The mantle is called MARID for it is composed of mica, amphibole, rutile, ilmenite and diopside minerals and is most commonly known to occur in South Africa, where orangeite rocks are found.
However, new evidence establishing the relationship between the magma and the diamond-bearing rock suggests orangeites may be present in much higher abundance worldwide, including in Australia, opening new opportunities for explorers.
“There is evidence in the literature of this type of MARID-mantle occurring globally [and] with our study we put forward a reasonable working hypothesis that states that this MARID mantle is the source of these orangeites,” Marco Fiorentini said. Fiorentini is co-researcher and associate professor at the University of Australia’s Centre for Exploration Targeting, and at the ARC Centre of Excellence for Core to Crust Fluid Systems.
“This has never been demonstrated before with the same type of geochemical and isotopic confidence that we have nowadays.”
Fiorentini said the team worked on MARID mantle and orangeite rock fragments from South Africa to analyse and compare their isotopic signatures, in other words their DNA, and found similarities.
“You look at that little material with very sophisticated techniques that characterise for example the age of these rock fragments and you can compare that with the age of the orangeite rock,” he said.
“From an isotopic point of view, we can say that the two are related.”
While Fiorentini stressed the finding should not be taken as a panacea for diamond exploration, he said it could help explorers refine their targets.
“Our study is very much focused on understanding the interior of the earth and what [it] looks like but if you apply an exploration targeting rationale, this type of knowledge could potentially be used in a predictive way.
“You could say: ‘Well we know where this type of mantle outcrops or is found so technically we could use this knowledge about the link between this type of mantle and the genesis of diamond-bearing orangeites to look at the globe to find areas that are more interesting than others’.
“The problem is that all the time in geology you are dealing with such a complex scenario that this is just an additional tool that you can add to your toolbox to analyse what the planet looks like.”
Opportunities for diamond exploration are real, but the benefits of this research don’t stop there.
With all magmas in the mantle related to each other, Fiorentini said that understanding more about the MARID mantle could help shed light on the source of other magmas and the role they play in transporting other metals from the depth of the lithospheric mantle to the crust of the planet.
“We can’t access metals in the lithospheric mantle but we know that [it] is the source of different magmas and fluids that can focus and concentrate those metals in the upper part of the crust, where locally around the planet they can become available and searchable,” he explained.
“From a broader geological perspective, all these types of mineral systems are related to each other so understanding more about the sources of these magmas can indirectly shed more light on the source of other magmas, and in general more about the composition of the lithospheric mantle which is what we’re ally after.”
The research was led by the University of Melbourne in collaboration with UWA, the Australian National University and the University of Tasmania.
It was published this week in <i>Nature Communications</i>.