FLUID INCLUSION FREEZING/MELTING BEHAVIORS IN ACID AND NEUTRAL LAKE HALITES FROM SOUTHERN WESTERN AUSTRALIA

Freezing/melting behaviors of unaltered primary fluid inclusions in chemical sediments can be used to determine past surface water compositions. Identification of phases in a fluid inclusion and the specific temperatures at which they exist characterizes the salinity and major ion composition of the parent water. However, there is no documentation on the freezing/melting behavior of natural, acid fluid inclusions, and little data about how sulfur compounds affect freezing/melting characteristics. In this study we compare fluid inclusion freezing/melting behaviors from natural halite from two acid salt lakes, Twin Lake West (pH 2.8) and Lake Magic (pH 1.7), and two neutral salt lakes, Gastropod Lake (pH 8.0) and Lake Polaris (pH 7.3), in southern Western Australia. To better determine the components that are influencing the freezing/melting behavior of these natural fluid inclusions, we compare the freezing/melting data to the geochemistry of the parent lake waters. In general, the lake waters are Na-Mg-Cl-SO4 rich, but also contain a variable and complex range of other elements, such as Ca, Br, Fe, Al, and Si. Additionally, this data from natural halite will be compared to that from acid and neutral synthetic halite. Preliminary freezing/melting runs on natural and synthetic acid and neutral fluid inclusions show distinguishable differences. Natural acid fluid inclusions have the most complex freezing/melting behaviors. Acid fluid inclusions tend to have more metastable phases at specific melting temperatures than the neutral fluid inclusions, and develop a discrete, definable, fuzzy boarder on inclusion rims. The acid inclusions also have higher eutectic temperatures, higher and wider temperature range of hydrohalite rims, and more complex final melting behaviors of various crystals. The data that is collected herein can be used to identify past acid water chemistry in these acid salt deposits and from ancient halite.