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Chimneys
made of sulfide minerals form around focused hydrothermal vents. The composition
of the chimneys depends on the composition of the vent fluids. As
a chimney structure ages and continues to grow, some conduits become sealed
off while others grow anew. Flanges and spires form in response to changing
fluid flow patterns within the chimney, creating beautiful, delicate,
and sometimes astonishingly large edifices. Sulfide chimneys exist at
Castle, ASHES, and CASM in the NeMO study area. Some sulfide chimneys
reach over 10 meters in height. Eventually the structures become unstable
and fall, resulting in piles of sulfide rubble at the base of many chimneys.
However, chimneys can grow back very rapidly, some at upwards of a meter
per day!
Chimney
growth often begins with anhydrite precipitating in a zone of high temperature
mixing between the vent fluid and the surrounding seawater. The anhydrite
structure grows upward like a collar around the venting fluid stream.
As the chimney evolves, hydrothermal fluid begins to diffuse out horizontally
through the porous anhydrite walls. A copper-iron sulfide lining (such
as chalcopyrite) is typically deposited on the inner walls of the chimney.
Then as horizontal fluid flow continues, a gradual inward-thickening of
the walls takes place with partial replacement of the original anhydrite
with sulfide, and outward deposition of predominantly zinc-sulfides.
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