--> ABSTRACT: Geological Setting of Hydrothermal Vents on Loihi Submarine Volcano, Hawaii, by Alexander Malahoff; #90097 (1990).

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ABSTRACT: Geological Setting of Hydrothermal Vents on Loihi Submarine Volcano, Hawaii

Alexander Malahoff

Loihi submarine volcano is an active submarine hot spot volcano with a summit depth at 969 m below sea level. It is located 28 km southeast of the Island of Hawaii. The geology of Loihi has been extensively mapped with photogeological multibeam acoustic and submersible-based sampling and photographic techniques. Analysis of the data assemblage suggests a structural setting of a 31 km, northwest-to southeast-striking rift. The elongate Loihi edifice located at the southern end of the rift (at 18°56^primeN, 155°16^primeW) has been built through periodic eruptions from a water depth of 4023 m. Magnetic data suggest that the bulk of the edifice below the summit has a temperature of above 550°C. The summit of Loihi is characterized by the presence of two pit cra ers and a number of smaller rifts, paralleling the principal rift system. The hydrothermal vents are located along the fissures and at sites of young, pillow cones located along the southern rift. The entire hydrothermal system on Loihi is dominated by low-temperature (30°C) hydrothermal vents that have built either blankets for fields of nontronite or nontronite chimneys. These fields appear to be short-lived and there is evidence that they are frequently destroyed by downslope mass wasting processes. Talus is the dominant rock cover of the edifice flanks. The wide distribution of the hydrothermal fields over the summit area is seen in the presence of nontronite in the intersticial voids between exposed pillow lavas and talus. The nontronite located over Loihi is an iron-rich smect te with high Mn (1700 ppm), Cu (700 ppm), and Ni (215 ppm) content. To date, polymetallic sulfides in the form of iron pyrite have been found only at the base of the normal fault outlining the western pit crater. Higher temperature phases may be forming deeper within the edifice.

AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990