Geological indexes of hydrothermal venting

Hydrothermal venting occurs on mid-ocean ridge axes with a diverse array of morphological, structural, and petrological characteristics. Consequently, it is not clear if certain geological environments are more conducive than others to hydrothermal activity. This paper uses complementary data sets recently collected on multiple-tectonic-segment scales along intermediate to superfast spreading ridge sections to systematically examine this issue. The spatial density of venting is gauged by plume incidence, the fraction of linear ridge crest overlain by hydrothermal plumes. Five geological indexes are compared with plume incidence: ridge-axis elevation, ridge-axis cross-sectional area, the percentage of ridge axis underlain by an axial magma chamber (AMC) reflector, MgO wt% in basaltic glass, and spreading rate. A mean value for plume incidence and each index, where available, is calculated for each of 14 second- to fourth-order tectonic segments from the Juan de Fuca Ridge (44 degrees 30' to 48 degrees 45'N) and the northern (9 degrees to 11 degrees 50'N) and southern (13 degrees 50' to 18 degrees 40'S) East Pacific Rise. Plumes are most common on segments with a cross-sectional area > 3.5 km(2), a net elevation > 0.35 km, AMC coverage > 60%, and MgO > 7 wt%. Spreading rate is not deterministic on the segment scale. The data suggest two classes of ridge segments. Segments with relatively low geological index values have a uniformly low plume incidence indicative of a presently feeble magmatic budget. Those with high index values have a plume incidence ranging from low to complete coverage, apparently a consequence of a magma supply rate sufficient to produce frequent dike intrusions and highly variable hydrothermal activity. The best individual predictors of hydrothermal activity are cross-sectional area and net elevation. Applying these indexes to the East Pacific Rise between 18 degrees N and 33 degrees S suggests that the unexplored segments with the highest probability of activity are those at 15 degrees 30'-16 degrees N, 6 degrees-9 degrees N, 5 degrees-6 degrees S, 27 degrees-28 degrees S, and 30 degrees 30'-33 degrees S.