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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 μm) 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−1 for fine‐grained lithologies and ~1 °C h−1 for coarse‐grained lithologies. Mg‐rich olivines (~Fo64) were found only in fine‐grained lithologies. Crystallization of NWA 7203 started in the fine‐grained lithologies with Mg‐rich olivine grains acting as seeds for crystallization. Coarse‐grained 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 ± 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]