Properties of an age spectrum:

• The x-axis is the cumulative percent ³⁹Ar_K released from the sample over the course of the entire step-heating analysis (e.g. step F was heated to 975 degrees celcius and released approximately 20% (70-50%) of the total ³⁹Ar).
• The y-axis on the lower plot is apparent age in millions of years (Ma) in ascending order from 700 thousand years ago (0.700 Ma) to one million years ago (1.00 Ma).
• The individual boxes on the lower plot are the age plus/minus error for each of the heating steps. For example, step B (700 degrees) has a calculated apparent age of approximately 0.820 plus/minus 0.005 Ma. The age is corrected for extraction line and mass spectrometer contributed blank, nucleogenic isotopes and atmospheric contamination (assuming a trapped ⁴⁰Ar/³⁶Ar value of 295.5). The error box associated with the ages may be plotted as 1 or 2 sigma, depending on the preference of the user.
• The margin including steps D through G is called a plateau. It has been commonly defined by the strict criteria of Fleck et al (1977): 3 or more contiguous heating steps comprising 50% or more of the ³⁹Ar_K released and overlapping at the two sigma confidence level. Steps D through G define a plateau meeting these criteria. Steps C and H do not because, while overlapping the immediately adjacient heating steps, they do not overlap step F. The NMGRL no longer relies on the plateau definition of Fleck et al (1977), but rather tests the statistical precision of a number of contiguous heating steps. The plateau age is calculated by weighting each step by the inverse of the variance. The plateau error is calculated using the method of Taylor (1982).
• The age immediately above the x-axis is the total gas or integrated age. This is analagous to a laser-fusion age or conventional K/Ar age and therefore is not used frequently. The integrated age is calculated by weighting the individual steps by the fraction of ³⁹Ar released.
• The box immediately above the age information graph is for plotting the K/Ca ratio of each individual heating step. Commonly plotted as log values, these ratios help to indicate what mineral phases may be degassing at certain temperatures. For instance, the K/Ca ratio for the above sample remains relatively constant at about 15-20 for the initial 975 degrees of heating. But then at 1075 and greater the K/Ca ratio drops, at one point to about 1, indicating a phase with either a relatively larger calcium content or relatively smaller potassium content. In the case of basalts, this lower K/Ca may arise from amphiboles or pyroxenes. K/Cl ratios may also be plotted in this auxilliary box. Changing K/Cl ratios may help determine when fluid inclusions are degassing from a plutonic potassium feldspar.
• The auxilliary plot on the top of the age spectrum graphically represents the radiogenic ⁴⁰Ar (⁴⁰Ar*) yield from the individual heating steps. In the case of the above sample, the first two heating steps have radiogenic yields below 50% (ie. 50% of the ⁴⁰Ar is radiogenic while the remainder is atmospheric and/or nucleogenic). The radiogenic yields for the remaining heating steps is significantly greater than 50%. Ideally, we would like to see radiogenic yields approaching 100%. However, effects such as alteration often contaminate the sample with large quantities of atmospheric ⁴⁰Ar overwhelming the radiogenic ⁴⁰Ar. This problem can be particularily troublesome for very young samples that already have very little ⁴⁰Ar*.

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Fleck, R.J., Sutter, J.F., and Elliot, D.H., 1977. Interpretation of discordant ⁴⁰Ar/³⁹Ar age-spectra of Mesozoic tholeiites from Antarctica, Geochim. Cosmochim. Acta, 41, 15-32.

Taylor, J.R., 1982. An Introduction to Error Analysis: The Study of Uncertainties in Physical Measurements, Univ. Sci. Books, Mill Valley, Calif., 270 p.

 

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