Effects of heating on the geochemistry of biogenic carbonates

Heating and leaching experiments were performed on biogenic carbonates to investigate the behavior of trace elements during diagenetic and metamorphic processes. Samples were treated with H2O2 to remove organic compounds and heated in vacuum in the range 100–500°C. The effects of heating on the geochemistry of these shells were studied by measuring: (1) Cl−, SO42−, Na+, Ca2+, Mg2+ and Sr2+ concentrations in leachates; and (2) the amount of CO2 released by the carbonate during heating experiments. nificant increase in Mg2+ and Sr2+ was observed in the leachates for heated samples relative to the blank reference which corresponds to the leaching of unheated samples. An enrichment in Ca2+ from 100 to 700 ppm is only detectable at high temperatures (> 400°C) as a consequence of a decarbonation process. The effects of heating on the chemistry are especially sensitive for the aragonite shells. Leachates were obtained on aragonites heated above 200°C and enrichment factors up to 100, 200 and 500 were measured for Cl−, SO42− and Na+, respectively. Na+ contents display a positive correlation with temperature whereas Cl− ion contents are rather constant. urce of Cl− is probably the fluid inclusions which represent ∼ 1 wt% of the shell. Na contents in samples treated with H2O2 are lower than those untreated, this result suggests that a fraction of Na+ is linked to the organic matrix of the shells. On the basis of increasing Na+ and SO42− contents with peak temperatures, another fraction of Na+ and SO42− ions is considered associated with defects in the crystal lattice. These ions are thus expelled during the heating of samples which is responsible for the annealing of defects and the observed progressive aragonite-calcite inversion. Mass-balance calculations using the chemistry of shell fragments reveal that heating and leaching of aragonites above 200°C are responsible for reductions by up to 70%, 55% and 25% of initial Cl−, SO42− and Na+ contents, respectively. Sr2+ and Mg2+ ions are more stable in the crystal structure of the biogenic carbonates and are probably more reliable indicators of paleoenvironment than SO42−, Cl− and Na+ ions. In skeletal carbonates, especially aragonites, the abundance of the latter elements can be strongly modified when they are exposed to temperatures of only 200°C and leached by low-salinity fluids.