ALBERT

Description: Pseudobrookite microphenocrysts occur in cognate inclusions in the ~305 ka Coleman Pinnacle hornblende andesite flow from the Mount Baker volcanic field, Washington. Pseudobrookites are associated with hornblende phenocrysts and glomerophyric clusters of orthopyroxene, clinopyroxene, plagioclase, ilmenite, titanomagnetite, apatite, and zircon in a matrix of fresh rhyolitic glass. Grains of pseudobrookite are rimmed by or intergrown with ilmenite. These textures are analogous to those observed between armalcolite and ilmenite in high-Ti lunar basalts. In a unique occurrence, pseudobrookite, and titanomagnetite form a symplectitic intergrowth surrounding a core of ilmenite. Mass balance calculations show that the pseudobrookite + titanomagnetite assemblage is not an isochemical decomposition of ilmenite. In the TiO 2 -FeO-Fe 2 O 3 system (Mg-free), pseudobrookite and titanomagnetite solid solutions do not coexist. However, all three Fe-Ti oxides in the symplectitic assemblage contain significant amounts of Mg. In the TiO 2 -MgO-FeO-FeO 1.5 system at high oxygen fugacities, the Mg-rich pseudobrookite + titanomagnetite assemblage is stable relative to the conjugate pair of Mg-bearing ilmenite solid solutions. At lower $${f}_{{O}_{2}}$$ , Fe 2+ increases, Mg/(Mg+Fe 2+ ) (Mg no.) decreases and the conjugate ilmenite pair becomes the stable assemblage at Mg no. 〈 ~0.6. The compositions of coexisting ilmenite + titanomagnetite pairs in the Coleman Pinnacle andesite yield T = 900–1000 °C and $${f}_{{O}_{2}}$$ = NNO + 1.5 to + 1.75, one of the highest redox states on record for arc magmas. The calculated $${f}_{{O}_{2}}$$ range is consistent with the composition of the ilmenite in equilibrium with pseudobrookite ± rutile and with Fe 3+ -rich cores in hornblende phenocrysts.

Description: The northern Cascade arc is an end-member ‘hot’ subduction zone where slab dehydration may be essentially complete prior to reaching sub-arc depths, presenting a potential problem for a flux melting origin for arc basalts. Nevertheless, mafic lavas from the Mt. Baker volcanic field, the most magmatically productive volcano in the northern Cascade arc, record subduction input and compositions typical of calc-alkaline magmas. Relative to normal mid-ocean ridge basalt, the most primitive lavas at Mt. Baker show elevated abundances of large ion lithophile elements (LILE), Pb and light rare earth elements (LREE). High field strength elements (HFSE) and heavy REE (HREE) are depleted relative to LILE. Reconstructed primary magmas define three groups: Group I calc-alkaline basalts (Sulphur Creek, Lake Shannon, Hogback) have lower SiO 2 , (La/Yb) N and LILE/HFSE, and are olivine- and plagioclase-phyric; Group II high-Mg basaltic andesites (Tarn Plateau, Cathedral Crag) also contain clinopyroxene phenocrysts and have higher H 2 O, SiO 2 , (La/Yb) N and LILE/HFSE; Group III low-K olivine tholeiite (Park Butte) has the lowest (La/Yb) N but highest LILE/HFSE. The redox states of all groups lie between QFM (quartz–fayalite–magnetite) and QFM +1·4. Pb, Sr and Nd isotope compositions are similar among all the analysed samples and are consistent with a depleted mantle source modified by input from the subducting slab. Trace element–isotope modeling indicates a subduction component composed predominantly of metabasalt-derived fluid with lesser amounts of sediment melt and metabasalt melt. Group I basalts record the smallest melt fractions (~5–7%), lowest water contents (1·5–2·1 wt %), and highest temperatures and pressures of mantle segregation (up to 1354°C, 1·5 GPa) from lherzolitic (± spinel) residues. Group II basaltic andesites show the greatest extents of mantle metasomatism, the highest water contents (2·7–3·7 wt %) and partial melt fractions (10–12%), and segregated from harzburgite at ~1270°C, 1 GPa, consistent with pooling of melts at the Moho. Group III records P–T conditions similar to Group I (~1·4 GPa, 1326°C) but melt fractions (12%) and mantle residues (harzburgite) are more similar to Group II, and H 2 O contents (~2·1 wt %) are intermediate. Melting beneath Mt. Baker was initiated by dehydration melting of amphibole peridotite at ~95 km and 1020°C, within the stability field of garnet lherzolite. Initial melt fractions were small (~1–2%) and near water-saturation. Phase equilibria and trace element modeling show no evidence for garnet-bearing mantle residues, indicating that progressive melting during ascent of diapirs through the hot core of the convecting mantle wedge reduced H 2 O contents and erased any residual garnet signature. Because fluid release from the slab is restricted to the forearc, mantle hydrated at shallow depths in the serpentine and/or chlorite stability fields must be down-dragged to the region of amphibole stability to initiate dehydration melting.