09:30 〜 09:45
[SCG51-03] TIMS Pb–Pb dating of ulvospinel in the angrite meteorite NWA 4590
キーワード：表面電離型質量分析計、鉛–鉛年代、鉄–チタン酸化物、ウルボスピネル、アングライト
Introduction: Fe-Ti oxides are complex ternary systems in which FeO-Fe2O3-TiO2 are in solid solutions. For Fe-Ti oxides, three solid solution series exist; (i) ulvospinel (Fe2TiO4)-magnetite (Fe3O4) series, (ii) ilmenite (Fe2TiO3)-hematite (Fe2O3) series, (iii) ferropseudobrookite (FeTi2O5)-pseudobrookite (Fe2TiO5) series. The complexity and diversity of these chemical compositions are mainly due to the variation in the ratio of ferric to ferrous iron due to changes in oxygen fugacity. It has been pointed out that Fe-Ti oxides have potential as a U–Pb chronometer (Burton and O’Nions, 1990; Noyes et al., 2011), but previous studies have been limited due to their low U concentrations (< 100 ppb) and complex relationships with coexisting minerals. Furthermore, there are no previous studies on Pb–Pb dating of Fe-Ti oxides in extraterrestrial samples. Considering that Fe-Ti oxides have low U and radiogenic Pb concentrations, the main challenge for high–precision Pb–Pb chronology of Fe-Ti oxides is to obtain Pb isotope compositions of Fe-Ti oxides that are free from contaminant Pb derived from impurities such as other minerals. In this study, we have developed a new method for extracting Pb from Fe-Ti oxides by acid leaching method using HNO3 + HCl and applied it to ulvospinel in the angrite meteorite, Northwest Africa (NWA) 4590.
Sample and Method: Northwest Africa (NWA) 4590 is a small, very fresh, fusion-crusted fragmented single stone of 213 g. It has a unique coarse-grained, cumulate texture and contains ulvospinel with a high abundance of 18 vol.% (Kuehner and Irving, 2007). Pb–Pb ages of pyroxenes and silico-phosphate in this meteorite were precisely determined to be 4558.07 ± 0.31 Ma and 4557.73 ± 0.26 Ma, respectively (Amelin et al., 2011; Huyskens et al., in prep). Given the rapid cooling rate of ~0.15 °C/year (Mikouchi et al., 2011), it is expected that ulvospinel has an identical Pb–Pb age to that of pyroxenes and silica-phosphate, thereby allowing for the assessment of the accuracy of our Fe-Ti oxide Pb–Pb dating. We analyzed 8 ulvospinel fractions (1–2 mg) from NWA 4590. Sample fractions were leached in 0.5 M HNO3 with ultrasonic agitation (L-1), hot 6 M HNO3 (L-2), and ulvospinel was decomposed by hot 12 M HCl (L-3). Some insoluble minerals were further digested in a mixture of 25 M HF and 12 M HNO3 (R). Pb isotopes were measured on a Thermo Scientific TRITON Plus TIMS at the University of California, Davis, USA.
Results & Discussion: Pb content of ulvospinel was 0.026–0.151 ppm. Pb-isotopic composition of ulvospinel was radiogenic (206Pb/204Pb between 175–1750), albeit lower than those of pyroxene (206Pb/204Pb between 2160 to over 100,000; Amelin, 2008). Model 232Th/238U ratios calculated from radiogenic 208Pb/206Pb ratios: ulvospinel (7 data from L-3, 3 data from L-2) has 232Th/238U = 0.01–0.43, which was significantly lower than the bulk rock value (3.39 ± 0.17; Tissot et al., 2017). Th/U in Fe-Ti oxides is expected to be lower than the value of the bulk rock because Fe-Ti oxides selectively incorporate U rather than Th (e.g., Klemme et al., 2006; Snape et al., 2022). Hence, the low Th/U ratio in L-3 (and some L-2) indicates that radiogenic Pb from ulvospinel was successfully extracted by HNO3 washing and HCl decomposition method.
Pb isotope data from ulvospinel yielded a precise Pb–Pb isochron age of 4558.09 ± 0.88 Ma (n = 10, MSWD = 1.4); where the 238U/235U isotope ratio from the bulk rock value of 137.772 ± 0.025 for NWA 4590 (Tissot et al., 2017) was taken for age calculation. The isochron passes through primordial Pb (Tatsumoto et al., 1973). The Pb–Pb age of ulvospinel agrees within error with the Pb–Pb ages of pyroxenes and silica-phosphate (Amelin et al., 2011; Huyskens et al., in prep).
Conclusion: This is the first report of Pb–Pb dating of extra-terrestrial Fe-Ti oxide. The efficient decomposition of Fe-Ti oxides can be performed by HCl. Ulvospinel is a major U carrier with low Th/U in the angrite NWA 4590 and has radiogenic Pb isotopic composition. The Pb–Pb age of 4558.09 ± 0.88 Ma is consistent with the ages of pyroxenes and silico-phosphate obtained in previous studies.
Sample and Method: Northwest Africa (NWA) 4590 is a small, very fresh, fusion-crusted fragmented single stone of 213 g. It has a unique coarse-grained, cumulate texture and contains ulvospinel with a high abundance of 18 vol.% (Kuehner and Irving, 2007). Pb–Pb ages of pyroxenes and silico-phosphate in this meteorite were precisely determined to be 4558.07 ± 0.31 Ma and 4557.73 ± 0.26 Ma, respectively (Amelin et al., 2011; Huyskens et al., in prep). Given the rapid cooling rate of ~0.15 °C/year (Mikouchi et al., 2011), it is expected that ulvospinel has an identical Pb–Pb age to that of pyroxenes and silica-phosphate, thereby allowing for the assessment of the accuracy of our Fe-Ti oxide Pb–Pb dating. We analyzed 8 ulvospinel fractions (1–2 mg) from NWA 4590. Sample fractions were leached in 0.5 M HNO3 with ultrasonic agitation (L-1), hot 6 M HNO3 (L-2), and ulvospinel was decomposed by hot 12 M HCl (L-3). Some insoluble minerals were further digested in a mixture of 25 M HF and 12 M HNO3 (R). Pb isotopes were measured on a Thermo Scientific TRITON Plus TIMS at the University of California, Davis, USA.
Results & Discussion: Pb content of ulvospinel was 0.026–0.151 ppm. Pb-isotopic composition of ulvospinel was radiogenic (206Pb/204Pb between 175–1750), albeit lower than those of pyroxene (206Pb/204Pb between 2160 to over 100,000; Amelin, 2008). Model 232Th/238U ratios calculated from radiogenic 208Pb/206Pb ratios: ulvospinel (7 data from L-3, 3 data from L-2) has 232Th/238U = 0.01–0.43, which was significantly lower than the bulk rock value (3.39 ± 0.17; Tissot et al., 2017). Th/U in Fe-Ti oxides is expected to be lower than the value of the bulk rock because Fe-Ti oxides selectively incorporate U rather than Th (e.g., Klemme et al., 2006; Snape et al., 2022). Hence, the low Th/U ratio in L-3 (and some L-2) indicates that radiogenic Pb from ulvospinel was successfully extracted by HNO3 washing and HCl decomposition method.
Pb isotope data from ulvospinel yielded a precise Pb–Pb isochron age of 4558.09 ± 0.88 Ma (n = 10, MSWD = 1.4); where the 238U/235U isotope ratio from the bulk rock value of 137.772 ± 0.025 for NWA 4590 (Tissot et al., 2017) was taken for age calculation. The isochron passes through primordial Pb (Tatsumoto et al., 1973). The Pb–Pb age of ulvospinel agrees within error with the Pb–Pb ages of pyroxenes and silica-phosphate (Amelin et al., 2011; Huyskens et al., in prep).
Conclusion: This is the first report of Pb–Pb dating of extra-terrestrial Fe-Ti oxide. The efficient decomposition of Fe-Ti oxides can be performed by HCl. Ulvospinel is a major U carrier with low Th/U in the angrite NWA 4590 and has radiogenic Pb isotopic composition. The Pb–Pb age of 4558.09 ± 0.88 Ma is consistent with the ages of pyroxenes and silico-phosphate obtained in previous studies.