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A petrologic study of basalts from the Magic Mountain hydorthermal area, Southern Explorer Ridge, northeast Pacific Ocean Shea, Gregory Thomas Francis


The Magic Mountain Hydrothermal Area (MMA) is a 5 km portion of the Southern Explorer Ridge (SER) centered at 49°46' and 130°20'W. It is a region of active hydrothermal activity located near the culmination of an unusually high standing spreading centre. Seafloor photographs, conductivity-temperature surveys and acoustic images have been used to determine the nature and extent of axial volcanism and tectonism as well as the associated hydrothermal activity. Major element and trace element whole rock analyses were obtained by x-ray flourescence spectrometry (XRF) for 25 recently formed basalts collected by submersible from the MMA. These were compared with analyses of basalts dredged from other locations on the ridge axis. The mineral phases of selected samples were analysed by electron microprobe. Basalt chemical variations observed along axis reveal that some of the MMA samples are the most highly fractionated and incompatible-element-enriched basalts so far obtained from the SER. These trends, along with variations in ridge morphology and lava flow type, indicate that the volcanism of the MMA is affected by a hot spot centred at the axial topographic high five km to the north. This hot spot may be associated with the propagation of the ridge segment. Several quantitative tests of fractional crystallization indicate that the 25 MMA basalts represent at least 13 discrete lavas derived from at least 5 distinct source magmas. Mixing between magmas is indicated by disequilibrium mineral textures and compositions observed in some samples. A relationship between hot spot activity and degree of magmatism with the location and duration of axial hydrothermal activity on spreading centres is indicated by these findings. Further investigation of these relationships as well as the relationship between the tectonic processes of ridge propagation and hot spot activity is recommended.

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