Clay mineral characteristics in volcanic tuffs of Dokdo, South Korea: implication on their genesis and evolution

Abstract

Secondary alteration minerals like clays accompanied by volcanic activity can provide an important information on eruption cycle. Representative volcanic tuffs and seabed sediments were collected from the Dokdo volcanic island in East Sea, South Korea, to interpret eruption cycle based on clay mineral characteristics and other geochemical proxies (sulfur isotope composition, Si/Al, K/Al, among others). Tuffs are mainly composed of volcanic glass and hydrothermally altered phenocrysts. Secondary minerals such as clay mineral species in volcanic tuffs mostly were identified as smectite comprising dioctahedral sheets, based on expanding to 15.8 angstrom after treatment with 1 M MgCl2 solution, and 060 reflections near at 1.48 angstrom. Moreover, the results of infrared bands, based on (Al, Al)-OH (904-912 cm(-1)), (Al, Fe)-OH (865-877 cm(-1)), (Fe, Mg)-OH (778-798 cm(-1)) indicate the presence of dioctahedral sheets in clay mineral structure. Partially, occurrence of kaolinite and relatively low pH values and K/Al ratios at the Eolgulbawi tuff support chemical flexibility of clay mineral species. The delta S-34 value of Dongdo tuff (H2O-S: 15.3 parts per thousand) is close to water leached sulfate, whereas Seodo tuff II including pyrite and native sulfur (- 5.5 parts per thousand) shows a value of H2S-bearing volcanic gases. The similarity of chemical compositions between clay fractions and surrounding volcaniclastic rocks suggests that the smectite formed in low-temperature hydrothermal environments, where alkali elements were supplied under reducing conditions. Clay fractions separated from seabed sediments include diatom frustule with higher SiO2/Al2O3 ratios, and clay minerals contain mineral nitrogen originated from organic-rich seafloor sediments. Moreover, the delta S-34 values of seabed sediments (1.8 parts per thousand, 8.4 parts per thousand) indicate that the different origin of clay mineral species between tuffs and seabed sediments. It can be concluded that the alteration types and clay mineral characteristics are similar to the Surtseyan volcano, generally known as shallow-marine phreatomagmatic eruption and deposits. It further suggests that clay minerals sensitive to environmental change will be useful to interpret the volcanic environment characterized by explosive hydrovolcanic activities that occurred at the time of magma in contact with water before erupting into the atmosphere.