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

Abstract

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, ¹⁶O, ¹⁸O; and (2) ¹⁶O, ¹⁶O¹H, ¹⁸O, ¹⁷O¹H, ¹⁶O²H. 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 ¹⁸O/¹⁶O ratio in static multi-collector mode without trading off its analytical precision, and ¹H/¹⁶O (or ¹⁶O¹H/¹⁶O) and ²H/¹H (or ¹⁶O²H/¹⁶O¹H) 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 δ¹⁸O and δD is better than 0.56‰ (2SD) and 54‰ (2SD), respectively. After eliminating the outlier (beyond 3SD error), the external precision of ¹⁶O¹H/¹⁶O or ¹H/¹⁶O 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 ¹⁶O¹H/¹⁶O or ¹H/¹⁶O 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.