No abstract

Timing, depth, and extent of high‐temperature hydrothermal alteration in the ocean crust is key to understanding how lower oceanic crust is cooled after accretion. Epidote veins were collected in spatially recurring zones of intense alteration of the lower crustal Wadi Gideah section and the root zone of sheeted dykes at Wadi Amadhi. 87Sr/86Sr ratios feature a narrow range from 0.70429 to 0.70512, while O isotope compositions vary between −0.7‰ and +4.9‰ in δ18OSMOW. These compositions indicate water‐rock ratios between 1 and 5 and formation temperatures in the range of 300 to 450 °C. Fluid inclusion entrapment temperatures for a subset of samples linearly increase from 338 °C to 465 °C in lowermost 3 km of crust of the Wadi Gideah section and record 340 °C for the Wadi Amadhi basal sheeted dike sample. Salinities are uniform throughout and scatter closely around seawater values (3.2 ± 0.2 wt%). A model in which cooling down to 2 km (i.e., the depth of the melt lens) is followed by slow off‐axis hydrothermal cooling of the lower crust to 1 Myr predicts a thermal gradient for the lower crust that matches this observed trend for ages between 1 and 2.5 Myr. We suggest that the epidote veins formed in off‐axial hydrothermal systems that reach the base of the crust within 50‐ to 100‐km off axis. This deep circulation provides an efficient mechanism for mining heat that escapes the crust in the young flanks of mid‐ocean ridges.