Utility and limitations of apatite phenocryst chemistry for continent-scale correlation of Ordovician K-bentonites

Two prominent, Ordovician, altered volcanic layers, the Deicke and the Millbrig K-bentonites, crop out over a wide area of North America. These beds have been correlated over selected portions of North America but have not been successfully correlated throughout the entire Mississippi Valley, southern Appalachians, and New York-Ontario region. A dual approach that used trace element compositions and Sr isotopes in apatite phenocrysts was applied to Deicke and Millbrig candidates from this large region. Major (Ca, P, F) and minor (Mg, Mn, Cl, Fe, Sr, Si) concentrations within apatite phenocrysts were determined by electron microprobe analysis. Mg, Mn, and Cl proved to be the most diagnostic elements to distinguish apatite between the Deicke and Millbrig K-bentonites. These three elements have a restricted range in concentration within Deicke apatite whereas they vary significantly within Millbrig apatite and define three major clusters. Two of the Millbrig clusters appear to be mostly limited to the middle portions of the bentonite, whereas the third cluster appears to be largely restricted to apatite from the upper portion of the bentonite, based on analyses from a vertical profile within a thick bed from Hagan, Virginia. It is thus suggested that the Millbrig K-bentonite is composed of multiple eruptions or was derived from a chemically zoned magma chamber. Sr isotopic data are consistent with, these scenarios (decreasing ⁸⁷Sr/⁸⁶Sr with stratigraphic height) but are too variable within a single bed to be reliable discriminators of these two K-bentonites.