Characterization of Rock and Sulfide Pore Structure on the Ultraslow-Spreading Southwest Indian Ridge (49-52E) Using Nuclear Magnetic Resonance

We applied Nuclear Magnetic Resonance (NMR) to analyze the pore structure of basalt, altered mafic rock, altered ultramafic rock and polymetallic sulfide samples which were acquired on the Southwest Indian Ridge (SWIR) during the Chinese Dayang cruises. Pore structure analysis is essential for the understanding of seafloor rocks and sulfides formation, which is closely related to seafloor hydrothermal circulation. NMR is a method based on sensing the signal of hydrogen nucleus from rock fluid, which has advantages such as being non-destructive to samples and short measuring period, etc. The samples to be measured were saturated with 35 NaCl under a vacuum pressure of 30Mpa for 12h. These samples were measured to obtain raw echo data, which were then inverted to transverse relaxation time (T2) distributions. The T2 distribution characteristics consisting of the size, position, and number of T2 spectrum peaks, can be applied to understand rock and sulfide pore types. The results of NMR indicate that the rock and sulphide samples from the SWIR exhibit strong heterogeneity. Porosity measurements were also obtained from NMR. Basalts exhibit micropores pore structure with the lowest porosity, and small pore size distribution, while sulfides have the largest porosity, more macropores, fewer micropores, larger pore size distribution, and good connectivity between pores. The porosity of altered mafic rocks and altered ultramafic rocks is between that of basalt and sulfide, with more micropores and less macropores, and the connectivity between pores is poorer than that of sulfides. This study has shown that NMR can represent not only the porosity but also the pore structure of seafloor sulfides and rocks. It is a useful tool for the quantitative description of pore connectivity and calculation of seafloor hydrothermal flux in the future.