Atmospheric and environmental interpretations of Early Paleozoic halite (Upper Ordovician, Hudson Bay Basin, & Upper Silurian, Michigan Basin)
Halite sourced from Upper Ordovician strata (Red Head Rapids Formation) in the Hudson Bay Basin and Upper Silurian strata (Salina Group A-2 and B units) in the Michigan Basin was used to interpret atmospheric and environmental conditions during their respective depositional periods. Primary halite was distinguished from secondary or diagenetically altered halite using screening parameters consisting of petrography, trace element analysis, microthermometry and gas analysis of fluid inclusions. Primary halite preserves liquid and/or liquid-vapour cubic fluid inclusions within chevrons and fluid inclusion bands whereas secondary halite contains large, irregularly-shaped fluid inclusions. Petrographic analysis highlights the coexistence of primary and secondary fluid inclusions within some sample sets. Trace element analysis suggests that the deposits of the Hudson Bay and Michigan Basins were likely affected by periodic freshening that altered the trace element concentrations. Homogenization temperatures of the Upper Silurian halite range from 13.3 – 35.2°C which agree with expected paleoclimatic conditions. The contemporaneous occurrence of primary, depositional halite with secondary, diagenetic halite is noted during the crush fast scan method of gas analysis by mass spectrometry. Diagenetic influences are clearly reflected in the gas analysis results of some halite samples. In part, they reflect the influence of degradation and/or thermal maturation of organic matter and hydrocarbon migration subsequent to halite deposition. Atmospheric oxygen content in the primary halite from the Red Head Rapids Formation (Upper Ordovician) ranges from 13.5 to 19 %, whereas in the Salina Group A-2 and B units (Upper Silurian) it falls between 20 and 26 %. The measured values of atmospheric oxygen based on Upper Ordovician halite overlap with the range of oxygen modelled by Berner (2006) and Algeo & Ingall (2007), and the values of atmospheric oxygen of Upper Silurian halite correspond to values modelled by Berner (2006; 2009).