Reconstruction of Magma Plumbing System and Regional Magmatic Processes via Chemical and Structural Zoning of Biotite in Rhyolite from Long Valley, CA

Minerals with compositional zoning in volcanic products are widely used to decipher the history of magmatic evolution. However, structural information, which reflects physical conditions and crystallization equilibrium, has often been overlooked. This study presents the first report on the structural zoning of deep-derived biotite phenocrysts through investigations of metaluminous rhyolite from Long Valley, CA. Biotite is enriched in Si, Mg, and K and depleted in Fe³⁺, Ti, and Al^IV in core zones compared with rims. In situ structural analyses, including micro X-ray diffraction, Raman spectroscopy, and transmission electron microscopy, were conducted to identify cores with perfect 2M₁ polytype and disordered rims of biotite. The results demonstrate the effectiveness of these methods in revealing various (micro)polytypes of a single species, which occur at different crystallization temperatures, pressures, supersaturation levels, and oxygen fugacities. The concept of structural zoning is introduced here to describe the different structural features distributed systematically in various parts of minerals. By combining structural and chemical zoning, we illustrate a two-step growth for samples: equilibrium crystallization of the highly ordered cores in a deep magma reservoir with high temperature and pressure, followed by rapid growth of disordered rims during magma mixing in a crystal mush. We further discuss the implications of these findings for reflecting the plumbing system structure and eruption history of rhyolitic magma over extended periods. Our study underscores the remarkable sensitivity of structural zoning in delineating the crystallization conditions of minerals and documenting the environmental changes within magma.