LAUSR

Since the discovery of hydrothermal activity on the seafloor at mid-ocean ridges (MORs) more than 40 years ago, questions about the relative role of magmatic and tectonic processes on the chemical and physical evolution of vent fluids have been expressed with increasing frequency. These questions have largely been motivated by geological and geophysical observations at MORs characterized by slow and intermediate spreading rates. In PNAS, McDermott et al. (1) use a highly integrated approach taking full advantage of deep submersible assets, geophysical data, and hydrothermal vent fluid temperature and chemical data, to document the existence of a large, tectonically controlled vent field on the fast-spreading East Pacific Rise (EPR). In contrast to previous observations of vents on the EPR at 9°N to 10°N (2, 3), which are largely, if not entirely, associated with the magmatically active axial summit trough (AST) (4), the newly discovered YBW-Sentry vent field is located ∼750 m east of the EPR axis, and about 7 km north of the previously studied on-axis vents. Finding more sparsely distributed vents in an area that has traditionally been ignored in favor of axial sites requires high-resolution bathymetry possible only with the autonomous vehicle technology developed by the namesakes of the YBW-Sentry site (Yoerger, Bradley, and WaldenYBW-Sentry (Yoerger, Bradley and Walden, engineers at Woods Hole Oceanographic Institution). Moreover, the proximity of the vents to a normal fault links hydrothermal and tectonic activity in a compelling way, underscoring the importance of the discovery, while broadening implications. Thus, McDermott et al. propose very different heat sources and fluid pathways for the off-axis YBW-Sentry vent fluids and on-axis vents located farther south on the EPR. In effect, the on-axis vents are thought to derive heat from localized diking (magmatic) events that enhance vertical fluid flow within zones of high permeability that are largely restricted to the ridge axis, while the YBW-Sentry vent fluids tap heat sources more closely related to the underlying axial magma lens (AML) and provide fluid flow paths that diverge from the ridge axis. The authors are correct in recognizing the discovery of the off-axis YBW-Sentry vent field as an alternative explanation for vent systems elsewhere where considerations of spreading rate alone might dispel the role of tectonics in the source of fluids and fluid pathways. Off-axis hydrothermal vents associated with fast-spreading MORs could serve as a presently underappreciated source of heat and chemicals that might be significant on a global scale. The interpretations of divergent chemical and physical processes described by the authors (1) for the off-axis YBW-Sentry vents and on-axis vents, south of the EPR study area, are supported by extensive analysis of vent fluid chemistry obtained at each of the hydrothermal vent sites. For example, recent chemical data reported by McDermott et al. (1) for hydrothermal fluid issuing from YBW-Sentry vent structures indicate a temperature of ∼368 °C. Importantly, the dissolved chloride composition of the vent fluid is 518 mm/kg, which is about 4% below that of seawater, indicating phase separation in the NaCl–H2O system at conditions just slightly offset from the seawater two-phase boundary. The authors use this information, with constraints imposed by chemical geothermometers and geobarometers, to estimate the temperature–pressure conditions in the subseafloor from which the vapor-dominated fluids issuing from YBW-Sentry vent structures are derived. In contrast to the observed seafloor vent fluid temperature of 368 °C, model data indicate subseafloor temperature and pressure for the YBW-Sentry vent fluid of 437 °C and ∼380 bar, respectively. The calculated pressure is equivalent to a depth of ∼1.3 km beneath the seafloor, within ∼160 m of the seismically imaged AML. Broadly similar approaches applied to on-axis vents reveal a very different picture. In this case, data indicate origin temperatures and pressures significantly lower than for the off-axis YBW-Sentry vent fluids, with evidence of relatively near-seafloor phase separation, especially in the aftermath of eruptive events, well illustrated and carefully documented by the authors of the present study. Thus, inherent differences in location and relative stability of the heat sources fueling onand off-axis vents at EPR 9°N to 10°N can be expected to result in corresponding differences in chemical and physical (temperature, pressure) stability of respective vent fluid systems, as observed. The YBW-Sentry vents and their geological context contribute to the understanding of the construction and evolution of oceanic crust. Although 750 m may not seem like a great distance from the axis, known vents are so tightly localized in the AST that this discovery stands out as something very different. Although high-temperature vents in other spreading systems occur in diverse settings (5), in some cases, in association with AMLs, and on fault scarps, the connection between these elements has not been made previously. Axial vents have a high probability of being modified by dike intrusion, clogging with mineral precipitates, and burial by lava flows near the AST. The