Measurements link magma melting rate to tectonic plate subduction rate

November 8, 2007

Determining the origin and rate of magma production in subduction zone volcanoes is essential to understanding the formation of continental crust and the recycling of subducted materials back into Earth’s mantle.

Now, geologists at the University of Illinois report new measurements of rock samples from Kick’em Jenny, a submarine volcano in the Caribbean, that link the rate at which magma is produced beneath subduction zone volcanoes to the rate at which tectonic plates converge in this plate tectonic setting.

“We can use the geochemical measurements to constrain a geophysical parameter, the melt production rate; we then relate the melting rate at an individual subduction zone to its plate convergence rate, which can also be measured,” said Craig Lundstrom, a UI professor of geology. “We can then use this information in similar situations to understand the rate at which magma is produced in other settings.”

Lundstrom and graduate research assistant Fang Huang report their findings in the November issue of the journal Geology, which is published by the Geological Society of America.

The geochemical technique is based on uranium decaying to lead through a long decay chain of short-lived nuclides. For example, U-235 (a “parent” with a half-life of 700 million years) will decay to Pa-231 (protactinium-231: a “daughter” with a half-life of 33,000 years). By measuring the ratio of parent and daughter species in a rock sample (a technique called uranium-series dating), scientists can determine whether the rock is in secular equilibrium (and quite old), or in uranium series-disequilibrium (and very young).

Using multiple-collector inductively coupled plasma-mass spectrometry, Huang and Lundstrom analyzed 12 rock samples from Kick’em Jenny, a submarine volcano located about 8 kilometers north of Grenada in the southern Lesser Antilles arc.

At Kick’em Jenny, the Atlantic oceanic plate is being pushed beneath the Caribbean plate at a rate of 2-4 centimeters per year, one of the lowest convergence rates of any subduction zone.

In Kick’em Jenny lavas, the researchers found there was twice as much protactinium than should be present if the system was in secular equilibrium. This is the largest protactinium-uranium disequilibrium found in any subduction-zone volcano.

The relationship between melting rate and convergence rate centers on the role of water during melting. “An essential part of all volcanoes at subduction zones is the amount of water involved in the mantle melting process,” Huang said. “During subduction, water is released from the subducting slab into the mantle wedge, which lowers the melting point of the rock. When less water is transported to the mantle, less melt is produced.”

At Kick’em Jenny, water is being added very slowly, because the subducting plate is going down very slowly, Lundstrom said. This results in a slower melting rate, which produces a higher ratio of protactinium to uranium 235.

“This is the first study to show that there is a straightforward relationship between this uranium disequilibrium system and the rate of tectonic plate convergence,” Lundstrom said. “No doubt these short-lived nuclides can be used for a variety of other processes in volcanoes, from determining how fast crystals form to how fast magma moves under mid-ocean-ridge volcanoes.”

Source: University of Illinois at Urbana-Champaign

Explore further: Why did the Earth's ancient oceans disappear?

Related Stories

Why did the Earth's ancient oceans disappear?

November 10, 2017

We think of oceans as being stable and permanent. However, they move at about the same speed as your fingernails grow. Geoscientists at CEED, University of Oslo have found a novel way of mapping the Earth's ancient oceans.

Residual strain despite mega earthquake

December 13, 2017

On Christmas Day 2016, the earth trembled in southern Chile. In the same region, the strongest earthquake ever measured occurred in 1960. A comparison of data from seismic and geodetic measurements during and after both earthquakes ...

Plate tectonics coming of age

November 24, 2011

(PhysOrg.com) -- Plate tectonics in its current form is believed to have started one billion years ago. A study of two billion year old rocks from African gold mines has now shown that the same process of subduction we observe ...

Plate tectonics may take a break

January 3, 2008

Plate tectonics, the geologic process responsible for creating the Earth’s continents, mountain ranges, and ocean basins, may be an on-again, off-again affair. Scientists have assumed that the shifting of crustal plates ...

Release of water shakes Pacific Plate at depth

January 11, 2017

Tonga is a seismologists' paradise, and not just because of the white-sand beaches. The subduction zone off the east coast of the archipelago racks up more intermediate-depth and deep earthquakes than any other subduction ...

Recommended for you

Weather anomalies accelerate the melting of sea ice

January 16, 2018

In the winter of 2015/16, something happened that had never before been seen on this scale: at the end of December, temperatures rose above zero degrees Celsius for several days in parts of the Arctic. Temperatures of up ...

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.