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[MTT44-P05] Rutile in the Miles IIE iron meteorite as a potential mineral providing precise 92Nb-92Zr isochron data
Keywords:Iron meteorite, Rutile, Niobium-92, Extinct nuclide
The extinct nuclide 92Nb decays to 92Zr with a half-life of 37 Ma. This decay system has been recognized as a promising tool to provide chronological information in the early Solar System [e.g., 1,2]. The application of this chronometer to iron meteorites that contain Nb-enriched rutiles is particularly useful because dating of iron meteorites is notoriously difficult. A recent study utilizing an internal isochron of mesosiderite rutiles indicates that the 92Nb/93Nb ratio of the Solar System started with (1.66 ± 0.10) × 10-5 [3]. In contrast, a higher initial ratio has been reported for carbonaceous chondrites, which likely originate from farther away from the Sun [4]. Thus, precise determination of this ratio using various meteorites is necessary for the application of the Nb-Zr chronometer to early Solar System materials.
Rutiles in iron meteorites are known to have high Zr and Nb contents with large variations [5], thereby they might allow to describe an internal isochron by themselves. This would result in avoiding some difficulties when utilizing internal isochrons, such as demonstrating the mineral phases for isochrons to have the same formation age and initial 92Zr/90Zr ratio. In this study, we used the Miles IIE iron meteorite whose formation age has been determined as 4542 ± 2 Ma [6] and tried to prepare three rutile fractions having different Nb/Zr ratios to obtain a precise internal isochron.
Occurrence and chemical composition of rutiles in Miles were examined using FE-EPMA at Tokyo Tech. Rutiles were separated from ~100 g of Miles by dissolving the metal parts in concentrated HCl and the silicates in concentrated HNO3-HF mixture. Subsequently, rutile grains with sizes of 60-150 μm in diameter were handpicked from the residues and were investigated on the Nb and Zr concentrations of individual grains using FE-EPMA. The 37 rutile grains were allocated to three fractions depending on their Nb/Zr ratios, so as to contain enough Zr amounts for Zr isotopic analysis (~30 ng) and obtain a large variation of the 92Nb/90Zr ratios. The three fractions were then dissolved in HNO3-HF using Parr bombs. The 93Nb/90Zr ratios of rutiles and reference materials (BHVO-2, NIST SRM 154c) were measured with a Thermo X series 2 quadrupole ICP-MS at Tokyo Tech by taking up a few % sample aliquots after confirming negligible contributions from procedural blanks and from argide interferences on Zr and Nb ions.
The rutiles are located in the silicate inclusions, and most of them exist at the boundary between the silicate and metal parts. Their round shape is quite different from the needle-shape rutiles observed in chondrites [7]. The rutiles in iron meteorites are likely to have formed by decomposition of ilmenite under strongly reducing condition [5]. Therefore, the rutiles in Miles are considered to have formed by decomposition of ilmenite in progenitor and potentially have recorded the formation age if the 92Nb-92Zr decay system was not disturbed by later impacts. The FE-EPMA analysis revealed that the rutiles contain up to 2.1 wt.% Nb and 200-8300 ppm Zr and the 93Nb/90Zr ratios range from 0.05 to 20.1. Then, the three fractions were prepared to have the 93Nb/90Zr ratios of <1, 1-4, and <4 by combining the rutile grains. The preliminary result from ICP-MS analysis shows that the three fractions have the 93Nb/90Zr ratios of 0.58 ± 0.05, 3.1 ± 0.3, and 16.6 ± 0.4 (2σ). This variation is the largest among the previous 92Nb-92Zr studies using internal isochrons [1,2,3,4] and indicates that our method using rutiles would provide a precise initial 92Nb/93Nb ratio of the parent body.
References: [1] Schönbächler et al. (2002) Science, 295, 1705. [2] Iizuka et al. (2016) EPSL, 439, 172. [3] Haba et al. (2021) PNAS, 118, e2017750118. [4] Hibiya et al. (2019) 82nd MetSoc, p. 6370. [5] El Goresy (1971) EPSL, 11, 1. [6] Kirby et al. (2016) 47th LPSC, p1938. [7] Buseck & Keil (1966) Am. Mineral., 51, 1506.
Rutiles in iron meteorites are known to have high Zr and Nb contents with large variations [5], thereby they might allow to describe an internal isochron by themselves. This would result in avoiding some difficulties when utilizing internal isochrons, such as demonstrating the mineral phases for isochrons to have the same formation age and initial 92Zr/90Zr ratio. In this study, we used the Miles IIE iron meteorite whose formation age has been determined as 4542 ± 2 Ma [6] and tried to prepare three rutile fractions having different Nb/Zr ratios to obtain a precise internal isochron.
Occurrence and chemical composition of rutiles in Miles were examined using FE-EPMA at Tokyo Tech. Rutiles were separated from ~100 g of Miles by dissolving the metal parts in concentrated HCl and the silicates in concentrated HNO3-HF mixture. Subsequently, rutile grains with sizes of 60-150 μm in diameter were handpicked from the residues and were investigated on the Nb and Zr concentrations of individual grains using FE-EPMA. The 37 rutile grains were allocated to three fractions depending on their Nb/Zr ratios, so as to contain enough Zr amounts for Zr isotopic analysis (~30 ng) and obtain a large variation of the 92Nb/90Zr ratios. The three fractions were then dissolved in HNO3-HF using Parr bombs. The 93Nb/90Zr ratios of rutiles and reference materials (BHVO-2, NIST SRM 154c) were measured with a Thermo X series 2 quadrupole ICP-MS at Tokyo Tech by taking up a few % sample aliquots after confirming negligible contributions from procedural blanks and from argide interferences on Zr and Nb ions.
The rutiles are located in the silicate inclusions, and most of them exist at the boundary between the silicate and metal parts. Their round shape is quite different from the needle-shape rutiles observed in chondrites [7]. The rutiles in iron meteorites are likely to have formed by decomposition of ilmenite under strongly reducing condition [5]. Therefore, the rutiles in Miles are considered to have formed by decomposition of ilmenite in progenitor and potentially have recorded the formation age if the 92Nb-92Zr decay system was not disturbed by later impacts. The FE-EPMA analysis revealed that the rutiles contain up to 2.1 wt.% Nb and 200-8300 ppm Zr and the 93Nb/90Zr ratios range from 0.05 to 20.1. Then, the three fractions were prepared to have the 93Nb/90Zr ratios of <1, 1-4, and <4 by combining the rutile grains. The preliminary result from ICP-MS analysis shows that the three fractions have the 93Nb/90Zr ratios of 0.58 ± 0.05, 3.1 ± 0.3, and 16.6 ± 0.4 (2σ). This variation is the largest among the previous 92Nb-92Zr studies using internal isochrons [1,2,3,4] and indicates that our method using rutiles would provide a precise initial 92Nb/93Nb ratio of the parent body.
References: [1] Schönbächler et al. (2002) Science, 295, 1705. [2] Iizuka et al. (2016) EPSL, 439, 172. [3] Haba et al. (2021) PNAS, 118, e2017750118. [4] Hibiya et al. (2019) 82nd MetSoc, p. 6370. [5] El Goresy (1971) EPSL, 11, 1. [6] Kirby et al. (2016) 47th LPSC, p1938. [7] Buseck & Keil (1966) Am. Mineral., 51, 1506.