Petrologic and Microstructural Constraints on Focused Melt Transport in Dunites and The Rheology of the Shallow Mantle

Michael Braun, Ph.D., 2004
Peter Kelemen and Greg Hirth , Advisors

Observations at mid-ocean ridges indicate that magmas are focused to the ridge axis by a network of porous dunites in near chemical isolation. This thesis investigates several of the outstanding questions regarding the mechanisms of melt transport and its effects on the shallow mantle. Chapter 1 details the current understanding of melt migration from observations at mid-ocean ridges and ophiolites. Chapter 2 uses the size distribution and abundance of dunites measured in the Oman ophiolite to place limits on the potential mechanisms by which dunites from and subsequently estimate the flux of chemically unequlibrated melt which a network of dunites can supply. Chapter 3 characterizes the chemical composition of dunites and harzburgites from Oman to further constrain the process by which dunites form and relates to the observed trends within dunites to variations in the time-integrated melt/rock ratio. Chapter 4 examines the microstructures of peridotites in Oman to constrain the deformation mechanisms which determine the viscosity of shallow mantle. Chapter 5 is a numerical investigation of advection beneath ridges incorporating the rheology inferred from the observed microstructures. Chapter 6 integrates the conclusions of the previous chapters, reevauating the potential melt flux through dunites and constraining the permeability of the shallow mantle.