ORDOVICIAN SEAWATER COMPOSITION: EVIDENCE FROM FLUID INCLUSIONS IN HALITE

Fluid inclusions in halite can directly record the major composition of evaporated seawater, however Ordovician halite is very rare. Ordovician is a key time during the evolution history. However there are no Ordovician seawater data reported except from the data of the halite from the boundary of Ordovician and Silurian. In this study, we report the major compositions from middle Ordovician halite in Majiagou Formation from Ordos Basin of China to give the exact composition of Ordovician seawater. The basic ion composition (K⁺, Mg²⁺, Ca²⁺, and SO₄^2-) of inclusion brines was established with the use of ultramicrochemical analysis (UMCA). The data on the chemical composition of the brines in the primary inclusions indicated that the brines of that basin were of the Na-K-Mg-Ca-Cl-(Ca-rich) type, and average K⁺content is 20.9 g/L; average Mg²⁺ is 41.3g/L. average Ca²⁺ content is 66.1 g/L in primary fluid inclusions of halite with almost no SO₄^2-.The chemical composition of the primary inclusion brine in halite coincided with the earlier results for the Cambrian and Silurian halite from other salt basins and indicated a higher potassium content in the Lower Paleozoic seawater than in the seawater of other periods of the Phanerozoic.