Re: Early Earth likely had continents, was habitable, according to new study

In article <1132435134.485984.230710@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
John Curtis wrote:
> The age of zircons (zirconium silicates)
> on the ocean floor correlates with magnetic striping.
> http://www.newswise.com/articles/view/515684/
>
OK, now I see where you're coming from.
To precis the article: U-Pb dating of zircons extracted from
(unspecified mid-ocean ridge rocks) reveals a generally good
correlation with ages estimated from geophysics readings (magnetic
changes, deduced spreading rates), but with variations in detail. That
adds up to - where a constant spreading rate has been *assumed* between
reversals, application of a novel dating technique shows that rate is
not as constant as the assumption implies.
No-one (on the sane side of creationists) disputes that zircon
is a good material for dating - chemically resistant, relatively high
initial concentration of radioactive nuclides (Th and U) and low
initial concentration of daughter isotopes (variations on Pb). The only
unfortunate factor is the low decay constant of the decay sequences
that operate. That's why SHRIMP techniques seem to have been necessary.
What does appear to have been novel in this work is that the
authors appear to have been the first people to (successfully) extract
zircon from mid-ocean ridge material. That's not terribly strange,
since zirconium is an "incompatible" element - it doesn't fit well into
the lattices of common iron-magnesium-aluminium silicates, so it gets
concentrated into the residual liquors. In consequence, the content of
zirconium in granite and granitoids is factors of <several> greater
than in granites, and so the basalts of a mid ocean ridge are not an
area that you would go looking for zircons.

This is a useful and valuable result for the detail of
geochronology, and in particular for studying the detail of mid-ocean
ridge processes and development. But at $250,000 for 17 data points,
it's not going to replace aeromagnetic (or even satellite magnetic?)
surveying for bulk work. I wonder how much of the cost went to doing
the SHRIMP analyses, how much to sample recovery (time on ROVs and/ or
subs is not cheap), and how much to sample processing (crushing
substantial numbers of kilos of rock, sieving the powder down, then
doing a heavy liquid separation to concentrate the (small)
concentration of zircons, and finally hand-picking the zircons under
the microscope. A lot of work in the fume cupboard and a lot of hand
work under the microscope.).

I still don't see where the claim that
> To form zircons, or any other silicates, requires oceanic
> depths around 2 km.
comes from though. I've got an idea of what *I* think *you* are
*trying* to say, but I'd prefer you to tell us what you're trying to
say.

--
Aidan Karley,
Aberdeen, Scotland,
Location: 57°10'11" N, 02°08'43" W (sub-tropical Aberdeen), 0.021233

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