"I don't know the greater context, but you're textbook quote is a simplified view of the U/Pb process. Hopefully, it was presented as such."
--Yes it was very simplified. While simplification is fine, to go without mentioning or hinting at discordant findings and other things along that line is a bit misleading as a student learns these things. What is also given in the book is that the decay of radioisotopes has been shown to be unaffected by environmental conditions. While this is in the majority, true, in the students mind this is a stumbling block because they now may have the impression that a statement such as 'the greater the percentage of lead present in the sample, the older the rock is' and then going on to say 'The U-Pb ratio can be used only when all the lead in the rock is known to have come from the decay of uranium' gives them the impression that the latter is irrelevant but was mentioned anyways.
"Now, as I understand it, the U/Pb process is a pretty strong method. It is usually done on zircons, which are quite solid containers of the elements involved."
--Zicons themselves in SHRIMP analytical technique, for instance, radiogenic Pb is shown to vary within most tested zircon grains on a 20um spatial scale[such as in: Compston; 1997]. Some spots are found to show excess Pb of up to 30 times expected values so I wouldn't agree that they are very uniformly trustable as a dating method for zircon isochrons. Heterogeneity is vastly out of order. There is also an approximately sinusoidal, variation in
206Pb/
238U apparent ages with orientations in baddeleyite crystals which are not detected in zircon or monazite crystals. Though radiogenic
208Pb/
206 and
232Th/
238U both vary with orientation. Usually attributed to real compositional variation reflecting zones of anisotropic primary crystal growth. Different effects in mineral grains and zones within them renders U-Th-Pb dating in high question and is pretty much up to the investigators' interpretations. Interpretations which are usually based on expectations determined by the geological contexts of the rocks being dated.
--There are also problems with Zircon Inheritance and chemical weathering.
--What do you think of the patterns in mineral U-Th-Pb Ages for Pitchblende, uranitite, monazite, zenotime, monazite, zircon and tanite, samarskite, thorite, titanite, euxenite, etc.?
"Regardless, detailed study of radiometric dating methods sure seems to be beyond the scope of a high school class. Where I encountered in was in an upper-level college geology course."
--I can agree with you here, however for the geochemical processes to go
unmentioned isn't all too good. These types of class discussions with the teacher should be encouraged.
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[This message has been edited by TrueCreation, 07-01-2002]
[This message has been edited by TrueCreation, 07-01-2002]