The Timing and Nature of Volcanological Processes as Captured by Bimodal Composition Eruptions

University Of Hawaii, Honolulu

Investigators

Abstract

Rubin EAR-0106463 One critical yet still incompletely resolved volcanic parameter is the time period involved in crustal processes leading to both magmatic evolution and eruption. These likely vary greatly amongst magmatic systems of differing style, tectonic setting and composition. There are a number of ways to potentially get at the time scales of crustal magmatic process. This proposal suggests a novel approach based upon U-series disequilibrium age contrasts between primitive and evolved magmas that erupted simultaneously. Evidence for such " bimodal" composition eruptions has become commonplace in the volcanological literature. The magmas involved may have formed through a variety of processes and over a range of time scales, which is an important aspect of this investigation. Three inter-related issues are: (a) petrological: mechanism of formation of evolved magmas; (b) volcanological: eruption triggering of relatively cool, viscous, chemically evolved magmas (dacite and rhyolite); and c) time scales: how long do these processes take in different settings. This study would examine both magmatic evolution and one of several proposed mechanisms for initiating silicic eruptions (i.e. the basalt injection trigger) using bimodal composition eruptions. Such eruptions provide a unique snapshot of processes in complex and evolving magmatic systems because the two end members magmas were erupted simultaneously and therefore are closely-related in space and time. Three working hypotheses would be tested regarding time scales and processes captured in 4 historical (or geologically very recent) bimodal-composition eruptions that are believed to represent various scenarios for petrological evolution and the role of mafic magma in causing or aiding in eruption of chemically evolved magmas. The eruptions to be studies are: 1875 Askja; 1724 Krafla; 1902 Santa Maria; and 1943-1952 Paricutin. Although there are numerous others examples (e.g., historical or recent eruptions of Vesuvius, Hekla, Popocatepetl, Colima, Novarupta, Mt. St. Helens, Nevado del Ruiz, Montserrat, Pinatubo), the eruptions proposed here were picked because they have been previously well-studied volcanologically and petrologically, have excellent temporal control on the eruptions themselves, have something close to compositional end members physically separable from pumices or lavas, and have available sample types suited for dating with U-series disequilibrium by internal mineral isochrons. Eruption deposits will be examined volcanologically, and samples will be examined petrographically and geochemically to arrive at as complete a picture as possible for the volcanic/magmatic processes involved. Time permitting a 5th eruption (the circa 1305 Mt Tarawera "Kaharoa" eruption in New Zealand) would also be examined.