Hydrothermal cooling along the Eastern Lau Spreading Center: No evidence for discharge beyond the neovolcanic zone

Heat transported from the mantle beneath spreading centers creates an astonishingly narrow ribbon of convective heat discharge at plate boundaries, as apparently demonstrated by exhaustive exploration for hydrothermal discharge sites over the last three decades. Recent observations and models are now challenging this assumption of exclusively axis-centric high-temperature venting. One example is the proposal that intense cooling along the vertical boundaries of a broad low-velocity volume (LVV) of hot crust could generate high-temperature fluids several kilometers off axis. To test the hypothesis that substantial hydrothermal discharge might occur beyond the LVV, we conducted a dense survey grid of the ridge and surrounding seafloor (up to +/- 5 km) along 175 km of the Eastern Lau Spreading Center and Valu Fa Ridge (similar to 1800 km of track line). Our sampling array extended from similar to 50 to 400 m above bottom and included light-scattering, oxidation-reduction potential, and hydrographic sensors attached to the tow line and beneath the IMI120 sonar mapping system. The surveys successfully mapped plumes from several vent fields in the neovolcanic zone (similar to +/- 1.5 km about the axis) but did not detect evidence of significant discharge anywhere farther off-axis. At a few locations on the Valu Fa Ridge, however, we did record oxidation-reduction potential anomalies with hydrographic density signatures that imply low-temperature hydrothermal sources on the axial flank. Although these sites are hundreds of meters deeper than the adjacent crest, they are above, not beyond, the previously mapped LVV. Our results thus do not support a simple picture of high-temperature fluids ascending undiluted through the crust to the seafloor several kilometers off-axis. However, we cannot exclude the possibilities that the largely unmapped LVV is narrower here than seen on other ridges, that hydrothermal fluids formed beyond the LVV are channeled to the axis, or that discharge beyond the neo-volcanic zone occurs only as dispersed, very low-temperature fluids. Our observations do demonstrate that high-temperature discharge predominantly exits the seafloor within a narrow (similar to +/- 1.5 km) axial ribbon, regardless of the presence or absence of an axial magma chamber.