Diamonds that originated deep within Earth may have formed from the remains of fish or decayed matter on the ocean floor, according to a new study. The findings, say researchers, are the first evidence for an organic origin of deep diamonds and show that shallow material is being recycled through Earth's bowels.
Most diamonds are born about 100 to 200 km below Earth's surface. At this depth, the temperature is so high that rocks melt. Given the right pressure, carbon atoms in the melting rocks bond together to form diamond crystals. However, some diamonds form much deeper, as evidenced by the presence of majorite garnet, a silica-rich mineral that can only form more than 250 km deep. Surprisingly, these garnets contain high levels of iron and calcium, and low levels of magnesium, suggesting an origin much closer to the surface.
To resolve the mystery, Ralf Tappert, a diamond geologist at the University of Alberta, Canada, and his colleagues looked at deep diamonds from the Jagersfontein mine in South Africa. Their study showed that deep diamonds have a much more C-12, the carbon isotope that comprises almost all organic matter, than C-13, the other isotope. The results suggest, say the researchers, that life on the ocean floor supplies deep diamonds with their carbon when ocean plates slip under other plates and plunge into Earth's mantle. The team reports its findings in the July issue of Geology.
The study is nice proof that material from subducted slabs eventually returns to Earth's surface, says Steven Shirey, an isotope geochemist with the Carnegie Institution of Washington.