NEWS RELEASE #14618, 8/28/97
Precise dating of the destruction of Pompeii proves argon-argon method can reliably date rocks as young as 2,000 years
Berkeley -- A powerful geologic dating technique called argon-argon dating has pegged the 79 A.D. eruption of Vesuvius so precisely that it establishes one of the most solid and reliable anchors for any dating method.
With such validation, the radioactive argon dating technique now can reliably establish the age of rocks as old as the solar system or as young as 2,000 years, say researchers from the University of California at Berkeley and the Berkeley Geochronology Center.
"Argon-argon dating is by far the most important technique in documenting the history of human evolution, and this new result is an important validation of the technique," says Paul Renne, adjunct associate professor of geology and geophysics at UC Berkeley and director of the privately funded Berkeley Geochronology Center.
The center has used the argon-argon method to date many recent important fossil finds, from the highly touted human ancestor dubbed "Lucy" and the major Ethiopian discoveries of UC Berkeley anthropologist Tim White to Homo erectus remains from Java.
Argon-argon dating also has been used to establish the age of meteorites several billion years old, mass extinctions, climate changes and other geologic events in the last several hundred million years.
The new results are published in the Aug. 29 issue of Science magazine. Renne's co-authors are Warren D. Sharp and Alan L. Deino of the Berkeley Geochronology Center, and Giovanni Orsi and Lucia Civetta of the Department of Geophysics and Vulcanology at the University of Naples. Civetta also is head of the Vesuvian Vulcanological Observatory.
Orsi and Civetta are working with the center to obtain argon-argon dates for numerous past volcanic eruptions in the Campi Flegrei or Phlegraean Fields surrounding Naples, in search of clues to the periodicity of activity that might allow prediction of future eruptions. Naples and vicinity, with more than two million inhabitants, is one of the world's most vulnerable populations to volcanic hazard.
According to the Roman historian Pliny the Younger, Vesuvius erupted in the early afternoon of Aug. 24, 1,918 years ago, destroying Pompeii, Herculaneum and other Roman cities.
The certainty of the date tempted the team to test the ability of the argon-argon dating technique to establish the age of recent historic events. If it gave an accurate age for the pumice thrown out by the volcano, it would be by a wide margin the youngest rock ever dated by the technique.
The most common method for obtaining the age of objects as young as this is carbon-14 dating, a technique limited to organic material such as wood or bone.
To everyone's surprise the date given by the argon-argon dating technique was 1,925 years ago -- off by only seven years. The scientific error on the estimate was plus or minus 94 years.
"We nailed the date to five percent on our first attempt, so we could probably get the error down to one percent or less," Renne says.
The result is so amazing because every dating technique invokes assumptions or involves uncertainties that limit its ability to pinpoint dates with extreme precision. With carbon-14, for example, the changing ratio of carbon-12 to carbon-14 in the atmosphere over time puts a limitation on how precise a date can be established.
Scientists at the Berkeley Geochronology Center have improved the argon-argon technique so as to identify and correct for many of these uncertainties, thereby obtaining improved estimates.
The argon-argon technique is useful only with rocks containing appreciable amounts of potassium, because it is based on the fact that the naturally occurring isotope potassium-40 decays to argon-40 with a 1.25 billion year half-life.
In an older method known as potassium-argon dating the quantity of each isotope in the sample was measured to obtain an estimate of the its age. Precision was limited, however, by how accurately the argon could be extracted from the sample, which typically is melted to 1,600 to 1,700 degrees Celsius.
In argon-argon dating, first developed at UC Berkeley in the 1960s, samples are irradiated with neutrons to convert potassium-40 to argon-39, which is normally not present in nature. The ratio of argon-40 to argon-39 gives a measure of the age of the sample less subject to experimental error.
Thanks to refinements in equipment, the technique can now be used with milligram samples instead of the tens of grams -- nearly an ounce -- of rock needed in potassium-argon dating.
Sharp says too that a new carbon dioxide laser allows gradual heating of small samples, in a way that provides internal reliability tests, and can help improve the precision of the age estimate.
"With potassium-argon dating you had to assume that any argon trapped in the sample had the same isotopic composition as the atmosphere at the time of formation," Sharp says. "Argon-argon dating allows us to correlate with other air isotopes to tell if there is any deviation from that caused by non-atmospheric argon, such as contamination from argon in the rocks."
With the new dating system it should be possible to date even younger samples, perhaps a mere 1,000 years old with 10 percent accuracy.
"Dating things that are really young has always been the Holy Grail of potassium-argon and argon-argon dating," Renne says.
For this experiment the researchers obtained samples of potassium-rich sanidine, a type of feldspar, from a recent excavation of the Villa di Poppea at Pompeii, and brought them to Berkeley for analysis.
The work was supported by grants from the National Science Foundation, the Ann and Gordon Getty Foundation, and the Osservatorio Vesuviano. The Berkeley Geochronology Center is an independent, non-profit scientific institute specializing in state-of-the-art geologic dating.
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