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15 Abstract Obsidian samples from three separate volcanic provinces in Papua New Guinea have been exposed in normal air and water vapor pressure to controlled temperatures of 10 ºC, 20 ºC, 30 ºC, and 40 ºC, for up to 26 years. The purpose of the experiment was to provide hydration rate constants at normal terrestrial temperatures for archaeological dating purposes. When the experiment began, nuclear profiling was being used to measure obsidian hydration depths to below the approximately 0.7 μm thickness and approximately 0.2 μm resolution for conventional optical methods. This was seen as an opportunity for determining hydration rates for obsidian artifacts without the use of radiocarbon dates from a specific archaeological site or stratigraphic location, that being the usual practice at the time. The trial was not intended to continue for 26 years but the promise of nuclear profiling kept it running. Greatly improved instrumental analysis through secondary ion mass spectrometry, SIMS, has now allowed hydration depth and resolution measurements to be made below the optical threshold, and therefore over relatively short exposure times and at normal terrestrial temperatures, including those applied in the present study. The results confirm the value of the strategy by providing basic hydration rate constants for obsidian specimens from each of the three major Papua New Guinea sources. In an archaeological context these data still require calibration for optical microscopy hydration measurement. SIMS provides an enhanced measurement system and is combined here with experimentally determined rate constants to independently cross-check radiocarbon dates and site temperature calculation at an approximately 2100 BP archaeological site from Papua New Guinea. 1. Introduction Obsidians from the Papua New Guinea sources of West New Britain and Manus (figure 2.1) have the distinction of a distributional range of over 6000 kilometers (km) from Borneo to Fiji between 2000 and 3000 years ago. Within these two main source regions, obsidian has been used for at least 30,000 and 12,000 years respectively, while obsidians from the D’Entrecasteaux Islands have a briefer history of use and a shorter distributional range. The chemistry of the Papua New Guinea obsidian sources has been extensively analyzed during the last 40 years by most of the standard procedures from XRF, INAA, PIXE-PIGE (proton-induced X-ray emission– proton induced gamma-ray emission), SEM (scanning electron microscopy), and LA-ICP-MS so that the main outline of the changing distributional range of obsidians as artifacts over time can be presented (Summerhayes 2004). The chronology of obsidian source exploitation in Melanesia is usually found from archaeologically associated radiocarbon dates (Fullagar and Torrence 1991; Summerhayes 2004; Torrence and Summerhayes 1997); in contrast, hydrationdatinghasbeenusedonalimitedscaleemploying Chapter 2 Obsidian Hydration Chronometrics Using SIMS and Optical Methods from 26-year Temperaturecontrolled Exposures Wallace Ambrose and Steven W. Novak 16 | OBSIDIAN HYDRATION DATING standard microscopy for the basic hydration depth measurement (Torrence and Stevenson 2000) and digital microscopy (Ambrose 1994; Ambrose and McEldowney 2000). The tropical climate of the region can promote rapid obsidian hydration but also brings aggressive surface weathering that can reduce the developing hydration depth, sometimes with its complete removal. This reduction is not a problem confined to tropical environments, but by being more obvious it does emphasize the need for weathering losses to be taken into account in any program of obsidian hydration dating. 2. Empirical rate determination The well-known modifiers of obsidian hydration rate are temperature, chemical composition, and water vapor pressure. While temperature and vapor pressure can be independently related to rate changes (Jones et al. 1997; Rogers 2007), the effect of chemical composition on hydration velocity is not so easily found. General element oxide contributions to the hydration rate have been indicated (Liritzis 2006; Liritzis and Ganetsos 2006), and in particular the crucial role of compositional water has been underlined (Stevenson et al. 1993, 1998) and reaffirmed (Stevenson and Novak 2011). However, obsidians still need to be independently assessed for specific rate determination to validate these models. Four obsidian sources from three source islands in Papua New Guinea were exposed to normal water vapor pressure in controlled temperatures within the normal terrestrial range of 10 ºC, 20 ºC, 30 ºC, and 40 ºC for up to 26 years in order to find rate constants for samples of these chemically different sources (table 2.1). The samples included are from West New Britain (Mt. Bao), from West Fergusson Island in the D’Entrecasteaux group (Igwageta), and from Lou Island, Manus (Umleang and Wekwok). 3. Method Several small obsidian flakes...

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