Geochemical monitoring of groundwater in seismically-active regions has been carried out since 1970s. Precursors were well documented, but often criticized for anecdotal or fragmentary signals, and for lacking a clear physico-chemical explanation for these anomalies. Here we report – as potential seismic precursor – oxygen isotopic ratio anomalies of +0.24‰ relative to the local background measured in groundwater, a few months before the Tottori earthquake (M 6.6) in Southwest Japan. Samples were deep groundwater located 5 km west of the epicenter, packed in bottles and distributed as drinking water between September 2015 and July 2017, a time frame which covers the pre- and post-event. Small but substantial increase of 0.07‰ was observed soon after the earthquake. Laboratory crushing experiments of aquifer rock aimed to simulating rock deformation under strain and tensile stresses were carried out. Measured helium degassing from the rock and 18O-shift suggest that the co-seismic oxygen anomalies are directly related to volumetric strain changes. The findings provide a plausible physico-chemical basis to explain geochemical anomalies in water and may be useful in future earthquake prediction research.

The δ17O and δ18O values of a number of terrestrial minerals and rocks have been determined using laser fluorination method worldwide. For the comprehensive and congruous interpretation of oxygen isotope data, the δ-values should be normalized by the two-point method (i.e., the VSMOW-SLAP scale) to eliminate inter-laboratory bias. In this study, the δ17O and δ18O values of VSMOW and SLAP were measured to calibrate our laboratory working standard O2 gas. The O2 gas liberated from the water samples was purified using the preparation line normally employed for solid samples, and analyzed by the same mass spectrometer. From the analyses of VSMOW and SLAP, the oxygen isotope compositions of the international silicate standards (UWG2 garnet, NBS28 quartz, and San Carlos olivine) were normalized to the VSMOW-SLAP scale (two-point calibration), and then the Δ′17O values were determined. Using the δ-values obtained in this way, the inter-laboratory discrepancy of the δ17O and δ18O results of the silicate standards could be reduced. The VSMOW-SLAP scaling for δ17O and δ18O analysis of silicates provides the most effective way to obtain accurate and precise data. In reporting the Δ′17O values, it is important to make the choice of the reference fractionation line into account because the Δ′17O value is quite variable owing to the slope and y-intercept of the linear relation of the δ-values. The reference fractionation line obtained from the measurement of the low- and high-δ18O reference silicates would help to compare ∆′17O values. We confirmed that the ∆′17O results of the international silicate standards based on the two-point silicate reference line were consistent with the results from other laboratories.

Rationale The three oxygen isotopes in terrestrial/extraterrestrial silicates can provide geochemical and cosmochemical information about their origin and secondary processes that result from isotopic exchange. A laser fluorination technique has been widely used to extract oxygen from silicates for δ17O and δ18O measurements by isotope ratio mass spectrometry. Continued improvement of the techniques is still important for high‐precision measurement of oxygen‐isotopic ratios. Methods We adopted an automated lasing technique to obtain reproducible fluorination of silicates using a CO2 laser‐BrF5 fluorination system connected online to an isotope ratio mass spectrometer. The automated lasing technique enables us to perform high‐precision analysis of the three oxygen isotopes of typical reference materials (e.g., UWG2 garnet, NBS28 quartz and San Carlos olivine) and in‐house references (mid‐ocean ridge basalt glass and obsidian). The technique uses a built‐in application of laser control with which the laser power can be varied in a programmed manner with a defocused beam which is in a fixed position. Results The oxygen isotope ratios of some international reference materials analyzed by the manual lasing technique were found to be isotopically lighter with wider variations in δ18O values, whereas those measured by the automated lasing technique gave better reproducibility (less than 0.2‰, 2SD). The Δ17O values, an excess of the δ17O value relative to the fractionation line, also showed high reproducibility (±0.02‰, 2SD). Conclusions The system described herein provides high‐precision δ17O and δ18O measurements of silicate materials. The use of the automated lasing technique followed by careful and controlled purification procedures is preferred to achieve satisfactory isotopic ratio results.

Northwest Africa (NWA) 7203 is a quenched angrite, showing mineralogical features typically not present in other quenched angrites. NWA 7203 exhibits textures whose grain size varies from fine grains (<10 lm) to coarse grains (~3 mm), while other quenched angrites show only single-sized textures. Fine-grained and coarse-grained lithologies have nearly the same bulk compositions. Cooling rates were estimated to be ~80 °C h À1 for finegrained lithologies and ~1 °C h À1 for coarse-grained lithologies. Mg-rich olivines (~Fo 64 ) were found only in fine-grained lithologies. Crystallization of NWA 7203 started in the finegrained lithologies with Mg-rich olivine grains acting as seeds for crystallization. Coarsegrained lithologies were subsequently formed under conditions of slower cooling. NWA 7203 shows clear shock metamorphic textures unlike other quenched angrites except for NWA 1670. We confirm that the oxygen isotopic ratios of NWA 7203 plot on the angrite fractionation line within uncertainty. However, the obtained Pb-Pb age of NWA 7203 is 4543 AE 19 Ma, younger than the ages of other quenched angrites, which might be a result of disturbance by shock metamorphism. The finding of shock metamorphism of NWA 7203 suggests that some angrites might be derived from asteroids that remained large (>10 km in diameter) during the late heavy bombardment.

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