Super-deep diamonds, which form more than 380 km deep in Earth’s mantle, are invaluable tools for deep carbon scientists.

Super-deep diamonds, which form more than 380 km deep in Earth’s mantle, are invaluable tools for deep carbon scientists. Not only do they harbor clues about how they formed and therefore the reactions taking place inside Earth, they also trap small samples of mantle minerals, so-called inclusions, within their carbon crystal structure as they grow. These tiny samples of Earth’s deep interior from the region where the diamond forms are preserved under high pressure within a super-strong, unreactive diamond shell.

Many super-deep diamonds are small, have poor clarity, and are not generally used as gemstones. However, in a paper published in the journal Science, a team of researchers led by Evan Smith of the Gemological Institute of America (GIA) and including Deep Carbon Observatory DMGC (Diamonds and Mantle Geodynamics of Carbon) collaborators Steven Shirey (Carnegie Institution for Science, USA) and Fabrizio Nestola (Università degli Studi di Padova, Italy), suggests giant gemstone diamonds, like the 3106 carat Cullinan, are super-deep diamonds formed under special mantle conditions [1].

Co-author Professor Fabrizio Nestola explains the method of X-ray diffraction at the University of Padova, Italy. This method was used to first identify the presence of cohenite (an iron-nickel carbide) within the metallic inclusions. (credit Chiara Anzolini and Fabrizio Nestola)

When gem diamonds are polished and cut, expert diamond cutters often remove sections of the stones with inclusions. These offcut diamond pieces are not normally made available to scientists, and are usually considered waste, but the team made special efforts to get their hands on some.

“The project started with our collaborators at the GIA who have the opportunity to observe a number of large gem diamonds and have access to some of their offcut pieces,” said Shirey. “Evan Smith, a GIA postdoctoral researcher had a hypothesis that large diamonds could form deep in the mantle from metallic liquid, but we needed to the samples to figure it out.”

When they analyzed the offcuts, the team discovered multi-mineral metallic inclusions containing iron-nickel metal, an iron-carbide mineral known as cohenite, and the iron-sulfide mineral pyrrhotite. There were also traces of fluid methane and hydrogen in the thin space between the mineral phases and the encasing diamond. At the original pressure and temperature deep in Earth’s mantle, the composition of these multi-mineral inclusions suggested to the research team that a much larger mass of molten metallic liquid existed from which pure carbon crystallized to form diamonds. As each diamond grew, small droplets of the metallic liquid got trapped. As the diamonds were brought to Earth’s surface by volcanic eruption, the liquid droplets crystallized to the individual minerals.

“My motivation in this work was to solve this long-standing mystery about how these especially large and alluring diamonds form,” said Smith. “Everything about them suggests they form in a special way and that means they might tell us something new about the behavior of mantle carbon. In this research I was chasing an idea that I published a couple years ago, that the low nitrogen content and large size of these (CLIPPIR) diamonds might be linked to metallic iron in the mantle. I was thrilled when I started finding the first few inclusions. With the expertise of everyone involved we saw the observations unfold into an amazing story from the deep Earth.”

As well as diamonds with only the metal inclusions, the team found additional similar diamonds with silicate mineral inclusions –that coexisted with smaller amounts of metal. This assemblage suggests that all the metal-containing diamonds formed between 360 and 750km deep inside Earth. This is much deeper than most other gem diamonds, which form in the lower part of continental tectonic plates at depths of 150–200 km.

These two observations together show not only that Earth’s largest gemstone diamonds form extremely deep in the mantle, but also in regions of the mantle with metallic iron, the first time these aspects of the largest gem diamonds have been recognized.

“The idea of metallic iron in the silicate mantle at far shallower levels than Earth’s iron core , is something Earth scientists have expected for a while,” said Shirey. “A number of experiments and simulations predicted it, but now we have physical evidence that this is the case.”

Previous experiments and theory suggested for many years that small amounts of metallic iron existed in parts of the deep mantle below about 250 km depth. Though it’s still unclear how much metallic iron is present in the lower mantle, this is a key observation for our understanding of Earth and the conditions under which it formed and evolved. Because the metallic liquid at these pressures and temperatures contains carbon and hydrogen it plays a hitherto undetected role in the geochemical cycles of these elements in the deep mantle.

“This result provides a direct link between diamond formation and deep mantle conditions, addressing a key goal of the Deep Carbon Observatory,” said DCO Executive Director Robert Hazen (Carnegie Institution for Science, USA). “The fact that it was made possible by a hugely successful collaboration between our Diamonds and Mantle Geodynamics of Carbon group and the Gemological Institute of America is also very exciting, highlighting the importance of academic connections with industry and their important role in providing postdoctoral funding and the key specimens for this research.”

The 15 November 2016 edition of the journal Lithos delves into the role of natural diamonds in deep Earth research.

Titled “The nature of diamonds and their use in Earth’s study,” the 15 November 2016 edition of the journal Lithos delves into the role of natural diamonds in deep Earth research. This special issue is edited by DCO scientists involved the Reservoirs and Fluxes initiative, Diamonds and the Mantle Geodynamics of Carbon (DMGC).

This special issue of Lithos was compiled in recognition of the second International Diamond School, which took place in January 2015 in Bressanone, Italy. The Deep Carbon Observatory and the Gemological Institute of America sponsored the school, which brought together more than 80 graduate students and postdocs from around the world. Many of the students of the school are authors in the special issue.

Fabrizio Nestola (Università degli Studi di Padova, Italy), Matteo Alvaro (Università degli Studi di Pavia, Italy), Graham Pearson (University of Alberta, Canada), and Steven Shirey (Carnegie Institution for Science, USA) edited a selection of 30 original research articles from 134 authors for the issue. The papers cover four main research areas: geochemistry, diamond forming fluids, and diamond origin sources; geothermo-barometry and geochronology of diamonds; super-deep diamonds, carbonado-like diamonds and diamondites; and innovative methods for the investigation of diamonds.

 

CONTENTS

GEOCHEMISTRY, DIAMOND FORMING FLUIDS AND DIAMOND ORIGIN SOURCES

The Third International Diamond School took place at the University of Alberta, with the Deep Carbon Observatory as the main event sponsor (together with De Beers and IsoMass).

The Third International Diamond School took place at the University of Alberta, with the Deep Carbon Observatory as the main event sponsor (together with De Beers and IsoMass). DCO’s Graham Pearson (Reservoirs and Fluxes Scientific Steering Committee member; University of Alberta, Canada), Steve Shirey (Carnegie Institution for Science Department of Terrestrial Magnetism, USA), Thomas Stachel University of Alberta, Canada), Bob Luth (University of Alberta) and Fabrizio Nestola (University of Padua, Italy) were the principal conveners. The event continued in the tradition of having a mixed participation of students, senior academics, and industry. Seventy-five delegates, including 2 BSc students, and 30 Ph.D and Masters students from Canada, USA, Australia, and the UK attended, along with 18 delegates from industry and Government/Provincial Geological Surveys.

Prior to the conference 21 people (a mix of academics and students) attended a 2 day field trip to the Northwest Territories diamond mines and to see Archean geology around Yellowknife. The field trips were made possible by the generosity of Dominion Diamonds and Rio Tinto (Diavik Diamond Mine), as well as the staff of the Northwest Territories Geological Survey.

The scientific program consisted of 34 presentations, including 12 student talks, and 4 student posters that spanned a range of topics from the fundamentals of diamond formation through new mantle thermometry methods to aspects of diamond exploration and deposit evaluation. DCO early career scientists (non-students) who presented or attended included Emilie Thomassot (Nancy, France), Yakov Weiss (Columbia, USA) and Andrea Guiliani (Melbourne, Australia). The science program was supplemented by laboratory facility tours at the University of Alberta.

In a recent letter published in Nature, researchers propose that most carbon goes no deeper than about 300 to 500 kilometers, at which point a carbon barrier limits carbon recycling into the deeper mantle.

Carbon is cycled from Earth’s surface to its depths, emerging through the crust from volcanoes, and descending to the mantle in subducting ocean floor. But how far down is the carbon subducted? In a letter by Andrew Thomson, Michael Walter, Simon Kohn, and Richard Brooker (University of Bristol, UK) published in Nature, the authors propose that most carbon goes no deeper than about 300 to 500 kilometers, at which point a carbon barrier limits carbon recycling into the deeper mantle [1].

Downwelling slabs of mid-ocean ridge basalt (MORB) efficiently dehydrate at sub-arc depths, but may retain a considerable portion of their carbon cargo. Thomson et al made high pressure-temperature melting experiments on materials that replicated carbonated basalt from the IODP 1256D site on the East Pacific Rise. They show that upon reaching transition zone depths carbonatite melts are produced along a deep solidus depression. The melts infiltrate and react with the overlying mantle, causing diamond production, refertilization and associated metosomatism of the surrounding mantle. This melting of recycled crust in the transition zone is an effective barrier to carbon transport into the lower mantle.

The major difference between this work and other melting studies of carbonated MORB above 8GPa is the different phase assemblage resulting from lower and more realistic CO₂ and CaO contents of this study’s bulk composition. The resulting change in phase relations produces a deep solidus depression in carbonated oceanic crust at upper-most transition zone depths. The authors estimate that melting would occur to depths of at least 7 kilometers into the crustal section, and that only the coldest modern-day slabs would survive the solidus depression and carry carbonate beyond the transition zone.

The compositions of superdeep diamond-hosted inclusions provide strong evidence of carbonate melt-peridotite reaction. These diamonds form at transition zone depths, and have isotopic characteristics consistent with subducted carbon. The diamonds confirm that carbon must survive subduction beyond sub-arc dehydration reactions, and may record the process of slab melting in the transition zone.

Dr. Andrew Thomson said, “superdeep diamonds are a unique pristine snapshot of the deepest portions of the Earth’s carbon cycle. They contain a wealth of information that makes them invaluable and unparalleled tools for better understanding the interior of our planet”.

In a study published in Nature, a team of scientists describes an unexpected mechanism for diamond formation relying on ancient, subducted seawater.

Diamonds are crystals of carbon, formed deep in Earth. As diamond crystals grow, they sometimes trap fluids or other mineral crystals, micro-samples of their surrounding environment. In a study published in Nature, a team of scientists, including DCO’s Graham Pearson (University of Alberta, Canada), describes an unexpected mechanism for diamond formation relying on ancient, subducted seawater [1].

The team, lead by Yaakov Weiss (Columbia University, USA), analyzed 11 diamonds from the Ekati mine in the Northwest Territories of Canada. These diamonds, so called fibrous diamonds, are less than a millimeter in diameter. The center of many of the stones is familiar, a gem-like diamond. But surrounding this core the diamond is studded with millions of minute inclusions, giving it a “fuzzy” or fibrous appearance under a microscope.

The inclusions in the 11 diamonds studied provided the authors with new information about how, when, and where in Earth this carbon crystalized. For diamonds to have inclusions like these, they must have formed quickly, trapping surrounding fluids and minerals. Through a series of measurements, some involving a unique laser ablation method developed by Pearson’s research group, Weiss and colleagues showed that many of the inclusions contained fluids rich in chlorine and sodium.  The source of such high levels of these two elements, combined with their isotopic fingerprint, are strongly indicative of ancient seawater that reacted with oceanic crust, that was subducted to depth.

During subduction, water, in the form of salty fluids or “brines” was transferred into the deep mantle beneath the Northwest Territories, as oceanic lithosphere descended beneath the overlying tectonic plate. The reaction of these brines with particular rock types in the mantle root appears to be a critical part of the diamond forming process.

“These results are particularly interesting to the Deep Carbon Observatory because they point to a new mechanism whereby carbon is cycled into, and stored in, deep Earth,” said Pearson. “Before now, it was unclear what the starting compositions were for the unusual fluids that form these diamonds. Diamonds with “salty” inclusions appear to be common beneath the Northwest Territories. Similar fluid compositions in diamonds from other parts of the world indicate that this diamond forming reaction is widespread beneath the deepest continents around the world.”