NOTE: The ideal sample is as coarse in grain size as possible. After irradiation very fine grained separates become radioactive powder and present serious concerns for contamination in the lab.

Whole Rock/Volcanic Groundmass

It is usually best to avoid whole rock analysis if possible, but for certain fine grained plutonic/metamorphic or aphyric volcanic rocks this may be the only material available. It is highly desirable in this situation (really for all samples which may be dated) to look at a thin section first. Potassium, and thus argon, is concentrated in certain phases; these materials are thus the carrier of the K/Ar isotopic clock and must be unaltered (i.e. a closed system) in order to obtain an age which can be interpreted with confidence. In plutonic or metamorphic rocks look particularly closely at the condition of the feldspars, micas, and amphiboles. Any visible alteration of these phases in thin section means that sample is best avoided in general. For volcanic rocks look for interstitial glass. Potassium typically behaves as an incompatible element, especially in mafic rocks, and tends to be concentrated in the melt (which becomes glass upon eruption). Glass is notorious for exhibiting open system behavior; as it hydrates K moves in, radiogenic argon may be lost, and atmospheric argon may be gained. Thus, the presence of glass in your volcanic rock sample will typically yield questionable ⁴⁰Ar/³⁹Ar ages. A holocrystalline groundmass sample is what your after, one with no calcite or zeolite present, and olivine phenocrysts must be removed as they may carry excess argon. Whole rock samples may be treated in dilute nitric acid (~5%), for ~5-10 minutes, to remove traces of calcite or minor alteration phases, but never in hydrochloric acid as traces of HCl in your sample may contaminate our mass spectrometers. Approximately 100 mg of ~0.3-1.0 mm sized material is usually sufficient, unless the sample is a Pleistocene basalt, in which case we may need up to 200 mg.

Mineral Separates for Step Heating Analysis

The general rule is that the coarsest size fraction which can be separated from your rock without composite (multi-mineralic) grains is desirable. Coarse material is easier to handle (especially the micas) and less dangerous after it has been irradiated. Pure mineral separates are critical, especially for thermochronology studies where a temperature appropriate to the mineral being analyzed is assigned to the age obtained. Coarse-grained samples may often be most easily separated by hand picking under a binocular microscope. Samples separated using heavy liquids such as bromoform must be rinsed thoroughly in acetone afterwards to avoid potential contamination of our vacuum system. Zapping the mineral separates in acetone followed by distilled water in an ultrasonic bath will aid in final cleanup also. We need approximately 50-100 mg of a typical mineral separate for analysis.

Amphibole: Amphiboles (~3.1-3.3 specific gravity or G) can typically be separated by settling out of heavy liquid with a G of ~3.10-3.15 whereas muscovite and biotite (see below) will float. A permanent magnet passed over the amphibole separate will remove magnetite. Final purification is usually done by hand picking. Beware of biotite inclusions in your hornblende! In many rocks a good size fraction for amphiboles is ~100-200 µm.

Biotite and Muscovite: Biotite (~3.0-3.1 G) may be settled out of your heavy liquid and removed by using a S.G. of ~3.10, this will leave muscovite and other lighter minerals floating. Muscovite (~2.8-3.0 G) can then be settled out by reducing the G of your heavy liquid to ~3.0, this will basically leave quartz and feldspars floating. Note that if you have a rock with only biotite or muscovite in it they can sometimes be separated easily by the "paper shaking" technique. Micas will be in the size range 200-500 µm in many rocks.

Orthoclase/Microcline: If you have gone through the above operations you will end up with the light minerals quartz and feldspars floating in your heavy liquid. Orthoclase and microcline (~2.5-2.6 G) can usually be separated at this point by sinking quartz (~2.7 G) in a heavy liquid with a G of ~2.61. K-feldspars can be obtained from the 100-300 (µm size fraction in most rocks.

Sanidine/Anorthoclase/Plagioclase for Laser Fusion Analysis: These minerals, when present as phenocrysts in volcanic rocks, may be separated by conventional heavy liquid techniques (using G above) or by hand picking. If your sample is pumice it is often simple to crush the sample to 1-2 mm size fraction and hand pick the phenocrysts under a binocular scope. Final treatment should include a brief treatment in dilute HF (~5%), for ~5-10 minutes, to remove glass if present.