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?


2 Answers 2


Gordon's answer is correct to a first order approximation.

However, the zircons that were found in Jack Hills are from the Hadean, more than 4 billion years ago. The fact that zircons that old exist is interesting - zircon is not a mineral you find in mafic rocks. Zirconium is an incompatible element in the mantle, so to get enough of it to form its own mineral you need some partial melting events. This may also be of interest:

What are the high field strength and large ion lithophile (HFS or HFSE & LIL or LILE) elements?

When you melt the mantle, you end up having basalt or more felsic rocks. If you have enough zircon in mafic rocks, you form baddeleyite: ZrO2. Zircon, ZrSiO4 will only form in more felsic rocks, where you have enough Si. This alone suggests that some parts of the early crust was somewhat more felsic than a basalt.

There's more to it: inclusions of quartz and oxygen isotope data indicate that not only the zircon was in a felsic rock, it has evidence to suggest liquid water and hydrothermal alteration.

This entire thing is heavily debated, because there's only so much you can tell from a few grains of zircon. However, the basics are there - some felsic rocks existed as far as the Hadean.


Apart from a relatively thin veneer of sediments, today's crustal material is mainly granitic in nature, with a melting temperature of about 850 deg C. We don't know the early proto-Earth composition, but further to collision with the putative 'Theia' (see https://en.wikipedia.org/wiki/Giant_impact_hypothesis ) the combination of thermal energy from the primordial gravitational accretion and kinetic energy of collision, would have raised the temperature to well in excess of the 2000 deg C melting point of ultramafics. The pre-fractionated precurser to the crust and mantle would have been somewhat alkaline, with water, K Al, Na etc all depressing the freezing point. Nevertheless the first crust to fractionate would have been at a temperature far above 850 deg C, and hence there were probably several cycles of fractionation, subduction and remelting of progressively more acidic (with respect to silica) crust before any recognizable 'Sialic' crust was able to stabilize.

One could therefore argue, purely on temperature grounds, that the first crust must have been mafic (boninite volcanics?), even leaning towards ultramafic, and since zircon is so refractory, one would expect the first zircons to have mafic associations, unlike the later granitic association.


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