October 11, 2013
Scientists Look At Supervolcano Life Cycle In Whole New Way
Lee Rannals for redOrbit.com - Your Universe Online
Scientists say they've discovered a new way to look at the life cycle of a supervolcano at a Yellowstone hotspot.University of Oregon researchers said that crystals of zircon from the Snake River Plain helped to solidify evidence of "recycled" super-volcanic magma chambers. This Yellowstone hotspot creates a conveyor belt style of volcanism because of the southwest migration of the North American plate at about 0.8 to 1.6 inches annually over the last 16 million years of volcanism. The plume interaction with the crust leaves footprints in the form of caldera clusters in this area.
The "Picabo" volcanic field was active between 10.4 and 6.6 million years ago and experienced at least three violent caldera-forming eruptions. The field has been difficult to assess because the calderas have been buried by as much as a mile of basalt since its eruption cycle died.
The team believes that basalt from the mantle plume, rocks from the Earth's crust, and previously erupted volcanoes are melted together to help form the rhyolites that erupted in the Snake River Plain. Rhyolite magma is stored in dispersed pockets throughout the upper crust before an eruption.
"We think that this batch-assembly process is an important part of caldera-forming eruptions, and generating rhyolites in general," said Dana Drew, a UO graduate student and lead author of the paper published in the journal Earth and Planetary Science Letters.
The researchers analyzed radiogenic and stable isotopic data in zircons detected in rhyolites found at the margins of the Picabo field and from a deep borehole. This data helped to provide framework to understand the region's ancient volcanic past.
"There is a growing database of the geochemistry of rhyolites in the Yellowstone hotspot track," Drew said in a statement. "Adding Picabo provides a missing link in the database."
The team identified a diversity of oxygen ratios occurring in erupted zircons near the end of the Picabo volcanic cycle. These oxygen ratios are referred to as delta-O-18 signatures based on oxygen 18 levels relative to seawater. This approach helped to provide a glimpse into the connection of surface and subsurface processes at a caldera cluster. The interaction of erupted rhyolite with groundwater and surface water causes hydrothermal alteration and the change in oxygen isotopes.
"Through the eruptive sequence, we begin to generate lower delta-O-18 signatures of the magmas and, with that, we also see a more diverse signature," Drew said. "By the time of the final eruption there is up to five per mil diversity in the signature recorded in the zircons."
"Researchers believe these signatures are due to the mixing of diverse magma batches dispersed in the upper crust. When the pockets of melt are rapidly assembled, the process could have triggered caldera forming eruptions," according to UO geologist Ilya N. Bindeman.
"That leads to a homogenized magma, but in a way that preserves these zircons of different signatures from the individual pockets of melt," he said.
Bindeman added that their research highlights the importance of using new micro-analytical isotopic techniques to relate geochemistry at the crystal-scale to processes occurring at the crystal-wide scale in generating and predicting large-volume rhyolitic eruptions.