Inclusions rock dating
“Most mines have a very low frequency of inclusion-bearing diamonds to gem-quality diamonds, and so in some mines it’s almost impossible to get inclusion-bearing stones.” These diamond crystals, which bear sulfide inclusions decorated by rosette fracture systems, are suitable candidates for dating using the rhenium-osmium decay system.
Pedro Padua © GIA, courtesy Carnegie Institution of Washington Once the researchers have enough inclusion-bearing diamonds for a valid study, work can begin.
It involves ingenious solutions to fashion the tiny diamonds into plates for study, cleave out the sulfides, separate the isotopes of rhenium and osmium by chemistry, and count the various isotopes with sophisticated laboratory instrumentation for the final age determination.
These “superdeep” diamonds provide Shirey and other scientists a tantalizing window into the workings of the deeper mantle.
Peter Johnston © GIA paper “Recent Advances in Understanding the Geology of Diamond,” coauthored by Shirey and GIA’s Dr. Shirey is a one of a collaborative group of geoscientists from institutions all over the world using diamonds as a means to sample the deep earth.
“So in one mineral species,” he says, “you have the deepest, the oldest, and the most resistant to secondary effects mineral that you can get, so it makes them very, very unique specimens.” Shirey tells us that the osmium-rhenium radioactive decay system allows researchers to date individual inclusions.
Previous dating techniques required researchers to collect inclusions of other minerals—including garnets and zircon—from many different diamonds to obtain an age.