Abstract
The smectite conversion can be divided into diagenesis and low degree hydrothermal metamorphism stages in the Nanatani formation (depth: 4250-6300m; middle Miocene) basalt and interstratified rocks in the Niigata sedimentary basin, Japan. In the diagenesis stage, the features of smectite conversion are crystallization and recrystallization from glass or primary minerals, precipitation as fillings of vesicle and crevice in basaltic rocks. In the low grade hydrothermal metamorphism stage, the prior smectites are dissolved and recrystallized as corrensite on the early hydrothermal progression and as chlorite and talc associated with epidote, prehnite and others on the late hydrothermal progression.
The thin section, XRD, SEM, TEM, EDS and bulk rock chemical analysis have documented that the mixed-layer Chl/S conversion from smectite to chlorite via corrensite is a discontinuous transformation process between diagenesis and low degree hydrothermal metamorphism. Although smectite and randomly mixed-layer Chl/S exhibit crystal growth from fine to coarse size in diagenetic process, the chlorite layer percentage has been only slightly changed in the mixed-layer Chl/S (chl, <15%). The low grade hydrothermal metamorphic corrensites are close to ideal 1:1 interstratified structure of mixed-layer Chl/S (chl, 45-60%) and the discrete chlorites contain a little smectite layers (chl, >90%).
The Al2O3-FeO-MgO relation and ratio of octahedral Fe/Fe+Mg indicate that the smectite and precipitated chlorite composition are controlled by host rocks and environmental fluid in diagenesis. However the hydrothermal corrensite and chlorite have similar compositions to those in other hydrothermal metamorphism.
