Discovery of lanthanide tetrad effect in an oceanic plagiogranite from an Ocean Core Complex at the Central Indian Ridge 25°S

抄録

This paper reports the first discovery of lanthanide tetrad effect in an oceanic plagiogranite recovered from an Ocean Core Complex at 25°S along the Central Indian Ridge. The plagiogranite consists mainly of sodic plagioclase, quartz, and amphibole with accessory minerals of epidote, titanite, zircon, apatite, and allanite. The chondrite-normalized REE pattern of the studied sample exhibits noticeably high REE abundances with a conspicuous negative Eu-anomaly, indicative of very high degrees of fractional crystallization. In addition, the convex (M-type) tetrad effect is clearly recognizable in the REE pattern, implying an unusually volatile-rich (e.g., H₂O, CO₂, Li, B, F, and/or Cl) parent magma for the plagiogranite. Because MORB sources are known to have significantly low volatile content, interaction with an external fluid originating from seawater is suggested to be responsible for the presence of the tetrad effect in the oceanic plagiogranite.