Constraint on the geochemical stage causing tetrad effect in kimuraite: Comparative studies on kimuraite and its related rocks, from REE pattern and Nd isotope ratio

Our careful analysis (Akagi et al., 1993) disclosed a W type tetrad effect in kimuraite, an REE carbonate mineral. To put constraint on the genetic stage responsible for the appearance of the effect, rare earth element (REE) abundances were compared between kimuraite and its paragenetic mineral (lanthanite) and between kimuraite and its host rock. The Nd isotopic compositions of kimuraite and the host rock were also compared. Neodymium isotopic analyses conclude that the REE in kimuraite has a different source to its host rock. The similar magnitude of the tetrad effect among the paragenetic minerals of kimuraite rules out the process of mineralization as the stage responsible for the tetrad effect. It is inferred that kimuraite and lanthanite formed in situ from aqueous solutions enriched in REE which had infiltered the fissures of the host rock. This solution is considered to be responsible primarily for the tetrad effect of kimuraite.