Tracing mantle-crust fluid interactions in a lithospheric extension zone: Insights from the Betic Cordillera, Spain.

The Betic Cordillera, located in southeastern Spain, underwent a complex geodynamic history that contributed to the Messinian salinity crisis in the Mediterranean. The Alboran margin is characterized by crustal thinning, linked to slab retreat, tearing, and delamination processes during the Miocene. These processes, combined with alkaline to calc-alkaline volcanism and exhumation of metamorphic domes, are thought to drive a dynamic fluid system. The relative contributions of magmatism, crustal thinning and slab tearing to the uplift of the Betics remain however unclear. Understanding these deep fluid systems has significant scientific and industrial implications, particularly for deep geothermal and hydrogen systems. Active lithospheric faults, such as the Carboneras-Palomares strike-slip fault systems, in the eastern Betics potentially act as major conduits for deep fluids (gases, water) and heat sourced from the mantle.
In this work, we aim to characterize the influence of these faults on the fluid system, both in the past and now. Paleofluids are studied through calcite and quartz mineralization in fault zones, while modern-day fluids are collected in thermal waters (20-50°C) where gas species are sampled (as bubbles or dissolved in water). Multiple tracers are studied in mineralization (Microthermometry, carbonate isotopy, cathodoluminescence, U-Pb dating, ³He/⁴He as well as in modern-day fluids (major compounds geochemistry and their δ¹³C, ³He/⁴He). Preliminary results in modern-day fluids indicate high levels of N₂ (up to 92%) with associated CO₂ (4 to 6%) and some CH₄ (around 1% when present). δ¹³C (CO₂) (-10 to -7‰) are compatible with a deep origin. Microthermometry results indicate hydrothermal temperatures of ~300°C in quartz and ~120°C in calcite. These temperature data, combined with isotopic analyses (δ¹⁸O_CaCO3 value around 12‰ VPDB) also point to a deep fluid source. All these results illustrate the role of large-scale structures on driving the origin pathways and calendar of the fluids in the upper crust.