Abstract. 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.
Abstract. 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.
Abstract. 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.
<p>The Aar Massif is a mid-crustal basement section of the European plate and it was intensely deformed during the Alpine orogeny. Alpine deformation of Aar Massif granitoids is expressed by a pervasive network of ductile shear zones consisting of fine-grained polymineralic (ultra)mylonites dominated by viscous granular flow processes (Wehrens et al., 2016). Fluid circulation and hydration reactions are recorded by both granitic protoliths and shear zones. In particular, they are made evident by the alteration of feldspar into hydrous minerals like epidote and mica. The timing of this hydration event and, consequently, whether Alpine deformation was initiated in already altered granitoids or in fresh ones were unclear. This lack of time constraints led to a pivotal question: did deformation initiate in rocks that were altered by pre-kinematic hydration, or was hydration syn-kinematic and driven by the formation of Alpine shear zones?</p><p>&#160;</p><p>Laser ablation ICP-MS U&#8211;Pb geochronology applied to epidote in hydrothermal veins provides new evidence for pre-Alpine hydration of granitoids in the Aar Massif. Two veining events are recognized: 1) one at ca. 276 Ma occurring during Permian transtension and rifting, and 2) another at ca. 14 Ma related to late Alpine exhumation phases. Initial <sup>207</sup>Pb/<sup>206</sup>Pb ratios of all Permian epidote samples overlap within uncertainty, suggesting only one fluid source and equilibration path. Also, these ratios are more radiogenic than those of the host rocks at the time of vein formation. The hydrogen isotopic composition of the Permian fluids was calculated from measurements in bulk epidote separates by high-temperature conversion elemental analyzer. With a temperature range of epidote crystallization estimated between 200&#8211;300 &#176;C, the calculated &#948;D<sub>fluid</sub> value is -57 to -44 &#8240;. An external source for the Permian fluids is suggested by the disequilibrium of Pb isotopes between hydrothermal epidote and host granitoids. Percolation of meteoric water along transtensional faults and interaction with syn-rift sediments before reaching the granitoids underneath is suggested by the Permian transtensional geodynamics and supported the hydrogen isotopic composition of the Permian fluids.</p><p>&#160;</p><p>The occurrence of Permian fluid circulation in the Aar Massif granitoids indicates that these rocks were altered before the onset of Alpine deformation. In fact, it can be inferred that fluid circulation caused not only veining, but also pervasive flow and thus the alteration of magmatic feldspar into fine-grained hydrous minerals throughout the granitoids of the present-day Aar Massif. This enabled pre-Alpine storage of water and the creation of numerous additional grain boundaries, both favoring viscous granular flow during Alpine deformation. In this context, the localization of strain in polymineralic aggregates containing hydrous minerals can recycle stored pre-kinematic (i.e., Permian) water. Thus, the initiation of Alpine deformation did not necessarily require the addition of syn-kinematic (i.e., Alpine) fluids, although their presence is confirmed by this and previous studies (e.g., Ricchi et al., 2019; Peverelli et al., 2021).</p>
Hydrothermal veins and altered feldspar are evidence for fluid circulation in granitic rocks in the continental crust. The hydrothermal alteration of feldspar affects the deformation behavior of granitoids, especially if it occurs before orogeny. Geochronology can establish the timing of fluid circulation to determine if this fluid-driven alteration plays a role in crustal deformation. Although existing dating techniques cannot be applied to feldspar alteration directly, absolute ages of fluid circulation can be obtained from hydrothermal veins. We combined U-Pb geochronology and hydrogen isotope data (δD) from epidote [Ca2Al2(Al,Fe3+)Si3O12(OH)] to unravel the hydration of post-Variscan granitoids in the Alpine orogen. The recent protocol for epidote U-Pb dating proves for the first time that fluids of meteoric origin infiltrated the granitoids in Permian times by exploiting synrift faults, consistent with the δD values of the epidote-forming fluids. This hydration event caused at least some degree of feldspar hydration and weakening of the granitic rocks ~260 m.y. before their pervasive structural overprint by the Alpine orogeny. The preservation of Permian U-Pb ages despite Alpine orogenic processes confirms epidote as a powerful tool with which to unveil pre-orogenic hydration events in metagranitoids. Our analytical approach broadens insights into the water cycle in the middle continental crust in orogens.
<p>The widespread presence of epidote-bearing veins and hydrous minerals such as micas in meta-granitoid rocks attests to the large extent of hydration of the exhuming continental crust. The ability of epidote (Ca<sub>2</sub>Al<sub>3</sub>Si<sub>3</sub>O<sub>12</sub>(OH) &#8211; Ca<sub>2</sub>Al<sub>2</sub>Fe<sup>3+</sup>Si<sub>3</sub>O<sub>12</sub>(OH)) to incorporate a wide variety of trace elements renders this mineral a promising geochemical tracer of circulating fluid(s).</p><p>We report trace element and microstructural data on epidote-bearing veins from the Aar Massif (Central Alps) and from the Albula Pass (Eastern Alps). We characterized and classified the epidote-bearing veins based on their extent of deformation, shape and size of the epidote grains, coexisting minerals, and degree of dynamic recrystallization of associated quartz. Laser ablation ICP-MS data of individual epidote crystals reveal prominent zoning, confirmed by electron probe maps for Sr and Mn. Overall, low to very low Th/U ratios (down to 0.0005 in the Aar Massif veins and 0.001 in the Albula ones) with Th often below limits of detection (< 0.1 &#181;g/g at 16 &#181;m beam size) go along with variably LREE-depleted patterns (and CI Chondrite-normalized La<sub>N</sub>/Yb<sub>N</sub> ~0.35 in the Aar Massif veins and ~0.60 in the Albula Pass veins). Strontium contents are variable (hundreds to thousands of &#181;g/g) and mostly high (up to 10100 &#181;g/g in the Aar Massif samples and 12800 &#181;g/g in the Albula Pass samples). The in-situ geochemical data are linked to the microstructures in order to assess whether microstructures can be related to variations in trace elements, also considering the role of coexisting phases. Moreover, trace element data of samples from the Aar Massif are compared to metamorphic host-rock epidotes and cleft epidotes from the same massif.</p><p>We find that REE patterns of Aar Massif vein epidotes are clearly different than those of metamorphic host-rock epidotes and of cleft epidotes. In addition, REE patterns vary based on the microstructural characteristics of veins. Overall REE patterns of the Albula Pass vein epidotes resemble those from the Aar Massif. Different veins and microstructures define clusters in Sr vs. Y, Eu anomaly vs. Th/U ratios, and Eu anomaly vs. U values. Geochemical heterogeneities are observed among sampling localities within the Aar Massif.</p><p>The fact that the geochemical characteristics of retrograde hydrothermal vein epidotes are clearly different than those of high-grade metamorphic and metamorphic host-rock epidotes, and the relationship between geochemical characteristics and microstructures support the hypothesis that the deformation did not alter the original geochemical record through neomineralization. Our data argue for the potential of epidote as a powerful fluid tracer in the granitoid continental crust.</p>
<p>Sample description, methods, and measured U-Pb isotopic data. </p>
<p>Sample description, methods, and measured U-Pb isotopic data. </p>
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