Fluid systems captured by giant amethyst-hosted fluid inclusions from continental flood basalts

Abstract

Continental flood basalt (CFB) provinces, defined by the rapid emplacement of large volumes of mafic lava, represent some of the most significant large-scale magmatic events in Earth’s geological record. Gaining insight into the origin and evolution of fluid systems within these settings is critical for understanding the geodynamic architecture of large igneous provinces (LIPs) and accompanying mineralisation processes. However, the provenance and reservoirs of hydrothermal fluid systems in CFB settings remain poorly constrained. This study provides new evidence on the origin and sources of fluids responsible for the formation of amethyst mineralisation within a CFB context. Unique, giant fluid inclusions entrapped in amethyst crystals enabled reconstruction of variations in fluid system dynamics during incremental crystal growth in the geodes, which developed in the LTZ.L series Goboboseb basalts in an active continental flood basalt volcanic setting of the Paraná-Etendeka CFB Province.

Three compositionally different types of fluid inclusions, namely H₂O-NaCl-N₂-CH₄, N₂-CH₄-CO₂ and N₂-CH₄, preserved in individual amethyst and quartz crystals have archived an intricate fluid evolution history. Amethyst crystallisation commenced at moderate temperatures (281–327 °C) from low salinity (0.3–8.7 wt% NaCl equiv.) aqueous H₂O-NaCl-N₂-CH₄ fluid. As crystal growth rates decreased and the fluid system cooled to 160–211 °C, some crystals captured fluid boiling/phase separation in the environment of ongoing magmatic CO₂ degassing. Fluid properties and fluid-host rock thermal equilibrium, coupled with high SiO₂ and H₂O contents of the low Ti/Zr LTZ.L series basalts, support a magmatic-hydrothermal origin of the aqueous fluid. For the first time, fluid inclusion noble gas data reveal unequivocal evidence of deep primordial mantle contributions (³He) to magmatic-hydrothermal fluids involved in amethyst formation. The results provide convincing evidence for the Tristan da Cunha plume-related magmatism in the Paraná-Etendeka CFB Province and confirm that deep-sourced volatiles can migrate into shallow crustal levels via magmatic degassing. Strongly elevated ⁴⁰Ar*/⁴He, nucleogenic ²¹Ne/²²Ne isotope ratios and crustal δ¹⁵N signatures of fluids indicate significant assimilation of continental crust and entrainment of old crustal fluids during magma fractionation in feeder chambers at different crustal levels above the Tristan da Cunha mantle plume, supporting models of lithospheric recycling and magma-crust interaction. The engagement of multi-source fluid systems, predominantly mantle and crust-derived, across different evolution stages shows the complexity of fluid systems in CFB volcanic settings. Moreover, the findings of this study demonstrate that not only post-magmatic, but also syn-magmatic fluids are capable of generating gemstone-grade amethyst mineralisation and thus bring to light the untapped mineral potential of underexplored areas in the CFB settings worldwide.