Re: Age of the Earth

The formation of the
>>>GP> Earth is supposedly determined by this method as 4.6 BYA.  Yet I
>>>GP> undestand that the giant impact which caused to formation of the
>>>GP> Moon occurred at about 4.3 BYA. 

Where does this 4.3 Ga number for the formation of the moon come from? 
Some of the returned rock samples may be at least that old, and these 
ages may reflect shock processing rather than crust formation (which 
itself obviously post-dates 'moon formation'). I don't think the moon 
forming event is all that well constrained, but it could have been 
substantially earlier, maybe only several tens of millions of years 
after the first solids formed in the solar system. For a summary of the 
implications of recent tungsten-hafnium dating see 
www.lpi.usra.edu/meetings/LPSC98/pdf/1759.pdf
and references therein (interpretation of the radiochemistry is somewhat 
model dependent - you have to be very careful about what you mean by 
'formation age').

-- 
Dr. James Whitby           Tel: +41 (0)31 6314409
Physikalisches Institut    Fax: +41 (0)31 6314405
Universitaet Bern
Sidlerstrasse 5
3012 Bern, Switzerland.