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.
Highlights
It is not surprising that high-Mg olivine in association with chromite is recorded in equilibrium with felsic melts of dacitic composition in our low-pressure experiments (0.2 GPa)
The hydrous intermediate to felsic melts produced in our experiments are likely formed by the fertilization of olivine-rich zones due to the presence of aqueous fluids produced by the interaction of serpentinite and basaltic melt at 0.2–1.0 GPa
We propose that basaltic melts were introduced in the primitive ultramafic protocrust at the very end of the crystallization of the magma ocean, and/or by heat-pipe volcanoes through a stagnant lid (Moore and Webb 2013), during the earliest stages of Earth evolution, or as proto-rifts at the end of the Hadean (Capitanio et al, 2020)
Summary
The conditions and mechanisms that led to the production of the earliest intermediate to felsic (Si- and Al-enriched) crust on Earth and Mars are the subject of intense debate (Rudnick and Gao, 2003; Harrison, 2009; Reimink et al, 2014, 2016; Sautter et al, 2015, 2016; Burnham and Berry, 2017). Ancient granodiorite rocks have been identified on Mars (Sautter et al, 2015; Sautter et al, 2016), indicating that formation of felsic crust occurred on other terrestrial planets even without plate tectonics and subduction In this context, melting of dry peridotite at shallow depths does not produce appropriate intermediate to felsic melt compositions comparable to that of bulk continental crust (Hirschmann et al, 1998; Rudnick and Gao, 2003; Harrison, 2009; Reimink et al, 2014, 2016; Burnham and Berry, 2017). Trondhjemite-tonalite veins are common features throughout the cores in the few sites of the Deep Sea Drilling Project, and Ocean Drilling, Integrated Ocean Drilling and International Ocean Discovery Programs (DSDP, ODP, and IODP) where the deep oceanic crust has been drilled along present-day spreading ridges These rocks have been ascribed to local remelting processes in the presence of hydrothermal fluids along hightemperature normal shear zones. Their origin is enigmatic, understanding the origin of TTG rocks situated in modern oceanic lithosphere may open new perspectives on the formation of the first planetary felsic crust
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