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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 backscattered electrons (BSE), to in situ dating by secondary ion mass spectrometry (SIMS) or by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). 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heterochemical mineral generations, interpretations of mineral ages, based on the exclusive role of diffusion, are superseded, and closure temperatures of zircon and monazite are irrelevant in geological reality.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-ICP-MS 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. Ar 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. Ca-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. Pb 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: Data reported in 40Ar/39Ar geochronology studies are commonly insufficient to allow computation of ages. This deficiency renders it difficult to compare ages based on different standards or constants, and often hinders critical evaluation of the results. Herein are presented an enumeration of the data that should be reported in all 40Ar/39Ar studies, including a discussion in support of these requirements. The minimum required data are identified and distinguished from parameters that are useful but may be derived from them by calculation. Finally, recommendations are made for metadata needed to document age calculations (e.g., from age spectrum or isochron analyses). [Copyright &y& Elsevier]

Abstract: Magmatism in the Gardar Province, South Greenland, is related to two main rifting events at 1280 Ma and 1180 to 1140 Ma. Little is known about the duration of the magmatic activity in a specific complex. The Ilímaussaq intrusion belongs to the second period of rifting and comprises an extraordinary diversity of granitic and syenitic rock types, which intruded and fractionated in three successive magmatic events. As the intrusion contains some of the most evolved, incompatible element-rich rocks on Earth, it was chosen for a detailed geochronological study to quantify the duration of melt production, intrusion, fractionation and cooling. Amphiboles, which are abundant in all rock types, including pegmatites and late magmatic veins, were dated using the 40Ar/ 39Ar technique. Since the solidus temperature of the most evolved melts is below the closure temperature of amphibole, and no later heating event occurred, the 40Ar/ 39Ar ages reflect the magmatic crystallisation and can be used to determine the duration of igneous differentiation. The 40Ar/ 39Ar plateau ages range between 1142.6±2.2 Ma and 1152.3±3.7 Ma using the Steiger and Jäger [Steiger, R.H. and Jäger, E., (1977). Subcommision on Geochronology: convention on the use of decay constants in geo- and cosmochronology. Earth Planet. Sci. Lett. 36, 359–362.] 40K decay constant. These ages are younger than an U–Pb age of 1160±5 Ma on baddeleyite from the first magma batch. Our results indicate that the 40K decay constant of Steiger and Jäger may be too high; a lambda similar to that proposed by Kwon et al. [Kwon, J., Min, K., Bickel, P. and Renne, P.R., (2002). Statistical methods for jointly estimating decay constant of 40K and age of a dating standard. Math. Geol. 34, 457–474.] is required to make the 40Ar/ 39Ar match the U–Pb age. [Copyright &y& Elsevier]

Abstract: Earth sciences deal with physical entities spanning 15 orders of magnitude, from nanometer (crystal lattice unit cells) to megameter (faults and orogens). The understanding of megameter-scale processes is greatly improved by the study of the nanometer-scale processes: the growth of crystals, their deformation under shear, and the transport of isotopes used for dating. From a physical point of view, the transport of atoms across a crystal with high potential barriers and no interatomic voids is not accurately described by Fick''s equations, which were developed for dilute solutions with unhindered particle mobility. A variety of more general descriptions have been recently proposed, all of which share features such as complex temperature dependence as opposed to linear Arrhenian behaviour. From a petrological point of view, the ubiquitous recognition of microstructures in chemical and isotopic disequilibrium underscores the fact that diffusive reequilibration is a much slower process than creating the structures themselves. Diffusion in an inert matrix (“Fickian” diffusion) is observed to be negligible with respect to the growth of discrete mineral generations. As the latter frequently involves aqueous fluids, isotope geoscientists should follow petrologists and stress the geohygrometric properties of minerals (dissolution/reprecipitation). The fact that mineral reactions and microstructure development do not solely depend on temperature may explain why some “thermochronological” models from the literature have made predictions whose field tests have failed. The demise of one class of models might initially seem unfortunate; actually it is a great benefit because it shifts our attention to the reality of petrogenesis and to the sequence of events that any mineral assemblage records. [Copyright &y& Elsevier]

Abstract: A monzogranite, here referred to as the Montieri pluton, was encountered along two depth profiles from ca. 2 to 4 km depth in wells of the Larderello–Travale geothermal system, Italy. We obtained high-resolution petrographic and microchemical data on seven fresh samples of the Montieri pluton. These data are required to correctly interpret the combined isotope data for the Rb–Sr and K–Ar systems. Biotite and K-feldspar from all samples were analyzed by Rb–Sr and 39Ar–40Ar. Rb–Sr analyses gave extremely variable two-point Rb–Sr apparent ages and Sr initials, indicating that biotite did not equilibrate with feldspar. Bulk dissolution of K-feldspars does not define a Rb–Sr alignment that could have proved a common origin of all K-feldspar samples from a single magma batch. The leachable fractions of K-feldspar separates have much more homogeneous 87Sr/86Sr ratios around 0.712, which may reflect the isotopic composition of a late-stage circulating fluid. Because of the anatectic origin of the granites from the Larderello–Travale geothermal system, it is likely that every K-feldspar separate contains at least three isotopically distinct feldspar generations: relics of the Hercynian gneiss basement, Pliocene magmatic minerals, and hydrothermal retrogression products. Such heterogeneity can be confirmed and quantified by petrographical observations and electron microprobe analyses. The 39Ar–40Ar age spectra of the biotite samples show some internal discordance. Because of deviations from stoichiometry, biotite discordance can be attributed both to chlorite intergrowths, as predicted from the active fluid circulation, and to the presence of multiple biotite generations, such as have already been documented from other micas from the Larderello–Travale geothermal system and as indeed confirmed by high-resolution petrography. Total K–Ar biotite ages on cluster around 3 Ma and broadly agree with a Rb–Sr age obtained by regressing only the biotite analyses. This suggests that the Montieri pluton, whose known volume is at least 25 km3, was emplaced at that time. The K-feldspar spectra are strongly discordant; no step age approaches the zero-age which would be predicted by Ar diffusivity modelling. The extraneous Ar in the feldspars is not excess Ar, but inherited Ar instead. Ar inheritance in the feldspar separates correlates with inheritance of Sr. Arrhenius trajectories of the apparent Ar diffusivities for all seven feldspars are astonishingly identical, despite the heterochemism amongst samples. However, the apparent diffusivity calculated from the step-heating experiments led to irreproducible models for the thermal history along the depth profile, as well as time-scales shorter than the historical record. This indicates that Ar diffusivities calculated from laboratory experiments must not be extrapolated to geological conditions. [Copyright &y& Elsevier]

Abstract: Combining Lu–Hf garnet geochronology with in situ trace element analyses in garnet allowed us to gain new insight into the metamorphic evolution of UHP–UHT rocks in the Stary Gierałtów region, in the Polish Sudetes. Prograde garnet growth recorded by Rayleigh-type heavy REE (HREE) zoning in the felsic granulites indicates that the obtained 386.6±4.9 Ma Lu–Hf age represents the time of garnet crystallization on a prograde UHP metamorphic path. The surrounding rocks were metamorphosed at the same time as indicated by 381.2±6.7 Ma Sm–Nd garnet age obtained for the mid-crustal metapelites. The second metamorphic episode, which affected most of the lower crust in the Orlica–Śnieżnik Massif (OSM) occurred at ca. 340 Ma as determined by U–Pb zircon and Sm–Nd garnet dating of granulites in this and previous studies is interpreted as a high temperature event, which took place on a retrograde path. Trace element distribution in garnets from the layered granulites showed significant differences in distribution of medium and HREE in garnets from mafic and felsic protoliths over the course of the metamorphic evolution. This had strong impact on the isotopic dating results and led to “decoupling” of the Sm–Nd and Lu–Hf clocks, which recorded timing of the two different metamorphic episodes separated by as much as 40 Ma. Moreover, the preservation of the HREE growth zonation profile in garnets from the felsic granulites whose minimum metamorphic temperature was established at 900 °C implies that the Lu–Hf system under relatively dry conditions does not undergo significant diffusional re-equilibration even at such extreme temperatures and therefore it sill provides the age of prograde garnet growth. Under hydrous conditions, at least some resetting will take place, as documented by the partially relaxed HREE zonation profile in the amphibolitised mafic granulite, which yielded a 10 Ma younger age. The HREE distribution study appeared to be a particularly valuable and essential tool, which allowed us to distinguish garnet growth from post-growth complexities and hence, provide improved age interpretation. Medium REE, on the other hand, did not show any obvious correlation with the isotopic signature of garnet. Two distinct metamorphic episodes recorded in the Stary Gierałtów region show that buoyancy-driven uplift of UHP rocks can be arrested at the base of a continental crust if not supported by any additional force. In our case study, the UHP rocks would have never reached the surface if their uplift had not been resumed after a long pause under a different tectonic regime. The multistage, discontinuous uplift revealed by the UHP rocks of the OSM provides a new scenario for the exhumation of continental crust from mantle depths distinct from the fast-track exhumation histories recognized in UHP terranes elsewhere. [Copyright &y& Elsevier]

Abstract: This study presents the chronological evolution of the upper amphibolite facies Orue Unit in NW Namibia. Metasedimentary and meta-igneous rocks of the Orue Unit were investigated using the Pb–Pb stepwise leaching technique on garnet and rutile, U–Pb multi-grain analysis on rutile, Sm–Nd–Lu–Hf leaching technique on garnet, SHRIMP analysis on zircon and Ar–Ar dating on amphibole. Each of these techniques pertains to different processes that occurred before, during, or after the metamorphic peak. Our age data can be integrated with petrological constraints to provide a more complete understanding of the metamorphic cycle. Our pre-peak metamorphic zircon ages, peak metamorphic garnet ages and peak to late peak metamorphic amphibole 39Ar–40Ar ages bracket the upper amphibolite facies metamorphic event including hydration or dehydration processes into a time span of only ca. 20 Ma. The age data obtained by peak metamorphic mineral analyses cluster around 1340–1320 Ma. Based on age data and field observation, we interpret the upper amphibolite facies metamorphism as a large-scale regional mid-crustal event. Spot analyses of inherited zircon cores obtained by SHRIMP reflect the sedimentary origin of the respective rocks of the Orue Unit and derivation from Palaeoproterozoic protoliths. The metamorphic rocks south of the anorthositic Kunene Intrusive Complex (KIC) have previously been ascribed to the Palaeoproterozoic Epupa Complex at the SW margin of the Congo craton and were thus thought to be older than the Mesoproterozoic KIC. Our data show that the high-grade metamorphic overprint took place 30–50 Ma after emplacement of the KIC. Rutile growth ages of 1248 Ma in one sample reflect fluid activity which seems to be a local phenomenon since there is no other evidence of geological activity throughout the Orue Unit at that time. The rutile ages predate the emplacement of satellite intrusions in that area by 30 Ma and there is no causal relation between these two events. [Copyright &y& Elsevier]