17:15 〜 19:15
[SMP27-P05] Mo isotopes as a proxy for understanding the later accretion of Earth? A case study of Acasta Complex
★Invited Papers
キーワード：early Earth, nucleosynthetic Mo isotopes , later accretion, Acasta complex
The later accretionary process(es), that occurred after Moon forming Giant impact, have significant implications for understanding the general dynamics of the Earth formation and the origin of its habitability. Due to the ongoing convective mixing of the mantle since its formation, it is not easy to find relicts of this history on Earth. Siderophile elements and their isotopes are the best available tools to decipher such information from the terrestrial materials. Mo is a promising proxy due to its moderately siderophile (MSE) character. Mo partitioned largely into the core and the present-day bulk silicate Earth Mo isotopic composition is an integration of the original and the late accreted material [1]. Mass-independent (nucleosynthetic) Mo isotope anomalies are a valuable tool in constraining genetic relationship among meteorites and planetary materials [2, 3].
In this regard, constraining the Mo isotopic composition of the bulk silicate earth (BSE) and mantle retaining proto-Earth signatures, is essential. We focused on the later and the potential target terrestrial samples for this can be classified into two categories: (a) rocks derived from the preserved primordial mantle domains and (b) old Archean rocks. In our attempt, we investigated Archean rocks from Acasta complex. The Acasta Gneiss complex hosts the oldest rocks preserved on Earth with a crystallization age of 4.03 Ga [4]. Located on the North-West Territories of Canada in the Slave Province, rocks from this complex offer an invaluable opportunity to directly study the mantle processes during the early history of our planet.
We have conducted a Mo isotopic investigation of several mafic and intermediate rocks from the Acasta Complex, that have been least affected by later events involving widespread migration of fluid-mobile elements [5]. Three samples exhibited anomalous mass-independent compositions for Mo, although the results were inconclusive , since spurious Mo mass-independent anomalies could result from a mass-dependent fractionation (MDF) of Mo isotopes in terrestrial samples [6]. We performed an additional measurement of mass-dependent Mo isotope fractionation for these samples, to better understand this behavior of Mo isotopes in these pristine rock samples. We observe a correlation between Mo mass-independent anomalies with respective mass-dependent fractionation of these samples. It is a reflection of non-exponential mass fractionation of the Mo isotopes in these rock samples. This observation could have implications to reconsider Mo isotopes as the best proxy for understanding later accretionary processes using terrestrial materials. To expand on this, we are working on determining MSE abundances in these rocks along with their additional Mo isotopic compositions.
References:
[1] N. Dauphas, “The isotopic nature of the Earth’s accreting material through time,” Nature, vol. 541, no. 7638, pp. 521–524, 2017, doi: 10.1038/nature20830.
[2] T. Kleine et al., “The Non-carbonaceous–Carbonaceous Meteorite Dichotomy,” Space Sci. Rev., vol. 216, no. 4, 2020, doi: 10.1007/s11214-020-00675-w.
[3] T. Yokoyama, Y. Nagai, R. Fukai, and T. Hirata, “Origin and Evolution of Distinct Molybdenum Isotopic Variabilities within Carbonaceous and Noncarbonaceous Reservoirs,” Astrophys. J., vol. 883, no. 1, p. 62, 2019, doi: 10.3847/1538-4357/ab39e7.
[4] J. R. Reimink, A. M. Bauer, and T. Chacko, The Acasta Gneiss Complex. Elsevier B.V., 2019.
[5] K. Koshida, A. Ishikawa, H. Iwamori, and T. Komiya, “Petrology and geochemistry of mafic rocks in the Acasta Gneiss Complex: Implications for the oldest mafic rocks and their origin,” Precambrian Res., vol. 283, pp. 190–207, 2016, doi: 10.1016/j.precamres.2016.07.004.
[6] G. Budde, F. L. H. Tissot, T. Kleine, and R. T. Marquez, “Spurious molybdenum isotope anomalies resulting from non-exponential mass fractionation,” Geochemistry, vol. 83, no. 3, p. 126007, 2023, doi: 10.1016/j.chemer.2023.126007.
In this regard, constraining the Mo isotopic composition of the bulk silicate earth (BSE) and mantle retaining proto-Earth signatures, is essential. We focused on the later and the potential target terrestrial samples for this can be classified into two categories: (a) rocks derived from the preserved primordial mantle domains and (b) old Archean rocks. In our attempt, we investigated Archean rocks from Acasta complex. The Acasta Gneiss complex hosts the oldest rocks preserved on Earth with a crystallization age of 4.03 Ga [4]. Located on the North-West Territories of Canada in the Slave Province, rocks from this complex offer an invaluable opportunity to directly study the mantle processes during the early history of our planet.
We have conducted a Mo isotopic investigation of several mafic and intermediate rocks from the Acasta Complex, that have been least affected by later events involving widespread migration of fluid-mobile elements [5]. Three samples exhibited anomalous mass-independent compositions for Mo, although the results were inconclusive , since spurious Mo mass-independent anomalies could result from a mass-dependent fractionation (MDF) of Mo isotopes in terrestrial samples [6]. We performed an additional measurement of mass-dependent Mo isotope fractionation for these samples, to better understand this behavior of Mo isotopes in these pristine rock samples. We observe a correlation between Mo mass-independent anomalies with respective mass-dependent fractionation of these samples. It is a reflection of non-exponential mass fractionation of the Mo isotopes in these rock samples. This observation could have implications to reconsider Mo isotopes as the best proxy for understanding later accretionary processes using terrestrial materials. To expand on this, we are working on determining MSE abundances in these rocks along with their additional Mo isotopic compositions.
References:
[1] N. Dauphas, “The isotopic nature of the Earth’s accreting material through time,” Nature, vol. 541, no. 7638, pp. 521–524, 2017, doi: 10.1038/nature20830.
[2] T. Kleine et al., “The Non-carbonaceous–Carbonaceous Meteorite Dichotomy,” Space Sci. Rev., vol. 216, no. 4, 2020, doi: 10.1007/s11214-020-00675-w.
[3] T. Yokoyama, Y. Nagai, R. Fukai, and T. Hirata, “Origin and Evolution of Distinct Molybdenum Isotopic Variabilities within Carbonaceous and Noncarbonaceous Reservoirs,” Astrophys. J., vol. 883, no. 1, p. 62, 2019, doi: 10.3847/1538-4357/ab39e7.
[4] J. R. Reimink, A. M. Bauer, and T. Chacko, The Acasta Gneiss Complex. Elsevier B.V., 2019.
[5] K. Koshida, A. Ishikawa, H. Iwamori, and T. Komiya, “Petrology and geochemistry of mafic rocks in the Acasta Gneiss Complex: Implications for the oldest mafic rocks and their origin,” Precambrian Res., vol. 283, pp. 190–207, 2016, doi: 10.1016/j.precamres.2016.07.004.
[6] G. Budde, F. L. H. Tissot, T. Kleine, and R. T. Marquez, “Spurious molybdenum isotope anomalies resulting from non-exponential mass fractionation,” Geochemistry, vol. 83, no. 3, p. 126007, 2023, doi: 10.1016/j.chemer.2023.126007.