Title

Apatite trace element tephrochronology

Date of Award

2010

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Earth Sciences

Advisor(s)

Scott D. Samson

Keywords

Apatite, Tephrochronology, Bentonite, Ash fall beds

Subject Categories

Geology

Abstract

Initial studies of Late Ordovician tephra (K-bentonites) have demonstrated the potential for utilizing trace element concentrations in apatite as ash-fall bed discriminators. To test the utility of apatite trace element tephrochronology on a large scale we studied over 200 K-bentonite samples from the Late Ordovician of eastern North America and Scandinavia. In addition we studied unaltered volcanic samples from various locations around the world. On the basis of electron microprobe (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) results, the resulting apatite trace element data are shown to provide unique bed discriminators despite within-crystal variability, intra-sample heterogeneity, and effects of low-grade regional metamorphism. Each of the tephra layers possesses unique and reproducible trends in Mg, Cl, Mn, Fe, Ce and Y concentrations in apatite. Results from EPMA show comparable precision for Mg and Mn against analyses from LA-ICP-MS. Further results from LA-ICP-MS show the potential of Sr, U, Pb, and Rare Earth Elements (REE) ratios as powerful bed discriminators. Results from apatite analyses of the Late Ordovician have provided 12 correlated K-bentonite beds over an estimated 5 Ma-duration, which provide a significant improvement of stratigraphic resolution. Five of these beds are new (Wintergreen Park, Queen Street, Riverton, Fischer Hill, and Nittany K-bentonites). In addition, our data indicate that the Brannon K-bentonite (formerly known only in the Lexington Limestone of Kentucky) is equivalent to the Paradise K-bentonite (which is locally developed in NY within the lower part of the Indian Castle Shale); and that the Haldane K-bentonite (located within the upper Dunleith Formation in the upper Mississippi Valley) is equivalent to the Manheim K-bentonite (uppermost Dolgeville Formation in New York and lower Antes Shale in central Pennsylvania). Our findings also indicate that the Millbrig (North America) and Kinnekulle (Scandinavia) K-bentonites are not from a single eruption. The stratigraphic relations revealed by these new K-bentonite fingerprints substantially alter previous interpretation of sequence and chemostratigraphic correlations in eastern Laurentia.

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