5.6 Discussion of Tracer-Specific Issues

Aside from the uncertainty in age discussed in Section 5.5, a significant limitation of the [latex]{SF}_{6}[/latex] dating method results from subsurface sources referred to as terrigenic [latex]{SF}_{6}.[/latex] For example, Harnisch and Eisenhauer (1998) found [latex]{SF}_{6}[/latex] in naturally occurring fluorite and igneous rocks containing fluorite. Busenberg and Plummer (2000) measured [latex]{SF}_{6}[/latex] concentrations in excess of atmospheric solubility in hot springs located in volcanic areas that indicated the existence of a natural subsurface source. Busenberg and Plummer (2000) extracted [latex]{SF}_{6}[/latex] from fluid inclusions in crushed rocks and minerals and found detectible amounts in most of the samples tested, especially those from igneous rocks. Koh and others (2007) found evidence for terrigenic [latex]{SF}_{6}[/latex] on Jeju Island, Korea (which is of volcanic origin), by comparing [latex]{SF}_{6}[/latex] tracer ages with ages from CFC‑12. They found that [latex]{SF}_{6}[/latex] ages were biased young relative to CFC‑12 ages by typically 20 years but as much as 30 years.

The presence of terrigenic [latex]{SF}_{6}[/latex] is obvious when aqueous concentrations exceed the solubility plus excess air concentration for modern water. Small additions of terrigenic [latex]{SF}_{6}[/latex] that bias age are much harder to discern. As a practical matter, an [latex]{SF}_{6}[/latex] tracer age should be considered a minimum age unless the presence of terrigenic [latex]{SF}_{6}[/latex] can be ruled out. Potential ways to evaluate the existence of terrigenic [latex]{SF}_{6}[/latex] include the following.