HR: 1340h
AN: V13B-0544    [Abstracts]
TI: Pseudobrookite-Magnetite-Ilmenite Assemblages in Highly Oxidized Hornblende Andesite of the Coleman Pinnacle Flow, Mount Baker, Cascade Range, WA
AU: * Mullen, E K
EM: emullen@u.washington.edu
AF: University of Washington, Department of Earth and Space Sciences, Seattle, WA 98195 United States
AU: McCallum, I S
EM: mccallum@u.washington.edu
AF: University of Washington, Department of Earth and Space Sciences, Seattle, WA 98195 United States
AB: Pseudobrookite microphenocrysts occur in cognate xenoliths in a 305 ka hornblende andesite flow from Coleman Pinnacle, Mount Baker, WA. Pseudobrookite (Psb)-ferropseudobrookite (Fpb)-karrooite (Kar) solid solutions are associated with hornblende or clusters of orthopyroxene, clinopyroxene, plagioclase, ilmenite (Ilm)-hematite (Hem)-geikielite (Gk) solid solutions, ulvospinel (Usp)-magnetite (Mt) solid solutions, apatite and zircon in a matrix of rhyolitic glass. Grains of Psb_46-49Fpb_25-28Kar_24-28 occur with Ilm₄₇Hem₄₂Gk₁₀ rims or intergrown with Ilm_45-47Hem_40-43Gk₁₂. These textures are analagous to those observed between armalcolite and ilmenite in lunar basalts. In a unique occurrence, Psb₄₈Fpb₂₀Kar₃₂ and Usp₂₂Mt₇₈, an assemblage previously unrecorded in natural systems, form an apparently stable symplectitic intergrowth surrounding a core of Ilm₄₃Hem₄₂Gk₁₄. Mass balance calculations show that the Psb_ss + Usp_ss assemblage is not an isochemical decomposition of ilmenite and the following explanation for the unique intergrowth is proposed. In the Fe-Ti-O system (Mg-free), the lower stability limit of Psb_ss lies at a temperature higher than that of the Ilm-Hem solvus, most likely precluding the stable existence of a Psb_ss + Usp_ss assemblage. However, all three phases (Ilm_ss, Psb_ss, Usp_ss) in the symplectitic assemblage contain significant amounts of Mg, e.g., the Psb_ss contains 32 mol% MgTi₂O₅ (karrooite) component. The effect of addition of Mg to the Fe-Ti-O system is to increase the temperature of the solvus and tricritical point along the Ilm-Hem-Gk join. The symplectitic Psb_ss has a lower stability limit of ~710°C based on linear interpolation between the lower stability limits of Fpb (1140°C), Psb (585°C) and Kar (646°C), allowing the Psb_ss+Usp_ss tie line to lie inside the solvus and thereby stabilizing the Mg-bearing Psb_ss + Usp_ss assemblage relative to an exsolved Mg-bearing Ilm_ss + Hem_ss pair. Experiments are being conducted to test this model. The compositions of Ilm_ss + Usp_ss pairs coexisting with Psb_ss are used to calculate the temperatures and oxygen fugacities of formation. The most recent refinement of the Fe-Ti oxide geothermometer-oxygen barometer (Mark Ghiorso, personal communication) applied to the Psb_ss-bearing assemblages yield T = 990-1130°C and fO₂ = NNO +1.5 to +1.75. Coexisting orthopyroxene and clinopyroxene in the Psb_ss-bearing cognate xenoliths record a temperature of formation of ~960°C. The calculated fO₂ range is consistent with the composition of the Ilm_ss in equilibrium with Psb_ss ± rutile and is also consistent with the presence of Fe³⁺-rich oxyhornblende phenocrysts (Fe³⁺/σFe = 0.8) in the andesite.
DE: 3625 Petrography, microstructures, and textures
DE: 3651 Thermobarometry
DE: 8412 Reactions and phase equilibria (1012, 3612)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005