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Ingrid Aarnes, Physics of Geological Processes, University of Oslo (Norway)
Yuri Y. Podladchikov, Physics of Geological Processes, University of Oslo (Norway)
Else-Ragnhild Neumann, Physics of Geological Processes, University of Oslo (Norway)
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We present the first steps in developing a new model for differentiation in sills using a combination of field observations, geochemical data, numerical modeling, and dimensional analysis. Geochemical data from mafic sills reveal a reversed differentiation process which causes the floor and roof of the sill to have the most evolved composition, with progressively more primitive composition towards the center of the sill. The vertical profiles of whole-rock Mg-number (Mg# = Mg/(Mg+Fe)) associated with this differentiation trends typically exhibit a D-shape (i.e. low value at the margins and progressively higher towards the center). Differentiation by fractional crystallization, as developed for large plutons, is known to produce the opposite trend (i.e. C-shaped profiles) and hence cannot be applied directly to the sills. Our goal is to test the feasibility of having a substantial post-emplacement flow in natural magma intrusions and to show that such flow can be an alternative mechanism of differentiation in sills.
Porous melt flow within a stationary crystal network provides a mechanism for melt and crystals to segregate and such flow may cause differentiation by advection. The numerical model shows that a significant flow is feasible under natural occurring conditions. The driving force for the flow is underpressure associated with the crystallization of interstitial melt in a rigid crystal network. We show that the total amount of melt flow is primarily controlled by 1) viscosity of the melt, which is dependent on the melt composition; 2) permeability of the crystal network, which is dependent on cooling time which in turn is related to grain sizes. We have estimated the conditions for when substantial flow (i.e. >10% of the sill thickness) and insignificant flow can occur. Data from D-shaped and I-shaped profiles coincide well with our numerical predictions on the conditions for substantial and insignificant flow, respectively.
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