To determine the effect of early freshwater diagenesis on Mg distributions in former high Mg calcite (HMC) cements, Mg distributions were determined by electron microprobe analysis of two groups of samples as follows: (1) HMC in limestones not yet exposed to fresh water, and (2) HMC and recrystallized equivalents after freshwater exposure. Mg varies little in pristine HMC, and traverses through the cements show no trends or large-scale zonations. Mg content is not related to inclusion abundance. Former HMC from the Miocene of Eniwetok Atoll, most of which are now radiaxial, prismatic calcites, have a much greater variation in Mg content, and traverses through the calcites display definite trends. Generally, the Mg content of the calcite is inversely correlated with abunda ce of inclusions.

The small variation in Mg content in original HMC cements is a valuable point to be aware of when interpreting Mg trends in recrystallized equivalents. Because Mg content can be related to inclusion abundance in recrystallized, former HMC, but not in pristine HMC, Mg trends in the altered materials most likely reflect dissolution/reprecipitation processes, not original Mg trends. A likely cause for the low Mg content associated with inclusion-rich growth layers in the recrystallized Mg calcites is relatively high rates of dissolution and reprecipitation in these relatively permeable layers. In some places, the cloudy growth layers and increased permeability are due to infestation by endoliths. Such organisms may often cause the cloudiness observed in other radiaxial calcites.

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