The isotope (Sr and O) and elemental (Mg, Ca, Mn, Fe, and Sr) compositions of the various dolomites in the Middle Ordovician St. Peter Sandstone in the Michigan Basin are determined and the variations are modeled in terms of fluid-rock interaction or as mixing relations. These geochemical models, combined with the paragenetic sequence of the dolomites and late anhydrite cement, suggest the existence of at least four distinct diagenetic fluids in the St. Peter Sandstone during the Paleozoic. Fluid 1 (Sr = 30 + or - 10 ppm, Sr/Ca = 0.019, 87 Sr/ 86 Sr = 0.7093 + or - 1) has a composition consistent with a modified older (pre-Middle Ordovician) seawater origin, which indicates that the flow path for this fluid had a major upward component. This fluid resulted in the first and volumetrically most important burial dolomitization event, producing dolomite in both carbonate and quartz sandstone lithofacies in the St. Peter Sandstone. Fluid 2 (Sr = 30 + or - 10 ppm, Sr/Ca = 0.019, 87 Sr/ 86 Sr = 0.7084 + or - 1, low Fe) has a composition consistent with a modified Middle to early Late Ordovician seawater origin, suggesting a major downward component for fluid flow. Fluid 2 produced dolomite cement in the carbonate lithofacies that postdates Fluid 1 dolomite. The composition of Fluid 3 (Sr > 50 ppm, Sr/Ca = 0.044-0.110, 87 Sr/ 86 Sr = 0.710-0.711, high Fe) is best interpreted to reflect a heated, deep basinal brine that had previously interacted with the K-feldspar-rich rocks near the Cambrian-Precambrian unconformity in the Michigan Basin, indicating a major upward component for fluid flow. Fluid 3 produced dolomite cement in quartz sandstone lithofacies that postdates Fluid 1 dolomite. Fluid 4 ( 87 Sr/ 86 Sr = 0.7085 + or - 1) resulted in precipitation of late anhydrite in fractures. The 87 Sr/ 86 Sr ratio of the anhydrite is consistent with Fluid 4 originating as a dilute fluid that interacted extensively with Silurian gypsum in the Michigan Basin; this indicates that the flow path of Fluid 4 had a major downward component. The St. Peter Sandstone was an important migration pathway for many different types of fluids in the Michigan Basin, and the proposed fluid origins indicate that both upward and downward cross-formational fluid flow occurred during the Paleozoic. Older and younger modified seawater produced most of the burial dolomite in the St. Peter Sandstone.