Secular variations in seawater chemistry as a control on the chemistry of basinal brines: test of the hypothesis

The hypothesis that basinal brines inherited their major ion chemistries and elevated salinities from evaporated paleoseawaters is tested by comparing the compositions of basinal brines in Silurian (Michigan basin, Illinois basin, Appalachian basin in eastern Ohio) and Jurassic/Cretaceous (Central Mississippi Salt Dome basin, Arkansas shelf, and south‐central Texas) host rocks, when the world oceans were ‘CaCl₂ seas’, with those from Permian and Pennsylvanian rocks (Palo Duro basin, Central Basin Platform, and Delaware basin, Texas and New Mexico) when the world oceans were ‘MgSO₄ seas’. Basinal brines examined are assumed to have originally formed from evaporation of the same seawaters that produced major evaporites. Sulfate, Mg and K levels in basinal brines are below the concentrations expected from evaporation of seawater of any type, which emphasizes the importance of diagenetic mineral–brine interactions in controlling basinal brine chemistry. There are no major differences in SO₄, Mg and K concentrations between basinal brines hosted by rocks originally formed during ages when the world oceans were MgSO₄ seas versus CaCl₂ seas. Basinal brines in Pennsylvanian–Permian rocks are compositionally distinct (relatively high Na and low Ca) from basinal brines in Silurian, Jurassic and Cretaceous host rocks, which may reflect original differences in seawater chemistry. Basinal brines enriched in Ca and depleted in Na relative to evaporated seawater of any type have traditionally been interpreted to form by albitization of plagioclase feldspar. A new explanation for Ca enrichment and Na depletion of basinal brines is the mixing of evaporated CaCl₂‐type seawater with more dilute water. Some basinal brines are similar in major ion composition to evaporated seawater of a particular age, for example basinal brines in the Cretaceous Edwards Group carbonates, Texas, where dolomitization is the only reaction required to convert evaporated Mesozoic CaCl₂ seawater into Edwards Group brine.