadomian crust-forming event. Most igneous rocks formed between ca 570 and 525 Ma. Cadomian crust is well-known from western and southern Europe and from eastern North America but is much less well-known from Iran and Anatolia. We use published age and compositional data and contribute new data in order to better understand this ancient magmatic system. Cadomian magmatism included calc-alkaline igneous rocks of arc affinity in the main arc and alkalic igneous rocks that formed in a back-arc setting; these igneous rocks are associated with sedimentary rocks. Geochemical and isotopic modelling reveals that basaltic magmas were the main input, that these formed by partial melting in the upper mantle, and that basaltic magmas evolved further in deep crustal hot zones to form granitic magmas through a combination of assimilating older continental crust and fractional crystalization of basaltic magmas.This study was funded by the “ National Key Research and Development Program of China ( 2016YFE0203000 )” and by “ Chinese Academy of Sciences , President's International Fellowship Initiative (PIFI, 2019VCB0013 ). Financial support was also received from the Alexander von Humboldt Foundation in the form of a senior research grant and GEOMAR Helmholtz Centre while preparing these results for publication. FL gratefully acknowledges the PRIN2017 Project 20177BX42Z_001 (Intraplate deformation, magmatism and topographic evolution of a diffuse collisional belt: Insights into the geodynamics of the Arabia-Eurasia collisional zones) and the grant to Department of Science, Roma Tre University (MIUR-Italy Dipartimenti di Eccellenza, ARTICOLO 1, COMMI 314 – 337 LEGGE 232/2016 ). We thank Semih Gürsu for providing us bulk rock data from Derik complex of Turkey. Zircon U–Pb geochronology and and Lu–Hf isotope data were obtained using instrumentation funded by DEST Systemic Infrastructure Grants, ARC LIEF, NCRIS/AuScope, industry partners, and Macquarie University. All logistical support for field studies came from Damghan University. This is contribution 1544 from the ARC Centre of Excellence for Core to Crust Fluid Systems ( http://www.ccfs.mq.edu.au ) and 1412 in the GEMOC Key Centre ( http://www.gemoc.mq.edu.au ), and 1380 from UTD Geosciences and is related to IGCP-662. from the ARC Centre of Excellence for Core to Crust Fluid Systems ( http://www.ccfs.mq.edu.au ), xxxx from the GEMOC Key Centre ( http://www.gemoc.mq.edu.au ), and xxxx from UTD Geosciences and is related to IGCP-662

Pyroxenites: a key geochemical vector in the mantle

Conferencia plenaria en Geosciences Colloquium Series, Goethe Universit

Petrology and geochemistry of pyroxenites from the Cabo Ortegal Complex, Spain

Les roches mafiques telles que les pyroxénites jouent un rôle majeur dans le développement des hétérogénéités du manteau supérieur et le recyclage des éléments entre la Terre interne et ses enveloppes externes. Les pyroxénites sont notamment impliquées dans la différenciation des magmas d'arc primitifs dont la genèse a eu un impact important sur la formation des continents depuis l'Archéen. La rareté des affleurements de sections profondes d'arc limite cependant notre compréhension de la migration des liquides magmatiques et de leur interaction avec les roches encaissantes. Dans la suture Varisque du nord-ouest de l'Ibérie, le massif de Herbeira du complexe de Cabo Ortegal représente un affleurement unique d'harzburgites, de dunites et chromitites, et d'abondantes pyroxenites issues du manteau sub-arc. Cependant, les processus responsables de leur abondance et de leurs caractéristiques géochimiques ne sont pas bien contraints, en particulier en raison d'une histoire tectono-métamorphique polyphasée. Cette étude présente de nouvelles observations pétrographiques et de terrain, et la composition en éléments majeurs et traces, et en isotopes radiogéniques (Sr, Nd, Hf et Os) de ces pyroxénites. Après avoir contraint les effets des processus secondaires, il est démontré que la faible teneur en Al des pyroxènes, la forte abondance des éléments compatibles et l'absence de plagioclase sont liées au caractère hydraté et primitif des magmas parents. Les webstérites et clinopyroxénites à olivine (de type 1) qui préservent des lentilles de dunite, et leurs dérivés foliés (de type 3), ont enregistré le remplacement partiel de péridotites à rapports magma/roche décroissants, suite à l'intrusion de magmas picritiques à relativement faible profondeur (< 1,2 GPa), potentiellement au sein d'une racine d'arc. Les webstérites massives (de type 2) représentent le produit final de cette réaction à des rapports magma/roche plus élevés, potentiellement sous forme de veines et filons cristallisés à partir de magmas boninitiques différenciés des magmas initialement picritiques. L'interaction entre ces magmas enrichis en Si et les dunites a produit de rares webstérites à opx (de type 4). La rééquilibration chromatographique a accompagné la cristallisation d'amphibole à partir de liquides magmatiques résiduels, percolant ou piégés, ce qui a produit une large gamme de profils de terres rares, en accord avec la composition du Nd progressivement non-radiogénique. Les forts rapports CaO/Al2O3 et l'enrichissement spécifique des LILE par rapport aux HFSE suggèrent la génération de magmas parents riches en Ca par des faibles degrés de fusion d'une lherzolite réfractaire à < 2 GPa, suite à la percolation de fluides issus du slab et/ou de carbonatites. La systématique des isotopes radiogéniques indique l'implication d'un manteau ancien et métasomatisé, suggérant la remobilisation d'une marge continentale par ce magmatisme d'arc, et de sediments recyclés. La géochronologie de ces mêmes isotopes suggère que les pyroxénites de Cabo Ortegal se sont formées autours de 500 Ma, impliquant que les âges précédemment reportés à ~ 390 Ma correspondent probablement à un épisode métamorphique. Les pyroxénites et leurs péridotites hôtes ont enregistré le développement de plis en fourreaux et de mylonites faisant eux-mêmes suite à un épisode de déformation à haute température. Nous proposons que cette transition correspond à la délamination de la racine d'arc, associée à du métamorphisme prograde jusqu'à 1,6-1,8 GPa et 780-800°C. L'exhumation dans une zone de subduction a ensuite été accompagnée par de l'hydratation, comme le suggère l'abondance d'amphiboles automorphes, et par l'addition de S et la mobilisation du Re, comme en témoignent la composition isotopique de l'Os. Le complexe de Cabo Ortegal préserve donc un exemple unique de racine d'arc délaminée qui illustre le rôle de l'interaction magma-roche pendant la différentiation des magmas d'arc primitifs en profondeur.Mafic lithologies such as pyroxenites play a major role in the development of chemical heterogeneity in the upper mantle and in the recycling of elements between the deep Earth and its external envelopes. Pyroxenites are notably involved in the differentiation of primitive arc magmas whose genesis has had a major impact on continent formation in post-Archean times, but the rarity of exposures of deep arc sections limits our understanding of melt migration and melt-rock interaction. In the Variscan suture of north-western Iberia, the Herbeira massif of the Cabo Ortegal Complex represents a unique km-scale section of sub-arc mantle harzburgites, dunites and chromitites, and abundant pyroxenites. However, the processes responsible for their abundance and geochemical characteristics are not well established, notably due to a particularly complex tectonothermal history. This work combines new field and petrographic observations with the characterization of major-, trace-element and radiogenic-isotope (Sr, Nd, Hf and Os) compositions of these pyroxenites. After constraining the effects of secondary processes by studying small-scale heterogeneities, it is shown that the low-Al content of pyroxenes, the high abundance of compatible elements and the absence of plagioclase are related to primitive hydrous parental melts. Olivine websterites and clinopyroxenites preserving dunite lenses (type 1, and their foliated type-3 products) record the partial replacement of peridotites at decreasing melt/rock ratios, following intrusion of picritic melts at relatively low pressure (< 1.2 GPa), potentially within an arc root. Massive websterites (type 2) represent the final products of this reaction at higher melt/rock ratios, potentially as veins and dykes crystallized from boninitic melts differentiated from the initial picritic melts. Interaction between these Si-rich melts and dunites produced rare opx-rich websterites (type 4). Chromatographic reequilibration accompanied the late-magmatic crystallization of amphibole from migrating or trapped residual melts, producing a range of REE patterns from spoon-shaped to strongly LREE-enriched, consistent with increasingly unradiogenic Nd compositions. Particularly high CaO/Al2O3 and the selective enrichment of LILE/HFSE in Cabo Ortegal pyroxenites suggest the generation of their Ca-rich parental melts via low-degree, second-stage melting of a refractory lherzolite at < 2 GPa, following percolation of slab-derived fluids and/or carbonatite melts. Radiogenic-isotope systematics indicate the involvement of an old metasomatized mantle domain, suggesting the reworking of a continental margin during this arc magmatism, and of recycled sediments. Their geochronology suggests that Cabo Ortegal pyroxenites formed at around 500 Ma, which implies that previously reported ages around 390 Ma probably correspond to a metamorphic event. Pyroxenites and their host peridotites then recorded the development of sheath folds and mylonites following high-temperature deformation. We suggest that this episode corresponds to the delamination of the arc root associated with prograde metamorphism peaking at 1.6-1.8 GPa and 780-800°C. Exhumation of the arc root after its intrusion into a subduction zone was accompanied by hydration, as suggested by abundant post-kinematic amphibole, and by addition of S and mobilization of Re, as suggested by Os-isotope systematics. The Cabo Ortegal Complex may thus preserve a unique example of delaminated arc root, bearing evidence of the role of melt-peridotite interaction during the differentiation of primitive arc magmas at depth

Pétrologie et géochimie des pyroxénites du Complexe de Cabo Ortegal, Espagne

Mafic lithologies such as pyroxenites play a major role in the development of chemical heterogeneity in the upper mantle and in the recycling of elements between the deep Earth and its external envelopes. Pyroxenites are notably involved in the differentiation of primitive arc magmas whose genesis has had a major impact on continent formation in post-Archean times, but the rarity of exposures of deep arc sections limits our understanding of melt migration and melt-rock interaction. In the Variscan suture of north-western Iberia, the Herbeira massif of the Cabo Ortegal Complex represents a unique km-scale section of sub-arc mantle harzburgites, dunites and chromitites, and abundant pyroxenites. However, the processes responsible for their abundance and geochemical characteristics are not well established, notably due to a particularly complex tectonothermal history. This work combines new field and petrographic observations with the characterization of major-, trace-element and radiogenic-isotope (Sr, Nd, Hf and Os) compositions of these pyroxenites. After constraining the effects of secondary processes by studying small-scale heterogeneities, it is shown that the low-Al content of pyroxenes, the high abundance of compatible elements and the absence of plagioclase are related to primitive hydrous parental melts. Olivine websterites and clinopyroxenites preserving dunite lenses (type 1, and their foliated type-3 products) record the partial replacement of peridotites at decreasing melt/rock ratios, following intrusion of picritic melts at relatively low pressure (< 1.2 GPa), potentially within an arc root. Massive websterites (type 2) represent the final products of this reaction at higher melt/rock ratios, potentially as veins and dykes crystallized from boninitic melts differentiated from the initial picritic melts. Interaction between these Si-rich melts and dunites produced rare opx-rich websterites (type 4). Chromatographic reequilibration accompanied the late-magmatic crystallization of amphibole from migrating or trapped residual melts, producing a range of REE patterns from spoon-shaped to strongly LREE-enriched, consistent with increasingly unradiogenic Nd compositions. Particularly high CaO/Al2O3 and the selective enrichment of LILE/HFSE in Cabo Ortegal pyroxenites suggest the generation of their Ca-rich parental melts via low-degree, second-stage melting of a refractory lherzolite at < 2 GPa, following percolation of slab-derived fluids and/or carbonatite melts. Radiogenic-isotope systematics indicate the involvement of an old metasomatized mantle domain, suggesting the reworking of a continental margin during this arc magmatism, and of recycled sediments. Their geochronology suggests that Cabo Ortegal pyroxenites formed at around 500 Ma, which implies that previously reported ages around 390 Ma probably correspond to a metamorphic event. Pyroxenites and their host peridotites then recorded the development of sheath folds and mylonites following high-temperature deformation. We suggest that this episode corresponds to the delamination of the arc root associated with prograde metamorphism peaking at 1.6-1.8 GPa and 780-800°C. Exhumation of the arc root after its intrusion into a subduction zone was accompanied by hydration, as suggested by abundant post-kinematic amphibole, and by addition of S and mobilization of Re, as suggested by Os-isotope systematics. The Cabo Ortegal Complex may thus preserve a unique example of delaminated arc root, bearing evidence of the role of melt-peridotite interaction during the differentiation of primitive arc magmas at depth.Les roches mafiques telles que les pyroxénites jouent un rôle majeur dans le développement des hétérogénéités du manteau supérieur et le recyclage des éléments entre la Terre interne et ses enveloppes externes. Les pyroxénites sont notamment impliquées dans la différenciation des magmas d'arc primitifs dont la genèse a eu un impact important sur la formation des continents depuis l'Archéen. La rareté des affleurements de sections profondes d'arc limite cependant notre compréhension de la migration des liquides magmatiques et de leur interaction avec les roches encaissantes. Dans la suture Varisque du nord-ouest de l'Ibérie, le massif de Herbeira du complexe de Cabo Ortegal représente un affleurement unique d'harzburgites, de dunites et chromitites, et d'abondantes pyroxenites issues du manteau sub-arc. Cependant, les processus responsables de leur abondance et de leurs caractéristiques géochimiques ne sont pas bien contraints, en particulier en raison d'une histoire tectono-métamorphique polyphasée. Cette étude présente de nouvelles observations pétrographiques et de terrain, et la composition en éléments majeurs et traces, et en isotopes radiogéniques (Sr, Nd, Hf et Os) de ces pyroxénites. Après avoir contraint les effets des processus secondaires, il est démontré que la faible teneur en Al des pyroxènes, la forte abondance des éléments compatibles et l'absence de plagioclase sont liées au caractère hydraté et primitif des magmas parents. Les webstérites et clinopyroxénites à olivine (de type 1) qui préservent des lentilles de dunite, et leurs dérivés foliés (de type 3), ont enregistré le remplacement partiel de péridotites à rapports magma/roche décroissants, suite à l'intrusion de magmas picritiques à relativement faible profondeur (< 1,2 GPa), potentiellement au sein d'une racine d'arc. Les webstérites massives (de type 2) représentent le produit final de cette réaction à des rapports magma/roche plus élevés, potentiellement sous forme de veines et filons cristallisés à partir de magmas boninitiques différenciés des magmas initialement picritiques. L'interaction entre ces magmas enrichis en Si et les dunites a produit de rares webstérites à opx (de type 4). La rééquilibration chromatographique a accompagné la cristallisation d'amphibole à partir de liquides magmatiques résiduels, percolant ou piégés, ce qui a produit une large gamme de profils de terres rares, en accord avec la composition du Nd progressivement non-radiogénique. Les forts rapports CaO/Al2O3 et l'enrichissement spécifique des LILE par rapport aux HFSE suggèrent la génération de magmas parents riches en Ca par des faibles degrés de fusion d'une lherzolite réfractaire à < 2 GPa, suite à la percolation de fluides issus du slab et/ou de carbonatites. La systématique des isotopes radiogéniques indique l'implication d'un manteau ancien et métasomatisé, suggérant la remobilisation d'une marge continentale par ce magmatisme d'arc, et de sediments recyclés. La géochronologie de ces mêmes isotopes suggère que les pyroxénites de Cabo Ortegal se sont formées autours de 500 Ma, impliquant que les âges précédemment reportés à ~ 390 Ma correspondent probablement à un épisode métamorphique. Les pyroxénites et leurs péridotites hôtes ont enregistré le développement de plis en fourreaux et de mylonites faisant eux-mêmes suite à un épisode de déformation à haute température. Nous proposons que cette transition correspond à la délamination de la racine d'arc, associée à du métamorphisme prograde jusqu'à 1,6-1,8 GPa et 780-800°C. L'exhumation dans une zone de subduction a ensuite été accompagnée par de l'hydratation, comme le suggère l'abondance d'amphiboles automorphes, et par l'addition de S et la mobilisation du Re, comme en témoignent la composition isotopique de l'Os. Le complexe de Cabo Ortegal préserve donc un exemple unique de racine d'arc délaminée qui illustre le rôle de l'interaction magma-roche pendant la différentiation des magmas d'arc primitifs en profondeur

Derivation of Hawaiian rejuvenated magmas from deep carbonated mantle sources: A review of experimental and natural constraints

International audienceThe role of carbon-rich or carbonatitic melts as an important metasomatizing agent in the Earth's mantle is supported by direct and indirect evidence of their involvement, ranging from the presence of erupted carbonatitic lavas to metasomatic reactions documented in minerals and melt inclusions from mantle xenoliths. Carbonatite metasomatism in hot-spot settings, and more particularly in the mantle sources of rejuvenated Hawaiian lavas, has long been suspected. However, an unequivocal geochemical tracer of carbonated mantle sources in alkaline volcanic suites is still missing. We here examine high-quality major-and trace-element compositions of ~400 primitive Hawaiian lavas (MgO = 8.5-21 wt%, SiO 2 = 37-50 wt%) and associated xenoliths, focusing on those erupted during rejuvenated stages of activity of the Hawaiian hot spot, Pacific Ocean. The rejuvenated-stage alkaline lavas are the most enriched in volatile elements among the four-stage Hawaiian lavas. Our compilation shows that these rejuvenated-stage lavas range from melilite/nephelinite to transitional basalts and are characterized by low-Si and high-Na,-K and-Ca contents, along with the enrichment of REE, Th and Ba relative to K, Hf, Zr, Ti and Nb. Their trace-element systematics argues against derivation from a homogeneous lherzolitic or pyroxenitic source, regardless of the involvement of residual garnet or hydrous phases. Based on a comprehensive review of natural and experimental constraints on partitioning between carbonatites and mantle minerals and numerical simulations of open-system melting, we show that it is rather consistent with carbonatite metasomatism in their source. Variations in SiO2 , CaO, alkali contents and trace-element proxies such as Hf/Sm also specify temporal variations in the depth of melting and/or the respective contribution of lherzolites and pyroxenites in a hybrid (probably asthenospheric) source fluxed by carbonatitic melts. We suggest that this episode took place ≤4.2 Ma at temperatures and pressures in excess of 1100 • C and 2 GPa. The lowsolidus carbonatite melts were likely generated following a time lag which allowed for cooling of the plume and likely derived from an ancient (>1Ga), recycled mantle, or lower mantle, source in the Hawaiian plume, in good agreement with Sr-Nd-Hf-Os isotope systematics and other chemical and mineralogical features of Hawaiian rejuvenated lavas and xenoliths. The identification of a deep carbonated mantle source for Hawaiian rejuvenated volcanic series is also in line with noble gas and light stable isotope systematics and suggests that the interaction between carbonatite melts and peridotites/pyroxenites may be a critical process explaining the compositional variability of many oceanic island magmas

Fluid circulation and deformational gradient in north-Pyrenean flyschs : example from the Saint-Jean-de-Luz basin (France)

The relationships between fluid circulation and deformation are one of the issues of the sedimentary basin study. In the Cretaceous flysch of the Saint-Jean-de-Luz basin, the evolution of folds geometry and the increasing volume of calcite-filled fractures and veins evidence a northward deformational gradient along the French Basque coast. A combined approach is proposed to assess the corresponding physical and chemical conditions: the microthermometric study of fluid inclusions in calcite sampled in different generations of veins and fractures and the X-ray diffraction analysis of clay minerals from adjacent marl layers. Salinity of the trapped H₂O-CaCl₂-NaCl fluids increases with depth in the series, in good agreement with salinity gradients reported in sedimentary basins. Dispersion of the data also increases with depth from 0.3 to 13 wt.% NaCl eq. in the shallowest formation (Haizabia flysch) to 9.1 to 23.0 wt.% NaCl eq. in the deepest formation (Guethary flysch). Minimal trapping temperatures of the fluids in the Haizabia and Socoa flyschs (79 and 102 °C, respectively) are consistent with the temperatures estimated from the depth of burial, which did not exceed 5 km, in good agreement with the stability of the smectite-illite-kaolinite assemblage found in marls. In addition, the kaolinite proportion significantly decreases with depth in the series, as a potential consequence of climate changes and diagenetic transformations, whereas the increasing dispersion of illite crystallinity data might indicate fluid-mineral interactions. We propose a synthetic model of fluid circulation in the folded series that involves the mixing of mainlyhorizontal fluid circulation (potentially meteoric) with an upward flow of high-salinity fluid throughout the deepest formations (potentially related to underlying evaporite-rich layers). The northward deformational gradient, as exposed along the French Basque coast, is likely to be responsible for such a vertical circulation, by increasing the volume of fracture (particularly cross-cutting fractures) in the deeply buried formations.15 page(s

A Disequilibrium Reactive Transport Model for Mantle Magmatism

Besides standard thermo-mechanical conservation laws, a general description of mantle magmatism requires the simultaneous consideration of phase changes (e.g. from solid to liquid), chemical reactions (i.e. exchange of chemical components) and multiple dynamic phases (e.g. liquid percolating through a deforming matrix). Typically, these processes evolve at different rates, over multiple spatial scales and exhibit complex feedback loops and disequilibrium features. Partially as a result of these complexities, integrated descriptions of the thermal, mechanical and chemical evolution of mantle magmatism have been challenging for numerical models. Here we present a conceptual and numerical model that provides a versatile platform to study the dynamics and nonlinear feedbacks inherent in mantle magmatism and to make quantitative comparisons between petrological and geochemical datasets. Our model is based on the combination of three main modules: (1) a Two-Phase, Multi-Component, Reactive Transport module that describes how liquids and solids evolve in space and time; (2) a melting formalism, called Dynamic Disequilibirum Melting, based on thermodynamic grounds and capable of describing the chemical exchange of major elements between phases in disequilibrium; (3) a grain-scale model for diffusion-controlled trace-element mass transfer. We illustrate some of the benefits of the model by analyzing both major and trace elements during mantle magmatism in a mid-ocean ridge-like context. We systematically explore the effects of mantle potential temperature, upwelling velocity, degree of equilibrium and hetererogeneous sources on the compositional variability of melts and residual peridotites. Our model not only reproduces the main thermo-chemical features of decompression melting but also predicts counter-intuitive differentiation trends as a consequence of phase changes and transport occurring in disequilibrium. These include a negative correlation between Na2O and FeO in melts generated at the same Tp and the continued increase of the melt's CaO/Al2O3 after Cpx exhaustion. Our model results also emphasize the role of disequilibrium arising from diffusion for the interpretation of trace-element signatures. The latter is shown to be able to reconcile the major- and trace-element compositions of abyssal peridotites with field evidence indicating extensive reaction between peridotites and melts. The combination of chemical disequilibrium of major elements and sluggish diffusion of trace elements may also result in weakened middle rare earth to heavy rare earth depletion comparable with the effect of residual garnet in mid-ocean ridge basalt, despite its absence in the modelled melts source. We also find that the crystallization of basalts ascending in disequilibrium through the asthenospheric mantle could be responsible for the formation of olivine gabbros and wehrlites that are observed in the deep sections of ophiolites. The presented framework is general and readily extendable to accommodate additional processes of geological relevance (e.g. melting in the presence of volatiles and/or of complex heterogeneous sources, refertilization of the lithospheric mantle, magma channelization and shallow processes) and the implementation of other geochemical and isotopic proxies. Here we illustrate the effect of heterogeneous sources on the thermo-mechanical-chemical evolution of melts and residues using a mixed peridotite-pyroxenite source. © 2020 The Author(s) 2020. Published by Oxford University Press. All rights reserved.B.O. and J.C.A. acknowledge funding from Macquarie University, ARC Grant DP160103502 and ARC Linkage Grant LP170100233. B.O., J.C.A. and R.T. also acknowledge support from the ARC Centre of Excellence Core to Crust Fluids Systems (CCFS), the Centre for Earth Evolution and Dynamics (CEED) and the European Space Agency via the ‘3D Earth—A Dynamic Living Planet

Geodynamic evolution of the Tethyan lithosphere as recorded in the Spontang Ophiolite, South Ladakh ophiolites (NW Himalaya, India)

The authors thank the Head, Department of Geology and Inter-disciplinary School of Science (IDSS) , SPPU for providing necessary facilities. MKJ acknowledges the financial support received from Science Education and Research Board (SERB) and Department of Science and Technology (DST) by way of its Young Scientist Scheme (Ref. No. SR/FTP/ES-2/2014) Women's scientist scheme (Ref No. SR/WOS-A/EA-14/2017) . S. Mounic and A. Marquet respec-tively from Toulouse TIMS and ICPMS facilities. RT acknowledges a "Juan de la Cierva-formacion" Fellowship (FJC2018-036729) granted by the Spanish Ministry of Science and Innovation and co-funded by the European Development Fund and the European Social Fund."The Spontang Ophiolite complex represents the most complete ophiolite sequence amongst the South Ladakh ophiolites and comprises mantle rocks (depleted harzburgites, dunites and minor lherzolites) as well as crustal rocks (basalt, isotropic gabbros, layered gabbros etc.). In the present study, detailed geochemistry (whole rock as well as mineral chemistry) and Sr-Nd isotopic analyses of thirty-six ultramaficmafic samples have been attempted to constraint the evolution and petrogenetic history of the Tethyan oceanic crust. Major, trace-element and REE patterns of the peridotites and their minerals indicate that the lherzolites experienced lower degrees of partial melting resembling abyssal peridotites (at higher temperatures, TREE = 1216 C) than the harzburgites (6%–8% versus 15%–17%). Elevated eNd(t) and variable 87Sr/86Sr(t) ratios along with REE patterns suggest that the Spontang mafic rocks display N-MORB affinity with negligible participation of oceanic sediments in their genesis are originated from a depleted upper mantle with little contribution from subduction-related fluids. MORB-type Neotethyan oceanic crust is associated with the earliest phase of subduction (of older Jurassic age) through which a younger intra-oceanic island arc (Spong arc) subsequently developed. Harzburgites REE display typical U-shaped patterns, suggesting that these rocks have been metasomatized by LREE-enriched fluids. On the other side, mafic rocks are characterized by heterogeneous (Nb/La)PM and (Hf/Sm)PM and relatively homogeneous eNd(t), indicating interaction of subduction-related melts with the upper mantle during the initiation of subduction, in Early Cretaceous times.Science Education and Research Board (SERB)Department of Science & Technology (India) SR/FTP/ES-2/2014Women's scientist scheme SR/WOS-A/EA-14/2017"Juan de la Cierva-formacion" Fellowship FJC2018-036729Spanish GovernmentEuropean Social Fund (ESF)European Development Fun

Evidence of ghost plagioclase signature induced by kinetic fractionation of europium in the Earth’s mantle

Crustal recycling in the Earth’s mantle is fingerprinted by trace-element and isotopic proxies in oceanic basalts. Positive Eu and Sr anomalies in primitive lavas and melt inclusions that are not otherwise enriched in AlO are often interpreted as reflecting the presence of recycled, plagioclase-rich oceanic crust in their mantle source – referred to as “ghost plagioclase” signatures. Here, we report natural evidence of Eu anomalies and extreme crystal-scale heterogeneity developed kinetically in mantle peridotite clinopyroxene. Numerical modelling shows that diffusional fractionation between clinopyroxene and melts can account for this intra-crystal heterogeneity and generate Eu anomalies without requiring plagioclase. We demonstrate that kinetically induced Eu anomalies are likely to develop at temperatures, redox conditions and transport timescales compatible with the genesis of mid-ocean ridge and ocean island basalts. Our results show that, in the absence of converging lines of evidence such as radiogenic isotope data, ghost plagioclase signatures are not an unequivocal proxy for the presence of recycled crust in oceanic basalt sources.We are grateful to J.-L. Bodinier, J.-M. Dautria, A. Louni-Hacini and A. Azzouni-Sekkal for field work and sampling. This study was supported by the Consejo Superior de Investigaciones Científicas (CSIC) and by the Ministerio de Ciencia e Innovación and the Agencia Estatal de Investigación through grants IJC2020-044739 (to R.T.) and AEI-PID2021-122792NA-I00 (to R.T.) funded by MCIN/AEI/10.13039/501100011033, “ESF, Investing in your future” and “ERDF, A way of making Europe”. This is contribution 1754 from the ARC Centre of Excellence for Core to Crust Fluid Systems (www.ccfs.mq.edu.au) and 1523 from the GEMOC Key Centre (www.gemoc.mq.edu.au)