Collecting samples for detrital zircon and
apatite in the field
Matthias
Bernet
Yale University, Department of Geology & Geophysics
210 Whitney Ave.
New Haven, CT 06520-8109, USA
Phone: (203) 432 5686, fax: (203) 432 3134
email: matthias.bernet@yale.edu
Detrital zircon fission-track analysis can be done on modern river and beach sediment as
well as on ancient sandstone. However,
sampling techniques are different for both kinds of samples and sampling
strategies depend on the purpose of the intended study.
Sampling modern river and beach sediments
For exhumation and provenance studies one should carefully
consider where in the field samples are collected. Geologic maps usually
provide reasonable information about the potential zircon yield that can be
expected in a given drainage area. Lithologies with usually low zircon yield
are carbonate, mafic and ultramafic rocks etc., while areas with siliciclastic
sedimentary rocks, metasediments, and felsic volcanic rocks should give a
reasonable zircon yield. The highest yield can be expected from granitic rocks
and orthogneisses. In many river basins the selection of gravels will provide a
quick overview about the lithology in the sediment source area. The variety of
gravels, if available, should therefore always be recorded.
Collecting detrital zircon samples from modern
sediment can be very easy. By using gold washing pans (Fig.1) it is possible to
enrich the sample's heavy mineral content already in the field. This allows to
reduce sample material to 200-300g instead of several kg. In general it is
sufficient to pan between 12-14 pans of material, but the final outcome depends
on the zircon yield in the drainage.
Fig. 1: Gold pan with heavy mineral
enrichment, indicated by garnet and magnetite.
Preferably samples should be collected from sand
bars and beaches with medium to fine sand grain sizes. They can be directly
processed in the gold pans. However, it is also possible to collect samples
from gravel bars. In this case garvel and all finer grained material inbetween
can be run through a sieve. The fine fraction (coarse to fine sand size and
smaller) should be caught and processed further in the gold pan, while all
coarse material can be discarded. All clay content in the fine material should be
washed out first. Then simply by "gold washing" the lighter
material (quartz, feldspar, micas etc.) is removed and heavy minerals like
zircon, garnet, magentite and even gold (if one is lucky) are enriched (Fig.1).
It is worthwhile to look for heavy mineral placer
deposits, which can be easily recognized by black and reddish colors from
magnetite and garnet (Fig. 2). If placer deposits are available it is
sometimes not even necessary to use gold pans. If they are enriched enough,
like in Fig. 2, the top layers can be scraped from the surface using a
plastic card for example (but not credit cards) and put directly into a sample
bag.
Fig. 2. Gold pan on sand bar of the Toce River in Italy.
The sand shows strong enrichment in heavy minerals
like garnet and magnetite (red and block colors).
If neither gold pans are used nor placer deposits can be found, one should
take at least 2-7 kg of sample material. However, even this size of sample may
not have sufficient zircon if the lithology is not zircon-bearing.
Sampling sandstone
Collecting detrital samples form sandstone outcrops is very
straight forward: locate the correct unit, and collect enough sample for
analysis. However, it is unfortunate when you go to all the trouble of
getting to an outcrop, taking the sample, and processing the sample and in the
end you don't have enough zircon for analysis. This is a common problem,
because most geologists inexperienced with FT dating tend to collect samples
that are too small and perhaps the wrong grain size. The best samples are
medium-grained arkosic sandstones, but many sandstone compositions are
appropriate for collection and zircon extraction. In general you should
look for quartz in the sandstone, and quartz-rich lithologies require smaller
samples. Our rule of thumb is the following:
(A) Arkose - 2-4 kg
(B) Quartzo-feldspathic sandstone - 4 kg
(C) Quartz-bearing volcaniclastic sandstone - 4-7 kg
(D) Lithic sandstone - variable, but collect 4-7 kg
(E) Silicic volcaniclastic (2-4 kg)
If you are uncertain about the sample, 4-7 kg is a good
starting point. Note that this is a big sample, bigger than most
geologists would consider taking. As you can imagine, one can run into a
number of logistical problems in the field with samples this size. To be
certain in the field, we over-collect in the field on daily traverses. We
then break up the samples (at camp for example), into pieces that are 10x10 cm
or smaller. We fill a large sample bag, and then weigh out a
pre-determined amount for each samples to be carried out of the field. To
do this we use a durable hand-held scale (used for fishing - available from Bass Pro
in the USA for under $10) that goes to 25 kg. Sandstones can be coarse- to
fine-grained, but the best are medium grained. Zircon is a minor accessory
mineral in sandstones (> 0.5%).
Our studies have shown that there is little
difference between beds in a sequence deposited at the same time (i.e.
sandstone beds in a turbidite sequence close to one another will give a similar
result). It is important that you collect medium to coarse
sandstone and try to avoid the fine sandstones, when collecting
turbidites. For turbidites, this observation requires that you take
only part of the bed. Take your time when collecting the sample and make
sure that all included pieces have visible grains - a good rule of them that it
is coarse enough. We like to wash all samples in the field.
If you plan to extract zircon and apatite, it is
important to avoid altered sandstones, especially those with excessive iron
oxide ("rusty weathering"). These samples may have very poor
yields of apatite if they have experienced interstratal dissolution, and there
may be an annoying amount of heavy secondary minerals (pyrite, siderite,
barite, etc.) that slow down the separation process. Zircon is very
durable, and as far as we know, we have not run across any common diagenetic or
metamorphic condition that fundamentally alters zircon concentration in a
sandstone.
Questions? Please contact garverj@union.edu
.
Back to Union Fission Track Home
page
Back to Home page for J.I.
Garver
Back to Union Geology Department
This document can be located
from
http://idol.union.edu/~garverj/FT/FThome.html
All rights reserved. No part
of the document can be copied and/or redistributed, electronically or otherwise, without written permission
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.