Structure and petrophysics of oceanic Crust and Lithosphere, Atlantis Massif. Study of cores from IODP Expedition 357 (CLAM)

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Seismic anisotropy of the Earth´s oceanic crust and lithosphere is globally observed, and usually explained by crystallographic preferred orientations (CPO) in peridotites and/or deformed gabbros. While this model may be adequate for fast-spreading at mid oceanic ridges, deformation in slow-spreading environments brings about hydration and fundamental changes in mineralogy that alters elastic properties. Here it is proposed to conduct a neutron and synchrotron diffraction study of CPO in a suite of samples from the mafic-ultramafic rocks of Atlantis Massif, to gain fundamental insight in petrophysical properties of oceanic basement in a slow-spreading environment. Atlantis Massif was drilled and cored during IODP Expedition 357, and the rocks recovered are considered an ideal test case because of its excellent exposure and structural control through the network of drillsites. Elastic constants of the protolith minerals and those of the alteration products predict an increase of elastic and, thus, seismic anisotropy with deformation and hydration. This prediction is in contrast to observations made by seismological studies pointing to lower anisotropies of slow-spreading crust than those observed in normal or fast spreading environments. To analyze and explain this important disconnect between tectonic models and seismological observations is the main motive of the planned study.