Basalts and serpentinite from the Puerto Rico Trench, 2. Rare-earth geochemistry

Seven tholeiitic basalts from the Puerto Rico Trench walls (three from the south wall and four from the north wall) were investigated for rare-earth abundances. All these rocks, with the exception of a pigeonite-bearing rock (RC8-RD1–12), were found to possess similar rare-earth features. Their rare-earth patterns resembled those of usual abyssal basalts, and showed no Eu anomalies. There is no difference between the rare-earth abundances of fine-grained and coarse-grained rocks. It is clear that the pigeonite-bearing rock (RC8-RD1 type 2) differs in origin from the abyssal basalts. The rocks with rare-earth patterns similar to that of RC8-RD1–12 are found among the Japanes “geosynclinal” basalts (Tanaka and Sugisaki, 1973).A serpentinite from the same trench was also analyzed for the same elements. The resultant rare-earth pattern suggests that the relevant original rock was separated from a magma having the same relative rare-earth abundances as a chondritic meteorite. The markedly high Eu positive anomaly of this serpentinite is also discussed.