Tephrochronology of highly altered ash beds: the use of trace element and strontium isotope geochemistry of apatite phenocrysts to correlate K-bentonites

Tephrochronological studies of extremely altered ash beds (bentonites) are hampered by the difficulty of determining the original volcanic ash compositions. Correlations of individual Ordovician K-bentonites within the Taconic Foreland basin of the northern Appalachian orogen, however, are possible based on trace element and strontium isotopic analyses of apatite phenocrysts contained within the altered ash beds. Apatite phenocrysts from most of the K-bentonites have characteristic Cl, Fe, Mg, and Mn contents; thus, these elements are good discriminators. Rare earth element concentrations of apatite phenocrysts are not as distinct and are less useful discriminators. The strontium isotopic composition of apatite phenocrysts is the single most diagnostic parameter for distinguishing the Taconic basin K-bentonites. The range in measured ⁸⁷Sr/⁸⁶Sr ratios for the apatite phenocrysts from different K-bentonites is 0.7056-0.7090. Some apatite phenocrysts have undergone secondary alteration, indicated by a frosted, opaque appearance. These grains have slightly different ⁸⁷Sr/⁸⁶Sr ratios compared to unaltered apatites from the same K-bentonite layer collected at different localities. The cause and timing of the alteration is unknown but it may reflect interaction with fluids that equilibrated with the surrounding country rock. Exchange with seawater Sr cannot solely account for the observed effect because altered samples are displaced both toward and away from Ordovician seawater ⁸⁷Sr/⁸⁶Sr values. The alteration effects appear to be limited to the outer portion of the apatite grains because the effects were overcome by abrading the apatites in an air-abrasion device. Determining the ⁸⁷Sr/⁸⁶Sr ratios of abraded apatites may become a general method of determining the strontium isotopic composition of a wide variety of altered felsic volcanic rocks. Previous correlations of the K-bentonites within the Taconic Foreland basin were based on the physical matching of individual beds and groups of beds. These correlations led to the assertion that several graptolite species thought to be important in biostratigraphic studies were coeval and thus provided no time information. The new geochemical correlations of the K-bentonites, however, demonstrate that the graptolite zones are indeed sensitive time indicators. Many of the previous correlations are thus incorrect, and paleo-environmental models based on them need to be reevaluated.