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2.3.8  Normal and Inverse Isochron Menus

Individual menus are available for the Normal and Inverse Isochrons, allowing you to change a variety of Display options for each isochron separately and to toggle the calculations between Weighted/Unweighted YORK-2 Fits.

Display Labels for Selected Data Points

You can show or hide Labels for the Selected data points (green squares) in your isochron diagrams by toggling this menu item. The labels always show you the Temperature for a specific (incremental heating) data point. For example, 0.06W or 1500°C.

Display Labels for Deselected Data Points

Alternatively, you can show or hide the same Temperature Labels for the Deselected data points (blue squares).

Display Isochron Line

With this menu item you can show or hide the calculated Isochron line (pink line). The isochron line is shown by default, except if less than 3 data points are included in the calculations.

Display Reference Line

You can also show or hide the Reference line (dark grey line) that depicts the line between the Plateau Age Intercept on the 39Ar/40Ar axis and the Atmospheric Argon Intercept (295.5) on the 36Ar/40Ar axis. In a perfect case, the reference and isochron lines overlap on the 95% confidence level. The reference line is shown by default, except if less than 3 data points are included in the calculations.

Display Error Ellipses

To show or hide Error Ellipses in the isochron plots toggle this menu item. Since the ratios on the axes of the isochron diagrams have errors that are correlated, the ellipses always become (slightly) inclined. This is more clear in normal isochrons than in inverse isochrons, where in the normal isochron diagrams the ellipses often get elongated extensively and rotated to line up with the isochron itself. In that case, it is often easier to hide the error ellipses from view, which also is the default setting in ArArCALC for normal isochrons. For the inverse isochrons, the ellipses are shown by default.

Display Total Fusion Point

The Total Fusion Point (red circle) can be shown or hidden in both isochrons. By default ArArCALC shows this point, because in a closed system, this point is expected to coincide with (i.e. falling on) the isochron line. These points thus provide you with an easy visual check on this particular characteristic for your sample.

Apply Weighted YORK-2 Fit

By default the Isochron calculations are performed using Weighted Errors (York, 1969).

Apply Unweighted YORK-2 Fit

In some cases, you might want to use an Unweighted YORK-2 Fit. A good example is, if one of your data points has an unusual small error (because you employed a too large incremental heating step) and thus (unrealistically) outweighs all other data points in your isochron calculations. By choosing this option, a linear fit without any weighting gets performed, and thus ignores the unevenness in your heating schedule.