Mineralogical and petrological study of pyrope megacrysts and related minerals from the Four Corners Area, Colorado Plateau, and an experimental study of iron-magnesium order-disorder in magnesian orthopyroxenes

This dissertation includes detailed mineralogical and petrological studies of pyrope crystals and their mineral inclusions from the Four Corners Area on the Colorado Plateau, and an experimental study of Fe-Mg order-disorder in magnesian orthopyroxenes using high-sensitivity Mössbauer spectroscopy. These studies provide insights into the understanding of processes in the deep interior of the earth, the mineralogy of the mantle, phase equilibria, and thermodynamics of mineral solid solutions and reactions. Thermobarometric calculations suggest that the pyrope crystals studied formed at depths ranging from 50 km (where T ≈ 600°C and P = 15 kbar) to 95 km (where T ≈ 800°C and P = 30 kbar) along the local geotherm. Many mineral inclusions, including nearly all mineral phases that are believed to be present in the upper mantle, were identified in these pyrope crystals. Abundant titanate minerals constitute a potential repository for high field strength elements (HFSE), large ion lithophile elements (LILE) and light rare earth elements (LREE) in the upper mantle. They may have an impact on the liberation or sequestration of these elements during partial melting of the mantle. The solidified fluid/melt inclusions and exotic oxide minerals strongly suggest the role of mantle metasomatism in the genesis of pyrope crystals and associated mineral inclusions. The process provides a possible mechanism for HFSE depletion in arc basalts. A new hydroxyl-bearing titanate mineral, carmichaelite, was identified and characterized. In addition to being a potential repository for water and HFSE in the upper mantle, its newly determined structure features may have implications for silicate phases under lower mantle conditions. Garnet inclusions were found in one pyrope crystal, and coexisting pairs were interpreted to represent immiscible pairs. This is the first direct observation of immiscibility in the pyrope-almandine-grossular garnet system, and has major implications for thermodynamic mixing properties of garnet solid solutions. The thermodynamic mixing properties for orthopyroxene, also a common inclusion mineral in pyrope crystals, were analyzed based on the intracrystalline Fe-Mg distribution coefficients obtained on magnesian orthopyroxenes. These coefficients were experimentally determined as a function of temperature and orthopyroxene composition using Mössbauer technique. They were found to be independent of orthopyroxene composition, contrary to previous studies.