Steven Shirey is a fellow of the Geological Society of America and a past president of the Geological Society of Washington. He has been a Staff Member at the department since 1985.

This is an optical photomicrograph of a sulfide- inclusion-bearing rough diamond from the Jwaneng Mine, Botswana. Below the natural diamond growth surface, at center, is a hexagonal grain of iron sulfide (Fe-S) surrounded by an irregular black rim. This rim is caused by internal fracture of the diamond on its 150-km ascent to the Earth’s surface in the explosive volcanism of the kimberlitic magma. Sulfide grains like these are removed for rhenium-osmium isotopic analysis to reveal the age of the diamond and the composition of the sulfide. The diameters of these sulfide grains are about 300 microns.

(Image courtesy J. W. Harris.)

Igneous rocks are formed when molten material, often rising from great depths, solidifies. These rocks contain clues of past processes in Earth’s interior in the form of different radioactive isotopes. Over time, these isotopes decay and beget daughter isotopes. Measuring these changes provides scientists with information about ancient geologic events. Geochemist Steven Shirey uses isotopic analyses to learn about the evolution and composition of the Earth’s crust and mantle, as well as the history of the continents and ocean basins. He travels the world to search for rock samples and develops new tools and techniques in the laboratory to extract this valuable information. One of his goals is to understand how the continents were first created from the ancient mantle.

Shirey is particularly interested in the rhenium-osmium (Re-Os) isotopic decay system and has contributed significantly to its development and application. The Re-Os method is enormously useful for studying geological problems because Re and Os differ radically in melting behavior from the isotopes in other dating systems. Shirey, DTM colleague Richard Carlson, and others have applied the Re-Os method to a diversity of volcanic/magmatic processes and to the formation of the stable interiors of continents, called cratons. Cratons, containing the oldest rocks on Earth, anchor the amalgamation of younger continental material and contain much of the planet’s mineral wealth, including diamonds, gold, and platinum.

Shirey and colleagues recently developed a microchemical technique to determine the Re-Os composition of single small sulfide inclusions (at 10^-15 grams) embedded in diamonds from the Kaapvaal Craton in Botswana and South Africa. These inclusions are the most informative capsules of materials from the deep mantle. Their analyses have led to the construction of a geochronological framework for diamonds and have provided a link to their source regions in the cratonic mantle keel. The results are important to diamond exploration and represent an important way to sample and analyze mantle material. Scientists now can link diamond growth to geological processes, such as craton stabilization, and to the role mantle keels played in continent development.

Shirey is also interested in specific magmatic processes in the ancient and modern Earth. To investigate this area, he has studied unique 2.7- to 3.3-billion-year-old high-temperature volcanic rocks known as komatiites. Recently he has turned his attention to the rocks from the volcanically active midocean ridges and helped develop new methods for measuring their boron isotopic composition in situ.

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