Geochemical evidence for repetitive intracrystal recrystallization during the mineralogical stabilization of some biogenic Mg calcites

Mineralogical stabilization of porcellaneous foraminifera is unique relative to other types of bioclasts in that these foraminifera is unique relative to other types of bioclasts in that these foraminifera can undergo stabilization to low-Mg calcite without any textural change. Holocene porcellaneous foraminifer from the freshwater diagenetic zone of the Schooner Cays, Bahamas, are in the midst of this alteration and thus provide a rare insight into the stabilization process. These bioclasts exhibit Mg loss and oxygen isotopic changes with no textural alteration at any scale. The mineralogical stabilization, or recrystallization, is a repetitive intracrystal process. Each recrystallization produces a calcite with a slightly lower Mg content than its predecessor. The stabilization rate is dependent on time and hydrologic flux; older phreatic-zone material is the most altered and younger vadose-zone material is the least altered. Numerical modeling of the chemical diagenesis suggests that the molar water:rock ratio of a single recrystallization is about 1:100 and that the resultant precipitate is not in equilibrium with the ambient pore waters. Repetitive recrystallizations, however, eventually yield a mineralogically stable low-Mg calcite that can be in equilibrium with the bulk pore waters. Complete mineralogical stabilization to LMC should occur at cumulative molar water:rock ratios ofmore » about 16 and requires hundreds to thousands of recrystallizations, each reducing Mg content by less than 0.01 mole %. The large number of recrystallizations with incrementally small chemical changes per recrystallization makes alteration of these foraminifera significantly different from single-step recrystallization of other types of bioclasts.« less