Why olivine and diamonds are best friends

Gi­uliani has been study­ing the form­a­tion and oc­cur­rence of the gem­stone since 2015. When still at the Uni­ver­sity of Mel­bourne, he ana­lysed count­less samples of kim­berlite, a bluish-​black rock of mag­matic ori­gin. Dia­monds are only ever found where there’s a kim­berlite, which in turn oc­curs only on very old con­tin­ental blocks that have re­mained geo­lo­gic­ally un­changed for bil­lions of years: pre­dom­in­antly in Canada, South Amer­ica, cent­ral and south­ern Africa, Aus­tralia and Siberia. “Just look­ing for a kim­berlite is like look­ing for a needle in a hay­stack,” Gi­uliani says. “Once you’ve found it, then the ar­du­ous search for dia­monds really gets un­der­way.”

Lots of iron, no dia­monds

Gi­uliani and his col­leagues have now de­veloped a method that will sim­plify the de­tec­tion of dia­mond de­pos­its. Their pro­cess re­lies on the chem­ical com­pos­i­tion of kim­berlites. Gi­uliani real­ised early on that there is a con­nec­tion between the oc­cur­rence of dia­monds and oliv­ine, a min­eral that makes up around half of kim­berlite rock.

Oliv­ine con­sists of vary­ing pro­por­tions of mag­nesium and iron. The more iron oliv­ine con­tains, the less mag­nesium it has and vice versa. “In rock samples where the oliv­ine was very rich in iron, there were no dia­monds or only very few,” Gi­uliani says. “We star­ted to col­lect more samples and data, and we al­ways got the same res­ult.” Their in­vest­ig­a­tions ul­ti­mately con­firmed that oliv­ine’s iron-​to-magnesium ra­tio is dir­ectly re­lated to the dia­mond con­tent of the kim­berlite.

Gi­uliani took these find­ings back to the big dia­mond pro­du­cer and trader De Beers, which had provided him with the kim­berlite samples. De Beers was in­ter­ested; it provided the sci­entific study with fin­an­cial sup­port and asked the re­search­ers not to pub­lish the res­ults for the time be­ing. In 2019, Gi­uliani came to ETH Zurich as an Am­biz­ione Fel­low and, sup­por­ted by the Swiss Na­tional Sci­ence Found­a­tion, began to look for ex­plan­a­tions for the con­nec­tion between oliv­ine’s mag­nesium and iron con­tent and the pres­ence of dia­monds.

Dia­monds don’t like stop-​and-go

To this end, the ETH Zurich re­search­ers ex­amined how the pro­cess of meta­so­mat­ism, which takes place in the Earth’s in­terior, af­fects dia­monds. In meta­so­mat­ism, hot li­quids and melts at­tack the rock. The min­er­als present in the rock re­act with the sub­stances dis­solved in the flu­ids to form other min­er­als.

The geo­lo­gists ana­lysed kim­berlite samples that con­tained oliv­ines with a high iron con­tent – and hence no dia­monds. They dis­covered that oliv­ine be­comes richer in iron where melt pen­et­rates the litho­spheric mantle and changes the com­pos­i­tion of mantle rocks sig­ni­fic­antly. And it is pre­cisely in this layer, at a depth of around 150 kilo­metres, that dia­monds are present. The In­filt­ra­tion of the melt that makes oliv­ine richer in iron des­troys dia­monds. If, on the other hand, no or only a small amount of melt from un­der­ly­ing lay­ers pen­et­rates into the litho­spheric mantle and thus no meta­so­mat­ism takes place, the oliv­ine con­tains more mag­nesium – and the dia­monds are pre­served.

“Our study shows that dia­monds re­main in­tact only when kim­berlites en­train mantle frag­ments on their way up that haven’t ex­tens­ively in­ter­ac­ted with pre­vi­ous melt,” Gi­uliani says. A key point here is that kim­berlites don’t nor­mally reach the Earth’s sur­face in one go. Rather, they be­gin to rise as a li­quid mass, pick up frag­ments of the mantle on the way, cool down and then get stuck. In the next wave, more melt swells up from the depths, en­train com­pon­ents of the cooled mantle, rises higher, cools, gets stuck. This pro­cess can hap­pen mul­tiple times. “It’s a real stop-​and-go pro­cess of melt­ing, as­cent and so­lid­i­fic­a­tion. And that has a de­struct­ive ef­fect on dia­monds,” Gi­uliani says. If, on the other hand, con­di­tions pre­vail that al­low kim­berlites to rise dir­ectly to the sur­face, he ex­plains, then this is ideal for pre­ser­va­tion of the dia­monds.

De Beers is already us­ing oliv­ine ana­lysis

Oliv­ine ana­lysis is as re­li­able as pre­vi­ous pro­spect­ing meth­ods, which are mainly based on the min­er­als clinopyroxene and gar­net. How­ever, the new method is easier and faster: it takes only a few ana­lyses to get an idea of whether a given kim­berlite field has dia­monds or not.

The ana­lysis of oliv­ine com­ple­ments pre­vi­ous meas­ure­ments and helps to form a more com­plete pic­ture. “The great thing about this new method is not only that it’s sim­pler, but also that it fi­nally al­lows us un­der­stand why the pre­vi­ous meth­ods worked,” Gi­uliani says. He adds: “De Beers is already us­ing this new method.”

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