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.

High‐magnesium calcite skeletons provide magnesium for burrow‐selective dolomitization in Cretaceous carbonates

“…Petrographic and stratigraphic analyses have ruled out other potential sources of Mg for burrow dolomitization, such as surrounding evaporites (Zenger, 1996a), volcanic ash (Callen & Herrmann, 2019) and clay minerals (Chatalov, 2018), all of which are conspicuously absent from the Rumaila Formation and surrounding strata. Therefore, echinoderm fragments initially composed of HMC are the sole candidate for a potential Mg source.…”

“…The most prevalent source is dense saltwater with a high Mg concentration, pumped by shallow seepage reflux and/or evaporitic tidal pumping (Morrow, 1978;Saller & Yaremko, 1994;Zenger, 1996a;Baniak et al, 2014;Baldermann et al, 2015;Niu et al, 2020;Rahim et al, 2020;Alibrahim et al, 2021;Ye et al, 2021;Yang et al, 2022a). Moreover, Callen & Herrmann (2019) found that the leaching of volcanic ash could provide Mg for burrow dolomitization. Jones et al (1979) proposed that the upward migration of burial brine brought Mg ions, and Zenger (1996a) suggested that Mg was sourced from the overlaying evaporites in the burial realm.…”

The Diagenetic Alteration of the Carbonate Rocks from the Permian Qixia Formation as Response to Two Periods of Hydrothermal Fluids Charging in the Central Uplift of Sichuan Basin, SW China

“…Barite was observed under a microscope. Meanwhile, there was the obvious Eu positive anomaly of the two types of dolostone and the heavy rare earth elements were relatively enriched in the meso-coarse crystalline dolostones, indicating a strong reducing environment probably caused by the hydrothermal fluids [32,[58][59][60]. These characteristics indicate that the digenesis of the two types of dolostones were influenced by hydrothermal fluids.…”

“…According to the Formulas (1) and ( 2) [27][28][29]32], the ancient seawater temperature of the Qixia period in the study area was calculated (Table 5). As shown in Table 5, the Ts of the Qixia period was between 11.35 and 25.87 • C, with an average value of 21.26 • C. At the same time, the oxygen isotope values could also be used to calculate the temperature of the precipitation of carbonate minerals (Tc) [33].…”

“…Through the statistics of a large number of hydrothermal dolostones in the world, Davies and Smith (2006) found that nearly 80% of hydrothermal dolomitization occurs during early diagenesis or shallow burial [32,39,65]. The homogenization temperatures of fluid inclusions within fine-medium crystalline dolostone were mostly between 95 and 125 • C (Figure 7a), indicating that the formation of dolomite may be related to the early stage of hydrothermal fluid activity, before the timing of meso-coarse crystalline dolomite formation.…”