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2002 Denver Annual Meeting (October 27-30, 2002)

Paper No. 11
Presentation Time: 1:30 PM-5:30 PM

THE ROLE OF ANHYDRITE DEPOSITION IN SUB-SEAFLOOR SILL-DRIVEN HYDROTHERMAL SYSTEMS


CARR, Patrick M.1, CATHLES III, Lawrence M.1, IOANNOU, Stefan2 and BARRIE, C. Tucker3, (1)Earth and Atmospheric Sciences, Cornell Univ, Snee Hall, Ithaca, NY 14853-1504, (2)Earth Sciences, Univ of Toronto, 22 Russell Street, Toronto, ON M5S3B1, Canada, (3)23 Euclid Ave, Ottawa, ON K1S 2W2, Canada, pmc1@cornell.edu

Although anhydrite is uncommon in the footwall of volcanogenic massive sulfide deposits in the rock record, it is common in modern seafloor hydrothermal systems and the effect of its deposition can have a substantial effect on the evolution of the convective system. The initial structure of a black smoker chimney is formed by the precipitation of anhydrite where cooler, sulfate-rich seawater mixes with the hot venting plume and the retrograde solubility of anhydrite results in crystallization in that boundary layer (Haymon, 1983). Likewise, anhydrite precipitates below the seafloor, where inwelling seawater contacts the upwelling hydrothermal plumes.

Numerical models of this convection allow us to study the evolution of the system with permeability changes based on the diagenesis and redissolution of anhydrite in the sub-seafloor. Previous models using an infinite heated base to approximate a mid-ocean ridge show the system being "choked" by a barrier produced by the anhydrite precipitation front, with venting temperatures not exceeding 80 degrees Celsius (Fontaine, et al., 2001). This leads to the conclusion that other factors such as tectonism or subsequent volcanism are required to alter the permeability profile and allow the hotter venting required to produce black smokers. In our calculations, the heat source intrusion or magma chamber is modeled with a thickness and an edge, as in an isolated sill emplacement, and the free convection at that edge dominates the system and vents at temperatures greater than 300 degrees Celsius. In this system, the anhydrite effectively "sheathes" the hydrothermal plumes, and allows them to vent at hotter temperatures for longer than in simulations without anhydrite calculations.

References

Fontaine, F., M. Rabinowicz, and J. Boulegue. 2001. Permeability changes due to mineral diagenesis in fractured crust: implications for hydrothermal circulation at mid-ocean ridges. Earth Planet. Sci. Lett., 184: 407-425.

Haymon, R.M. 1983. The growth history of hydrothermal black smoker chimneys. Nature, 301: 695-696.

Session No. 153--Booth# 35

Geochemistry (Posters) II
Tuesday, 29 October 2002: 1:30 PM-5:30 PM
Geological Society of America Abstracts with Programs. Vol. 34, No. 6, September 2002, p.341
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