Since the discovery of hydrothermal activity on the sea-floor 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.
展开▼
