Abstract:Brothers submarine volcano is very hydrothermally active and sits in the Kermadec arc, New Zealand. The International Ocean Discovery Program (IODP) Expedition 376 was intended to explore the Cu, Au etc. mineralization model, they drilled five sites in caldera and recovered about hundreds of meters of cores between May and July 2018. The hole U1530A dilled to 453. 1meter below sea floor and recovered 76. 77 meter samples. Some of them show different degrees of blue color, which is usually called “blue clay”. We attempt to find out the coloration mineral and mechanism, and further discuss the geological significance. Methods: A series of analytic methods, involved X-ray powder diffraction, Diffused Reflection Spectrum -Ultra Violet-Visual (DRS-UV-VIS), Scanned Electron Microscopy (SEM) and Transmission electron microscope (TEM) were applied to 11 cores from 16.97 mbsf to 309.22 mbsf. Results: There is different amounts of vivianite in the blue samples. According to these test results, mineral assemblages can be divided into two types: ① illite + chlorite +opal-C + pyrite + anhydrite + quartz ± vivianite; ② pyrophyllite + quartz +illite ± vivianite. SEM and TEM images suggest that the vivianite grains in the blue samples shows ultra-fine xenomorphic crystal, or even none single grain with size of micron or bigger.Conclusions: Vivanite is the coloration mineral and it filled in the intergranular of other mineral grains as matrix or cement, which caused blue color of samples. Discovery of vivanite in the IODP drill cores suggests that brothers volcano hydrothermal fluid characterized by low temperature and reducibility, and still maintains relative reduction condition after the formation of vivanite, which makes vivianite exists stably.