Amphibole as window on the Archean mantle composition

The Archean mantle is among the few reservoirs on Earth preserving records of the early differentiation of the planet. Because the Archean mantle is poorly preserved, only Archean komatiites and few other mantle-derived rocks of Archean age are the only source of information on the Archean mantle composition we presently have (Sossi et al., 2016). However, these rocks generally have strong metamorphic overprints that cause the bulk rock composition to be poorly representative of the original chemical signature, in particular for volatile elements and elements with high fluid mobility. Furthermore, olivine, orthopyroxene or clinopyroxene - the major constituents of these ultramafic rocks - are all minerals that operate a significant filter in the uptake of trace elements, thus hindering precious information on the Archean mantle. Here we propose a novel approach in deciphering the Archean mantle geochemical signature based on the geochemical characterisation of amphibole which is a mineral capable to incorporate a large number of petrologically relevant elements including volatiles (Tiepolo et al., 2007; Giesting et al., 2014). Several studies have shown the presence of primary hydro-magmatic amphiboles among accessory minerals of Archean ultra-mafic rocks (Stone et al., 2003; Fiorentini et al., 2012). We have selected amphibole-bearing samples from komatiites of the Agnew-Wiluna greenstone belt (Western Australia), from the tholeiitic and Fe-picritic suites of the Abitibi greenstone belt (Canada) and from the ferropicrites of the Pechenga complex (Russia). Phanerozoic amphiboles from mafic and ultramafic rocks were also considered for comparison. We have characterised amphiboles for major-, trace- and volatile element contents. The δD and δ18O isotopic composition of amphibole has been also determined by HR-SIMS. Results suggest plume-related origin for the selected Archean ultramafic rocks and point to a significant change in mantle source composition at the origin of Archean and Phanerozoic primary mantle melts. Moreover, elemental and isotopic results on volatiles reveal the occurrence of hydrous deep mantle sources in the Archean. Hence results have significant implications for the debated “wet or dry” signature of komatiites.