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The department is equipped with mineral separation facilities. We can process your sample prior to analyses for a fee and generally recommend that samples are prepared by us for the best results. If preferred, samples can be sent already prepared. Some general guidelines are given here regarding sample processing and treatment, size fractions needed, and amount of material needed for 40Ar/39Ar analysis; it is recommended that you contact us for more detailed information prior to sending your samples.
NOTE: It is best to avoid using fine grained (<180 μm sieve size) grains. After irradiation, very fine grained separates become radioactive powder and present serious concerns for contamination in the lab.
Groundmass represents the matrix of a igneous rock between any phenocrystic phases. The phase is prone to alteration but often contains the highest concentrations of K within the inter-crystalline mesostasis. When working with groundmass (really for all samples which may be dated), it is highly suggested to look at a thin section first. For volcanic groundmass, holocrystalline (e.g. sugary) textures provide the best results as the coarser grain sizes tend to more efficiently preserve the mesostasis. Zeolite inclusions (most often found as vesicle infill) are detrimental to the analyses and typically invalidate the sample as a groundmass candidate. A glassy groundmass is analyzable if it is generally unaltered. However, glass is prone to open-system behavior and alteration by water and therefore tends to less reliably produce statistically valid age plateaus.
When preparing a groundmass (or any separate) homogeneity in grain size (300-250um is a typically a good size) and appearance is vital. This ensures that you are degassing the same phases between grains at the same time and increases the likelihood a statically valid plateau develops. To reduce interferences from secondary alteration (e.g. clay recrystallization) we employ acid leaching treatment to samples prior to irradiation. Acid leaching procedures are outlined in the methods of Konrad et al. (2018-Nature Communication). Briefly, they consist of subjecting the crushate to sequentially heated (50oC) and sonicated baths in 3N HCl, 6N HCl, 1N HNO3 and 3N HNO3 for 1 hour each, with rinsing between each step. The samples are then subjected to a final 60-minute bath in ultrapure H2O prior to drying overnight in a 50oC oven. It is imperative that HCl never be the final stage in the acid leaching procedure as the Cl will contaminate the mass spectrometer. Samples are then re-sieved and handpicked to generate a pure separate. Micro- and phenocrysts, especially olivine, must be removed as they may carry excess argon.
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 or should be subjected to acid leaching procedures. Given that the K/Ar isotopic clock in plutonic and metamorphic rocks can be reset at partially or fully at different temperatures for different mineral phases it is almost always better to generate individual phase separates as opposed to attempting a whole rock analysis.
Depending the age and K-concentration of your sample, ~40 mg is usually sufficient. For ultra-depleted basalts (like mid-ocean basalts) or very young (e.g. Pleistocene or younger), we may need up to 100 mg. We are able to measure as little as 1 mg of material in some instances and if you are working with microsamples we ask you contact us to discuss prior to sending your samples.
Mineral Separates for Step Heating Analysis
The general rule is that the coarser size fractions (0.3 – 1 mm), 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. Rinsing the mineral separates in acetone followed by distilled water in an ultrasonic bath will aid in final cleanup also. For sanidine grains we recommend lightly leaching the grains in HF (~5% for 5-10 minutes) after heavy liquid separation. In our laboratory we subject plagioclase, hornblende and clinopyroxene mineral separates to the same acid leaching procedures as outlined in the groundmass section above. Plagioclase feldspar grains are subjected to an additional 10-15 minutes of leaching in 5% HF to remove any potential sericite alteration. We need approximately 20-40 mg of a typical mineral separate for analysis. We are able to perform single crystal sandine analyses.
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 or running the sample through a magnetic separator 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 ~180-212 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. It is not recommended that mica be subjected to acids and instead sonicating and rinsing in acetone prior to irradiation is performed.
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 180-300 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.