2026-2027 Lecturer: Steven Shirey

Steven B. Shirey

Carnegie Institution for Science, Earth and Planets Lab

Biography

An enduring question in geology is when plate tectonics, in its present form, began. Tied to this are more detailed questions about mantle chemical evolution, the creation of the continents, and the emergence of life. A knowledge of how plate tectonics works, especially in the deep mantle, is paramount. Geologist and isotope geochemist, Steven Shirey, has long been involved researching these topics over a 42 year career at the Carnegie Institution for Science, especially with the 39 postdoctoral fellows and 26 students he has actively mentored. In the latter half of his career, he has used diamonds and their inclusion cargo to investigate the origins of ancient continentalstabilizing mantle keels under the continents and the penetration of subducted plates as deep as the top of Earth's lower mantle. These processes, visible far below Earth's surface by extracting microscopic inclusions from encapsulating diamonds, reveal how life-essential elements such as N, H, C, B, and S are recycled into the mantle and the nature of deep earthquakes. Shirey is a Fellow of the American Geophysical Union (AGU), the Geochemical Society, the Geological Society of America, and the Mineralogical Society of America (MSA). He was the 96th President of MSA and the 100th President of the Geological Society of Washington. Shirey received AGU's 2025 Harry H. Hess Medal, for "...seminal contributions in geochemistry and mantle petrology and the origin, evolution, and consequences of plate tectonics on Earth". He was educated at Dartmouth College, the University of Massachusetts and Stony Brook University.


Abstract: The petrogenesis of Earth's deepest diamonds: A look at plate tectonics in the third dimension

     Sublithospheric diamonds and the inclusions they may carry crystallize in Earth's asthenosphere, mantle transition zone, or uppermost lower mantle (from 300 to ~800 km). They are the deepest minerals so far recognized to form by plate tectonics. These diamonds are distinctive in their deformation features, low nitrogen content, and inclusions of high pressure mantle minerals such as majorite, ringwoodite, perovskite, ferropericlase, and bridgmanite or their equivalent compositions transformed to lower-pressure minerals. The isotopic composition of boron, carbon, and nitrogen in the diamonds and/or oxygen, iron and magnesium in inclusions are typically well outside normal mantle compositional ranges. These differences from normal mantle indicate that the diamond-forming fluids were modified by interaction with seawater in so-called oceanic slabs before the slabs were subducted.

     Metamorphic minerals formed in oceanic slabs, especially when they are cold, are effective hosts that transport carbon as carbonate and hydrogen as water, hydroxyl, or methane below the mantle wedge of the island arc–mantle system. Warming of the slab generates carbonatitic melts, supercritical aqueous fluids, or metallic liquids from these minerals, forming three basic types of sublithospheric diamonds. Diamond crystallization occurs by movement and reduction of these mobile fluids as they pass through host mantle via fractures —a process that creates chemical heterogeneity in the mantle and may promote deep focus earthquakes. The geobarometry of majorite and diamond ages suggest mobility of mantle carrying the diamonds upwards, perhaps to the base of mantle lithosphere as part of the supercontinent cycle. From there, diamonds are finally transported to Earth’s surface by eruptions of kimberlite magma.

     Mineral assemblages in sublithospheric diamonds directly trace a deep volatile cycle, demonstrating how the hydrosphere of a rocky planet such as Earth can connect to its solid interior.

Shirey, S.B., Pearson, D.G., Stachel, T., Walter, M.J. (2024) Sublithospheric Diamonds: Plate Tectonics from Earth's Deepest Mantle Samples, Annual Review of Earth and Planetary Sciences 52, 9.1-9.45