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	Complex Magmatic Processes Operating in Deep Volcanic Plumbing Systems. (April - 2002)

[photo] First lava flow from Parícutin, the "volcano born in a Mexican cornfield", moving northward over two fields prepared for planting; a rock wall separates the fields. Photo taken by Instituto de Geología scientists from the north on the eruption's fifth day (Feb. 25, 1943). A dark, ash-rich plume rises from the new volcano, whose flanks are obscured by fine dust and vapor. (Click here for larger image.)

A recent analysis of decades-old volcanic cinder samples has provided a glimpse into the complex interplay among the volcanic processes of degassing, crystallization, and crustal assimilation during the rise of magma (molten rock) through Earth’s crust just prior to eruption.

Using samples collected on specific dates by a previous NMNH researcher during the nine-year eruption (February 1943 - February 1952) of Parícutin Volcano, Mexico, James Luhr (Department of Mineral Sciences) analyzed the chemical composition of glass inclusions in olivine crystals from small cinders. For modern studies of this type it is critical to have samples that quenched quickly; therefore explosively erupted cinders are ideal whereas slowly cooled lavas are not. It is also critical to know the date of the eruption, and this information was carefully recorded at the time of the eruption.

[photo]
Post-eruption airplane view from the north shows the main cone and the NE-flank vent mound of Nuevo Juatita, which was the main source of lava during the last 5 years of the eruption. Photo taken by Jim Luhr in 1997. Note that vegetation is beginning to gain a foothold on the 45-year-old lava flows, particularly where fine ash and cinders accumulated in crevices. (Click here for larger image.)

Parícutin Volcano is an excellent study subject due to the unprecedented detail in the documentation of the complex evolution of its cinder cone and associated lava field. Explosive eruptive activity was especially vigorous during the volcano’s early years, and lavas came to dominate in the later years. Earlier studies indicated that the erupted compositions evolved over time, with the rate of change being particularly rapid in 1947, midway through the eruption. Gravitational settling of crystals formed in the magma was incapable of quantitatively explaining these changes; however, when combined with the addition or assimilation of granites known to exist in the upper crust, successful fits to the rock compositions were achieved.

[photo]
An olivine crystal from Parícutin, about 0.5 mm across, contains numerous trapped inclusions of brown glass (G) with vapor bubbles, as well as inclusions of the mineral chromian spinel (C). Most glass inclusions have spherical to elongated oval shapes, but the two largest glass inclusions have conical shapes symmetrically aligned along the crystallographic a axis, forming an hourglass configuration. Such inclusions are ideal for FTIR analysis of pre-eruptive H₂O and CO₂ contents. Photomicrograph by Jim Luhr of crystal lying in immersion oil. (Click here for larger image.)

Luhr built upon that earlier study, revisiting the collected samples using state of the art Fourier transform infrared (FTIR) spectroscopy to measure the concentrations of water, carbon dioxide, and other gaseous components trapped in glass inclusions. As they grow in a magma, crystals engulf some of the liquid they are growing from, encapsulating it and preventing further changes.

Upon eruptive quenching, these tiny pockets of silicate liquid freeze to form glass, and provide evidence for the original volatile contents of the magma. Luhr found some inclusions with up to 4 wt.% H₂O and 300 parts per million of CO₂, likely close to the original magmatic values. These critical inclusions were probably trapped in olivine at a depth of about 9 km beneath the volcano. Most glass inclusions showed lower H₂O and CO₂ concentrations, evidence for degassing as the magma rose from that depth to erupt at the surface. One of the goals of modern volcanology is to better understand how degassing occurs beneath volcanoes, and how the original volatile contents of magmas are related to eruptive behavior. Studies such as Luhr conducted at Parícutin provide insight into the complex magmatic processes operating in deep volcanic plumbing systems.

[photo]
(Right) An ash-rich eruption column roils above Parícutin volcano sometime during 1946-48. A thick blanket of ash and cinders mantles the foreground, where two people provide a scale. These deposits exceeded 15 cm in thickness over an area of 300 square km surrounding the volcano. Collection of ash and cinders as they fell, and their careful documentation and preservation in the National Rock and Ore Collections, allowed this study to be conducted 50 years later. Photo by Ray Wilcox (U.S. Geological Survey). (Click here for larger image.)

To learn more about this Volcanism research, visit these sites:

NMNH Department of Mineral Sciences - Global Volcanism Program (GVP)
http://www.volcano.si.edu/

NMNH Department of Mineral Sciences
http://mineralsciences.si.edu/

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April 2002 - NMNH Webmaster