diamond birth

Diamonds are forever... diverse diamond birth

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A post-doctoral fellow at McGill University has discovered that diamond may well be forever, but their origins are not necessarily as clear-cut as commonly believed.

found at McGill University March 30, 2007 . En Français : Naissance diamants

Emilie Thomassot, a member of McGill’s Department of Earth and Planetary Sciences, studied nearly 60 diamonds extracted from one fist-sized sample of Earth’s mantle found in a South African diamond mine.

Her findings are highlighted as an Editor’s Choice in the March 2007 issue of Science magazine.

This unique sample originated at a depth of about 160 km and breached the surface during an explosive volcanic eruption more than one billion years ago. Thomassot measured nitrogen content and carbon and nitrogen isotopes in the diamonds, which ranged up to nearly 0.2 carats in size.

“This is the first time that anyone has observed such large chemical variation in diamonds extracted from a single sample,” said Thomassot. “The observation was completely unexpected and really turns conventional interpretations of how diamonds form on their head.”

The counterintuitive implication of the study is that gem diamonds may form from methane-bearing fluids circulating locally within the deep Earth, rather than through geodynamically induced mixing of different global carbon reservoirs. Although the existence of a methane-rich diamond-forming fluid has been predicted theoretically, direct evidence had been lacking up until now

GEOLOGY: Diamond Diversity

The chemistry of diamonds brought up from Earth's mantle--notably their widely ranging nitrogen contents and nitrogen and carbon isotope values--has complicated understanding of their origins. It is commonly thought that many diamonds form from the movement of carbon-rich fluids into deep mantle rocks of a contrasting composition, thereby inducing diamond precipitation. To better constrain these fluids and sources, Thomassot et al. studied in detail nearly 60 diamonds contained within one small (<30 cm³) mantle sample carried to the surface in a kimberlite volcano in South Africa. Surprisingly, the nitrogen contents and isotopic values of these diamonds in this one sample spanned a large part of the ranges observed from all diamonds worldwide. The covariations of the data imply that these diamonds formed from a methane-rich fluid, not a more oxidized fluid as commonly assumed. The wide variation can be produced by the fractionation of nitrogen and carbon during growth of the diamonds over time. Such fluids may also account for the variable oxidation state of the mantle beneath Earth's most ancient crust. -- BH

Earth Planet. Sci. Lett. 10.1016/j.epsl.2007.02.020 (2007).

related: Un boursier de recherches postdoctorales de McGill fait éclater le mythe entourant la naissance des diamants et l’origine des pierres précieuses

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