Silver Isotopes to Date the Earth

The Earth is depleted in some elements, such as hydrogen, carbon, nitrogen and silver, compared to the Solar System as a whole. What scientists can not tell us is when this depletion occurred. Carnegie Institute scientists have used the isotopic ratios of silver in primitive meteorites and rocks from the Earth's mantle to determine the history of Earth's volatiles relative to the formation of Earth's iron core.

Silver has two stable isotopes of which silver-107 was produced in the early Solar System by the rapid nuclear decay of palladium-107, which is so unstable that virtually all of it decayed within the first 30 million years. Silver is more volatile than palladium, while palladium is more likely to bond with iron. The silver isotope evidence suggests that the core formed between 5 and 10 million years after the origin of the Solar System.

Studies using hafnium and tungsten isotopes indicate that the core formed between 30 and 100 million years after the origin of the Solar System.

These apparently contradictory results support the "Heterogeneous Accretion" model of planetary growth in which the Earth's building block's changed composition as the planet accreted. So, at first the Earth accreted volatile-depleted material until it reached about 85% of its final mass and then accreted volatile-rich material in the last stages of its formation, about 26 million years after the Solar System's origin.

Reference:
Carnegie Institution (2010, May 14). Water was present during birth of Earth, study of silver suggests. ScienceDaily. Retrieved May 17, 2010, from http://www.sciencedaily.com­ /releases/2010/05/100513143457.htm