Natural diamonds and their inclusions provide crucial information over a range of depths for geologists studying Earth’s mantle, from the base of the continental lithospheric mantle (>150 km), the mantle transition zone (440-660 km), to the top of the lower mantle (~700 km). They can also provide important clues as to how Earth’s continents formed billions of years ago, and how the mantle convects today. In a recent review in Gems and Gemology, Steve Shirey (Carnegie Institution of Washington, USA) and James Shigley (Gemological Institute of America, USA) summarize the latest technological advances in the field to demonstrate how natural diamonds are used in geologic research.
This comprehensive review begins with an overview of widely accepted features of diamond formation in Earth, and outlines some of the big questions that remain unanswered. Shirey and Shigley delve into the geology of kimberlitic magmas. Although these eruptions have never been witnessed, they are known to carry to diamonds from the mantle to Earth’s surface. The authors also describe ways that modern science can inform mining efforts to find new diamond deposits, new techniques for accurately dating diamonds, and the specific chemical and physical conditions required for diamond formation.
“This review, written primarily for the practicing gemologist, will hopefully provide a comprehensive resource for non-scientists and those scientists studying these geologic processes who are not familiar with what diamonds can tell us,” said DCO’s Steve Shirey. “In the process of writing it, I myself discovered new ways to explain what we know, why we know it, and how to describe what we may yet learn in the future.”
The review is illustrated with beautiful geologic drawings, photographs of diamonds, their inclusions, and the rocks in which they are found. It is a very accessible read for those interested in mantle diamonds and their formation, growth, and distribution. The review is available in open access format.
Photo credit: A red garnet inclusion in diamond. Stephen H. Richardson, University of Cape Town