Geothermometry and cathodoluminescence of quartz reveal crystallization histories of Katmai magmas

Abstract

"This study combines electron microprobe analysis of titanium and aluminum in quartz, cathodoluminescence imagery, and titanium-in-quartz geothermometry to constrain the growth histories of quartz grains from three magmatic rock suites of Katmai, Alaska: (1) high-silica rhyolite from the 1912 Novarupta lava dome; (2) the Mageik rhyolite sills, suggested as an analog for Novarupta's feeder; and (3) the Pinnacle Porphyry granodiorite, which has mingled field relations with the Mageik sills. Oscillatory zoning and constant titanium concentrations across Novarupta quartz phenocrysts suggest continuous growth at temperatures of ~800-850°C. Thin, aluminum-rich, high-cathodoluminescence intensity rims may represent rapid growth during magmatic ascent or dome emplacement. Euhedral matrix quartz of the Mageik sills has cores enriched in both titanium and aluminum, yielding minimum temperatures of ~900°C, and possibly representing rapid growth. Titanium-poor rims likely represent continued growth to subsolidus temperatures after emplacement of the sills. Quartz in the Pinnacle Porphyry show early crystallization at minimum temperatures of ~820-870°C, followed by partial resorption and continued growth at ~800-815°C. Magma mixing events, as evidenced by mafic enclaves, may be responsible for the dissolution, while ascent and uninterrupted crystallization to solidus may be responsible for resumed growth at lower temperatures"--Leaf iii