Ferroan calcite replacement indicates former magnesian calcite skeletons

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Abstract
The replacement by ferroan calcite with preservation of the original structures can be used as a new criterion for identifying skeletons originally composed of high‐magnesian calcite. This applies to bryozoa, rugose corals, echinoderms, many foraminifera, most ostracods, red algae, and serpulids. On the other hand, skeletons originally composed of low‐magnesian calcite were never replaced by ferroan calcite, as shown by belemnites, brachiopods, and most of the pelecypods.Using this criterion, an original low‐magnesian calcite composition is inferred for Tentaculites and some ostracods and foraminifera, whereas a previous high‐magnesian calcite composition is inferred for trilobites, oligostegina and certain ooids. Chemical instability of high‐magnesian calcite is suggested to be the driving force of the replacement by ferroan calcite. In most of the thirty‐seven samples investigated, of Oligocene to Devonian age, the ferrous iron concentration of the interstitial fluid increased during diagenesis, as shown by well established sequences of cement A and B and fissure fill. This offers a relative time scale for diagenetic processes.Ferroan calcites contain up to 6 mol % FeCO3 and up to 5 mol % MgCO3. In this range of concentration, the distribution coefficients for Fe and Mg between calcite and solution at about 25°C are about 1 to 0‐03, respectively, according to experiments. Possible sources of iron are iron oxides and hydroxides as well as clay minerals including glauconite.Though a submarine origin below the sediment surface is conceivable for ferroan calcite, there are serious limiting conditions such as low Eh and, at the same time, lack in sulphate‐reducing bacteria. On the other hand, ferroan ‘dedolomite’, compositional zonality in individual ferroan calcite overgrowths, low δ18C and δ18O values, and low Mg concentrations point more to a meteoric‐phreatic origin of many ferroan calcite occurrences.