Cumulates and phase diagrams

Phase diagrams are graphical representations of the stability of different minerals, compounds and melts in plots with different intensive parameters (pressure, temperature, composition) on the different axes. If you need an introduction to phase diagrams, you are advised to refer to any undergraduate textbook in igneous petrology.

Cumulates successions are closely linked to the phase relations of the crystallising magma. A closed system crystallisation will produce a succession of cumulates with progressively lower temperature mineral assemblages during crystallisation. The animation below shows a situation where minerals are separated from the fractionating magma (by deposition or crystallisation) on the floor of an intrusion generating a layered succession of cumulates. The composition of the magma is illustrated with the light red colour.

The geology of layered intrusions is tightly tied in with phase relations. Most layered intrusions formed from basaltic magmas that undergo significant fractionation during their crystallisation. The phase diagram above was used by Irvine (1970) to explain layering successions in the Muskox intrusion in northern Canada.

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