✦ A 3 billion year old quartz pebble conglomerate from the Jack Hills, Yilgarn Craton, Western Australia. This metasedimentary rock contains the oldest known minerals on Earth, zircons that formed only 100 million years after the Earth accreted. (photo: Steve Shirey)
Formation of Continents
A main research interest of Steve's is how the continents of Earth first formed from the mantle, became stable, and have since been modified. These questions are among the most basic in Geology -especially since Earth's continents, with their complex and long geologic record, are unique features our planet. The process of nascent continent formation involved some form of mantle recycling in a water-rich-world. Such early geodynamic processes were fundamental to the formation of life on Earth. Continents thus hold a glimpse of how Earth became a habitable planet.
Steve's past, current and future research reflect the diversity of continental rocks, encompassing a range of studies such as continental volcanic rocks, ancient and present subduction zones, crust-mantle evolution now and in the past, the mantle keels to the continents, and rocks from the present oceanic mantle -an analog to pre-continental, oceanic mantle of the Hadean to Paleoarchean (4500 to 3200 million years ago).
Diamonds and the Minerals of the Deep Mantle
The study of continents using the deepest-derived rocks (xenoliths in kimberlite) led to Steve's recent interest on diamonds. Diamonds and their inclusions are rare specimens that usually come from the mantle keels of continents at depths greater than 150km. But a small fraction, known as superdeep diamonds, derive from greater depths including the mantle transition zone and the top of the lower mantle. Diamonds and their inclusions afford a remarkable chance to study mantle mineralogy, the migration of carbon-bearing fluids and melts, and pressure, temperature, and redox conditions in the deep mantle. A particular contribution of Steve’s is to use patterns of diamond formation age, inclusion mineralogy, and location relative to geologic setting to extract large-scale constraints on continent evolution and mantle geodynamics.
Select the Project Areas menu above to read about some current larger research efforts that Steve is involved in and the Publications menu above to read about Steve's relevant papers grouped by subject area. Under Other Resources in the navigation bar, select the Geochemistry & Cosmochemistry Group menu to learn about this research group and the analytical capabilities available to investigators working in it. Select the Postdocs & Visitors menu to learn about visitor and postdoctoral fellowship programs.
Scale of research -- from continental to sub-microscopic
One of the interesting aspects of Steve's research is the variety of size scales that it encompasses. On one hand consideration of continent formation in the context of the ancient Earth is global in scope. But many of the most significant observations and analyses are made at the submicroscopic scale. This scale range requires thinking big and small -from geodynamics to mineralogy and using a wide range of analytical techniques on whole rocks and minerals.
✦ Plot of diamond type, E-type or eclogitic (red) vs P-type or peridotitic (green) with seismic wave velocity structure of the Kaapvaal craton at the 150 km depths where diamonds crystallize.
✦ Optical microscopy image (fov = 1.5 mm) of a shiny, elongate metallic inclusion in a diamond fragment cut from a large, gem-quality, superdeep diamond, Letseng mine, Lesotho.
✦ Elemental map by scanning electron microscopy of a polished diamond surface (black) that exposes a metallic inclusion. These metallic inclusions contain an assemblage of iron carbides, sulfides, and alloys that crystallize at low pressure upon eruption in their host kimberlite. (Left figure from Shirey et al., Science 2002. Middle and right figures from Smith et al., Science 2016).