1. Preliminary study of Halite-hosted Fluid Inclusions from the Evaporite deposits of the Crotone Basin, Calabria, Southern Italy
MARA CIPRIANI1*, ALESSANDRA COSTANZO2, GIUSEPPE CIANFLONE3, ROCCO DOMINICI1, MARTIN FEELY2
1DiBEST, University of Calabria, I Arcavacata di Rende (CS), Italy
2Earth and Ocean Sciences, School of Natural Sciences. National University of Ireland Galway
3E3 - EalCUBO (Environment, Earth and Engineering) Start-up - University of Calabria
INTRODUCTION
This preliminary work has set as its objective the study and analysis of Messinian evaporites outcropping in the Crotone basin using Transmitted Light Microscopy, Ultraviolet (UV) Light Microscopy and Laser Raman Spectroscopy. The evaporites formed during the Messinian Salinity Crisis (~6 Ma). The halite crystals display a range of morphologies and sizes (range from millimeters to centimeters in length) and contain primary fluid inclusions (FIs) with trapped organic chemical species. These deposits can be used to interpret the evolution of the Crotone basin.
GEOLOGICAL FEATURES
The Crotone basin is located on the Ionian side of the Calabria Peloritan Arc (CPA). The CPA is located in the Central-Western Mediterranean area and represents the segment connecting the orogenic southern Apennines with the Sicilian Maghrebides. The Crotone basin, located in the northern sector of the CPA, began to form during the Serravallian and Tortonian times ( Ma). The depositional history of the Crotone Basin reflects an extensional tectonic regime that was periodically interrupted by short compressional or traspressional events (Fig.1). In addition to the evaporite horizons, the stratigraphic sequence consists of clay and marl horizons, deltaic sands and
conglomerates.
Fig.1. The location and geological setting of the Crotone Basin, Southern Italy (adapted from Van Dijk et al., 2000).
RESULTS
The halite crystals display a range of morphologies and sizes (range from millimeters to centimeters in length) and contain primary fluid inclusions (FIs) with trapped organic chemical species. Primary FIs (~20 and 100 microns in their longest dimension) are present in all samples (Fig.2) and many host organic matter (Fig.3). The main FIs are type 1 (monophase liquid – L or monophase vapour – V), type 2 (multiphase liquid ± vapour ± organic matters - L ± V ± Org) and type 3 (multiphase liquid ± vapour ± solid - L ± V ± S). Halite deposits are classified into three different facies i.e. banded facies (microcrystalline halite), white facies (macrocrystalline halite) and clear facies (macrocrystalline halite). These facies match the “Saline pan cycle” shown by Lowenstein et al., 1985 (Fig.4).
Fig.2. Primary Fluid inclusions in Halite crystal. b) Linear trail parallel to crystal growth. c) Detail of type 1 FIs (L).
Fig.3. Photomicrographs of organic matter trapped in halite hosted Type 2 FIs. a) Single halite crystal with entrapped FIs. b), c) and d) Different distribution of organic matter trapped in primary FIs displaying a brown colour and irregular shapes under plane polarised light (left) and a bright blue to dull orange fluorescence under UV light (right).
SUMMARY AND CONCLUDING REMARKS
The morphologies and relative clarities of the studied halites can be used to link them with the different stages of the saline pan cycle. The banded microcrystalline halites correspond to a “flooding stage”, the white facies macrocrystalline halites correspond to an “evaporate concentration stage” and the clear facies macrocrystalline halites correspond to a “desiccation stage. All halite crystals, both micro- and macrocrystalline types, possess primary FIs mainly with negative crystal shapes and indicate that the evaporites are primary in origin. In addition, UV light Microscopy confirms the presence of organic matter within the FIs (type 2). Type 3 inclusions contain solid halite crystals. Laser Raman Microspectroscopy generated raman spectra with peaks attributable to organic chemical species, in particular the carotenoids lutein and lycopene groups. In summary, the halite morphologies, the fluid inclusion petrographic data combined with the presence of fluid inclusion hosted carotenoids indicates that all evaporites are primary in origin.
REFERENCES
Goldstein, R.H., Petrographic analysis of fluid inclusions. In. Samson, Anderson, Marshall eds. Fluid inclusions: Analysis and interpretation. Mineralogical Association of Canada, Short Course, 32, 9-53.
Lowenstein, T.K., Hardie, L.A., Criteria for the recognition of salt-pan Evaporites. Sedimentology, 32,
Van Dijk, J.P., Bello, M., Brancaleoni, G.P., Cantarella, G., Costa, V., Frixia, A., Golfetto, F., Merlini, S., Riva, M., Torricelli, S., Toscano, C., Zerilli, A., 2000.A regional structural model for the northern sector of the Calabrian Arc (south Italy). Tectonophysics, 324,
Fig.4. Schematic representation of the Saline Pan Cycle (modified after Lowenstein et al., 1985) with photographs of the different types of halite crystals associated with each stage.