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Hydrothermal dolomitization of corals, gastropods, pelecypods, echinoderms, forams, and coralline algae indicates that grain size is more important than mineralogy in determining (1) whether or not a fossil will be dolomitized and (2) whether or not the dolomite will pseudomorphically replace a fossil.
Dolomite commonly selectively replaces matrix and/or specific fossils. When dolomite replaces fossils, certain fossils retain their optical characteristics (i.e., pseudomorphic replacement). These selective characteristics have been attributed to both grain size and mineralogy and have been used to make inferences about the predolomitization diagenetic history of sediments.
Fossils were dolomitized at 250°C (482°F) in Ca/MgCl2 solutions for periods of time from 4.5 to 320 hours. Aragonitic corals, gastropods, and pelecypods formed stoichiometric, microcrystalline, xenotopic dolomite and low Mg-calcite (LMC). The dolomite was not pseudomorphic after the aragonite. The conversion of AR ^rarr LMC is more rapid than the AR ^rarr DOL in these experiments. For instance, gastropods run for 23 hours formed dolomite and LMC in a ratio of 1/10, at 170 hours the ratio was 1/4, and at 340 hours the ratio was 1/1.
HMC coralline algae, forams, and echinoderm fragments were dolomitized before and after conversion to LMC. The dolomite formed was cryptocrystalline and pseudomorphic after the forams and echinoderms regardless of the mineralogy. We attributed this to the cryptocrystalline nature of the substrate.
Oyster fragments composed of microcrystalline LMC formed non-stoichiometric, poorly ordered dolomite even after 320 hours. None of the other reactants were as resistant to dolomitization.
Our results indicate that grain size is more important than mineralogy in determining the fabric of dolomite replacement crystals. Both HMC and LMC can be pseudomorphically replaced. Pseudormophic replacement requires (1) abundant nucleation sites and (2) a regular crystallographic relationship between the calcite and dolomite. Argonite was not pseudomorphically replaced, probably because it was microcrystalline rather than cryptocrystalline. Also, most of the aragonite converted to LMC prior to dolomitization.
Selective replacement characteristics of many natural dolomites are readily explained as being an effect of the grain size of the material replaced. Freshwater diagenesis of a sediment prior to dolomitization may retard dolomitization if the grain size of the CaCO3 is increased. However, conversion of LMC without appreciable increase in grain size may not retard dolomitization.
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