A study of origin and history of magma by trace element partitioning and U-Io dating method.

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We have found in the lst-year research, that zircon is very difficult to synthesize from silicate melts. Thus we used Li molybdate flux to synthesize zircon containing trace elements at the level of natural rocks. Good agreement in solia-liquia partition coefficients of trace elements was found between the synthesized zircon and natural zircon, when latter values were calculated by assuming the concentrations of trace elements in glass are equal to those in the liquid magma.For the purpose of studying the diffusional change of distributions of elements in zircon crystals, synthesized zircon (ZrSiO_4) attached to synthesized hafnon (HfSiO_4) and double layered Zircon-Hafnon crystals synthesized by epitaxial growth technique were heated to 1600ﾟC for diffusion annealing. It is found that the distribution of Zr and Hf in zircon do not change significantly within 10^5 years. Taking diffusivities of U and Th are equal to those of zr and Hf in zircon, U and Th isotopes in zircon which had been crystallized in the early stage of formation of the magma did not move significantly durina-residence of zircon in magma at temperatures 800-900ﾟC for the order of 10^4 years.The U-Io age of pumtice of Nevado del Ruiz volcano erupted in 1985 in Columbia South America determined for zircon-glass pair exceeded 10^4 years. The reason for this unreasonably old age was examined by partition coefficients and major mineral compositions etc. We concluded that the zircon crystals in the pumice are mixture of phenocryst and xenocryst zircons.