Re: Do scientists agree on the age of the universe and earth?


John Harshman wrote:
> Stuart wrote:
>
> > Noone Inparticular wrote:
> >
> >>Stuart wrote:
> >><snip>
> >>
> >>>If you define the Earth as being formed when it was
> >>>closed to the U-Pb isotope system, then the age is ~4.55 billion years.
> >>
> >>Stuart, what (in a nutshell) does "closed to the U-Pb system" mean?
> >
> >
> > "Closed" means the Earth stopped accumulating U,Pb and became a closed
> > system with respect to those isotopes.
>
> Presumably you really mean that the individual zircons being measured
> stopped accumulating U and Pb; i.e. they cooled enough that diffusion
> stopped, and never got hot enough thereafter for diffusion to start up
> again. That says nothing about when the entire planet became closed,
> unless you assume that the zrcons must have formed at such a time.

The quoted age of the Earth is not determined from zircons, the oldest
of which give ages of ~4.4Gyr and those old zircons are the oldest
terrestrial materials. But the age of the Earth cited is based on the
meteorite isochron and some old terrestrial lead ores. In actuaility,
the age represents the age of the Earth-meteorite system or when the
lead isotope composition for this system was last uniform.

Stuart

.

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