Water is carried into subduction zones with the subducting oceanic plate either as sedimentary pore water, or bound to mineral phases.
Up to now it has been assumed that most of the water associated with sediments, particularly the sedimentary pore water is released during early phases of the subduction process within the first few kilometers of subduction. In contrast, it is assumed that mineral-bound subduction zone water input has the potential to penetrate deep into the subduction zone and be released under the overlying volcanic arc or be recycled into the mantle.
Our paper sets out to test the first of these assumptions by analysing water-rich fluids that were trapped in the form of fluid inclusions in minerals that come from the convecting mantle above the subducting plate. The fluid inclusions are small (0.01-0.02 mm diameter), spherical bubbles of water that are trapped in the mineral olivine (Fig. 1), a key constituent of peridotite (Fig. 2), which is the predominant rock type in the mantle.
The water inside the fluid inclusions is believed to have been distilled from the “water mineral” serpentine, released as the plate descends into the mantle and is subjected to increasing temperature and pressure. Serpentine is an alteration mineral formed from basalt and occurs within deep fractures and faults produced as the ocean plate bends and cracks as it descends into the mantle. Water is driven into theses fractures and reacts with the surrounding basalt filling the space with serpentine and other alteration minerals. As the serpentine descends further into the mantle it begins to release water (and other volatile species) and it is this water that we believe is trapped as fluid inclusions in our olivine samples.
Our study has analysed the halogens (chloride [Cl] , iodine [I] and bromine [Br]) and noble gases contained in the fluid inclusions in peridotite samples from volcanoes from two volcanic arcs: the Kuril-Kamchatka arc in the north-west Pacific Ocean and Luzon (Philippines) arc in the western Pacific Ocean. Halogens and noble gases are highly effective tracers of the source of water (Fig. 3) and our paper concludes that the “signature” we obtain is of sedimentary pore fluid. This means we now know that, contrary to the previous assumption that sedimentary pore fluid is lost from the subduction system at relatively shallow levels, some of this constituent of subduction water is being transported to depths of at least 150-200 km into the mantle, and perhaps even further down.
Kobayashi, M., Sumino, H., Nagao, K., Ishimaru, S., Arai, S., Yoshikawa, M., Kawamoto T., Kumagai Y., Kobayashi T., Burgess R., Ballentine C.J. (2017). Slab-derived halogens and noble gases illuminate closed system processes controlling volatile element transport into the mantle wedge. Earth and Planetary Science Letters 457, 106-116. doi.org/10.1016/j.epsl.2016.10.012
Our paper can be downloaded for free at the following website.