Abstract

Cretaceous rudists, with thick, multilayered, bimineralic (aragonite and low-Mg calcite) shells, have been studied for the effects of meteoric diagenesis on their textural and chemical attributes. The original fine, compact, prismatic or cellular-prismatic microstructures of low-Mg calcitic layers have been preserved frequently, despite the vagaries of their subsequent diagenetic histories. These layers usually suffered only partial recrystallization, which caused fusion of crystal units, rounding of prism comers, and partial cementation confined to intercrystalline and intraparticle pore spaces. In contrast, the originally aragonitic parts of the shell usually suffered either complete dissolution or, at best, retained some vestiges of their crossed-lamellar or complex crossed-lamellar, microstructures. Synsedimentary submarine diagenetic events, such as borings by endolithic fauna, internal micritic and/or peloidal sedimentation, and submarine aragonite and/or high-Mg calcite cements within the primary inter- and intraparticle pore spaces, were coeval with rudist accretion. The subsequent meteoric cementation has been characterized by the formation of bladed and equant ferroan calcites which occluded the residual primary and secondary porosities. Diagenetic stabilization of aragonite to diagnetic low-Mg calcite and low-Mg calcite to diagenetic low-Mg calcite is believed to have been a two-stage process. Initially, Sr and Na have been partially depleted during an early meteoric diagenetic phase (calcitization of metastable phases), with expulsion of Sr at a slower rate than that of Na. This stage is followed by incorporation of more Mn, Fe, and Mg due to precipitation of late ferroan calcite. Stabilization of aragonite causes more pronounced trace-element repartitioning than the stabilization of low-Mg calcite. However, the degree of repartitioning depends not only on mineralogy, but also on the structural buildup of a given shell layer, suggesting that factors such as water/rock ratio (surface kinetics) are of considerable significance for the process of diagenetic stabilization. Rudist skeletal components with preserved original mineralogy and texture have trace-element chemical signatures analogous to Holocene marine bivalves. This similarity argues for comparable chemical composition of the Cretaceous and Holocene seawater, as well as for similar modes of incorporation of trace elements into mollusc shells.

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