Development of a method for Sr isotopic analysis of apatite by LA-ICP-MS to constrain the chemical composition of the original magma

Kagami, Saya   ; Yokoyama, Tatsunori   ; Sasao, Eiji   ; Yuguchi, Takashi*; Chang, Q.*

The fracture frequency distribution in granitic plutons depends on lithofacies, which reflect the chemical composition of the original magma. The initial Sr isotopic ratio of the whole rock is a valid indicator to identify the chemical composition of the magma. The apatite Sr isotopic composition probably records a primary Sr isotopic composition of the original magma because Rb is incompatible element for apatite. In this study, we developed a method for in situ Sr isotopic analysis of apatite by LA-ICP-MS. We adopted the technique of introducing desolvated solutions and solid samples into the ICP mass spectrometer. The correction for mass discrimination effects in the solid sample analysis by LA was attempted based on the isotope analysis of the solution. A gas line for mixing N into Ar sample carrier gas was installed to suppress the oxide production rate and improve the ionization efficiency of Sr. Our results of the analysis on the apatite reference materials were consistent with previous studies. In addition, we confirmed that this method can be applied to apatite samples with lower Sr mass fraction than above apatite reference materials and grain sizes of approximately 100 m. The developed method enables the evaluation of differences in the chemical composition of the original magma and then provides the constraint on the relationship between lithofacies and fracture frequency distribution by combination with chemical and geological information of granitic rocks.