Oxygen isotope variations in Paleogene volcanic rocks from southern New Mexico: Insight on crustal contamination and magmatic sources
— M. Hoffman and G.S. Michelfelder

This study reports oxygen isotope values determined by laser fluorination of mineral separates from basalt through rhyolite composition volcanic rocks erupted from the Rubio Peak and Bell Top formations and the Uvas Volcanic Field. Plagioclase phenocrysts from the Uvas Basalts are altered and have δ¹⁸O values of 12.87‰. Pyroxene phenocrysts from the basalts are considered magmatic with δ¹⁸O values of 4.29–5.64‰; quartz contains δ¹⁸O values of 2.10–7.52‰ and is only found as amygdule filling crystals. Phenocrysts from the Rubio Peak Formation basalts and andesites contain δ¹⁸O values of 4.88-5.40‰. Two rhyolite samples from the Bell Top Formation are ash flow tuffs from the Kneeling Nun Tuff and Cooney Canyon Tuff. Plagioclase and quartz phenocryst δ¹⁸O values are restricted, ranging from 6.69–6.73‰ and 8.01–8.23‰, respectively. Biotite phenocrysts from these samples exhibit a greater range in δ¹⁸O values, from 5.21– 6.18‰. Calculated magmatic δ¹⁸O values for the Uvas basalts and Rubio Peak andesites range between 6.12 and 6.40‰ corresponding to fractional crystallization of a primary mantle melt, while Bell Top Formation volcanic rocks exhibit higher δ¹⁸O magmatic value of 8.60‰, representing partial melting of a granitic composition crustal source.