ジルコンをめぐる最近の話題 ＩＩ． 重鉱物の晶出条件と希土類元素の分配

Abstract

The saturation of Zr, P2O5 and TiO2 in felsic to intermediate silicate melts, and the REE partitioning into zircon and other accessory minerals (apatite and sphene) are reviewed in this article. As discussed in Part I, the latent primitive crust can be characterized by the study of REE in the xenocrystic cores of Archean zircons, such as the 4.2 Ga zircon from Mt Narryer. In order to better understand the geochemistry of the REE, we review the REE partitioning data for zircon, apatite, allanite and sphene in natural felsic to intermediate rock studies and in high pressure experiments. We also review the experimental results for Zr, P2O5 and TiO2 saturation conditions. The saturation of P2O5 and TiO2 is important for the discussion of the REE partitioning in zircon, because the REE contents in the silicate melt coexisting with zircon is affected by the crystallization of the other REE rich accessory minerals like apatite or sphene. Based on our reviews, we conclude that REE partitioning cannot be constrained from the studies of natural rocks because the partitioning of the REE is affected by temperature and pressure which we do not know precisely. The SiO2 content in the silicate melt also affects the partitioning of the REE because it changes the structure of the silicate melt, as briefly reviewed in this article. In addition, REE partitioning data obtained by conventional “bulk analysis” of zircon is becoming less important. This is because a bulk analysis, especially for LREE, is easily affected by inclusions in zircon, where the LREE are enriched in these inclusions. Only SIMS analyses can eliminate the effect of the inclusions and reveal the actual partitioning of REE in zircon.