We present the first steps of a new explanation model for
differentiation in sills, using a combination of geochemical data and
field observations, numerical modeling and dimensional analysis.
Geochemical data from a saucer-shaped dolerite sill intruded into the
Karoo basin, South Africa reveal a differentiation process which causes
D-shaped profiles. The geometry name is based on the variation in
whole-rock Mg-number (Mg#=Mg/(Mg+Fe)) from floor to roof in a sill; the
D-shaped geochemical profiles represent sheet-intrusions with the most
primitive composition (i.e. high Mg#) in its center, and progressively
more evolved composition (i.e. low Mg#) towards the upper and lower
margins. The differentiation is reversed compared to the normal
differentiation produced by fractional crystallization (C-shaped
profiles). C-shaped profiles are believed to be formed by segregation of
crystals from the magma. We propose that the opposite, the D-shaped
profile, may result from melt segregation from the crystal mush. This is
achieved by porous melt-flow through a consolidated crystal network
after the main phase of emplacement, and before complete solidification.
We show that a significant flow is feasible under natural occurring
conditions. An underpressure of magnitude 10$ Pa develops at the
cooling margins due to volume reduction of the crystallizing porous
melt. The resulting pressure gradient is the driving force for the
melt-flow towards cooling margins considered in this work. As a result
the margins will be enriched in the incompatible elements associated
with the melt phase, while the center will be depleted. We show that the
amount of flow is primarily a function of viscosity of the melt and
permeability of the crystal network, which in turn is a transient
phenomenon dependent on a number of parameters. Diagrams have been
constructed to evaluate the feasibility of substantial melt extraction
in terms of these poorly constrained parameters. Data from the Golden
Valley Sill and many other natural occurrences of D- and I-shaped
geochemical profiles show a reasonable agreement with our predictions of
melt flow potential, and are thus well explained by the presented model.
We conclude that in order to fully understand differentiation processes
occurring in sheet intrusions, we need to account for postemplacement
segregation of melt from crystals, anti not only segregation of crystals
from melt. (C) 2008 Elsevier B.V. All rights reserved.