A look inside of diamond-forming media in deep subduction zones.

Geologists have "known" for many years that continental crust is buoyant and cannot be subducted very deep. Microdiamonds 10-80 microm in size discovered in the 1980s within metamorphic rocks related to continental collisions clearly refute this statement, suggesting that material of continental crust has been subducted to a minimum depth of >150 km and incorporated into mountain chains during tectonic exhumation. Over the past decade, the rapidly moving technological advancement has made it possible to examine these diamonds in detail, and to learn that they contain nanometric multiphase inclusions of crystalline and fluid phases and are characterized by a "crustal" signature of carbon stable isotopes. Scanning and transmission electron microscopy, focused ion beam techniques, synchrotron infrared spectroscopy, and nano-secondary ion mass spectrometry studies of these diamonds provide evidence that they were crystallized from a supercritical carbon-oxygen-hydrogen fluid. These microdiamonds preserve evidence of the pathway by which carbon and water can be subducted to mantle depths and returned back to the earth's surface.