Hydrated Peridotite – Basaltic Melt Interaction Part I: Planetary Felsic Crust Formation at Shallow Depth

Abstract

Current theories suggest that the first continental crust on Earth, and possibly on other terrestrial planets, may have been produced early in their history by direct melting of hydrated peridotite. However, the conditions, mechanisms and necessary ingredients for this crustal formation remain elusive. To fill this gap, we conducted time-series experiments to investigate the reaction of serpentinite with variable proportions (from 0 to 87 wt%) of basaltic melt at temperatures of 1,250–1,300°C and pressures of 0.2–1.0 GPa (corresponding to lithostatic depths of ∼5–30 km). The experiments at 0.2 GPa reveal the formation of forsterite-rich olivine (Fo90–94) and chromite coexisting with silica-rich liquids (57–71 wt% SiO2). These melts share geochemical similarities with tonalite-trondhjemite-granodiorite rocks (TTG) identified in modern terrestrial oceanic mantle settings. By contrast, liquids formed at pressures of 1.0 GPa are poorer in silica (∼50 wt% SiO2). Our results suggest a new mechanism for the formation of the embryonic continental crust via aqueous fluid-assisted partial melting of peridotite at relatively low pressures (∼0.2 GPa). We hypothesize that such a mechanism of felsic crust formation may have been widespread on the early Earth and, possibly on Mars as well, before the onset of modern plate tectonics and just after solidification of the first ultramafic-mafic magma ocean and alteration of this primitive protocrust by seawater at depths of less than 10 km.

Hadean zircon formed due to hydrated ultramafic protocrust melting

“…Experiments performed at 1250-1300 °C, 0.2 GPa (corresponding to depth of ∼5 km), and redox conditions from ΔQFM+1.8 to ΔQFM+4.4 (expressed in log 10 f O2 relative to the quartz-fayalite-magnetite [QFM] oxygen fugacity [f O2 ] buffer) indicate that partial melts produced at these conditions are enriched in SiO 2 (average 66 ± 4 wt% SiO 2 ; Figs. 1A and 1B; Tables S1 and S2 in the Supplemental Material 1 ) and other lithophile elements (Al, alkalis), with abundances broadly similar to those of continental crustal rocks (Borisova et al, 2021).…”

“…In the experimental study of Borisova et al (2021), modern tholeiitic mid-ocean ridge (MOR) basalt (δ 18 O VSMOW = 5.8‰, VSMOW-Vienna standard mean ocean water) and a serpentinite with 9.7‰ δ 18 O VSMOW (suggesting hydration below ∼120 °C; Fig. 3; see the Supplemental Material) were used as the starting products.…”

“…This hydrated protocrust was subsequently buried by later komatiitic and basaltic lava flows and finally remelted due to radioactive self-heating to generate the magmas that crystallized the Hadean zircons. As an alternative scenario, Borisova et al (2021) assumed that a primitive protocrust quenched on top of the magma ocean, as observed in a permanent lava lake (Helz, 2009). In the case of the Hadean, this protocrust would have been of peridotite composition and become rapidly hydrated (e.g., serpentinized) in its uppermost part by the readily available liquid hydrosphere (Valley et al, 2002;Elkins-Tanton, 2012), as proposed by Albarède and Blichert-Toft (2007).…”

“…The residual part of the magma ocean then evolved beneath the early quenched crust to yield basaltic magmas, likely enriched in incompatible elements, with some similarity to lunar KREEP basalts (e.g., Cronberger and Neal, 2018). Borisova et al (2021) performed experiments on the interaction between serpentinite and basaltic magma that demonstrated that tonalite to granodiorite compositions can be formed in this way at shallow depths (<10 km). In this scenario, partial melting is triggered by dehydration driven by intrusion of basaltic magmas or by meteorite impacts.…”

“…Our study tested the hypothesis that an early felsic crust may have formed by low-pressure, fluid-present melting of hydrated peridotite interacting with basaltic magma through assessment of the capacity of experimental felsic melts (Borisova et al, 2021) to crystallize zircon (referred to here as "model zircon") and then comparison of the temperature conditions, co-crystallizing mineral assemblages, and isotopic and trace-element patterns of these model zircons with data from detrital Hadean zircons.…”