An Oxygen Isotope Study of Aluminum Silicate 'Triple Point' Rocks

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EAR-0087336SharpThe goal of this project is to use oxygen isotope geochemistry to quantify the temperature of crystallization of coexisting aluminum silicates (AS), andalusite, kyanite and sillimanite. These are important index minerals, and their coexistence has been used to infer pressure-temperature conditions of metamorphism. Previous investigations of "triple point" localities, defined by the occurrence of all three polymorphs, have relied on textural evidence for assessing whether or not the AS minerals crystallized in equilibrium. In this study, the oxygen isotope values of quartz and AS minerals from quartzites will be measured in classic triple point assemblages from New Mexico in order to address four goals: 1) Determine the temperatures of aluminum silicate formation: 2) Evaluate the degree of fluid exchange following AS crystallization: 3) Determine whether 'triple point' assemblages actually formed over the narrow P-T range defined by the invariant equilibrium: 4) Compare the d18O values of sillimanite and fibrolite to better understand the temperature interval over which the latter forms. It is envisioned that the study will demonstrate the feasibility of a powerful tool for deciphering the timing and temperature of AS crystallization. This work will constitute the work of a Ph.D. student's thesis at the University of New Mexico.