Abstract
In subsurface samples of Wilcox (Eocene) sandstones, calcite cements occur above 2315 m depths, whereas ankerites occur at depths from 2560 m (temperatures 125 ° C) to at least 4650 m (temperatures 210 ° C). Microprobe analyses indicate that some shallow ankerites have appreciable excess calcium, analogous to protodomites. Ankerites at depths greater than 3200 m have compositions of about CaMg0.5Fe0.5(CO3)2.
Oxygen isotope data suggests that the ankerites are similar to low temperature hydrothermal dolomites and that they have probably formed in pore fluids with higher O18/O16 ratios than sea water. The isotopic data also suggest that the ankerites have formed over a more limited temperature interval than they occur today.
The ankerite is believed to have formed from calcite by the reaction 4CaCO3+Fe2++Mg2+=2CaMg0.5Fe0.5(CO3)2 +2Ca2+. Iron and magnesium for this reaction was apparently released by the breakdown of smectite to illite in mixed-layer clays. Bulk chemical analyses suggest that some iron was transferred from shales into sandstones. Mass balance and chemical considerations are compatible with this model.
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