We used to think that diamonds were everywhere. New research suggests they have always been rare

New research sheds light on the tumultuous processes that give rise to diamonds by searching for a distinct purple companion that exists alongside them.

Diamonds are highly valued for their qualities, but also for their rarity. One way to look for them is to search for associated minerals that occur more commonly, such as the chromium-rich pyrope garnet.

This vibrant purple garnet is easily found by diamond exploration companies, in sediment downstream from potentially diamond-bearing volcanic pipes and in the pipes themselves. The presence of purple garnet is an indicator, diamonds may also be present.

Moreover, this garnet is not only found near diamonds, but is also consistently found inside them. So by improving our understanding of pyrope garnet and how it forms, we can also improve our understanding of diamond formation.

It was previously thought that this type of garnet could not form very deep in the Earth. The theory was that it came from another chromium-rich mineral, called spinel, which formed at a shallow depth in the mantle and was then pushed down where temperatures and pressures were higher – leading to the garnet’s formation.

Our latest research, published today in Nature, uses a new model to revisit an old theory suggesting that these pyrope shells actually formed much deeper in the mantle, about 100 km-250 km below the current surface. It also suggests that diamonds may be rarer than we think.

Pyrope garnets vary in color from purple to violet.  Their color reflects high metal chromium content

Pyrope garnets vary in color from purple to violet. Their color reflects high metal chromium content. Image credit: Vvoe/Shutterstock.com

How diamonds and pyrope garnet are formed

Diamond is the crystalline form of elemental carbon, stable at very high pressures and relatively low temperatures – accidentally brought to the surface through powerful volcanic eruptions.

The necessary conditions to form diamond at great depth in the Earth’s mantle are only met in a few places. The geographical distribution of diamonds is very uneven, with notable concentrations in southern Africa, Congo, Tanzania, Canada, Siberia and Brazil. All these places are characterized by an ancient continental crust between 2.5 and 3.5 billion years old.

This crust is underlain by deep solid “roots”—like the keel of an iceberg—made of mantle that has been highly chemically depleted through intense melting over time.

It is here in this depleted mantle, which extends as deep as 250 km into the hotter, churning mantle beneath, that diamonds have the best opportunity to form. So what about their chromium-rich companions?

Using a thermodynamic computer model, we were able to demonstrate that pyrope garnets can form very deep in the Earth, at the same depths as diamonds. Specifically, these garnets would have formed during intense heating events with extreme pressures and temperatures in excess of 1,800 ℃.

How the continents grew their roots

While this is a very exciting finding in itself, what makes it more relevant is that it informs two other important theories.

The first concerns why the continents formed the way they did—a point experts have long speculated about.

As mentioned above, pyrope garnets form in extremely hot upwellings that come from great depths. Our results suggest that these upwellings then melted the upper mantle into place and formed the stable base of the continents.

In other words, the “roots” that help continents remain stable for billions of years are remnants of the same mantle melting events that produced pyrope garnets.

Diamond rarity

The other major conclusion concerns the rarity of diamonds.

Some researchers believe that diamonds were not originally rare, but that many were destroyed as the mantle root was eroded and modified by continental plates moving across the globe. Our model offers the alternative perspective that diamonds have indeed always been rare.

How can we assess whether the necessary cradles of diamond – bits of highly depleted mantle in the continental roots – were once common and became rare over time, or whether they have always been rare?

This kaleidoscopic image is a diamond cradle under a microscope. In this view, the garnet is the black mineral. Author specified

When intense melting events occurred on the early Earth, the melts themselves erupted onto the continental surface as highly fluid lavas called “komatiites”. These lavas are preserved and widely analyzed. They have different compositions, and our model predicts which of these could have formed together with chromium-rich pyrope garnet.

We know from tens of thousands of chemical analyzes of komatiite that the particular composition associated with this pyrope garnet is very rare. This is because, in order for it to form, magma must interact with exceptionally depleted mantle that has undergone many melting events. Only between 8%-28% of komatiite fits this bill.

From this we can infer that both pyrope garnets and the highly depleted mantle domains they come from have always been rare—even back on the early Earth. And because diamonds have an affinity for these special stones, they must also have always been rare – making them even more remarkable.The conversation

Carl Walsh, PhD Candidate, Queensland University of Technology; Balz Kamber, Professor of Petrology, Queensland University of Technology, and Emma Tomlinson, Associate Professor, Trinity College Dublin

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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