John Hernlund wrote:

> This is great stuff!  For many years now people have been trying to defend the
> idea that mantle plumes, which may be responsible for many "hot spots" such as
> the Hawaiian islands, come from the lower mantle.  Some people have even
> proposed the base of these plumes to be at or right above the core-mantle
> boundary.  The evidence for this specific case is very solid, with crystals of
> MgSiO3 type perovskite contained in them.  Perovskite is thought to be the
> dominant crystalline MgSiO3 phase in the lower mantle, making it probably the
> single most abundant material in the Earth.  The other metal oxides
> present suggest a depth even deeper than 670 km which is where the famous
> discontinuity is present presumably due to the transformation of
> ringwoodite, clino pyroxene, majorite garnet and a few other minerals into
> perovskite and magnesiowustite.  Only problem is that it is terribly difficult
> to get it up to the surface, hence its rare appearance on the surface of the
> Earth.
>

John,

Think you're absolutely right on with the rising plume answer.  Theres really
no other way it could have made this mass transition otherwise.  Speculate
it must have been a extra large plume to have made the trip up, without the
interaction of other lava mixing it up into less recognizable components
chemically
and its resulting minerology composition.  Time and erosion likely presented it
in its present state.

The article also mentioned possible diamond mining possiblities.  What struck
me was the size of the garnet crystal mentioned.

Thanks, Meredith Lamb




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