A QUANTITATIVE ANALYSIS OF HYDROTHERMAL CIRCULATION AROUND MID-OCEAN RIDGE MAGMA CHAMBERS.

Abstract

Hydrothermal activity is one of the dominant processes affecting the chemical and thermal evolution of oceanic crust at the mid-ocean ridge (MOR), but little is known about the sub-surface portions of ridge hydrothermal systems. These systems can be investigated using numerical modeling techniques, and models of two-dimensional cross-sections are utilized in this study to investigate the behavior of MOR hydrothermal systems. The influence of magma chamber geometry is explored by modeling two extremes of proposed 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 above the distal tip of the intrusion for the lifetime of the system, (4) effective hydrothermal activity ends by 70,000 yrs. Petrological evidence supports a wide sill-like chamber 15 km in half-width, and models of this chamber indicate that: (1) complete crystallization of the magma requires 100,000 yrs, (2) hydrothermal vents are present at the ridge axis, but most of the vents are located 5-10 km away from the axis, (3) a large hydrothermal cell develops at the distal tip of the magma chamber, while a series of small but vigorous cells develops directly above the intrusion, both features migrate toward the ridge axis as the magma solidifies, (4) effective hydrothermal activity ends by 170,000 yrs. Substantially different hydrothermal systems develop around these two chamber geometries and comparison of the models shows this is because different patterns of near-critical P-T conditions developed around them. The fundamental influence on the nature and pattern of hydrothermal circulation at MOR is the distribution of near-critical conditions.