Etching detrital zircons for
fission-track analysis
John I. Garver
Geology Department
Union College
Schenectady NY, 12308-2311 (USA)
garverj@union.edu
Matthias Bernet,
Department of Geology & Geophysics
Yale University
210 Whitney Ave.
New Haven, CT 06520-8109 (USA)
The
first published paper to describe sample preparation, etching, and track
density determination in zircon crystals for fission-track dating used cleaved
(split) crystals immersed in boiling phosphoric acid to reveal tracks
(Fleischer et al., 1964). Since the introduction of mounting zircon
grains in Teflon and etching with a strong base (KOH:NaOH, NaOH and water, or
KOH:NaOH:LiOH), there have not been any dramatic changes in the basic
methodology for handling zircon for routine fission-track analysis (i.e.
Gleadow et al., 1976; Naeser 1976; Gleadow, 1978; Zaun and Wagner, 1985).
By the early 1980's most labs had settled on using a technique where zircon
grains are embedded in Teflon¨ and etched in a KOH:NaOH eutectic between
~200-230 C, but most studies lacked a quantative criteria for track revelation.
Our group has been dating detrital zircon suites using a KOH:NaOH eutectic at
exactly 228oC and the following discussion assumes these etching
conditions. For full discussion of etching zircons at different labs around the
world see Methodology
for etching zircons .
Three
principle techniques are used for etching zircon. The Muti-Etch
technique assures both random selection and optimal etching by repeatedly
etching and counting a single mount at regular intervals (Hasebe et al.,
1993). This method provides an unbiased distribution of grain ages, but
it has a major drawback in that it is very time intensive an operationally
difficult (see Hasebe et al., 1993, p. 124). The Multi-Mount
technique optimizes the total range of countable grains by insuring that
all grain populations are well etched by etching several mounts different times
and counting grains from both (Naeser et al. 1987). The principal
advantage of this technique is that it quickly and reliably provides the full
fission-track grain age (FTGA) spectrum, but results in an inadequate
quantative samplling of the FTGA distribution. The optimal etch technique
attempts to maximize a certain population of grains from a single sample (e.g.
Garver and Brandon, 1994b; Kowallis and others, 1986). This approach
requires that a particular population is optimally etched at the expense of all
other populations, and it has largely been used to date the young population of
grains so mounts are given a long, rather than short etch. This technique
is particularily useful when only a certain population of grains is of interest
and other grains can be safely ignored, as is the case in dating the young
population of grains to constrain the depositional age of sediments.
Detrital
zircon suites are characterized by their complex distribution of fission-track
cooling ages and variable U content of the zircon grains. To reveal
countable fission-tracks in as many grains as possible, it is
necessary to select reseaonable etch times. We give here a first-order
orientation for selecting the correct etch time. Therefore, we show the
FT age - U content data from over 1000 zircons, etched for different intervals
(Fig.1). Theses samples include zircons from modern river sediments and
acients sandstones from all around the world (European Alps, Apennines,
Southern Alps of New Zealand, Himalayas, Mississippi etc.)
Fig.
1) U content versus FT age with respect to etch time. Zircons were etched at
exaclty 228oC in a KOH-NaOH eutectic melt.
In
general zircons should be etched slowly, which means it is better to start out
with a short etch time and then increase the etch time progressively until a
satisfactory result is obtained. After each etching step the zircons should be
carefully inspected to see if the zircons are still under etched, if they
are well etched or over etched. Etch times should be selected with
respcet to the expected FT ages. Long etch times tend to reveal
tracks in grains with "young" cooling ages and high U content,
while short etch times should be used for grains with "old" cooling
ages and lower uranium content. Table 1 gives approximate etch times that
should be selected to reveal tracks in certain FT age groups.
Table
1. approximate etch times for certain FT age groups
|
expected FT age (m.y.) |
etch time (hours) |
|
1-10 |
40-60 |
|
10-100 |
15-30 |
|
100-1000 |
5-10 |
Nervertheless,
it is important to note that there is a good deal of overlap between different
etch times and the FT ages that are revealed in these etches. Therefore, it is
not necessary to worry too much about "the absolute exact etch time".
Questions? Please contact garverj@union.edu
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from: J.I.Garver, Geology
Department, Union College, Schenectady NY, 12308-2311, USA.
Last
Revised: 3 January 2003
Some of this
material is based upon work supported by the US National Science Foundation
under Grant No. 9614730. Any opinions, findings, and conclusions or
recommendations expressed in this material are those of the author(s) and do
not necessarily reflect the views of the National Science Foundation.