Microscopy of Magmatic Rocks

Abstract

This course provides practical knowledge in magmatic microscopy. It includes the identification of common igneous minerals in thin section and in crystal separates, but also aims at providing a deeper understanding of mineral equilibrium assemblages and disequilibrium textures. These are useful skills in studying magmatic processes and reconstructing igneous conditions.

Objective

The main objectives are to acquire expertise in: (1) Optical determination of minerals in igneous rocks; (2) Identification of igneous rocks and their emplacement history based on mineralogy, structure and texture; (3) Identifying the igneous processes that are revealed by the rock record, and understanding how to use the minerals to reconstruct magma chamber physical-chemical conditions; (4) Application of phase diagrams to natural rocks.

Content

In this class, we’ll look together at how to identify plutonic and volcanic rocks and at what their minerals and textures can tell us about the igneous conditions and styles of eruption. We’ll follow different magmatic lines of descent to understand the evolution of magmas formed in different conditions and tectonic settings, focusing on the tholeiitic, calc-alkaline and alkaline series. We’ll look at how magmatic conditions affect the order of crystallization and the chemistry of minerals, and how we can use this knowledge to reconstruct magmatic processes. We’ll learn about equilibrium assemblages, which allow us to see which minerals grew together and record the same magmatic conditions (this is key for petrology and mineral geochemistry), but we’ll also learn to interpret disequilibrium textures, which relate to processes commonly responsible for volcanic eruptions. The plan is to equip the participants of the course with a basic understanding of what we need to look for in the mineral and textural record of igneous rocks, of how to identify those features and how to use them correctly when studying magmatic systems.

Resources