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Experimental and isotopic constraints on the formation of Archean continental crust

Remnants of Earth’s juvenile continental crust are preserved in the form of Archean Tonalite-Trondhjemite-Granodiorites (TTGs). However, much controversy surrounds the composition of TTG protoliths and whether the geodynamic setting involved convergent-style plate tectonics. Thus, a combination of high-pressure and high-temperature experiments combined with robust geochemical proxies are required to gain insight into TTG petrogenesis. Numerous experimental studies have demonstrated that partial melting of hydrated basalt at 0.8 to 2 GPa is sufficient to produce TTG-like melts. The compositions of starting materials used in these experiments vary significantly between studies and has profound implications for the solidus, melt composition, and the type, composition and modal abundance of solid phases. This is problematic especially given most studies have utilised MORB-like starting compositions, which differ significantly in composition to the least altered Archean metabasalts which posses higher MgO and lower Al2O3. We present the results of partial melting experiments conducted at 1-1.5 GPa and 940 to 1100 °C in a piston cylinder apparatus using synthetic starting materials with varying H2O (4-6 wt.%), based on the compositions of Eoarchean metabasalts from the Isua supracrustal belt in southern West Greenland. The run products of these experiments will be analysed to assess their phase assemblages and melt composition, and to constrain the magnitude of mineral-melt trace element and isotopic (Ti) fractionation during partial melting. Ultimately this experimental campaign will ascertain if partial melting of high-Mg, low-Al metabasalts is a viable mechanism to produce melt compositions resembling those of Archean TTGs.


Liam Hoare1, Christopher Beyer1, J. Elis Hoffmann2, Raúl O.C. Fonseca2
1Ruhr-Universität Bochum, Germany; 2Freie Universität Berlin, Germany
GeoBerlin 2023