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.

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.