On the conditions for lower crustal convective instability

Abstract

Continental crust is thought to be formed as a result of arc magmatism, but many of the lavas produced in these settings are basaltic, while those that are silicic are commonly evolved, with lower Mg #s than the continental crust. The bulk composition of continental crust can be produced by mixing of end‐member basaltic and silicic compositions, via magma mixing or in mechanical, tectonic juxtaposition, but some process is required to remove the cumulates and residues formed during generation of the silicic, “granitic” end‐member. We consider convective instability of dense mafic and ultramafic lower crust as a means to remove mafic residues of basalt differentiation in order to produce end‐member compositions that can mix to form the bulk composition of the continental crust. Using a range of lower crustal and mantle bulk compositions, ranging from mafic and ultramafic cumulates to primary liquid compositions, we calculated the subsolidus phase assemblage and resulting density. The results show that densities of likely lower crustal lithologies can exceed those of the mantle (by ∼50–250 kg m−3), but the density contrast is a strong function of composition, temperature, and pressure. For a “cold” geotherm with a Moho temperature of 300°C, relevant to cratonic settings, densities of all of the lower crustal compositions that we considered, except granulite, exceed the density of the underlying mantle at pressures as low as 0.8 GPa. For a “hot” geotherm with a Moho temperature in the range of 800–1000°C, the density of the lower crust is much more variable, with gabbroic and granulite compositions having lower densities than the mantle, while “arc gabbronorite” and ultramafic cumulate compositions having higher densities than the mantle at pressures similar to that for the cold geotherm. Instability times calculated for a two‐dimensional Rayleigh‐Taylor convective instability, where a dense lower crustal layer sinks into a lower‐density mantle, show that high temperatures (>700°C, or >500°C with a background strain rate) are required for this process to occur on a timescale of 10 Myr with rheological parameters expected for the crust and mantle. The high temperature required for dense lower crustal mafic‐ultramafic cumulates to sink into the mantle suggests that this process is restricted to arcs, volcanic rifted margins, and continental regions that are undergoing extension, are underlain by a mantle plume, or have had part of the conductive upper mantle removed.