[Rockhounds] when volcanoes drop bombs: giant crystal payload

[Rik Dillen]

Pete, is the siderophilic character of metallic iridium (and many other metals) due to the fact that they (Ir + Fe) form
intermetallic eutectic alloys which act locally (micro-scale ?) as some kind of efficient extraction medium ? If not,
what is the mechanism of the enrichment of iridium in the iron phase ? 

Grts,

Rik DILLEN 
Doornstraat 15,  B-9170 Sint-Gillis-Waas 
Belgium 
E-mail rik.dillen at skynet.be 
Homepage : http://users.skynet.be/rik.dillen 
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-----Original Message-----
From: rockhounds-bounces at lists.drizzle.com [mailto:rockhounds-bounces at lists.drizzle.com] On Behalf Of pmodreski at aol.com
Sent: Wednesday, August 22, 2007 9:35 PM
To: ajs at frii.com; rockhounds at lists.drizzle.com
Subject: Re: [Rockhounds] when volcanoes drop bombs: giant crystal payload

Axel,

No, that's not correct, that kimberlites are only exposed by erosion.? It's true for some; some kimberlites are only
intrusive plugs that did not make it to the surface; but for most, they did make it to the surface and erupt.? That's
how they form what we call "diatremes", which are pipes that exploded at the surface forming a crater, and with a lot of
rubble (breccia) that fell back into the crater.

And Alan, the reason there's not much iridium in rocks we see in the crust, is that when the Earth's core formed, the
portion of the iron that was in the reduced, metallic state sank to form the core, and with it, almost all of the
elements like iridium that are strongly attracted to the metallic iron (we normally call them "siderophile" elements).?
This removed nearly all the iridium down into the core.? But not all the iron was metallic iron, some was in the form of
oxidized iron, so this did not sink but remained in the crust and mantle, providing the iron that exists in olivine,
pyroxenes, magnetite, etc.? Iridium is only attracted to metallic iron, not to the oxidized forms of iron, so nearly all
the Ir really did settle with the iron in the core.? And any material that is in the core rarely or probably, never,
makes it back up into the mantle or crust.

Pete




-----Original Message-----
From: Alan Silverstein <ajs at frii.com>
To: rockhounds at lists.drizzle.com
Sent: Wed, 22 Aug 2007 11:40 am
Subject: Re: [Rockhounds] when volcanoes drop bombs: giant crystal payload



A recent article in Science News shared a theory that it's some kind of
carbon dioxide phase change and rapid depressurization that sets off the
kimberlite ascent.

I guess if you cool/depressurize a diamond too slowly, it decomposes,
but faster, it "freezes" in place.

Apparently kimberlite pipes are relatively common, although hard to find
after lots of weathering, but only about 1% contain diamonds.  I think
the South African deposits are alluvial reconcentrations (now buried) of
diamonds weathered out of ancient pipes.

Given that we live on "floating scum" rocks (the continental crust), I
find it amazing that we have such a wide variety of elements and
minerals present at all.  Heavier stuff sinks down, but enough of it
either chemically bonds (like iron I suppose) or is churned back up to
be found on or near the surface.

Hmm...  I read that iridium, liking to bond with iron (ferrophile?), is
almost all in the core, which is why it's rare on the surface and a
hallmark of newly fallen space debris.  But why doesn't iridium stick
around or reappear on the surface just like iron or other "rare, heavy
metals"?

Alan Silverstein
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