Biotites from granitic rocks of the central Sierra Nevada batholith, California

Biotites from plutonic rocks of the central Sierra Nevada and Inyo Mountains, California, have been examined and characterized by powder X-ray diffraction and optical and chemical methods.

Compositions of the biotites define a trend in the compositional triangle Fe+3 Fe+2Mg. When related to the experimentally studied ternary system KFe₃+3AlSisO₁₂H-₁-KFe₃+2 AlSi₃O₁₀(OH)₂-KMg₃AlSi₃O₁₀(OH)₂ and coupled with the estimated positions of biotite solid solutions for different oxygen buffers, the trend suggests that oxygen fugacities in magmas during biotite crystallization were slightly higher than those defined by the Ni-NiO buffer. The compositional data also suggest that magmas were ‘buffered’ with respect to oxygen by oxides existing within the magmas themselves.

Correlation between the Fe/(Fe+Mg) ratio, an inferred temperature indicator, and other elements is generally poor, which suggests that factors other than temperature at the time of crystallization exerted an important influence on compositions.