✦ A transparent, white, octahedral, rough, lithospheric diamond from South Africa contained in its brown to black kimberlite matrix. This cover photo for the Winter 2013 issue of Gems and Gemology (v. 49, pp. 188-222) highlights the article, Recent advances in understanding the geology of diamonds by Steven Shirey and James Shigley http://dx.doi.org/10.5741/GEMS.49.4.188 (VRL# 157869). Photo by Robert Weldon. © 2014 Gemological Institute of America).
Diamonds: the deepest probe of plate tectonics
✦ This is a vertical slice of the mantle under Japan and Korea from the original study (Li, C., Hilst, R., Engdahl, E., Burdick, S. (2008). A new global model for P wave speed variations in Earth's mantle. Geochemistry, Geophysics, Geosystems). Colder mantle with faster P wave travel times is shown in blue colors, warmer mantle with slower travel times is shown in red colors. Superdeep diamonds form in association with (Modified from an original figure by Josh Wood, Deep Carbon Observatory).
Independently, and with postdoctoral fellows and students, I explore geological questions about the evolution of the solid Earth and the active tectonic processes that have shaped it. (See a Carnegie Newsletter article here.) The formation of lithospheric and superdeep diamonds has been an area of interest and research for me for nearly two decades. My studies of mineral inclusions in these 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 to see how plate tectonics works at depth.
Diamonds are the premier container for mineral inclusions, effectively isolating them completely from reactions with fluids and magmas. A special class of diamond, estimated to comprise less than 10% of diamonds mined from kimberlite, derives from hundreds of kilometers below the lithospheric mantle. These so-called "superdeep" diamonds carry distinctive retrograded mineral assemblages that give a valuable look at deep mantle mineralogy not attainable any other way. Recent studies of microscopic mineral inclusions extracted from diamond hosts support new findings: 1) the recycling of surface-derived constituents to the top of the lower mantle, 2) a measurably high water content for nominally anhydrous high-pressure minerals, and 3) a mantle that gets progressively reduced with depth yet harbors distinct oxidized and reduced regions.
Igneous evolution of the Earth
✦ 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).
In my work, 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. This quest has led me to work on topics as diverse as arc volcanism, Archean crustal evolution, continental volcanism, meteorites and impacts, mantle heterogeneity, and analytical method development in geochemistry. A recurring research theme has been to show how ancient episodes of mantle-derived magmatism and crust formation differ (or not!) from those today.
The emergence of continents
✦ The ages of the rocks on Earth's surface are displayed in this map from the United States Geological Survey (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. The important thing to note is the pattern of the age distribution; the oldest rocks are at the center of the continents and the youngest are at the edges.