Marion Rise

The principle objective of the cruise is to investigate the geology of a major ocean hot spot associated rise on an ultraslow spreading ridge and compare it to the Icelandic and Azores Rises. Whereas the Icelandic Rise is commonly believed to be supported by enhanced asthenospheric flow and a large mantle plume, existing data (Zhou & Dick, 2013 ) suggest that the Marion Rise is a region of thin or missing crust, punctuated by large local ridge volcanoes. If this is the case, then mantle flow from the Marion Island mantle hotspot cannot support the Marion Rise, which is a 1,000 m deeper in water depth, but a thousand kilometers longer than the Icelandic Rise, bringing into question the hypothesis that all ocean rises are supported by mantle plumes originating in the lower mantle (Wilson, 1963; Wilson, 1965; Morgan, 1972; Morgan, 1973).

Additional objectives include investigating the tectonics and morphology of adjacent sections of the SW Indian Ridge, which show contrasting asymmetric (with core complexes) and oblique spreading. The latter is believed due to amagmatic spreading at very low melt flux, and the former with detachment faulting due to low to moderate melt flux. Post cruise research will also investigate how these different tectonic styles may reflect variations in mantle source composition as inferred from the residues and products of mantle melting. Geochemical studies of Hf-isotopes will be used to test the hypothesis that the rise is supported by highly depleted harzburgitic mantle at depth, which may reflect the absence of garnet in the deep mantle source. A complementary program will be to use MAPR’s on the dredge wire and AUV Sentry to locate hydrothermal activity, which would then be visited by the ROV on the Sonne cruise.