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 .
 


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© Geology Department, Union College, Schenectady N.Y. 12308-3107.

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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.