Mullen, E. and McCallum, I.S., 2005, Coexisting pseudobrookiteŚilmeniteŚmagnetite solid solutions in highly oxidized hornblende andesite of the Coleman Pinnacle flow, Mt. Baker, WA. EOS Transactions AGU, 86(52), Fall Meeting Suppl., Abstract V13B-0544.

Coexisting pseudobrookiteŚilmeniteŚmagnetite solid solutions in highly oxidized hornblende andesite of the Coleman Pinnacle flow, Mt. Baker, WA. EOS Transactions AGU, 86(52), Fall Meeting Suppl., Abstract V13B-0544.

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 Psb46-49Fpb25-28Kar24-28 occur with Ilm47Hem42Gk10 rims or intergrown with Ilm45-47Hem40-43Gk12. 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 Psbss + Uspss 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 Psbss lies at a temperature higher than that of the Ilm-Hem solvus, most likely precluding the stable existence of a Psbss + Uspss assemblage. However, all three phases (Ilmss, Psbss, Uspss) in the symplectitic assemblage contain significant amounts of Mg, e.g., the Psbss 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 Psbss 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 Psbss+Uspss tie line to lie inside the solvus and thereby stabilizing the Mg-bearing Psbss + Uspss assemblage relative to an exsolved Mg-bearing Ilmss + Hemss pair. Experiments are being conducted to test this model. The compositions of Ilmss + Uspss pairs coexisting with Psbss 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 Psbss-bearing assemblages yield T = 990-1130░C and fO2 = NNO +1.5 to +1.75. Coexisting orthopyroxene and clinopyroxene in the Psbss-bearing cognate xenoliths record a temperature of formation of ~960░C. The calculated fO2 range is consistent with the composition of the Ilmss in equilibrium with Psbss ▒ rutile and is also consistent with the presence of Fe3+-rich oxyhornblende phenocrysts (Fe3+/σFe = 0.8) in the andesite.