BH250-15
Title
Subject
Description
Minor Mineral: glass
Texture: amphibole dehydration, trachytic flow
BH250-15b is an andesite from Mt. Shasta, California. When plotted on the TAS (Total Alkali–Silica) diagram, it falls within the dacite field, indicating a slightly more evolved composition than typical andesite. Twinned amphiboles and plagioclase feldspar are common. Its mineralogy includes phenocrysts of hornblende and plagioclase feldspar. The matrix consists of fine-grained feldspar and volcanic glass. Both phenocryst types exhibit zoning, suggesting changes in magmatic conditions during crystallization. The hornblende phenocrysts display dehydration reaction rims, with the extent of alteration ranging from thin margins to rims so extensive that they have nearly consumed the amphibole entirely.
In this sample, you can observe hornblende dehydration, a key mineral reaction that records changing magma conditions during ascent. Hornblende is a hydrous amphibole, stable only at relatively high pressure and in water-rich magmas. As magma rises, pressure decreases and dissolved water exsolves, destabilizing hornblende. It then breaks down into minerals stable under drier, lower-pressure conditions, typically forming pyroxene, plagioclase, and iron oxides such as magnetite, while releasing water vapor.
This process commonly produces reaction rims around hornblende crystals. These dark, opaque “opacite rims” consist of very fine-grained oxides and pyroxenes and indicate that the dehydration reaction began but did not go to completion, usually due to relatively rapid magma ascent. In contrast, complete alteration occurs when hornblende is entirely replaced by these secondary minerals, preserving only the original crystal shape as a pseudomorph. In some cases, the breakdown products may also include minor glass or extremely fine-grained intergrowths, reflecting rapid quenching near the surface.
The thickness and development of these rims provide important clues about magma dynamics. Thin rims suggest rapid ascent and limited time for reaction, whereas thick rims or complete replacement indicate slower ascent, longer residence at shallow depth, or more extensive degassing prior to eruption. These textures are therefore considered classic disequilibrium features, showing that minerals did not have time to fully adjust to changing conditions.
Compared to BH250-15b and BH250-120, where most of the hornblende is completely dehydrated and replaced, this sample preserves earlier stages of the reaction. The presence of partial reaction rims here suggests that magma ascent was faster or that the crystals experienced less time under low-pressure, degassing conditions. In contrast, the more complete dehydration observed in BH250-120 indicates longer residence at shallow depths or more extensive pre-eruptive degassing, allowing the reaction to proceed to completion.
Overall, hornblende dehydration textures are powerful indicators of magma evolution. Because hornblende stability is strongly tied to water content, its breakdown provides direct evidence of pre-eruptive degassing and decompression in volcanic systems. These features are especially common in intermediate magmas such as andesite and dacite and are widely used to interpret ascent rates, storage conditions, and eruption dynamics in volcanic arcs.
Optics: pleochroism in amphibole, Carlsbad twining