Mass Balance

When material is subducted into Earth's mantle, a certain amount is recycled by magmatic activity, which returns it to the surface either contained in volcanic rock or associated with geothermal fluids such as fumaroles or bubbling hot springs. Some of it stays behind in the mantle. Determining mass balance - how much goes into the mantle vs. how much comes back out - is a key objective of the MARGINS science team in Japan.

Scientists assigned to the task chose two locations that they believe hold the greatest potential for yielding breakthroughs in the understanding of the Subduction Factory. In 2001, the Central American margin was targeted and work commenced on the string of volcanoes from the Mexico/Guatemala border through Honduras, El Salvador and Nicaragua to Costa Rica in the south.

Among the most important volatiles to track through the subduction process are water and carbon dioxide. Carbon dioxide, a greenhouse gas, is emitted by volcanoes that use carbonates from the subducting crust as raw materials. Water in its vapor form is the most abundant gas that comes from volcanoes. The researchers are interested in where that water comes from in the first place, and will be looking at a source they believe is most important: the water-bearing minerals within crustal rock that gets subducted. Encased as it is, the water within these minerals can survive the hot and violent journey into the mantle. Careful laboratory analysis of water can tell researchers where it came from and what journey it took. The examination can give clues to how the water influences other properties of the Subduction Factory such as magma melting temperature, the speed of seismic wave movements and the heat conductivity of materials in the factory.

We are also interested in the nitrogen mass balance through subduction zones. Nitrogen gas emitted from volcanoes can come from two sources: "primordial" nitrogen from the mantle wedge or organically-derived nitrogen from subducted sediment. These two sources have very different nitrogen isotopic compositions, allowing us to model the amount of nitrogen contributed from the mantle and the amount contributed from the sediments. One of the main objectives of the MARGINS initiative is to quantify how much of the subducted material is recycled to the surface through the subduction zone and how much is subducted into the deep mantle. We will calculate this for nitrogen by estimating the amount of nitrogen gas released from the entire IBM system using the composition of our gas samples and our sulfur dioxide flux measurements from mini-DOAS. We will compare this estimate with the amount of nitrogen that is being subducted in the oceanic sediments. If the amount of nitrogen emitted by the arc is less than the amount carried into the mantle via the subducting plate then organically-derived nitrogen with a distinct isotopic composition in the sediments is being introduced to the mantle, transporting the signature of life into the deep Earth.