Estimating the time scales of magmatic processes

A number of techniques have been developed to estimate time scales from magmatic rocks. The techniques may be usefully subdivided into those that yield absolute ages, as from short lived isotopes of the U-series, and those that yield relative ages. The latter include major and trace element profiles modified by diffusion, and they are calculated on the basis of how long individual crystals have been at magmatic temperatures as inferred from the mineralogy of the host rock. They have the advantage that individual rocks can be characterised in terms of the age profiles of their crystal populations.

Age information can be obtained from crystals, the whole rocks and the groundmass, and each yield information on different aspects of the evolution of the magmatic system. They range from eruption ages, best determined on minerals from the groundmass, to the pre-eruption crystallisation history inferred from phenocrysts, the residence of magmas in the crust, and the time scales of magma differentiation. One way to assess the geological reliability of the calculated ages is to see if similar ages are obtained from other age dating systems. It is argued that if crystals are very young at the time of bulk rock crystallisation, there may have been insufficient time for fractional crystallisation to have taken place. The estimated time scales for magma differentiation range from 1000 years to 200,000 years, and if differentiation is linked to fractional crystallisation/crustal melting, it is likely to be thermally controlled. Longer time scales are inferred for magmas that crystallised deep in the crust, and there appear to be fewer evolved magmas in areas of high melt generation rates. It is less clear whether there is any link between different magma series and their time scales of differentiation.