Earth’s earliest events

The Earth has a core made, probably, of alloyed iron, nickel and sulphur.  Much evidence points to the core having formed very early in our planet’s history, probably in its first 100 million years.  Core formation explains the depletion in iron of mantle rocks and magmas derived from them, compared with iron’s abundance in the cosmos.  Because some rarer elements have a 10 000 times greater tendency to partition into melts containing metallic iron than into silicates, such siderophile (‘iron-loving’) metals are also highly depleted in the outer Earth.  That is one of the reasons why gold and the platinum-group metals are so rare and highly prized at the Earth’s surface.  In fact, such noble metals are a lot more abundant than the presence of a metallic core could have allowed; they should be at vanishingly low abundances.

One solution to this paradox is that the ‘extra’ gold and PGEs arrived after core-formation had finished, the agency of delivery being continual bombardment by meteoritic debris in the first half billion years of the Solar System’s history.  The other is that somehow, the affinity of such metals for iron drops off at extremely high pressures.  German, Canadian and Australian geochemists  (Holzheid, A. et al., 2000.  Evidence for a late chondritic veneer in the Earth’s mantle from high-pressure partitioning of palladium and platinum.  Nature, v. 406, p. 396-399) have shown experimentally that such a decrease doesn’t occur, at least in the outermost 500 km of the Earth.  This points strongly to impacts having seeded the upper mantle with noble metals, and therefore, perhaps, with lots more besides.  This re-opens the old controversy between homo- and heterogeneous accretion of the Earth, tempered by the fact that more common siderophile metals, such as nickel and cobalt do not show mantle abundances that are in disequilibrium with core formation.  The distinction is not trivial, for much of Earth’s evolution has been driven by its internal composition, most especially its content of radioactive isotopes and water.

The Moon seems to have formed as a result of a gigantic impact of a Mars-sized body with the early Earth.  Since the Moon has neither a core nor its full cosmic complement of iron, such a catastrophic beginning (effectively ‘Year Zero’ for the geochemistry of both bodies) must have taken place after core formation in the Earth.  Because lunar rocks are so little changed by later events, its age is known with considerable accuracy – the Lunar Highlands are about 4450 million years old.  It would be interesting to compare gold and PGE abundances between Earth and its Moon, for that might reveal the period during which bombardment delivered siderophile elements.  Up to 3.8 billion years ago, both bodies received lots of visitors, culminating in a bout of huge impacts between 4.0 and 3.8 billion years ago that formed the huge lunar craters, that early astronomers termed maria or ‘seas’.


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