Effective hydration temperature of obsidian: a diffusion theory analysis of time-dependent hydration rates

Archaeological Age Estimation Based on Obsidian Hydration Data for Two Southern Andean Sources

“…In 1960, Friedman and Smith observed that a freshly exposed surface of obsidian takes on ambient water at a knowable rate that can be used to calculate the time elapsed since exposure and, therefore, the date of an obsidian artifact's production (Doremus 1995(Doremus , 2002Friedman and Smith 1960;Rogers 2007;Stevenson et al 1998). Subsequent research has clarified our understanding of the "diffusion-reaction" hydration process (Doremus 2002;Rogers 2008a;cf.…”

“…While such data are best collected using temperature cells buried at the site of interest, they can also be approximated using temperatures measured at a surrogate site, adjusted according to an adiabatic lapse rate. Since temperature also varies with depth below ground surface, the correction factor applied to each observed hydration rim value includes an adjustment for depth (Riddings 1996;Rogers 2007;Rogers and Yohe 2011). However, no known approach accounts for prehistoric conditions; significant, long-term climate changes (e.g., Medieval Warm Period); or temperature histories affected by sediment turbation (see Rogers 2010a; Rogers and Yohe 2011).…”

“…The rate at which water diffuses into glass can be determined by either of two methods: (1) empirically, by assessing the relationship between hydration depth (however it is measured) and the 14 C dates with which the measured samples are clearly associated; or (2) by inducing hydration in a laboratory and calculating a hydration rate based on the activation temperature and Arrhenius equation for reaction kinetics (Friedman and Long 1976;Friedman and Trembour 1983;Rogers 2007;Rogers and Duke 2011;Stevenson and Novak 2011;Stevenson and Rogers 2014). Laboratory-induced rates, while potentially more precise and less timeconsuming, have only recently been shown to reliably predict the ages of artifacts independently dated by radiocarbon or other methods (Ambrose and Novak 2012;Rogers and Duke 2011;cf.…”

“…Anovitz et al 1999;Rogers 2006). Empirical rates, on the other hand, generally take longer to establish and may be more subject to error, but provide reasonable predictive accuracy within the range of rim readings and radiocarbon ages used to generate the equations (Rogers 2007(Rogers , 2008(Rogers , 2010b. Friedman and Long (1976) proposed that obsidian hydration proceeds as the square root of time and that the time elapsed since an obsidian surface was last exposed could be estimated by the hydration equation…”

“…where x is hydration rim thickness measured in microns (x preferred here to the more common r to avoid confusion with the coefficient of determination, R 2 ), D is the hydration constant, and t is time in thousands of years, typically measured by radiocarbon (Anovitz et al 1999;Friedman and Long 1976;Rogers 2007). The hydration constant, D, is a function of temperature, and its value must be estimated before obsidian hydration can be used as a chronometer.…”