Secular variations in seawater chemistry: A new technique for analyzing fluid inclusions in marine halites from the modern, Permian, and Cretaceous.

摘要

Secular changes in the composition of seawater may be examined by chemical analyses of samples of ancient seawater brines preserved as fluid inclusions within crystals of halite in marine salt deposits. A new technique has been developed (Environmental SEM-X-ray-EDS) that can produce rapid and accurate major element chemical analyses of fluid inclusions in crystals of halite greater than about 30 microns in diameter. Halite from the marginal marine saline pan of Salina Omotepec, Baja California, Mexico, was analyzed via ESEM-X-ray-EDS to verify that primary fluid inclusions in chevron halite from a modern setting contain unmistakable samples of evaporated seawater. The major ions in fluid inclusions from Baja California halites, and from another modern marine setting (Great Inagua Island, Bahamas), display linear trends when plotted against one another, which closely track the evaporation path of seawater as it evolved during crystallization of halite. Fluid inclusions in halites from three geographically-separated evaporite basins (Salado Formation, New Mexico and San Andres Formation, Texas, analyzed using the ESEM-X-ray-EDS method and from the Zechstein Formation, Poland) all of late Permian age (ca. 250 Ma) have overlapping major ion chemistries which indicates they formed from a common “global” seawater parent systematically different in composition from modern seawater (relatively enriched in K and depleted in Mg and SO4). These results support the hypothesis that changes in the flux of seawater circulation through the global mid-ocean ridge system, along with global river inflow, are the two first order controls on the chemistry of seawater. The chemical composition of Cretaceous seawater was determined by analyses of fluid inclusions in Cretaceous marine halites. Three large evaporite deposits studied (Early Cretaceous Loeme Formation, Congo basin and Muribeca Formation, Sergipe basin, Brazil and Middle Cretaceous Maha Sarakham Formation Khorat Plateau, Thailand and Laos) all had parent waters highly enriched in Ca and depleted in Na, Mg, and SO₄ compared to modern seawater. The results from this study confirm that the very low Mg/Ca ratio in Cretaceous seawater was responsible for calcite being the dominant nonskeletal marine carbonate of the Cretaceous.