Vadose and Phreatic Diagenesis: Processes, Products and their Recognition in Corals

ABSTRACT Pleistocene reef corals from Barbados, West Indies, have provided petrographic and chemical criteria for distinguishing the products of mineralogic equilibration (aragonite to calcite transformation) under vadose and phreatic conditions. Petrographically, corals that have equilibrated to subaerial conditions in a phreatic zone contain statistically more calcite cement and secondary porosity than their vadose counterparts. Chemically the phreatic-altered corals contain more magnesium and less strontium than vadose-altered samples. Discriminant function analysis of combined point count and chemical data allows clear distinction of vadose and phreatic populations. Texturally the phreatic calcite mosaic often consists of coarse crystals of calcite (up to several millimeters in maximum dimension) which extend out from the coral skeleton into void filling cement. These mosaics are termed cross-cutting for the manner in which the former walls of filled primary voids do not serve as crystal boundaries. No vadose-altered corals were observed with such cross-cutting mosaics. In contrast the vadose mosaics are termed fabric selective since the boundaries of filled primary voids usually serve as crystal boundaries in the calcite mosaic. These two mosaics result from different modes of aragonite/calcite transformation in the vadose and phreatic zones. Vadose transformation takes place across micron (or less) width films of water, aragonite being dissolved on one side and calcite precipitated on the other. Textural detail is preserved to the level of viewing at low magnification in a light microscope. Phreatic diagenesis often involves the development of zones of chalky aragonite (several millimeters across) between fresh coral skeleton and the growing calcite mosaic. The chalk is a region where extensive dissolution of the coral leaves a very white, friable framework. As the chalk front moves through the coral a calcite product with areas of secondary porosity and calcite cement results. Since water is always present, single calcite crystals may grow to large size and extend beyond the coral skeleton into primary void, yielding the characteristic cross-cutting mosaic. A two water model is proposed in both zones. Dissolution-reprecipitation takes place in a static water held immobile in thin films (vadose) or wider chalk zone (phreatic). In the vadose this water intermittently comes in contact with water percolating through the larger connected pores of the reef sediments. Thus chemical exchange is limited and episodic; strontium values in the calcite build up a la Kinsman (1969) and little magnesium is externally introduced. The immobile water of the chalk areas is constantly in contact with a large reservoir of water passing through the reef; the site of transformation acts as a relatively open chemical system. The resultant calcites have low strontium values due to diffusion from the site and high magnesium values as this ion is constantly introduced from the surrounding high-magnesium calcite sediments. The relative chemical homogeneity of the phreatic zone in time and space is borne out in microprobe analysis of phreatic corals; far more variability is encountered in vadose probe runs.