Off-axis vents

5 Biogeochemistry of off-axis vents and seafloor basalt 8.2.5.1 Off-axis vents 

Little microbiological and biogeochemical work has been done on off-axis vents to date. At off-axis seamounts, development of microbial mats and 

---

Because few detailed studies of off-axis venting have been undertaken to date and many have been exclusively near the Juan de Fuca Ridge (JDFR see references in German etal., 1995 Becker etal., 2000 Wheat Mottl, 2000), attempts to estimate the overall chemical flux to the ocean through hydrothermal activity and its importance to ocean chemistry, relative to the contribution from rivers, have been hampered by an overall lack of knowledge (Elderfield Schultz, 1996). Discussion of the relative contributions of off- and on-axis fluxes has centred on comparing the chemical output on the axis to the greater heat (and probably water) flux on flanks (Von Damm, 2001 Mottl, 2002). 

Deposits which are forming are frequentiy characterized by venting streams of hot (300°C) mineralized fluid known as smokers. These result in the local formation of metalliferous mud, rock chimneys, or mounds rich in sulfides. In the upper fractured zone or deep in the rock mass beneath the vents, vein or massive sulfide deposits may be formed by the ckculating fluids and preserved as the cmstal plates move across the oceans. These off-axis deposits are potentially the most significant resources of hydrothermal deposits, even though none has yet been located. 

A schematic representation of the categories of heat and water convection through the sediments and crust. Axiai (ridge crest) heat convection is divided into that from hlgh-temperature (c. 350 °C) and lower-temperature vents, and the rest is off-axis flow through the thin veneer of sediments on the ridge flank. 

Off-axis diffuse flow versus axial venting... 

Since hydrothermal element outputs are generally derived by multiplying the mean vent fluid composition for that element by an estimate of global high-temperature fluid flux, accurate estimates of the latter are essential. Unfortunately, such estimates are not presently well constrained. In addition to episodic events, the relative importance of axial and off-axis fluxes needs to be quantified. This could have important consequences for estimating global fluxes from hydrothermal vents (Elderfield Schultz, 1996). Increased exploration and subsequent studies of off-axis sites (e.g. Kelley etal., 2001) will contribute greatly to this poorly studied issue. 

Significant hydrothermal sites are known from a number of on- and off-axis seamounts. These include the Axial Volcano site on the JFR, a large sulfide deposit on a near-axis volcano at 13°N EPR, Loihi seamount in the Hawaiian-Emperor chain, the Lucky Strike hot-spot-related seamount site on the MAR, and a number of other localities. Axial Volcano and Lucky Strike have been studied most thoroughly, and have high-temperature hydrothermal systems. The Ashes vent field on the summit of Axial Volcano was the first to show effects of boiling at the reduced pressures encountered on the seamount relative to a normal ridge crest (Massoth et al. 1989). Many ridge-crest vent fields have been discovered in the last decade that show the effects of phase-separation into low-salinity vapor and more saline fluid (Butterfield 2000). 

---