The Messinian halite facies: Insights into halite crystallisation and depositional environments using geochemical, petrographic and fluid inclusion studies

• Multidisciplinary approach on three different Messinian halite facies. • Each facies reflects different crystallisation conditions and hydrological balance. • Their coexistence provides new evidence for the comprehension of Messinian Salinity Crisis. The Crotone Basin (Calabria, Southern Italy) is a representative area in the Italian peninsula where Messinian halite deposits preserve three distinct crystal facies: (i) banded composed of cumulate halite and mud-rich interlayers, (ii) white consisting of bottom-growth crystals with chevron fabrics, and (iii) transparent made up of massive, optically pure crystals. The transparent facies appears to be undocumented in other Mediterranean Messinian basins, offering new perspective on halite crystallisation under variable environmental conditions. Microscopic observations (optical and scanning electron microscopy), supported by high-resolution 3D imaging through synchrotron-based X-ray microtomography, revealed a lack of pervasive recrystallisation in all facies, enabling the non-destructive visualisation of internal fabrics and inclusions. These methods provided critical insights into halite growth dynamics and the environmental conditions prevailing during deposition. Microthermometric data indicated that all halite crystals precipitated from a NaCl-MgCl 2 -H 2 O salt-water system under extreme evaporative conditions, with fluid salinity exceeding 340 ‰ – 360 ‰ and a distinct brine temperature for each facies (∼35 °C in the banded, ∼45 °C in the white, and ∼ 20 °C in the transparent). Strontium isotope ratios ( 87 Sr/ 86 Sr) placed halite formation within the TG14-TG12 interval (ca. 5.61 – 5.55 Ma), with progressively increasing values from banded to transparent facies, suggesting enhanced continental input and brine dilution in the later stages of deposition. Organic matter was detected both in primary fluid inclusions and within the halite lattice, particularly in the white and transparent facies. Raman spectroscopy and UV-epifluorescence revealed amorphous organic compounds, including carotenoids and aliphatic functional groups such as methyl and methylene, which are commonly associated with microbial activity. These findings suggested that organic-rich brines may have played an active role in crystal nucleation and growth dynamics. The chemical immaturity and heterogeneous distribution of organic compounds imply a combination of autochthonous microbial input and episodic allochthonous influx, pointing to complex organic-mineral interactions during halite formation. The coexistence of three petrographically distinct halite facies within a confined area, each linked to specific environmental and geochemical conditions, supports the view that halite precipitation was modulated by fluctuations in hydrological balance, brine composition, and organic matter availability. These data contribute to a better understanding of the environmental and geochemical processes that controlled evaporite deposition during the Messinian Salinity Crisis.