At subduction zones, oceanic lithosphere that has interacted with sea
water is returned to the mantle, heats up during descent and releases
fluids by devolatilization of hydrous minerals. Models for the formation
of magmas feeding volcanoes above subduction zones require largescale
transport of these fluids into overlying mantle wedges(1-3). Fluid flow
also seems to be linked to seismicity in subducting slabs. However, the
spatial and temporal scales of this fluid flow remain largely unknown,
with suggested timescales ranging from tens to tens of thousands of
years(3-5). Here we use the Li-Ca-Sr isotope systems to consider fluid
sources and quantitatively constrain the duration of subduction-zone
fluid release at similar to 70 km depth within subducting oceanic
lithosphere, now exhumed in the Chinese Tianshan Mountains. Using
lithium-diffusion modelling, we find that the wall-rock porosity
adjacent to the flowpath of the fluids increased ten times above the
background level. We show that fluids released by devolatilization
travelled through the slab along major conduits in pulses with durations
of about similar to 200 years. Thus, although the overall slab
dehydration process is continuous over millions of years and over a wide
range of pressures and temperatures, we conclude that the fluids
produced by dehydration in subducting slabs are mobilized in
short-lived, channelized fluid-flow events.