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In the last quarter-century we have progressed from the discovery of vent fields, to conceptually straightforward hydrothermal surveys along many fast-spreading ridges, to the challenge of cataloging hydrothermal activity throughout Earth’s oceans. This progress allows us to test, on a global scale, the proposition that hydrothermal activity increases linearly with the magmatic budget. About 20% of the ~67,000 km global ridge system has now been explored, to some degree, for hydrothermal venting, along with ~3000 km of submarine volcanic arcs and a few intraplate volcanoes. We can precisely locate ~145 confirmed vent sites and have indications of another ~130 based on water column observations alone. Fast ridges (no rift valley), spreading at full rates >55 mm/yr, presently account for 125 sites, slow ridges (rift valley, 20–55 mm/yr) for 55 sites, and ultraslow ridges (<20 mm/yr) for 34 sites. Vent site exploration has long been focused in the eastern Pacific and northern Atlantic ridges. Along a 30,000 km span of ridge from the equatorial Mid-Atlantic Ridge, through the Indian Ocean, to 38°S on the East Pacific Rise we know of only two confirmed sites, separated by <150 km, although more than 20 further likely targets have been identified from plume studies. If our present distribution of sites is representative across all spreading rates, then the expected total global population of active vent fields on the ridge system is ~1000.
Of the explored ridge, on only 13 sections totaling ~7400 km do we have reasonable confidence in our estimates of the relative frequency of hydrothermal activity. Using either plume incidence, ph, or site frequency, Fs, and regressing the estimates from each section (except two hotspot-affected ridge sections) against the crustal magmatic budget, Vm, yields statistically robust linear trends spanning spreading rates from 10–150 mm/yr. These direct correlations are consistent with the hypothesis that Vm, rather than any spreading-rate-dependent variability in the bulk crustal permeability, for example, is the principal control on the distribution of hydrothermal activity. This conclusion is supported by the oceanic pattern of deep (He%), an unequivocal hydrothermal indicator, which agrees with a ridge source distribution that is a function of spreading rate.
In addition to this first-order trend, the data also suggest that ultraslow and slow ridges support 2–4 times as many vent sites, for a given Vm, than do faster ridges. We interpret this result as an indication that ultraslow ridges, as well as slow ridges to a lesser degree, use deep and enduring faults, strongly three-dimensional magma delivery, and additional heat sources such as direct cooling of the upper mantle, cooling gabbroic intrusions, and serpentinization to supplement the heat supplied by the crustal cooling of basaltic melt.
It is important to remember that these conclusions assume that the average vent field heat flux does not vary systematically with spreading rate. While flux measurements are rare and arduous to obtain, the steady accumulation of information on the tectonic setting, size, and nature of vent fields will eventually permit a reliable test of this assumption.
What are some of the critical problems that call for our attention in the next decade? Several key questions evolve from this review:
With continued attention to both spatial and temporal variability, perhaps in another decade we can derive a confident and quantitative relationship between hydrothermal activity and the magmatic budget at ridges of all types.
Acknowledgments. This review was supported by the NOAA VENTS Program (ETB) and by NERC (CRG). We expressly acknowledge the countless colleagues who have contributed time and energy into the collection of these data. The impetus for this paper was the 2001 InterRIDGE Theoretical Institute at the University of Pavia in Italy. We thank the conveners and participants for channeling our energies into this effort. Thanks also to helpful reviews from R. Haymon, L. Parson, and J. Lin. PMEL contribution number 2544.
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