CONODONTS PRESERVED AS ALUNITE CASTS AND WHY THERE ARE NO APATITE PHENOCRYSTS IN K-BENTONITES FROM THE KATIAN (LATE ORDOVICIAN) GSSP SITE

The Katian (Late Ordovician) GSSP site at Black Knob Ridge near Atoka, Oklahoma contains two prominent K-bentonite beds within the Womble Shale. These K-bentonite beds are within a biostratigraphic interval that suggests they could correlate with the widely distributed and geochemically characterized Deicke or Millbrig K-bentonite beds. Such a correlation would assist in tying the biostratigraphy from the GSSP section into other successions in North America and around the world. In an effort to chemically fingerprint these K-bentonite beds by means of trace elements in apatite, the beds were collected and separated for heavy-minerals (minerals with a density greater than 2.967 g/mL). No apatite phenocrysts or other volcanogenic heavy minerals with the exception of zircon were discovered. Preliminary U-Pb zircon data suggest that these beds could be correlative with the Millbrig and Deicke K-bentonites. There are relatively abundant conodonts, which are originally composed of carbonate fluorapatite, preserved on bedding planes in the Womble Shale. The conodonts are a matte white color, which is a peculiar color for conodonts, or they are preserved as molds. Energy dispersive spectroscopy of the conodonts indicate that the matte white material is not original conodont carbonate fluorapatite, but is most likely the mineral alunite, KAl₃(SO₄)₂(OH)₆, based on the K/Al ratio. Thus, they are casts of the conodonts. This indicates that acidic fluids moved through the rock, dissolved conodont carbonate fluorapatite, and left conodont molds that in some cases were filled with alunite. The acidic fluid was likely sulfuric acid generated from weathering pyrite in the shale. Such an acidic fluid moving through this part of the Womble Shale explains why there are no apatite phenocrysts in the K-bentonite beds, as the volcanogenic fluorapatite would likely be dissolved by the same fluids that dissolved the conodont carbonate fluorapatite.