INSIGHT INTO INTERRELATIONSHIPS AMONG DENSITY, VISCOSITY, CRYSTALLINITY AND CHEMICAL COMPOSITION WITHIN HYPOCRYSTALLINE INTERMEDIATE LAVAS, MT. HOOD VOLCANO, OREGON

Study of the relationships between rheological properties and the mineralogical and chemical compositions of lavas is important to understanding numerous magmatic processes, such as magma buoyancy, magma mixing, and rate of magma emplacement. Rate of crystal movement throughout a magma also may effect lava bulk composition. This study examines the relationships between density, viscosity, chemical composition, mineralogical content, and percent crystallinity in intermediate lavas erupted at Mt. Hood volcano in the Cascade Range. Mt. Hood lavas were chosen for study because many contain glass in the groundmass, suggesting a possible systematic relationship between, crystallinity, density, viscosity, and chemical composition. The lavas chosen for analysis were erupted during the Main Stage period (~700,000 to 30,000 ybp). Most are two-pyroxene andesites (SiO₂ = 58 – 61 wt %) and basaltic andesites (SiO₂ = 52 – 58 wt %) characterized by hypocrystalline groundmasses with plagioclase, clinopyroxene and orthopyroxene (in basaltic andesites) as major phases. Temperature-dependent melt viscosities were calculated from whole rock compositions using the method of Giordano et al. (2008). Melt densities were calculated using partial molar volumes of oxide components using the method of Bottinga and Weill (1970). Both density and viscosity were calculated at temperatures ranging from 700 ^oC to 1200 ^oC. Densities vary from 2.41 g/cm³ at 1200 ^oC to 2.54 g/cm³ at 700 ^oC. Viscosities range from log η = 2.23 PaS at 1200 ^oC to log η = 12.62 PaS at 700 ^oC. Preliminary calculations of percent crystallinity using XRD sample spectra vary from 63% crystalline at log η = 2.23 PaS (SiO₂ = 52.49 wt %) to 55% crystalline at log η = 3.58 PaS (SiO₂ = 61.34 wt %). Density and viscosity data, when graphically correlated to crystallinity and chemical composition, provide insight into interrelationships among these physical properties within the volcano’s pre-eruptive magmatic system.