Insights Into Magma Reservoir Dynamics From a Global Comparison of Volcanic and Plutonic Zircon Trace Element Variability in Individual Hand Samples

Abstract

Trace element compositional trends in zircons separated from single hand samples have been used to infer dynamic processes in magma reservoirs. Here, we compile published zircon trace element chemistry to quantify any systematic difference between the range of compositions observed in zircon from individual volcanic and plutonic hand samples and compare these results with geochemical modeling to derive implications for magma reservoir dynamics. We find that both rock types span a wide range of hand-sample scale variability (i.e., wide range of coefficients of variation), but there is no systematic difference in the average variability between plutonic and volcanic samples (i.e., no difference in the mean coefficient of variation). This indicates that dynamic processes related to eruption are not necessarily required as a fundamental process to create hand sample-scale compositional heterogeneity beyond what is present due to dynamic processes in the reservoir recorded in plutonic samples. Modeling of felsic systems (>68.5 wt.% SiO₂) indicates that the similar average variability in felsic volcanic and plutonic hand samples cannot be reproduced by closed-system crystallization of compositionally distinct melts locally within a magma reservoir (i.e., isolated melt pockets in a crystal mush) but requires mixing of at least two felsic melt compositions at a small spatial scale. This study provides a framework for focused studies on individual volcanic-plutonic systems exploring how plutonic and volcanic zircon compositional variability records the time and length scales of magma reservoir processes.