TOWARD FLOW-SPECIFIC OBSIDIAN HYDRATION RATES: COSO VOLCANIC FIELD, INYO COUNTY, CALIFORNIA Jonathon E. Ericson INTRODUCION As a result of research conducted under proprietary contact in 1981-83, it was observed that the hydration measurements among Coso obsidian artifacts showed definable clusters which correlated to chemical groups. The trace element values for Coso obsidian artifacts determined by x-ray fluorescence analysis, following techniques presented by Jack (1976), appeared to be concentrated in three centroids, which were nominally called Coso A, B and C. After testing petrographic characterization, it was determined there was signifi- cant clustering of crystal fabric among the three sub- groups so these data, along with chemical characeriza- ton data, were used to discriminate sub-groups (see Hughes 1988). In the course of this research, what appeared to be flow-specific hydration rates among the different Coso flows were observed. This paper presents results verifying these observations. It is now apparent that the next development in the refinement of obsidian hydration dating will require the formulation of flow-specific hydration rates in complex volcanic fields, such as the Coso volcanic field. When Friedman and Smith (1960) first introduced obsidian hydration dating, they suggested that rhyolitic glasses would hydrate at different rates than basaltic glasses, but they did not suggest that the chemical and physical variability among different rhyolitic sources would change hydration rates significantly. Three data sets indicated that hydration rates would vary among rhyolitic obsidian sources. Aiello (1969) observed differential hydration associated with changes of bulk chemical composition of a single artifact from Grimes Canyon fused shale, and bimodality in hydration and bulk chemistry composition were indicated for one cache from Oregon and one stratigraphic layer form Amapa, Mexico (Ericson, MacKenzie, and Berger 1976). These preliminary findings provided the justification for further research to study the relation- ships among the chemical and physical propertes of obsidians and their rates of hydration (Ericson 1977; 1981a, b). In response to these findings, California archaeolo- gists have incorporated obsidian hydration dating and chemical characterization as part of their research strategies. As a consequence there have been a number of empirical obsidian hydration rates formulated with available archaeological data. The Coso obsidian source is one of the better examples of the variation of rate determinations (Ericson 1978; Meighan 1978; 1981; 1983; Friedman and Obradovich 1981; Findlow et al. 1982; Michels 1983; Elston and Zeier 1984; Koerper et al. 1986). This variation is now a cause for concem among practicing field archaeologists. There are a number of plausible explanations which may account for the number of hydration rates Contributions of the Archaeological Research Facility Number 48, December 1989 for the Coso source. Each condition may act singly or in concert with other conditions to produce the ob- served variation of rates; 1) errors intruced using relative chronological markers to formulate a hydration rate; 2) misidentification of obsidian source during the chemical characterization process (type II error); 3) non-standardization of hydration measurements among laboratories; 4) lack of true temporal association between obsidian atifacts and associated chronological samples; 5) different effective temperatures among different study areas; 6) the affects of unidentified environmental (soil) variables among different study areas; and 7) the affects of different chemical and physical variables among different flows within a single source of obsidian. The last point is a major focus of this paper. Hopefully, all of these effects will become the focus of future studies and reexamination of earlier research. At this point, it is important to define terminology. The term, source, as employed in source-speciflc hydration rates (Ericson 1977; 1981), refers to one or more eruptive events within a volcanic field which have valid geochemical entity relative to other sources. For example, Coso (Ericson 1977; 1981) refers to both West Sugarloaf and Sugarloaf which are geologically superimposed yet whose eruptions are separated by approximately 100,000 years (Bacon et al. 1981; Friedman and Obradovich 1981). From the standpoint of chemical characterization of obsidian artifacts by short half-life neutron activation analysis and x-ray fluorescence analysis, Coso is a viable geochemical source relative to other sources (Ericson and Kimberlin n.d.). Obsidian artifacts from Coso have been grouped together for purposes of rate formulation. The term, flow, as used herein to refer to flow- specific hydration rates, refers to an eruptive event which provided a geological source of obsidian for prehistoric exploitation. The term flo may refer to a dome, a flow, or a primary deposit of ejecta (bombs, blocks, lapilli, etc.) or other structure. Thus, a flow- specific hydration rate is the average hydration rate for a particular geographical and geological structure. The use of geographical terms to further define the flow is the most preferable approach if available, e.g., West Sugarloaf flow-specific rate, Stigarloaf Mountain flow- specific rate, etc. SCIENTIFIC BASIS FOR FLOW-SPECIFIC RATES New evidence indicates that flow-specific hydra- tion rates may lead to the refinement of obsidian hydration dating in California. The scope of the authors earlier research was limited to source-specific empirical obsidian hydration rates which combined artifacts from different flows as one source (Ericson 1977; 1981). New evidence presented herein indicates that most Pleistocene and recent volcanic fields in California have multiple flows and other volcanic structures which provided obsidian to prehistoric Indian groups (Ericson, Hagan and Chesterman 1976). The scientific basis for flow-specific obsidian hydration rates within a single obsidian source can be documented by appeal to the effects of chemical and physical parameters on the rates of hydration (Ericson 1981). The reproducibility of the same chemical and physical parameters for each eruptive event within a volcanic field is considered to be relatively low - that was apparent from initial stages of field work noted in 1970 (Ericson, Hagan and Chesterman 1976). There may be important variations of properties among flows which may affect the hydration rates; 1) the intrinsic water content, a known variable of hydration (Ericson 1977; 198 la, b), may vary tremendously between within eruptive events; 2) chemical interaction of two magmatic melts such as rhyolitic and basaltic or rhyolitic and andesitic may cause changes in the chemistry, e.g., Modoc Glass Mountain; 3) differential remelting of different host rocks forming the magma chamber and vents may cause chemical variations; 4) phase separation by differential settling of crystals within the magma chamber or flow may cause vari- ations in the chemistry between events; 5) the melting of the contact surface during eruption may change the obsidian chemistry of the flow. Each of these factors, uniquely or in combination with other factors, can alter the chemical composition of the obsidians within a volcanic field. In addition, physical factors like viscosity and temperature of the melt may cause variations among obsidians. In this case, variations may occur as a reuslt of; 1) differential release of water from the obsidian structre; 2) differential crystallization both of the amount and type of crystals formed; 3) degree of annealing and degree of mechanical srss; 4) the amount and type of glass phase separation in the obsidian; 5) the conditions of cooling history of the obsidian. It is to be expected that flow-specific obsidian hydration rates will vary, significandy in some cases, given the above chemical and physical parameters which will change the propeties of obsidian flows within a volcanic field. The Coso volcanic field seemed to be a good target for further study given initial data. 14 Flow-Specific Obsidian Hydration Rates, Coso Volcanic Field TABLE 1 POTASSIUM-ARGON AGES OF OBSIDIAN FLOWS, COSO VOLCANIC FIELD, CALIFORNIA, REFERRED TO IN FIGURE 1 Flow Name West Sugarloaf Sugarloaf Mountain Age (Myrs.) 1.08 + .06 0.044 ? .022 n.d. 0.093 ? .026 n.d. 0.99 + .12 or 0.244 + .028 RESEARCH DESIGN AND OBJECTIVE For the objectives of this research- investigation of potential variability in the hydration rates for obsidian flows in the Coso volcanic field- the laboratory-induced hydradon experiments were considered the best experimental approach to control temperature, pressure, and other variables of hydration. If significant differ- ences were observed among the flows in the Coso volcanic field, it would support the development of a set of flow-specific hydration rates. THE STUDY AREA AND THE SAMPLES The Coso volcanic field has been the focus of the geological study for some time (Ross and Yates 1943; Chesterman 1956), and recently the volcanic field has been studied to establish its geothermal potential (Austin and Pringle 1970; Lanphere et al. 1975;Bacon etal. 1981; 1982). On the basis of a cursory petrogra- phic examination without chemical analyses, Lanphere etal. (1975) suggest that a single rhyolitic magma of rlatively homogeneous composition fed all domes. More recently, Bacon et al. (1981) indicate that seven geochemically distinctive eruptive episodes occurred within the Pleistocene of the Coso volcanic field. Granodiorite and quartz monzonite of the Sierra Nevada batholith form the bedrock of the Coso volcanic field (Lanphere et al. 1975; Duffield and Bacon 1981). The geologic ages of the domes and flows determined by potassium argon dating are presented in Table 1. Several research trips were conducted in 1981-82 to resurvey the volcanos in the Coso area. Pilot Knob to the southeast of Coso and Jawbone Canyon to the southwest were surveyed without success in locating viable obsidian. Pilot Knob had altered to perlite and was too brittle for tool manufacture, and the Jawbone Canyon "obsidian" source reported by Ericson et al. (1976) was not relocated. Apache tears from Fort Irwin were provided by Mr. Russell Kaldenberg of the Bureau of Land Management. He mentioned that a new obsidian source had been found some distance away at Mid Hills, but this source was not investigated. A second tip (1982-1983) to the Coso volcanic field was more extensive than reported earlier (Ericson et al. 1976). Multiple sources of obsidian and prehistoric workshops were observed, and flow samples were collected for experimental purposes (see Figure 1). Dr. Kenneth Pringle, China Lake Naval Weapons Center and Clay A. Singer of Santa Monica, CA accompanied the author in the field. The flow of West Sugarloaf (Hughes 1988) is Flow 2-1. The dome of Sugarloaf (Hughes 1988) is Flow 2-2, which was sampled at two locations. EXPERIMENTAL PROCEDURES The obsidian samples were cut into 1 cm cubes with a lapidary diamond saw and assigned a random code for the double-blind experiments. Obsidian cubes were fractured into halves, labeled, weighed, washed in doubly deionized water, and placed upright at the base of a 23 ml Teflon container of a Parr reaction vessel. Flow No. 2-1 2-2 2-3 2-4 2-5 2-6 Location Southern Southeem Central Central Central Westem is Contributions of the Archaeological Research Facility Number 48, December 1989 For experiment 3, one-half cube of each obsidian sample was placed with 14 ml doubly deionized water in the Parr reaction vessel for the two-phase system test. Twenty-five Parr reaction vessels were loaded for each run. Each vessel was placed in at a predeternined shelf position in a programmable oven pre-set at 174 degrees + 1 degree C for 137 hours. For experiment 5, the matched half cube was placed with approximately 5 drops of doubly deionized water, to provide a saturated vapor environment without excess water. The actual amount of water was pre-calculated from volume measurements of the sample and reaction vessel and weighted to + 0.00005 gm accuracy. The shelf position of the Parr reaction vessels, time of hydration and temperature were reproduced for experiments 3 and 5. Following the procedures outlined above, additional obsidian samples were hydrated in the oven at 174 + 1 degree C for 88 hours and 240 hours. The hydrated surface of the experimental samples was prepared by cutting a slice from the cube, grinding one cut surface with aluminum oxide grits (nos. 95 and 400) on glass plates, mounting the ground surface on a pre-labeled flat (pre-ground) petrographic glass slide with epoxy cement, grinding the surface down to 75 micron thickness using optical birefringence colors of crystal inclusions, and covering with cover glass and Canada balsam. Using these prepared slides, the hydration bands were measured for the induced hydration bands, which were measured on a Vickers split image system pre-calibrated with a stage microme- ter mounted on a petrographic microscope. The results of the experiments are presented in Tables 2 and 3. EXPERIMENTAL RESULTS AND ANALYSIS The hydration measurement data, presented in Table 2, were used in a matched pairs difference test to determine whether there is a significant difference if the system is single-phase (saturated vapor only) or two- phase (liquid plus vapor). The null hypothesis of this statistical test was that the mean of the differences was zero or (Ho: j1d= 0). The critical value of t for the two- tailed statistical test is 2.064 ( at the 0.05 alpha-level) with 24 degrees of freedom. The matched pairs test was performed on the data. It was found that t = 1.674 where d = 0.2972, n = 25, and Sd= 0.8876. Thus, the t- test results were not significant, given these experimen- tal data. It was concluded that the hydration process was not significantly different whether the system was single-phase (saturated vapor) or two-phase (vapor and liquid). The data presented in Table 3 were grouped by flow. The standard errors of the means were calculated for each experiment for each flow. West Sugarloaf (Flow 2-1), Sugarloaf Mountain (Flow 2-2), the northerly flows (Flows 2-3 and 2-5) have standard errors below 16% and appear to have excellent internal consistency. Flows 2-4 and 2-6 to the north and northwest appear to have greater internal variability relative to hydration. Further research may reveal why this variability was observed. Hydration data were plotted in Figure 2. It is clear from this figure that the hydration rates for West Sugarloaf (Flow 2-1) and Sugarloaf Mountain (Flow 2- 2) are very similar. Based on calculating chemical indexes (Friedman and Long 1976) derived from data provided by Bacon et al. 1981 (Table 1a-d), Hughes (1988) predicted that the rate of hydration for West Sugarloaf should be slower than Sugarloaf Mountain. However, the experimental data generated here do not support a signiflcant difference in hydration rates between the two flows. The northerly flows (Flows 2-3 and 2-5) appear rather similar to each other, and their rates are approximately 40% faster than the West Sugarloaf and Sugarloaf Mountain rates. The north- westerly flows (Flow 2-4 and Flow 2-6), which appear to have more variable hydration rates, are intermediate between West Sugarloaf/Sugarloaf Mountain and the northerly flows (Flows 2-3 and 2-5). DISCUSSION Although the results of this research are prelimi- nary, the findings are significant. It does appear that induced hydration experiments can be used to evaluate whether obsidians from different flows will hydrate at different rates. In cases where inter-flow variation is significant, flow-specific rates will have to be deter- mined using archaeological data. Whether laboratory- induced hydration rates will be effective and accurate enough for archaeological application is conjecture at this point. Preliminary evaluation of the laboratory- induced hydration rates presented so far for Coso (Michels 1983; Stevenson and Scheetz [this volume]) suggests that discrepancies exist relative to archaeologi- cal data (Meighan 1981; 1983; Elston and Zeier 1984). The experimental results suggest that intra-flow variation of hydration can be evaluated, but it may be negotiable or variable depending on homogeneity of the obsidian. 16 Flow-Specfc Obsidian Hydradon Rates, Coso Volcanic Field TABLE 2 HYDRATION RESULTS IN MICRONS OF ISOTHERMAL MATCHED SAMPLE PAIRS IN SATURATED VAPOR AND - VAPOR/LIQUID PHASES (174 DEGREES C) Exp. 3 Vapor/Liquid 6.77 ? .14 5.99 ? .06 4.73 ? .06 4.62? .08 4.94 + .16 4.72 ? .11 3.97 + .09 5.72 + .09 6.49 ? .06 6.27 ? .08 6.24 + .08 6.87 ? .08 6.84 ? .08 4.91 +.15 5.09 ? .11 4.41+.09 4.97 + .07 6.24 ? .15 5.78 ? .06 7.93 ? .12 7.17 ? .08 6.19 ? .14 6.28 ? .05 6.86 + .08 6.31?.07 Exp. 5 Satuated Vapor 7.41+.28 5.63 + .08 4.52 ? .09 5.40 ? .14 4.63 + .12 5.94 + .08 3.34 + .14 4.86?.07 6.73 + .09 7.00? .20 7.55 + .07 6.78 + .13 6.83 ? .06 5.64 ? .05 5.31?.05 4.26?.09 5.22+ .04 6.88 ? .19 6.91?.10 7.39 + .05 6.12+ .10 6.09+ .08 6.77 ? .07 10.16 ? .23 6.37 ? .06 Blind Code A B C D E F G H I K L M N 0 p Q R S T U V w x y Sample Field No. 2-3-3-5 2-3-1-3 2-2-1-2 2-2-2-2 2-1-0-2 2-1-0-3 2-4-1-3 2-4-1-2 2-5-1-3 2-5-1-1 2-6-1-5 2-6-1-3 2-4-1-5 2-1-0-5 2-1-0-5 2-2-2-1 2-2-1-3 2-6-1-1 2-6-1-2 2-5-1-2 2-5-1-5 2-3-1-4 2-3-2-5 2-3-2-4 2-3-3-4 17 Contributions of the Archaeological Research Faciity Number 48, December 1989 TABLE 3 OBSIDIAN HYDRATION MEASUREMENTS IN MICRONS GROUPED BY FLOW FOR THREE EXPERIMENTAL PERIODS FOR THE COSO VOLCANIC FIELD Sample Members 0E,F,N C,Q,P,D B,V,X,W,Y,A H,G,M J,T,I,U R,S,L,K Sample Members 0,E,F,N C,Q P,D B,V,X,W,Y,A H,G,M J,T,I,U R,S,L,K Sample Members 0E,F,N C,Q,P,D B,V,X,W,Y,A H,G,M J,T,I,U R,S,L,K Group Hydration (88 hours) 3.72 + 0.22 3.32 + 0.53 5.12 + 0.25 4.12+ 1.22 5.04 +0.65 4.34 + 0.48 Group Hydration (137 hours) 4.92 + 0.15 4.68 +0.23 6A0 + 0.34 5.51+ 1.45 6.97 + 0.75 6.28 + 0.45 Group Hydration (240 hours) 6.49 + 0.99 6.20 + 0.33 9.24 + 0.88 6.77 + 0.46 8.63 +0.60 7.15 ? 1.45 Flow No. 2-1 2-2 2-3 2-4 2-5 2-6 Flow No. 2-1 2-2 2-3 2-4 2-5 2-6 Flow No. 2-1 2-2 2-3 2-4 2-5 2-6 Coeff. Var. 6. 16. 5. 29. 13. 11. Coeff. Var. 3. 5. 5. 26. 11. 7. Coeff. Var. 15. 5. 10. 7. 7. 20. 18 Flow-SpecfIc Obsidaia Hydration Rates, Coso Volcanic Field FIGURE 1 THE STUDY AREA SHOWING SAMPLE LOCATIONS WITHIN THE COSO VOLCANIC FIELD (AFTER DUFFIELD AND BACON 1981) T.21 CACTUS PEAK - . 1(5 4P4, <^ # II _ _ I -..LOA e_* IOUNTAI , ' I, -d - T.22 / II 3 4 5 S Miles It wv xoutW rVet n 19 0 %F I v oftliumelarw I I I/ --- i I I I f I I I N OAIU% n I t S g Contribudons of the Archaeological Research Facdii Number 48, December 1989 FIGURE 2 PLOTS OF FLOW-SPECIFIC HYDRATION MEASUREMENTS AND EXPERIMENTAL TIME CONCLUSIONS AND RECOMENDATIONS At present, researchers routinely apply source- specific hydration rates to obsidian artifacts whose source has been determined by chemical characteriza- tion. This preliminary study suggests that flow-specific rates may improve the accuracy of obsidian hydration dating for the Coso source; preliminary findings for the Coso volcanic field suggest that flow-specific rates may vary as much as 40%. There are numerous sore- specific hydration rates for Coso obsidian in the literature, most derived from available archaeological data. hese will have to be reevaluated, given these new findings. The variation of these published rates may, in part, be explained by combining data which should have been stratified, given the operation of flow- specific hydration rates. The role of environmental variables, other than effective temperature, in explain- ing variations of hydration rates, remains to be re- solved. This paper is offered in the hope that the findings herein will lead to the refmement of obsidian hydration dating. The determination of flow-specific hydration rates can be implemented in three reseach phases: 1) laboratory-induced obsidian hydration experiments can identify the range of rates within complex volcanic fields with multiple flows or eruptive events. Whether these rates are accurate enough for chronological application will have to be determined by long-term archaeological evaluation; 2) if significant rate vari- ations are found, chemical characterizaon will have to discriminate and identify the obsidian flow of each artifacLt Here, recent refinements in chemical charac- terizaton by x-ray fluorescence analysis (Hughes 1988) and long half-life instrumental neutron activation analysis (Ericson and Kimberlin, n.d.) provide the techniques to identify specific flows within complex volcanic fields. Quarry analysis (Ericson and Purdy 1984) may reveal Xt a single flow was the sole source for obsidian in a complex volcanic field; 3) hydration measurements of obsidian artifacts from specific flows combined with associated radiocarbon dates can be used to determine flow-specific hydration rates. Even with the above controls, obsidian hydration rates may vary due to other environmental variables that have not been considered heretofore. Understanding these new variables may be the ultimate step in refinement of the obsidian hydration dating technique. A new protocol is being developed which will improve evaluation of environmental variation effects (Ericson 1988). Cl o '0 2-1 y= 0.56 + 0.18x R2= 0.998 E 60 - 2-2 y= 1.11 + 0.16x R2= 0.992 o 2-3 y= 3.15 + 0.36x R2= 0.988 U 2-4 y= 0.87 + O.l9x R2= 0.989 W 40 8 Z a 2-5 y= 0.34 + 0.32x R2= 0.988 ? 0 2-6 y= 1.76 + 0.22x R2= 0.950 00 200 TIME - HOURS 20 Flow-Specflc Obsidian Hydration Rates, Coso Volcanic Field ACKNOWLEDGMENTS The author would like to thank Dr. Chester King of Topanga for accompanying him on the 1981-82 Obsidian Expedition, Clay A. Singer of Santa Monica, California, for assistance in the field, and Dr. J. Kemeth Pringle of China Lake Naval Weapons Center for arranging access to and transportation within the Coso volcanic area in 1982-83. I would like to thank Candice Hooper of C.A.R.D., Harvard University, for measuring the hydration, Professor Joseph W. Michels, Pemsylvania State University, for training Candice on the Vicker's split image system, and Marco Van Gameran of Harvard University, Geology, for preparing this section. I would also like to thank Donor Landon T. 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