Plumbing the depths of magma crystallization using 176Lu/177Hf in zircon as a pressure proxy

Abstract

Extensional tectonics are marked by shallow magma crystallization depths, whereas compressional tectonics are associated with deeper crystallization depths. This implies that variations in crystallization depths can be used to track changes in Earth’s dominant tectonic regimes through time. We therefore developed a new “pressure of crystallization” proxy based on the variation of the 176Lu/177Hf ratio in zircon. This ratio is controlled by zircon fractionation and residual garnet, and it can be used to monitor the evolution of a crystallizing magma ascending within the crust. The secular evolution of the 176Lu/177Hf ratio in zircon is characterized by cyclical oscillations that are broadly in tune with the δ18O record in zircon and with periods of continental collision and supercontinent amalgamation. The apparent mean depth of crystallization of zircon-bearing igneous rocks has decreased with time over the last ~3.0 b.y. This can be linked to shallowing of the primary crystallization depths and/ or to the effect of time-integrated erosion in the geologic record. Prior to ca. 3.0 Ga, crystallization depth maxima and oscillations in apparent depth are less clear, perhaps suggesting that the nature of tectonic interactions was different in the Mesoarchean and earlier