麻豆传媒

THAT ring on your finger may be made of unearthly stuff. According to a
German researcher, the gold and platinum in the Earth鈥檚 crust fell to Earth in
meteorites after the planet鈥檚 core formed.

Earth is known to have coalesced around 4.5 billion years ago from a cloud of
dust and gas that originally had the same composition as stony meteorites known
as chondrites. Over a period of about 50 million years, dense molten metals
settled to form the mainly iron core. Molten silicate rock that floated on the
core cooled to become the mantle.

But the Earth鈥檚 mantle and crust present a puzzling anomaly: they contain
significant amounts of gold and elements of the platinum group, including
palladium. These elements are attracted to iron, and dissolve far better in
liquid metal than in molten rock. If the core formed at the same time as the
whole mantle, it should have soaked up virtually all these metals. But the
mantle contains about 10 parts per billion of platinum-group elements, says
Richard Walker, a geologist at the University of Maryland.

鈥淵ou think that鈥檚 not much, but it should be two orders of magnitude or more
lower, in the parts per trillion range,鈥� he says. At these levels the metals
would be impossible to mine or recover.

Some experts have claimed that the anomaly could be explained if we had more
information about how such precious metals behave under extreme pressure. So
Astrid Holzheid, now at the University of M眉nster, decided to
investigate.

Holzheid melted small samples of rock and metal at pressures never before
attempted in similar experiments, reaching 16 gigapascals鈥攅quivalent to
conditions 500 kilometres below the surface. Her measurements show that platinum
and palladium are as soluble in metal at high pressures as they are at surface
pressures, and therefore should have moved toward the iron core. 鈥淵ou need to
have something which adds siderophile [iron-loving] elements after core
formation,鈥� she says.

She believes that this source was an extra dash of meteorites鈥攁mounting
to about 0.7 per cent of the Earth鈥檚 mass鈥攚hich arrived after the core
formed. Roughly the mass of the Moon, this 鈥渓ate veneer鈥� of chondritic material
would have mixed into the mantle before the still-hot Earth formed a thick
crust, topping up gold and the platinum-group elements to present levels.
Relative concentrations of platinum-group elements in the crust and mantle
鈥減erfectly match the ratios you get out of analyses of these meteorites鈥�,
Holzheid says.

鈥淚t鈥檚 one of the missing pieces of information that people have been waiting
for,鈥� says Walker. He adds that it may point to similar late veneers on the Moon
and Mars.