NEXT time you鈥檙e digging in the garden, keep your eyes peeled for bright, hard little stones. There might be diamonds in your soil.
OK, maybe it鈥檚 not quite that simple, but the scenario isn鈥檛 as far-fetched as it sounds, according to Ken Collerson, a geochemist at the University of Queensland. He reckons that traditional ideas about how and where diamonds form are wrong. In fact, Collerson believes he now has evidence that diamonds aren鈥檛 confined to just a few places on Earth 鈥� they could be lurking literally anywhere under land and sea.
This claim has brought him face to face with civil war, bitter academic disputes and even death threats. But, though it hasn鈥檛 yet brought him valuable gems, he鈥檚 not backing down. Next month, after a two-year wait, a new prospecting process is going to start up. Collerson believes that hard, sparkling proof of his theory might not take too long to find.
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His confidence stems from minerals discovered on the small South Pacific island of Malaita, part of the Solomon Islands. The treasure trail started in 1957 when Australian geologist Frank Rickwood found garnets 鈥� some the size of a fist 鈥� in streams on the island. Diamond hunters have known for centuries that garnets often mean diamonds are nearby, and they鈥檝e led to the discovery of diamond-rich river deposits in South Africa, India and Brazil. If there were garnets on Malaita, surely diamonds couldn鈥檛 be far away?
Prospectors soon moved in. But for more than 40 years, all their efforts to find diamonds have failed. The world鈥檚 geologists have nodded sagely at this failure: there are widely acknowledged reasons why diamonds could never turn up on an oceanic island. But then, in 1998, Collerson found some.
To be more precise, they were handed to him. Alfred Sasako, then a Solomon Islands minister, had invited Papua New Guinea-based company Zorba Mining and Exploration to prospect on the island. The company sent its mineral samples to Collerson for analysis.
Working with Queensland colleagues Balz Kamber and Sarath Hapugoda, Collerson analysed the samples in his laboratory. He also sent them to Quentin Williams, a mineralogist at the University of California in Santa Cruz. The analyses identified microscopic diamonds just tens of microns across in the rock samples, as well as other minerals such as garnets and zircons. Comparison of the Malaitan rocks with samples from existing diamond mines showed up strong similarities: Malaita seemed to be a good prospect for big, 鈥渕acro鈥� diamonds.
With these results, Collerson, Zorba Mining, and UniQuest, the commercialisation arm of the University of Queensland, formed a Solomon Islands-based company called Solsearch to exploit the discovery. In 2000, Solsearch gained approval from the Solomon Islands government to begin prospecting. But four months later, just as an expedition was set to go to Malaita, a violent ethnic uprising overturned the government. 鈥淲e were all ready to go over and do fieldwork,鈥� Collerson says. 鈥淏ut, the very week before, the coup happened. We walked away from it and haven鈥檛 been able to get in since.鈥�
Death threats
It鈥檚 been a frustrating couple of years. Malaita is still in the throes of a violent war. Recent attempts to establish an amnesty and a surrender of arms have caused some of the ethnic factions to splinter, making the situation less stable than ever. Collerson and Nicos Violaris, who owns Zorba, say they have both received death threats from people hoping to muscle in on the prospecting. All this trouble has scared away foreign investors.
In the meantime, all Collerson and his colleagues have been able to do is try to persuade the academic community that, in spite of everything geologists believe, there should be diamonds on Malaita.
There are at least some hard facts to work with. To make diamonds you need two specific things: carbon and extremely high pressure 鈥� high enough to squeeze the carbon atoms into the orderly tetrahedral arrangement that characterises diamond.
The pressure is no problem. Go anywhere more than 150 kilometres down into the Earth and there it is. That鈥檚 where diamonds are traditionally thought to form, in the part of the mantle just beneath the continental crusts. The carbon originates at the surface as organic detritus and ends up in the mantle by a process called subduction. Where the tectonic plates of the crust collide, oceanic crust covered in carbon-rich sediment gets pushed right underneath continental crust, dragging some carbon with it. This abundance of carbon in a high pressure environment is thought to be why all the diamond-bearing rocks 鈥� volcanic magmas known as 鈥渒imberlites鈥� 鈥� have only ever been found under continental crust.
The diamonds make their way to the surface on currents of molten kimberlites. Like a soda pop from hell, the kimberlites tear vertical fissures through the crust 鈥� known as kimberlite pipes 鈥� as pressurised gases within the magma force their way towards the surface. As they go, the kimberlites sweep along a rich mixture of minerals, including any diamonds they contain.
But kimberlites are rare, and only a small fraction of pipes contain diamonds. These pipes are widely accepted as pretty much the only source of diamonds, and that鈥檚 why geologists have dismissed Collerson鈥檚 claims: Malaita has no kimberlite pipes, they say (though Collerson disagrees) and there is certainly no reason to believe that there is carbon-rich subducted crust beneath it at the traditional diamond-making depths (see Diagram).
So how can Collerson defend his claim that there are diamonds on the island? Quite simply: he says diamonds form deeper in the Earth than anyone thought 鈥� and they can form whatever sort of crust lies above them.
Collerson came to this controversial conclusion after studying Malaitan rock samples. They contain majorite, a form of garnet that鈥檚 rich in silica and forms only under very high pressures. The majorite serves as a barometer that indicates the pressure, and hence depth, from which the Malaita rocks came: in this case it was 670 kilometres down into the mantle. These rocks have come from deeper within the Earth than any others ever found (Science, vol 288, p 1215). And, since they contain micro-diamonds, Collerson concludes that diamonds must also form at such depths.
The immediate implication of his find is that diamonds could turn up anywhere, not just near subduction zones. Collerson thinks the only reason we find more diamonds on older continental crust is simply that these areas have had been hit by rising kimberlites for longer. He cites Africa, which hasn鈥檛 moved much for 3 or 4 hundred million years, and is our richest source of the gems.
But how could carbon end up so deep inside the Earth? It might be left over from when the Earth formed, Collerson suggests, or maybe it comes from oceanic crust that was subducted long ago and sank much further than anyone thought possible. If either theory is true, it would have huge implications for our understanding of the processes going on inside our planet. Collerson is currently preparing papers that discuss those implications.
He is convinced that our understanding of the processes behind diamond formation has to change: many diamonds come from much, much deeper than traditionally thought, he says. Collerson will present results supporting his theory at the annual Goldschmidt Conference on geochemistry in Davos, Switzerland, next week.
Of course, it would take a considerable amount of work to get the diamonds to the surface from such depths, and Collerson realises that the traditional mechanism of hot magma plumes originating in the upper mantle wouldn鈥檛 do the trick. It would take a plume of hot material from very deep in the mantle, perhaps even from the edge of the core, to push this diamond-making mix to the surface. But that鈥檚 not unheard of.
In April, Barbara Romanowicz and Yuancheng Gung of the University of California, Berkeley, reported the existence of two 鈥渟uperplumes鈥� of hot, fluid magma rising from the core-mantle boundary (Science, vol 296, p 513). One of those superplumes is located exactly under southern Africa, the other is located in the western Pacific, just to the east of Malaita. Collerson believes it鈥檚 more than coincidence.
But while the predicted diamonds are proving hard to find, sceptics are not. 鈥淭here鈥檚 been a hell of a lot of good prospecting done there by locals,鈥� says diamond prospector John Ferguson of Citation Resources, a mining company based in New South Wales. 鈥淭hey鈥檝e never come up with any [large] diamonds. That tells you the 鈥榮parklies鈥� aren鈥檛 there.鈥� But Ferguson admits he can鈥檛 convince himself that Collerson鈥檚 got it all wrong. 鈥淚t is an enigma at this point,鈥� he admits. 鈥淚t would be marvellous if he鈥檚 right.鈥�
Among Collerson鈥檚 academic peers, Clive Neal, a geochemist at the University of Notre Dame in Indiana, is the most vocal. He disputes Collerson鈥檚 conclusions, and has been in a long-running argument with Collerson over the Malaita data and its interpretation (Science, vol 292, p 1013). Neal doesn鈥檛 believe there are diamonds buried on the island. But he does concede that 鈥渢here鈥檚 definitely something going on there鈥�.
Neal thinks that Malaita鈥檚 unusual geological history might account for Collerson鈥檚 mineral discoveries. Neal reckons the garnets and zircons that Collerson found were created at between just 90 and 100 kilometres down, which he contends is deep enough for those minerals to form. They could then have risen to the surface some 34 million years ago via pipes of aln枚ite 鈥� a type of igneous rock. These pipes are common on the Ontong Java Plateau, on which the Solomon Islands sit, and although they are similar to kimberlite pipes, they don鈥檛 bear diamonds, he says. Neal also questions the authenticity of Collerson鈥檚 micro-diamonds.
But Collerson dismisses Neal鈥檚 doubts. He believes his evidence is, well, rock solid. Ray Jeanloz, a geochemist at the University of California at Berkeley, agrees: he says Collerson鈥檚 samples are better documented as truly deep samples than any others. And, although Jeanloz admits that the discovery of minerals from the deep mantle in the middle of an ocean is surprising, he says he has never been entirely convinced that the mantle plumes that power kimberlite pipes can only occur under continental masses.
Despite any and all doubters, Collerson is still optimistic. 鈥淭he data continues to point to Malaita as being a great target for diamonds,鈥� he says. And things are beginning to look more hopeful on the ground. Solsearch is now hopeful that it will finally be able to send someone back to the island this month to collect more samples.
The Solsearch partners will not be too worried if Malaita turns out to be empty of marketable macro-diamonds. Collerson鈥檚 new theory of diamond formation implies there could be diamonds anywhere a plume of carbon-rich material from the mantle has reached the surface 鈥� regardless of whether it鈥檚 under the ocean or in the middle of a continent. They鈥檝e already identified some of these 鈥渉ot spots鈥�, Violaris says: he is confident they will soon be digging up diamonds in the most unexpected places.