Iron-Titanium Oxide Phenocrysts in Compositionally Zoned Ash-Flow Sheets from Southern Nevada

Electron-microprobe analyses of coexisting magnetite and ilmenite phenocrysts from four compositionally zoned ash-flow sheets of the Paintbrush and Timber Mountain Tuffs, southern Nevada, show systematic variations in Fe-Ti oxide composition that correlate closely with bulk-rock composition and with the inferred position in the source magma chambers. Comparison with experimental data of Buddington and Lindsley (1964) indicates that the Fe-Ti oxides of rhyolitic upper parts of the differentiated magma chamber crystallized at 700° ± 50° C, whereas oxides of quartz latitic lower parts of the magma column crystallized at temperatures as high as 900° C. The trend of temperature and oxygen fugacity is parallel and intermediate to trends for Ni-NiO and MnO-Mn₂O₃ buffers. The low indicated temperatures for the rhyolites suggest that they were saturated, or nearly saturated, with water at the time of crystallization.