5. Models of Continental Growth and Destruction

We don’t know with confidence the mechanisms by which primitive arc basalts are modified to produce the broadly granodioritic continental crust but there is widespread agreement that plate tectonics has been doing just that for at least the past billion years. We also don’t fully understand the structure of the continental crust; popular layered models lack mechanisms to produce such structures or to recover them following tectonic homogenization. The geochemical community long favored the view that early crust was mafic, in part owing to misconceptions regarding feldspar buoyancy on a hydrous magmatic substrate and the deep stabilization of garnet (which retards crystallization of more buoyant aluminous phases from the magma). But early felsic crusts with the potential for long term stability could have emerged via crystallization of tonalitic liquids fractionated from ultramafic magmas in equilibrium with olivine or differentiating magma sheets following large impacts into early basaltic crusts. The remarkable range of estimates of the growth history of continental crust reflects a number of influences but, generally speaking, earlier growth has been increasingly favored as new age survey methodologies became available and as the effects that crustal reworking and recycling have on apparent surface age provinces became better appreciated. Isotopic data once thought to support rapid growth at ~2.7 Ga are now recognized as equally consistent with constant volume continental crust. The longstanding misapprehension that the present-day distribution of crust formation ages is equivalent to the growth history of continents strongly influenced some estimates. Instead, today’s crust represents a running balance between new growth, internal overprinting, and crustal recycling. The difficulty in deconvolving these processes is one of the two principal challenges in establishing the growth history of continental crust. The other is that crust recycled back into the mantle and thoroughly mixed leaves no trace of its past incarnations. Although the rock record has yet to yield clear, direct evidence from which to constrain the magnitude of Hadean continental crust, optimal solutions to modelling mantle isotopic data are at least as consistent with constant volume continental crust since ca. 4.4 Ga as with slow monotonic growth. Radiogenic isotopic data used to argue for an early mafic crust are contradicted by stable isotopic results that appear to support a continuously felsic continental crust of unknown volume.