The Cooling History of a Simple Intrusion from Nipigon, Ontario

Abstract:

One of the critical tenets of physical petrology is that chemical processes cannot be considered in isolation from the physical properties and history of the magma. Particularly through the development of the solidification front framework, we recognize the fundamental significance of isotherm propagation through a magma body as the governing control on crystallization. Except in experimental simulations and certain fortuitous natural examples, the primary data we have to reconstruct the thermal evolution of a magma body is the texture of the rock.

We have measured crystal sizes of oxide minerals (primarily magnetite) from a thin (1.4 m), compositionally uniform, diabase sill to better characterize the relationship between thermal history and rock texture. In the outer 30 cm of the sill (upper and lower margins), crystal sizes increase from ~2 to ~9 µm, and then remain nearly uniform at 9.5(±1.5) µm through the central 80 cm. There is significant alteration in the lowermost 15 cm of the sill, which has largely obliterated the original oxide textures, but otherwise the textural profile is remarkably symmetrical, indicating that cooling occurred by simple conduction through the upper and lower margins. Reasonable cooling models, coupled with a simple linear crystal growth model, generate theoretical textural profiles that closely resemble the measured profile. The parameters of the growth model are consistent with experimental and observational data from a wide range of igneous bodies.

We suggest that the observed crystal size variations in this thin sill can be considered a type example of a simple dike or sill in which there were no phenocrysts intruded with the magma, no significant redistribution of crystals during cooling, and simple conductive cooling through the margins. Our results can be used as an example of the extent and pattern of textural variations that can be expected in a simple intrusion, and thus a null hypothesis for proposed complex intrusions.