Hadean, Archean, and modern Earth: Zircon-modeled melts illuminate the formation of Earth’s earliest felsic crust

The magmato-tectonic environment(s) of origin for Earth’s earliest crust are enigmatic and fiercely debated. Revealing the composition of the melts from which Hadean (>4.02 Ga) zircons crystallized might clarify conditions of initial crust construction. We calculate model melts using Ti-calibrated zircon/melt partition coefficients (KdZrc(Ti)) and published trace element data for Hadean and Archean zircons. The same treatment is applied to zircons from possible analogue environments (MORB, Iceland, arcs, lunar), to constrain potential petrogenetic similarities and distinctions between the early and modern world. Model melts from oceanic environments (MORB, oceanic arc, Iceland) have higher heavy rare earth element (HREE) contents and shallower middle REE (MREE) to HREE/chondrite (ch) slopes than those from continental arcs and tonalite-trondhjemite-granodiorite suites (TTGs). Hadean and Archean model melts are nearly indistinguishable from one another, both resembling TTGs and continental arcs, with pronounced depletion of HREE and slope reversal in heaviest REE. A limited number of samples > 4.25 Ga yield model melts with broadly similar characteristics to those from younger Hadean and Archean zircons, but with relatively elevated REE (~half order of magnitude) and higher LREE and MREE relative to HREE. Rare earth element patterns of early Earth model melts suggest a common petrogenetic history in the Hadean and Archean, involving garnet +/-amphibole in relatively low-temperature, high-pressure, environments.