EFFECTS OF DRYING, HEATING, ANNEALING, AND ROASTING ON CARBONATE SKELETAL MATERIAL, WITH GEOCHEMICAL AND DIAGENETIC IMPLICATIONS

Carbonate skeletons subjected to drying, heating, annealing, or roasting at elevated temperatures as part of routine sample preparation for chemical analyses or geochemical experiments differ significantly from skeletal materials as they occur in nature. Heating of skeletal samples can degrade organic material, expel H2O and OH-, reduce the concentration of some trace elements, and change the mineralogy and texture of the material. Thermal degradation of organics and expulsion of water in inclusions, which can occur at temperatures of 100-105-degrees-C, cause fracturing and pitting of skeletal samples; areas of pitting reflect original concentrations of volatile phases within the skeleton. Coralline aragonites are partially or completely altered to calcite at temperatures of 150-degrees-C or higher; the degree of alteration varies with temperature and duration of heating, and genus of the coral. High Mg calcites (HMCs) tend to form calcian dolomite and multiple HMCs of lower Mg content on heating, but the rate of alteration is related to the taxonomic group rather than the Mg content; echinoids alter very rapidly (dolomite detectable by X-ray diffraction formed in Clypeaster heated for 6 h at 200-degrees-C), while the coralline red alga Neogoniolithon showed no alteration after heating at 400-degrees-C for 23 h. Mineralogical alteration of coralline aragonites and echinoid HMCs is positively correlated with water loss. Skeletal carbonates comprise a very diverse and heterogeneous suite of materials, and their diversity and heterogeneity are reflected in their responses to heating. Variations in rate and degree of alteration on heating, in many cases between different subsamples of material produced by the same organism, make it difficult to obtain a consistent product from heat treatment. Many workers have used high temperature experiments to model diagenetic processes, and the results of this study have implications for diagenetic alteration as well as for laboratory analyses and experiments.