High quality geochronology begins at the outcrop where care must be taken to avoid weathered or altered rocks. The amount of sample needed depends on a variety of factors: (1) how old the rocks are, (2) are there potassium-rich minerals like sanidine, biotite, or amphibole and if so, what is the modal %, (3) for lava flows, is the groundmass texture holocrystalline, rather than glassy or vesicular.
Once you have collected your samples, they must be crushed to isolate either mineral grains, or in the case of lava flows the groundmass (essentially you will remove the phenocrysts and attempt to date the fine-grained part of the rock that cooled and solidified at the earth’s surface. If the sample contains water, as do many bentonites that are clay-rich, they must be dried before crushing. Once you have a crushed sample, it can be passed through wire mesh sieves with a variety of sizes, so that you can obtain several size fractions between 80 and 500 microns to work with further.
A dedicated room in the basement of Weeks Hall houses rock preparation equipment including:
- a jaw crusher for creating granular material from hand samples,
- a tungsten carbide hydraulic rock press for iron-free preparation of coarse grained material from hand specimens,
- a disk mill to grind samples into a range of size fractions from about 80 microns to 500 microns – depending on the nature of the material (bentonite ash vs. volcanic lava vs. granitic rock),
- a shatterbox for pulverizing granular material to very fine grained powder for bulk rock geochemical and/or isotopic analysis. Dust created during processing of samples using the he disk mill and jaw crusher is vacuumed away into a dedicated filter system.
To purify your sieve-sorted mineral or groundmass concentrate, several techniques may be used that distinguish different phases in your sample on the basis of magnetic susceptibility, density, or hydrodynamic behavior in a water flume. Each sample will require different application of magnets, heavy liquids, Gold Table, to purify several milligrams of mineral or groundmass for irradiation.