U.S. Dept. of Commerce / NOAA / OAR / PMEL / Publications
The "magmatic budget hypothesis" proposes that variability in magma supply is the primary control on the large-scale hydrothermal distribution pattern along oceanic spreading ridges. The concept is simple but several factors make testing the hypothesis complex: scant hydrothermal flux measurements, temporal lags between magmatic and hydrothermal processes, the role of permeability, nonmagmatic heat sources, and the uncertainties of vent-field exploration. Here we examine this hypothesis by summarizing our current state of knowledge of the global distribution of active vent fields, which presently number ~280, roughly a quarter of our predicted population of ~1000. Approximately 20% of the global ridge system has now been surveyed at least cursorily for active sites, but only half that length has been studied in sufficient detail for statistical treatment. Using 11 ridge sections totaling 6140 km we find a robust linear correlation between either site frequency or hydrothermal plume incidence and the magmatic budget estimated from crustal thickness. These trends cover spreading rates of 10–150 mm/yr and strongly support the magma budget hypothesis. A secondary control, permeability, may become increasingly important as spreading rates decrease and deep faults mine supplemental heat from direct cooling of the upper mantle, cooling gabbroic intrusions, and serpentinization of underlying ultramafics. Preliminary observations and theory suggest that hydrothermal activity on hotspot-affected ridges is relatively deficient, although paucity of data precludes generalizing this result. While the fullness of our conclusions depends upon further detailed study of vent field frequency, especially on slow-spreading ridges, they are consistent with global distributions of deep-ocean He, an unequivocally magmatic tracer.
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