From message 48:
quote:
One of the strongest pieces of evidence I know of for accelerated decay is the high retention of radiogenic 4He in microstopic zircons. Figure 5 summarizes the data [Gentry et al., 1982b]. These zircons, about 75 microns long, are embedded in crystals of biotite (black mica). In turn, the biotite is embedded in hot Precambrian basement granodiorite (granitic rock) below the Jemez volcanic caldera near Los Alamos, New Mexico. Radioisotopic (Pb-Pb) dating of zircons recovered from deep boreholes in the formation give an age of "1.5 billion years" [Zartman, 1979].
My bolds.
and
quote:
Diffusion rates of radiogenic He through bare zircons, not embedded in other crystals, have been measured, as Figure 6 shows [Magomedov, 1970]. Those rates are too fast to retain the He for more than a few decades even at room temperatures. However, the biotite crystals in which the Jemez zircons were embedded could "bottle up" the He in the zircons, causing longer retention times. So the real question is: how fast does He diffuse through biotite?
I am presuming that these difusion rates were measured at 1 atmosphere of pressure.
Now, all this discussion is much to messy and complicated for a meer Moose brain to follow, but I will ask the question:
Were presure considerations made, in studying the diffusion rates?
Apparently at least some of the zircon samples were taken from a considerable depth (and higher much higher than atmospheric presure).
Moose