ACID FLUID INCLUSIONS IN BEDDED HALITE OF THE TRIASSIC MERCIA MUDSTONE, NORTHERN IRELAND

Some Permian red beds and evaporites have previously been interpreted as products of acid saline lake systems. The early Triassic Mercia Mudstone Group of Northern Ireland shows sedimentological and mineralogical similarities to known Permian acid red bed and evaporite deposits. The Gaelectric Carnduff 02 core, drilled in 2014 in County Antrim, preserves 591.6 meters of Mercia Mudstone red beds and evaporites. Primary fluid inclusions in bedded halite from Larne Halite Member were investigated for evidence of acidity in parent lakes and ground waters.

Primary fluid inclusions from twelve beds of chevron and cumulate halite crystals in the Carnduff 02 core were studied using petrography, microthermometry, and laser Raman spectroscopy. Primary fluid inclusions were identified based on their alignment along growth bands parallel to crystal faces and their negative crystal shape. Most primary fluid inclusions are ~20-50 microns long and are all-liquid inclusions. Double rims were observed on all primary fluid inclusions. Some primary inclusions contain daughter crystals. During freezing-melting runs, 36 inclusions failed to freeze when exposed to temperatures of –190 °C for several minutes. Laser Raman spectra of liquid phase in 45 primary inclusions shows peaks at ~422 cm^-1, ~490 cm^-1, ~986 cm^-1, and ~1017-1020 cm^-1, as well as water peaks.

The lack of a vapor bubble in most primary inclusions indicates that they grew from waters at typical surface temperatures (~0-70°C) and are well-preserved. When compared to synthetic and other natural brine inclusions, only inclusions with low pH values contained a double rim. This suggests that double rims form due to a reaction between host halite and acid, and may be a new criterion for determining acidity. Failure of inclusions to freeze shows that waters were extremely acid and/or saline. Laser Raman spectra contain peaks indicative of bisulfate and hydronium alunite. This combination of double rims, failure to freeze, and Raman identification of bisulfate and hydronium alunite, suggest that the primary fluid inclusions of the Larne Halite are acid.

This study provides the first evidence that acid saline lakes existed in northeastern Pangea. Acid saline environments may have been regionally extensive and long-lived in the Permo-Triassic.