Sr-Nd isotope re-equilibration within augite phenocrysts

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On the basis of whole-rock and mineral chemistry and ^<87>Sr/^<86>Sr and ^<143>Nd/^<144>Nd ratios, Bishamondake adakitic rocks were revealed to be magma-mixing products. Samples richer in silica tend to be higher in Mg/Fe and Sr/Y and poorer in Y.In addition, the pyroxene phenocrysts show bimodal distributions in Mg/Fe, and rocks richer in silica tend to be high in a ratio between high-Mg/Fe and low-Mg/Fe pyroxenes. It follows that one end-member was silica-rich adakitic magma with high-Mg/Fe pyroxenes. The ^<87>Sr/^<86>Sr and ^<143>Nd/^<144>Nd ratios of the high-Mg/Fe augite separates and their host rocks are the same within the analytical uncertainties in the augite separates with mean mg#(=100Mg/(Mg+Fe)=81.6 to 84.2. Thus evidence were obtained for my expectation that Sr and Nd isotopic diffusion would be faster than Mg and Fe elemental diffusion. On the other hand, the augite separates with mean mg#=84.8 have distinctly different isotope ratios form their host rock. Rather, the ^<87>Sr/^<86>Sr and ^<143>Nd/^<144>Nd are close to calculated isotope compositions of a restite of late Cretaceous volcanic activity of the area. It collectively follows that the adakitic nature of Bishamondake rocks are inherent in the presumed silica-rich end-member magma derived by remelting of the areal restite.Additional case studies were carried out on the Ueno basaltic rocks and the Setouchi high-Mg andesites to understand the isotopic modifications of primary magmas within the crust in this project.