Petrology field note
Olivine phenocrysts preserved in basalt
The large olivine crystals formed before the fine-grained basaltic groundmass, preserving part of the magma's journey from the mantle to the surface.
- Mineral
- Olivine
- Host rock
- Basalt
- Texture
- Porphyritic
Partial melting in the mantle
Olivine is one of the common minerals of the mantle. Partial melting in the upper mantle produces mafic magma that can carry an olivine-rich chemical signature toward the crust.
Early crystallization
As magma rises and its pressure and temperature change, olivine crystallizes at relatively high temperature. Slow cooling at depth gives these early crystals time to grow into comparatively large phenocrysts before the surrounding melt solidifies.
Transport and interaction during ascent
During ascent, fluid motion and buoyancy redistribute the phenocrysts through the magma. The crystals may continue to exchange elements with the surrounding melt, and magma mixing can promote further growth or modify their composition.
Rapid cooling during eruption
After eruption, the remaining melt cools rapidly and forms a fine-grained groundmass. The olivine crystals that had already grown at depth remain much larger, producing the characteristic porphyritic texture seen in this sample.