ROLE OF HYDROTHERMAL SYSTEMS IN VOLATILE RECYCLING IN SUBDUCTION ZONES

Most of hydrothermal systems located within volcanic belts of subduction zones are associated with intrusive magma bodies feeding the systems by heat and volatiles from the degassing and crystallizing magma. In contrast to volcanic gases those output rates can be measured instrumentally either from the ground-based remote sensing of SO₂ and other gases using COSPEC, DOAS, FTIR or from the space using satellite-based equipment (OMI, TOMS, ASTER), the contribution of hydrothermal systems to the total magmatic degassing of subduction zones is still poorly known. The acidic part of magmatic volatiles released from a magmatic intrusion (HCl, S, CO₂, HF) dissolves in groundwater forming hydrothermal solutions equilibrated with the rock matrix. The non-reactive fraction of magmatic gases (N₂ and noble gases) comes out unchanged but mixed with the dissolved air and crustal gases. ³He is an ideal indicator of the mantle fraction in a discharging fluid. If the output rate of ³He from a hydrothermal system is known, the discharge of other magmatic components can be estimated by the ³He/X method, where X is any magmatic volatile specie, and values of ³He/X are known within a certain range of errors. However, for many hydrothermal systems it is difficult to measure the natural total gas output, but it is possible to measure the flow rate of thermal water. Chlorine is another non-reactive component of magmatic-hydrothermal solutions. It does not form insoluble minerals like S, CO₂ and F and transported by thermal waters to the surface where it is easy to measure its output rate. The flux of any magmatic component X from a hydrothermal system then can be estimated using “magmatic” Cl/X ratios. An example from the Kamchatka-Kuril subduction zone demonstrates that the contribution to the total magmatic volatile flux from hydrothermal systems is nearly the same as from the non-eruptive volcanic degassing