Seismic and hydrothermal evidence for a cracking event on the East Pacific Rise crest at 9° 50′ N

Interaction between the hydrothermal system and the axial magma chamber at a mid-ocean ridge spreading centre takes place in a boundary layer of crust that separates circulating sea water from basaltic melt1. The nature of heat flow through this region is critical because it determines the pressure–temperature conditions of the water–rock interaction and regulates the total heat flux through the system2. Here we combine seismic, thermal and chemical time-series data from high-temperature vents on the East Pacific Rise axis at 9° 50.2′ N to link a microearthquake swarm with changes measured in vent fluids. Four days after the earthquake swarm opened fractures near the base of the circulation system, a sudden increase in fluid temperature in the overlying ‘Bio9’ black-smoker vent was observed. Temperatures peaked at the vent 11 days after the swarm and gradually declined back to just above pre-swarm levels (365 °C) over the next 70 days. These observations are consistent with the Bio9 hydrothermal system tapping a previously isolated region of crust, and an upflow fluid residence time of 4 days, compared to previous lower-resolution estimates of 3 years or less3.