Chemistry of fluid inclusions in halite from the Salina Group of the Michigan basin: Implications for Late Silurian seawater and the origin of sedimentary brines

Fluid was extracted from 18 fluid inclusions in halite of the Late Silurian Salina Group exposed in the Crystal Mine on the outskirts of Detroit, Michigan. Compared with modern seawater evaporated to the same degree, the inclusion fluids are severely depleted in SO₄^-2, somewhat depleted in Na⁺ and Mg⁺², and greatly enriched in Ca⁺². The composition of the inclusion fluids can be derived from Silurian seawater with a composition close to that of modern seawater, if it is assumed that the composition of the Silurian seawater was modified by dolomitizing CaCO₃-rich sediments and by albitizing silicate minerals during its evolution into evaporite brines. Since the evolution of the brines involved a number of chemical reactions, it is impossible to recover the initial concentration of all of the major ions in the parent Silurian seawater from the composition of the inclusion fluids alone. It is likely, however, that the m_k⁺ m_Br^- ratio and the functions [m_Ca⁺² + m_Mg⁺² - m_SO4^-2 - m_HCO3^- 2m_Br^- and [if m_cl^- - m_Na⁺ m_Br^-] in Late Silurian seawater had values close to those of modern seawater. Measurements of the isotopic composition of sulfur and of Sr in anhydrite within and associated with the halite host of the fluid inclusions are consistent with previous measurements of δ³⁴S in Silurian marine anhydrites and with the ⁸⁷Sr ⁸⁶Sr ratios of Late Silurian marine carbonates.