BH250-120
Title
Subject
Description
Minor Mineral: alted mafic minerals
BH250-120 is a Trachydacite sample (collected by Mike Smith, Class of 1999) from near Flagstaff, Arizona. This rock represents a relatively uncommon but geologically significant volcanic lithology within the San Francisco Volcanic Field (SFVF), a region otherwise dominated by basalt, basaltic andesite, and andesite. Trachyte in this setting reflects a more evolved, silica-undersaturated, alkali-rich magma composition, indicating advanced stages of magmatic differentiation. BH250-120 is a fine-grained, light-colored volcanic rock rich in alkali feldspar—primarily sanidine or anorthoclase—with minor amounts of plagioclase, biotite, amphibole, and pyroxene. Quartz is rare or absent, and the rock contains ~60–65 wt% silica, with elevated concentrations of alkali oxides (Na₂O + K₂O). The texture is typically porphyritic, with large alkali feldspar phenocrysts set in a groundmass that exhibits a trachytic texture, where feldspar laths are flow-aligned due to magmatic movement. In thin section, BH250-120 reveals large phenocrysts of plagioclase feldspar, remnants of what were once mafic minerals and volcanic glass. The original ferromagnesian minerals are now completely altered, and the outlines of amphiboles have been replaced by iron oxides. The flow alignment of the plagioclase feldspars is especially prominent and adds to the evidence of viscous, evolved magma flow during emplacement.
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 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 adjust fully to changing conditions.
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.
Coverage
GPS Coordinates: 35.253810, -111.623994