Widespread felsic volcanism as a possible step towards Archean subaerial landmass: Insights from combined oxygen and hafnium isotopes in zircon

Abstract

Subaerial land today is mainly formed by continental crust, but before the stabilization of the first cratons, at ca. 3 Ga, volcanic structures (e.g., oceanic islands) may have been the first subaerial regions of the early Earth. Understanding the onset of felsic magmatism is crucial for constraining the formation of both continental crust and hypothetical early volcanic islands. Studies of ancient zircons suggest that subaerial land likely emerged at least by 3.5 Ga, but how long before that it began remains unknown. Our work examines circa 3.5 Ga old felsic volcanic rocks, the oldest known in the Kaapvaal (South Africa) and Singhbhum (India) cratons. We analyzed oxygen and Lu-Hf isotopes in zircon as they are effective proxies for distinguishing the melt source between mantle-derived and crustal (remelting of altered rocks and sediments). Oxygen isotopes ratios (δ18O) were measured by Secondary Ion Mass Spectrometry (SIMS) in coeval felsic units of the Kaapvaal Craton (i.e. Theespruit, Sandspruit, and Toggekry formations), and of the Singhbhum Craton (Daitari and Gorumahisani greenstone belts). This new data was compared with a newly compiled global Archean δ18O dataset (ca. 13,000 data points). Our felsic volcanic rocks display the averaged δ18O values ranging between 5.1 and 5.8 ± 0.24 ‰ (2 sd), which are purely mantle-like values. The only exception is a Toggekry formation sample (δ18O 3.9 ± 0.24 ‰), which reflects remelting of hydrothermally altered rocks. Published εHf values for the same rocks fall between CHUR and Depleted Mantle trends, implying juvenile melt signatures. In this context, we highlight the significance of the early Earth's first felsic rocks, whose formation is usually attributed to partial melting of a hydrated basaltic oceanic crust. In contrast, our data emphasizes the importance of purely mantle-derived felsic melts in the Archean. These felsic melts can be a result of extensive fractional crystallization (ca. 80%) of a stalled basaltic melt. Such relatively dry melting (possessing only juvenile water) requires elevated heat flow, and thick lithosphere. During the Archean, these conditions may have prevailed in a thick basaltic oceanic plateau setting. Reworking (i.e., melting) of such ancient oceanic plateaus could have led to the renewed generation of felsic melts producing buoyant silicic rocks and ultimately result in the consolidation and emergence of the earliest continental crust. The global Archean δ18O values compilation suggests that the mantle and seawater-altered rocks are both important sources of felsic melts during the Archean. This highlights the significance of global Archaean tectonic regimes that may have led to the formation of the first subaerial landmass in brief stints.