WHAT CAN MINERAL CRYSTAL NEIGHBOURS TELLS US ABOUT THE PETROLOGICAL PATH TAKEN DURING THE SOLIDIFICATION OF IGNEOUS ROCKS? (Invited Presentation)

A fundamental problem in igneous petrology is the solidification path, which is the petrographic route taken from magma to rock. This is not generally easy to infer from rock samples, as crystals may grow, dissolve and move during solidification in response to kinetic, equilibration and mechanical forces. Here I examine how looking at mineral neighbours, the crystals that surround a crystal or bubble, can clarify the path. Most textural descriptions are qualitative, but less common quantitative studies can give much more information. One useful metric is a stereologically exact method (2D measurement) that gives a measure of the area of crystal contacts per unit volume for pairs of different or identical phases.

The spatial association of minerals in rocks can be produced by reconstructive, destructive and constructive processes, alone or in combination. The most common reconstructive process is exsolution, commonly in response to cooling, such as the formation of plagioclase lamellae in K-feldspar (perthite). Partial destruction of a mineral phase, by interactions with another phase, solid or fluid, can result in a spatial association, such as the partial conversion of pyroxene to amphibole. Constructive processes are probably the most important path for the production of mineral associations. Interpretations are complex because minerals may co-crystallise over a significant temperature range with different growth rates. The simplest process is the simultaneous crystallisation of phases. Early associations are mono or poly-phase crystal clusters observed in volcanic rocks and some plutonic rocks. These may represent disrupted cumulates or partially solidified magmas. They may also be evidence of heterogeneous nucleation. Aligned crystal clusters may form in the magma by synneusis or other processes. Some mineral associations result from the crystallisation of immiscible liquid droplets, such as sulphides and possibly other compositions. Growth of oikocrysts can seal in early textures and are probably the most-used mineral association. However, the process may not be simply displacing the liquid component of the magma, but also heterogeneous nucleation of phases on the oikocryst surface. Finally, phases may be associated in late, physically isolated pockets of evolved magma.