What was the likely composition of Earth's early crust (how did crustal composition evolve)?

Question

Was the first crust entirely mafic (oceanic) crust which was later distilled via volcanic arcs along subduction zones to make felsic rocks or did the earth start with bits of felsic crust? Are the ancient zircons most likely from felsic or mafic rocks?

Answer 1

This is a fantastic question! One which continues to spur debates. That said, most conventional isotopic evidence suggests that early Earth's crust was predominantly mafic. The existing catalog of oldest preserved rocks generally do not indicate reworking of felsic materials. However, there are some ancient mineral records which point to the existence of high-silica source materials. The origins of these high-silica materials themselves are debated, a potential source could be linked to extraterrestrial input (asteroid impacts, not aliens).

As your question suggests, reworking of crustal material is one of the strongest theories for development of felsic crust. And all of this is to say: the ancient zircon you mention most likely come from mafic sources.