In general, my research draws on the magnetic behavior and crystal orientation of minerals to understand questions about Earth processes operating on global, tectonic, outcrop, and nanometer scales.  In order to connect observations made at small scales to processes occurring on larger scales, my research incorporates a strong multi-disciplinary approach including fieldwork, rock magnetism and paleomagnetism, mineral texture studies, electron microscopy, and numerical modeling.

Magnetic Mineralogy

Mineral Microstructures

Mineral microstructures at the nanometer and micrometer length scales can profoundly influence the magnetic properties of a rock sample.  Certain kinds of mineral microstructures can even play an important roll at regional (aeromagnetic) and planetary (satellite) observation scales.  I use a variety of microscopy tools to understand the processes that create such mineral microstructures and their effects on bulk magnetic properties. 

Exsolution Processes Many magnetic minerals contain impurities such as Cr, Mg, Al, and Ti, which unmix from their host crystals to form exsolution products.  The difference in a mineral’s magnetic properties before and after this unmixing occurs is dramatic.  Often such processes increase the intensity and stability of magnetization.  Determining the temperatures and the kinetics of such processes is critical for correctly interpreting the magnetization (Feinberg et al., 2005).

Magnetic Mineral Fabric Electron backscatter diffraction (EBSD) is a scanning electron microscopy (SEM) technique that determines the three dimensional orientation of a crystal lattice to within 0.5 degrees. My work with Hans-Rudolf Wenk at the UC Berkeley Texture Laboratory focuses on the crystallographic relationships of magnetite inclusions exsolved within silicate minerals such as pyroxene (Feinberg et al., 2004) and plagioclase. We also use EBSD to examine the preferred orientation of minerals in mafic plutonic rocks of the Bushveld Layered Intrusion in South Africa and the Duluth Complex of northern Minnesota.

Characterization of Magnetic Minerals in Environmental Samples The composition, concentration, and properties of magnetic minerals in geologic samples (soils, sediments, airborne particulate matter, etc.) are tools that can help illuminate environmental research problems such as climate change, volcanic ash dispersal, soil evolution, and atmospheric air pollution.  Members of my research group are using rock magnetic techniques to study environmental samples from Mexico, Texas, California, and Minnesota.   

Magnetic Imaging

Magnetic Force Microscopy (MFM) By scanning a flexible, magnetized cantilever across a highly polished surface, MFM produces an image of a sample's out-of-plane magnetization. Ongoing work at the Institute of Rock Magnetism and the Magnetic Microscopy Center at the University of Minnesota focuses on nanometer-scale mineral structures within exsolved magnetite inclusions in silicate minerals such as pyroxene and plagioclase (Feinberg et al., 2005).

Off Axis Electron Holography Electron holography is an advanced transmission electron microscopy (TEM) technique that produces a map of a sample's in-plane magnetization. In collaboration with Richard Harrison and Takeshi Kasama at the University of Cambridge and Rafal Dunin-Borkowski at the Technical University of Denmark, I am investigating magnetostatic interactions within finely exsolved magnetite inclusions (Feinberg et al., 2006).

Archeological Applications of Rock Magnetism and Paleomagnetism

Impressions found in a basaltic tuff (volcanic ash and debris) in the Valsequillo Basin in the Puebla state of México have been variably interpreted as either human footprints that are 40,000 years old, or as modern tool marks left behind in an abandoned quarry.  If the impressions are footprints that were preserved shortly after the tuff’s deposition, then it is important to accurately determine the age of the tuff.  Paleomagnetic analyses combined with 40Ar/39Ar ages indicate that the tuff is 1.3 ± 0.03 Ma (Renne et al., 2005, Feinberg et al., 2009).