Preparation of samples for ⁴⁰Ar/³⁹Ar analyses

Sample size
The amount of mineral separate needed for ⁴⁰Ar/³⁹Ar analyses varies depending upon the age of the sample, mineral of interest, rock type, and grain size. Typically, for lavas and tephras that are < 1 Ma and do not contain a K-bearing phase, groundmass separates are preferred. Furnace incremental heating experiments require 100-150mg of clean, hand-picked groundmass. Single incremental heating experiments often yield precise ages; however, most samples commonly require replicate experiments to achieve 2σ analytical precisions of ± 2-10%. Therefore, we recommend sending at least 500mg of purified groundmass.

For samples that are >1 Ma, incremental heating experiments are performed using a 25W CO2 laser. Samples for laser extraction analyses range from <1 mg to ~40 mg.

NOTE: If you do not have the facilities to prepare mineral separations, we can process samples at UW-Madison. Please send a “fist-sized” whole rock hand sample. Raw material required for mineral separations is largely dependent on mineral abundance in the sample.

Sample preparation
The following are recommended guidelines for preparing samples:

Holocrystalline groundmass separates are prepared from porphyritic lava samples by crushing and sieving to the appropriate size fraction. The best results are usually obtained from groundmass that is 180-500 microns. Following sizing, the groundmass is separated from the phenocrysts using a strong hand magnet or a Frantz magnetic separator. The separate is then rinsed with de-ionized water several times and placed in an ultrasonic bath for 5 minutes. Repeat ultrasonic wash until water is clear.  Decant the water, rinse the separate with acetone, and dry under a heat lamp. Hand-picking under a binocular microscope is then performed until visible phenocrysts are completely removed.  Further separation using heavy liquids is sometimes required. Groundmass separate should be shipped in plastic, leak-proof vials. Taping the cap to the vial for additional security is strongly recommended.

For sanidine and plagioclase, it is best to try and isolate the largest crystals possible. Feldspar and quartz can be isolated from other phenocrysts using a Frantz magnetic separator. The sample should be etched in ~5-10% HF to remove adhered glass and/or matrix and then rinsed thoroughly with deionized water. Separation of sanidine from plagioclase is achieved using heavy liquids set to a density of 2.62-2.65. Quartz is removed from the sanidine/plagioclase separate via hand-picking under a binocular microscope. We often check the purity of the feldspar separate using an SEM.

NOTE: Do not use HCl for acid treating samples Use acetic acid to remove carbonates.