MOST of us think of volcanoes as awesome or tranquil, depending on whether or not they are erupting. But we seldom consider that the magma belched up during an eruption represents a recycling of our Earth's most basic components--molten rock and gases that formerly lay deep within the mantle migrate up and finally are thrust skyward to become part of the crust and atmosphere. This activity not only forms new islands, mountain ranges, and large lava plains but also provides a brief glimpse into the dynamic processes that shape our Earth. Volcanoes that lie along the edge of the Earth's great tectonic plates, like those in Japan, Indonesia, and the Aleutian Islands, constitute over 75% of all volcanoes that erupt above sea level. Known as island-arc volcanoes, they are the dramatic result of continuous interactions between the oceans, crust, mantle, and atmosphere. Along deep oceanic trenches, the oceanic slab is thrust or subducted into the mantle bringing with it water from the ocean (see Figure 1). Water reduces the melting temperature of rock so that, as water is introduced to the mantle, the mantle melts. This melted material, known as magma, rises buoyantly until it erupts on the Earth's surface to form island-arc volcanoes. These eruptions are thought to be the major process by which mantle material is transferred to and becomes part of the Earth's crust. The gases from these eruptions also contribute to the formation of our atmosphere. Understanding the chemical recycling at subduction zones has practical implications. For example, these volcanic eruptions affect global climate by releasing greenhouse gases into the atmosphere. The 1991 eruption of Mt. Pinatubo in the Philippines provided a stunning example of the effect that a volcano can have on global climate. Annie Kersting, a geochemist with Livermore's Institute for Geophysics and Planetary Physics, recently completed a study of island-arc volcanoes on the Kamchatkan Peninsula, Russia, and on the island of Honshu, Japan, with scientists from Australia and Japan (see Figure 2). Her studies were designed to learn more about the processes that control the generation of new crust at island arcs. The consensus among geologists is that island-arc magmas are composed mostly of material from the mantle, with fluids from the subducting oceanic crust providing the mechanism for melting. But they are unsure to what extent the subducting oceanic sediments and/or oceanic crustal material mixes with the mantle and to what extent the thin arc crust immediately beneath the volcanoes contributes to the chemistry of the lavas that these volcanoes produce.

To determine the components of the magmas being studied, Kersting used long-lived isotopes of lead (Pb), strontium (Sr), and neodymium (Nd) as tracers. The so-called parental materials of lava--the mantle, oceanic crust, sediments, and arc crust--are isotopically distinct from one another, and the erupted lava will have the isotopic tracer or fingerprint of one or more of these parents. Are Oceanic Sediments Involved? Kersting's study of Klyuchevskoy volcano in Kamchatka, Russia, evaluated the influence of oceanic sediments in the generation of island-arc volcanoes. Klyuchevskoy is the most active island-arc volcano in the world (see Figure 3). A recent eruption blew volcanic material 15 to 18 kilometers above sea level and required the diversion of international airline traffic as a safety precaution. The team first measured Pb, Sr, and Nd isotope ratios in basaltic rocks from Klyuchevskoy. They used Livermore's thermal ionization mass spectrometer, which can measure extremely low levels of these isotopes. The same ratios were measured in oceanic sediments from the North Pacific, parallel to the Kamchatkan arc. These sediments are the best analog for sediments that might have previously subducted with the oceanic slab beneath Kamchatka.

In all cases, the isotopic ratios of the lavas and Pacific sediments were different. Even a 1% sediment contribution to the lavas would be detectable, but there was none. Instead, the isotopic fingerprint was that of a purely mantle source. To verify these figures, the team also looked at three other Kamchatkan volcanoes and found similar results.1 "One area of controversy among geologists is whether oceanic sediments are carried down with the subducted oceanic slab and if so, whether they are melted and recycled into the arc crust via volcanism or scraped off and not subducted. This study indicates that sediments are not required in the production of island-arc volcanoes," says Kersting. In contrast, previous studies of other island arcs indicate that sediments are involved in magma generation. This work has shown that the involvement of sediment in the chemistry of arc magmas varies from arc to arc. The Arc Crust's Contribution In Japan, Kersting's team evaluated the effect that a relatively thin (30-kilometer) arc crust has on the magmas that pass through it. This research tested the widely accepted theory that only very thick (70-kilometer) arc crusts, such as those in the Andes, can influence the chemistry of lavas from the mantle. Northeastern Honshu, Japan, is an excellent place to study this theory. The Tanakura Tectonic Line, a fault that penetrates the arc crust, cuts across an arc of active volcanoes so that the volcanoes form on two different arc crusts. The volcanoes are close together, which helps to minimize variations in other parameters that might influence the volcanoes' chemistry--the depth to the subducting oceanic slab, distance to the oceanic trench, thickness of the arc crust, and composition of the mantle and subducting oceanic crust and sediments. Thus, any differences in the isotopes from volcanoes on opposing sides of the fault must result from the compositional differences in the arc crusts through which the magmas rise. After collecting rock samples, the team measured isotope ratios at Livermore and found significant differences in the Pb, Sr, and Nd ratios in the lavas from either side of the fault. According to Kersting, the different geochemical signatures between the volcanoes immediately north and south of the fault must result from chemical "contamination" by the arc crust as the magmas traverse it. More recently analyzed data from Mt. Fuji, which lies on a third type of arc crust on Honshu, substantiates the team's findings, adding strength to the argument that the mixing of basalts from the mantle with the arc crust is an important process in island-arc volcanism. Even a thin arc crust is an active geochemical filter for magmas that move upward through it.2 Scientists are still a long way from fully understanding island-arc volcanoes and their effects on our planet and its atmosphere. But Kersting's work contributes to the body of knowledge that helps scientists define how the chemical exchange between the crusts, mantle, and atmosphere at subduction zones influences crustal growth and global climate dynamics.

--Katie Walter

Key Words: crustal growth, global climate, island-arc volcanoes.

References
1. A. B. Kersting and R. J. Arculus, "Pb Systematics of Klyuchevskoy Volcano, Kamchatka, and North Pacific Sediments: Implications for Magma Genesis and Sediment Recycling in the Kamchatkan Arc," Earth and Planetary Science Letters 136, 133-148 (1995).
2. A. B. Kersting et al., "Lithospheric Contributions to Arc Magmatism: Isotope Variations Along-Strike in Volcanoes of Honshu, Japan," Science 272, 1464-1468 (1996).

For further information contact Annie Kersting (510) 423-3338 (kersting@llnl.gov) or Frederick Ryerson (510) 422-6170 (ryerson1@llnl.gov).