Obsidian hydration dating
Encyclopedia
Obsidian hydration dating (OHD) is a geochemical
method of determining age in either absolute or relative terms of an artifact
made of obsidian
.
Obsidian is a volcanic glass
that was sometimes used as a raw material in the manufacture of stone tools such as projectile points, knives, or other cutting tools through the process of flintknapping
. Obsidian obeys the property of mineral hydration
, and absorbs water
when exposed to air. When an unworked nodule of obsidian is initially fractured, there is typically less than 1% water present. 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) a high-power microscope
with 40-80 power magnification
b) depth profiling with SIMS secondary ion mass spectrometry
. 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. as a result of radiocarbon dating
of associated materials.)
and Robert Smith of the U.S. Geological Survey . Their initial work focused on obsidians from archaeological sites in western North America.
The use of Secondary ion mass spectrometry
(SIMS) in the measurement of obsidian hydration dating was introduced by two independent research teams in 2002.
Today the technique is applied extensively by archaeologists to date prehistoric sites and sites from protohistory
in California
and the Great Basin
of North America. It has also been applied in South America, the Middle East, the Pacific Islands, including New Zealand and Mediterranean Basin.
outfitted with some method for measuring distance, typically in tenths of micrometers. The technician measures the microscopic
amount of water absorbed on freshly broken surfaces. The principle behind obsidian hydration dating is simple–the longer the artifact surface has been exposed, the thicker the hydration band will be.
is known to speed up the hydration process. Thus, artifacts exposed to higher temperatures, for example by being at lower elevation
, seem to hydrate faster. As well, obsidian chemistry, including the intrinsic water content, seems to affect the rate of hydration. Once an archeologist can control for the geochemical signature of the obsidian (e.g., the "source") and temperature (usually approximated using an "effective hydration temperature" or EHT coefficient), he or she may be able to date the artifact using the obsidian hydration technique. Water vapor
pressure may also affect the rate of obsidian hydration.
The reliability of the method based on Friedman’s empirical age equation (x
=kt, x the hydration rim, k the diffusion coefficient and t the time) is questioned from several grounds regarding temperature dependence, square root of time and determination of diffusion rate per sample and per site, apart of some successful attempts on the procedure and applications.
Several commercial companies and university laboratories provide obsidian hydration services.
Geochemistry
The field of geochemistry involves study of the chemical composition of the Earth and other planets, chemical processes and reactions that govern the composition of rocks, water, and soils, and the cycles of matter and energy that transport the Earth's chemical components in time and space, and...
method of determining age in either absolute or relative terms of an artifact
Artifact (archaeology)
An artifact or artefact is "something made or given shape by man, such as a tool or a work of art, esp an object of archaeological interest"...
made of obsidian
Obsidian
Obsidian is a naturally occurring volcanic glass formed as an extrusive igneous rock.It is produced when felsic lava extruded from a volcano cools rapidly with minimum crystal growth...
.
Obsidian is a volcanic glass
Glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...
that was sometimes used as a raw material in the manufacture of stone tools such as projectile points, knives, or other cutting tools through the process of flintknapping
Flintknapper
Knapping is the shaping of flint, chert, obsidian or other conchoidal fracturing stone through the process of lithic reduction to manufacture stone tools, strikers for flintlock firearms, or to produce flat-faced stones for building or facing walls, and flushwork decoration.- Method :Flintknapping...
. Obsidian obeys the property of mineral hydration
Mineral hydration
Mineral hydration is an inorganic chemical reaction where water is added to the crystal structure of a mineral, usually creating a new mineral, usually called a hydrate....
, and absorbs water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
when exposed to air. When an unworked nodule of obsidian is initially fractured, there is typically less than 1% water present. 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) a high-power microscope
Microscope
A microscope is an instrument used to see objects that are too small for the naked eye. The science of investigating small objects using such an instrument is called microscopy...
with 40-80 power magnification
Magnification
Magnification is the process of enlarging something only in appearance, not in physical size. This enlargement is quantified by a calculated number also called "magnification"...
b) depth profiling with SIMS secondary ion mass spectrometry
Secondary ion mass spectrometry
Secondary ion mass spectrometry is a technique used in materials science and surface science to analyze the composition of solid surfaces and thin films by sputtering the surface of the specimen with a focused primary ion beam and collecting and analyzing ejected secondary ions...
. 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. as a result of radiocarbon dating
Radiocarbon dating
Radiocarbon dating is a radiometric dating method that uses the naturally occurring radioisotope carbon-14 to estimate the age of carbon-bearing materials up to about 58,000 to 62,000 years. Raw, i.e. uncalibrated, radiocarbon ages are usually reported in radiocarbon years "Before Present" ,...
of associated materials.)
History
Obsidian hydration dating was introduced in 1960 by Irving FriedmanIrving Friedman
Irving Friedman was a U.S. Geological Survey scientist and a pioneer in geochemistry. Born in New York City, New York, he obtained a B.S. degree in chemistry from Montana State University, a M.S. degree in chemistry from Washington State University, and a Ph.D...
and Robert Smith of the U.S. Geological Survey . Their initial work focused on obsidians from archaeological sites in western North America.
The use of Secondary ion mass spectrometry
Secondary ion mass spectrometry
Secondary ion mass spectrometry is a technique used in materials science and surface science to analyze the composition of solid surfaces and thin films by sputtering the surface of the specimen with a focused primary ion beam and collecting and analyzing ejected secondary ions...
(SIMS) in the measurement of obsidian hydration dating was introduced by two independent research teams in 2002.
Today the technique is applied extensively by archaeologists to date prehistoric sites and sites from protohistory
Protohistory
Protohistory refers to a period between prehistory and history, during which a culture or civilization has not yet developed writing, but other cultures have already noted its existence in their own writings...
in California
California
California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...
and the Great Basin
Great Basin
The Great Basin is the largest area of contiguous endorheic watersheds in North America and is noted for its arid conditions and Basin and Range topography that varies from the North American low point at Badwater Basin to the highest point of the contiguous United States, less than away at the...
of North America. It has also been applied in South America, the Middle East, the Pacific Islands, including New Zealand and Mediterranean Basin.
Conventional procedure
To measure the hydration band, a small slice of material is typically cut from an artifact. This sample is ground down to about 30 micrometers thick and mounted on a petrographic slide. The hydration rind is then measured under a high-power microscopeMicroscope
A microscope is an instrument used to see objects that are too small for the naked eye. The science of investigating small objects using such an instrument is called microscopy...
outfitted with some method for measuring distance, typically in tenths of micrometers. The technician measures the microscopic
Microscopic
The microscopic scale is the scale of size or length used to describe objects smaller than those that can easily be seen by the naked eye and which require a lens or microscope to see them clearly.-History:...
amount of water absorbed on freshly broken surfaces. The principle behind obsidian hydration dating is simple–the longer the artifact surface has been exposed, the thicker the hydration band will be.
Secondary Ion Mass Spectrometry (SIMS) procedure
In case of measuring the hydration rim using the depth profiling ability of the secondary ion mass spectrometry technique, the sample is mounted on a holder without any preparation or cutting. This method of measurement is non-destructive.Limitations
Several factors complicate simple correlation of obsidian hydration band thickness with absolute age. TemperatureTemperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
is known to speed up the hydration process. Thus, artifacts exposed to higher temperatures, for example by being at lower elevation
Elevation
The elevation of a geographic location is its height above a fixed reference point, most commonly a reference geoid, a mathematical model of the Earth's sea level as an equipotential gravitational surface ....
, seem to hydrate faster. As well, obsidian chemistry, including the intrinsic water content, seems to affect the rate of hydration. Once an archeologist can control for the geochemical signature of the obsidian (e.g., the "source") and temperature (usually approximated using an "effective hydration temperature" or EHT coefficient), he or she may be able to date the artifact using the obsidian hydration technique. Water vapor
Water vapor
Water vapor or water vapour , also aqueous vapor, is the gas phase of water. It is one state of water within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice. Under typical atmospheric conditions, water vapor is continuously...
pressure may also affect the rate of obsidian hydration.
The reliability of the method based on Friedman’s empirical age equation (x
=kt, x the hydration rim, k the diffusion coefficient and t the time) is questioned from several grounds regarding temperature dependence, square root of time and determination of diffusion rate per sample and per site, apart of some successful attempts on the procedure and applications.Several commercial companies and university laboratories provide obsidian hydration services.
See also
- Dating methodology (archaeology)Dating methodology (archaeology)Dating material drawn from the archaeological record can be made by a direct study of an artifact or may be deduced by association with materials found in the context the item is drawn from or inferred by its point of discovery in the sequence relative to datable contexts...
- Absolute datingAbsolute datingAbsolute dating is the process of determining an approximate computed age in archaeology and geology. Some scientists prefer the terms chronometric or calendar dating, as use of the word "absolute" implies an unwarranted certainty and precision...
- Relative datingRelative datingRelative dating is the science determining the relative order of past events, without necessarily determining their absolute age.In geology rock or superficial deposits, fossils and lithologies can be used to correlate one stratigraphic column with another...
- Water vaporWater vaporWater vapor or water vapour , also aqueous vapor, is the gas phase of water. It is one state of water within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice. Under typical atmospheric conditions, water vapor is continuously...
- Water vapor feedback
External links
- National Park ServiceNational Park ServiceThe National Park Service is the U.S. federal agency that manages all national parks, many national monuments, and other conservation and historical properties with various title designations...
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