The connectivity of multicomponent fluids in subduction zones

November 12, 2020

A team of researchers has discovered more about the grain-scale fluid connectivity beneath the earth's surface, shedding new light on fluid circulation and seismic velocity anomalies in subduction zones.

Lithospheric plates collide at convergent boundaries. Here, the less dense oceanic lithosphere subducts below the continental plate, and releases an abundance of water due to a progressive metamorphic reaction at high pressure and high temperature. The released water can infiltrate into the mantle wedge which lies between the subducting oceanic lithosphere and the continental crust.

Fluids that circulate in subduction zones have a significant effect on magma genesis, global material exchange between the Earth's interior and surface, and seismicity. The dihedral angle (θ)--the angle between two intersecting planes--holds the key to revealing the fluid connectivity and migration regime for a fluid-bearing, deep-seated rock in the Earth's interior known as pyrolite--a rock mainly composed of olivine.

Although H2O is the predominant composition of subduction-zone fluids, minor components in the fluid can have a dramatic impact on the wetting properties of olivine. This is evidenced in the dihedral angle between olivine and fluid.

Salt (NaCl) and non-polarized gases such as CO2 are two crucial components of subduction-zone fluids that significantly affect the dihedral angle between olivine and fluid. CO2 is known to increase the olivine-fluid θ under conditions in which the olivine does not react with CO2. Whereas, a recent study showed that NaCl can effectively decrease the olivine-fluid θ even with a low NaCl concentration. NaCl and CO2 have opposite effects on the olivine-fluid θ, and this factor has inhibited researchers in their understanding of fluid migration in subduction zones.

Clarifying the competing effects of NaCl and CO2 on θ in an olivine + multicomponent (H2O-CO2-NaCl) fluid system can help researchers understand the connectivity of aqueous fluid with more realistic compositions of the mantle wedge; thus making mapping of fluid distribution easier.

To do this, doctoral student Yongsheng Huang, professor Michihiko Nakamura, and postdoctoral researcher Takayuki Nakatani from Tohoku University worked alongside professor Catherine McCammon from the University of Bayreuth. The research team sought to constrain θ in olivine + H2O-CO2 fluid and olivine + H2O-CO2-NaCl (multicomponent) fluid systems at 1-4 GPa and 800-1100 °C.

The results in the H2O-CO2 system showed that CO2 tends to increase θ at 1 GPa and 800-1100 °C and at 2 GPa and 1100 °C. In contrast, CO2 reduced the θ to below 60° at relatively high-pressure and low-temperature conditions. Here, the olivine partly reacts with CO2 to form magnesite and orthopyroxene (opx).

Additional experiments on olivine-magnesite + H2O and olivine-opx + H2O systems showed magnesite or opx decreased the olivine-fluid θ. This implies that coexisting minerals affect the olivine-fluid interfacial energy by changing fluid chemistry. The multicomponent system results showed that the effect of NaCl on θ is much more significant than CO2. Strikingly, θ was smaller than 60° in all the magnesite- and opx-bearing multicomponent systems.

"Our study has revealed that CO2-bearing multicomponent aqueous fluid can infiltrate the overlying plate through an interconnected network at pressures above 2 GPa, which facilitates significant fore-arc fluid circulation and confirms the origin of the high electrical conductivity anomalies detected in the fore-arc mantle wedge," said Nakamura.

The contrasting effects of aqueous fluid and silicate melt on the seismic wave velocity may allow for mapping partial melt in the mantle wedge.
-end-


Tohoku University

Related Subduction Articles from Brightsurf:

The connectivity of multicomponent fluids in subduction zones
A team of researchers has discovered more about the grain-scale fluid connectivity beneath the earth's surface, shedding new light on fluid circulation and seismic velocity anomalies in subduction zones.

New fault zone measurements could help us to understand subduction earthquake
University of Tsukuba researchers have conducted detailed structural analyses of a fault zone in central Japan to identify the specific conditions that lead to devastating earthquake.

Japan's geologic history in question after discovery of metamorphic rock microdiamonds
A collaboration of researchers based in Kumamoto University, Japan have discovered microdiamonds in the Nishisonogi metamorphic rock formation in Nagasaki Prefecture, Japan.

A review of ridge subduction, magmatism and metallogenesis
Ridge subduction events are very common and important geodynamic processes in modern oceanic plate tectonics (Figure 1), and play an important role in the generation of arc magmatism, material recycling, growth and evolution of continental crust, deformation and modification of overlying plates and metallogenesis.

Plate tectonics goes global
A research team led by Dr. WAN Bo from the Institute of Geology and Geophysics (IGG) of the Chinese Academy of Sciences has revealed that plate tectonics went global 2 billion years ago.

Curtin study could rewrite Earth's history
Curtin University-led research has found new evidence to suggest that the Earth's first continents were not formed by subduction in a modern-like plate tectonics environment as previously thought, and instead may have been created by an entirely different process.

Geoscientists create deeper look at processes below Earth's surface with 3D images
Geoscientists at The University of Texas at Dallas recently used massive amounts of earthquake data and supercomputers to generate high-resolution, 3D images of the dynamic geological processes taking place far below the Earth's surface.

Scientists review the metallogenesis and challenges of porphyry copper systems above subduction zone
Porphyry copper ± molybdenum ± gold deposits (PCDs) are the most economically important magmatic-hydrothermal metallogenic system above subduction zones, which have supplied nearly 3/4 of the world's copper, 1/2 of the molybdenum and 1/5 of the gold, as well as large amounts of silver, zinc, tin and tungsten, with however their metallogenesis remaining controversial.

New clues to deep earthquake mystery
A new understanding of our planet's deepest earthquakes could help unravel one of the most mysterious geophysical processes on Earth.

Does accelerated subduction precede great earthquakes?
A strange reversal of ground motion preceded two of the largest earthquakes in history.

Read More: Subduction News and Subduction Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.