Mixing and compositional stratification produced by natural convection 2. Applications to the differentiation of basaltic and silicic magma chambers and komatiite lava flows
Am. Jellinek et Rc. Kerr, Mixing and compositional stratification produced by natural convection 2. Applications to the differentiation of basaltic and silicic magma chambers and komatiite lava flows, J GEO R-SOL, 104(B4), 1999, pp. 7203-7218
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.