THE COMPOSITION OF PERMIAN SEAWATER

Forty-nine brine inclusions in marine halite from the Ochoan Salado Formation in the Delaware Basin and fifteen inclusions in halite from the Leonardian Wellington Formation in the Kansas Basin were extracted, and their chemical compositions were determined. The brines are of the Na-K-Mg-Cl-SO4 type; their compositions resemble those of evaporated modern seawater. The values of (m(Cl-) - m(Na+))/m(Br-) and (m(Mg2+) + m(Ca2+) - m(SO4)2- - 1/2m(HCO3-))/m(Br-) of the inclusion brine from the two formations are equal to or slightly higher than those of modern seawater. The original m(Na+)/m(Br-) and m(Cl-)/m(Br-) ratios of the inclusion brines were probably equal to or slightly larger than those of modern seawater. The values of m(Mg2+)/m(Br-) of the inclusion brines from the Salado Formation are very close to that of modern seawater; the ratios of inclusion brines from the Wellington Formation are slightly lower, probably due to the formation of dolomite/magnesite. The m(Mg2+)/m(Br-) ratio in the initial seawater was probably close to the parent seawater of the Salado brines. The values of (m(SO4)2- - m(Ca2+) + 1/2m(HCO3-))/m(Br-) of the inclusion brines appear to be reduced by the formation of dolomite/magnesite, and the value of this ratio in Permian seawater was probably similar to that of modern seawater. The m(K+)/m(Br-) ratios of the inclusion brines are variable, but the original ratios are probably close to or slightly larger than that of modern seawater. If the Br- concentration of Permian seawater was equal to that of modern seawater, the composition of Permian seawater can be narrowly constrained; in mmol/kg H2O, 460 less-than-or-equal-to m(Na+) < 630, 550 less-than-or-equal-to m(Cl-) < 730, m(Mg2+) = 54 +/- 6, m(K+) congruent-to 11, (m(SO4)2- - m(Ca2+) + 1/2m(HCO3-)) greater-than-or-equal-to 17, 20 < m(SO4)2- < 45, 5 < m(Ca2+) < 20, and 0.15 < m(HCO3-) < 5. The composition of Permian seawater was therefore quite similar to that of modern seawater.