Mixing and compositional stratification produced by natural convection 2. Applications to the differentiation of basaltic and silicic magma chambers and komatiite lava flows

The petrogenesis of igneous rocks can be controlled significantly by the mi xing of dissimilar magmas. Within the contexts of basaltic and silicic magm a chambers and komatiite lava flows we identify circumstances in which the extent to which contrasting magmas are mixed by natural convection potentia lly controls their differentiation. To evaluate the amount of mixing in eac h context, we apply the experimental results from part 1 [Jellinek et al., this issue] of this study, in which we quantified the conditions under whic h fluids could be mixed by; convection at large Rayleigh numbers (> 10(11)) . When our laboratory results are applied to basaltic magma chambers, we fi nd that convection driven by compositionally buoyant magma released during floor crystallization or floor dissolution will produce partial to nearly c omplete mixing of the ascending fluid in chambers that are tens of meters t o kilometers high, respectively. We also conclude that substantial floor me lting (with extensive mixing) is expected only for basaltic chambers emplac ed in the deep crust. During the turbulent flow of Archean komatiites, unde rlying sediments melted by forced convective heat transfer are predicted to have been mixed nearly completely into the overriding lavas. During the re plenishment of silicic magma chambers by basaltic magmas we predict that th e convection of buoyant silicic magma overrun by a spreading injection of d enser basalt will cause little mixing. However, after emplacement, heat tra nsfer from a basalt layer will gradually melt and mobilize its felsic floor , producing a small flux of buoyant felsic liquid that will be mixed extens ively.