1999 年 49 巻 1 号 p. 15-28
Major and minor element concentrations of fresh to altered basaltic rocks dredged from near the Rodriguez Triple Junction in the Indian Ocean are reported in order to clarify the elemental behavior of these rocks during the hydrothermal alteration at mid-oceanic ridges and low-temperature seafloor weathering. Dredged samples are classified into three groups based on the degree of alteration; relatively fresh basalt, altered basalt with a brown secondary mineral, and highly altered chloritized rocks (basalt and dolerite). Whole-rock chemical compositions indicate that the fresh basalt is mid-oceanic ridge basalt (MORB) although only Nb is slightly depleted. Compared to the fresh basalt, altered basalt containing a brown alter-ation mineral is enriched in K, Rb, and U, suggesting that this type of basalt is subjected to low-temperature seafloor weathering. On the other hand, chloritized rocks show an enrichment of Mn, Mg, Na, Cu, and Zn and a depletion of Ca, K, Co, Rb, Sr, and Ba. These geochemical signatures are likely to have been attributed to an addition of base metals from hydrothermal solutions and to the hydrothermal alteration such as chloritization and albitization by virtue of seawater cycling through oceanic crust at mid-oceanic ridges. Geochemical features of these altered basaltic rocks can be comparable with the geochemical variations of the pillow basalt zone and the transitional zone toward the sheeted dike zone in the IPOD/ODP Hole 504B.