Paradigmnoia wrote "Regarding Earthly circumstances, there is a huge amount of data available, and no sign of the changes occurring that you suggest."
It depends on what you mean by Earthly circumstances.
The conditions in the mantle are completely different from those at the surface of the Earth
There are no decay rate data conducted in a combination of high hydrogen concentrations, in the presence of nickel and iron lattices under conditions of cavitation, crystallisation and recrystallization from molten silicates, varying electrochemical currents and magnetic fields, as far as I know
However there is some indication that nuclear decay rates can change.. unfortunately you'll need a subscription to access these 4 articles and I have no idea how reliable the data is.
Phenomenological Rules for Nuclear Metabarysis
Riccardo Capotosto and Francesca Rosetto
J. Adv. Phys. 5, 80-83 (2016)
Deformed Space-Time Reactions: Towards Nuclear Metabarysis
Gianni Albertini and Riccardo Capotosto
J. Adv. Phys. 5, 84-89 (2016)
Isotopical Changes in Piezonuclear Iron
Fabio Cardone, Monica Lammardo, Andrea Petrucci, Alberto Rosada, and Emilio Santoro
J. Adv. Phys. 5, 90-96 (2016)
On the Possibility of Induced -Decay of Uranium
L. I. Urutskoev, D. V. Filippov, A. A. Rukhadze, K. A. Alabin, and A. A. Levanov
J. Adv. Phys. 5, 97-103 (2016)
The proper examination of zircon U-decay rates in mantle circumstances is a major undertaking.
We need to mimick the journey of the uranium ions into the zircon crystal and its subsequent tortuous passage to the Earth's surface. Along the way we might examine the following questions
What are the permeabilities for H2, H in silicate melts and crystals?
What are the concentrations for H2, H in silicate melts and crystals?
What is the role of Zirconium.?Can it act as an catalytic generator of protiums from hydrogen
as nickel is postulated by Piantelli to do in Ni/H2 LENR? Will it be converted to heavier atoms?
What isotopic reactions are generated for a range of different keV protiums, alpha particles?
What ash might be left? Can full isotopic analysis of zircons yield clues to isotopic reactions.
What Helium isotopes are expected to be generated. Can these be detected in Zircons.
W hat is the effect on zirconium isotope ratios?
What is the role of and isotopic effect on silicon and oxygen?
What alternative pathways to Pb for Thorium, U-235. U-238 might be possible under LENR conditions.
Before this major undertaking a few simple experiments to show that LENR does provide quicker, alternative pathways for decay might be useful starting with less dangerous Beta emitters such as
Sr-90 Cs-137.
Surface circumstances are much easier than mantle circumstances. We already know that at the surface zircon U-decay rates are constant, and that is what is used for rockdate calculations. But rocks are not made on the surface... they are made in the mantle