Trace element composition and distribution of Yamato-691, an unequilibrated enstatite chondrite

Twenty one trace elements (Re, Ir, Pd, Au, Ag, Se, Te, Zn, In, Cd, Tl, La, Ce, Nd, Sm, Eu, Gd, Tb, Tm, Yb and Lu) were determined by radio-chemical neutron activation analysis for the bulk sample and acid (nitric acid and aqua regia)-soluble and residual phases of the Yamato-691 (EH3) meteorite. No fractionation was confirmed in siderophile elements among Y-691,EH4-5s and EL6s. Among volatile chalcophiles, Zn and In in Y-691 are in the ranges of their abundance variations in EH4-5s, whereas Cd and Tl were observed to be more enriched in Y-691 than in EH4-5s. About 15% of light rare earth elements (REEs) and 30% of heavy REEs were confirmed to be distributed in acid-residual phases (pyroxene-dominant silicates). A small but difinite negative anomaly of Yb was found in acid-residues, but neither Eu nor Ce anomalies were confirmed. Distributions of the REEs with a negative Yb anomaly in acid-residues must be a strong constraint on formation hypotheses for enstatite chondrites.