For a number of large volume ash-flow tuffs, the temperatures from two-feldspar geothermometry are consistently lower than temperatures obtained by iron-titanium oxide geothermometry by as much as 150°C (assuming low pressure). The iron-titanium oxide temperature is essentially independent of pressure, but the two-feldspar temperature has a pressure dependence of 18°C/kbar. The pressure for which the calculated temperatures correspond should indicate the depth of origin of the phenocryst assemblage. Although the present experimental calibration of these geothermometers is uncertain, other types of data suggest that useful, qualitative estimates of the pressure and depth of origin for certain ash-flow tuffs can be made. The phenocryst assemblage in the Fish Canyon Tuff (>3000 km3, San Juan volcanic field) appears to have originated at depths of about 25 km. The phenocrysts in the Bishop Tuff (500 km3, eastern California) appear to have originated at about 15 km depth. Even with allowance for a large error in this determination, it suggests that ash-flow magmas may develop at much deeper levels in the crust than previously thought. However, deeper sources would be consistent with some experimental work and geophysical measurements on silicic volcanic centers. This suggests that some, but not necessarily all, large volume ash-flow tuffs may have developed their chemical characteristics at mid-crustal levels. Models for the differentiation and eruption of such magmas should consider the possibility that the magmas may not have resided in shallow chambers for any significant period of time.

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