Diamond geologists now have evidence for a wide range of diamond source fluid compositions, including carbonate-bearing fluids, boron-rich fluids and metallic melts. On the left is a cubo-octahedral diamond from Zimbabwe that formed from water-rich fluids that contain both methane and carbon-dioxide. On the right is a diamond from Sierra Leone that formed from water-rich fluids containing methane and ethane. Photos by Joshua Balduf and Karen Smit (GIA).
How do diamonds grow in the mantle? While Hollywood’s depiction of Superman squeezing coal captured the public’s imagination, in reality this does not work. Coal is a crustal compound and is not found at mantle pressures. Also, we now know that diamond does not form through direct conversion of solid carbon, even though the pressure and temperature conditions under which diamond forms have traditionally been studied experimentally as the reaction of graphite to diamond.
Generally, two conditions are needed for diamond formation: Carbon must be present in a mantle fluid or melt in sufficient quantity, and the melt or fluid must become reduced enough so that oxygen does not combine with carbon.
But do diamonds all grow by the same mechanism? What does their origin reveal about their growth medium and their mantle host rock? Surprisingly, diamonds do not all form in the same way, but rather they form in various environments and through varying mechanisms.
Through decades of study, we now understand that diamonds such as the rare blue Hope, the large colorless Cullinan, and the more common yellow “cape” diamonds all have very different origins within the deep Earth.
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