Journal Article

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

Cooper, Kari M., Ratschbacher, Barbara C., Keller, C. Brenhin

2024

Overview

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. Plain Language Summary Studies of volcanic rocks (erupted magmas) and plutonic rocks (unerupted magmas) provide insights into dynamic processes operating in magma reservoirs (e.g., mixing, crystal‐melt separation, etc.). However, contemporaneous volcanic and plutonic rocks of the same magmatic system are rarely exposed together, thus conceptual models of magma reservoir dynamics are seldom integrated directly between volcanic and plutonic studies. Zircon is a common mineral in crustal magmas (volcanic and plutonic) and is capable of recording melt evolution via its trace element chemistry. This study aims to gain insights into magma reservoir dynamics by systematically comparing trace element compositional variability of zircon separated from individual volcanic and plutonic hand samples. Our study shows that there is no systematic difference in the average compositional variability between plutonic and volcanic hand samples. This indicates that processes leading to eruptions do not necessarily introduce compositional heterogeneity beyond what is present due to dynamic processes in the reservoir recorded in plutonic samples. We further show using geochemical modeling that the observed similar average variability of zircon in felsic volcanic and plutonic hand samples cannot be reproduced by closed‐system crystallization (i.e., isolated melt pockets in a crystal mush) but requires mixing of at least two felsic melts (i.e., open‐system behavior). Key Points Zircon trace element chemistry from volcanic and plutonic hand samples does not show a difference in their average compositional variability Similar variability suggests that processes leading to eruption do not introduce systematically more heterogeneity than present in unerupted parts of the reservoir Crystallization modeling requires open‐system behavior at a scale of decimeters to reproduce the average variability in both volcanic and plutonic hand samples

• Publisher: John Wiley & Sons, Inc,Wiley
• Subject: Coefficient of variation / Crystallization / Geochemistry / Heterogeneity / Lake dynamics / Lava / Magma / Magma chambers / magma reservoir dynamics / Modelling / Reservoirs / Silica / Trace elements / Variability / Volcanic eruptions / Volcanic rocks / volcanic‐plutonic connection / Zircon / zircon trace elements