Thursday, August 4, 2016

Photomicrograph- Late Ordovician Calcite Cement Stratigraphy In Cathodoluminescence

Cathodoluminescence (CL) brings out beautifully the hidden growth history of calcite crystals. This photomicrograph is of a Late Ordovician pore space from the Fernvale Limestone, Georgia, Southern Appalachians. It is showing calcite cement grown syntaxially over echinoid fragments. Echinoid skeletons are monocrystalline. A syntaxial overgrowth means that pore filling precipitated calcite has maintained the same crystallographic orientation over this monocrystalline substrate. As a result, successive crystal masses even if precipitated at different times under different conditions appear to be one continuous block under polarized light and under crossed nicols. It takes CL to reveal these different growth phases.

The black growth zones were precipitated in oxidizing conditions by fresh water in the vadose zone (above the groundwater table). The black zones are pendant, hanging on the underside of skeletal grains. They are in essence micro-stalactites.

This was followed by another growth phase in suboxic conditions with the incorporation of divalent Mn(+2) in the calcite lattice. Divalent Mn is an activator of CL, hence the bright yellow growth bands interspersed with a thin black bands indicating periodic return to Mn poor oxidizing conditions.

The last phase is a pore filling phreatic ferroan calcite cement precipitated by reducing meteoric fluids in deeper burial conditions. Fe+2 is a quencher of CL. The cement appears dull brown.

The pore space is a couple of millimeters across.


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