23. SEISMIC VELOCITY–POROSITY RELATIONSHIP OF SULFIDE, SULFATE, AND BASALT SAMPLES FROM THE TAG HYDROTHERMAL MOUND1Rainer J. Ludwig,2 Gerardo J. Iturrino,3 and Peter A. Rona4 |
ABSTRACTPhysical properties and ultrasonic velocity measurements on 24 minicores recovered at the Trans-Atlantic Geotraverse (TAG) hydrothermal mound during Ocean Drilling Program Leg 158 provide a unique reference data set for subsurface sulfide, sulfate, and basalt samples from an active sediment-free hydrothermal system. Seismic velocities, densities, porosities, and the distribution of pore shapes are extremely variable both horizontally and vertically within the mound. However, if the sulfide and sulfate samples are classified according to their mineralogy and their location within major internal lithologic zones of the mound, the measured physical properties exhibit distinct characteristics. Ultrasonic seismic velocities of the samples were determined at confining pressures of 5 to 100 MPa. Compressional-wave (Vp) and two orthogonal shear-wave (Vs) velocities were measured with a single transducer. The transducer geometry allowed for testing of anisotropy effects, observed only for massive sulfide samples recovered in close proximity to the active Black Smoker Complex. Vp/Vs ratios for rock types of the different internal zones are distinctive: high values of 1.9 for massive sulfides near the seafloor, medium values averaging 1.7 for anhydrite-rich samples, and low values of 1.55 for the deeper, more silicified samples. Seismic velocities of basalt samples, which were recovered at the edges of the mineralized upflow zone, are relatively low (Vp = 6.1 km/s; Vs = 3.4 km/s) compared with "normal" mid-ocean ridge basalts. The relatively low basalt porosity (~1.5%) tends to indicate that low velocities are caused by hydrothermal alteration rather than porosity effects. We present for the first time results of an iterative model using rock physics theories that relate seismic velocities to porosity structure on a hand-sample scale and estimate the distribution of pore-aspect ratios for minicore samples, which are representative of each major lithologic zone within the TAG mound. Microscopic observations on thin sections and index properties provided effective constraints as initial models. Computed porosity models yield satisfactory results that match both the visual observations of pore shapes and the seismic velocity measurements. This analysis will provide useful constraints on future studies exploring permeability, fluid flow, and alteration mechanisms of hydrothermal systems. |
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