Identification of secondary minerals crystallized by low and high temperature alteration in the Northern Kyushu-Palau Ridge volcanic rocks

Seafloor rocks were affected by hydrothermal alteration and low temperature seawater weathering display various elemental behaviors, necessitating detailed investigations to evaluate primary bulk rock compositions without the effect of elemental behaviors during alteration. Seafloor alteration entails primary minerals being changed into hydrous minerals. Bulk chemical compositions of seafloor igneous rocks are changed by high- temperature hydrothermal alteration and low-temperature seafloor weathering. In this study, I report the secondary mineral identifications by XRD analyses in the rocks from the northern Kyushu-Palau Ridge, and consider to condition of alteration processes. Volcanic rocks dredged from the Northern Kyushu Palau Ridge during cruise by Tansei-maru, ORI, University of Tokyo show petrological and geochemical characteristics of low and high temperature alterations. These rocks are classified into bulk water content, that is, low H2O- and LOI samples at the Miyazaki Seamount, high H2O- samples at the Nichinan Seamount, and high LOI samples at the Komahashi-Daini Seamount. The Nichinan Seamount samples show flesh phenocrysts, low altered groundmass minerals, and high degree alteration of groundmass glass, assumed to replace into clay minerals. These altered phenocrysts are identified by XRD to be serpentine, saponite, and talc. And these altered groundmasses are identified by XRD to be saponite with primary plagioclase and clinopyroxene. These results are assumed to replacement of glass into clay minerals under low temperature seafloor weathering. Nichinan Seamount rocks show high alkali-elements contents. The remarkable movement of bulk composition is not occur under the low temperature seafloor weathering except for K and Rb, and these enrichments reflect secondary deposition of celadonite, K-rich smectite (e.g. Nakamura, 2001). Saponite is typical identified, but celadonite is not identified in the Nichinan Seamount rocks. Therefore, the characteristics of bulk composition of the Nichinan Seamount rocks are assumed similarity to primary signature. On the other hand, the Komahashi-Daini Seamount samples show completely re-crystallization, and igneous textures are observed to pseudomorph. These are identified by XRD to be quartz, clinochlore (one of chlorite), and albite. Secondary mineral assemblage is homogeneous in these rocks. The temperature of replacement by chlorite accompanied by enrichment in MgO is estimated to be more than 150°C on the basis of experimental studies (e.g. Mottle 1983). And interpreted two types of albitization, low temperature (< 50°C) and high temperature (> 100°C), are identified on the basis of study of ODP Leg 123 Site 765 igneous rocks (Gillis et al. 1992). Therefore, it is considered that volcanic rocks from the Komahashi-Daini Seamount were under effect of hydrothermal alteration more than 150°C. Many elements show significant movement under high temperature hydrothermal alteration (e.g. Laverne et al. 1996). That is, re-crystallization of chlorite under high temperature hydrothermal alteration accompanied addition of magnesium from seawater and remarkable bulk MgO enrichment (e.g. Nakamura, 2001). Therefore, it is considered that the Komahashi-Daini Seamount rocks show significant MgO-enrichment because of secondary mineralization of chlorite, and assumed to significant movement of other elements. These observations suggest that geochemical investigation of highly altered rocks must be made with caution.