Zircon destiny during a lifespan of a magma chamber: insights from numerical simulations

Séminaire d’Oleg Melnik (Russian Academy of Sciences).

Sur la base de modèles numériques de formation des chambres magmatiques, Oleg discutera l’importance des différents paramètres contrôlant l’évolution de ces chambres, l’éruptabilité des magmas et comment ces paramètres sont enregistrés par les zircons.

Abstract

Formation of a crustal magma chamber involves periodic supply of heat and material from a deeper source. Dike injection into country rocks is a main mechanism of magma transport. Developed 2D numerical model of magma body formation in granitic crust by injection of rhyolitic or andesitic dikes and sills allows to understand the role of governing parameters such as magma supply rate, temperature, and the size of injection zone on the evolution of the system. The model provides temperature histories in individual batches of magma and country rocks that are used for prediction of zircon crystallization/dissolution, trace element and zirconium isotopes profiles.

Simulations reproduce incremental accumulation of intruded magma into magma chambers generating eruptible and interconnected magma batches with melt fraction >50 vol% that form in clusters. The rate of melt production is highly variable in space and time. High injection intensities produce magma reservoirs capable of large eruptions, while repetitive eruptions lead to shrinkage of magma bodies. Due to rock melting most of zircons loose significant portion of their old cores and, thus, average age. Magmatic zircons in the periphery of the intrusion form very quickly while in its central part crystals contain old cores and young rims and can grow during several hundreds of ka. A case study of Elbrus long-lived volcanic system will be presented.