In situ Re-Os dating of molybdenite by LA-ICP-MS/MS

Molybdenite is the principal host mineral for rare metals such as molybdenum and rhenium and is widely distributed in various hydrothermal deposits. Owing to its high rhenium content and negligible common osmium, molybdenite is an ideal target for Re–Os isotopic dating. Re–Os ages of molybdenite can directly constrain the timing of metal sulfide mineralization.

In recent years, the development of inductively coupled plasma tandem mass spectrometry (ICP-MS/MS) has provided a new analytical approach for in situ β-decay isotopic dating, including Rb–Sr, Lu–Hf, K–Ca, and Re–Os. In this study, an i CAP ICP-MS/MS coupled with a 193 nm G2 laser and N₂O as the reaction gas was employed (Fig. 1). Based on systematic characterization of the reaction products between N₂O and Re and Os, the reaction gas flow rates were optimized, and in situ Re–Os dating of molybdenite was established.

This present protocol was applied to Re–Os age determinations of various molybdenite with ages ranging from 2.7 Ga to 0.15 Ga. The obtained results are consistent with those from ID-NTIMS or ID-ICP-MS. The study demonstrates that reliable in situ Re–Os ages can be achieved when the Re content of molybdenite more than 5 ppm. When combined with trace-element geochemical characteristics, in situ Re–Os dating of molybdenite provides important constraints on the timing of mineralization and genetic processes, offering valuable insights into the detailed geological evolution of metal sulfide deposits.

Figure 1. Schematic illustration of the basic principles of plasma tandem mass spectrometry. In the Re–Os system, reactions between rhenium (Re) and osmium (Os) with nitrous oxide (N₂O) produce oxide species exhibiting a mass shift of 64 amu. However, a small proportion of Re also reacts with N₂O to form ¹⁸⁷ReO₄, which interferes with the target ion ¹⁸⁷OsO₄, requiring appropriate interference correction.