Sample Preparation and Amounts

Here in the OSU Argon Geochronology Lab we carry out 40Ar/39Ar experiments through three principal modes of operation. In the first two modes we use either a Heine resistance furnace or a CO2 laser to carry out incremental heating experiments. In both modes we measure the isotopic composition of the released argon gas on the MAP 215-50 single-collector by peak-hopping and using a multiplier. In the third mode we also use a CO2 laser for incremental heating but we measure the argon gas on the ARGUS VI multi-collector simultaneously on four Faradays and one ion-counting multiplier.

Standard Methods

We employ a variety of standard methods to prepare samples for age determinations as included in the $750 or $1,000 base price for an incremental heating analysis. Fine grained, unaltered whole-rocks (basalts, andesites) to be analyzed with the Heine resistance furnace on the MAP 215-50 are cored with a 5 mm diameter diamond-tipped drill bit, then sectioned into disks of 100-300 mg. All whole rock core preparations will be performed by the OSU Argon Geochronology Lab personnel to ensure proper sizing for the irradiation packaging. Please send 1-2 kg of a hand-sample for a single whole rock core analysis. The relatively large sample size allows us to avoid areas containing xenocrysts and xenoliths, filled vesicles, alteration, cracks or veins. Aphyric volcanic rocks can be processed from smaller hand-samples, if they are relatively unaltered.

Groundmass samples to be analyzed with either the MAP 215-50 or the ARGUS VI are crushed and sieved to typically a grain size of 200-300 µm (or alternatively 300-500 µm if the rock is relatively unaltered). The groundmass samples are then rinsed with distilled water several times until the water remains clear and these are dried in an oven at ~80 °C for several hours. Separations by primarily hand-picking, magnetic separation, heavy-liquids or other means are then performed, followed by acid treatment. For subaerial and unaltered submarine samples we employ a mild leach in an ultrasonic bath with ~5% HNO3 for 20 minutes, followed by an ultrasonic bath of DI water for 20 minutes. In between the samples are rinsed thoroughly with DI water for three times. Finally, the samples are dried in an oven that is no hotter than 80 °C.

Unaltered phenocrysts in otherwise altered rocks to be analyzed with the CO2 laser on either the MAP 215-50 or the ARGUS VI are separated by crushing, sieving, washing and drying, then passing through a Frantz magnetic separator, followed by mild acid cleaning (1 N acetic acid to remove carbonates, 1 N HNO3 to remove minor alteration), ultrasonic washing in DI water, drying, and a final hand-picking under binocular microscope. Feldspar mineral separates (plagioclase) are further cleaned with a short 5 minute wash in 5% HF. Sediments (clastic, clays) are separated by size fraction (2-20 µm, 20-63 µm) then treated with acetic acid to remove carbonate, washed and dried. Raw material required for mineral separations is dependent on mineral abundance, however, most are successful using ~2 kg of raw hand specimen material. When providing crushed materials make certain these are coarser than 100 µm in grain size, preferably coarser than 500 µm.

Groundmass samples and mineral separates from more (severely) altered submarine volcanic rocks are treated with a more extensive (stronger) acid leaching procedure designed by Koppers et al. (2000) and further refined in Koppers et al. (2011).

Read More about our Acid Leaching Procedures

When considering the type of sample preparation (whole rock core, groundmass or mineral separate), sample amount, irradiation duration and subsequent 40Ar/39Ar analysis, the use of thin-sections and elemental (XRF, INAA or ICP) data are highly recommended prerequisites for 40Ar/39Ar dating. If available, these should be provided to the OSU Argon Geochronology Lab.

When Preparing Your Own Samples

Please consider the following important pointers when preparing your samples to send to us:

  • When acid leaching with ~5% HF, rinse thoroughly with DI water afterwards and ONLY do this leach for plagioclase separates.
  • Following a mild acid treatment, or the stronger Koppers et al. (2011) procedure, always end with a 20-60 minute ultrasonic bath of DI water, followed by a rinsing of your sample for three times in DI water, and drying in an oven that is no hotter than 80°C.
  • Continue your groundmass or mineral separation techniques, ending with a final hand-picking under the binocular microscope.
  • Before shipping your sample, re-sieve the final fraction, wash with acetone followed by DI water, do a final wash with acetone and dry the sample in an oven that is no hotter than 80°C.
  • Pre-separated sample should be shipped in a leak-proof vial. Highly recommended is a ~14×45 mm size vial capped with a tightly fitting plastic push closure. For further security tape cap to vial using clear tape. Screw capped vials are NOT recommended as these often leak sample during shipping, even when taped.

When ready, please send your samples to our Shipment Address.

Sample Amounts

In the table below we show the typical sample amounts we load for 40Ar/39Ar incremental heating analyses.

MINIMALLY REQUIRED CLEAN SAMPLE MATERIALS

Instrument
Set Up
High-K
Minerals
Low-K
Minerals
Groundmass
Whole Rock
> 1 Ma
Low-K
Rocks
< 1 Ma
Furnace
MAP 215-50
100 mg 300 mg
CO2 Laser
MAP 215-50
5-10 mg 50-90 mg 50-90 mg 150 mg
CO2 Laser
ARGUS VI
from single crystals
up to 2 mg
10-20 mg 10-20 mg 50 mg

The above amounts are the final amounts of clean geological material being analyzed during incremental heating experiments. If you send us already prepared samples, we request you send us double the above amounts to allow for final sample cleaning, inspection and hand-picking conducted by the OSU Argon Geochronology Lab personnel.

Instrument Selection

Selecting whether to have your samples analyzed using the MAP 215-50 single-collector or the ARGUS VI multi-collector depends on the age and composition of your sample, the required end precision of the age determination, and how much and the type of high-quality material you have available. Below you find two tables (one for older and one for younger geological materials) with our general suggestions, but please Contact Us if you have special requirements.

OLDER GEOLOGICAL ROCKS AND MINERALS (> 1 Ma)

Material
Types
Furnace
MAP 215-50
CO2 Laser
MAP 215-50
CO2 Laser
ARGUS VI
Groundmass
Whole Rock
X X X
Low-K Minerals
(plagioclase)
X X
Low-K Rocks
(tholeiitic basalt)
X
High-K Mineral Separates
(sanidine, hornblende, biotite, leucite)
X X
High-K Single Crystals
(sanidine, hornblende, biotite, leucite)
X
High-K Rocks
(granite, diorite)
X X
High-K Alteration Minerals
(sericite, celadonite)
X
High-K Ore Minerals
(adularia)
X

YOUNGER GEOLOGICAL ROCKS AND MINERALS (< 1 Ma)

Material
Types
Furnace
MAP 215-50
CO2 Laser
MAP 215-50
CO2 Laser
ARGUS VI
Groundmass
Whole Rock
X
Low-K Minerals
(plagioclase)
X
Low-K Rocks
(tholeiitic basalt)
X
High-K Mineral Separates
(sanidine, hornblende, biotite, leucite)
X X
High-K Single Crystals
(sanidine, hornblende, biotite, leucite)
X
High-K Rocks
(granite, diorite, tonalite)
X X
High-K Alteration Minerals
(sericite, celadonite)
X
High-K Ore Minerals
(adularia)
X