Space & Astronomy/Space and Astronomy (October 1993).iso/mac/TEXT/JPLNEWS1/0994.PR

Text File | 1993-04-23 | 10KB | 381 lines

PUBLIC INFORMATION OFFICE
JET PROPULSION LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIF. 91109. PHONE [phone]



                                     FOR RELEASE JUNE 1, 1982



          A new technique makes it possible to determine
reli-

able ages for some very young volcanic rocks, a Jet
Propulsion

Laboratory geologist told the American Geophysical Union
meet-

ing in Philadelphia today.

          Dr. Alan R. Gillespie said he has dated a 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
fault-

ing history of the largest single mountain range in the
continen-

tal United States.

          Gillespie says his 119,000-year-old lava flow lies

beneath moraines from two of the major glacial periods of the
î
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
consid-

erable 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 a yet-older glacial moraine in
the same Sawmill Canyon. That 465,000-year date argues for
the

presence of a 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,
put-

ting accurate dates to those events. By understanding the
chron-

ology 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
  îfaulting along the eastern slope of the Sierra Nevada. The
lava

flowed down Sawmill Canyon, across a 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
occur-

red 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
î
40,000 years ago is difficult to measure with conventional

methods.

          Gillespie said the dating technique he developed is
a

variation on two established methods: Comparing ratios of
radio-

active potassium and 40argon, and comparing ratios of two
iso-

topes of argon --40argon and 39argon. 40Argon is created by
the

radioactive decay of potassium, so is a direct measurement of

the rock's age.  In the second method, some of the potassium

is converted to 39argon in a research reactor.

          By finding a piece of ancient granite in the
younger

basaltic lava (the piece is called a xenolith, or "foreign
rock"),

irradiating it in a nuclear reactor and then measuring the
ratio

of 40argon (radiogenic) to 39argon (created in the reactor),
Gil-

lespie 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. Theyare


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
re-

tain a few percent of their argon during heating. Since the
xeno-

liths are so much older than their host basalt, even that few

percent can result in an apparent age many times too great.
Gil-

lespie and his colleagues, J.C. Huneke and G.J. Wasserburg of

the California Institute of Technology, theorized that the
"mem-

ory" 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 non-

retentive sites since the host lava erupted would be free of

"memory," and would yield a 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
  îthe 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 a sample of what he believed
was

young lava from Sawmill Canyon, near Independence, Calif. He

then removed a large granitic xenolith -- a 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` C (2,000` F), the heat released argon from the
sample.

          Using a 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 î
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
po-

tassium, which scientists call its half-life, is about 1.5

billion years.

          ` In addition to being present inside the rock
crystal

as a 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
39argon-

40argon 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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