[meteorite-list] Earth Scientists Iron Out Their Differences

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:41:53 2004
Message-ID: <200101041729.JAA21320_at_zagami.jpl.nasa.gov>

http://helix.nature.com/nsu/010104/010104-6.html

Earth scientists iron out their differences
PHILIP BALL
Nature Science Update
January 4, 2001

Most substances that make up the planet can be accounted for by standard
theories of how the Earth formed, but some elements are just not where
they're expected to be. In this week's Nature1, J. Wade and B. J. Wood of
the University of Bristol suggest that they may have tracked down one of the
errant elements.

They believe that niobium, which is not, as had been predicted, in the rocky
mantle surrounding the Earth's molten core, but is instead lurking in the
liquid iron at the centre. At the same time, researchers at the University
of Bayreuth have unravelled the behaviour of other elements - vanadium,
chromium and manganese - that don't always appear where they should.

Like a fine Scotch whisky, the provenance of the Earth can be discerned from
the subtle blend of substances that it contains. Most of the solid Earth
consists of just a handful of different elements. The hot core, about 5,600
kilometres across, is mostly iron. Surrounding this is the mantle, 8 to 50
kilometres thick; and atop the mantle is the crust.

The Earth was created when large rocky blobs called planetesimals collided
and coalesced in the early Solar System. The energy released by these
impacts left most, perhaps all, of the planet molten, whereupon the rocky
magma separated from the molten iron. The iron sank to the core, and the
rock (mostly silicate) floated on top.

Some elements are more soluble in iron than in magma. These are called
'siderophiles', meaning iron-loving, and they are thought to have segregated
to the Earth's core when the world was a young fiery ball. Because of this,
the mantle is low in siderophile elements. This becomes clear if we compare
the chemical composition of the mantle with those of a certain kind of
meteorite called chondrites.

Chondrites are almost as old as the Earth. They are thought to be left-over
lumps of the planetesimals from which the planets formed. But they did not
segregate into iron-rich and silicate-rich regions, so their elemental
abundances tell us what the Earth's total inventory of elements would be
like if we could measure it.

Some siderophile elements, such as vanadium and chromium, are more abundant
in chondrites than in the Earth's mantle, suggesting that they must indeed
have flocked to the iron core. But other iron-lovers, such as calcium and
scandium, are surprisingly abundant in the mantle, even though they are
known to dissolve better in iron than in molten silicate. These odd elements
are said to be 'refractory' siderophiles.

And then there is niobium. The chemical properties of this rare element mean
that it should be a refractory siderophile - there should be as much of it
in the mantle as in a chondrite. But, mysteriously, there isn't.

Some geochemists have proposed that the 'missing' niobium is hidden away in
a reservoir in a deep part of the mantle, like a pocket of cocoa in a poorly
mixed chocolate cake. But it is hard to explain how such reservoirs could
have arisen.

Deciding that most of what is known about niobium is assumption and analogy,
Wood and Wade studied the element's behaviour experimentally, at the high
temperatures and pressures characteristic of the deep Earth.

They report that, contrary to popular belief, niobium behaves more like the
siderophiles vanadium and chromium than 'refractory' elements such as
calcium. So, the researchers say, like the other siderophiles, niobium
probably dissolved in the iron core.

In a similar set of experiments reported in Earth and Planetary Science
Letters2, Christine Gessmann and David Rubie of the University of Bayreuth
in Germany looked at the puzzling partitioning of vanadium, chromium and
another siderophile, manganese.

These elements are not as under-represented in the mantle as earlier
attempts to measure their solubility in iron suggest they should be. So
Gessmann and Rubie took measurements at higher temperatures and pressures
than before to get a better idea of how these elements might really have
behaved in the young Earth.

They find that if the temperature is high enough (over 3,300 oC), the
solubility of vanadium, chromium and manganese changes enough to solve the
puzzle. Such temperatures could have been reached following a collision
between the Earth and a large planetesimal - the like of which is widely
thought to have formed the Moon.

                   --------------------------------------

  1. Wade, J. & Wood, B. J. The Earth's 'missing' niobium may be in the
     core. Nature 409, 75-78 (2001).

  2. Gessmann, C. K. & Rubie, D. C. The origin of the depletions of V, Cr
     and Mn in the mantles of the Earth and Moon. Earth and Planetary
     Science Letters 184, 95-107 (2000).
Received on Thu 04 Jan 2001 12:29:03 PM PST