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



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



Geochimica et Cosmochimica Acta 59(12): 2527-2536



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.

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Accession: 020211536

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DOI: 10.1016/0016-7037(95)00147-6


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