Abstrakt
The order of crystallization of minerals from melt is of prime importance for an understanding of magma fractionation and chemical differentiation from the magma chamber to the planetary scale. Determination of the crystallization sequence based on petrographic observations, however, is often ambiguous; especially in multiply saturated, nearly eutectic felsic melts.
This paper presents a novel approach to estimate the order of crystallization of minerals in igneous systems based on a quantitative study of their spatial distributions. Statistical modelling of crystallization demonstrates that later crystallizing mineral phases are generally more clustered.
A simple inversion model is then derived to calculate the crystallization sequence and the volume fraction of older minerals present in the system at the onset of crystallization of a later (younger) phase. Application of the model to a sample of equigranular granodiorite (Fichtelgebirge granite batholith, Germany) indicates that plagioclase was the first liquidus phase.
It was followed by biotite, K-feldspar, and quartz at 41, 48, and 63 vol. % crystallized, respectively, which is in qualitative agreement with experimental phase equilibria results for moderately hydrous granitic melts. If phase equilibria for a given composition are known or assumed, the crystallization sequence thus constrains the intensive variables (e.g., water content) and their evolution during magma solidification.
The model thus provides a novel and independent approach to reconstruct the magma crystallization path that would be inaccessible by other methods.