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

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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.
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