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Crustal rocks involved in orogenic processes frequently bear evidence for widespread fluid circulation. The hydration history of the granitic continental crust in inverted passive continental margins is of particular interest, as granitoids experience rheological weakening by fluid-rock interaction processes. Regrettably, it is often unclear if hydration occurs during rifting or during tectonic inversion. Hence, it is difficult to appreciate the interplay of pre- and syn-orogenic fluids inside continental crustal segments of rifted margins. The geochemical fingerprint of ancient hydration events is stored in hydrous minerals that crystallized directly from circulating paleo-fluids. Thus, such minerals can shed light on the nature of these ancient fluids, as well as provide temporal constraints if they can be dated. Hence, advances in geochronological methods applied to hydrous minerals may prove pivotal in untangling the history of fluid circulation in the granitic continental crust in orogens.We applied U–Pb geochronology of epidote [i.e., Ca2Al2(Al,Fe3+)Si3O12(OH)] in hydrothermal veins hosted by a late Carboniferous/early Permian calc-alkaline granodiorite in the inverted Adriatic passive continental margin (hereafter “Err nappe”), both located in the eastern Swiss Alps. During Jurassic rifting leading to the break-up of Pangea, the continental crust in the Err nappe was hydrated, as seawater-derived fluids percolated along syn-rift faults. However, geochronological data of epidote reveal that the hydration of the granitic continental crust in the Err nappe occurred also later during inversion. Epidote U–Pb geochronology returned two age clusters: (1) 85.2 ± 9.7 Ma, related to Late Cretaceous compression; and (2) 59.9 ± 2.7 Ma, related to subsequent Paleocene extension. These age clusters unveil two distict events of fluid circulation, which are consistent with the timing of tectonic inversion and deformation proposed in the literature. As confirmed by Pb–Sr–O–H isotope geochemistry of epidote, Late Cretaceous fluid circulation was likely mediated by fluids released by underlying units undergoing metamorphism during Eo-Alpine compression. Notably, the Paleocene fluids circulating during extension were most likely surficial in origin (i.e., meteoric water and/or modified/connate seawater), and they percolated into the granitic continental crust by exploiting extensional faults.In the context of existing data, our results show that the hydration of the granitic continental crust of the Adriatic passive continental margin was mediated by a repeated series of fluid circulation events. Our work advocates that the use of a multi-methodological approach, combining new geochemical and geochronological, tools provides unprecedented insight into complex processes of fluid circulation in the continental crust, and beyond.

Deformation of polymineralic aggregates can be accommodated by viscous granular flow, a process mediated by the interplay among intracrystalline plasticity and dissolution–precipitation, each active in specific minerals under given P–T conditions. Some rock-forming minerals like quartz and feldspars have been intensively studied in terms of deformation processes. Instead, the deformation behavior of epidote and its role during viscous granular flow is not well investigated, although this mineral is ubiquitous in granitic rocks deforming under greenschist-facies conditions. In this contribution, we provide microstructural and geochemical evidence for the occurrence of dissolution–precipitation of epidote during deformation of an epidote–quartz vein. The main part of the vein is deformed, producing a fold, which is visible due to relicts of primary-growth layering inside the vein. The deformation mechanisms active during deformation include dynamic recrystallization of quartz by subgrain rotation recrystallization, producing grain size reduction in the primary vein quartz. Recrystallization occurs contemporaneously with dissolution and (re)precipitation of epidote and quartz grain boundary sliding, leading to a combined process described as viscous granular flow. The combination of grain boundary sliding and dissolution locally and repeatedly produces creep cavities. These represent not only loci for nucleation of new epidote grains at the expense of dissolved ones, but they also allow fluid-mediated transport of elements. The same trace element patterns between old epidote relicts and newly formed grains, with much narrower variability in the latter, indicate a process of chemical homogenization. The nature of the fluid that mediates deformation is investigated using Pb–Sr isotope data of epidote, which suggest that deformation is assisted by internally recycled fluids with the addition of a syn-kinematic external fluid component.

Eclogite-hosted metamorphic veins mark former fluid migration pathways during a subduction-exhumation cycle, and allow to trace fluid-mediated element transfer across lithologies, to ultimately metasomatize the mantle wedge. Fluids can be generated by dehydration reactions at different P-T conditions in various lithologies, all influencing how different chemical constituents are dissolved and re-precipitated. Here we present a study of eclogite-hosted quartz-rich metamorphic veins in the Variscan Münchberg Massif. The eclogites probably represent subducted continental crust that was variably hydrated and subjected to amphibolite facies conditions before reaching eclogite facies peak conditions of ca. 3 GPa and 700°C.Isolated, mm-sized quartz pockets with euhedral high-pressure minerals are common in the Münchberg eclogites, but continuous veins that may have allowed focused fluid flow beyond specimen scale are rare. Nevertheless, where such veins occur, they can contain high-pressure minerals such as garnet and omphacite, but also rutile, zircon, and allanite, indicating high field strength-element (HFSE) mobility at least on the specimen scale. Garnet- and omphacite-bearing veins are typically 1-10 mm thick with average crystal sizes of 1 mm and less. A different vein type is mostly similar in thickness, but consists of quartz + phenocrysts (sometimes >1 cm long) of kyanite, phengite, and/or rutile. Symplectite-rich selvages surrounded by mostly fresh host eclogite are common.Oxygen isotope thermometry of quartz-garnet, quartz-phengite, and quartz-kyanite pairs yield temperatures around 700°C, interpreted to represent vein crystallization. δ18O values of vein quartz (+6.1 to +10.5‰) from all vein types are identical to δ18O values of host rock quartz (the latter were predicted from mass balance modelling at 700°C based on host rock δ18O values from +4.0 to +7.9‰). While it is evident that the garnet- and omphacite-bearing veins were formed under eclogite facies conditions, pressures are uncertain for the quartz-rutile, quartz-phengite, and quartz-kyanite veins. Still, vein formation at relatively high pressures seems probable, as solubilities of chemical components tend to increase with pressure, facilitating HFSE mobilization from the source rock. We propose that internal fluids were generated by dehydration of phengite and/or zoisite and/or amphibole from the eclogites. Isolated quartz-rich pockets formed in eclogites that may have released only small amounts of fluid, whereas continuous metamorphic veins were formed in eclogites that produced more fluid, probably reflecting a more intense hydration before eclogite facies metamorphism. The internal origin of the fluids supported by oxygen isotope evidence argues against fluid transport over large distances. The fluids may have largely remained in place before being consumed for symplectite formation upon retrogression to amphibolite facies conditions.

Serpentinization of ultramafic mantle rocks is one of the main reactions leading to a significant water incorporation into the oceanic lithosphere. The multiphase hydration is yet poorly constrained, in terms of sequence of events, their chemical and isotopic compositions, and the reaction conditions (temperature, fluid composition and the water/rock ratio). We present here the first study on Iberia and Newfoundland passive margins samples (oceanic drill core samples, ODP, from Site 1070 and Site 1277) that closely correlates petrographic observation, in situ oxygen isotopic data and major and trace elements mobility in serpentine phase.The serpentine minerals lizardite and chrysotile are the main hydrous phases formed during the serpentinization reaction. These minerals crystallize in specific textures, depending on the primary minerals being replaced: (i) serpentine in mesh texture after the alteration of olivine, and (ii) serpentine as bastite pseudomorphing pyroxenes. As the textural control is the key to detect multistage fluid uptake during progressive hydration, we used Secondary Ion Mass Spectrometry (SIMS) to achieve a good spatial resolution of ∼20 µm for in situ oxygen isotope measurements. The trace elements analyses were analyzed with laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and a spot size of ∼38 µm. Newfoundland samples show less variability in the oxygen isotope composition than Iberia samples and have values higher than the mantle composition, suggesting low temperature of serpentinization (110-200°C). One of the Iberia samples shows similar variability of the oxygen isotope composition but lower values than 𝛿18Omantle (temperature around 140-200°C). The second sample has the highest variability: (i) homogeneous mesh rim texture with 𝛿18O = 4.4 ± 0.8 ‰ (2𝜎), (ii) wide compositional range for mesh centers with 𝛿18O = 4.0 - 7.7 ‰, and (iii) bastite with large isotopic variation from 5.5 to 13.5 ‰. These values suggest a large hydration temperature range from 60-200°C within a single sample.Texturally controlled rare-earth element (REE) analyses of the serpentine minerals reveal inherited, typical melt depletion patterns of the protolith, for both localities. The trace element composition of serpentine in the different textural domains displays typical signatures related to the precursor minerals (olivine vs. pyroxene), particularly in terms of Ni/Cr ratio. Positive anomalies of fluid-mobile elements (e.g. B, Sr, As) confirm hydration of the mantle rocks during oceanic serpentinization.

Mechanisms such as crystallization differentiation, subduction erosion, delamination, or relamination that are responsible for the formation and modification of modern crust with an on average andesitic composition are actively debated (Hacker et al. 2015). Isotope fractionation associated with igneous processes is documented for many non-traditional stable isotope systems, making them promising tools to advance our understanding of modern arc crust formation. Titanium isotopes are especially promising, as volcanic and plutonic arc rocks show a trend from light to heavy isotope values with increasing SiO2 concentration due to the fractionation of minerals with light Ti isotopes.We present new Ti isotope data on medium K calc-alkaline to shoshonitic magmatic differentiation suites from the Adamello Batholith (N-Italy), Kos (Agean arc), Torres del Paine (Patagonia) and the Dolomites (N-Italy) in addition to crust-derived mafic cumulates. The Ti isotopic composition of dacites and granites range between δ49TiOL-Ti ≈ 0.3 to 1.1‰, with heavier values for more alkaline granitic melts in agreement with published data (Hoare et al. 2020). Mafic cumulates from related and additional localities are overall isotopically lighter than (their) granitic counterparts ranging between δ49TiOL-Ti ≈ -0.15 and +0.08‰. Cumulates of studied crustal sections enriched in Fe-Ti oxides (>5 modal %) show δ49Ti values lighter than the depleted MORB mantle (DMM, δ49TiOL-Ti ≈ +0.002 ± 0.007‰) and counterbalance the isotopically heavy composition of felsic rocks. The occurrence of cumulates heavier than DMM may have several reasons: (i) “heavy” cumulates may represent late-stage relicts of progressive magma differentiation containing trapped intercumulus melt or (ii) they experienced overprinting, e.g., by mafic rejuvenation.We therefore find that the Ti isotopic composition of cumulate rocks and likely also the magmatic lower continental crust is influenced by their mineralogical composition. How this impacts the Ti isotopic composition of the bulk continental crust in the light of delamination and relamination processes needs further work. REFERENCESHacker, B. R., Kelemen, P. B., & Behn, M. D. (2015). Continental lower crust. Annual Review of Earth and Planetary Sciences, 43, 167-205.Hoare, L., Klaver, M., Saji, N. S., Gillies, J., Parkinson, I. J., Lissenberg, C. J., & Millet, M. A. (2020). Melt chemistry and redox conditions control titanium isotope fractionation during magmatic differentiation. Geochimica et Cosmochimica Acta, 282, 38-54.

The Aar Massif is a mid-crustal basement section of the European plate and it was intensely deformed during the Alpine orogeny. Alpine deformation of Aar Massif granitoids is expressed by a pervasive network of ductile shear zones consisting of fine-grained polymineralic (ultra)mylonites dominated by viscous granular flow processes (Wehrens et al., 2016). Fluid circulation and hydration reactions are recorded by both granitic protoliths and shear zones. In particular, they are made evident by the alteration of feldspar into hydrous minerals like epidote and mica. The timing of this hydration event and, consequently, whether Alpine deformation was initiated in already altered granitoids or in fresh ones were unclear. This lack of time constraints led to a pivotal question: did deformation initiate in rocks that were altered by pre-kinematic hydration, or was hydration syn-kinematic and driven by the formation of Alpine shear zones? Laser ablation ICP-MS U–Pb geochronology applied to epidote in hydrothermal veins provides new evidence for pre-Alpine hydration of granitoids in the Aar Massif. Two veining events are recognized: 1) one at ca. 276 Ma occurring during Permian transtension and rifting, and 2) another at ca. 14 Ma related to late Alpine exhumation phases. Initial 207Pb/206Pb ratios of all Permian epidote samples overlap within uncertainty, suggesting only one fluid source and equilibration path. Also, these ratios are more radiogenic than those of the host rocks at the time of vein formation. The hydrogen isotopic composition of the Permian fluids was calculated from measurements in bulk epidote separates by high-temperature conversion elemental analyzer. With a temperature range of epidote crystallization estimated between 200–300 °C, the calculated δDfluid value is -57 to -44 ‰. An external source for the Permian fluids is suggested by the disequilibrium of Pb isotopes between hydrothermal epidote and host granitoids. Percolation of meteoric water along transtensional faults and interaction with syn-rift sediments before reaching the granitoids underneath is suggested by the Permian transtensional geodynamics and supported the hydrogen isotopic composition of the Permian fluids. The occurrence of Permian fluid circulation in the Aar Massif granitoids indicates that these rocks were altered before the onset of Alpine deformation. In fact, it can be inferred that fluid circulation caused not only veining, but also pervasive flow and thus the alteration of magmatic feldspar into fine-grained hydrous minerals throughout the granitoids of the present-day Aar Massif. This enabled pre-Alpine storage of water and the creation of numerous additional grain boundaries, both favoring viscous granular flow during Alpine deformation. In this context, the localization of strain in polymineralic aggregates containing hydrous minerals can recycle stored pre-kinematic (i.e., Permian) water. Thus, the initiation of Alpine deformation did not necessarily require the addition of syn-kinematic (i.e., Alpine) fluids, although their presence is confirmed by this and previous studies (e.g., Ricchi et al., 2019; Peverelli et al., 2021).

Eclogite‐facies breccias from the Monviso metaophiolite complex (N‐Italy and W‐Alps) represent a unique opportunity to study the triggering mechanisms for transient brittle deformation at eclogite‐facies conditions in ductilely deforming subducted oceanic lithosphere. Here the intact foliation of Fe‐Ti‐rich and Mg‐Al‐rich metagabbro clasts, cemented by unfoliated eclogitic matrices (paragenesis: omphacite ± garnet ± lawsonite), demonstrates pristine brecciation at eclogite‐facies conditions. Successive generations of high‐pressure veins and eclogitic matrices reveal multiple brittle rupture events. Pseudosection modeling, textural observations, and geochemical data suggest that minerals of both prograde veins and first brecciation event (M1 matrix) crystallized in presence of fluids buffered by the surrounding metagabbro minerals, while M2 matrix composition suggests an incipient infiltration of external fluids. Trace element composition of the third M3 matrix, associated to its impressive lawsonite pseudomorphs, clearly points to the massive ingression of external, serpentinite‐derived fluids. Therefore, these rocks record the progressive increase in the scale of fluid circulation along multiple, stepwise eclogitic brecciation events, from locally released fluids (closed system) to kilometer‐scale fluid infiltration. The successive brecciation steps promoted permeability creation (at least transiently), which resulted in progressive opening of the system to large‐scale sustained fluid circulation. Eclogite‐facies brecciation also controlled the initial stages of strain localization, which highlights the importance, in ductilely deforming low‐permeability eclogite‐facies rocks, of brittle deformation events for both strain localization initiation and creation of large‐scale fluid circulation pathways. Thus, Monviso breccia blocks record the complete, stepwise development of a major shear zone, via progressive strain localization, permeability creation, and increasing scale of fluid circulation.

Epidote – here defined as minerals belonging to the epidote–clinozoisite solid solution – is a low-μ (μ=238U/204Pb) mineral occurring in a variety of geological environments and participating in many metamorphic reactions that is stable throughout a wide range of pressure–temperature conditions. Despite containing fair amounts of U, its use as a U−Pb geochronometer has been hindered by the commonly high contents of initial Pb, with isotopic compositions that cannot be assumed a priori. We present a U−Pb geochronology of hydrothermal-vein epidote spanning a wide range of Pb (3.9–190 µg g−1), Th (0.01–38 µg g−1), and U (2.6–530 µg g−1) contents and with μ values between 7 and 510 from the Albula area (eastern Swiss Alps), from the Grimsel area (central Swiss Alps), and from the Heyuan fault (Guangdong Province, China). The investigated epidote samples show appreciable fractions of initial Pb contents (f206=0.7–1.0) – i.e., relative to radiogenic Pb – that vary to different extents. A protocol has been developed for in situ U−Pb dating of epidote by spot-analysis laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) with a magmatic allanite as the primary reference material. The suitability of the protocol and the reliability of the measured isotopic ratios have been ascertained by independent measurements of 238U/206Pb and 207Pb/206Pb ratios, respectively, with quadrupole and multicollector ICP-MS applied to epidote micro-separates digested and diluted in acids. For age calculation, we used the Tera–Wasserburg (207Pb/206Pb versus 238U/206Pb) diagram, which does not require corrections for initial Pb and provides the initial 207Pb/206Pb ratio. Petrographic and microstructural data indicate that the calculated ages date the crystallization of vein epidote from a hydrothermal fluid and that the U−Pb system was not reset to younger ages by later events. Vein epidote from the Albula area formed in the Paleocene (62.7±3.0 Ma) and is related to Alpine greenschist-facies metamorphism. The Miocene (19.2±4.3 and 16.9±3.7 Ma) epidote veins from the Grimsel area formed during the Handegg deformation phase (22–17 Ma) of the Alpine evolution of the Aar Massif. Identical initial 207Pb/206Pb ratios reveal homogeneity in Pb isotopic compositions of the fluid across ca. 100 m. Vein epidote from the Heyuan fault is Cretaceous in age ( 107.2±8.9 Ma) and formed during the early movements of the fault. In situ U−Pb analyses of epidote returned reliable ages of otherwise undatable epidote–quartz veins. The Tera–Wasserburg approach has proven pivotal for in situ U−Pb dating of epidote, and the decisive aspect for low age uncertainties is the variability in intra-sample initial Pb fractions.

Recently, the application of LA–ICP–MS has enabled U–Pb dating of epidote minerals within the epidote–clinozoisite solid solution series (Peverelli et al., 2020). Epidote crystallization ages can provide an absolute time frame of deformation sequences when combined with detailed microstructural and metamorphic P–T analysis. Epidote deformation occurs in a brittle manner over a wide range of conditions below its closure temperature for Pb diffusion (685–750 °C; Dahl, 1997); hence, such deformation will not affect its formation U–Pb age. Nevertheless, the possibility of isotopically resetting epidote via fluid–mineral interaction has to be taken into account even at low deformation temperatures.We investigated the geochemical and Sr–Pb isotopic characteristics of epidote in one hydrothermal vein in the Aar Massif (central Swiss Alps). The vein is associated with an Alpine shear zone and it is composed of aggregates of 0.1–1 mm anhedral to subhedral epidote grains (epidote-A) + green biotite within a quartz matrix. This quartz dynamically recrystallized by subgrain rotation at temperatures above 400 °C (Stipp et al., 2002) along with crystallization of a second epidote generation (epidote-B) made of tiny (< 0.1 mm) anhedral epidote grains in part mantling epidote-A and defining a fold. We address whether interaction with the fluid that precipitated epidote-B chemically affected epidote-A, i.e. whether the U–Pb age measured by LA–ICP–MS in epidote-A still dates its crystallization upon vein formation or displays age disturbance.LA–ICP–MS Sr and Pb concentration data overlap between epidote-A and epidote-B, as do their REE patterns, with (La/Yb)N ratios of 0.03–0.92. Lead and Sr isotopic signatures were measured respectively by solution MC–ICP–MS and by TIMS in epidote-A and in separates mixing different proportions of epidote-A and -B (no pure mechanical separates of epidote-B possible), and they are different. This requires open-system conditions during deformation, i.e., introduction of an external fluid with higher 87Sr/86Sr and 208Pb/206Pb ratios during crystallization of epidote-B. Despite the presence of an external fluid and the incorporation of external Sr and Pb in epidote-B, LA–ICP–MS U–Pb isotopic data for epidote-A define a regression in a Tera–Wasserburg plot indicating an age of 19.2 ± 4.3 Ma, consistent with epidote-A crystallization during original vein opening. The preservation of the crystallization age in epidote-A indicates that interaction with the fluid that formed epidote-B did not geochemically and isotopically affect epidote-A. The consistency in trace element contents between epidote-A and -B hints that the epidote-forming cations were inherited by the fluid from epidote-A, and thus suggests dissolution-precipitation as the formation process for epidote-B. Dahl, Earth Planet. Sci. Lett. 150, 277–290, 1997.Peverelli et al., Geochronology Discuss. [preprint], https://doi.org/10.5194/gchron-2020-27, in review, 2020.Stipp et al., Geological Society, London, Special Publications, 200(1), 171-190, 2002

Magnetite stability in ultramafic systems undergoing subduction plays a major role in controlling redox states of the fluids liberated upon dehydration reactions, as well as of residual rocks. Despite their relevance for the evaluation of the redox conditions, the systematics and geochemistry of oxide minerals have remained poorly constrained in subducted ultramafic rocks. Here, we present a detailed petrological and geochemical study of magnetite in hydrous ultramafic rocks from Cerro del Almirez (Spain). Our results indicate that prograde to peak magnetite, ilmenite–hematite solid solution minerals, and sulfides coexist in both antigorite-serpentinite and chlorite-harzburgite at c. 670 °C and 1·6 GPa, displaying successive crystallization stages, each characterized by specific mineral compositions. In antigorite-serpentinite, magnetite inherited from seafloor hydration and recrystallized during subduction has moderate Cr (Cr2O3 < 10 wt%) and low Al and V concentrations. In chlorite-harzburgite, polygonal magnetite is in textural equilibrium with olivine, orthopyroxene, chlorite, pentlandite, and ilmenite–hematite solid solution minerals. The Cr2O3 contents of this magnetite are up to 19 wt%, higher than any magnetite data obtained for antigorite-serpentinite, along with higher Al and V, derived from antigorite breakdown, and lower Mn concentrations. This polygonal magnetite displays conspicuous core to rim zoning as recognized on elemental maps. Cr–V–Al–Fe3+ mass-balance calculations, assuming conservative behavior of total Fe3+ and Al, were employed to model magnetite compositions and modes in the partially dehydrated product chlorite-harzburgite starting from antigorite-serpentinite, as well as in the serpentinite protolith starting from the chlorite-harzburgite. The model results disagree with measured Cr and V compositions in magnetite from antigorite-serpentinites and chlorite-harzburgites. This indicates that these two rock types had different initial bulk compositions and thus cannot be directly compared. Our mass-balance analysis also reveals that new magnetite formation is required across the antigorite-breakdown reaction to account for the mass conservation of fluid-immobile elements such as Cr–V–Al–Fe3+. Complete recrystallization and formation of new magnetite in equilibrium with peak olivine (Mg# 89–91), chlorite (Mg# ∼95), orthopyroxene (Mg# 90–91), and pentlandite buffer the released fluid to redox conditions of ∼1 log unit above the quartz–fayalite–magnetite buffer. This is consistent with the observation that the Fe–Ti solid solution minerals (hemo-ilmenite and ilmeno-hematite) crystallized as homogeneous phases and exsolved upon exhumation and cooling. We conclude that antigorite-dehydration reaction fluids carry only a moderate redox budget and therefore may not be the only reason why the magmas are comparatively oxidized.

Zircon occasionally crystallizes in evolved melt pockets in mafic large igneous province (LIP) magmas, and in these cases, it is used to provide high-precision age constraints on LIP events. The precision and accuracy of high-precision ages from LIPs are crucially important, because they may be implicated in mass extinctions. However, why zircon crystallizes in these magmas is not clearly understood, since their mafic compositions should limit zircon saturation. Here, we investigate the occurrence of zircon (and baddeleyite) in intrusive and extrusive mafic rocks from Central Atlantic Magmatic Province (CAMP) using petrography, trace-element analysis, Ti temperatures, Hf and oxygen isotopes, and high-precision U–Pb geochronology, along with petrological and thermal modeling. We provide new ages for CAMP sills that intruded into Paleozoic sediments in Brazil, indicating that the high and low Ti magmatism in this area occurred synchronously over 264 ± 57 ka. We show that upper crustal assimilation, especially of shales, during the emplacement of the CAMP likely led to zircon saturation. Assimilation of upper crustal sediments is also supported by high δ18O values and some rare negative εHf values in the zircon crystals. The only extrusive sample analyzed was the North Mountain basalt in Nova Scotia, Canada. This sample contains a large age variation in its zircon crystals (up to 4 Ma), and the older crystals have slightly more negative εHf values suggesting the presence of small (micron scale) xenocrystic cores associated with very late-stage sediment assimilation. However, the CAMP dataset as a whole suggests that the presence of xenocrystic cores is rare. Assuming no xenocrystic cores, and considering the zircon undersaturated nature of LIP mafic melts, the oldest zircon age clusters in a population should record the magma emplacement (or time when assimilation occurred), and the younger ages in a population are more likely to reflect Pb loss, especially given the high U concentrations of LIP zircon. Our identification of heterogeneous isotopic and elemental compositions in LIP zircon indicates that zircon in these magmas saturate in isolated minute melt pockets just before the system cools below its solidus.

The Cima Lunga unit in the Central Alps is dominated by quartzofeldspathic gneisses with subordinate mafic, ultramafic, and metacarbonate rocks. Only mafic and ultramafic lithologies were thought to preserve clear evidence of Alpine high-P metamorphism. This led to the questions of whether the different rock types were subducted and exhumed as a coherent unit or underwent different pressure���temperature (P���T) histories. New petrological and geochemical data from a metapelite associated with garnet peridotite from Cima di Gagnone (Cima Lunga unit, Switzerland) were obtained using major and trace element mapping. Complex zoning patterns in garnet and white mica are observed. In particular, high Ti content in phengite and increasing P, Zr, and HREE contents in pyrope-rich garnet indicate that this metapelite underwent high-P and high-T (HP���HT) metamorphism involving fluid-fluxed partial melting. A P���T path is reconstructed by combining textural analysis with petrological���geochemical data and thermodynamic simulations. We show that the mineral record preserves an evolution from prograde to HP���HT peak conditions (2.7 �� 0.1 GPa and 800���) followed by near-isobaric cooling (~2.5 GPa and 700���750���) prior to decompression (1.0 GPa and ~620���). The reconstructed P���T path suggests that the studied metapelites were subducted to depths where the slab gets heated by proximity to asthenospheric mantle related to slab break-off. This heating resulted in the dehydration of chlorite-to garnet peridotite and the liberated fluids triggered partial melting in the associated metapelites, which might have favoured the fast exhumation of the entire Cima Lunga unit. Metapelites and garnet peridotite from Cima di Gagnone underwent a common prograde to peak and retrograde P���T path without significant tectonic pressure difference between the different lithologies, and deviation from lithostatic pressure is excluded. Lastly, the peak metamorphic conditions of metapelite from Cima di Gagnone are comparable with P���T estimates of ultramafic lithologies from the southern Adula nappe and the Dascio Bellinzona zone, thus opening new scenarios for the geodynamic interpretation of the Central Alps.

The processes of carbon mobilisation at convergent plate boundaries are hotly debated. Recent findings suggest that carbon release along subduction geothermal gradients may well be more relevant than previously thought; however, it has remained difficult to achieve steady state atmospheric CO2 conditions over geological time scales based on current volatile cycle models. Here, we report on meta-ophicarbonate rocks in the contact metamorphic aureole of the Bergell intrusion, Val Malenco, European Alps, that reached T-P conditions of at least 650 °C at 0.35 GPa. We demonstrate by combined field evidence, geochemistry, and closed-system thermodynamic modelling that over 50% of the rock carbonate has been mobilised locally in response to reactive fluid flow upon progressive isobaric heating from 350 to >650 °C. Despite complete mineral transformation at the antigorite + calcite devolatilisation reaction, the resulting tremolite-ophicarbonate preserves the original ophicarbonate texture by olivine-chlorite clasts, representing the former antigorite-serpentinite fraction, embedded in a monomineralic tremolite matrix formed from the calcite fraction that often exceeded 50 vol%. Closed system thermodynamic modelling based on an ophicarbonate composition of 80 wt% serpentinite +20 wt% calcite reveals that rock-buffered fluid XCO2 values evolve from 700 °C – 1.0 GPa and >600 C – 0.3 GPa). Barrovian metamorphism, too, can reach T-P conditions of 800 °C – 1 GPa. Amagmatic CO2 mobilised this way will eventually reach the atmosphere via dispersion in the groundwater table and diffuse outgassing. Despite the huge uncertainties associated with quantification of such metamorphic carbon fluxes, geological time scale global carbon cycling models should more rigorously explore the significance of these contributions.

Terrestrial and extraterrestrial rocks exhibit significant variations in their mass-dependent Ti isotopic compositions, with basalts being isotopically lighter than evolved lithologies. The observed trend from light to heavy Ti isotopic compositions from more primitive to more differentiated rocks agrees with theoretical predictions that light Ti isotopes are sequestered in Fe–Ti oxides. However, there are lingering questions about the exact extent of this fractionation and whether it is influenced by the nature of oxides and silicate melt. To improve on this matter, we measured the Ti isotopic compositions of mineral separates and bulk rocks from the calc-alkaline Kos volcano-plutonic system, Aegean arc, Greece. Bulk rock Ti isotopic compositions (δ49Ti) increase with differentiation of the magmatic system, from δ49Ti of +0.042 ± 0.033‰ in basalt to +0.654 ± 0.034‰ in rhyolite. We document two different Ti isotope trends produced by (i) fractional crystallization, and (ii) mixing between a basaltic melt and an evolved (rhyolitic) magma. Trend (i) can be explained by a melt-cumulate Ti isotopic fraction factor α of 0.9998 (i.e., the bulk cumulate is on average 0.20‰ lighter than the melt). The mineral separates reveal variable δ49Ti values, with magnetite having the lightest 49Ti/47Ti isotopic composition, biotite being intermediate and neso- and tectosilicates (i.e., olivine, plagioclase and quartz) heaviest. Comparing the TiO2 concentrations of the low-Ti minerals olivine, plagioclase and quartz determined with LA-ICP-MS and isotope dilution shows that the δ49Ti values measured in these minerals reflect their isotopic compositions, and contamination by inclusions is minimal. The difference in δ49Ti between different minerals is smallest in a basalt (Δ49Tiolivine-magnetite = +0.426) and largest in two rhyolites (Δ49Tiquartz-magnetite = +1.083; both ± 0.046‰). Our data agree with theoretical predictions that Fe–Ti oxides have a light δ49Ti signature, and neso/tectosilicate minerals are heavy. Furthermore, the measured difference in δ49Ti between magnetite-olivine, magnetite-plagioclase and magnetite-quartz agree to first order with theoretically predicted inter-mineral Ti isotopic fractionation factors, thus suggesting that the measured inter-mineral Ti isotopic variations are equilibrium in nature.

Following publication of the original article (Ricchi et al. 2020), multiple typesetting errors were identified in the article. The updated sections/sentences are given below and the changes have been highlighted in bold typeface.

Monoclinic epidote is a low-µ (µ = 283U / 204Pb) mineral occurring in a variety of geological environments, participating in many metamorphic reactions and stable throughout a wide range of pressure–temperature conditions. Despite containing fair amounts of U, its use as a U–Pb geochronometer has been hindered by the commonly high contents of initial Pb with isotopic compositions that cannot be assumed a priori. We present U–Pb geochronology of hydrothermal-vein epidote spanning a wide range of Pb (3.9–190 µg g−1), Th (0.009–38 µg g−1) and U (2.6–530 µg g−1) contents and with µ values between 7–510 from the Albula area (eastern Swiss Alps), from the Grimsel area (central Swiss Alps) and from the Heyuan fault (Guangdong province, China). The investigated epidote samples show appreciable fractions of initial Pb that vary to different extents. A protocol has been developed for in situ U–Pb dating of epidote by spot-analysis laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) with a magmatic allanite as primary reference material. The suitability of the protocol and the reliability of the measured isotopic ratios have been ascertained by independent measurements of 238U / 206Pb and 207Pb / 206Pb ratios respectively by quadrupole and multicollector ICP–MS applied to epidote micro-separates digested and diluted in acids. For age calculation, we used the Tera–Wasserburg (207Pb / 206Pb–238U / 206Pb) diagram, which does not require corrections for initial Pb and provides the initial 207Pb / 206Pb ratio if all intra-sample analyses are co-genetic. Petrographic and microstructural data indicate that the calculated ages date the crystallization of vein epidote from a hydrothermal fluid and that the U–Pb system was not reset to younger ages by later events. Vein epidote from the Albula area formed in the Paleocene (62.7 ± 3.0 Ma) and is related to Alpine greenschist-facies metamorphism. The Miocene (19.1 ± 4.0 Ma and 16.9 ± 3.7 Ma) epidote veins from the Grimsel area formed during the Handegg phase (22–17 Ma) of the Alpine evolution of the Aar Massif. Identical initial 207Pb / 206Pb ratios reveal homogeneity in Pb isotopic compositions of the fluid across ca. 200 m. Vein epidote from the Heyuan fault is Cretaceous in age (108.1 ± 8.4 Ma) and formed during the early movements of the fault. In situ U–Pb analyses of epidote returned reliable ages of otherwise undatable epidote-quartz veins. The Tera–Wasserburg approach has proven pivotal for in situ U–Pb dating of epidote and the decisive aspect for low age uncertainties is the variability in intra-sample initial Pb fractions.

Author(s): Bali, E; Guðfinnsson, GH; Aradi, LE; Szabo, A; Szabo, C; Zierenberg, R; Diamond, LW; Pettke, T; Friðleifsson, G | Abstract: The ∼4500-m-deep Iceland Deep Drilling Project (IDDP) borehole IDDP-2 in Iceland penetrated the root of an active seawater-recharged hydrothermal system below the Mid-Atlantic Ridge. As direct sampling of pristine free fluid was impossible, we used fluid inclusions to constrain the in situ conditions and fluid composition at the bottom of the hydrothermal convection cell. The fluid temperature is ∼600 °C, and its pressure is near-hydrostatic (∼45 MPa). The fluid exists as two separate phases: an H2O-rich vapor (with an enthalpy of ∼59.4 kJ/mol) and an Fe-K–rich brine containing 2000 μg/g Cu, 3.5 μg/g Ag, 1.4 μg/g U, and 0.14 μg/g Au. Occasionally, the fluid inclusions coexist with rhyolite melt inclusions. These findings indicate that the borehole intersected high-energy steam, which is valuable for energy production, and discovered a potentially ore-forming brine. We suggest that similar fluids circulate deep beneath mid-ocean ridges worldwide and form volcanogenic massive sulfide Cu-Zn-Au-Ag ore deposits.

Subducted metapelites are more prone to re-equilibrate during exhumation than mafic or ultramafic rocks to the point that recognizing high-pressure (HP) relicts is often very challenging. Geologic evidence from the Cima Lunga Unit (Central Alps) show this apparent discrepancy between high to ultra-high pressure metamorphism (28 kbar and 780 °C) recorded in mafic/ultramafic lenses, and Barrovian metamorphism ( 700 °C). We combined detailed textural analysis with petrological-geochemical data and thermodynamic modelling to reconstruct the metamorphic evolution of the studied rock. We show that, thank to incomplete re-equilibration, the rock documents an evolution from prograde to UHP-HT peak (27 kbar and 800 °C) to retrograde (Barrovian) conditions (10 kbar and 620 °C). Noteworthy, peak metamorphic conditions of metapelite coincide with peak metamorphic conditions of the garnet metaperidotite. Lastly, geochemical evidence for minor wet melting of the studied metapelite at HP-HT conditions was recognized and is likely linked to the dehydration of chlorite to form garnet peridotite in the adjacent ultramafic body. We propose that metapelites and ultramafic rocks were coupled before subduction or at least in its early stage. This finding opens new scenarios for the geodynamic interpretation of the Cima Lunga unit. We propose that the ultramafic lenses at Cima di Gagnone were parts of the exhumed and serpentinised mantle emplaced at the hyper-extended European continental margin of the Piemont-Ligurian ocean. Slices of the margin were detached and tectonically mixed in the subduction channel. These new constraints call for re-evaluation of the paleogeographic position of the Adula-Cima Lunga nappe.

Ion probe 208Pb/232Th fissure monazite ages from high pressure regions of the Western Alps and from the Argentera Massif provide new insights on the tectonic evolution of the Western Alps during Cenozoic times. Fissure monazite is a hydrothermal mineral crystallizing during cooling/exhumation in Alpine fissures, an environment where monazite is highly susceptible to fluid-mediated dissolution-(re)crystallization. Fissure monazite ages directly record chemical disequilibrium occurring in a fissure environment, but growing evidences indicate that fissure monazite commonly register tectonic activity. Fissure monazite age domains from this study show that monazite crystallization occurred between ~32-30.5 Ma and ~31.5-30 Ma in the Piémontais and Briançonnais Zone of the High Pressure regions, and between ~17-15 Ma and in the north-eastern border of the Argentera Massif. So far, monazite ages were recorded between ~32-23 Ma and at ~20.5 Ma in the Briançonnais Zone and in the south-western border of the Argentera Massif respectively. Thus the presented dataset corroborate and complement already reported fissure monazite 208Pb/232Th ages from the Western Alps. This new fissure monazite ages compilation supports that Late Oligocene thrusting affected the High Pressure regions of the Western Alps, and that Early and Middle Miocene dextral strike-slips movements respectively affected the south-western and north-eastern margins of the Argentera Massif. Chemical observations provide new hints on fissure monazite growth conditions (e.g. leached host-rock minerals, oxidation conditions) encouraging to pursue chemical studies with a larger dataset on natural fissure monazite to better understand growth conditions under cleft environment.

The widespread presence of epidote-bearing veins and hydrous minerals such as micas in meta-granitoid rocks attests to the large extent of hydration of the exhuming continental crust. The ability of epidote (Ca2Al3Si3O12(OH) – Ca2Al2Fe3+Si3O12(OH)) to incorporate a wide variety of trace elements renders this mineral a promising geochemical tracer of circulating fluid(s).We report trace element and microstructural data on epidote-bearing veins from the Aar Massif (Central Alps) and from the Albula Pass (Eastern Alps). We characterized and classified the epidote-bearing veins based on their extent of deformation, shape and size of the epidote grains, coexisting minerals, and degree of dynamic recrystallization of associated quartz. Laser ablation ICP-MS data of individual epidote crystals reveal prominent zoning, confirmed by electron probe maps for Sr and Mn. Overall, low to very low Th/U ratios (down to 0.0005 in the Aar Massif veins and 0.001 in the Albula ones) with Th often below limits of detection (< 0.1 µg/g at 16 µm beam size) go along with variably LREE-depleted patterns (and CI Chondrite-normalized LaN/YbN ~0.35 in the Aar Massif veins and ~0.60 in the Albula Pass veins). Strontium contents are variable (hundreds to thousands of µg/g) and mostly high (up to 10100 µg/g in the Aar Massif samples and 12800 µg/g in the Albula Pass samples). The in-situ geochemical data are linked to the microstructures in order to assess whether microstructures can be related to variations in trace elements, also considering the role of coexisting phases. Moreover, trace element data of samples from the Aar Massif are compared to metamorphic host-rock epidotes and cleft epidotes from the same massif.We find that REE patterns of Aar Massif vein epidotes are clearly different than those of metamorphic host-rock epidotes and of cleft epidotes. In addition, REE patterns vary based on the microstructural characteristics of veins. Overall REE patterns of the Albula Pass vein epidotes resemble those from the Aar Massif. Different veins and microstructures define clusters in Sr vs. Y, Eu anomaly vs. Th/U ratios, and Eu anomaly vs. U values. Geochemical heterogeneities are observed among sampling localities within the Aar Massif.The fact that the geochemical characteristics of retrograde hydrothermal vein epidotes are clearly different than those of high-grade metamorphic and metamorphic host-rock epidotes, and the relationship between geochemical characteristics and microstructures support the hypothesis that the deformation did not alter the original geochemical record through neomineralization. Our data argue for the potential of epidote as a powerful fluid tracer in the granitoid continental crust.

Serpentinites are central to carry water and fluid-mobile elements down subduction zones. The breakdown of antigorite represents the most prominent aqueous fluid release, boosting fluid-mediated element cycling from the slab to the mantle wedge. At Cerro del Almirez, Spain, an antigorite dehydration reaction front is preserved in subducted serpentinites. Bulk rock and mineral major element chemistry linked to detailed petrography reveals that silicate mineral Mg# (100∗[Mg/(Mg+Fe)]molar) are higher than bulk rocks due to the presence of magnetite, and olivine Mg# are lower in Chl-harzburgite than in Atg-serpentinite. The amount of magnetite is lower in Chl-harzburgite (1.4 vol%) than in Atg-serpentinite (2.8 vol%), resulting in reactive bulk rock compositions with Mg# of 92.7 and 96.0, respectively. Pseudosection modelling employing these reactive bulk compositions yields a small temperature field at 670◦C, 1.6 GPa where Atg-serpentinite and Chl-harzburgite coexist at the same metamorphic conditions. Thus, the antigorite dehydration front represents a compositional boundary rather than a thermal front (isograd). We suggest that this compositional boundary represents an oxidation front established upon serpentinisation at the ocean floor. Previous studies have shown that with increasing extent of serpentinisation increasing amounts of magnetite are formed concomitant with an increase in the Mg# of coexisting silicates. During subduction of such heterogeneous ultramafic rocks antigorite dehydration will be a continuous reaction that can occur over up to ∼40◦C, controlled by variations in Mg# of the reactant silicates. The difference in the amount of magnetite between Atg-serpentinite and Chl-harzburgite is thus not related to a change in redox budget of subducting serpentinites at Almirez imposed by the antigorite dehydration reaction. Rather, mass balance considerations suggest that ferric iron from antigorite may actually be the most prominent contribution to the redox budget of the antigorite dehydration reaction. Our findings also imply that direct comparison between Atg-serpentinites and Chl-harzburgites to infer geochemical changes associated with prograde dehydration reactions may lead to erroneous conclusions, including estimates on element loss mediated by aqueous fluid escape and associated changes in redox budget based on Fe3+/Fetot.

Of the isotopically distinctive mantle domains, the so-called HIMU (“high-μ”; μ = 238U/204Pb) source is the most extreme, and its genesis continues to be debated. We report very strong U enrichment at unchanged Th concentrations in Cretaceous oceanic serpentinites with exceptionally high 206Pb/204Pb (reaching 56) but unchanged 208Pb/204Pb. Similar, but less extreme, features are found in 1.9 billion years old altered oceanic crust (AOC). Forward modelling demonstrates that mantle, if metasomatised by supercritical liquids derived from AOC and serpentinites, evolves to the HIMU Pb isotope signatures, while satisfying experimental and empirical constraints on subduction zone element processing. By contrast, no model solutions for the conventional proposal of the HIMU source representing residual igneous altered oceanic crust can be reconciled with 208Pb/204Pb, strengthening the need for a paradigm shift regarding HIMU OIB genesis. Over time, the net U addition to the convecting mantle via deeply subducted serpentinite has expressed itself as the so-called second terrestrial Pb isotope paradox, or kappa conundrum.

Relationships between cation substitution and hydrogen-bond system in hydrous pyroxenoids with a twisted chain consisting of three SiO4 tetrahedra are investigated by means of chemical and X-ray single-crystal structural analysis. Minerals with the common crystal-chemical formula M3(Me1þ)M1,M2(Me2þ)2[Si3O8(OH)], Me1þ = Na, Li and Me2þ = Ca, Mn, Mg, comprising pectolite, Li-bearing pectolite, murakamiite, marshallsussmanite, serandite, and tanohataite were studied. In pectolite and murakamiite two structurally independent octahedrally coordinated sites (M1 and M2) are occupied by Ca. In marshallsussmanite, serandite, and tanohataite, the site occupancies at M1 and M2 are M1[Ca0.764(7)Mn0.236]M2[Mn0.910(8)Ca0.090], M1[Mn0.758 (4)Ca0.242]M2[Mn1.0], and M1[Mn0.87(2)Ca0.13]M2[Mn0.943(9)Mg0.057], respectively. The Ca and Mn contents primarily influence and distances and hence the unit-cell parameters. The a, b, and c dimensions increase linearly with Ca content. Moreover, high Li concentration at M3 reduces the cell dimensions. The incorporation of smaller Liþ ions for larger Naþ ions at M3 modifies the coordination polyhedron due to the shift of the atomic position, and it decreases the coordination number of M3. The oxygen atoms at O3 and O4 host the hydroxyl group, and both oxygen sites may act as donor and acceptor of the hydrogen bond. With increasing Mn content the O3···O4 distance reduces from 2.487(3)A in murakamiite to 2.447(7)A in tanohataite. A short O3···O4 distance also facilitates disordered hydrogen bonds. The hydrogen-bond system in pyroxenoids with three-periodic single-chain of SiO4 tetrahedra depends on (1) the Na and Li position at the M3 site and (2) the O3···O4 distance.

The subduction-related volcanic front in Nicaragua consists of the Tertiary “Coyol” member in the eastern highlands and the Quaternary to recent volcanic arc within the Nicaraguan depression. Although the Holocene to recent explosive volcanism has been studied extensively no detailed work has been done on the products of explosive volcanism from Quaternary volcanic complexes comprising also the Malpaisillo and Monte Galan Calderas, the focus of this study. The 11 km-wide Malpaisillo Caldera and ~3.5 km-wide Monte Galan Caldera, located ~50 km northwest of Managua, are surrounded by tens of meters of rhyolitic tephras. These pyroclastic flow and fall deposits extend proximally at least 11 km to the southeast and 23 km to the southwest, with observed depositional thicknesses of >16 m for a single ignimbrite unit (or >25 m for the entire section). Distal deposits are found as far as 350 km offshore in the Pacific. At least twelve highly explosive large-volume eruptive phases with corresponding tephra deposits (LPT = La Paz Centro Tephra, PPT = Punta de Plancha Tephra, LCbT = Lower Chibola Tephra, GT = Guacucal Tephra, UCbT = Upper Chibola Tephra, FeT = La Fuente Tephra, ST = Sabanettas Tephra, MgT = Miralago Tephra, ToT = Tolapa Tephra, LMT, MMT, UMT = Lower, Middle, and Upper Maderas Negras Tephras) are distinguished based on geochemical correlations and similar depositional characteristics. Radiometric 40Ar/39Ar ages indicate that most activity related to the large Malpaisillo Caldera occurred between ~570 and ~420 ka. The large Pleistocene Malpaisillo and Monte Galan Calderas are characterized by a long-lived history and, if evolved, a distinctly alkaline (K2O = 2.3–3.8 wt%; Na2O = 4.0–4.9 wt%) geochemical signature compared to the other Nicaraguan tephra deposits. As a result, the previously defined Malpaisillo Formation has been considerably extended and revised. Our findings contribute to fill a considerable gap in the long-term eruptive history of Nicaraguan volcanoes, with prominent implications for volcanic hazard evaluation for Nicaragua.

International audience; Low-temperature chronometers offer potential to gain insights into the temporal evolution of hydrothermal systems. The long-lived fault-bound Grimsel pass hydrothermal system (including a fossil and an active part) in the Central European Alps serves here as a key site to test such an application. Zircon and apatite grains were separated from samples collected along a fault transect. The resulting zircon (U-Th)/He ages are homogenous along the profile at 8-9 Ma and thus record the regional cooling evolution, remaining unaffected by the younger hydrothermal activity. In contrast, the apatite (U-Th)/He ages show three age groups: One Group (1) of ca. 5 Ma inside and outside the hydrothermal zone matches the low-temperature part of the regional cooling trend, while group (2) with ages as young as 1-2 Ma occurs in a central narrow zone associated with hydrothermal activity. One sample (group 3) displays older apparent ages compared to the regional cooling trend. Group (2) apatite samples reveal a different cathodoluminescence texture and trace-element chemistry, which we interpret together with the young age as apatite growth or re-crystallization within the hydrothermal system. Forward 1D modelling of He diffusion indicates that apatite (U-Th)/He ages should always be reset when exposed to hot thermal waters (up to ~140 • C) present over ka timescales or to intermediate temperature waters (~90 • C) over Ma timescales. Combining our measured apatite (U-Th)/He ages with forward modelling results highlight that, besides regional cooling trends, local heat anomalies within hydrothermal zones are very variable in space and time. Combined trace-element geochemistry and (U-Th)/He dating shows local occurrence of newly-formed apatites crystals, which are best described as geochronometers rather than thermochronometers. Such information is important to explore the longevity of hydrothermal systems and associated spatial distributions of heat anomalies.

Molybdenum isotopes are an established proxy for paleoredox conditions in low-temperature surface systems. However, the mechanisms behind demonstrated Mo isotope fractionation during igneous and hydrothermal processes at elevated temperatures are still controversial. This study focusses on a comprehensive dataset documenting the late stage magmatic-hydrothermal evolution of Mo isotope systematics in miarolitic cavities and their host granite from a shallow arc-related intrusive system, the Torres del Paine laccolith in Chile. Molybdenum isotopic compositions (δ98MoSRM3134) were measured for (i) granitic bulk with or without petrographic signs of fluid loss, (ii) magmatic-hydrothermal fluids, and (iii) successively crystallised hydrothermal minerals and range from −1.6 to +1.8‰. The observed variability in δ98MoSRM3134 for individual miarolitic cavities approaching closed system conditions are smaller than the overall range in our dataset but still exceed 1.5‰. The Mo isotopic signature of magmatic fluids was directly measured for the first time by bulk dissolution of magmatic fluid inclusion bearing quartz. Absolute values for magmatic-hydrothermal fluids vary between +0.6 to +1.8‰ δ98MoSRM3134, which is significantly heavier than the granitic bulk rock signatures of −0.1 to +0.46‰ δ98MoSRM3134. Hydrothermal minerals in contrast exhibit variably light δ98MoSRM3134 between −1.6 and +0.6‰. Isotopic differences Δ98Mofluid-mineral between fluid and hydrothermal minerals coexisting in the sampled cavities are largest for plagioclase with 1.9–2.2‰ Δ98Mo, and amount to 1.6–1.9‰ Δ98Mo and 1.5–1.9‰ Δ98Mo for alkali feldspar and biotite, respectively. Smaller values of 1.2–1.5‰ Δ98Mofluid-siderite, 0.8–1.9‰ Δ98Mofluid-molybdenite, 0.4–1.2‰ Δ98Mofluid-titanite and 0.4–1.3‰ Δ98Mofluid-allanite are obtained for higher Mo concentration minerals. Given that fluid-mineral pairs coexisted in equilibrium the ranges in Δ98Mofluid-mineral values we report offer first constraints on the extent of hydrothermal Mo isotope fractionation. The magnitude and direction of these values agrees well with fractionation factors calculated based on an ionic bond-strength model for the incorporation of Mo6+ in hydrothermal minerals for crystallisation temperatures in miarolitic cavities (650–450 °C). This implies that significant fractionation effects can arise during hydrothermal processes even without changes in Mo redox state from oxidised fluid. We can summarise the Mo isotope evolution during magmatic-hydrothermal processes as follows: First, Mo is transferred into the fluid phase exsolving from solidifying magma during late stage igneous evolution. The exsolved fluid subsequently precipitates hydrothermal minerals upon cooling, which dominantly incorporate light Mo isotopes (at variable KDMo(fluid-mineral)). With progressive hydrothermal crystallisation, the remaining fluid evolves to increasingly higher δ98MoSRM3134 along with decreasing Mo concentration. Our data demonstrate that large variability in Mo isotopic signatures can be produced solely by primary magmatic-hydrothermal isotope fractionation processes at elevated temperatures. The generated large range in δ98Mo signatures implies that (i) Mo isotopic signatures of evolved samples cannot be employed for tracing sources or precursor processes unless isotopic fractionation during magmatic-hydrothermal stages is quantified and can be corrected for; and (ii) mass balance models for the global Mo cycle used in paleoredox reconstruction need to account for potentially heterogeneous and isotopically fractionated continental contributions from evolved or hydrothermally overprinted rocks.

Eclogites, metagabbros, and paragneisses from the Variscan M��nchberg Massif record a complex succession of igneous, hydrothermal and metamorphic processes. The geodynamic setting related to the protolith formation and the impact of different types of fluid-rock interactions have been uncertain up to now. We use major and trace element chemistry as well as oxygen isotopes to disentangle the geochemical signatures related to the different stages of the rocks' history. In the M��nchberg Massif, dark eclogites (kyanite-free; Fe-Ti-MORB signature) are distinguished from light eclogites (kyanite-bearing; higher Mg#, Al2O3, and Cr; lower incompatible element contents; positive Eu anomalies; MORB to arc basalt signature). The ��18O values for both types (+5.0 to +10.8���) are equal to, or higher than those of MORB. Amphibolite facies metagabbros have a more enriched, almost OIB-like trace element signature and high ��18O values (+9.4 to +10.3���). Good linear correlations between fluid-immobile elements throughout the eclogite types confirm their derivation from a common, N-MORB to E-MORB-like parental magma. We interpret the light eclogites as former plagioclase-rich cumulates and the dark eclogites as their complementary differentiates. This relationship is partly obscured by variable degrees of magma contamination by sediments, which also affected the metagabbros. However, the metagabbros originated from a more enriched mantle source than the eclogites. Following intrusion, the eclogites were subjected to hydrothermal alteration under the influence of seawater, as indicated by positive correlations between Li, B, Sb, and ��18O. Metamorphic fluid-rock interactions appear to be mostly of limited extent, probably due to the lack of lawsonite dehydration as a fluid source. Nevertheless, the contents at least of some fluid-mobile elements, such as LILE, Li, and Pb, were probably modified during the subductionexhumation cycle of the eclogites. The crustal contamination of the protolith magmas argues against derivation of the eclogites and metagabbros from typical oceanic crust. Instead, a rift-drift transition setting related to the opening of the Rheic or Saxothuringian Ocean seems most likely. The eclogites and metagabbros, alongside with similar rocks in the Mari ��ansk��e L��azn��e complex and other resembling high-pressure massifs, may record different stages of this rift-drift transition.

Recent studies have demonstrated that soils formed on pyroclastic ash deposits are much more common in the Mediterranean area than previously assumed. These soils are an important key to understanding past volcanic events and landscape evolution. Chronological information in soils of Quaternary volcanic events, however, remains still poorly understood in southern Italy. Using a multi-method forensic approach, we explore the origin and age of volcanic deposits (soils) in Sicily and Calabria. The geochemical signature of the soil was compared to the chemical fingerprint of the magmas of potential source areas of southern Italian volcanoes. The results indicate that the investigated soils on the Nebrodi (Sicily) and Sila (Calabria) mountains were both impacted by materials having a high-K calc-alkaline series volcanism. The Aeolian Islands (in particular Lipari and Vulcano) are the most likely source of origin, but contributions also from the Etna (particularly the Biancavilla ignimbrites and Plinian eruptions) occurred. Weathering and leaching processes, along with a potential contribution from the underlying non-volcanic bedrock, has altered the main chemical composition of soils, often precluding direct relation to potential source areas. Immobile elements and their ratios (e.g. the Nb/Y vs Zr/Ti plot) or trace elements (Co, Th) and rare earth elements (laser ablation ICP-MS analyses of glass particles, volcanic clasts and pumice-like materials) gave precious hints of the origin of the volcanic deposits. Radiocarbon dating of the H2O2 resistant soil organic fraction indicates a minimum age of 8–10 ka of the soils. The weathering index WIP (weathering index according to Parker) and the chemical composition of volcanic glasses and clasts were tested as proxies for the age of the volcanic deposits and time for soil formation. The soils and landscape are characterised by multiple volcanic depositional phases for the last about 50 ka in the Sila mountains and about 70 ka or more in the Nebrodi mountains. Chemical-mineralogical analyses enabled the detection of deposition phases during the Pleistocene and also Holocene. The multi-method approach enabled the identification of potential source areas, provided a tentative age estimate of the start (and in part duration) of ash deposits and therefore improved our understanding of volcanic landscape evolution.

Fluid-mobile element (FME) systematics in serpentinites are key to unravel the environments of mantle rock hydration, dehydration, and element recycling in subduction zones. Here we compile serpentinite geochemical data and, for the first time, report discriminative FME enrichment trends for mid ocean ridge vs. forearc serpentinisation by applying alkali element-U ratios. Characteristic element fractionations are thereby governed by redox-dependent differential U mobility at mid ocean ridges and in forearcs, and by high Cs input in forearcs due to fluids equilibrated with sediments. Simple modelling reproduces the observed enrichment trends in serpentinites that range over several orders of magnitude. From these systematics, first constraints on potentially discriminative fractionation trends for unconventional fluid tracers such as B, As, and Sb can be deduced. Prominent W enrichments that correlate with FMEs suggest significant W mobility in low-temperature serpentinising environments. Application of the alkali element-U systematics to the subducted metaperidotites of Erro Tobbio (recording initial brucite + antigorite breakdown during subduction) and Almirez (recording final antigorite breakdown) reveal that pre-subduction FME enrichment signatures are retained in progressively subducted hydrous mantle rocks to beyond subarc levels. Associated dehydration veins and fluid inclusions reveal subordinate alkali element-U fractionation trends during dehydration. Subducted hydrous mantle rocks therefore may introduce characteristic element signatures and thus contribute towards mantle geochemical heterogeneities.

Th-Pb age dating of zoned hydrothermal monazite from alpine-type fissures/clefts is a powerful tool for constraining polyphase deformation at temperatures below 350°C and presents an alternative to K/Ar and 40Ar/39Ar dating techniques for dating brittle tectonics. This study considers the relationship between cleft orientations in ductile shear zones and cleft mineral crystallization during subsequent brittle overprinting. In the Grimsel area, located in the Aar Massif of the Central Alps, horizontal clefts formed during a primary thrust dominated deformation, while younger and vertically oriented clefts developed during secondary strike-slip movements. The change is due to a switch in orientation between the principal stress axes σ2 and σ3. The transition is associated with monazite crystallization and chloritization of biotite at around 11.5 Ma. Quartz fluid inclusion data allow a link between deformation stages and temperatures to be established and indicate that primary monazite crystallization occurred in both cleft systems at 300–350°C. While cleft monazite crystallization ceases at ~11 Ma in inactive shear zones, monazite growth, and/or dissolution-reprecipitation continues under brittle deformation conditions in vertical clefts during later deformation until ~7 Ma. This younger shear zone activity occurs in association with dextral strike-slip movement of the Rhone-Simplon fault system. With the exception of varying Th/U values correlated with the degree of oxidation, there is only limited compositional variation in the studied cleft monazites.

The chemical and physical processes operating during subduction-zonemetamorphismcan profoundly influence the cycling of elements on Earth. Deep-Earth carbon (C) cycling and mobility in subduction zones has been of particular recent interest to the scientific community. Here, we present textural and geochemical data (CO, Sr isotopes and bulk and in-situ trace element concentrations) for a suite of ophicarbonate rocks (carbonate-bearing serpentinites) metamorphosed over a range of peak pressure-temperature (P-T) conditions together representing a progradesubduction zoneP-Tpath. These rocks, in order of increasing peakP-Tconditions, are the Internal Liguride ophicarbonates (from the Bracco unit, N. Apennines), pumpellyite- and blueschist-facies ophicarbonates from the Sestri-Voltaggio zone (W. Ligurian Alps) and the Queyras (W. Alps), respectively, and eclogite-facies ophicarbonates from the Voltri Massif. The Bracco oceanic ophicarbonates retain breccia-like textures associated with their seafloor hydrothermal and sedimentary origins. Their trace element concentrations and δ18OVSMOW(+15.6 to +18.2‰), δ13CVPDB(+1.1 to +2.5‰) and their87Sr/86Sr (0.7058 to 0.7068), appear to reflect equilibration during Jurassic seawater-rock interactions. Intense shear deformation characterizes the more deeply subducted ophicarbonates, in which prominentcalciterecrystallization and carbonation ofserpentiniteclasts occurred. Theisotopic compositionsof the pumpellyite-facies ophicarbonates overlap those of their oceanic equivalents whereas the most deformed blueschist-facies sample shows enrichments in radiogenic Sr (87Sr/86Sr = 0.7075) and depletion in13C (with δ13C as low as −2.0‰). These differing textural and geochemical features for the two suites reflect interaction with fluids in closed and open systems, respectively. The higher-P-metamorphosed ophicarbonates show strong shear textures, with coexistingantigoriteand dolomite, carbonate veins crosscutting progradeantigoritefoliation and, in some cases, relics of magnesite-nodules enclosed in the foliation. These rocks are characterized by lower δ18O (+10.3 to 13.0‰), enrichment in radiogenic Sr (87Sr/86Sr up to 0.7096) and enrichment in incompatible and fluid-mobile element (FME; e.g., As, Sb, Pb). These data seemingly reflect interaction with externally-derived metamorphic fluids and the infiltrating fluids likely were derived from dehydrating serpentinites with hybrid serpentinite-sediment compositions. The interaction between these two lithologies could have occurred prior to or after dehydration of the serpentinites elsewhere. We suggest thatdecarbonationand dissolution/precipitation processes operating in ancient subduction zones, and resulting in the mobilization of C, are best traced by a combination of detailed field and petrographic observations, C, O and Sr isotope systematics (i.e., 3D isotopes), and FME inventories. Demonstration of such processes is key to advancing our understanding of the influence ofsubduction zonemetamorphismon the mobilization of C in subducting reservoirs and the efficiency of delivery of this C to depths beneathvolcanic arcsand into the deeper mantle.

Molybdenum isotopes have emerged as novel tracers for high-temperature igneous and metamorphic processes. The debate remains to what extent different subducted slab lithologies, such as oceanic crust and marine sediments, contribute to the Mo isotope signature of arc magmas and, hence, exert different controls on the terrestrial Mo cycle. Here we investigate Mo isotope systematics from input to output at the Tongan subduction zone: Arc lavas from different Tongan islands, pelagic sediments and altered oceanic crust (AOC) samples from DSDP site 595/596 on the subducting Pacific plate. For complementary insights into the fate of Mo and its isotopic signatures during prograde subduction metamorphism, we also present data of metasediments and variably altered AOC-type eclogites from the Zermatt-Saas ophiolite, Switzerland and Italy, and the Schistes Lustres Complex in Alpine Corsica. Manganese oxide-rich pelagic sediments from DSDP site 595/596 show variable, depth-dependent Mo/Mn ratios and Mo isotope compositions controlled by diagenetic reactions. As subducted equivalents, Mn-rich eclogitic metapelites display lower Mo contents and δ98/95Mo ratios compared to their non-subducted protolith. This indicates prominent loss of Mo along with isotope fractionation during early subduction metamorphism. In comparison to unaltered MORB, low temperature seafloor alteration has shifted Mo/Ce and δ98/95Mo in studied AOC samples towards lower ratios, in the range of most mafic eclogites published so far. However, some mafic eclogites show even lower Mo/Ce and δ98/95Mo ratios compared to AOC, likely due to fluid-related Mo loss upon early subduction and preferential incorporation of light Mo into residual rutile. Our data document a prominent loss of isotopically heavy Mo before and upon early subduction metamorphism at shallow depths in the forearc region. Moreover, when prograde rutile crystallizes at ~30 km depth, it fixes the largest fraction of Mo in the subducting material. This creates an “arc Mo-conundrum” as devolatization of the slab at subarc depths is not able to account for the fluid-mobile Mo source responsible for observed higher Mo/Ce and δ98/95Mo in Tongan arc lavas compared to the mantle. As an alternative scenario, Mo mobilization by slab-derived aqueous fluids during the early stages of subduction into the forearc mantle produces serpentinites enriched in Mo with a possible heavy Mo isotopic signature. Mechanical transport and devolatization of forearc serpentinites at subarc regions is a plausible alternative recycling process accounting for the observed Mo systematics in Tongan arc lavas. This is supported by positive covariations of Mo/Ce and δ98/95Mo with elements such as As, Sb, and Cs, which are thought to be mostly released from the subducting material during early stages of subduction. We propose that a multi-stage recycling of metasomatized forearc mantle can be an important process in recycling of Mo and possibly other elements.

The Erro Tobbio olivine-antigorite serpentinites and associated dehydration veins represent hydrated oceanic mantle rocks that escaped complete dehydration and recycling into the mantle after subduction to ~ 550-600 °C and 2.0-2.5 GPa. These rocks thus offer valuable insights into the petrological evolution of a slice of hydrated oceanic mantle and the geochemical cycling down to intermediate subduction zone depths. Our study emphasises the role of brucite upon rock-buffered hydration and subduction dehydration employing bulk and in situ chemical data sets combined with petrology. Bulk rock data reveal a coherent mantle peridotite slice affected by variable melt depletion and refertilisation. Subsequent fluid-rock interaction stages proceeded isochemically with respect to SiO2, i.e., without significant SiO2 enrichment characteristic for hydrothermal ocean floor serpentinisation. Relicts of low-T mesh textures after olivine and preservation of precursor mineral and low-T hydration geochemical features indicate a lack of subsequent fluid and metamorphic overprinting, even on scales of tens of micrometres. Fluid-mobile element enrichments are modest with exceptions for B and W. Enrichment signatures of U/Cs << 1 and Rb/Cs of 4-26 are characteristic of shallow forearc hydration within or atop the slab by fluids derived from breakdown of clays or first dehydration of altered oceanic crust with a subordinate sedimentary pore fluid component. Overall, the geochemical and petrological changes of the Erro Tobbio peridotites during fluid-rock interactions were rockbuffered, in contrast to fluid-buffered hydration accompanied with significant SiO2 metasomatism at, e.g., mid ocean ridges. Silica-neutral rock-buffered serpentinisation resulted in prominent brucite formation upon olivine hydration. In absence of excess SiO2, subsequent serpentine transformation of chrysotile/lizardite to antigorite likely produced even more brucite. Rock-buffered fluid-rock interactions thus provide a mechanism for stabilising brucite in subduction zone serpentinites, presumably along hydration fronts and within deeper sections of the oceanic lithospheric mantle. Finally, brucite + antigorite dehydration produced up to 40 vol. % of metamorphic olivine and prominent olivine + Ti-clinohumite + magnetite vein networks at temperatures < 550-600 °C, prior to complete antigorite breakdown. Wall rocks released alkali elements, B, Cr, As, Sb, and Ba into the dehydration fluids, along with substantial Sr, REE and HFSE redistribution into vein minerals.

Mineralium Deposita, 39 (8), ISSN:0026-4598, ISSN:1432-1866

Recent studies report a large Mo isotope variability of up to 1‰ (expressed in δ98/95MoNIST3134) in convergent margin lavas. These isotopic variations have been associated with subduction zone processes and ultimately may account for heavy and variable isotope signatures in evolved continental crust. Arc lavas show both lighter and heavier Mo isotopic values when compared to the chondritic average (δ98/95MoNIST3134 −0.16 ± 0.02‰), with a concentration weighted mean isotopic composition (δ98/95MoNIST3134 +0.03‰) distinctly heavier than that of the mantle (δ98/95MoNIST3134~−0.21 to −0.16‰). The absence of isotopic fractionation during tholeiitic igneous differentiation indicates that heavy Mo isotope signatures in convergent margin rocks are intrinsic to subduction zone processes, caused either by recycling of subducted components, by Mo isotope fractionation during magmatic differentiation, or by a combination thereof. In order to gain a more detailed understanding of the Mo isotope variability in arc lavas, we have studied Mo isotopes and concentrations in calc-alkaline lavas sampled along the Banda Arc in Indonesia, an intra-oceanic subduction zone well known for variable contributions of subducting continental meterial. Banda Arc lavas exhibit Mo isotopic compositions covering a large range from −0.48 to +0.24‰ in δ98/95MoNIST3134. We find that, combined with published data from other subduction zones, the majority of arc data are isotopically heavier compared to its presumed mantle wedge source or mid-ocean-ridge basalts. Furthermore, arc lava Mo isotope signatures show an apparent covariation between indices of amphibole-clinopyroxene fractionation from the melt. Generally, heavier Mo isotopic signatures are associated with higher degrees of REE fractionation expressed as λ1, λ2, and Dy*/Dy, high Ca/Al, and higher Sc contents. These observations can be best explained with an open system magma reservoir where fractional crystallisation, recharge and eruption are tightly linked. Notable exceptions in the Banda region are four samples with isotopically light Mo. Three of these isotopically light lavas indicate the assimilation of an isotopically light Mo reservoir with heavy δ18O and radiogenic 87Sr/86Sr-206Pb/204Pb, tentatively identified as lower arc crust. Together, our data demonstrate, in line with previous findings, that magmatic processes modify the Mo isotopic composition in arc lavas, thus adding complexity to the Mo isotope systematics inherent to subduction zones.

Feldspars are the most abundant rock-forming minerals in the Earth’s crust, but their magnetic properties have not been rigorously studied. This work focuses on the intrinsic magnetic anisotropy of 31 feldspar samples with various chemical compositions. Because feldspar is often twinned or shows exsolution textures, measurements were performed on twinned and exsolved samples as well as single crystals. The anisotropy is controlled by the diamagnetic susceptibility and displays a consistent orientation of principal susceptibility axes; the most negative or minimum susceptibility is parallel to [010], and the maximum (least negative) is close to the crystallographic [001] axis. However, the magnetic anisotropy is weak when compared to other rock-forming minerals, 1.53 × 10−9 m3 kg−1 at maximum. Therefore, lower abundance minerals, such as augite, hornblende or biotite, often dominate the bulk paramagnetic anisotropy of a rock. Ferromagnetic anisotropy is not significant in most samples. In the few samples that do show ferromagnetic anisotropy, the principal susceptibility directions of the ferromagnetic subfabric do not display a systematic orientation with respect to the feldspar lattice. These results suggest that palaeointensity estimates of the geomagnetic field made on single crystals of feldspar will not be affected by a systematic orientation of the ferromagnetic inclusions within the feldspar lattice.

Rutile (TiO2) is an important host phase for high field strength elements (HFSE) such as Nb in metamorphic and subduction zone environments. The observed depletion of Nb in arc rocks is often explained by the hypothesis that rutile sequesters HFSE in the subducted slab and overlying sediment, and is chemically inert with respect to aqueous fluids evolved during prograde metamorphism in the forearc to subarc environment. However, field observations of exhumed terranes, and experimental studies, indicate that HFSE may be soluble in complex aqueous fluids at high pressure (i.e., >0.5 GPa) and moderate to high temperature (i.e., >300 degrees C). In this study, we investigated experimentally the mobility of Nb in NaCl- and NaF-bearing aqueous fluids in equilibrium with Nb-bearing rutile at pressure-temperature conditions applicable to fluid evolution in arc environments. Niobium concentrations in aqueous fluid at rutile saturation were measured directly by using a hydrothermal diamond-anvil cell (HDAC) and synchrotron X-ray fluorescence (SXRF) at 2.1 to 6.5 GPa and 300-500 degrees C, and indirectly by performing mass loss experiments in a piston-cylinder (PC) apparatus at similar to 1 GPa and 700-800 degrees C. The concentration of Nb in a 10 wt% NaCl aqueous fluid increases from 6 to 11 mu g/g as temperature increases from 300 to 500 degrees C, over a pressure range from 2.1 to 2.8 GPa, consistent with a positive temperature dependence. The concentration of Nb in a 20 wt% NaCl aqueous fluid varies from 55 to 150 mu g/g at 300 to 500 degrees C, over a pressure range from 1.8 to 6.4 GPa; however, there is no discernible temperature or pressure dependence. The Nb concentration in a 4 wt% NaF-bearing aqueous fluid increases from 180 to 910 mu g/g as temperature increases from 300 to 500 degrees C over the pressure range 2.1 to 6.5 GPa. The data for the F-bearing fluid indicate that the Nb content of the fluid exhibits a dependence on temperature between 300 and 500 degrees C at >= 2 GPa, but there is no observed dependence on pressure. Together, the data demonstrate that the hydrothermal mobility of Nb is strongly controlled by the composition of the fluid, consistent with published data for Ti. At all experimental conditions, however, the concentration of Nb in the fluid is always lower than coexisting rutile, consistent with a role for rutile in moderating the Nb budget of arc rocks.

Pyroxenes constitute an important component in mafic igneous and metamorphic rocks. They often possess a prismatic habit, and their long axis, the crystallographic c axis, helps define a lineation in a textured rock. Anisotropy of magnetic susceptibility (AMS) serves as a fabric indicator in igneous and metamorphic rocks. If a rock’s AMS is carried by pyroxenes, it can be related to their crystallographic preferred orientation and degree of alignment. This requires knowing the intrinsic AMS of pyroxene single crystals. This study provides a comprehensive low-field and high-field AMS investigation of chemically diverse orthopyroxene and clinopyroxene crystals in relation to crystal structure, chemical composition, oxidation state of Fe, and the possible presence of ferromagnetic inclusions. The paramagnetic anisotropy, extracted from high-field data, shows clear relationships to crystallographic directions and Fe concentration both in clinopyroxene and orthopyroxene. In the diopside-augite series, the intermediate susceptibility is parallel to b, and the maximum is at 45° to the c axis. In aegirine, the intermediate axis remains parallel to b, while the maximum susceptibility is parallel to c. The AMS of spodumene depends on Fe concentration. In enstatite, the maximum susceptibility aligns with c and the minimum with b, and in the case of hypersthene, the maximum susceptibility is normal to the exsolution lamellae. Magnetite inclusions within augite possess a ferromagnetic anisotropy with consistent orientation of the principal susceptibilities, which dominates the low-field anisotropy. These results provide better understanding of magnetic anisotropy in pyroxenes and form a solid basis for interpretation of magnetic fabrics in pyroxene-bearing rocks.