The inclusions in diamonds are direct and unaltered evidence of the processes that took place in the Earth's mantle billions of years ago, at depths that would otherwise be impossible to investigate. This is why their presence, even if it can devalue a diamond as a gem, has an immense scientific value which consists in the possibility of obtaining very important information on the past of our planet.

The typical gem-quality diamonds are formed in the deep Earth until over three billion years ago, and they have risen to the surface thought ascending deep magmas during rapid volcanic events, called “kimberlitic”. This special condition, combined with the extraordinary properties of extremely rigid and chemically inert material, permits diamonds to preserve information essential to study their formation in Earth’s mantle. Such makes this mineral of a great interest for scientists, and they represent an important brick of the study of the evolution of our planet’s interior.

In detail, the study of diamonds has provided important data about the geodynamics and the deep carbon cycle until 3.5 billion years ago. On this view, the diamond age is crucial to correctly place the retrieved information within the Earth’s history. So far, diamond alone cannot be dated, but its age may be calculated through the application of a radiogenic dating system on the minerals that it could have entrapped during its growth, which are called “inclusions”. In other words, to date diamonds we must first date their inclusions and second assume that the calculated age can actually be associated to the diamond formation. This can be easily demonstrated when diamond and inclusion formed at the same time from the same process, but what if the inclusion predates diamond? This case has to be evaluated in terms of time needed by the isotopic system to re-equilibrate, and therefore, to reset the radiogenic clock. If it is rapid, the mineral may isotopically re-equilibrate during diamond formation and provide age data correlated to their entrapment during diamond crystallisation.

A new study carried out at the Department of Geosciences, University of Padua, and recently published in Geology, has recently evaluated of reliability of the Sm-Nd dating system applied on clinopyroxene inclusions for dating diamonds if they are pre-existent. In fact, there are three main radiogenic systems for dating diamonds: Re-Os system in sulphide, Sm-Nd in garnet and Sm-Nd in clinopyroxene inclusions. It has been demonstrated that sulphide and garnet are reliable timekeeper for dating diamonds, but assessments were never done before for clinopyroxene, although it has been widely used. In this work, the authors studied 54 clinopyroxene inclusions in 18 diamonds worldwide with a crystallographic approach and have provided the evidence that clinopyroxene inclusions formed before diamond are not a rare case. The numerical modelling of the Nd diffusion in clinopyroxene computed geologically long times for the re-equilibration of this isotopic system at the typical conditions for diamond formation in the cratonic mantle (between 900 and 1400°C) and for the most common inclusions sizes (from 0.05 to 0.5 mm, with average values of 0.1 mm). For instance, at the temperature of 1150°C, the radiogenic clock in clinopyroxene grains larger than 0.05 mm reset in more than 3.5 million years, but these times are two orders of magnitude longer at 100 °C lower.

These results indicate that the ages calculated through the Sm-Nd dating system on clinopyroxene inclusions, if pre-existent, may be older than the actual diamond age. And lower the temperature, higher the possibility that the yielded ages are incorrect. On this view, the authors have suggested to consider that, if it is possible, a careful selection of inclusions of small sizes and high temperatures is desirable, as well as an accurate analysis on the temporal relationships for the diamond and inclusion growth.

Leonardo Pasqualetto, PhD student of the Department of Geosciences and first author of this paper, commented: “Since the 80ies, this method has represented one of principal ways to date diamonds, but an evaluation of its reliability, if the clinopyroxene has predated diamonds, has not been accurately done before. It is thanks to these methodologies that we know that diamonds may be so old, but it is necessary to be aware of their limitations. This work gives an idea of such, and quantifies, although if preliminarily, how much the ages calculated by using this method may deviate from the actual diamond age at certain conditions.”

This work has also been conducted by Prof. Fabrizio Nestola, Prof. Paolo Nimis and Dr. Martha Pamato Department of Geosciences, in collaboration with an international team composed by Prof. Dorrit Jacob from the Research School of Earth Sciences of the Australian National University (Canberra, Australia), Dr. Benat Oliveira from the Department of Earth and Planetary Sciences of the Macquarie University (North Ryde, Australia), Dr. Samantha Perritt and Dr. Ingrid Chinn from the De Beers Exploration (Southdale, South Africa), Dr. Sula Milani from the Department of Earth Sciences of the University of Milano (Milan, Italy) and Dr. Jeffrey Harris from the School of Geographical and Earth Sciences of the University of Glasgow (Glasgow, UK).