11:30 〜 11:45
[MZZ45-03] Evaluation of the initial 92Nb abundance in the inner Solar System
キーワード：ニオブ-92、鉄隕石、ルチル、Nb-Zr年代測定法、消滅核種
The short-lived isotope 92Nb decays to 92Zr with a half-life of 37 Ma. The 92Nb-92Zr decay system has been recognized as a promising tool to provide chronological information on the evolution of planetary materials over a relatively long period in the early Solar System [1,2]. For applying this chronometer to various types of meteorites, it is important to evaluate the heterogeneity of 92Nb distribution in the early Solar System. Meteoritic rutiles are a suitable mineral for 92Nb-92Zr dating because of their high Zr and Nb contents with large variations. A recent study utilizing an internal isochron of mesosiderite rutiles and zircons indicates that the 92Nb/93Nb ratio of the formation region of asteroid 4 Vesta started with (1.66 ± 0.10) × 10-5 (2σ) [3]. In this study, we used rutiles from an IIE iron meteorite to evaluate the initial 92Nb/93Nb ratio for the IIE parent body which is likely related to the parent bodies of H chondrites [4] inside Vesta in the protoplanetary disk.
Thirty-two rutile grains were collected from the Mile iron meteorite and dissolved with a mixture of concentrated HF and HNO3 in 3 mL PFA vials individually to check the Nb/Zr ratio and Zr amount in each grain. Using the solutions less than 1%, the Nb/Zr ratio and Zr amount in each grain were checked with a triple quadrupole ICP-MS (iCAP TQ, Thermo Fisher Scientific). Five rutile fractions were prepared to maximize the 93Nb/90Zr variation (93Nb/90Zr = 0.8, 2.8, 3.4, 5.4, 16.2). The Zr of rutile fractions and reference materials (BHVO-2 and NIST SRM 154c (synthetic TiO2 powder)) were separated using a three-stage ion-exchange procedure [2,5]. After ascertaining the absence of interferences from argides and isobaric isotopes, the Zr isotopic ratios of BHVO-2, NIST SRM 154c and rutile samples were measured on a Thermo Scientific Neptune Plus multicollector ICP-MS (MC-ICP-MS) coupled with an Aridus II introduction system installed at ETH Zurich. Each sample measurement was bracketed by analyses of a 30 ppb Zr standard solution (NIST SRM 3169).
The ε92Zr values of five rutile fractions are 0.19 ± 0.28, 0.84 ± 0.21, 1.00 ± 0.31, 1.72 ± 0.26, and 5.27 ± 0.21 (2σ). The rutile fractions show the largest variation of ε92Zr value obtained so far among published MC-ICP-MS data. These data define a single, well defined isochron yielding a 92Nb/93Nb ratio of (1.10 ± 0.07) × 10-5 (2σ) at the time of rutile formation (Fig. 1). The isochron indicates that rutiles in the Miles iron meteorite formed at the same time and that the Nb-Zr decay system was not modified by later impact events. Using the initial 92Nb/93Nb ratio estimated from mesosiderites for the inner Solar System, the rutile Nb–Zr data yields a 92Nb-92Zr age of 22 ± 2 Ma after the formation of Ca- and Al-rich inclusions (4,567.3 Ma [6]). This corresponds to an absolute age of 4,545 ± 2 Ma, which excellently agrees with the 207Pb–206Pb age of Zr-oxide and phosphate minerals formed during the melting event generating silicate inclusions in the iron meteorite (4,542.3 ± 4.0 Ma (2σ) [7]). Using the absolute age of 207Pb–206Pb age yields the initial 92Nb/93Nb ratio in parent body formation region of IIE iron meteorites, which is the inner region than asteroid Vesta, is calculated to be (1.76 ± 0.12) × 10-5 (2σ). This is well consistent with the initial 92Nb/93Nb ratio in Vesta formation region ((1.66 ± 0.10) × 10-5) [3], suggesting that the 92Nb/93Nb ratio was homogeneous in the early inner Solar System.
References: [1] M. Schönbächler et al., Science, vol. 295, pp. 1705–1708 (2002). [2] T. Iizuka et al., Earth Planet. Sci. Lett., vol. 439, pp. 172–181 (2016). [3] M. K. Haba et al., PNAS, vol.118, e2017750118, (2021). [4] K. H. McDermott et al., Geochim. Cosmochim. Acta, vol. 173, pp. 97–113 (2016). [5] M. Schönbächler et al., Analyst (Lond.), vol. 129, pp. 32–37 (2004). [6] J. N. Connelly et al., Science, vol. 338, pp. 651–655 (2012). [7] R. S. Kirby et al., Geochim. Cosmochim. Acta, vol. 339, pp. 157–172 (2022).
Thirty-two rutile grains were collected from the Mile iron meteorite and dissolved with a mixture of concentrated HF and HNO3 in 3 mL PFA vials individually to check the Nb/Zr ratio and Zr amount in each grain. Using the solutions less than 1%, the Nb/Zr ratio and Zr amount in each grain were checked with a triple quadrupole ICP-MS (iCAP TQ, Thermo Fisher Scientific). Five rutile fractions were prepared to maximize the 93Nb/90Zr variation (93Nb/90Zr = 0.8, 2.8, 3.4, 5.4, 16.2). The Zr of rutile fractions and reference materials (BHVO-2 and NIST SRM 154c (synthetic TiO2 powder)) were separated using a three-stage ion-exchange procedure [2,5]. After ascertaining the absence of interferences from argides and isobaric isotopes, the Zr isotopic ratios of BHVO-2, NIST SRM 154c and rutile samples were measured on a Thermo Scientific Neptune Plus multicollector ICP-MS (MC-ICP-MS) coupled with an Aridus II introduction system installed at ETH Zurich. Each sample measurement was bracketed by analyses of a 30 ppb Zr standard solution (NIST SRM 3169).
The ε92Zr values of five rutile fractions are 0.19 ± 0.28, 0.84 ± 0.21, 1.00 ± 0.31, 1.72 ± 0.26, and 5.27 ± 0.21 (2σ). The rutile fractions show the largest variation of ε92Zr value obtained so far among published MC-ICP-MS data. These data define a single, well defined isochron yielding a 92Nb/93Nb ratio of (1.10 ± 0.07) × 10-5 (2σ) at the time of rutile formation (Fig. 1). The isochron indicates that rutiles in the Miles iron meteorite formed at the same time and that the Nb-Zr decay system was not modified by later impact events. Using the initial 92Nb/93Nb ratio estimated from mesosiderites for the inner Solar System, the rutile Nb–Zr data yields a 92Nb-92Zr age of 22 ± 2 Ma after the formation of Ca- and Al-rich inclusions (4,567.3 Ma [6]). This corresponds to an absolute age of 4,545 ± 2 Ma, which excellently agrees with the 207Pb–206Pb age of Zr-oxide and phosphate minerals formed during the melting event generating silicate inclusions in the iron meteorite (4,542.3 ± 4.0 Ma (2σ) [7]). Using the absolute age of 207Pb–206Pb age yields the initial 92Nb/93Nb ratio in parent body formation region of IIE iron meteorites, which is the inner region than asteroid Vesta, is calculated to be (1.76 ± 0.12) × 10-5 (2σ). This is well consistent with the initial 92Nb/93Nb ratio in Vesta formation region ((1.66 ± 0.10) × 10-5) [3], suggesting that the 92Nb/93Nb ratio was homogeneous in the early inner Solar System.
References: [1] M. Schönbächler et al., Science, vol. 295, pp. 1705–1708 (2002). [2] T. Iizuka et al., Earth Planet. Sci. Lett., vol. 439, pp. 172–181 (2016). [3] M. K. Haba et al., PNAS, vol.118, e2017750118, (2021). [4] K. H. McDermott et al., Geochim. Cosmochim. Acta, vol. 173, pp. 97–113 (2016). [5] M. Schönbächler et al., Analyst (Lond.), vol. 129, pp. 32–37 (2004). [6] J. N. Connelly et al., Science, vol. 338, pp. 651–655 (2012). [7] R. S. Kirby et al., Geochim. Cosmochim. Acta, vol. 339, pp. 157–172 (2022).