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The IUPAC–IUGS joint Task Group “Isotopes in Geosciences” (TGIG) has evaluated the published literature on the half-lives of six long-lived, geologically relevant radioactive nuclides. Where conflicting literature estimates are present, it is necessary to first identify any systematic bias in accordance with metrological traceability and to exclude the biased estimates from further consideration. The TGIG recommends three robust half-life estimates: 49.61 ± 0.16 Ga for 87Rb, corresponding to a decay constant λ(87Rb) = (1.3972 ± 0.0045) × 10−11 a−1; 106.25 ± 0.38 Ga for 147Sm, and a corresponding decay constant λ(147Sm) = (6.524 ± 0.024) × 10−12 a−1; 4.4683 ± 0.0096 Ga for 238U, i.e. a decay constant λ(238U) = (1.55125 ± 0.00333) × 10−10 a−1. All cited uncertainties have a coverage factor k = 2. For other radionuclides of Sm and U, no unambiguous consensus value can be endorsed at present by TGIG, which limits its evaluation to a status report highlighting unaccounted-for potential sources of bias. The improved repeatability of mass spectrometric measurements has revealed systematic bias effects that had been dismissed as subordinate in the past. These issues can only be resolved by future dedicated investigations.

Pseudotachylytes are solidified frictional melts produced by seismic fault slip. Being melts that solidified in seconds or minutes after the seismic slip event, they have always been considered a very favourable tool to date brittle deformation. However, since all pseudotachylytes are composed of inherited clasts, melt-derived matrix and (quite often) also alteration products, it is necessary to discriminate the Ar contribution of these three reservoirs to obtain meaningful ages. This can be done by analyzing Ca/K and Cl/K signatures provided by Ar systematics. Furthermore, microstructural analysis and microCT allow quantifying the clast-to-pseudotachylyte matrix ratios for each sample, and XRPD allows detecting potential alteration phases. Here we present the results of step-heating 40Ar/39Ar analyses performed on pseudotachylytes of the Trois Villes Fault and the Quart Fault, which crop out in a region of the Western Alps (Aosta Valley) affected by three different post-metamorphic brittle deformation phases: D1 characterized by NW-SW extension, D2 with NE-SW extension, and D3 showing N-S extension. The relative chronology of these deformation phases is based on consistent cross-cutting relationships. D1 ages of 29–32 Ma have been inferred from syn-kinematic magmatic dikes and hydrothermal veins. However, no absolute ages were so far available for D2 and D3, as direct radiometric dating of fault rocks has never been performed before in the area. Our results are consistent with the relative chronology and greatly improve our understanding of the tectonics of this area.

The Montecristo district, northern Chile, is one of the few places worldwide where there is a direct relationship between magnetite-(apatite) (MtAp) mineralization and iron oxide-copper–gold (IOCG) mineralization. The MtAp mineralization includes Ti-poor magnetite, fluorapatite, and actinolite and is crosscut and partially replaced by a younger IOCG mineralization that includes a second generation of actinolite and magnetite with quartz, chalcopyrite, pyrite, and molybdenite. The MtAp stage at Montecristo is interpreted as the crystallized iron-rich melts that used the pre-existing structures of the Atacama Fault System as conduits. These rocks later acted as a trap for hydrothermal IOCG mineralization. Geochronology data at Montecristo indicate that the host diorite (U–Pb zircon 153.3 ± 1.8 Ma, 2-sigma), MtAp mineralization (40Ar-39Ar in actinolite, 154 ± 2 Ma and 153 ± 4 Ma, 2-sigma), and the IOCG event (Re-Os on molybdenite, 151.8 ± 0.6 Ma, 2-sigma) are coeval within error and took place in a time span of less than 3.4 Ma. The εHfi and εNdi values of the host diorite are + 8.0 to + 9.8 and + 4.3 to + 5.4, respectively. The whole-rock 87Sr/86Sri values of the IOCG mineralization (0.70425 to 0.70442) are in the lower end of those of the MtAp mineralization (0.70426–0.70629). In contrast, εNdi values for the IOCG mineralization (+ 5.4 and + 5.7) fall between those of the MtAp rocks (+ 6.6 to + 7.2) and the host diorite, which suggests that the IOCG event was related to fluids having a more crustal Nd (εNdi < + 5.7) composition than the MtAp mineralization. This likely reflects the mixing of Nd from the MtAp protolith and a deep magmatic-hydrothermal source, very likely an unexposed intrusion equivalent to the host diorite. Sulfur isotope compositions (δ34S, + 0.3 to + 3.4‰) are consistent with a magmatic source., NSERC discovery grant to J.M. Hanchar (RGPIN/004649-2015), RTI2018-099157-A-I00 (MCI/AEI/FEDER, UE) research grant to F. Tornos.

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.

Table S1 (complete Ar isotope data), Table S2 (electron microprobe data), and Figure S1.

An accurate record of preindustrial (pre-1900 CE) sea level is necessary to contextualize modern global mean sea level (GMSL) rise with respect to natural variability. Precisely dated phreatic overgrowths on speleothems (POS) provide detailed rates of Late Holocene sea-level rise in Mallorca. Statistical analysis indicates that sea level rose locally by 0.12 to 0.31 m (95% confidence) from 3.26 to 2.84 thousand years (ka) ago (2σ) and remained within 0.08 m (95% confidence) of preindustrial levels from 2.84 ka to 1900 CE. This sea-level history is consistent with glacial isostatic adjustment models adopting relatively weak upper mantle viscosities of ~10 20 Pa s. There is virtual certainty (>0.999 probability) that the average GMSL rise since 1900 CE has exceeded even the high average rate of sea-level rise between 3.26 and 2.84 ka inferred from the POS record. We conclude that modern GMSL rise is anomalous relative to any natural variability in ice volumes over the past 4000 years.

The study of a huge repertory of sparse glazed ware from Donoratico (Livorno, Italy) dated back to the ninth century was carried out to investigate the production technology via petro- graphic, chemical, and isotopic investigations. The mineralogical and chemical evidence suggested the application of a lead oxide flux to an unfired non-calcareous ceramic body, in accordance with late Antique and early-Medieval traditions. The isotopic investigation of the glazes also suggested different sources for PbO, which was frequently applied as a mix of different metal batches. The Pb isotopic record identified the Southern Tuscany districts (Campiglia Marittima and Colline Metallifere) and a source controlled by the Carolingian kings in the northern districts of Central Europe (either in Aquitaine or in the Middle German ore districts) as the most reliable sources. SEM-EDS showed the presence of tin impurities in the form of secondary cassiterite agglomerates in most of the glazes obtained by the use of lo- cal lead. The presence of cassiterite veins in Southern Tuscany lead sulphide deposits was con- sidered as the main cause for tin impurities in the glazing mixture and, thus, an important marker for the exploitation of local lead.

The IUPAC-IUGS joint Task Group “Isotopes in Geosciences” recommends a value of (106.25 ± 0.38) Ga for the half-life of 147Sm, and a corresponding decay constant λ147 = (6.524 ± 0.024) × 10−12 a−1, both with a coverage factor k = 2. For the extinct radionuclide 146Sm two very different half-lives are used in the scientific community (c. 68 and 103 Ma), to such a degree that no consensus value can be endorsed at present by the Task Group. Pending dedicated re-investigations it is recommended that papers using the 146Sm decay to quantify the cosmo/geological evolution of (extra)terrestrial samples perform a twin set of calculations using both proposed half-lives.

Strata of Chilcatay and Pisco formations exposed in the Ica Desert (East Pisco Basin, southern Peru) preserve one of the most complete and rich records of Miocene marine vertebrates of the world. Despite its exceptional importance, the chronostratigraphy of these fossil-bearing deposits has been only sporadically studied in the literature until recently. This work presents a detailed reconstruction of the chronostratigraphic framework, achieved by mapping and measuring of seven sections (totaling a height of almost 1000 m) of the Miocene Chilcatay and Pisco formations along the western side of the Ica River. The Chilcatay Formation consists of two allomembers, namely Ct1 and Ct2, bounded at the base by unconformities CE0.1 and CE0.2, respectively. Similarly, the immediately overlying Pisco Formation is divided into allomembers P0, P1, and P2, bounded at the base by unconformities PE0.0, PE0.1 and PE0.2, respectively. The new 39Ar–40Ar results presented here, combined with ages of previous work, provide precise constraints on the age of several stratigraphically referenced volcanic ash layers intercalated in the studied fossil-bearing succession, placing its vertebrate fossil fauna within a refined temporal framework and laying the solid ground for its detailed regional and global comparison. The ages of the allomembers, and thus their associated faunas, can be reliably estimated by the combination of 39Ar–40Ar dating on tephra layers with diatom biostratigraphy. In the study area, two methods are mutually consistent and constrain the deposition of the Chilcatay Formation between 19.2 and 18.0 Ma, that of P1 between 9.5 and 8.6 Ma, and that of P2 between 8.4 and 6.7 Ma. In the absence of direct dating of the P0 allomember, which lacks both preserved tephra suitable for 39Ar–40Ar dating and microfossils, its age can be constrained to the temporal gap between the youngest age available from the underlying Chilcatay strata (18.0 Ma) and the oldest age available from the overlying P1 strata (9.5 Ma).

Vorticity estimates based on porphyroclasts analysis are limited by the extrapolation to three dimensions of two-dimensional data. We describe a 3D approach based on the use of X-ray micro-computed tomography that better reflects the real 3D geometry of the porphyroclasts population. This new approach for kinematic vorticity analysis in the Munsiari Thrust mylonites, the lower boundary of the Main Central Thrust zone (MCTz) in Indian Himalaya, indicates a large pure shear component during non-coaxial shearing. 40Ar/39Ar ages of micas along the mylonitic foliation of the Munsiari and Vaikrita thrusts (the upper boundary of the MCTz) constrain thrust activity to 5–4 and 8–9 Ma, respectively. Available kinematic vorticity analyses of the Vaikrita mylonites suggest the dominance of a simple shear component. Combining these data, we suggest that the southward and structurally downward shift of deformation along the MCTz was accompanied by a progressive increase in the pure shear component in a general shear flow.

Thirty-nine Neolithic and Early Bronze Age copper objects (primarily axe blades and daggers) from Central, southern and eastern Switzerland or eastern France were analysed typochronologically, chemically and with regard to their lead isotope ratios. This combination of methods allows for more differentiated and reliable conclusions about the provenances of the copper ores and metallurgies than the individual methods could provide in isolation. The primacy of the typochronological classification is paramount for assessing the scientific results appropriately. For the period between c. 4300 and c. 1800 BCE, there is evidence of at least seven different metallurgical traditions found in the Prealps, some of them stemming from far-away innovation hotspots. These metallurgies do not form part of a continuous tradition; instead, they emerge unexpectedly and disappear again just as abruptly. This could hint at the fragility and volatility of these strands of tradition and the groups who keep them. Between 4300 and 3500 BCE, there was a multitude of types, groups of copper compositions and isotope ratios. Arsenic plays an essential role in the composition of copper during this time. Such arsenic-enriched copper is generally referred to as ‘arsenic copper’. It is likely that a considerable share of the raw material used in arsenic copper from this first heyday of copper metallurgy originates from the Slovak or Saxon-Bohemian Ore Mountains. Nevertheless, it seems likely that arsenic copper was at least partially cast locally in the pile dwelling of the Prealps and forged into the shapes known from these areas. According to our analyses, the raw material of some of the objects found in the Swiss Prealps, whether they are arsenic copper or not, could also stem from the eastern Alpine region. Concentrations of crucible and axe blade finds are not necessarily evidence for ore deposits in close proximity, as was assumed in the case of Mondsee and Attersee; these finds could also be indicative of local salt sources, with the salt being exchanged for copper from other areas. The copper of the flanged axe blades from the last quarter of the 4th millennium BCE found mainly south of the Alps—in isolation also north of the Alps—is very similar to the early arsenic copper mentioned above. The axe blades’ distribution patterns and the ‘fingerprint’ of their lead isotope ratios suggest that this later arsenic copper has its origin in the metallurgical centres in South Tuscany. These innovations from the West Mediterranean inspired the metallurgical traditions in the South of France (e.g. Cabrieres, Montagne Noire) and the south-western Alps (Massifs des Grandes Rousses, Saint Veran).

Eocene dykes extending over 50 km along the southern margin of the Ladakh batholith (NW India) fall into two main groups showing different orientation as well as isotope and trace element geochemistry. Both dyke families formed in the same tectonic setting over a time span of approximately 4 Ma. However, each family is far from monolithic, and therefore we tested several statistical approaches to identify geochemical subgroups from REE data and relate them to magmatogenetic processes. Hierarchical Clustering and Multidimensional Scaling calculate similarities/dissimilarities among individuals of a population. Both statistical tools, when applied to the Ladakh dykes, reflect the east-west dichotomy. However, detailed quantification of the resulting grouping varies according to input data. Normalization to chondrites yields slightly different groupings from unnormalized concentration data. Population-internal REE normalization provides the most accurate grouping as revealed by the fact that multiple samples from the same dyke are assigned the closest relatedness. Independently from normalization, east-west dichotomy is mirrored by marked differences in degree of crustal assimilation and magma evolution, pointing to km-scale geological heterogeneity. Finally, dykes intruding the Ladakh batholith 150 km SE of the present samples are geochemically similar, but cluster as distinct group.

Dating structurally complex fault rocks often results in internally inconsistent ages, as several mineral generations are intergrown at scales << 10 µm and almost always altered to various degrees. We describe here 39Ar-40Ar stepheating using the combination of two independent indicators that allow the discrimination of coexisting mica generations from each other and from the ubiquitous retrogression/alteration phases. A necessary first step is electron probe microanalysis to assess both inventory and spatial distribution of the mineral phases that need to be distinguished a posteriori by 39Ar-40Ar systematics. One indicator is based on mica stoichiometry, which can be proxied by the 39Ar concentration in combination with the 37Ar/39Ar and 38Ar/39Ar (i.e. Ca/K and Cl/K) ratios. The other indicator is the furnace temperature, at which a degassing peak accompanying dehydration and structural collapse is observed. As dehydration rates depend on the average bond strength in the crystal structure, it is predicted (and indeed observed) that the temperature of the differential Ar release peak is variable among different minerals. As the Ca/Cl/K signatures of pure micas coincide with the Ar release peak, their combination identifies the isochemical steps that correspond to the degassing of pristine micas. Only these should be used to date the activity of shear zones.This procedure should become routine in analysing polydeformed metamorphic rocks.

Multiple sulfur (δ34Ssulfate, Δ33Ssulfate, & Δ36Ssulfate), nitrogen and oxygen (δ15Nnitrate & δ18Onitrate) and strontium (87Sr/86Sr) isotope compositions of precipitation collected from Seoul, South Korea were analyzed to study the sources, transportation and deposition of air pollutants in East Asia. The δ34Ssulfate values (from 1.9 to 14.6‰ with a median of 4.7‰) and the δ15Nnitrate values (from −2.0 to 13.3‰ with a median of 1.0‰) suggest that fossil fuel use (emission from coal combustion and vehicle exhaust) is a predominant source for sulfur and nitrogen, but other natural sources including biogenic contributions of DMS also add to their total budget. The seasonal variations are observed in δ34Ssulfate and δ15Nnitrate values (both higher in winter season), which is likely to result from the increase of coal use for domestic heating in China. The δ18Onitrate values also varied seasonally depending on the NOx oxidation pathway, being about 20‰ higher in winter than in summer season. The Δ33Ssulfate and Δ36Ssulfate values are not anomalous, showing the absence of MIF signals in precipitation of Seoul. The 87Sr/86Sr ratio of the precipitation samples range from 0.70988 to 0.71487 with a median of 0.71073, indicating the influence of at least three end-member (silicate dust, carbonate dust and anthropogenic emission). Ionic ratios such as (K++NH4+)/(Ca2++Mg2+) and Cl−/Na+ suggest the overwhelming effect of anthropogenic input rather than carbonate dust on the end-member with lower 87Sr/86Sr ratios.

International audience; Museum-grade mica megacrysts of the Phalaborwa phlogopite (ca. 2 Ga) and the Rubikon pegmatite (ca. 0.5 Ga) were screened for intra-grain compositional and chronological heterogeneities by electron probe microanalysis and 40Ar/39Ar dating, respectively. Both micas were known to have Rbsingle bondSr ages indistinguishable from Usingle bondPb ages. Even though step heating plateaus were obtained for nearly a hundred 100 μm scale subgrain chips, age variations of several percents were observed among individual chips. The age variations were unrelated to the position within the megacryst and to the position in the irradiation canister. Element mapping parallel and perpendicular to the (001) plane showed significant variations in concentrations of major elements Ti, Fe, Mn, K, Al. Cation transport, element redistribution and retrograde mineral formation that escaped visual detection require open-system, fluid-assisted chemical reactions. These recrystallization processes occurred during one or several post-magmatic hydrothermal event(s). The spatial distribution of sub-grain ages show that 40Ar/39Ar dating does not provide a simple, diffusion-controlled cooling age, but rather apparent ages controlled by mineral retrogression and recrystallization. The result was variable loss of Ar at the subgrain scale. These observations show that retrogression events cannot be detected by the presence or absence of plateau ages. Instead, recrystallization can be diagnosed (1) by a thorough petrological investigation based on microchemical maps, and (2) by Cl/K and Ca/K constraints from 40Ar/39Ar systematics. The quest for a natural mineral sufficiently homogeneous to act as a precise calibrator for the 40K half-life remains unsatisfactory.

The systematics of Ar release during in vacuo stepwise heating of hydrous minerals is examined. By means of common-denominator three-isotope correlation diagrams, in use for 60 years, it is possible to recognize the presence of polymineralic mixtures in a step-heated mineral separate and characterize their components. These correlation diagrams, when applied to literature studies proposing Fick's Law diffusion during in vacuo stepwise heating, show that in actual fact these studies were based on partly retrogressed micas that were intergrown with their alteration products, probably at a scale < 20 μm so as to escape recognition during handpicking. The degassing of hydrous minerals in vacuo does not follow Fick's Law in an inert, homogeneous matrix, and instead proceeds from the structural collapse during dehydration. An alternative presentation discussed here quantifies the differential release of the Ar isotopes. Their release follows the structural collapse of their host phases and gives rise to remarkably well resolved patterns even in polymineralic samples.

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

The northern Adamello is crosscut by ductile shear zones and pseudotachylyte-bearing faults, both compatible with the same stress field, with ductile shear zones crosscut by brittle faults. These relations are coherent with the re-equilibration of the pluton-related thermal anomaly to temperatures typical of the base of the seismogenic continental crust (T = 250 – 300°). Our new 40Ar-39Ar ages help to constrain the absolute age and duration of each deformation phase.Samples included wall-rock biotite, bulk ultramylonites and pseudotchylytes. Before stepwise heating 40Ar-39Ar measurements, samples were characterized by microstructural, geochemical and petrological analyses.The wall-rock biotite is 33.4±0.1 Ma old, independently of grainsize. Mylonites feature complex age spectra between 28-31 Ma, including biotite and altered feldspar. Four pseudotachylyte matrices are clustered around 30-31.5 Ma, and two samples have 25-26 Ma ages.Ductile shearing active 2 Ma after wall-rock emplacement indicates either low strain rates, or a long-lasting thermal anomaly, which might be due to high emplacement depth, and/or the progressive assemblage of adjacent plutons through small magma pulses. Seismogenic faulting overlaps with mylonitization around 31 Ma; younger pseudotachylyte ages may be due to late-stage reactivation.

The degree of early Archaean terrestrial differentiation into mantle and crust is long debated. A number of studies rely on bulk rock Hf and Nd isotope data to gain further insight. Here we present Hf-Nd data from two igneous basements suites of Barberton Mountain Land, namely a tonalite-trondhjemite-granodiorite (TTG) and granodiorite-granite-monzogranite (GGM) suite to further decipher crust formation and its timing in the Archaean. TTG show high initial e(Hf) and e(Nd) if corrected using their respective U/Pb zircon ages that can be interpreted in two ways: (i) early differentiation of bulk silicate Earth or (ii) overcorrection of these data caused by decoupling of zircon from the ‘zircon-free’ bulk rock due to metamorphic overprint. Indeed, regional metamorphism affected the early TTG ca. 250 Ma after their emplacement and disturbed their bulk rock Hf and Nd systematics. Correction of bulk rock Hf and Nd data to the age of metamorphism yields values between +0.9 and − 6.2 and from −1.3 to −4.3, respectively, thus indicating open-system behaviour. Lu-Hf and Sm-Nd mineral isochron ages of two TTG plutons are significantly younger than their corresponding U/Pb zircon age, but agree within uncertainty perfectly with external age constraints for the age of the metamorphic overprint. The same accounts for the 40Ar-39Ar systematics of the investigated plutons. Amphibole samples consist of a mixture of heterochemical generations that yield ages that are both older and younger than the metamorphic overprint at 3.2 Ga. The metamorphic minerals in each plutons provide a highly divergent age information from the coexisting zircon. Hence, the positive e(Nd) and e(Hf)-values that resulted from age-correction to the respective U/Pb zircon age are most probably over-corrected. Finally, if all data is age-corrected to the metamorphic overprint all samples agree with models for the Nd-Hf isotopic evolution in the mantle that do not require a large-scale early differentiation of Earth into enriched and depleted reservoirs. Mantle extraction ages (TDM) of TTG are interpreted as minimum ages for the generation of felsic Archaean crust whereas TDM of GGM are interpreted as maximum ages for the last major crust growth episode in the Barberton Mountain Land.

Lead isotope compositions were determined for 18 metal objects from the archaeological site of Sant’Imbenia, NW Sardinia, dating to the end of the ninth century BCE onwards. The provenance of some objects is unambiguously traced to SW Sardinia; other objects could derive either from central Sardinia or the Iberian coastal ranges. The variety of the provenances attests to a wide trade network that spanned the entire island of Sardinia and extended to the Iberian sites.

The release of Ar during stepwise heating 39Ar 40Ar dating experiments may be controlled by Fick's Law diffusion in an inert matrix, and/or by structural modifications of the host mineral. A ca. 1 mm3 irradiated cleavage fragment of a low sanidine crystal from Itrongay, Madagascar was degassed isothermally at 888 ± 2 °C for 264 h, acquiring 67 stepwise 39Ar release data. The 39Ar release was observed to follow a smooth sigmoid curve and not a line as would be predicted by Fick's Law. If the 39Ar release is controlled by crystallographic changes, the implication is that these changes undergo time-dependent variations. The long-term bulk degassing, being the sum of various structural modifications, approximates a Fickian behavior that is not verified in detail in short-term experiments, as it averages over different 39Ar release regimes. This would make the downslope extrapolation of laboratory data to geological conditions highly underconstrained. In order to constrain the behavior of the crystal structure of K-feldspar during laboratory heating, we measured the Raman spectra of a different, ca. 1 mm3 cleavage fragment of the same irradiated sanidine crystal. The sample was heated in air in a Linkam heating stage, and Raman spectra were acquired at temperatures increasing from 300 to 1000 °C, including a 6-h isothermal run at 900 ± 1 °C. Raman modes between 50 and 1200 cm−1 were observed to record two kinds of change, defining robust trends. The positions of most peaks were shifted towards lower wavenumbers (lower energies) and broadened; in addition, the relative heights of different peaks showed robust variation trends. The larger changes coincide with discrete temperature increases, but all changes also progressed over time at constant temperature. These peaks mirror the excitation of phonon modes, which are associated with interatomic bond stretching and deformation, with Si,Al ordering and with deformation and rotation of SiO2 tetrahedra. Most, but by far not all, of the change is reversible (such as e.g. the differential activation of phonon modes), but irreversible structure modifications (such as e.g. Al,Si disordering) are also recorded. We conclude that the K-feldspar structure violates mathematical requirements of matrix inertness during laboratory heating.

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.

In the eclogite facies shear zone of Bottarello, Monte Rosa, Western Alps [1], fault recrystallization around 600 °C gives concordant Lu-Hf (garnet) and 39Ar-40Ar (white mica, WM) 47 Ma ages whereas The island of Naxos (Cyclades, Greece) is the classic example of multiple, coexisting WM generations [10]: relict pre-eclogitic basement WM, and eclogitic phengite retrograded to muscovite. Electron microprobe element maps demonstrate intergrowths at a scale The in-sequence thrusts of the Garhwal Himalaya add one complication: thrusting was long-lived. Microstructures combined with chemical microanalysis distinguish three monazite generations (dated by U-Pb) and three WM generations: relicts in microlithons, foliation-defining mica, and static coronas. As in the previous examples, intergrowths are

International audience; wo-mica—biotite and muscovite—volcanics are particularly rare in the geological record. One of the several dozen volcanic ash layers from Central Andes volcanoes found in the upper Miocene marine succession of the Pisco Formation (Ica Desert, Peru) contains juvenile biotite and muscovite, sillimanite/andalusite, feldspars, and rhyolitic glass. 39Ar–40Ar dating on biotite and muscovite concordantly constrain an age of 7.96 Ma for this two-mica ash layer. A second tephra in the Pisco Formation has a similar biotite composition and an age between 7.45 and 6.93 Ma. The peculiar mineral assemblage and the chemical composition of biotite indicate a strongly peraluminous composition of the erupted magmas and, together with the 39Ar–40Ar ages, suggest to consider a correlation of these ash layers to the eruptions of the Miocene Macusani (Peru) or Morococala (Bolivia) volcanic complexes in the Eastern Cordillera of Central Andes. The major and trace element composition of glass supports the correlation with Macusani. A provenance from Morococala seems less likely given the large distances involved. These results provide new data on the volcanic activity of the Eastern Cordillera revealing ash that dispersed to over 500 km to the west, in the forearc marine basins. This finding highlights that the exhumed forearc East Pisco Basin is highly promising as an archive of distal ash for the reconstruction of the volcanic activity of Central Andes during the Miocene silicic flare-ups.

Microscale oxygen isotope analysis (18O/16O) of minerals can identify distinct events of fluid-rock interaction. This method is, however, still limited to a few major and accessory minerals of which most are anhydrous minerals. We present the systematic study of oxygen isotope distribution in white mica by Secondary Ion Mass Spectrometry (SIMS). Texturally and chemically distinct white mica populations in granitic and contact metamorphic rocks from the Larderello-Travale geothermal field (LTGF), Italy, record stages of magmatic crystallization, metamorphic-hydrothermal replacement and fluid-rock interaction. The large range in intra- and inter-grain white mica δ18O values between 1 and 14‰ reflects varying protoliths and degrees of fluid-mineral interaction at variable temperatures (180–450 °C present-day temperatures; p.d.T.). This variability reflects the large-scale circulation of both (1) magmatic, syn-intrusive to early contact metamorphic hydrothermal fluids with high-δ18O values, and (2) meteoric fluids with δ18O values of −7‰ during a post-intrusive, late hydrothermal stage. Metasedimentary rocks from the upper reservoir contain distinct white mica populations occurring in close proximity (μm-scale), including detrital grains (δ18O = 12–14‰; high Na, low Mg), partially altered white mica (δ18O = 8–9‰) and late hydrothermal white mica (1–6‰; low Na, mid Mg). The late hydrothermal white mica has similar δ18O values to other secondary minerals and is in equilibrium with meteoric-dominated fluids with a δ18O of −6 to 0.5‰, which circulated in the late hydrothermal stage. Downhole towards the lower reservoir, white mica from two contact metamorphic micaschist samples shows either (1) homogeneous δ18O values of ca. 9‰ likely due to recrystallization in the contact metamorphic hydrothermal stage (T ca. 600 °C), or (2) a large spread in δ18O from 2 to 12‰ within and across grains of variable texture and chemistry in the host rock and a cross-cutting quartz-white mica vein (ca. 300 °C, present day temperature; hereafter p.d.T.). This contrasting δ18O signature of white mica is also recorded in granite cored at up to 4.6 km depth. The Carboli granite contains white mica with a homogeneous magmatic δ18O of 10 ± 0.6‰, whereas older granite samples from Radicondoli have magmatic to hydrothermal white micas that vary in δ18O from 4 to 10‰. A pronounced intra-grain δ18O variability of up to 6‰ occurs in white mica domains with higher Fe-Mg-Ti halos around inclusions of chloritized biotite, as a result of interaction with dominantly meteoric fluids that infiltrated to depths of at least 4.6 km. In the Porto Azzurro granite on Elba, Italy, altered white mica has δ18O values of 2.6‰ down from 10‰ in unaltered grains. The distribution of oxygen isotope ratios in white mica is thus firstly a result of pervasive versus selective fluid alteration (at depth, sample and grain scale). Secondly, the actual preservation of these μm-scale variabilities indicates that volume diffusion is not detectable at microscale at p.d.T at or below 350 °C where most of the heterogeneous white mica is found. Selective, sample- and grain-scale fluid penetration occurs episodically and anisotropically, on micro- and megascale, along faults, fractures and cleavages, producing lower δ18O white mica at various times in zones of higher secondary permeability and active hydrothermal fluid circulation.

Tephra layers are a unique tool for stratigraphy. Their geologically instantaneous deposition together with their dispersion on wide areas make them a powerful instrument for dating and correlating simultaneous events at different localities. In this paper, we present and discuss the application of tephra fingerprinting in the upper Miocene portion of the Pisco Formation. The Pisco Formation is an important marine vertebrate Fossil-Lagerstätte that crops out in the Ica Desert, on the Peruvian Coast. The paleontological relevance of this formation has entailed the necessity of reconstructing a chronostratigraphic framework, using 40Ar/39Ar dating and correlations on volcanic ashes. Tephra layers from the volcanic activity of the Peruvian Andes are very frequent in the Pisco Formation and many represent a primary air-fall deposition on the sea floor. Therefore, many samples from tephra layers within measured stratigraphic sections at different sites in the study area were fingerprinted with a combined approach, using petrographic and chemical analyses on phenocrysts and volcanic glasses, as well as glass shard morphology and granulometric analyses, performed on 53 selected samples, collected from different localities. Based on the results obtained, we correlate some tephra layers in two localities at several km distance, where the correlation between the measured sections was previously only hypothetical. Major element chemistry of biotite proved a valuable tool to discriminate different tephra in the upper Miocene portion of the Pisco Formation, due to the presence of peraluminous as well as metaluminous compositions. This work shows that it is possible to apply tephra fingerprinting and tephrostratigraphy in a complex situation such as that represented by the Pisco Formation, where a high number of apparently similar volcanic ash layers were deposited in a shallow marine environment. The application of this correlation method allows us to increase the chronostratigraphic detail in the studied interval of the Pisco Formation, providing an accurate framework in which fossil marine vertebrates are stratigraphically located. The obtained high-resolution chronostratigraphy will greatly clarify the evolutionary history of Cetaceans.

An unusual cave is located in the hills southeast of the town of Thabazimbi in the Limpopo Province. This cave, a V-shaped shallow chamber measuring 110 × 40 m, was discovered serendipitously when an adit was driven into the hill in connection with iron ore mining. It has no natural entry point and no externally derived sediment is found in it. It is situated within the upper part of the Malmani Subgroup and although it was formed as a solution cavity, requiring a phreatic environment, dripwater flow must subsequently have been low: no large speleothems (stalactites, stalagmites or flowstones) were ever formed in the cave. Instead, evaporation produced crystallictitic frostwork bushes consisting of aragonite (metastably grown due to enhanced Mg-content) which were in part redissolved followed by the secondary formation of calcite corals and encrustations. This sequence of events can be understood in the context of an enclosed cave where the CO2 content of the air is high. U/Th disequilibrium dating of two aragonite frostwork samples from the cave yielded two very different ages (155.1 ± 7.4 and 67.2 ± 3.4 ka), thus showing that the formation of this type of frostwork occurred (probably episodically) over a long period of time. The two dates broadly coincide with the waning stages of periods of low total inorganic carbon (TIC) as well as high clastic input in the Tshwaing crater sediment record, both indicating enhanced rainfall at that locality. The younger age coincides with a brief negative excursion in chlorine content and a positive potassium anomaly at Tshwaing, both also indicating high rainfall. However, both ages are also coeval with glacial maxima, thus underlining earlier observations from the Tshwaing record that in southern Africa, wet periods are not necessarily coeval with northern hemisphere interglacials.

The Jurassic clay-rich aquitard sequence underlying the Swiss Molasse Basin is locally penetrated by unusually frequent veins consisting of calcite ± celestite, in particular in the Schlattingen-1 and Oftringen drill cores. The veins are linked to extensional brittle structures, with shear senses ranging between dip-slip and strike-slip. Deformation triggered transient and localised events of fluid flow within the aquitard followed by sealing of the structures by the vein minerals. Vein calcite was dated by LA-ICP-MS analysis using the U/Pb method. While this was a demanding undertaking, given the low U and high common Pb contents of calcite, 9 significant ages could be obtained. Two groups of ages can be distinguished, namely 31–39 and 11–18 Ma. Both age groups are consistent with the known regional tectonic evolution. The older age group corresponds to the main phase of rifting in the Upper Rhine Graben, a major crustal discontinuity, whose effects were not limited to the graben proper but affected large parts of the Molasse Basin. The younger age group is linked to crustal tilting and related extensional deformation. Isotopic compositions of O and C in vein calcite from the Schlattingen-1 core show substantial variability but do not discriminate between veins of the two age groups. In contrast, 87Sr/86Sr ratios in veins show different signatures. The depth trend of 87Sr/86Sr ratios in veins fits well with that in pore water obtained by rock squeezing. The two data sets define a trend of values increasing with depth, connecting the current values in the embedding Malm and Keuper aquifer waters. This trend is best explained as a near-steady-state diffusion profile. While advective mixing of waters originating from the embedding aquifers to produce the veins, implying structures that cross cut the whole low-permeability sequence, cannot be discarded on the basis of existing data, it is considered more likely that the veins have local fluid sources. These are mixtures of local pore water with a possible contribution of water from the nearest aquifer, implying maximum vertical advection distances in the order of 50–100 m.

We present a new interpretation of the Bashgumbaz Complex (BSC), a fragment of a large mafic-ultramafic nappe exposed along the Alichur Valley in the Pamir, whose significance is of paramount importance for the understanding of the Cimmerian orogeny in South Pamir. The BSC is exposed along the contact between the SE and SW Pamir and consists of a low-grade metamorphic association dominated by gabbros and serpentinized harzburgites with minor bodies of quartzdiorite and plagiogranite tectonically coupled to basaltic and rhyolitic volcanic products. Tectonic slices of metasedimentary rocks are interleaved within the complex. The BSC, with its upper part, is in contact with a sedimentary unit rich in olistostostromes with exotic blocks containing Lower Permian fauna showing affinity with Central Pamir. The BSC is intruded by a Lower Cretaceous granitic body with a U-Pb zircon age of 117 Ma, crosscutting the whole complex. Petrographic, mineralogical and geochemical data suggest a supra-subduction-zone affinity for the gabbros and diorites. Significant enrichment in LILE and LREE compared to HREE, coupled with negative anomalies of Nb, Ti, Zr and other HFSE support this interpretation. U-Th-Pb dating of zircon from a diorite provides a lower Norian (ca. 222 Ma) youngest-crystallization-age constraint, suggesting that the BSC formed in a supra-subduction setting on the South Pamir terrane prior to the collision between Central and South Pamir blocks and was later underthrust and then obducted onto the southern margin of the closing Rushan-Pshart Ocean, which separated these two Gondwanan terranes during the Permian–Triassic. The obduction of the Bashgumbaz ophiolites can be considered as a time constraint for the collision between the South and Central Pamir terranes, which occurred within the framework of the Cimmerian orogeny that shaped the southern margin of Eurasia

Observations of discordant ages, meaning that an age given by one mineral geochronometer is different from the age given by another geochronometer from the same rock, began in the early days of geochronology. In the late 1950s and 1960s, discordant U-Pb zircon ages were unquestioningly attributed to Pb diffusion at high temperature. Later, the mineralogical properties and the petrogenesis of the zircon crystals being dated was recognized as a key factor in obtaining concordant U-Pb ages. Advances in analytical methods allowed the analysis of smaller and smaller zircon multigrain fractions, then the analysis of individual grains, and even pieces of grains, with higher degrees of concordancy. Further advances allowed a higher analytical precision, a clearer perception of accuracy, and a better statistical resolution of age discordance. As for understanding the cause(s) of discordance, belief revision followed the coupling of imaging, cathodoluminescence (CL) and back-scattered electrons (BSE), to in situ dating by secondary ion mass spectrometry (SIMS), or by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). Discordant zircon and other accessory minerals (e.g., monazite, apatite, etc.) often consist of young rims accreted onto/into older cores. Age gradients are sharp, and no Pb diffusion gradients are observed. As U-Pb discordance in crystalline, non-radiation damaged grains is caused by diachronous, heterochemical mineral generations, interpretations of mineral ages, based on the exclusive role of diffusion, are superseded, and closure temperatures of zircon and monazite are irrelevant in geological reality. Other isotopic systems (Rb-Sr, K-Ar) were believed, since the 1960s, to be similarly controlled by the diffusivity of radiogenic daughters. When zircon and monazite discordance were recognized as zone accretion/reaction with sharp boundaries that showed little or no high-temperature diffusive re-equilibration, the other chronometric systems were left behind, and interpretations of mineral ages based on the exclusive role of diffusion survived. The evidence from textural-petrologic imaging (CL, BSE) and element mapping by electron probe microanalyzer (EPMA) or high spatial resolution SIMS or LA-ICPMS provides the decisive constraints. All microcline and mica geochronometers that have been characterized in detail document patchy textures and evidence for mineral replacement reactions. It is important not to confuse causes and effects; heterochemical microstructures are not the cause of Ar and Sr loss; rather, they follow it. Argon and Sr loss by dissolution of the older mineral generation occurs first, heterochemical textures form later, when the replacive assemblage recrystallizes. Heterochemical mineral generations are identified and dated by their Ca/Cl/K systematics in 39Ar-40Ar. Replacive reactions adding or removing Cl, such as, e.g., sericite overgrowths on K-feldspar, retrograde muscovite intergrowths with phengite, etc., are detected by Cl/K vs Ar/K isotope correlation diagrams. Calcium-poor reaction products, such as, e.g., young biotite intergrown with older amphibole, adularia replacing microcline, etc., can be easily identified by Ca/K vs Ar/K diagrams supported by EPMA analyses. Mixed mineral generations are observed to be the cause of discordant, staircase-shaped age spectra, while step-heating of crystals with age gradients produces concordant plateaus. Age gradients are therefore unrelated to staircase age spectra. There is a profound analogy between the U-Pb, Rb-Sr, and K-Ar systems. Lead and Ar diffusion rates are both much slower than mineral replacement rates for all T < 750 °C. Patchy retrogression textures are always associated with heterochemical signatures (U/Th ratios, REE patterns, Ca/Cl/K ratios). As a rule, single-generation minerals with low amounts of radiation damage give concordant ages, whereas discordance is caused by mixtures of heterochemical, resolvably diachronous, mineral generations in petrologic disequilibrium. This can also include (sub-)grains that have accumulated significant amounts of radiation damage. For accurate geochronology the petrologic characterization with the appropriate technique(s) of the minerals to be dated, and the petrologic context at large, are as essential as the mass spectrometric analyses.

This paper represents the first systematic Pb isotope investigation of Italian Medieval coins and aims to provide new parameters for a general historical interpretation of coin production and circulation in Medieval Europe. We collected more than one hundred specimens, minted in a period between 9th − 14th centuries AD and coming mostly from archaeological sites of Tuscany. Here we report the results on the oldest group of (44) coins, dated between the end of the 9th and 11th centuries. All coins where previously characterized with handheld X-ray fluorescence (pXRF) analysis and lead isotope composition (PbIC) was performed using an MC-ICP-Mass Spectrometer. The Carolingian coins have PbIC compatible with Melle silver district; the few Carolingian coins possibly minted in Italy (Venice and Milan) are also compatible with ore districts such as Melle and Harz Mountains. Coins in the names of Italian rulers (9th-10th century) from Lucca, Pavia and other uncertain mints show PbIC compatible with Melle, Black Forest and the Harz Mountains as well. A quite similar pattern applies to coins in the names of Otto I-III and Conrad II (10th-11th century) from Lucca and Pavia mints, although they show a better overlap with the Harz Mountains. The vast majority of early medieval coins issued by the Italian mints investigated in the present paper show isotope compositions that do not match with silver (lead-copper) mines from the Colline Metallifere district of southern Tuscany, notwithstanding their exploitation in the considered period is suggested by many settlements located near mining sites.

International audience; Medieval pavements composed of lead-glazed tiles decorated with a variety of techniques continue to inspire questions about the organisation of glaze manufacture, and the supply and origin of lead materials. The tiles analysed in this study are from Suscinio I (a 13th-century pavement) and Suscinio II (a 14th-century pavement), at the Château of Suscinio in Brittany, and also from the 14th-century pavement at the fortified manor house in Brain-sur-Allonnes, Anjou. Lead isotope analysis (LIA) was used to examine samples from 44 lead-glazed tiles, 29 of which are transparent, while 15 are tin-opacified (an exogenous technique in these regions during this period). Five out of the six LIA groups thus identified favour site-specific supply networks, while results for the remaining group, LIA5, indicate a multi-site supply network. After combining LIA results with archaeological and historical data, the most likely provenance for the lead materials in LIA5 is Derbyshire, in the British Isles. Both the importation of ready-to-use glazing mixtures and the use of lead from neighbouring mines can now therefore be discounted as plausible hypotheses for the production of the tin-opacified lead-glaze tiles analysed in this study.

U-Th-Pb petrochronology is based on the incontrovertible fact that the diffusion of radiogenic Pb is negligibly small relative to retrograde reaction rates. Multi-element maps demonstrate that patchy textures tightly correspond to (U + Th)-Pb age variations, requiring that fluid-induced dissolution/ reprecipitation is the principal cause of Pb mobility. Attempts to model intracrystalline core-rim Pb zonations as diffusive transport are not legitimate unless genuine bell-shaped diffusion profiles in minerals can be documented, which happens only exceptionally. Monazite and zircon intra-grain age maps confirm that coupled dissolution-reprecipitation and retrogression reactions assisted by fluids control (Th + U)-Pb ages, not temperature. The chemical zonations observed in many (Th + U)-bearing mineral chronometers (e.g. monazite, allanite, xenotime, zircon) provide petrological constraints. Linking petrology with textures and the isotope record allows reconstructing entire segments of the P-T-A-X-D-t history of a rock and its geodynamic environment. The dearth of mathematically sound diffusion profiles equally applies to the isotope records of micas and feldspars. The tight link between petrology, microtextures, chemical composition and geochronology also pertains to Rb-Sr and K-Ar. Overdetermined multi-mineral Rb-Sr isochrons with excess scatter, and spatially resolved/stepwise release 39Ar-40Ar results, demonstrate ubiquitous correspondence between relict phases and isotopic inheritance. Many rock-forming minerals are highly retentive of Sr and Ar, unless they are obliterated by retrograde reactions. The rates of dissolution in fluid-controlled reactions are several orders of magnitude faster at upper and mid-crustal levels than diffusive reequilibration rates. Thus, as a rule Rb-Sr and K-Ar chronometers date their own formation. Accurately establishing P-T paths of monometamorphic rocks requires assessing petrologic equilibrium using multivariate thermodynamic software. Dating complex parageneses of polymetamorphic, unequilibrated rocks requires labor-intensive disentangling by: (i) qualitative identification of relicts, retrogression reactions, and chemically open systems by imaging techniques (e.g. cathodoluminescence, element maps, etc.); (ii) microchemical analyses at the μm-scale quantifying heterochemical disequilibrium phases and assigning them to a P-T-A-X segment; (iii) spatially resolved/stepwise release, relating the chemical signature of the analyzed mineral to its age. K-Ar and Rb-Sr usually provide a different perspective on the P-T evolution of a rock than does (Th + U)-Pb, as K + Rb-rich minerals (phyllosilicates and especially feldspars) mostly form later and react/dissolve faster in the retrograde path than U-rich accessory phases. The present paper reviews these general principles by means of well-understood examples, both successful and unsuccessful in matching the independently known external constraints.

Carbon forms: paths and processes in the Earth is a thematic set of six papers arising from the lectures presented at the Lake Como School, held at Villa del Grumello, Como, Italy (15–20 October 2017), and organized by the Graduate School of Milano Bicocca. This collection of lecture notes focusses on the structure of carbon allotropes, the geodynamics of deep Earth's carbon transport and fixation at mantle conditions, carbon degassing by ascending magmas, and the vast tectonic carbon degassing at the Earth's surface. New science is emerging for carbon, one of the fundamental elements for life, energy, and global climate change on our planet. Carbon is cycled between surface, atmosphere, and oceans. Most of the recyclable carbon is stored deep in the Earth. Ingassing from the atmo-hydrosphere into the mantle occurs by subduction; in the mantle carbon resides in solids such as diamonds and is transported by fluids and melts that migrate upward. Outgassing into the atmosphere is effected by volcanism and tectono-metamorphic soil emissions. The forms and processes of carbon ingassing and outgassing are still poorly constrained. There is as yet no consensus on how and when carbon transforms from solid to fluid …

The Singhbhum Craton in eastern India, one of the four major Archean cratons of the Indian Shield, is underlain by Paleoarchean granitoids (trondhjemite-tonalite-granodiorite i.e. TTGs, and potassic granites) that are encircled by greenstone belts rich in banded iron formation known as Iron Ore Group. This study presents whole rock major-trace element geochemistry, Sr and Nd isotopes, K-feldspar common Pb isotopes, along with in-situ zircon U–Pb ages and Hf isotope data from the granitoids and the greenstone volcanic rocks to characterize their source(s) and to constrain the evolution of the Paleoarchean continental crust. Zircon U–Pb ages show that these granitoids were emplaced during two magmatic episodes at ca. 3.47–3.44 Ga and 3.36–3.28 Ga. The greenstone volcanic rocks in the eastern part of the craton are basalts and basaltic andesites with rare komatiites, whereas those in the western part are basaltic-andesites and andesites. Whole rock Sm–Nd isotope regression lines for volcanic rocks from the eastern (including komatiites) and western regions yield ages of 3746 ± 340 Ma (MSWD = 169) and 2961 ± 420 Ma (MSWD = 3.3), respectively. The major and trace element data show that the volcanic rocks of the eastern and western greenstone belts had a different petrogenesis. The granitoids are highly evolved with SiO2 contents up to 76 wt%. The major and trace element data indicate that the granitoids of the Singhbhum Craton were derived by partial melting of a mafic crust at variable depths (ca. 10–15 kbar). The isotopic compositions of Pb measured on leached K-feldspars from the granitoids show undepleted mantle-like characteristics and negate the possible involvement of much older evolved crust in the formation of these rocks. The whole rock initial Nd isotopic compositions (eNdi = −0.3 to +2.2) and zircon in-situ initial Hf isotopic compositions (eHfi = −0.3 to +2.0) of the granitoids calculated using zircon crystallization ages indicate that they were derived from a near-chondritic reservoir. These data suggest that only a limited volume of continental crust formed prior to ca. 3.5 Ga, and major crust extraction from the mantle started at ca. 3.5 Ga. Using this time as the start of major crustal growth and concomitant mantle depletion, Nd and Hf mantle depletion histories are proposed for the Singhbhum Craton that can be described by the equations eNdt = −2.85 × (tGa) + 10, and eHft = −4.68 × (tGa) + 16.4. These curves do not change the model ages for young rocks, but they make a significant difference for the Archean rocks.

International audience; The taphonomy and palaeoecology of the early Miocene (Burdigalian) vertebrate assemblage of Ullujaya (East Pisco Basin, Peru) is here described. Vertebrate remains are concentrated in marine facies (Ct1a association) of the exposed Chilcatay Formation (dated 19-18 Ma) deposited within a 30-40 m deep, semi-enclosed, offshore environment. Coupled with ichnological observations, the size distribution of pyrite framboid relics reveals fluctuation of euxinic and oxic-dysoxic conditions at the seafloor. The assemblage is dominated by toothed cetaceans (kentriodontids, squalodelphinids, physeteroids, and the eurhinodelphinid-like Chilcacetus), together with a large dermochelyid turtle, some bony fish, and diverse elasmobranchs, mostly juveniles of Carcharhinus brachyurus and Cosmopolitodus hastalis. The vertebrate assemblage comprises a coastal community, dominated by mesopredators, representative of a warm-temperate, sheltered embayment connected with riverine and open-ocean environments. Vertebrate skeletons are typically disarticulated and incomplete, and some bone elements display shark bite marks. Microborings are observed at the bone surface. Bones exhibit a good degree of apatite mineralisation and bone cavities are locally filled by Ca-Mg carbonates. Our taphonomic observations suggest prolonged flotation of carcasses during which they were subject to biogenic and physical processes of partial destruction (including scavenging by sharks), before final deposition on a soft compact substrate. Preservation was favoured by the oxygen-deficient bottom conditions that inhibited the action of benthic macro-scavengers.

The Southeastern Alps were an important source of copper metal in prehistory, at least from the Eneolithic and through the Bronze Age, as documented by the abundant and substantial presence of smelting slags. Evidence of mining activity is scarce, because of limited ad hoc investigation and because of the subsequent systematic erasing by post-Medieval exploitation. Moreover, until recently the profusion of archaeometallurgical and archaeological investigations focusing on the prehistoric exploitation of Northern Alpine, Central European, and Balkan ore sources has somehow obscured the early role of the Italian Southern Alps as a major copper producing area. The recent advances in the systematic characterization of the copper ores in the Southeastern Alps (including Alto Adige, Trentino, Veneto, and nearby regions) by lead isotope analysis, supported by mineralogical and geochemical interpretation, offer now the appropriate tools to re-evaluate the extent of prehistoric mining and the local patterns of ore exploitation. The developed database is a powerful tool to identify the metal derived from local production. It is suggested that (1) based on the abundance and chronological distribution of smelting slags evidence, two major periods of mining exploitation took place, the first in the middle of the 3rd millennium BC and the second during the Late Bronze Age; and (2) based on the discrimination of copper sources and the available analyses, most of the metal circulating in Northern Italy and in the greater Po Valley region was actually produced from Southern Alpine ores.

In solid-state physics and materials science Fick's Law diffusion is a well-established process. In Earth Sciences, laboratory experiments on garnet, olivine and other anhydrous minerals do document the intra-grain element concentration gradients that follow the functional form required by Fick's Law. Natural gradients in minerals have rarely been analyzed with the necessary spatial resolution. Reports of actual observations of diffusion profiles of element concentrations are rare in the literature, and diffusion profiles of isotope ratios in minerals used for geochronology are absent. An in-depth re-examination of recent and older literature suggests that isotope transport in minerals is instead often dominated by fluid-mediated retrogression reactions. Imaging microtextures by cathodoluminescence or back-scattered electron maps provides ubiquitous evidence of patchy or dendritic replacement structures, which correspond to multiple growth stages, in zircon, monazite, muscovite, biotite, K-feldspar, etc. The U–Pb, K–Ar and Rb–Sr systems in these partly retrogressed minerals show isotopic inheritance (that survived the retrograde reactions at least in part) in close correspondence with the petrologic relicts. Depending on the relative rates of the petrological processes relevant for isotope transport, geochronometers can be grouped in two classes: Class I (thermochronometers) are those for which the diffusivity of a given radiogenic isotope is faster than the rate of dissolution and/or reprecipitation, and Class II (hygrochronometers) are those for which aqueous dissolution/reprecipitation is the faster process. All of the abovementioned geochronometers, for which patchy/dendritic textures formed by diachronous mineral generations and isotopic inheritance were observed, must be assigned to Class II. Class II samples in petrologic equilibrium can (but need not) record purely thermal diffusion of daughter isotopes. The isotope record of Class II minerals in petrologic disequilibrium, being controlled by inheritance and retrogression reactions, depends chiefly on the reaction-promoting factors, water activity and strain. The dependence of Class II mineral ages on thermal diffusion is subordinate and never unique.

The Janggun lead-zinc-(silver) deposit in South Korea was formed by hydrothermal-metasomatic processes. We collected 12 samples over a 100 m transect in one of the five orebodies recognized in the deposit and analyzed a total of 68 sulfide minerals. Detailed mineralogical observations show successive modifications and overprint of pristine sulfide by replacement reactions, suggesting several distinct petrogenetic events. We distinguished four different phase assemblages, which occur in contiguous zones of the transect. This suggests multiple alteration fronts. Major and minor element concentrations of sulfide phases were measured for the different sulfide generations. Milligram-sized sulfide mineral samples were micro-drilled for Pb and S isotope analyses with a resolution of 200 µm to 1 mm. Pb and S isotopic compositions (206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb and δ34S) are variable, showing 206Pb/204Pb ranges from 18.334 to 18.879, 207Pb/204Pb; 15.667 to 15.692, and 208Pb/204Pb; 38.682 to 38.899, and δ34S ranges from + 4.4 to + 6.2‰. Although Pb isotopic compositions and δ34S data are similar to literature data from different areas of the Janggun deposit, the Pb isotopic variations between the different samples (at the 10 m scale), and within individual polished sections (at the cm scale) are well over 100 times larger than the analytical uncertainty. The scatter in sulfur isotopic compositions confirms the Pb isotopic heterogeneity at both the cm and 10 m scales. At least seven isotopically well distinct fluid circulation episodes are required in the study area to explain different projections of common-denominator Pb isotope diagrams. This suggests that fluid pulses of distinct chemical and isotopic compositions circulated in distinct spatial and temporal episodes. To investigate the role of country rocks as sources of the metals transported by the ore-forming fluids, we leached 13 samples of country rocks (Janggun limestone, Chunyang granite, granophyre, and dyke) with dilute acid and analyzed both leachates and residues. Correlations are observed between Pb isotopic compositions and trace ratios (e.g. Co/Ni, Cu/Ag, and Zn/Cd). The Pb isotopic compositions of country rocks and Precambrian basement suggest that leaching of country rocks by circulating fluids is able to produce the observed Pb isotope and trace elements variations in the sulfide minerals, whereby different end-members in the isotope correlation diagrams can be linked to different country rocks.