Sr-Nd isotope re-equilibration within augite phenocrysts

根据全岩和矿物化学特征，以及<87>Sr/ <86>Sr和<143>Nd/ <144>Nd比值，认为比沙门达克阿达克岩为岩浆混合产物。富二氧化硅样品的Mg/Fe和Sr/Y值较高，Y值较低。辉石斑晶在Mg/Fe值上呈双峰分布，富二氧化硅岩石辉石斑晶在高Mg/Fe值和低Mg/Fe值之间呈高比值分布。结果表明，其中一端为富硅、高镁铁辉石的埃达克质岩浆。在分析不确定度范围内，高mg /Fe辉长岩分选体及其寄主岩石的Sr <87>Sr/ Sr <86>Sr和Nd <143>Nd/ Nd <144>Nd比值相同，平均mg#(=100Mg/(mg +Fe))=81.6 ~ 84.2。这就证明了Sr和Nd同位素的扩散比Mg和Fe元素的扩散要快。另一方面，平均mg#=84.8的辉长岩分离物与其寄主岩的同位素比值存在明显差异。相反，^<87>Sr/^<86>Sr和^<143>Nd/^<144>Nd与该地区晚白垩世火山活动残余岩的同位素组成接近。综上所述，Bishamondake岩石的埃达克质性质是固有的，被认为是由区域岩石重熔产生的富含硅的端元岩浆。本项目还对上野玄武岩和濑户内高镁安山岩进行了额外的案例研究，以了解地壳内原始岩浆的同位素变化。

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.