Geology Reference
In-Depth Information
smaller than that of non-Antarctic meteorites implies
that smaller fragments stand a better chance of survival
in the Antarctic and that pairing is common there.
The identification of pairing groups has both practical
and scientific importance. An especially interesting
specimen may be so small that the mass available does not
meet the scientific demand. Under these circumstances a
researcher may willingly substitute a fall-paired specimen.
Beyond practical considerations, the identification of
pairs is essential for identifying ancient strewn fields and
reconstructing their total masses, sorting meteorite fluxes
by type, and associating individual lunar and martian
meteorites with specific ejection events.
How do we recognize pairing relationships? The first
clues are chemical and/or petrologic similarity and
geographical proximity. Based on these considerations,
Antarctic team members note tentative or preliminary
pairings as they collect the samples. Such pairing assign-
ments are not rigorous, however, and to raise confidence
in them it is desirable to show that two meteorites assigned
to a likely pair have the same terrestrial age and the same
CRE age. Unfortunately, with hundreds if not thousands
of potentially paired meteorites on hand, it is not possible
to obtain these data in every case. In practice then, when
a few tentative pairings in a larger group are confirmed
by terrestrial and CRE ages, it is then usually assumed
that the entire suite is paired.
Pairing studies of the meteorites harvested at the
Frontier Mountains (FRO) in Northern Victoria Land
show how terrestrial exposure ages can be used to sort
out distinct falls present in a single area. Expeditions
spanning several decades collected over 450 meteorites in
this classic “meteorite trap.” By interpreting the mea-
sured activities of several radionuclides in 23 samples,
Welten et al . [2006] were able to identify two large pairing
groups. One pairing group, associated with FRO 90174,
consists of H3-6 chondrites, and is identified as an H
chondrite breccia. Its terrestrial age is ~100 ka and the
group is estimated to include over 50% of all the chondrites
in this stranding area. A second pairing group suggested
by physical characteristics comprises samples associated
with FRO 90001, which has a terrestrial age of ~40 ka.
This group consists of seven members. None of them has
an extensive fusion crust, so it is likely that the breakup
occurred on or in the ice, rather than in the atmosphere.
A few of the FRO meteorites belong to neither of the
larger pairing groups. The paired stones FRO 93009 and
FRO 01172 are among the oldest at the site, having terres-
trial ages of ~500 ka. Interestingly, they were found on
opposite sides of the Frontier Mountains, suggesting an
origin in a large meteoroid that fragmented in the
atmosphere and fell at a location “upflow” (i.e., upstream
as the glaciers flow) from the two recovery locations.
After landing, the two fragments were transported to
opposite sides of the stranding area by separate ice flows.
Taken together, the pairings at FRO suggest that the basic
dynamics of ice flow in the region have not changed much
in the last 500 ka. As it happens, the Frontier Mountains
area is also the location of the oldest terrestrial find, FRO
01149. This sample was found on a bedrock surface,
where Welten et al . [2008] argue that it fell. It is safe to say
that only in Antarctica could one find a meteorite having
a terrestrial age of ~3 Ma on a bedrock surface! When
viewed in light of independent evidence [ Höfle , 1989]
showing that the bedrock was overridden by glacier at
some point in its history, the presence of this terrestrially
ancient meteorite indicates that the glacial event(s)
occurred before the meteorite landed >3 Ma ago.
Meteorites recovered from the stranding area at the
Queen Alexandria Range provide a second example
illustrating the significance of pairing studies [ Welten
et al ., 2011b] (Plate 9). This area has yielded about 3500
fragments, including 660 L5 chondrites and nearly 1500
LL5 chondrites. It was initially assumed these meteorites
came from two large showers. The terrestrial ages of 11
representative fragments (iron fractions) all have the
same terrestrial age, consistent with the inference of a
single L/LL5 chondrite fall.
As implied above, the terrestrial ages of Antarctic
meteorites have implications for regional glaciology [e.g.,
Spaulding et al ., 2012]. For further details, we refer the
reader to Cassidy et al . [1992] and Harvey et al . [2014 (this
volume)].
9.3. EXPOSURE AGES
There is no reason to doubt that solar and galactic
cosmic rays have permeated the solar system for
4.56 Ga. Attenuated one thousand fold by a few meters
of solid material and traveling at near-light speeds,
most of these high-energy particles leave few traces
that  we can detect. The exceptions occur when the
irradiated object is small, ≤1 m in size (as are most
meteoroids and all micrometeoroids), and when the
matter is located near an exposed surface. If a collision
excavates a meteoroid from deep below the surface of
an asteroidal or planetary parent body and the enforcers
of Murphy's Law are distracted, we may expect the
meteoroid to record a single, uniform period of irradiation
that ended when the object landed on Earth after a time
equal to its exposure age T Exp . This is the so-called
one-stage model for an exposure history and it works
surprisingly well for many meteorites. For example, it
explains to first order clusters in the CRE age distributions
of L and H chondrites [ Graf and Marti , 1995, and refer-
ences therein].
Herzog and Caffee [2014] discuss the calculation of
CRE ages in detail. The calculations depend on use of the
Search WWH ::




Custom Search