[Rockhounds] Source of heavy elements in crust?

Tue Sep 23 02:49:47 PDT 2008

Hi Brian & Alan & Rik & list ;-)

The metals discussed in the article are the so called HSE. Highly
siderophilic means that they bond to iron or prefer to stay dissolved in the
iron core of the planet. A bit like gold likes to stay in mercury?
The crust pulls itself down by it's weight when sheets of basaltic rock sink
in the mantle in subduction zones.  Basalt being heavier than granite.
Tectonic plates sink into the mantle so the heat of the friction would melt
both the sinking sheet and the contact zone with the mantle, wouldn't it? So
ejected material as a result of subduction zone volcanism would have to
contain stuff from both mantle AND crust (and much carbonates from the
seafloor).
Since all movement happens in the upper mantle and crust, it would be
logical to assume that none or very little of the HSE are redistributed.
They are in the core and they stay there, I guess. 

All the elements heavier that iron are formed in supernova explosions
(although Co and Ni may form through a process in which iron nuclei capture
slow moving neutron which decay through internal transition in to a proton
an electron and an antineutrino, thus producing cobalt and nickel).  
Some of the lighter elements like lithium form through cosmic radiation
bombardment of heavier elements. They are just fragments that are shot off
heavier unstable elements.
Other elements form as a decay product of unstable isotopes. Lead is a good
example.
Rare earth elements ARE the heaviest so they came into being "as is". 

Stars form in the wake of a compressing agent that sweeps through
interstellar material and disturbs its gravitational equilibrium. Usually
one of the spiral arms that passes through but a supernova remnant crashing
into the interstellar dust and gas has the same effect.  That material is
riddled with heavier elements from millions of past supernovae. It stands to
reason that any "heavenly body" that forms in such regions sweeps up these
elements.
In planets or large moons, the formation generates great heat which melts
them causing the molten material to segregate by gravity. Iron sinks to the
core, taking the HSE with it.
This does not happen in smaller bodies like asteroids. They are small and
cool too quickly to allow for segregation if they ever were in  a liquid
state to begin with. Their composition would be much more constant
throughout their volume.

So I guess that meteorites could indeed be a source of HSE in the crust of
our planet and some of the mantle (by deep impacts).

Cheers
Axel

> -----Oorspronkelijk bericht-----
> Van: rockhounds-bounces at lists.drizzle.com
[mailto:rockhounds-bounces at lists.drizzle.com]
> Namens J Bryan Kramer
> Verzonden: maandag 22 september 2008 22:18
> Aan: Rockhounds at drizzle.com: A mailing list for rock and gem collectors
> Onderwerp: Re: [Rockhounds] Source of heavy elements in crust?
> 
> I guess if this were true then there would be higher levels of heavy
> elements in the crust than in the mantle. How you would measure the metal
> content of the mantle I wouldn't know. Magma is from the crust isn't it?
> 
>  I think 'precious' was added by the headline writer who probably doesn't
> know an element from an aardvark.
> 
> BK
> 
> On Mon, Sep 22, 2008 at 16:01, Alan Goldstein
<deepskyspy at insightbb.com>wrote:
> 
> > It would be a safe bet to say the geology of Earth is a reflection of
the
> > composition of the formation of the Solar System. Whether precious
metals
> > were accumulated during the initial collision of planetesimals or the
rain
> > of meteorites during the first billion years of the Earth's history
doesn't
> > rate among the more pressing issues of geology for me. Have any precious
> > metals been found in any meteorite? They are "precious" because of their
> > scarcity. Since meteorites are relatively scarce as well, one would
expect
> > "scarce squared" would indicate that you are not likely to find gold,
> > silver, etc. in meteorites.
> >
> > On a related matter, geologists say that earth's oceans are from comets
> > colliding with our planet.
> >
> > Alan
> >
> >
> > ----- Original Message -----
> > From: "J Bryan Kramer" <codeburner at gmail.com>
> > To: "Rockhounds at drizzle.com: A mailing list for rock and gem collectors"
> > <rockhounds at lists.drizzle.com>
> > Sent: Monday, September 22, 2008 10:36 AM
> > Subject: [Rockhounds] Source of heavy elements in crust?
> >
> >
> > This fellow thinks the crust was seeded with  heavy elements from meteor
> > strikes 4 GYa
> >
> > <
> >
> >
http://www.universetoday.com/2008/09/22/earths-precious-metals-could-be-from
-
> meteorites-and-asteroids/
> > >
> >
> > BK
> >
> >
> > --
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> 
> 
> --
> 
> 
> "The thunderbolt falls on an inch of ground; but the light of it fills the
> horizon."
> 
> Ralph Waldo Emerson
> 
> J Bryan Krämer
> North Florida, USA
> photos at:
> http://pbase.com/photoburner
> 
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