Can crystal morphology indicate different generations of dolomites? Evidence from magnesium isotopes

Among various geochemical and petrographic approaches, dolomite crystal morphology and dolostone fabric have been widely applied in the study of ancient dolostones. It is proposed that dolomite crystal morphologies and the rock fabric may reflect the formation temperature, and thus can be used to distinguish different generations of dolomite. However, this scenario has also been challenged by some researchers. In order to test whether the dolomite crystal morphology can be used to differentiate different generations of dolomite, in this study, we measured the Mg isotopic compositions (δ26Mg) of dolomite with different crystal morphologies. δ26Mg of dolomite is controlled by a variety of factors, including temperature, magnesium isotopic composition of dolomitization fluids, and the flow rate of dolomitization fluids. If dolomite with distinct crystal morphologies were derived from different dolomitization processes, it is highly plausible that they would have different δ26Mg. Five types of dolomite with distinct crystal morphologies and rock fabric were recognized from three sampling intervals (S1, S2, and S3) in the middle Ordovician Majiagou Formation in North China. Different types of dolomite in the same sampling interval have similar δ26Mg values, suggesting that these dolomites might have derived during the same dolomitization event. Our study indicates that the crystal morphology alone may not unambiguously differentiate the generations of dolomites. We propose the following reasons: (1) the dolomite crystal morphology might be controlled by various factors rather than the formation temperature alone, or (2) the dolomite crystal morphology might be modified in diagenesis, but δ26Mg remains unchanged.