Analytical Approaches

Instruments in the WiscAr lab allow us considerable flexibility in how we obtain isotopic compositions, and thus determine the age, of various types of samples. The principal approaches include the following:

  1.  Laser fusion analysis of single crystals
  2.  Laser incremental heating of single crystals or small aliquots of K-bearing minerals
  3.  Laser incremental heating of volcanic groundmass samples
  4.  In-situ ultraviolet laser ablation microprobe (UVLAMP) analyses

Before any of these types of analysis are undertaken, the purified sample must be irradiated in the fast neutron flux of a nuclear reactor. The length of time needed to irradiate a sample is proportional to its age.  This is because we are converting some of the 39K into 39Ar as a proxy for the parent isotope of 40K, and it is desirable to generate a 40Ar/39Ar ratio in the sample as close to unity as possible.  The method works because the ratio of 39K to 40K is constant in the earth, so measuring 39Ar in turn gives us 40K.  We use the research reactor at Oregon State University.

Laser fusion analysis of single crystals

Total fusion of analyses of individual sanidine crystals from the V4 tephra in the Montalbano-Jonico marine section, Italy (Petrosino et al., 2015)

Laser fusion analysis is typically used to date pyroclastic rocks such as ash fall or flow deposits. The Noblesse analytical system is outfitted with fully automated CO2 lasers that emit energy with a 10.6 micron (infrared) wavelength.  This laser energy is focused through a ZnS (sphalerite) viewport into a sample chamber and vibrates bonds in crystal lattices and thus heats geological materials, releasing argon gas.   As many as 150 individual crystals can be loaded into the vacuum system on a copper tray in the sample chamber.  The system can be programmed to automatically analyze these crystals.

The Noblesse system is used for total fusion of monitor minerals (Alder creek sanidine, Fish Canyon sanidine, etc) associated with each irradiation and for total fusions of individual crystals that yield small Ar signals (i.e., Pleistocene samples, K-poor minerals such as plagioclase, or very small Cenozoic to Mesozoic minerals).   By fusing tens of crystals one at a time, enough data can be generated to potentially obtain age constraints and assess any the potential causes (geologic, analytical, mineralogic, etc.) of the data dispersion. It is common to undertake 30-60 individual laser fusion measurements of single crystals separated from pyroclastic rocks such as ash fall or welded tuff samples.

Laser incremental heating analysis of single crystals or small aliquots of K-bearing minerals

Age spectrum and corresponding K/Ca diagram showing results of incremental heating experiment on 15 mg of clinopyroxene from a Northwest Hawaiian Ridge basalt using the NGX system (Jicha et al., 2024). 

Incremental heating experiments are used widely to obtain ages for volcanic, plutonic, and metamorphic rocks because the resulting age spectrum can help reveal whether or not the sample is altered or affected by thermal disturbances. Incremental heating is accomplished by heating the sample in a series of successive steps, each using a higher laser power than the preceding step. The gas generated during each 60 second heating step with the defocused CO2 laser is then scrubbed free of reactive molecules on SAES non-evaporable getter pumps for several minutes along with exposure to a cryo trap for several more minutes before admission to the mass spectrometer.  Careful assessment of analytical blanks and other instrument parameters are required throughout an incremental heating experiment, which can last up to 24 hours or more depending on the age and size of the sample analyzed. The WiscAr lab can analyze many different minerals including alunite, amphibole, biotite, clinopyroxene, glass, muscovite, leucite, phengite, plagioclase, sanidine, and tourmaline. 

 

Laser incremental heating of volcanic groundmass

Incremental heating spectra from groundmass from a basaltic andesite on Mocho Choshuenco volcano, Chile (Moreno-Yaeger et al. 2024).

The sensitivity of the low-temperature ion source and the increased stability and large dynamic range of the ATONA®-backed Faradays makes the NGX-600 mass spectrometer ideally suited to analyze Plio-Pleistocene volcanic groundmass samples. About 20-35 milligrams of purified groundmass separate (150-250 microns in size) are placed into a 5-millimeter diameter well in a copper tray and incrementally heated with a defocused CO2 laser in ~15-35 steps. The laser beam is defocused to evenly heat the entire sample during each step of the experiment. The actual amount of groundmass required for an incremental heating experiment depends on the age and composition of the sample. A typical incremental heating experiment of groundmass including accompanying air and blank measurements takes approximately one day. Not all groundmass from lava samples yields a plateau. The success rate for a large suite of volcanic samples may vary based on the degree of alteration, age, and the thermal history of the sample suite.