The Earth is about 4.6 billion years old but no rocks exist that are older than about 3.8 billion years. However, zircons that were eroded from the sedimentary rock section in the Jack Hills of western Australia, which more than 3 billion years old, were eroded from rocks as old as about 4.3 billion years. These Jack Hills zircons are the oldest recorded geological material on the planet.
In 2007 and 2008, two research papers in Nature reported that a suite of zircons from the Jack Hills included diamonds, requiring a revision of early Earth history. The papers posited that the diamonds formed, somehow, before the oldest zircons — that is, before 4.3 billion years ago — and then were recycled repeatedly over a period of 1.2 billion years during which they were periodically incorporated into the zircons by an unidentified process.
Or not. A team of three researchers has discovered using electron microscopy that the diamonds in question are not diamonds at all but broken fragments of a diamond-polishing compound that got embedded when the zircon specimen was prepared for analysis by the authors of the Nature papers.
"The diamonds are not indigenous to the zircons," said Harry Green, professor UC Riverside, who was involved in the research. "They are contamination. This, combined with the lack of diamonds in any other samples of Jack Hills zircons, strongly suggests that there are no indigenous diamonds in the Jack Hills zircons."
"It occurred to us that a long-term history of diamond recycling with intermittent trapping into zircons would likely leave some sort of microstructural record at the interface between the diamonds and zircon," said Larissa Dobrzhinetskaya, researcher in the Department of Earth Sciences at UC Riverside and the first author of the paper. "We reasoned that high-resolution electron microscopy of the material should be able to distinguish whether the diamonds are indeed what they have been believed to be."
Using high-resolution secondary-electron imaging and transmission electron microscopy, the research team confirmed the presence of diamonds in the Jack Hills zircon samples they examined but could readily identify them as broken fragments of diamond paste that the original authors had used to polish the zircons for examination.
They also observed quartz, graphite, apatite, rutile, iron oxides, feldspars and other low-pressure minerals commonly included into zircon in granitic rocks.
"In other words, they are contamination from polishing with diamond paste that was mechanically injected into silicate inclusions during polishing," Green said.
Citation: Larissa Dobrzhinetskaya, Richard Wirth, Harry Green, 'Diamonds in Earthʼs oldest zircons from Jack Hills conglomerate, Australia, are contamination', Earth and Planetary Science Letters Volume 387, 1 February 2014, Pages 212–218 DOI: 10.1016/j.epsl.2013.11.02