The chert samples were prepared for Neutron Activation Analysis using the following procedures. It was necessary to acquire a small amount of homogeneous material from each chert specimen, free of inclusions, matrix, weathered material, or exterior contamination. Luedtke (1978) found that the geochemistry within individual nodules of Bayport chert had small variations from the center of the nodules to parts near the exterior. While this may not be true for all cherts, it presents a reason to be cautious about where within a nodule a sample is taken. She also found that serious geochemical sampling problems might arise when physical variations include the texture of the chert. Therefore samples for this analysis were carefully obtained by striking flakes of chert from the massive interiors of the nodules using an alundum (Al₂O₃) pestle. (Mullite or agate pestles could not be used due to the hardness of the chert and the danger of contamination from flakes of broken pestle material.)

Many samples had convenient interior surfaces already exposed by being broken open during the field collection; those that did not were broken open using a rock hammer. Care was taken to make sure that no samples were taken from surfaces which had made contact with the rock hammer, which might have contaminated the sample with iron and other elements. Also, any visible inclusions and weathered portions of the chert were avoided. The flakes struck from these interior surfaces were then ground to a fine powder using an alundum mortar and pestle.

Chert is a homogeneous (perhaps monotonous) geologic material, but it still contains occasional inclusions and inhomogeneities. Therefore, although only 200 milligrams of sample were required for the Neutron Activation Analysis, a much greater amount (generally approximately one gram) of material was flaked from each nodule and powdered to provide a more representative sample for that specimen. From each powdered sample, 200 ± 1 milligrams were weighed out on an electronic scale and put into a labeled 2/5 dram flip-top polyvial, which was then sealed. The batch number, vial number, sample number, and sample weight were all recorded in a sample log.

A total of 210 geologic samples were collected from the field, but during the sample preparation process four were found to be unusable for various reasons. Of the remaining 206, three samples were split and processed as replicates in order to provide a basis for verifying the consistency of the analyses. Also, seven donated samples of Prairie du Chien chert recovered from archaeological contexts were prepared for analysis. All together, 216 chert samples were prepared and then sent to the University of Wisconsin, Madison, Nuclear Reactor Lab (Richard Cashwell, director) for neutron activation and counting. Prior to irradiation, the sample vials were sealed by friction welding. The samples were then subjected to a thermal neutron flux of about 10¹² neutrons cm^-2 sec^-1. The gamma radiation was counted using a Ge(Li) detector (170 cc Ortec Intrinsic Germanium Detector) coupled to a PCAII PC-based multichannel analyzer, and the spectra were analyzed by a UWNR compiled basic program ("NAACALC").

At the Reactor Lab, the specimens were analyzed separately for both short-half-life (on the order of a day or less) and long-half-life (on the order of days or more) radioisotopes induced by the bombardment of neutron radiation. (See Table 1 for a list of the elements analyzed by each method.) Samples were irradiated in batches of eight, each along with 200 mg of a known standard (Canadian Standard Reference Material CRSO4). Quantitative elemental concentrations were then calculated by comparison of detected peak intensities with the intensities and elemental abundances in the known standard. Short half-life isotopes were measured for a period of 5 minutes after irradiation for 3 seconds and a 20 minute delay time; long half-life isotopes were counted for 60 minutes after being irradiated for 2 hours with 200 hours (8.3 days) of decay time. A total of 55 elements were analyzed by one method or both.

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Intro and Background     Fieldwork     Sample Prep     Data Analysis     PCA     Correspondence Analysis    Stepwise DA   

Discriminant Analysis     More PCA     Element Trends     Conclusions     Bibliography     Appendix A: Part 1 Part 2 Part 3 Part 4 Part 5     Appendix B     Appendix C

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