Earth's evolution over time revealed by the Nb/U, Ce/Pb and Nb/Th ratios in the sources of mantle plumes.

Magmas from mantle plumes are potentially the best monitors of Earth's compositional and thermal evolution over time. However, their erupted products are commonly modified by syn- and post-magmatic processes and thus do not fully retain original information about their mantle sources. Such data can be recovered from melt inclusions in olivine phenocrysts in the most primitive magmas from mantle plumes. Such inclusions, shielded by host olivine, retain original isotopic and critical trace element signatures of deep mantle sources even for Archean and Hadean Eons.

We will present the results of a study of chemical and Rb-Sr isotope composition (EPMA, LA-ICP-MS and RAMAN) of melt inclusions and chemical (EPMA, LA-ICP-MS) compositions of host olivines for komatiites and plume-related picrites with eruption age from 3.3 Ga to 1 Ka.

Recent advances in in-situ split stream LA-ICP-MS measurements of ⁸⁷Sr/⁸⁶Sr ratios and trace element contents of olivine-hosted melt inclusions revealed significant mantle source heterogeneities of magmas from individual plumes. The results are confirmed by geodynamic modelling (Jain et al., this meeting).

We show that the melt inclusions of most studied mantle plumes display heterogeneous populations in age-corrected ⁸⁷Sr/⁸⁶Sr ratios and include groups with model ages more than 1 Ga older than the emplacement age. The oldest inclusion groups found in Archean komatiites correspond to Hadean (4.3±0.2Ga, Vezinet et al., in review) and Eo-Paleoarchean (3.6±0.2 Ga) model ages. These and most inclusions from studied komatiites and picrites display Nb/U, Nb/Th and Ce/Pb significantly higher than in BSE.

Evolution over time of canonical proxies of continental crust generation (Nb/U, Th/U and Ce/Pb, Hofmann et al., 1986) in mantle plumes, combined with geodynamic modelling, suggests:

• Most of the continental crust was generated in several Hadean and Archean pulses by plume-induced subduction and melting of the hydrated mafic/ultramafic crust or mantle. Hadean continental crust was subducted or/and reworked.
• Restites left after extraction of continental crust were continuously subducted to the core-mantle boundary from the mid-Hadean and later recycled in Archean mantle plumes.
• Active formation of both continental and oceanic crust in Hadean was governed by plume-induced subduction, which ceased after cold subducted material hindered the propagation of large plumes at the core-mantle boundary. After heating the recycled lithosphere at the core-mantle boundary, the process repeats, producing oscillating subduction and crustal formation in Hadean-Archean.