Fluid in clu sions in ha lite can di rectly re cord the ma jor com po si tion of evap o rated sea wa ter; how ever, Or do vi cian ha lite is very rare. The Or do vi cian is a key time dur ing the evo lu tion his tory be cause pro found changes oc curred in the planet's ecosys tems. Ma rine life was char ac ter ized by a ma jor di ver si fi ca tion, the Great Or do vi cian Biodiversification Event and the Late Or do vi cian Mass Ex tinc tion, the first of the "big five" mass ex tinc tions. How ever, so far there is no data on the Or do vi cian sea wa ter. Data from the Or do vi cian-Si lu rian bound ary were avail able only. In this study, we re port the ma jor com po si tions from Mid dle Or do vi cian ha lite in China to give the ex act com po si tion of Or do vi cian sea wa ter. The ba sic ion com po si tion (K + , Mg 2+ , Ca 2+ , and SO 4 2-) of in clu sion brines was es tab lished with the use of ultramicrochemical anal y sis. The data on the chem ical com po si tion of the brines in the pri mary in clu sions in di cated that the brines were of Na-K-Mg-Ca-Cl (Ca-rich) type, and cover a huge gap in the evo lu tion of sea wa ter chem is try. The chem i cal com po si tion of the pri mary in clu sion brine in ha lite con firmed the ear lier re sults for the Cam brian and Si lu rian ha lite orig i nat ing from other salt bas ins and the pre vi ous spec u lation of "cal cite sea" dur ing the Or do vi cian, in di cat ing a higher po tas sium con tent in the Lower Pa leo zoic sea wa ter than in the sea wa ter of other pe ri ods of the Phanerozoic.

Fluid inclusions in halite can directly record the major composition of 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 from the halite from the boundary of Ordovician and Silurian. Ca 2+ and Mg 2+ concentration were determined in brines from large secondary inclusions, located on periphery of the chevron. In brine from the first inclusion the Са 2+ content was 87.9 and 75.8 g/l (81.9 g/l in average). In this study, we report the major compositions from middle Ordovician halite in China. Ordovician fluid inclusions give the direct evidence that the Ordovician seawater is Ca-rich seawater of calcite sea, with the Na-K-Mg-Ca-Cl type seawater composition (Kovalevich et al., 1998;Lowenstein et al., 2001), while late-stage potash salts in marine evaporates is the KCl types (Hardie, 1996).