Influence of magma chamber geometry on hydrothermal activity at mid-ocean ridges

Abstract

Two-dimensional models of the ridge environment are utilized in this study to examine the behavior of ridge hydrothermal systems, with particular emphasis on the influence of magma chamber geometry. Seismological evidence supports a dike-like 2 km half-width chamber, and models of this chamber indicate that: (1) complete crystallization of the magma requires 30,000 years, (2) hydrothermal upflow and hot springs are concentrated in a narrow band within 1.5 km of the ridge axis for the lifetime of the system, (3) a large hydrothermal cell forms and remains centered 2 km from the axis for the lifetime of the system, (4) hydrothermal activity ends by 70,000 years. Petrological evidence supports a wide sill-like chamber up to 15 km in half-width, and models of this chamber indicate that: (1) complete crystallization of the magma requires 120,000 years, (2) hydrothermal vents are present at the ridge axis, but most of the vents are located 5–10 km away from the axis, (3) one large hydrothermal cell develops 15 km from the axis, while a series of small vigorous cells form directly above the intrusion, all of these features migrate toward the axis as the magma solidifies, (4) hydrothermal activity ends by 170,000 years. Substantially different hydrothermal systems develop around these two chamber geometries because they generate different distributions of near-critical P-T conditions for H 2O. These zones of near-critical conditions are the primary control on driving forces for ridge hydrothermal systems, and provide the primary link between magmatic and hydrothermal events. Narrow magma chambers produce a very narrow near critical zone, and therefore produce hydrothermal and hot-spring activity that is strong focused at the ridge axis. Wide chambers produce broad near-critical zones, and widely distributed hot-spring activity that sweeps towards thèaxis with time.