BH250-112
Title
Subject
Vesicular Olivine Basalt with Vapor-Phase Mineralization
Olivine Basalt Showing Degassing Textures and Vapor-Phase Crystal Growth
Description
Vesicular Olivine Basalt with Vapor-Phase Mineralization and an Olivine-Rich Crystal Aggregate
Description
Major Minerals: plagioclase, olivine, volcanic glass
Minor Minerals: clinopyroxene, biotite/phlogopite, Fe-Ti oxides, vapor-phase minerals
Textures: porphyritic, vesicular, pilotaxitic, intergranular, glomeroporphyritic, vapor-phase crystallization, crystal accumulation
Overview
BH250-112 is an exceptionally instructive volcanic rock that preserves several stages in the evolution of a basaltic magma. The hand sample consists primarily of a dark gray vesicular basalt containing scattered phenocrysts and abundant gas cavities. Near the upper portion of the specimen, a highly vesicular scoriaceous zone records intense volatile exsolution during eruption. Adjacent to this zone is a conspicuous olivine-rich domain (BH250-112C), composed of densely packed green crystals that contrast sharply with the surrounding basalt.
Thin section observations reveal a complex history involving magma crystallization, crystal accumulation, degassing, vapor-phase mineral growth, and late-stage alteration. Together, these features make BH250-112 one of the most valuable volcanic teaching specimens in the BH250 Collection.
Hand Sample Characteristics
The basalt is dark gray to black and contains abundant vesicles ranging from less than one millimeter to several millimeters in diameter. The upper portion of the specimen exhibits a reddish-brown scoriaceous zone characterized by extreme vesicularity, indicating rapid volatile loss during eruption and cooling.
The most distinctive feature is the green olivine-rich inclusion exposed near the upper right side of the sample. This crystal-rich zone contains abundant olivine grains and appears texturally distinct from the surrounding basalt. The sharp contact suggests that it may represent an olivine-rich crystal aggregate, cumulate fragment, or crystal-rich enclave entrained within the lava prior to eruption.
Petrography of the Host Basalt
The basaltic groundmass consists primarily of plagioclase microlites, volcanic glass, minor pyroxene, and opaque oxide minerals. Many areas display a pilotaxitic texture in which plagioclase microlites show weak preferred orientation within a finer matrix, reflecting flow and crystallization during lava emplacement.
Larger plagioclase crystals occur as phenocrysts scattered throughout the rock. Under crossed polars, these grains display well-developed albite twinning and low first-order interference colors. Several crystals show fractures and local deformation features, including bent twins and irregular extinction patterns, suggesting mechanical stress during cooling or eruption.
Opaque minerals, likely titanomagnetite and related Fe-Ti oxides, occur throughout the groundmass and contribute to the dark color of the rock.
Olivine Mineralization
Olivine is one of the most important minerals present in BH250-112. Numerous grains exhibit the high birefringence and bright interference colors characteristic of olivine under crossed polarized light. Many crystals are subhedral to anhedral and show irregular fracturing produced during cooling and contraction.
The olivine-rich domain is particularly significant. In thin section, this region consists of closely packed olivine crystals forming a crystal-supported texture. Such concentrations commonly develop when early-formed olivine crystals accumulate within a magma chamber or lava conduit before eruption.
The texture suggests that the olivine-rich material crystallized prior to final emplacement of the host basalt. Whether this domain represents a crystal accumulation, a cumulate fragment, or a partially disaggregated xenolith cannot be determined with certainty, but its presence demonstrates the physical segregation of early-crystallizing minerals from the surrounding melt.
Vesiculation and Degassing
The abundance of vesicles throughout the specimen provides direct evidence for volatile-rich basaltic magma. As the magma ascended toward the surface, decreasing pressure caused dissolved gases to exsolve and form bubbles. These bubbles expanded rapidly during eruption, producing the highly vesicular texture observed in both hand sample and thin section.
The progression from relatively dense basalt to highly vesicular scoria records increasing gas concentration and decreasing confining pressure. The vesicles preserve a frozen record of magma degassing and provide valuable insight into eruptive processes.
Vapor-Phase Crystallization
Among the most spectacular features preserved in BH250-112 are the delicate crystal linings developed within many vesicles. Several photomicrographs show radiating sprays of euhedral crystals growing inward from cavity walls. Individual crystals are elongate, prismatic, and commonly arranged in fan-shaped or spherulitic aggregates.
These textures are classic examples of vapor-phase crystallization.
Following emplacement of the lava, residual magmatic gases circulated through the interconnected vesicle network. These gases carried silica, alkalis, and other volatile components released from the cooling magma. As temperatures declined, minerals precipitated directly onto vesicle walls and grew into open cavities.
The resulting textures indicate growth in an open-space environment rather than crystallization from a liquid melt. The minerals responsible for these crystal sprays may include silica polymorphs such as cristobalite or tridymite, alkali feldspar, or zeolitic phases. Definitive identification would require analytical work, but the morphology strongly supports a vapor-phase origin.
These features provide outstanding examples of post-eruptive mineral growth and demonstrate how volcanic gases can continue to modify a rock long after primary crystallization has ceased.
Mafic Minerals and Late-Stage Magmatic Evolution
Several photomicrographs contain brown pleochroic minerals interpreted as biotite or phlogopite. The presence of mica in a basaltic rock suggests that the magma contained elevated concentrations of volatiles, particularly water and potassium-rich components.
The occurrence of mica together with abundant olivine and vesicle-filling minerals suggests that BH250-112 may represent an alkaline basaltic magma rather than a strictly tholeiitic basalt. Such magmas commonly contain higher volatile contents and are capable of producing extensive vapor-phase mineralization during cooling.
Contact Relationships
Thin sections crossing the boundary between the dense basalt and the olivine-rich domain reveal abrupt changes in texture and mineral abundance. The olivine-rich region is markedly coarser and contains a much higher concentration of early-crystallized mafic minerals than the surrounding lava.
This relationship suggests that the crystal aggregate formed prior to eruption and was subsequently incorporated into the host magma. Such features provide valuable evidence for crystal sorting and physical processes operating within magma reservoirs.
Petrologic Interpretation
BH250-112 records a sequence of events that began with crystallization of olivine and plagioclase within a basaltic magma. Some of the early-formed olivine became concentrated into crystal-rich aggregates. As the magma ascended and erupted, dissolved gases exsolved and produced abundant vesicles. Rapid cooling generated the fine-grained pilotaxitic groundmass preserved in the basalt.
Following solidification, residual magmatic gases circulated through the vesicle network and deposited spectacular vapor-phase minerals along cavity walls. These late-stage minerals preserve evidence of continued chemical activity within the cooling lava and represent one of the most visually striking features of the specimen.
Educational Significance
BH250-112 is an outstanding teaching specimen because it preserves numerous volcanic processes within a single rock:
- Basaltic mineralogy
- Olivine crystallization
- Plagioclase twinning
- Pilotaxitic texture
- Crystal accumulation
- Vesicle formation
- Magmatic degassing
- Scoria development
- Vapor-phase crystallization
- Late-stage mineral growth
- Volatile-rich magmatic processes
Students can use this specimen to reconstruct the complete history of a volcanic rock, from crystallization within a magma reservoir to eruption, degassing, cooling, and post-eruptive mineralization. Few samples in teaching collections preserve this range of igneous processes as clearly as BH250-112.
Coverage
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View on ArcGIS Online here
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TAS diagram of sample BH250-112. <iframe width="100%" height="480" src="https://drive.google.com/file/d/1qr9725u4ylZ9hWadIu_Jl7qFc7TNTS8n/preview"