Trace and Minor Element Analysis of Obsidian From
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r r TRACE AND MINOR ELEMENT ANALYSIS OF OBSIDIAN r FROM THE SAN FRANCISCO VOLCANIC FIELD r USING X-RAY FLUORESCENCE r r [ r A Thesis Presented to the Graduate Faculty F' ! 1. Northern Arizona University r ~ l rL In Partial Fulfillment r of the Requirements for the Degree r Master of Science r ' r r \. by r Suzanne C. Sanders L April 1981 _[r r t f l I. r l l t I ABSTRACT . r l f ! f r I Obsidian from eight locations in the San Franciscan volcanic field in northern Arizona were analyzed for 20 minor and trace elements r using x-ray fluorescence analysis. The intensity ratios relative to iron were statistically analyzed r and the trace and minor element patterns established. The obsidian rL outcrops clustered into four well defined groups consisting of two localities apiece: Government Mountain/Obsidian Tank, Slate Mountain/ r Kendrick Peak, Robinson Crater/O'Leary Peak, and RS Hill/Spring Valley. r Each of the four distinct groups was treated individually to refine the separation between the similar sites. Classification function coeffi r cients were calculated for each locality, then these were used to identify the source of thirteen obsidian artifacts recovered from a Northern r Arizona archaeological site. r r r r r r r r l r ..r r r r I CONTENTS r t Page LIST OF TABLES .. iii r LIST OF FIGURES . v r Chapter 1. INTRODUCTION . 1 r, 2. SAMPLE COLLECTION 7 3. METHODS AND MATERIALS 12 r 4. ANALYSIS OF THE DATA .. 17 r 5. RESULTS ....... 26 6. DISCUSSION AND CONCLUSIONS • . 50 r 7. SUGGESTIONS FOR FUTURE WORK 53 BIBLIOGRAPHY . 54 r APPENDIXES r A. TABLES OF RAW DATA . • . • • • • . • • 58 B. CLASSIFICATION TABLES FOR SIX AND SEVEN r CASES PER GROUP. .. 67 C. COMPUTER PROGRAMS ........... 71 r ! I r :· r r r r ii r t TABLES r Table Page 1. Overview of the Data • • • • • • . • • • • • • • • • • • • • • 18 r 2. Typical Set of Raw Data for Government Mountain Samp 1e #165 • • . • • • • . 32 r 3. Intensity Ratios for Data in Table 2 {Sample #165) 33 r 4. Results of Oneway Analysis of Variance • 34 5. Eigenvalues •• . 37 ~ L 6. Classification Function Coefficients 39 7. Calculations of Government Mountain Classification Score for Data in Tables 2 and 3 r (Sample #165) •••. 40 r 8. Classification Matrix .•.• 42 9. Jackknifed Classification. 44 r 10. Individual Classification Function Coefficients .. 45 r 11. Classification of .. Known Unknowns" 47 r 12. Classification of Unknowns 49 APPENDIX A r A1 Government Mountain Data • 58 r A2 Slate Mountain Data .• 59 A3 Robinson Crater Data . 60 r A4 0 1 Leary Peak Data . 61 AS Kendrick Peak Data . 62 r A6 RS Hill Data ••.•. 63 r A7 Spring Valley Data . 64 r iii TABLES CONTINUED Page AS Obsidian Tank Data . 65 r A9 Data for Unknowns. 66 r APPENDIX B 81 Classification Coefficients for Total r Sample, Six cases/group ...•••. 67 82 Classification Coeffi.cients for Total r Sample, Seven case~/group .••••. 68 lfl!'l 83 Coefficients for Individual Classifications t Six cases/group ...••..•...•.• . 69 84 Coefficients for Individual Classifications r Seven cases/group. • • • . • • • . •.• 70 r APPENDIX C C1 Calculation of Obsidian Intensity Ratios . 73 r C2 Calculation of Classification Scores 79 C3 Posterior Probabilities. 98 r C4 Discriminant Analysis .• 101 CS Batch Discriminant Analysis. 101 rL C6 Oneway Analysis of Variance. 102 r C7 Batch Oneways .••. 102 r CB Pearson Correlation. 103 r r r r iv r r r FIGURES r Figure Page 1. Silicate Structure. 3 r 2. Obsidian Collection Sites 11 3. Elements in the Analysis 13 r 4. Visual Representation of the Data r a. Government Mountain and Obsidian Tank. • • • 27 b. Slate Mountain and Kendrick Peak . • • • 28 r c. Robinson Crater and O'Leary Peak . • • • . • 29 r d. RS Hill and Spring Valley. • . • • • 30 r 5. Plot of Canonical Variables . 38 r r r r r r r r r v r r r Chapter 1 r INTRODUCTION The large role that chemistry can play in interdisciplinary r studies is not always realized. The project described in this thesis is a multidimensional study in that it combines chemistry with archaeology, rl statistics, and geochemistry. By studying the patterns of minor and trace ~ t elements in obsidian samples, one can establish a chemical fingerprint for a rock outcrop and trace the origin of an obsidian artifact to its r geological source. The pattern recognition techniques are statistical r in nature. X-ray fluorescence spectroscopy was chosen in this investigation r because the method is nondestructive, an essential criterion for the r study of artifacts. Similar characterization studies of obsidian world I wide have been published in recent years in which a variety of techniques r I were used for the elemental analysis. Atomic absorption spectroscopy has r I been used by some investigators (Wheeler and Clark, 1977), as well as x-ray fluorescence (Leach and Anderson, 1978; Hester, 1975; Nelson et al, r : 1975; Kowalski et al, 1972; Stross et al, 1971; Jack, 1970), Moessbauer I spectroscopy (Longworth and Warren, 1979), proton-induced x-ray emission r ;. spectroscopy (Duerden et al, 1980, 1979; Scott et al, 1978), and neutron activation analysis (Ericson and Kimberlin, 1977; Osawa et al, 1977; r Bowman et al, 1973; Stross et al, 1971). Most authors used a simple r visual method utilizing only two or three elements to interpret the data. The following two XRF studies are of particular interest since they are r related to the work in this thesis. 1 r t r 2 r A previous study of obsidian in the San Francisco volcanic field t of Northern Arizona was conducted by Jack (1970), and this study has been r further investigated here. This earlier study was semi-quantitative and r the classification relied solely on the concentrations of Zr, Rb, and Sr. The validity of these results was questionable since only 18 standards r from nine localities were used, in some cases employing only one sample as the standard for a site. Due to the variability which can exist in r the obsidian in each sample site, it was felt that too much importance r was placed on too little data. Another study was done in which archaeological obsidian artifacts r were classified by applying pattern recognition techniques to trace element data (Kowalski et al, 1972). The obsidian used in this study r were from four sources in Northern California and were analyzed using r x-ray fluorescence spectroscopy. The data were computer-analyzed using four pattern recognition methods. Three of the methods utilize super r vised learning, which means that the classification parameters are adjusted by the computer to fit the data; the fourth is a visual display r using non-linear mapping. r i Obsidian is a result of viscous lava flow which cools so rapidly t that crystals do not have time to form. This natural glass is the fine r ' grained equivalent of granite and is made up chiefly of quartz, feldspar, and mica. These mineral are all silicates, composed of Si04 tetrahedra r ! 3 I· linked together with cations such as A1 + replacing the Si in some instances. r I Quartz has the composition Si02, in which Si04 tetrahedra are connected so that every oxygen atom is shared by two silicon atoms, given an infinite r 1 three-dimensional framework. In the feldspars, one fourth of the Si are r .. replaced by Al (Wells, 1962) (see Figure 1); this requires a monovalent r ~. r 3 r r . r r r r r r r r r Figure 1. Partial Structure of A1Si o r 1 3 8 r ~ r , r r r r 4 r cation in the interstices between the oxygen atoms to balance the charge on the A1Si o framework. Common feldspars include orthoclase (KA1Si o ), r 3 8 3 8 albite(NaC1Si 0 ), celsian (BaA1 si ) and anorthite (NaA1 s; ). In 3 8 2 2o8 2 2o8 r the latter two minerals, an additional Si has been replaced by Al, which requires the inclusion of divalent ions. Micas are silicates which have r introduced layers of hydroxyl groups into the structure. Three common micas are muscovite (KA1 s; o (0H) ), biotite (K{Mg, Fe) , and r 3 3 10 2 3 A1Si 3o10(0H) 2). In obsidian the structure has short range order (as in r a liquid) rather than long range order. r Obsidian is black because of sparsely dispersed grains of magne tite (Fe3o4), but may be red or brown from the additional oxidation of the iron by hot magmatic gases to give hematite, Fe o (Gilluly et al, r 2 3 1959). Thin pieces of obsidian are almost transparent. Phenocrysts, r large crystals, usually of feldspar or quartz which cooled more slowly, occur embedded in some obsidians. r Since the volcanic glass has no cleavage planes, it breaks with r a conchoidal (shell-like) fracture. This break produces a very sharp natural edge which made obsidian an ideal working material in prehistoric r times. Primitive peoples had several uses for obsidian. It was commonly r used for the manufacture of utilitarian items such as projectile points, knives, and scrapers. ,·In Mesoamerica,. this glass with its inherent beauty was also used as ornaments such as lip and nose plugs, ear spools, r I and pendants. It was used in religious ceremonies in the form of figurines, r mirrors, and sacrificial knives, or buried in pits as offerings to the r gods. It was also used as a prestige symbol in hunting and warfare (Michels, 1971}. The superior quality of many obsidians compared to other r silicate rocks stimulated widespread trade, and it is often found far r from the original source.