Trace element signatures in quartz reveal crystallization histories of Katmai magmas: a combined electron microprobe/cathodoluminescence study
Abstract
Trace concentrations of Ti4+ and Al3+ which substitute for Si4+ in the quartz lattice reflect conditions in the melt at the time of crystallization, and zones enriched in these ions show distinctive cathodoluminescent signatures. Ti content of quartz has been shown to be an effective metric for crystallization temperatures, as calibrated by the TitaniQ geothermometer (Wark and Watson, 2006). Al concentrations in quartz show a less consistent connection with crystallization temperature and may have a positive correlation to crystal growth rate (Dennen et al., 1970; Müller et al., 2002). Ti and Al concentrations were measured by electron probe microanalysis (EPMA) and corresponding cathodoluminescence (CL) images were made for quartz grains from three magmatic rock suites of the Katmai region of the Alaska Peninsula: (1) high-silica rhyolite from the Novarupta lava dome; (2) the Mageik rhyolite sills, suggested as an analog for Novarupta's feeder (Lowenstern and Mahood, 1991); and (3) the Pinnacle Porphyry granodiorite, which has mingled field relations with the Mageik sills. Ti data was applied to the TitaniQ geothermometer to reveal the thermal histories of the three Katmai magmatic systems. Quartz from each of the three rock suites shows a distinctive crystallization history. Weak oscillatory CL zoning and constant (within error) Ti concentrations across Novarupta quartz suggest continuous growth at small degrees of undercooling at a minimum temperature of ~770-800°C, consistent with previously reported Fe-Ti oxide temperatures (Hildreth, 1983; Coombs and Gardner, 2001). Late-stage dissolution was followed by growth of very thin Al-rich, high-intensity CL rims, perhaps reflecting subsequent rapid growth during dome emplacement. Euhedral quartz of the Mageik sills has cores enriched in both Ti and Al, yielding minimum temperatures of ~900°C, and possibly representing rapid growth. Dark CL rims low in Ti likely represent continued growth to subsolidus temperatures after emplacement of the sill. Quartz in the Pinnacle Porphyry granodiorite shows a complex crystallization history. Cores yielding temperatures of ~820-870°C were fragmented and partially dissolved, then overgrown by quartz crystallized at slightly lower temperatures of ~800-815°C. The presence of mafic enclaves suggests magma mixing events, which may be responsible for the fracturing and dissolution, while ascent and uninterrupted crystallization to solidus may be responsible for resumed growth at lower temperature. Despite other similarities, the quartz data indicate that the sill rhyolite was a hotter and therefore dryer magma than the rhyolite of 1912. Quartz in the granodiorite is not derived from the rhyolite sill with which it is associated, but rather crystallized late and at a lower temperature than quartz in the sill rhyolite. Quartz in 1912 rhyolite seems to convey the same message of crystallization as accompanying plagioclase: short and simple, except at the very end.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFM.V51A1653P
- Keywords:
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- 8400 VOLCANOLOGY