(February 19, 2023 at 11:40 am)Anomalocaris Wrote:(February 18, 2023 at 9:34 pm)polymath257 Wrote: It should also be pointed out that Rb-Sr dating is self-correcting in this way. You need to have several different crystals from the same rock, but you can derive the amount of the parent isotope from the measurements.
But Rb-Sr dating is still susceptible to the amount of Rb and various Sr be changed by low grade metamorphosis after rock body formation. You still need corroboration from other dating methods, or have ways for controlling for impact of low grade metamorphosis, such as good agreements in Rb-Sr dates across stratigraphically correlated rock formations over an area large enough to be implausible for the different rock samples to have undergone the same metamorphic history.
Metamorphism typically isn't a problem for Rb-Sr ages because the closure temperatures for most minerals are high enough that any event hot enough to reset them becomes pretty obvious. We actually use the closure temperatures of different minerals to produce different ages that result in chronologies of metamorphic events.
K-Ar and Ar-Ar systematics are much more prone to resetting or loss at lower temperatures and are rarely used much anymore except on pristine materials or when trying to actually track low temperature events. That's because the argon daughter product is a noble gas and doesn't bind into any crystal structure, so the system is leaky as hell.
For really robust ages go with U-Pb concordia ages in zircon. You get two independent uranium decay series working side by side in a mineral that gleefully refuses to reset in anything up to and including having its host rock melt. On a bright day you can pull a Th-Pb date as a bonus. Just make sure you aren't analyzing detrital grains by accident or you're totally buggered.
