Multifaceted orogenic fluid dynamics unraveled by hydrothermal epidote

Characterizing fluid circulation in orogens is key to understanding orogenic processes because fluid–
rock interaction modifies the physical properties of rocks, hence their response to deformation and, for example,
their suitability for radioactive waste storage. Fluid circulation can be dated by applying geochronological
methods to fluid-precipitated minerals. Fluid sources and associated pathways can be traced using isotope data
measured in the same or in other cogenetic minerals. We applied this concept to the Aar Massif (central Swiss
Alps), which was part of the former European passive continental margin that was deformed and exhumed during
the (Cenozoic) Alpine orogeny. Newly collected epidote from veins and from one cleft at several localities in
meta-granitoids in the Aar Massif yielded U–Pb ages ranging from 27.7 3.4 to 12.4 1.9 Ma, which complement
previously published geochronological data revealing Permian (278 29, 251 50, and 275 18 Ma) and
Miocene (19.2 4.3 and 16.9 3.7 Ma) epidote veins. We used Pb–Sr–O–H isotope geochemistry of epidote
to evaluate fluid sources and pathways during Permian rifting and the Miocene compressional phases of Alpine
orogeny. Strontium isotope data of Permian epidote are consistent with previous work suggesting meteoric water
infiltration along syn-rift faults and through syn-rift sediments. A more-complex structural framework existed
in the Miocene, when a sedimentary lid covered the Aar Massif. Strontium, O, and H isotope data of Miocene
epidote-forming fluids indicate (1) meteoric water, mixing with (2) fluids derived from sedimentary units being
compacted during orogenesis and/or (3) metamorphic water. All three fluid endmembers may have been circulating
and mixing in the Aar Massif during Miocene deformation. Strontium isotope data further indicate that
Miocene fluids contributed to imprinting a highly radiogenic Sr isotope composition onto Alpine shear zones
or that the fluids inherited a highly radiogenic Sr isotope component by dissolving the Rb-rich, high 87Sr = 86Sr
biotite therein. Both possibilities can coexist, and they imply that external fluids could modify the chemical
composition of the post-Variscan granitoids hosting the studied epidote veins by fluid–rock interaction processes
during deformation. Lead, Sr, and H isotopic differences among Miocene samples further suggest complexity of
large-scale fluid circulation. Our work supports the fact that the reconstruction of multifaceted and multi-stage
fluid circulation in highly deformed rocks benefits from extracting geochronological and isotope data from the
same mineral.