Steven B Shirey
Senior Staff Scientist
Department of Terrestrial Magnetism
Carnegie Institution for Science
✦ These rough diamonds, besides being the world's most valuable gemstones based on size and clarity, are remarkable for geological reasons. New research done at the Gemological Institute of America and the Carnegie Institution shows them to crystallize in the mantle transition zone (440-660 km deep) in native iron metal bearing regions characteristic of the deep mantle. These reducing regions, long hypothesized but always hidden by melting processes in shallower and more oxidized mantle, had never been sampled before this research. ††
✦ Acadia National Park, ME (2009)
✦ Fall AGU Meeting, San Francisco (2010)
✦ 4th Int'l Diamond School, Italy (2018)
Our scientific home in Washington, DC USA
At the Department of Terrestrial Magnetism, (known as 'DTM') research in geophysics, geochemistry, cosmochemistry, astronomy and planetary science is carrried out. DTM is one of the six departments that make up the Carnegie Institution for Science --the nation's oldest privately-endowed research organization dedicated to fundamental scientific research. Steven Shirey is the senior staff member of the four-person Geochemistry and Cosmochemistry Group of DTM.
My research interests
✦ Pink and grey banded gneisses from Point Lake, NWT, Canada —the oldest rocks studied in the Central Slave Basement Complex. Such obvious intermixing of contrasting rock types is one of the challenges in studying some of the oldest rocks on Earth. Rocks such as these are hallmarks of the oldest continental crust on Earth and are a direct record of what the Earth was like almost 4 billion years ago. (Photo: Jesse Reimink)
Independently and with the mentoring of student-interns, PhD students, and postdoctoral fellows, Steve researches geological questions about the igneous evolution of the solid Earth. Trace element and isotopic compositions of ancient minerals and rocks are employed to understand some of the deepest and oldest magmatic and geodynamic processes that have shaped Earth since its formation. The formation of diamonds has been an area of interest and research for Shirey for nearly two decades. His study of mineral inclusions in diamonds has been applied to global scale questions such as the creation of continental mantle, the onset of plate tectonics, and the recycling of surface materials into the deeper parts of the mantle. In addition to his work on lithospheric and superdeep diamonds, he has worked on topics as diverse as arc volcanism, Archean crustal evolution, continental volcanism, meteorites and impacts, mantle heterogeneity, and geochemistry method development.
The importance of continents in geologic research
✦ The ages of the rocks on Earth's surface are displayed in this map from the USGS. The age of continental rocks is as follows: older than 2.5 billion years old in orange; 2.5 to 1.6 billion years old in pink, 1.6 to 1.0 billion years old in green, 1.0 to 0.54 billion years old in purple and younger than 0.54 billion years old in blue and yellow. The ocean floor is in shades of blue and is less than 0.2 billion years old.
Steve's main research emphasis has been on the formation of the continents from Earth's mantle, a geologic process that spans most of Earth history and records the conditions on the ancient Earth that led to life. Earth is the only rocky planet in our Solar System with tectonic activity that can be traced back to its birth 4.5 billion years ago. Continents are the record of that activity.
A recurring research theme has been to show how ancient episodes of mantle-derived magmatism and crust formation differ (or not!) from those today. Field work on modern rocks is important also to understand how the Earth's geologic processes have changed.
BA, Geology (1972), Dartmouth College
MS, Geology (1975), University of Massachusetts Amherst
PhD, Geochemistry (1984), State University of New York at Stony Brook
Carnegie Fellowship, Isotope Geochemistry (1984-1985), Carnegie Institution for Science
Some of the newer diamond results
✦ A blue, boron-bearing diamond, with dark inclusions of a mineral called ferropericlase that was one of 26 inclusion-containing blue diamoinds examined as part of the study. This gem weighs 0.03 carats. Photo by Evan M. Smith/© 2018 GIA
A new study published in the August 2 issue of the journal Nature (Smith, E. M., Shirey, S. B., Richardson S. H., Nestola, F., Bullock, E. S., Wang, J., & Wang, W. (2018). Blue boron-bearing diamonds from Earth’s lower mantle. Nature, 560, 84–87. http://doi.org/10.1038/s41586-018-0334-5 ) demonstrates that the world's most valuable diamonds, those that are blue and thus boron-containing, are also perhaps the world's most deeply-derived diamonds. When these diamonds do contain mineral inclusions, the inclusions are low-pressure reaction products of high pressure minerals including bridgmanite, ferropericlase, Ca-pervoskite, and stishovite among others. These minerals allow an estimate of depths of origin around or below 660 km in Earth's mantle. See the Carnegie Institution for Science press release here. See the Deep Carbon Observatory (DCO) press release here.
††Photo in title page by Robert Weldon. © 2015 GIA. Courtesy of Gem Diamonds Ltd. This group of type IIa rough diamonds (VRL# 190894) shown here range from 14 to 91 carats and sometimes appear to be broken fragments of once larger diamonds.