title: Epidote U–Pb geochronology and isotope geochemistry to trace the hydration of the continental crust in orogens
Creators: name: Peverelli, Veronica
Publisher: name: Universität Bern
Year: 2022
Subjects: subject: 550 Earth sciences &amp; geology

description: This work investigates epidote veins in crystalline rocks to date fluid circulation in the granitic continental crust, and to gain insight into fluid pathways and sources. Fluids are crucial in the tectono-metamorphic evolution of rocks because they control deformation processes, mineral reactions and heat transfer. Earth’s continental crust is dominated by primarily water-poor granitic rocks. The observation that granitoids in many orogens (e.g., Alps, Himalayas) contain metamorphic hydrous minerals attests to the introduction of fluids into these primarily “dry” rocks. However, whether this occurred during prograde, peak or retrograde metamorphism is often unclear. One way to reconstruct the timing of fluid circulation is by dating veining events. In addition to time constraints, many vein-filling minerals return useful information regarding fluid sources and pathways by stable and radiogenic isotope geochemistry.

We assess and demonstrate the potential of epidote [i.e., Ca2Al2(Al,Fe3+)Si3O12(OH)] in veins to date and trace hydration of the granitic continental crust of the Alps from Permian to Miocene times. Epidote U–Pb geochronology is made possible by combining U–Pb isotope measurements by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) with the Tera–Wasserburg diagram. This approach is suitable for high-initial Pb minerals like epidote, and it returns geologically meaningful ages dating the formation of epidote during veining. Allanite (i.e., REE-rich monoclinic epidote) is used as an LA-ICP-MS reference material with no noticeable matrix effects at the available analytical precision. This is a fundamental aspect of U–Pb dating by in-situ techniques, because no well-characterized epidote standard exists for U–Pb isotope analyses, and uncorrected matrix-effects cause grossly inaccurate U–Pb ages in high-initial Pb minerals. The application of this new protocol for epidote U–Pb dating to selected case studies establishes epidote as a powerful geochronometer and fluid tracer in the granitic continental crust.

One group of investigated epidote-bearing veins are located in an inverted passive continental margin (i.e., Err nappe) in the eastern Swiss Alps. Surprisingly, in the Err nappe, such epidote veins formed in Late Cretaceous to Paleocene times during the compressional and extensional phases of the Eo-Alpine orogeny, and not during rifting. Epidote Pb–Sr isotope geochemistry reveals that the epidote-forming fluids interacted with carbonatic rocks during percolation into the crust along extensional faults, and O–H isotope data in epidote suggest that the fluids originated as modified seawater with the addition of a sedimentary-water component.
The second group of studied epidote veins are from the Aar Massif, the inverted European passive continental margin. Here, epidote veins were produced by pre-orogenic Permian and syn-orogenic Miocene fluid circulation events. Hydrogen isotope data of Permian epidote indicates a meteoric origin for the Permian fluids, which likely percolated into the crust along rift-related normal faults and modified its hydrogen isotope composition interacting with syn-rift sediments.
The combination of epidote U–Pb geochronology and Pb–Sr isotope geochemistry with microstructural analysis and trace element data also reveals epidote dissolution–precipitation processes during viscous granular flow in an epidote-quartz vein. Such processes are mediated by a combination of recycled and newly added fluids, whose mixed isotope composition is recorded by newly precipitated syn-kinematic epidote. This study demonstrates that epidote is an active participant in deformation processes in deforming polymineralic aggregates.

This work demonstrates that the application of the newly developed protocol for epidote U–Pb dating, coupled with microstructural analysis, trace element data and isotope geochemistry, allows to characterize fluid circulation in inverted passive continental margins from rifting and across orogenic cycles. Epidote is ubiquitous in Earth’s crust: not only as metamorphic mineral, but also in ore deposits, in geothermal fields, and as alteration mineral in basalts and feldspars. The application of the tools presented here to epidote from other geological contexts may further establish this mineral as the key tracer of low-temperature fluid circulation.