合成実験による角閃石，単斜輝石，ザクロ石と珪酸塩メルト間のHf・Zrの分配

抄録

Partition coefficients for Hf and Zr (D_Hf, D_Zr) have been determined experimentally between HFSE-bearing mafic minerals such as amphibole, clinopyroxene and garnet and basanitic andandesitic melts doped with Hf and, in some cases Zr as well, in a range of pressure from 0.5 to 3.0 GPa, and 1000-1050°C (basanite) or 900-950°C (andesite). D_Hf and D_Zr for amphibole/melt decrease with increasing P, and this pressure effect on D's is most noteworthy between 0.5 and 1.5 GPa. The D_Hf and D_Zr correlate positively with the relevant D_Ti. Also amphibole/melt pairs show D_Hf>D_Zr consistently through the pressure range 0.5-2.5 GPa. Thus Zr and Hf are decoupled to some extent by amphibole crystallization. A similar partitioning behaviour of Hf and also possible Zr/Hf fractionation occurs for clinopyroxene/melt pairs. In contrast, garnet/melt pairs indicate D_Zr>D_Hf. This implies that a distinctive Hf/Zr fractionation would occur with garnet involvement, compared with clinopyroxene (or amphibole). Also a modest change in Hf/Zr ratios is supposable in magmas, especially during initial stages of the partial melting process in which clinopyroxene is postulated as a residual phase and/or if amphibole fractionation occurs during shallower-level differentiation. However, results of our partial melting and fractionation modeling indicate that the change is only minor to negligible through such mineral(s)/melt separation processes involving clinopyroxene and amphibole. Co-existence of garnet with clinopyroxene in the residue further restrains the change of Hf/Zr ratios in partial melts, because the two minerals tend to fractionate Hf and Zr in opposite directions.