Someone I know recently said that when they die, they want to be turned into a diamond. I didn’t know this was a thing, but it turns out that it is. Not a bad idea really. You could carry your ancestors around in a necklace. Gives the term family heirloom a new twist.
This week, I learned that a large number of the diamonds that exist in nature also came from organic material. Unlike those diamonds you can make from the cremated ashes of your loved-ones, they were probably formed from some kind of ancient bacterial life living on the sea bed.
It’s interesting to think about how these gems were created, although once you get down to describing the science, it turns out there are many concepts and terms to be unpicked that may be unfamiliar to even the most avid reader of popular science.
I gave it my best shot for a story for the ABC this week. I’m not sure it’s my best work. I feel the story gets bogged down in the technicalities, and in trying to explain how the latest work fits into the context of a wider dispute. I’d be glad to hear what anyone thinks.
Diamonds traced to ancient sea floor
Monday, 11 March 2013
Many of the world’s diamonds, including most of those found in Western Australia’s famous Argyle mine, may have begun as organic matter on the ocean floor, a study shows.
The research addresses a long-standing controversy over how some kinds of diamonds form, says study co-author Lynton Jaques, a visiting fellow at the Australian National University. Jaques and colleagues publish their findings in the journal Geology .
Most diamonds can be classified into two types, elcogitic or peridotitic, according to the type of rock they form in, and the nature of the chemical inclusions they contain. But the origin of elcogitic diamonds is a puzzle.
The chemical features of these diamonds are quite different from other materials found in the Earth’s upper mantle – the thick layer where diamonds form between the Earth’s inner core and the outermost solid crust.
“The isotope ratios in these diamonds are way outside mantle compositions,” Jaques explains. “They’ve deviated a long way from what you’d expect from mantle materials, and the question is how to explain that deviation.”
One explanation is that high temperatures in the mantle lead to diamonds crystalising from carbon-rich fluids, Jaques explains. “But we, amongst many others, have felt that this is not the full answer.”
Evidence of subduction
In recent years, Jaques and colleagues have gathered evidence suggesting eclogitic diamonds originated as organic matter on the ocean floor which was thrust down into the mantle by the shifting of tectonic plates, in a process known as subduction. There, deep beneath the surface, extreme heat and pressure formed diamonds from the carbon.
“In fact if you look at the range of evidence, it’s pretty clear that us that subduction processes were involved,” Jaques says.
In their latest study, the researchers examined the oxygen composition of mineral inclusions within diamonds from Western Australia, Botswana and Venezuela. They found a previously unrecognised correlation – virtually all diamonds that had low carbon isotope ratios had silicate inclusions with unusually high oxygen isotope ratios. The compositions were exactly what would be expected occur on the ancient ocean floor.
“The only way we can explain this really is by subduction of organic carbon as the source for the carbon in the diamonds,” Jaques says. “This is the clincher for us. This is the evidence that these suite of diamonds…are in fact formed by subduction processes.”