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Ultra deep diamonds: What Jules Verne did not discover on his journey

by Micaela Longo (15.01.2010)

The observation of earthquakes provides most of the information on the deep interior of the Earth. Based on such studies, Earth is mainly divided into three layers: the crust, the mantle and the core. Our possibility to directly sample the Earth is limited only to about 10 km through the crust. However, some volcanic eruptions may bring samples from much deeper levels to the surface. In particular, natural diamonds are an important "window" to the deep Earth. During their formation, diamonds sometimes incorporate tiny minerals of less than 50 micrometers (0.05 mm) in size, and therefore the study of those inclusions provides a means to investigate the deep Earth. Most diamonds come from the uppermost part of the mantle, up to a depth of about 200 km. Only rather recently, some very rare types of diamonds have been discovered that appear to come from much deeper in Earth´s interior. They are the first samples from a part of the Earth that appeared to be more inaccessible than the surface of distant planets. These rare ultradeep diamonds are brought to the surface via kimberlitic pipes, formed as a consequence of violent and rapid eruptions. At the extreme pressure and temperature conditions where ultradeep diamonds form, the most abundant inclusion in diamonds is a mineral called (Mg,Fe)O ferropericlase (Fig. 1), and when it is found in association with MgSiO3 perovskite, it constrains the origin of diamond formation to depths greater than 660 km, i.e., in the lower mantle.

The oxidation state of iron in minerals contains important information on the environment from which these minerals formed and on the processes occurring there. In my thesis, I investigated for the first time (Mg,Fe)O inclusions from ultra deep diamonds from different localities, including  samples from Brazil and Australia. In order to be able to study these very small samples, I developed a new analytical method, the flank method, which is based on the principle of electron microprobe analysis. I discovered that the diamonds contained inclusions with very variable oxidation state  (Fe3+/ΣFe between 1 and 13%). This implies that material from different sources – from close to the Earth surface and from the deep interior of the Earth – is included in these diamonds. Surprisingly, several inclusions recovered from the same host diamond found in Australia experienced different oxygen fugacity conditions, covering almost the entire range of Fe3+/ΣFe observed so far for natural (Mg,Fe)O inclusions.

This important discovery combined with the geographical correlation observed among all inclusions measured in this study and from previous studies in literature leads to the suggestion that the ultra deep diamonds record several processes occurring deep inside the Earth. One process is deep subduction – material from the Earth surface is transported back deeply into the mantle and the carbon contained in some ultra deep diamonds probably originally came from the floor of an ancient ocean. Other processes may perhaps involve the presence of deep-seated carbonatitic fluids not necessarily associated to subduction or in combination to it. The variation in the oxygen fugacity recorded by different growth stages may therefore represent the fingerprint of different magmatic events that might have been involved in the diamond growth history during its journey through the Earth.

Ultra deep diamond from Machado River in Brazil[Bildunterschrift / Subline]: Figure 1: Ultra deep diamond from Machado River (Brazil) containing (Mg,Fe)O ferropericlase inclusion surrounded by a family of unidentified octahedral inclusions (a); in the same diamond fluid inclusions are also present which are visible at the centre of the stone (b).

Micaela Longo
Micaela Longo
* 1978, Italien

  • Sept - Dec 2008
  • Research Training Stage Abroad within the PhD program. University of British Columbia (UBC), Vancouver, Canada. Research project: Cathodoluminescence studies of alluvial diamonds from Archean conglomerates of Eastern Canada
  • since April 2006
  • Ph.D. Fellow supported by the European Commission under the Marie Curie Action for Early Stage Training of Researchers at Bayerisches Geoinstitut (BGI)
  • Thesis title: Iron oxidation state of (Mg,Fe)O: calibration of the Flank Method (EMPA) on synthetic samples and applications on natural samples from lower mantle diamonds
  • Jan-Aug 2005
  • Internship at Bayerisches Geoinstitut in the frame of “Fellowships for training abroad” supported by the University of Rome "La Sapienza". Research project: Crystal chemistry of clinopyroxenes from mantle eclogites
  • April 2004
  • Master Degree in Natural Sciences at the University of Rome "La Sapienza". Master thesis title: Fe2+ and Fe3+ site distribution in tourmalines by 57Fe Mössbauer Spectroscopy

  • *Nestola, F., Longo, M., McCammon, C., Boffa-Ballaran, T. (2007) "Crystal structure refinement of Na-bearing clinopyroxenes from mantle-derived eclogite xenoliths". American Mineralogist, 92, 1242-1245.
  • *Andreozzi, G.B., Bosi, F., Longo, M (2008). "Linking Mössbauer and structural parameters in elbaite-schorl-dravite tourmalines". American Mineralogist, 93, 658-666.
  • *Longo, M., McCammon, C. Jacobsen. S. "Iron oxidation state of (Mg,Fe)O using the Flank Method (I): calibration on synthetic samples and application for the detection of exsolution processes on micro-scale at the electron microprobe" (in preparation).
  • *Longo, M., McCammon, C., Bulanova, G., Kaminsky, F. "Iron oxidation state of (Mg,Fe)O using the Flank Method (II): application on natural inclusions from lower mantle diamonds with implication for oxygen fugacity studies" (in preparation).