We used to think diamonds were everywhere. New research suggests they’ve always been rare

New research sheds light on the turbulent processes that give birth to diamonds by targeting a distinct purple companion found alongside them.

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

This bright purple garnet is easily found by diamond exploration companies in sediments downstream of potentially diamond-bearing volcanic tubes and in the tubes themselves. The presence of purple garnet is an indicator that diamonds may also be present.

In addition, this garnet is not only found near diamonds, but also in them again and again. So, by improving our understanding of pyrope garnet and how it is formed, we can also improve our understanding of diamond formation.

It used to be thought that this type of garnet could not form very deep in the earth. The theory goes that it came from another chromium-rich mineral called spinel, which formed at shallow depths in the mantle and then was pushed down where temperatures and pressures were higher – resulting in the formation of the garnet.

Our latest research, published today in Nature, uses a new model to rethink an old theory that suggests these pyrope grenades are actually formed much deeper in the mantle, some 100km to 250km below today’s surface. It also suggests that diamonds are rarer than we think.

How diamonds and pyrope grenades are formed

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

The necessary conditions for forming diamonds at great depths in the earth’s mantle are only given in a few places. The geographic distribution of diamonds is very uneven, with notable concentrations in southern Africa, Congo, Tanzania, Canada, Siberia and Brazil. All of these places are characterized by an ancient continental crust that is between 2.5 and 3.5 billion years old.

Beneath this crust lie deep, solid “roots” – like the keel of an iceberg – of mantle that has been severely chemically depleted by intense melting over time.

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

Using a thermodynamic computer model, we were able to show that pyrope garnets can form very deep in the earth, at the same depth as diamonds. In particular, these garnets would have formed during intense heating events with extreme pressures and temperatures in excess of 1,800 °C.

As the continents grew their roots

While this is a very exciting finding in itself, it is all the more relevant because it affects two other major theories.

The first relates to why the continents formed the way they did – a point that experts have long speculated on.

As mentioned above, pyrope shells formed in extreme heat lifts from great depths. Our results suggest that these upwellings then melted the upper mantle in place, forming 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 shells.

diamond rarity

The second important conclusion relates to 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 altered as tectonic plates moved across the globe. Our model offers the alternative perspective that diamonds may have always been rare.

How can we judge whether the necessary diamond cradles—parts of the severely depleted mantle in the continental roots—were once common and have become rare over time, or have always been rare?

When intense melting events took place on early Earth, the melts themselves erupted at the continental surface as very fluid lavas called komatiites. These lavas are conserved and extensively analyzed. They have different compositions, and our model predicts which of these may have co-formed with the chromium-rich pyrope garnet.

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

From this we can conclude that both pyrope garnets and the very depleted mantle domains from which they originate have always been rare – even on early Earth. And because diamonds have an affinity for these special rocks, they too must always have been rare – which makes them all the more remarkable.

Karl Walsch, PhD student, Queensland University of Technology; Balz Kamber, Professor of Petrology, Queensland University of Technology, and Emma Tomlinson, Associate Professor, Trinity College Dublin

This article was republished by The Conversation under a Creative Commons license. Read the original article.