TEPHROCHRONOLOGY OF THE ORDOVICIAN DEICKE AND MILLBRIG K-BENTONITES BASED ON APATITE CHEMISTRY: WHAT WORKS AND WHAT DOESN'T

Tephrochronological studies of ancient ash deposits are rare because of substantial chemical alteration of volcanic glass. (i.e. transformation to bentonite). Two prominent Ordovician altered volcanic layers, the Deicke and Millbrig K-bentonites, crop out over a wide area of North America. These beds have been correlated over selected portions of N. America, but have yet to be successfully correlated throughout the entire Mississippi Valley, southern Appalachians, and New York-Ontario region. A dual approach, using Sr isotopes and minor element compositions of apatite phenocrysts, was applied to potential Deicke and Millbrig candidates from this large region. Samples were collected from throughout the region in outcrops and from drill cores in Ohio and Kentucky.

Major (Ca, P, F) and minor element (Fe, Cl, Mg, Mn, Sr, Si, Al) concentrations of apatite phenocrysts were determined by electron probe microanalysis (EPMA). In most cases F contents ( ~ 3%) are indistinguishable between the Deicke and Millbrig K-bentonites. Fluorine was useful in some cases, however, in demonstrating that some Deicke or Millbrig candidates were clearly misidentified (F < 2%). Si, Al, and Sr concentrations were wither too variable within a given bentonite or too similar between bentonites to be useful discriminators. Mg, Mn, Cl, and to a lesser extent Fe, did, however, prove to have diagnostic concentrations in the apatite between the two beds. These four elements have a restricted range in concentration in Deicke apatite whereas they define three major clusters in Millbrig apatite. Two of the clusters appear to be limited to the mid portions of the bentonite, whereas the third cluster appears to be restricted to apatite from the upper portion of the bentonite, based on analyses from a vertical profile within a thick bed from West Virginia. The two clusters from the lower bentonite portion are present in samples from the Mississippi Valley, but the third cluster is not. It is thus suggested that the K-Millbrig bentonite is composed of multiple eruptions or was derived from a chemically zoned magma chamber.

Sr isotopic ratios do not appear to be useful for discrimination between these two beds as the composition is too variable within a single bed. Isotopic heterogeneity has been noted for Cenozoic ash derived from eruption of major zoned magma chambers.