Abstract by Haley Mosher
Composition and texture of titanite in mafic enclaves in the Oligocene Little Cottonwood stock, Utah reveal its crystallization history
Titanite in the Little Cottonwood Stock (LCS) and its mafic inclusions (MI) were used to evaluate the role of magma mixing. Both MI and LCS titanite grains tend to have mottled cores with abundant ilmenite amoeboids (exsolved to rutile/anatase) surrounded by oscillatory and sector zoned mantles. Poikilitic, anhedral titanite precipitated around feldspar and rare quartz exclusively in MIs. MI titanites are larger (150-3000 μm vs. 100-2000 μm) and more abundant than in LCS. Chemically, REE, Ti, Si, Ca, Al, Fe, F, Y and Mn concentrations and δ18O (~5.5‰) in titanite are nearly identical, varying more by location rather than by LCS vs. MI. Nonetheless, Zr-in-titanite temperatures are higher for MIs by ~30°C. Thus, we propose that a hotter, reduced, ilmenite-rich mafic magma mixed into an oxidized felsic magma. This caused resorption of ilmenite and stabilization of titanite, forming mottled cores and then the mantles. Simultaneously, the mafic magma quenched, causing poikilitic titanite to precipitate after feldspars.