Geotimes - July 2003 - Highlights

Geotimes

Highlights

Igneous Petrology
Allen Glazner

Much effort in igneous petrology over the past year centered on using microanalytical methods to study big problems. Ever-improving analytical techniques are allowing precise measurement of elemental abundances and isotopic ratios on ever-smaller phases in rocks. As ICP-MS (inductively coupled plasma mass spectrometry) has become a routine analytical tool for analyzing low-levels of trace-elements, laser ablation ICP-MS, multicollector ICP-MS and ion microprobe instruments have opened new avenues of research.

Large volcanic eruptions remain a significant societal concern and focus of research, and several studies estimated the timescales over which magma accumulates and differentiates. James E. Gardner and others (Geology, April 2002, v. 30, p. 347-350) demonstrated that hornblende crystals in the massive eruption of Toba 74,000 years ago are not phenocrysts (a native, larger crystal within igneous rock) but instead are xenocrysts (a foreign crystal within the rock). Using argon diffusion data, they estimated that the xenocrysts may have been entrained as few as tens of years before eruption, raising the question of how these crystals could have been thoroughly and rapidly stirred into the magma, which had a large volume of 2,800 cubic kilometers. Pavel E. Izbekov and others (Geology, September 2002, v. 30, p. 799-802) showed that cores of plagioclase phenocrysts from andesite summit eruptions of Karymsky volcano in Kamchatka chemically match phenocrysts in basalts that erupted at the same time. Their findings indicate rapid mixing between disparate magmas shortly before eruption and suggest that introduction of basalt into the magma chamber triggered eruption. Microanalytical techniques will probably continue to ferret out "phenocrysts" that are out of equilibrium with their host magma. Coupled with diffusion studies, this detective work will allow greater insight into magma chamber processes and their rates.

The lifetimes of magma systems remain difficult to pin down. Using an ion microprobe on zircon, Jorge A. Vasquez and Mary R. Reid (Contributions to Mineralogy and Petrology, December 2002, v. 144, p. 274-285) estimated that rhyolites at the Yellowstone caldera that erupted after the caldera collapsed had been sitting in the magma chamber for times on the order of 60,000 years before they were erupted. Arnd Heumann and others (Geochimica et Cosmochimica Acta, May 15, 2002, v. 66, p. 1821-1837) estimated significantly longer intervals of storage of post-caldera, high-silica rhyolite atop the Long Valley system.

High-precision uranium-lead and argon-argon geochronology allowed Ian M. Coulson and others (Journal of Volcanology and Geothermal Research, May 30, 2002, v. 114, p. 331-356) to demonstrate that a composite plutonic system in the Yukon was assembled and cooled below the solidus in around 100,000 years, consistent with thermal models. In contrast, Drew Coleman and others (Geological Society of America fall 2002 meeting), using high-precision uranium-lead geochronology on small zircon fractions, showed that the Half Dome Granodiorite — a single, large, homogeneous pluton in California — was emplaced incrementally over 3 million years. This time span seems to require that the pluton was never molten all at once, but rather was emplaced in small batches.
Database development gained prominence in 2002 as the National Science Foundation began supporting information technology research. PetDB, a database of igneous geochemistry of oceanic rocks, and GEOROC, a global igneous database, are maturing. NAVDAT, an effort centered on igneous rocks in western North America, is underway. NAVDAT promises a comprehensive continent-scale synthesis of patterns of magmatism and how they relate to plate tectonic events at the continental margin, work that should tie directly to the recently funded EarthScope initiative. Those working on these databases are developing a common interface and gateway for searching them.

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Glazner specializes in igneous petrology and tectonics and is a professor of geological sciences at the University of North Carolina-Chapel Hill. E-mail: afg@unc.edu.

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