A new technique makes it possible to determine reliable ages for some very young volcanic rocks, Jet Propulsion Laboratory geologist told the American Geophysical Union meeting in Philadelphia today.
Dr. Alan R. Gillespie said he has dated basaltic flow that erupted 119,000 years ago. The lava flow, at Sawmill Canyon on the east slope of California's Sierra Nevada mountains, forced its way through the 100-million-year-old granite of the Sierra.
Determining accurate dates for recent geologic events will allow geologists to sort out the complex climatic and faulting history of the largest single mountain range in the continental United States.
Gillespie says his 119,000-year-old lava flow lies beneath moraines from two of the major glacial periods of the Sierra -- the recent Tioga and the earlier Tahoe. That, he says, puts an older limit on the the Tahoe glaciation (it can be no older than 119,000 years), which has been the subject of considerable controversy among geologists. Gillespie's results confirm that the Tahoe glaciation probably occurred during the last major ice age in North America and Europe -- the Wisconsin glaciation.
He has also dated -- at 465,000 years old -- another lava flow that lies beneath yet-older glacial moraine in the same Sawmill Canyon. That 465,000-year date argues for the presence of previously undated glacial period that occurred between the Tahoe and the still earlier Sherwin period.
The importance of recognizing and dating individual glacial periods in the Sierra is that they can then be related to the major glaciers that swept much of the United States, putting accurate dates to those events. By understanding the chronology of the ice ages in the past, scientists hope to better understand and predict climatic trends in the future.
The same 119,000-year-old lava that was used to date the glaciations was also used to place limits on the rate of faulting along the eastern slope of the Sierra Nevada. The lava flowed down Sawmill Canyon, across major earthquake fault and out onto the alluvial floor of the Owens Valley. But the lava has disappeared east of the fault line, and Gillespie says it was dropped by displacement along the fault, and later buried by rocks and soil carried out of the canyon by Sawmill Creek. The base of the lava flow is now 62 meters (200 feet) above the present site of the creek. Gillespie says, therefore, that the faulting rate along that region of the Sierra has averaged one-half millimeter (0.02 inches) or more per year.
Gillespie expects his age-dating technique may also have an application in paleoanthropology, the study of fossil remains of early human ancestors, because of the requirement for accurate dating of the young basalts often found near fossilized skeletons.
Important events in the development of man have occurred during the last few million years, from the oldest hominid fossils such as "Lucy" from Ethiopia (about 3.5 million years old), to the appearance of modern homo sapiens about 40,000 years ago. The period from about 250,000 years ago to about 40,000 years ago is difficult to measure with conventional methods.
Gillespie said the dating technique he developed is variation on two established methods: Comparing ratios of radioactive potassium and 40argon, and comparing ratios of two isotopes of argon --40argon and 39argon. 40Argon is created by the radioactive decay of potassium, so is direct measurement of the rock's age. In the second method, some of the potassium is converted to 39argon in research reactor.
By finding piece of ancient granite in the younger basaltic lava (the piece is called xenolith, or "foreign rock"), irradiating it in nuclear reactor and then measuring the ratio of 40argon (radiogenic) to 39argon (created in the reactor), Gillespie was able to determine how long ago the lava erupted onto the surface.
The chief difference between Gillespie's approach and conventional 39argon-40argon dating is his use of the xenolith rather than the lava itself.
As magma forces its way up through the older granitic rock around it, bits of the granite fall into the magma. They are strongly heated and may even be partially melted. That allows the xenolith's radiogenic argon to escape, and should reset the xenolith's atomic clock to zero.
In practice, it has been found that the xenoliths retain few percent of their argon during heating. Since the xenoliths are so much older than their host basalt, even that few percent can result in an apparent age many times too great. Gillespie and his colleagues, J.C. Huneke and G.J. Wasserburg of the California Institute of Technology, theorized that the "memory" argon is contained only in extremely retentive sites in the crystals of the xenolith, and that argon from the majority of sites would be completely lost.
They reasoned that argon accumulating in the nonretentive sites since the host lava erupted would be free of "memory," and would yield correct age for the eruption, if it could be extracted from the xenolith free of the argon from the retentive sites.
They thought that, by releasing the argon in several steps at successively higher temperatures, they could separate the argon of the time of eruption from that which included the ancient "memory" argon.
After testing his technique on older rocks whose age was known, Gillespie collected sample of what he believed was young lava from Sawmill Canyon, near Independence, Calif. He then removed large granitic xenolith -- piece of the ancient granite that had fallen into the lava and partially melted -- from the sample. Working with Wasserburg and Huneke in Wasserburg's Caltech laboratory, Gillespie heated the xenolith in several stages. At each stage, ranging from 350`C (660`F) to 1,100` (2,000` F), the heat released argon from the sample.
Using mass spectrometer, Gillespie measured the ratio of 39argon to 40argon at each heating step. He was able to determine, from the amounts of argon released in each heating step, that the xenolith had fallen into the lava, partially melted, and thus lost most of its original radiogenic argon about 119,000 years ago.
The argon released in the low-temperature laboratory steps, Gillespie says, was free of the ancient inherited argon, as predicted. The argon released in the last steps -- just as the sample melted -- was dominantly the ancient argon that goes back to 100 million years ago, when the granite first cooled.
There are several reasons why it is very difficult to use conventional dating techniques on extremely young basalts:
` Basalt is poor in potassium;
` Very little radiogenic 40argon has accumulated from the radioactive decay of potassium, since the decay rate of potassium, which scientists call its half-life, is about 1.5 billion years.
` In addition to being present inside the rock crystal as result of potassium decay, argon also is present in the atmosphere, and sticks to the surfaces of the rock, becoming trapped there and contaminating the sample. The effect is most serious when there is little radiogenic argon present.
` In attempts to date the lavas directly by 39argon40argon method, geologists find that crystals in the basalt rock are so tiny that when they bombard them with neutrons in the reactor, the argon atoms are knocked out of the sample crystals and may escape.
Earlier attempts to date extremely young rocks from the same region of the Sierra as Gillespie's sample, using the potassium-argon technique, resulted in large uncertainties, some as great as 100 percent. Because of the nature of argon, sizes of the basalt crystals, and other uncertainties, the laboratory results showed the lava could have erupted anytime between 300,000 years ago and yesterday, although the best estimates placed the age at about 53,000 years. But those were attempts to date the basalt itself, and not the granite xenoliths that, heretofore, had been considered one of the contaminating problems of the age-dating technique.
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