Argon argon dating limitations
When a rock undergoes metamorphism, some or all of its argon can be outgassed.
If all the argon was lost, this would reset the K-Ar clock to zero, and dating the rock would give us the time of metamorphism; and if we recognized the rock as metamorphic this would actually be quite useful.
If so, then the K-Ar and Ar-Ar "dating" of crustal rocks would be similarly questionable.
When muscovite (a common mineral in crustal rocks) is heated to 740°-860°C under high Ar pressures for periods of 3 to 10.5 hours it absorbs significant quantities of Ar, producing K-Ar "ages" of up to 5 billion years, and the absorbed Ar is indistinguishable from radiogenic argon ( In other experiments muscovite was synthesized from a colloidal gel under similar temperatures and Ar pressures, the resultant muscovite retaining up to 0.5 wt% Ar at 640°C and a vapor pressure of 4,000 atmospheres.
A second problem is that for technical reasons, the measurement of argon and the measurement of potassium have to be made on two different samples, because each measurement requires the destruction of the sample.
If the mineral composition of the two sample is different, so that the sample for measuring the potassium is richer or poorer in potassium than the sample used for measuring the argon, then this will be a source of error.
Finally, we must consider the possibility of argon loss.Argon, on the other hand, is an inert gas; it cannot combine chemically with anything.As a result under most circumstances we don't expect to find much argon in igneous rocks just after they've formed.Download Potassium-Argon and Argon-Argon Dating of Crustal Rocks and the Problem of Excess Argon PDF According to the assumptions foundational to potassium-argon (K-Ar) and argon-argon (Ar-Ar) dating of rocks, there should not be any daughter radiogenic argon ( represents primordial Ar carried from source areas in the earth's mantle by the parent magmas, is inherited by the resultant volcanic rocks, and thus has no age significance.However, are all other rocks in the earth's crust also susceptible to "contamination" by excess emanating from the mantle?