In situ geochemistry of middle Ordovician dolomites of the upper Mississippi valley

Abstract

The dolomitization and diagenetic history of Ordovician carbonates of southern Wisconsin is complex. Previous studies attributed dolomitization to various diagenetic factors and environments. In this study, high-resolution, in situ laser ablation inductively coupled mass spectrometry analysis of rare earth element patterns of dolomite was used to assess the diagenetic fluids responsible for dolomitization of the Ordovician Decorah Formation. Integrated geochemical data and petrographic evidence suggest that the dolostones are formed in two different diagenetic realms: shallow burial and hydrothermal. Shallow burial dolomites exhibit three distinct rare earth element patterns. Dolomite from the middle portion of the Guttenberg Member exhibits light rare earth element enrichment consistent with early burial dolomitization. Dolomites of the Carimona, Specht's Ferry and Lower Guttenberg members are often burrow associated and exhibit medium rare earth element enrichment associated with Fe-oxide desorption in anoxic porewaters. Leaching of Mg from co-occurring volcanic ash during alteration is a probable source that contributed to the dolomitization. Extensively dolomitized samples in the upper Guttenberg and Ion Member exhibit evidence of hydrothermal dolomitization. The relationship of these heavily dolomitized samples to interbedded limestones provides evidence for a recently proposed hydrothermal dolomitization model invoking pressure solution of calcite and precipitation of dolomite. These early burial and hydrothermal depositional models are consistent with models proposed for overlying and underlying Ordovician dolostones.