Aug
28
2007

Oldest diamonds may answer questions about early Earth

How old is the earth?

Oldest diamond puzzles scientists
Oldest diamond puzzles scientists
Scientists mostly agree that the Earth is about 4.54 billion years old (USGS.gov). This age represents a compromise between the interpretations of oldest-known terrestrial minerals – small crystals of zircon from the Jack Hills of Western Australia – and astronomers' and planetologists' determinations of the age of the solar system based in part on radiometric age dating of meteorite material and lunar samples. (Read why meteorites were used here.)

How fast did Earth cool?

Earth's beginning is known as the Hadean—because geologists speculate the planet's surface boiled and bubbled with molten lava under a steady bombardment of comets and meteorites. Diamonds would never form in such conditions yet diamonds over 4.2 billion years old have now been found. Does this mean that the Earth cooled enough that a cool crust and maybe even oceans existed 4.25 billion years ago?

Oldest diamonds within zircon may give the answers.

Because no rocks older than 4 billion years had previously been found, the Hadean period of Earth was thought to be at least 500 million years. Zircons found in Jack Hills Austalia changed this thinking.
When a zircon forms from a solidifying magma, atoms of zirconium, silicon and oxygen combine in exact proportions (ZrSiO4) to create a crystal structure unique to zircon; uranium occasionally substitutes as a trace impurity. Atoms of lead, on the other hand, are too large to comfortably replace any of the elements in the lattice, so zircons start out virtually lead-free. The uranium-lead clock starts ticking as soon as the zircon crystallizes. Thus, the ratio of lead to uranium increases with the age of the crystal. Scientists can reliably determine the age of an undamaged zircon within 1 percent accuracy, which for the early earth is about plus or minus 40 million years.
Furthermore, the ratios of radioactive isotopes of neodymium and hafnium--two elements used to determine the timing of continental-crust-forming events--suggest that significant amounts of continental crust formed as early as 4.4 billion years ago.Scientific American

Explainations for the diamonds are uncertain.

Craig O'Neill, a geochemist with Sydney's Macquarie University, at first thought the diamonds were due to heavy meteroite bombardment, forming in large impact craters due to the huge pressures reached. Dr Nemchin agreed that the birth of the diamonds was key, although he speculated that they formed when new-formed hunks of the Earth's crust collided. Study co-author Wilde said, "The bottom line is that we really honestly don't know why they're (the diamonds) there." The 4.25-billion-year-old diamonds "suggest the additional possibility that the diamonds have formed by some process that is not yet understood." He suggests the researchers should test the diamonds' carbon isotopic composition and whether nitrogen is present as single or paired atoms. This would indicate the time spent in the mantle and whether it was under relatively high or low temperatures.
Dr Nemchin says analysis of the carbon isotopes would be the next obvious step and could provide clues about the possible existence of life forms 4.2 billion years ago.

Sources:

The study, led by Martina Menneken, a master's student at the Westfaelische Wilhelms-Universitaet, appeared in the Aug. 23, 2007 journal, Nature. Alexander Nemchin from Curtin University of Technology also contributed.
I also recommend the Scientific American article, "A cool early earth?" (October, 2005). I now have a better understanding of how diamonds and zirconium crystals can tell us about what the Earth was like more than 4 billion years ago.

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