[Rockhounds] when volcanoes drop bombs: giant crystal payload

[Axel Emmermann]

Yes Pete, but a continuous ejection of matter wouldn't yield any sizeable
diamonds would it?
I was under the impression that the diamonds formed in a plug that is
(considerably) later ejected, after the diamonds have formed. Of course, the
plug can reach the surface in an eruption but I thought that the diamonds
have to form at great depth and gradual cooling over prolonged time. Not
formed during the eruptive flow of magma.
Is this wrong?

Axel


> -----Oorspronkelijk bericht-----
> Van: rockhounds-bounces at lists.drizzle.com 
> [mailto:rockhounds-bounces at lists.drizzle.com] Namens pmodreski at aol.com
> Verzonden: woensdag 22 augustus 2007 20:35
> Aan: ajs at frii.com; rockhounds at lists.drizzle.com
> Onderwerp: 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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