Ubiquitous dendritic olivine constructs initial crystal framework of mafic magma chamber

Abstract

Layered intrusions are fossilized mafic magma chambers in the Earth's crust. The pathways that led to crystallization and solidification of layered intrusions have been hotly debated as the growth model of primocrysts (the earliest-formed crystals) in mafic magma chambers remains enigmatic. In this study, we carried out high-resolution elemental mapping of mm-scale olivine primocrysts from the Sept Iles layered intrusion (Canada), the third largest one in the world, with a focus on phosphorus (P) zoning of olivine. Our results reveal that complex P zoning of olivine with intense dissolution textures is ubiquitous in the ∼4.7 km-thick Layered Series of the intrusion. The P-rich zones of olivine are featured with dendritic, hopper and sector-zoned patterns, which are attributed to significant magma undercooling. Thermal modeling based on a 1-D conductive cooling model suggests that initially hot parental magma intruding into cold country rocks would result in high degrees of undercooling (-ΔT >60 °C) in the margins (i.e., floor, roof and sidewalls) of magma chamber, facilitating rapid growth of dendritic olivine, which may be then spread within the magma chamber by dynamic convection and crucial to construct initial crystal framework of a solidifying magma chamber. Additionally, diffusion modeling based on the P gradients in olivine suggests a minimum cooling rate of 2.7 to 3.3×10−3 °C/year in the center of the intrusion, similar to the averaged cooling rate of other layered intrusions (e.g., Bushveld, Stillwater and Skaergaard) reported in previous studies. This indicates that rapid cooling (ca. 10−2 to 10−3 °C/year) at high temperature (>800 °C) may be predominant regardless of the size of magma chambers. Our study demonstrates that P zoning of olivine is powerful in decoding crystallization and thermal histories of mafic-ultramafic intrusions.