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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. |