Oxygen Isotope In Situ Microanalysis of Zircon by SHRIMP Ⅴ

Oxygen isotope analysis can provide important genetic information for the formation and evolution of rocks, hydrothermal fluids, and mineral deposits. It is a powerful tool for petrology and mineral deposits research. Secondary ion mass spectrometry (SIMS) is widely used for oxygen isotopes analysis of accessory minerals such as zircon with its advantage of its unique in situ microanalysis. However, since the advent of SHRIMP, there has always been a problem of sample chamber vacuum damage caused by heating and deflation during the movement of the sample stage. In order to maintain a stable high level of vacuum in the sample chamber and ensure the accuracy of oxygen isotope testing, an in situ microanalytical method of oxygen isotopes in zircon was established by using the fifth generation of sensitive high resolution ion microprobe (SHRIMP Ⅴ) with an upgraded piezoceramics driven high vacuum sample stage. The instrument conditions include: the primary ion intensity of 3nA, electron gun energy of −1.9keV, beam spot size of 25μm, source slit width of 120μm, and slit widths of both ¹⁸O⁻ and ¹⁶O⁻ receivers of 300μm. The system was adjusted by using zircon standard samples Temora 2 and Qinghu, and the obtained signal intensity of ¹⁶O was 1.2×10⁹cps. The standard samples R33, FC1 and Tanz were also tested. The results showed that the analysis values of δ¹⁸O for zircon standard samples were consistent with the reference values within the error range. Internal analysis accuracy was better than 0.30‰ (2SE), and the external accuracy was better than 0.50‰ (2SD). The overall testing accuracy of zircon oxygen isotope analysis by SHRIMP Ⅴ was comparable to that of existing SHRIMP series instruments in China and abroad, which verified the accuracy, precision and stability of SHRIMP Ⅴ. After upgrading the high vacuum platform, the sample chamber vacuum stability of the SHRIMP Ⅴ was maintained within 2.5×10⁻⁸torr (original SHRIMP is 4.0×10⁻⁸torr), which further improves the ultimate vacuum, repeat positioning ability and resolution. Besides, it is more conducive to the analysis of stable isotopes such as oxygen and provides the possibility for the future testing of water content in traditional anhydrous minerals.