SIMS simultaneous measurement of oxygen-hydrogen isotopes and water content for hydrous geological samples

摘要

In this study, we developed two new SIMS (secondary ion mass spectrometry) analytical protocols to simultaneously measure oxygen-hydrogen (O-H) isotopic compositions and water content for hydrous geological samples. These two protocols involve the measurement of two sets of secondary ion contents: (1) ~1H, ~2H, ~(16)O. ~(18)O; and (2) ~(16)O, ~916)OlH, ~(18)O, ~(17)O~1H. ~(16)O~2H. Both measurements utilize a hybrid dynamic multi-collector system of CAMECA IMS 1280-HR, which benefits from both the static multi-collector mode and peak-hopping mono-collector mode. These new methods can simultaneously measure (with high-precision) the ~(18)O/~(16)O ratio in static multi-collector mode without trading off its analytical precision, and ~1H/~(16)O (or ~(16)H/~(16)O) and ~2H/~1 (or ~(16)O~2H/~(16)O~1H) ratios in conventional peak-hopping mono-collector mode. Three glass samples (LBS7H, LBS5H and LBS6H-) with known water contents and two apatite samples (Kovdor, Durango) with known oxygen-hydrogen isotopes and water content were measured to verify the protocols' reliability. The olivine crystal San Carlos with ~1 ppmw water content was used for background monitoring. For the apatite samples, the external precision (spot-to-spot reproducibility) for 8~(18)O and 5D is better than 0.56%o (2SD) and 54‰ (2SD), respectively. After eliminating the outlier (beyond 3SD error), the external precision of ~(16)O~1H/~(16)O or ~1H~(16)O ratio improves to 10.27% (2SD). For the glass samples, the water content calibration curves, which were constructed by comparison of the known water content with the SIMS measured ~(16)H/~(16)O or ~1H/~(16)O ratios, yielded good correlations. It is noteworthy that the apatite and glass samples can have a uniform water content calibration curve for protocol 1, but not for protocol 2, indicating different matrix effects for the two protocols.