17:15 〜 18:45
[MZZ45-P02] Toward the determination of the initial 146Sm abundance of outer Solar System
キーワード：同位体
Recent studies on isotopic compositions of various elements such as O, Mo, Cr, and Ti have confirmed the isotopic dichotomy between carbonaceous (CC) and non-carbonaceous (NC) meteorites, which presumably originated from the outer and inner Solar System, respectively. Some ungrouped primitive achondrites are known to be classified in the CC meteorites depending on their isotopic compositions, unlike most achondrites derived from differentiated parent bodies. Northwest Africa (NWA) 6693 is one of the carbonaceous achondrites, showing cumulate texture and isotopic affinity to CR chondrites.
Samarium-146 (146Sm) is an extinct nuclide having a half-life of 103 million years. The accurate and precise initial 146Sm/144Sm ratio of the Solar System is needed for utilizing the 146Sm-142Nd chromometer to date the planetary material evolution in the early Solar System. The initial 146Sm/144Sm ratio of 0.00828 ± 0.00044 was obtained from a Ca-Al rich inclusion (CAI) in the Allende meteorite [1]. Another initial ratio was obtained from the oldest andesitic meteorite, Erg Chech (EC) 002, which yielded a 146Sm/144Sm ratio of 0.00830±0.00032 [2]. The objective of this study is to obtain the initial 146Sm/144Sm ratio from the carbonaceous achondrite NWA 6693 and compare it with those obtained from CAI and NC meteorites.
In this study, mineral separation was conducted for NWA 6693 to obtain the 146Sm-142Nd mineral isochron. The 3 g specimens of NWA 6693 were crushed in an agate mortal with a pestle. The powdered samples were sieved using nylon meshes having different mesh sizes (178–198 µm, 154 µm, 109 µm, 77 µm). In addition to the whole rock sample (<77 µm), the sieved grains from several fractions were further separated into mineral phases depending on their strength of magnetic properties by using a neodymium magnet. These procedures made fractions having different magnetic properties, which are expected to correspond to orthopyroxene-olivine (magnetic fraction) and plagioclase (non-magnetic fraction). To confirm the constituent minerals in each fraction, the representative grains were handpicked and made one sample from non-magnetic fraction and three samples from magnetic fraction, followed by dissolving with a mixture of concentrated HF and HNO3 in PFA vials. The abundances of rare-earth elements (REEs) of dissolved samples were measured with a triple quadrupole ICP-MS instrument (iCAP TQ, Thermo Fisher Scientific). The non-magnetic fraction yielded the CI-normalized REE pattern showing light REE (LREE) enriched trend with positive Eu anomaly, suggesting that this fraction is dominantly composed of plagioclase. In contrast, the REE patterns of two magnetic fractions yielded flat to LREE enriched trends with negative Eu anomalies. These trends are different from the REE pattern of orthopyroxene characterized by heavy REE enriched trend with a negative Eu anomaly [3]. This result suggests that the magnetic fractions contain not only orthopyroxene and olivine but also other mineral, possibly phosphate with an LREE enriched trend with negative Eu anomaly and high REE concentrations.
References:
[1] Marks, N.E., Borg, L.E., Hutcheon, I.D., Jacobsen, B., & Clayton, R.N. (2014). Earth Planet. Sci. Lett., 405, 15-24. [2] Fang, L., Frossard, P., Boyet, M., Bouvier, A., Barrat, J. A., Chaussidon, M., & Moynier, F. (2022). Proc. Nat. Acad. Sci., 119(12), e2120933119. [3] Hibiya, Y., Archer, G. J., Tanaka, R., Sanborn, M. E., Sato, Y., Iizuka, T., ... & Irving, A. J. (2019). Geochim. Cosmochim. Acta, 245, 597-627.
Samarium-146 (146Sm) is an extinct nuclide having a half-life of 103 million years. The accurate and precise initial 146Sm/144Sm ratio of the Solar System is needed for utilizing the 146Sm-142Nd chromometer to date the planetary material evolution in the early Solar System. The initial 146Sm/144Sm ratio of 0.00828 ± 0.00044 was obtained from a Ca-Al rich inclusion (CAI) in the Allende meteorite [1]. Another initial ratio was obtained from the oldest andesitic meteorite, Erg Chech (EC) 002, which yielded a 146Sm/144Sm ratio of 0.00830±0.00032 [2]. The objective of this study is to obtain the initial 146Sm/144Sm ratio from the carbonaceous achondrite NWA 6693 and compare it with those obtained from CAI and NC meteorites.
In this study, mineral separation was conducted for NWA 6693 to obtain the 146Sm-142Nd mineral isochron. The 3 g specimens of NWA 6693 were crushed in an agate mortal with a pestle. The powdered samples were sieved using nylon meshes having different mesh sizes (178–198 µm, 154 µm, 109 µm, 77 µm). In addition to the whole rock sample (<77 µm), the sieved grains from several fractions were further separated into mineral phases depending on their strength of magnetic properties by using a neodymium magnet. These procedures made fractions having different magnetic properties, which are expected to correspond to orthopyroxene-olivine (magnetic fraction) and plagioclase (non-magnetic fraction). To confirm the constituent minerals in each fraction, the representative grains were handpicked and made one sample from non-magnetic fraction and three samples from magnetic fraction, followed by dissolving with a mixture of concentrated HF and HNO3 in PFA vials. The abundances of rare-earth elements (REEs) of dissolved samples were measured with a triple quadrupole ICP-MS instrument (iCAP TQ, Thermo Fisher Scientific). The non-magnetic fraction yielded the CI-normalized REE pattern showing light REE (LREE) enriched trend with positive Eu anomaly, suggesting that this fraction is dominantly composed of plagioclase. In contrast, the REE patterns of two magnetic fractions yielded flat to LREE enriched trends with negative Eu anomalies. These trends are different from the REE pattern of orthopyroxene characterized by heavy REE enriched trend with a negative Eu anomaly [3]. This result suggests that the magnetic fractions contain not only orthopyroxene and olivine but also other mineral, possibly phosphate with an LREE enriched trend with negative Eu anomaly and high REE concentrations.
References:
[1] Marks, N.E., Borg, L.E., Hutcheon, I.D., Jacobsen, B., & Clayton, R.N. (2014). Earth Planet. Sci. Lett., 405, 15-24. [2] Fang, L., Frossard, P., Boyet, M., Bouvier, A., Barrat, J. A., Chaussidon, M., & Moynier, F. (2022). Proc. Nat. Acad. Sci., 119(12), e2120933119. [3] Hibiya, Y., Archer, G. J., Tanaka, R., Sanborn, M. E., Sato, Y., Iizuka, T., ... & Irving, A. J. (2019). Geochim. Cosmochim. Acta, 245, 597-627.