SEISMIC AND HYDROTHERMAL EVIDENCE FOR A CRACKING EVENT ON THE EAST PACIFIC RISE CREST AT 9-DEGREES-50'-N

Interaction between the hydrothermal system and the axial magma chambe r at a mid-ocean ridge spreading centre takes place in a boundary laye r of crust that separates circulating sea water from basaltic melt(1). The nature of heat now through this region is Critical because it det ermines the pressure-temperature conditions of the water-rock interact ion and regulates the total heat flux through the system(2). Here we c ombine seismic, thermal and chemical time-series data from high-temper ature vents on the East Pacific Rise axis at 9 degrees 50.2'N to Link a microearthquake swarm with changes measured in vent fluids. Four day s after the earthquake swarm opened fractures near the base of the cir culation system, a sudden increase in fluid temperature in the overlyi ng 'Bio9' black-smoker vent was observed Temperatures peaked at the ve nt 11 days after the swarm and gradually declined back to just above p re-swarm levels (365 degrees C) over the next 70 days. These observati ons ape consistent with the Bio9 hydrothermal system tapping a previou sly isolated region of crust, and an upflow fluid residence time of 4 days, compared to previous lower-resolution estimates of 3 years or le ss(3).