Obsidian hydration relative dating
However, the effect of the lower mean relative humidity experiences by surface samples tends to compensate for the elevated temperature, which may explain why obsidian hydration ages of surface samples usually approximate those derived from buried samples.Obsidian hydration dating typically yields a range of ages for a single chronometric measurement, even after controlling for source chemistry and effective hydration temperature.
When an unworked nodule of obsidian is initially fractured, there is typically less than 1% water present..ti~.crtior~ tit Pot C7Piiehlo itidic~utr rirtrt u heti etfc~c~rii~i, teniper.crrrrrc~ vcrrier sigtiifictitirl~ it,ith iirpth, c.vvovs in ohsriiitrti h? if ~~drutioti to cic.~)tli-.\j~ec~r/;c~ mte c~oti.stcrtir.s irre e~.rtrtip~ltrfrii e/ikcri~,e hj~iircrrioti trrnpertrtuluc This p~ohirnr u,i/l hr iilir* \i2r iri c~otitrtietitul ciiniuti~\. As the Pot Creek data will show, if an artifact has been buried over time (instead of very sud- denly), even an accurate EHT for the depth of recovery may not be representative of the hydra- tion history of that artifact. hut n.i I1 htr1.e .ronie rffkc,t crti~1r the trrnplituiic. the technique has become more complicated and expensive with increased understanding of potential errors, and development of analytical procedures to constrain them. obsidian hydration may still have potential as a valuable chronometric tool. In contrast, in areas where the daily and seasonal temperature varia- tions are small, such as near the Equator, varia- tions in effective hydration temperature (i.e., val- ues of T' ) will also be small. Chemical analysis of their trace elements now allows most of the sources to be distinguished (especially by neutron activation and x-ray fluorescence spectrometry), so that the pattern of trade spreading out from each can be traced.Two dating methods have been applied to obsidian: it generally went out of favor for everyday purposes (perhaps as a result of competition from metal tools) but it continued to be used for prestige objects in some areas, especially by the Minoans and Mycenaeans.Over time, water slowly diffuses into the artifact forming a narrow "band," "rim," or "rind" that can be seen and measured with many different techniques such as a high-power microscope with 40-80 power magnification b) depth profiling with SIMS (secondary ion mass spectrometry), and IR-PAS (infra red photoacoustic spectroscopy).
In order to use obsidian hydration for absolute dating, the conditions that the sample has been exposed to and its origin must be understood or compared to samples of a known age (e.g.
oftlie irr~r~~rtil 1e c~~c~i~i~iis cirvfcrc,e trr~ipc~rsrrlirt~ 2 jn(. qtre rc.sultcrti c~rrirr~do drtc.rnirtitrc~iorii.\cjri i.1 1(it. Itr\ cori.\ttrtitrs iitriizcrciti\ rri 111 c.c~rtirititi ciez dr.r.hetii~rs riginally, obsidian hydration dating was very attractive as an easy, inexpensive, yet powerful chronometric tool, particularly in regions where obsidian was abundant, but also where sites did not yield other dateable materials (Michels 1967; Michels and Bebnch 1971). if its lim- itations are considered realistically (Freter 1993 ). Since thermal cells have become more readily available. This contrast will be illustrated in the final section of this paper, using examples of effective hydration temperatures for Cheyenne, Wyoming, and Nairobi, Kenya.
Itivrvtigcrcrr~r~es(itccrmpo iicrti prociricrdo c~i,it Ii~ri~~itr cir el.l,ol.e\ poreric~itiic~\ c Jr7 la ti.c,~~icer tic. In spite of research efforts spanning four decades, archaeo- logical applications of obsidian hydration as an independent dating technique have not all been suc- cessful (e.g.. Recent efforts in technical research ha\,e focused on (I) digitizing hydration rind measure- ments by computer imaging (Ambrose 1993): (2) identifying heterogeneities in obsidian sources that may affect hydration rate constants (Hughes 1994: Stevenson et al. concern over the accuracy of site temperature estimates has decreased. Pot Creek was selected for a field comparison of the methods for estimating effective hydration temperature because of its well-developed den- drochronology, in addition to the presence of obsidian artifacts.
We have found that the thickness of the hydrated layer developed on obsidian outcrops exposed to the sun and to relative humidities of 30-90% is similar to that formed on other portions of the outcrop that were shielded from the sun and exposed to a relative humidity of approximately 100%.
Surface samples of obsidian exposed to solar heating should hydrate more rapidly than samples buried in the ground.
Obsidian breaks with a and is easily chipped into precise and delicate forms.