COMPLEX GROWTH HISTORY OF PHANG, A SULFIDE STRUCTURE FROM THE MOTHRA HYDROTHERMAL VENT FIELD

A detailed three-dimensional investigation of an intact sulfide structure from the Mothra Hydrothermal Field, along the Endeavour Segment of the Juan de Fuca Ridge, was conducted to examine the growth and evolution of sulfide-microbial habitats within this dynamic environment. Analyses included co-registered centimeter-scale mapping of mineralogy, chemistry, and physical features within this nearly 2 m long recovered portion of the structure. The interior of Phang shows no evidence of simple annular flow channels, but instead, it hosts a complex series of overlapping, porous miniature chimneys that coalesce. Continuous, gradual chemical-mineralogical evolution and episodic perturbations combined to cause a variety of conditions in this type of sulfide edifice. For example, during growth, hydrothermal fluids were progressively isolated from surrounding seawater and the interior became steadily hotter and more reducing. Temperature, redox conditions, and chemical constituents of the fluids in any one part of the sulfide structure were generally dominated by this gradient of internal hydrothermal fluid to external seawater. However, over time the microhabitat varied significantly on the centimeter to decimeter scale. Fractures and mineralogical evidence of changing conditions indicate abrupt, episodic changes in fluid pathways. Such perturbations may reflect earthquakes, alteration of conditions in the feeder zone and/or gradual over-pressuring of the system. Growth occurred as a result of these episodic changes, which allowed hydrothermal fluids to reach the exterior of the structure and form new parasitic growths along the outside and upper surfaces. Macro-biological activity is an important factor in accumulation of mineral material on the exterior of such structures. Moderate temperatures, redox gradients, mineral surfaces, and available porosity make this type of sulfide an excellent candidate for incubation of biological communities.