Alkali elements (Na, K, Rb) and alkaline earth elements (Mg, Ca, Sr, Ba) in the anoxic brine of Orca Basin, northern Gulf of Mexico

Concentrations of Rb, Sr, and Ba in the homogeneous brine, reported here for the first time, are enriched by factors of 1.5, 1.4, and ∼ 9, respectively, with respect to the overlying seawater. Unlike Ca and Sr, Ba concentrations in the brine are clearly controlled by the solubility of its sulfate salt (barite), causing a maximum of 670 nmol/kg at the interface. Several independent lines of evidence suggest that the brine is formed outside the basin, most likely by the interaction of seawater with regionally extensive evaporite deposits. A simple mass balance shows that the dissolution of about 280 g of halite per kg of seawater can account for the extreme concentrations of Na and Cl in the Orca Basin brine. The same mass balance was applied to a number of minor constituents (K, Rb, Mg, Ca, Sr, SO4, Br, total iodine) in order to calculate what abundances in the halite are required to reproduce their concentrations in the brine as measured in the present work and by others. The results are entirely compatible with the composition of the Jurassic Louann Salt, specifically with the median of compositions published for a transect spanning early to late stage halites. Elevated abundances of K and Rb point to contributions from bittern facies modified by prior diagenetic contact with seawater.