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Antarctica - Part 6

Smithsonian Contributions to the Earth Sciences, Number 23

Catalog of Antarctic Meteorites, 1977-1978

Terrestrial Ages*

J.C. Evans and L.A. Rancitelli

Approximate terrestrial ages have been measured on 27 Antarctic
meteorite specimens by nondestructive gamma ray analysis of their 26Al
contents (Evans, Rancitelli, and Reeves, 1979). The specimens include 20
chondrites collected from the Allan Hills region in the 1977-78 season,
4 chondrites and 1 achondrite collected from the Allan Hills in 1976-77,
a chondrite from the Yamato Mountains, and the chondrite discovered in
Adelie Land in 1912 by the Mawson Expedition. The 26Al contents of these
samples are compared with expected saturation values established by
measurements on ordinary chondrites from 14 finds and 22 contemporary
falls. Several of the Antarctic meteorites have also been analyzed for
36C1, 53Mn, and 21Ne.
The calculated terrestrial residence times for the 27 specimens range
from less than 0.1 to about 0.7 million years. These include the oldest
terrestrial ages ever measured on chondrites. When compared with the
contemporary find and falls, the Antarctic specimens show a perceptible
shift toward lower 26A1 values and hence older terrestrial ages (Figures
38, 39). The 26Al values also show a bimodal distribution in which 11 of
the 12 H-type chondrites have apparent terrestrial ages of less than
300,000 years, and 8 of the 12 L-type chondrites have estimated
terrestrial ages of more than 400,000 years. The three oldest finds,
with ages approaching 700,000 years, include an L3, an L5, and an L6
chondrite.
Two specimens, ALHA76008 and ALHA77002, which were suspected on the
basis of their low 26 Al contents of having terrestrial ages of 1
million years or more, proved to have 36Cl contents indicative of short
residence times. In both cases, the data are interpreted as reflecting a
two-stage history of cosmic ray bombardment involving a fairly recent
breakup in space of a body to its pre-atmospheric size, followed by a
short terrestrial residence time. The Yamato chondrite, 7301 (J),
appears to have had a similar history.
The old terrestrial ages of Antarctic chondrites have introduced a new
and exciting element, not only into meteorite research but, for the
first time. have provided the possibility of utilizing meteorites as
time probes of a terrestrial process. Measurements of isotope pairs
should make it possible to span a range of terrestrial ages extending
from the present back 15 million years to the middle Miocene when the
Antarctic icecap is believed to have been approximately its present
size, but was soon to undergo a significant expansion (Shackleton and
Kennett, 1975). The large concentrations of meteorites found on two
sides of the continent could well yield specimens old enough to give new
information on the age and motion of portions of the Antarctic icecap.


*NORTON R.O. (1998) They Are All Old Meteorites (RFS II, pp. 319 + 321):

Isotopic studies of the Antarctic meteorites reveal the length of time
they have spent on Earth. While in space, meteorites are constantly
bombarded by cosmic rays that interact with elements in them, producing
radioactive isotopes. The isotopes slowly decay, but since the cosmic
radiation in space is constant, new isotopes continually take their
place and the isotopic content remains constant. As soon as meteorites
reach Earth, where cosmic rays can no longer bombard them, their
radioactive isotopes decay slowly over time. By measuring the isotopic
content in these meteorites and knowing their rate of decay, scientists
can calculate how long they have been on Earth. The terrestrial ages
vary from 10,000 to more than 1 million years. Antarctic meteorites have
the oldest terrestrial ages of any known meteorites. Most of the
meteorites in collections around the world were picked up soon after
they fell and have terrestrial ages averaging one hundred to two hundred
years.
The meteorites in Antarctica provide an excellent opportunity to test
whether meteorites in the world's collections are typical of meteorites
that have fallen over the past million years. Of the eight hundred or so
meteorite falls recognized at Antarctica, the ratio of chondrites to
achondrites, irons, and stony-irons is close to the ratios in museum
collections worldwide. This means that the sampling of meteorites is
probably a true representation of the real meteorite populations in
space.
Antarctic meteorites provide another unique test. If all meteorites come
from the same sources, then the Antarctic meteorites should have
compositions identical to those in collections around the world. Some
Antarctic meteorites are new types never seen before or new specimens of
rare but known types. Recent research has shown that Antarctic
meteorites have significant chemical differences, suggesting that
meteorites falling to Earth do change with time. This may mean that
meteorites come from different sources over time.


Well, that's it for the 1977-78 field season in Antarctica.

Best regards,

Bernd

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