Measurements link magma melting rate to tectonic plate subduction rate
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鈥檚 mantle.
Now, geologists at the University of Illinois report new measurements of rock samples from Kick鈥檈m 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.
鈥淲e 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,鈥� said Craig Lundstrom, a UI professor of geology. 鈥淲e can then use this information in similar situations to understand the rate at which magma is produced in other settings.鈥�
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 鈥減arent鈥� with a half-life of 700 million years) will decay to Pa-231 (protactinium-231: a 鈥渄aughter鈥� 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鈥檈m Jenny, a submarine volcano located about 8 kilometers north of Grenada in the southern Lesser Antilles arc.
At Kick鈥檈m 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鈥檈m 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. 鈥淎n essential part of all volcanoes at subduction zones is the amount of water involved in the mantle melting process,鈥� Huang said. 鈥淒uring 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.鈥�
At Kick鈥檈m 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.
鈥淭his is the first study to show that there is a straightforward relationship between this uranium disequilibrium system and the rate of tectonic plate convergence,鈥� Lundstrom said. 鈥淣o 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.鈥�
Source: University of Illinois at Urbana-Champaign
