Cathodoluminescence in calcite cements: New insights on Pb and Zn sensitizing, Mn activation, and Fe quenching at low trace-element concentrations

The role of Pb and Zn as sensitizers of cathodoluminescence (CL) in sedimentary carbonates is still speculative, as is the relationship between Mn, Fe, and CL intensity at low concentrations of Mn and Fe, We address these questions using calcite cements from the Lower Oligocene Suwannee Formation of Florida, which have mean Mn, Pb, Zn, and Fe concentrations less than 45 ppm, Only a small fraction of these cements exhibit visible CL, but when present, the luminescence is orange to reddish orange. Trace-element contents were measured using a synchrotron x-ray fluorescence microprobe, and CL intensity was approximated by spot-metered photomicrograph exposure times. Based on Mn and Fe contents, four cement populations can be defined. Group I is luminescent calcite with Mn > 25 ppm and molar Mn/Fe > 1.8. Group II contains both luminescent and nonluminescent cements, all with Mn > 25 ppm, molar Mn/Fe < 1.8, and Fe < 100 ppm. Group III is dully luminescent cements with Mn > 25 ppm, molar Mn/Fe < 1.8, and Fe > 100 ppm. Group IV is nonluminescent calcite with Mn < 25 ppm and variable Fe. These groupings and statistical analyses of the data (Mann-Whitney tests and factor analyses) confirm that 25 ppm Mn is necessary to initiate visible CL, and that Mn is the sole control on CL intensity when Mn and Fe contents are very low (<100 ppm). The data also establish 100 ppm Fe as a new minimum for the quenching of CL when Mn is below 200 ppm. There is no evidence in our data that Pb or Zn concentrations in tens of parts per million either activate or sensitize CL. The Mann-Whitney test shows that there is no statistical evidence at the 95% confidence level to indicate that luminescent and nonluminescent cements reflect different populations of Pb and Zn, except for Group II cements that have more Pb in nonluminescent cements than in otherwise chemically equivalent luminescent cements. Factor analysis shows that CL intensity in all luminescent cements is unrelated to Pb or Zn concentrations, Given that Pb or Zn sensitizing is lacking in low-Mn calcites like the Suwannee cements, it is hard to imagine how these elements could play a significant role in calcites containing even more Mn, as is typical of most sedimentary carbonates.