Magma-hydrothermal interactions at the Costa Rica Rift from data collected in 1994 and 2015

We use co-located CTD/transmissometry casts and multichannel seismic reflection surveys conducted at the Costa Rica Rift (CRR) to provide a better understanding of magma-hydrothermal processes occurring at an intermediate-rate spreading center. Water column observations reveal an ∼200 m thick plume head ∼650 m above the seafloor, which corresponds to a hydrothermal heat output of ∼200 ± 100 MW at the ridge axis. Assuming a hydrothermal vent temperature of 350 °C and a discharge area between 104 and , this heat output implies a mean crustal permeability within the discharge zone of between and , and a conductive thermal boundary layer thickness of ∼20 m. The volume of magma required to maintain the current hydrothermal heat output over the past two decades should result in an across-axis axial magma lens (AML) width between 270 and 1300 m, depending on the amount of cooling and crystallization. However, seismic reflection images, acquired in 1994 and 2015, while showing an apparent along-axis growth of the AML from 2.4 to 6.0 km between surveys, also suggest that, as of 2015, the AML has an apparent across-axis width of no more than 300 m, and that magma delivery at the intermediate spreading rate CRR may be episodic on time scales of tens of years. The data on magma-hydrothermal interactions at the CRR collected in 1994 and 2015 suggest that the hydrothermal system may have significantly cooled and crystallized the AML, primarily in the across-axis direction, and that this hydrothermal system may also episodically turn on and off. The current pattern of microseismicity supports this conclusion, with events not only mirroring the AML depth and location beneath the ridge axis, but also having a temporally varying focus