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[SCG51-07] Geochronology of ultramafic complexes based on Pt–Os and Re–Os isotope systematics of platinum-group minerals
Keywords:LA-ICP-MS, Platinum-group elements, Os isotope systematics, Kurosegawa belt
Elucidation of hidden geophysical and geochemical processes within the lower crust and mantle of the Earth have been retarded due to the insufficiency of basic chronological data involved in the formation history of ultramafic complexes both in quality and quantity. This is mainly because of the low abundances of the elements commonly used for radioisotope dating methods, since most of them demonstrate lithophile and incompatible signature. Thus, the age determination of ultramafic rocks is difficult due to the subtle deviation of isotope ratios related to radioisotope dating methods, or the chronological data preserved in ultramafic rocks are susceptible to reset by alteration and metamorphic events with contamination of crustal materials, which are enriched in incompatible elements.
To derive primary chronological information of ultramafic complexes, lithophile elements must be excluded from chronological studies, and, in this study, the 190Pt–186Os and 187Re–187Os isotope systematics composed of siderophile elements are investigated. The combination of both the radioisotope systematics can be applied to the age determination of platinum-group minerals (PGMs), such as isoferroplatinum and tulameenite, occurring in ultramafic complexes.[1] Based on the geochemical nature of PGMs as enriched Pt and depleted Re compared to Os, the formation ages of PGMs can be acquired from the 190Pt–186Os isotope systematics, and the geochemical origins of PGMs can be constrained from the 187Re–187Os isotope systematics.
For the reliable determination of ages, in situ Os isotope analysis of PGMs with careful observation of internal textures reflecting formation histories is important. Then, in this study, a new analytical protocol for in situ Os isotope analysis using laser ablation ICP mass spectrometry (LA-ICP-MS) is developed. The LA system used in this study is composed of femtosecond ultraviolet laser for high sampling efficiency of metal samples,[2] whereas in situ Os isotope analysis is conventionally conducted by using ICP-MS combined with a nanosecond LA system.[3] For high-precision isotope ratio analysis, Os and W isotopes (188Os, 187Os+187Re, 186Os+186W, 184W, and 182W) are simultaneously measured by the multiple-collection (MC) system with two Daly detectors and three electron multipliers. Moreover, the trace-elemental analysis, especially for chalcophile elements, is conducted for the same analysis spots of minerals by diverting particles generated through LA and introducing them into quadrupole-based ICP-MS in addition to MC-ICP-MS (i.e., laser ablation-split stream; LASS).
The developed analytical protocol based on LASS-ICP-MS was applied to the age determination of PGMs occurring in the Kurosegawa belt. The PGMs are found as placer minerals emanating from a pyroxenite in the Kurosegawa belt,[4] and this is important for the elucidation of the early stage of the geotectonic history in Japan. Prior to the in situ age determination, elemental imaging analysis using LA-ICP-MS was performed to the PGMs. Based on the elemental maps, the analysed PGM grains have the core portion of isoferroplatinum, and the rim portion of tulameenite, which contains Cu as a major component and is enriched in other chalcophile elements as well. Subsequently, the Os isotope analysis was carried out for the distinct areas of the core-shell structure revealed by the elemental imaging analysis. Based on the 190Pt–186Os and 187Re–187Os isotope systematics, the formation timing of the PGMs and the geochemical reservoir of the source material can be constrained, and in the presentation, chronological evolution of the Kurosegawa belt will be discussed.
[1] Luguet, A. et al., 2019, Geochem. Perspect. [2] Russo, R.E. et al., 2002, JAAS. [3] Hirata, T. et al., 1998, Chem. Geol. [4] Nishio-Hamane, D. et al., 2019, JMPS.
To derive primary chronological information of ultramafic complexes, lithophile elements must be excluded from chronological studies, and, in this study, the 190Pt–186Os and 187Re–187Os isotope systematics composed of siderophile elements are investigated. The combination of both the radioisotope systematics can be applied to the age determination of platinum-group minerals (PGMs), such as isoferroplatinum and tulameenite, occurring in ultramafic complexes.[1] Based on the geochemical nature of PGMs as enriched Pt and depleted Re compared to Os, the formation ages of PGMs can be acquired from the 190Pt–186Os isotope systematics, and the geochemical origins of PGMs can be constrained from the 187Re–187Os isotope systematics.
For the reliable determination of ages, in situ Os isotope analysis of PGMs with careful observation of internal textures reflecting formation histories is important. Then, in this study, a new analytical protocol for in situ Os isotope analysis using laser ablation ICP mass spectrometry (LA-ICP-MS) is developed. The LA system used in this study is composed of femtosecond ultraviolet laser for high sampling efficiency of metal samples,[2] whereas in situ Os isotope analysis is conventionally conducted by using ICP-MS combined with a nanosecond LA system.[3] For high-precision isotope ratio analysis, Os and W isotopes (188Os, 187Os+187Re, 186Os+186W, 184W, and 182W) are simultaneously measured by the multiple-collection (MC) system with two Daly detectors and three electron multipliers. Moreover, the trace-elemental analysis, especially for chalcophile elements, is conducted for the same analysis spots of minerals by diverting particles generated through LA and introducing them into quadrupole-based ICP-MS in addition to MC-ICP-MS (i.e., laser ablation-split stream; LASS).
The developed analytical protocol based on LASS-ICP-MS was applied to the age determination of PGMs occurring in the Kurosegawa belt. The PGMs are found as placer minerals emanating from a pyroxenite in the Kurosegawa belt,[4] and this is important for the elucidation of the early stage of the geotectonic history in Japan. Prior to the in situ age determination, elemental imaging analysis using LA-ICP-MS was performed to the PGMs. Based on the elemental maps, the analysed PGM grains have the core portion of isoferroplatinum, and the rim portion of tulameenite, which contains Cu as a major component and is enriched in other chalcophile elements as well. Subsequently, the Os isotope analysis was carried out for the distinct areas of the core-shell structure revealed by the elemental imaging analysis. Based on the 190Pt–186Os and 187Re–187Os isotope systematics, the formation timing of the PGMs and the geochemical reservoir of the source material can be constrained, and in the presentation, chronological evolution of the Kurosegawa belt will be discussed.
[1] Luguet, A. et al., 2019, Geochem. Perspect. [2] Russo, R.E. et al., 2002, JAAS. [3] Hirata, T. et al., 1998, Chem. Geol. [4] Nishio-Hamane, D. et al., 2019, JMPS.