Emplacement and Post-Emplacement Dynamics of Magma Reservoirs: Implications for the Continental Crust Formation

The Earth is a unique planet in the solar system as it has an outermost layer composed by two types of crusts with different compositions, structures and ages. While the generation of the oceanic crust is relatively well understood, the processes involved in continental crust formation remain elusive. Bulk chemical compositions and petrological models suggest that the continental crust has lost a significant portion of its mafic components over its geological history. However, mafic and ultramafic intrusions, such as the Bushveld complex, South Africa, and the Great Dyke, Zimbabwe, have been preserved in the crust for billions of years. The question is how are mafic cumulates lost and, more specifically, what is the long-term evolution of a magma reservoir, possibly including post-emplacement and post-crystallization processes. This work aims at answering this outstanding question with the following approach: we review available geological, geophysical information of some well-studied mafic/ultramafic intrusions which testify to the penetration of mantle-derived melts in the crust. Crystallization of these magmas resulted in a dramatic buoyancy reversal which lead to a potentially unstable density distribution. We therefore performed laboratory experiments with viscous fluids to investigate such an instability associated to buoyancy reversal and we derive simple scaling laws governing the different dynamical regimes. Subsequently, we study the physical conditions under which mafic intrusions become unstable using extensive numerical simulations to reproduce crustal flows. The most important finding is that the main control on the instability is the temperature of the country rocks, and thus the emplacement depth. We show that many mafic intrusions preserve flow structures, proving that they were affected by gravitational instabilities consistent with the regimes observed in laboratory and numerical experiments. The results of this work imply that many of the mafic intrusions we observe today at the outcrop are the vestiges of much larger systems that became unstable in a post-emplacement phase. Consequently, foundering and sinking of mafic cumulates through the crust is shown to be a very common process in active volcanic regions. This mechanism was therefore a key process in the formation and differentiation of the Earth crust.