Geochemical Study of Fluid Pathways in Dilational Hydroshear Veins: A Record of Fossil Tremor and Slow Slip Events in Subduction Zones

Geophysical studies of subduction zones have identified deep episodic tremor and slow slip events (ETS), which frequently occur at depths >30 km in subduction zones. Recently, considerable attention has been devoted to the geological records of these events, with a particular focus on crack-seal fluid-mediated vein formation characteristic of these environments.

In this study, we examined continental metasediments from the Northern Apennines, Italy, where crack-seal quartz-carpholite veins are extensively developed. These dilational hydroshear veins predominantly align with the metamorphic foliation and consist of iso-oriented quartz and carpholite fibres. Thermodynamic modelling indicates that the formation of these veins and the associated mylonitic foliation occurred under high-pressure, low-temperature conditions (~1 GPa and 300–350°C), fitting to the identified ETS conditions in subduction zone.

The study emphasizes determining the composition and origin of fluids entrapped as fluid inclusions during the formation of quartz-carpholite veins. Raman spectroscopy revealed variability in the composition of the biphasic fluid-gas inclusions, showing commonly contents of H₂O, CO₂, alkanes (CH₄), and N₂. The research also focuses on reconstructing the major and trace element migration associated with the development of these fluid-mediated veins and examining the incorporation of trace elements into vein-associated phases (primarily carpholite). Elemental analyses conducted using WDS-EPMA and LA-ICP-MS revealed correlations and anti-correlations between trace elements, providing insights into the operating conditions, transferability and availability of elements during dilational hydroshear vein formation.

In-situ δ¹⁸O SIMS measurements constrain values between +18.4 and +19.2 ‰ for the quartz fibres in the veins. δ¹⁸O bulk rock analyses by laser fluorination range between +12.3 and +15.7‰ for the host metasediments and ca. +14‰ for the adjacent metabasites. These results suggest that the fluids responsible for vein formation were in O-isotope disequilibrium with the surrounding rocks, indicating at least partly derivation from an external source.

In conclusion, the study identifies the geochemical characteristics of fluid inclusions and pathways in deeply subducted metasedimentary rocks with quartz-carpholite veins, tracking forming conditions of a fossil record of deep ETS in subduction zones.