[S-GC33] Solid Earth Geochemistry, Cosmochemistry

[SGC33-P06] Mantle evolution in late Archaean probed by highly siderophile element content ~Constraints from ultramafic volcanic rocks from the Pyradie Formation, Western Australia~

*Ryu Sato¹, Akira Ishikawa¹, Yuichiro Ueno¹, Tetsuya Yokoyama¹, Marco Fiorentini² (1.Tokyo Institute of Technology, 2.University of Western Australia)

Keywords:Archaean, Mantle evolution, late veneer, komatiite

The Pilbara Craton in northwestern Australia is known to host several generations of Archean greenstone belts. The occurrence of komatiites has long been recognized in ~3.53 Ga Coonterunah, ~3.34 Ga Kelly, and ~3.18 Ga Ruth Well and Regal, and they have been studied with the aim of constraining the chemical evolution of the Archean mantle. Recently, Puchtel et al. (2022) estimated the abundances of highly siderophile elements (HSE) in their sources, and argued for a secular increase of HSE due to progressive mixing of late veneer materials, as proposed by Maier et al. (2009). To evaluate this scenario, we investigated the whole-rock major, trace, and HSE compositions of 2.7 Ga ultramafic volcanics in the Pyradie Formation of the Fortescue Group, which occurs at the southern margin of the Pilbara Craton. Our results demonstrate that the whole-rock data of Pyradie volcanics follow clear crystal fractionation trends. They provide a linear positive correlation between Ru and Cr abundances, which deviate significantly from the trends procduced by the data from the older Coonterunah and Kelly komatiites. This suggests that Ru abundances are controlled by spinel fractionation during magma evolution, but their primary magmas have significant variations in Ru abundances. If we can assume that Cr abundances of komatiites are less variable reflecting the high degree of melting of peridotites, the 2.7-3.2 Ga komatiites and their sources could contain higher Ru than those in older suites, supporting the secular HSE increase in mantle sources beneath the Pilbara Craton.