Re: how radioactive dating is used to find the age of Vishnu Schist

I'm not familiar with Vishnu Schist, so I can't speak to the specifics of 
how it has been dated.  However, radioactive dating in general is applied 
as follows:
Several common elements (e.g. Carbon, Argon, Potassium, etc.) are somewhat 
radioactive (one or more of the isotopes of the element are naturally 
radioactive).  As a particular item ages (whether it is a rock, a bone, or 
a piece of wood), the radioactive material in it decays into 
it's "daughter products," leaving the nonradioactive portions unchanged. 
By taking a precise measurement of the ratio of the nonradioactive portion 
of the item compared to the radioactive portion, the "decay time" of the 
element, and thus it's age, can be determined. (This is a fairly 
simplistic model.  If you're interested in researching this topic, please 
go to http://www.radiocarbon.org/ to find more in-depth information).

Elements behave differently in the biosphere (Carbon behaves differently 
than Argon, for instance).  Because of these differences, several systems 
of dating have been developed.  Carbon 14, for instance, is a convenient 
method to date organic materials less than about 50,000 years old.  This 
is because most living organisms acquire Carbon thoughout their lives 
(steady state conditions, steady state ratio between stable Carbon and 
Carbon 14), but when they die they stop acquiring Carbon.  Thus, the ratio 
between stable Carbon and Carbon 14 starts to change, as Carbon 14 decays 
away (it decays to an isotope of Nitrogen).  For the Carbon 14 system, 
the "clock" starts when an organism dies.  Because the half-life of Carbon 
14 is only about 5,700 years, it cannot be used to date very old materials
(essentially all of the Carbon 14 is gone in about 50,000 years, rendering 
the system useless for materials older than that).  In other systems the 
clock is started in other ways.  For instance, a rock which melts will 
lose any volatile elements that may be present in the rock.  Some 
radioactive materials happen to decay to volatile elements.  Therefore, 
when the rock resolidifies, the clock starts, because the ratio of the 
radioactive parent to the volatile daughter can be measured and the 
elapsed time determined.
Lots of information and links are available at the website of Journal of 
Amateur Paleontology: http://www.fossilnews.com/