13:45 〜 14:00
[SGC33-01] Highly siderophile elements in Earth's oldest rocks from Acasta Gniess Complex, Canada: an impact melt origin or not?
キーワード：強親鉄性元素、Re-Os同位体、アカスタ片麻岩体
The first billion years' history of the Earth is veiled in mystery due to the dearth of Hadean and earliest Archean rocks. Although this might be a natural outcome given the high efficiency of overprinting and reworking processes caused by plate tectonics, an important question of when plate subduction started on Earth still remains obscure. Alternatively, it has been proposed that the crust preservation was delayed by a barrage of asteroid impacts that caused global-scale melting and continuous resurfacing of Earth after a period referred to as "late heavy bombardment" on the Moon (~4.1-3.9 Ga).
A recent petrogenetic modelling suggests that the Earth's oldest evolved rocks, the 4.02 Ga Idiwhaa tonalitic gneisses of the Acasta Gneiss Complex [1], represent the predominant felsic crust at that time, which has been generated by impact melting of Hadean mafic crust [2]. We test this hypothesis by investigating abundances of highly siderophile elements (HSEs) and Re-Os isotope systematics for the Idiwhaa tonalitic gneisses collected from previously located outcrops and a newly identified exposure as a southern extension of the same unit. These tracers are considered to be sensitive for involvement of extraterrestrial components and relatively immune to secondary alteration.
Although quadruplicate measurements for each sample showed significant variations indicative of isotope and elemental heterogeneity inherent in studied samples, the preservation of Hadean HSE signatures in the Idiwhaa tonalites are supported by whole-rock Re-Os isotope data; extremely radiogenic 187Os (>94% of total Os) provides a weighted mean 187Re-187Os model age ~4.0 Ga consistent with the U-Pb ages of zircon. Regarding HSE abundances, the Idiwhaa tonalites display strongly fractionated HSE patterns. Concentrations of common Os, Ir and Ru are low with chondrite-normalizing values ranging from 10-5 to 10-6 level. These data show no contribution of the meteoritic components to the generation of Idiwhaa tonalites, and lend no support to the hypothesis of impact melt origin.
[1] J. R. Reimink et al., Earth's earliest evolved crust generated in an Iceland-like setting. Nature Geosci. 7, 529–533 (2014).
[2] T. E. Johnson et al., An impact melt origin for Earth's oldest known evolved rocks. Nature Geosci. 11, 795–799 (2018).
A recent petrogenetic modelling suggests that the Earth's oldest evolved rocks, the 4.02 Ga Idiwhaa tonalitic gneisses of the Acasta Gneiss Complex [1], represent the predominant felsic crust at that time, which has been generated by impact melting of Hadean mafic crust [2]. We test this hypothesis by investigating abundances of highly siderophile elements (HSEs) and Re-Os isotope systematics for the Idiwhaa tonalitic gneisses collected from previously located outcrops and a newly identified exposure as a southern extension of the same unit. These tracers are considered to be sensitive for involvement of extraterrestrial components and relatively immune to secondary alteration.
Although quadruplicate measurements for each sample showed significant variations indicative of isotope and elemental heterogeneity inherent in studied samples, the preservation of Hadean HSE signatures in the Idiwhaa tonalites are supported by whole-rock Re-Os isotope data; extremely radiogenic 187Os (>94% of total Os) provides a weighted mean 187Re-187Os model age ~4.0 Ga consistent with the U-Pb ages of zircon. Regarding HSE abundances, the Idiwhaa tonalites display strongly fractionated HSE patterns. Concentrations of common Os, Ir and Ru are low with chondrite-normalizing values ranging from 10-5 to 10-6 level. These data show no contribution of the meteoritic components to the generation of Idiwhaa tonalites, and lend no support to the hypothesis of impact melt origin.
[1] J. R. Reimink et al., Earth's earliest evolved crust generated in an Iceland-like setting. Nature Geosci. 7, 529–533 (2014).
[2] T. E. Johnson et al., An impact melt origin for Earth's oldest known evolved rocks. Nature Geosci. 11, 795–799 (2018).